WO1992001334A1 - Magnetic structure and power converter for light sources - Google Patents

Abstract

A light source for the powering of a gas lamp (26) having a power converter circuit (20) that converts a primary voltage source (30) to a high frequency ac voltage. The power converting circuit (20) having an oscillator circuit (48) that converts the primary voltage source to the high frequency ac voltage through the use of a pair of switching transistors (Q1, Q2) each connected to one of a pair of first primary windings (W3, W4) to establish a high frequency alternating current in a first secondary winding (W5). The converter circuit (20) also includes a continuous loop isolation circuit (60) that is coupled to the oscillator circuit (48) via the first secondary winding (45) and coupled to the load supplying output winding (W8) via a second primary winding (W6), the load supplying output winding (W8) being coupled to supply the high frequency ac voltage to the gas lamp (26).

Description

MAGNETIC STRUCTURE AND POWER CONVERTER

FOR LIGHT SOURCES

This application is a continuation in part of U.S. Patent

Application Serial No. 07/318,703 entitled CASSETTE LIGHT,

POWERING UNIT THEREFORE, MULTI-DYNAMIC SMART MAGNETIC STRUCTURE

AND METHOD by Beckrot et al. filed on March 3, 1989, which is a continuation-in-part of U.S Patent Application Serial No.

07/206,336 entitled SELF ILLUMINATED COOL LIGHT DISPLAY, METHOD

AND BALLAST by Helling et al. filed on June 13, 1988, which is a continuation-in-part of U.S. Patent Application Serial No.

07/131,752 entitled COMBINATION BALLAST AND COLD CATHODE SEALED

LAMP AND METHOD by Helling et al. filed on December 11, 1987.

TECHNICAL FIELD

This invention relates to the field of gas discharge lighting, such as fluorescent lighting, and to the field of light sources and circuits designed to convert electrical power derived from any low frequency single or multi-phase ac service, such as sixty cycle, 120 volts ac (Vac) to a high frequency ac voltage for driving fluorescent lamps, or other forms of high intensity discharge lamps, with a current limited high frequency source of ac voltage.

The present invention provides an efficient, low cost, light weight power converter circuit having high efficiency, high reliability and a minimal parts count. The invention converter circuit uses a novel magnetic structure and is

particularly useful for supplying a source of high frequency, and high amplitude ac voltage to gas discharge type lamp loads. The use of the invention circuit with a cold cathode fluorescent lamp load provides a light source with a sufficient life expectancy to make it economically practical to package the invention converter and lamp load in a sealed disposable unit as a cassette replacement for existing fluorescent lamp loads with the invention offering reduced power consumption and maintenance for an equivalent light requirement. As used herein, "cold cathode lamps" are meant to include neon and other forms of gas discharge lamps which employ electrodes that do not contain filaments or hot cathodes.

BACKGROUND ART

The primary light source for task lights in commercial light fixtures are standard fluorescent light tubes which are typically powered by an alternating current ("ac") of 120 volts.

Fluorescent light tubes are generally placed in a light fixture positioned in the ceiling of an area to be lighted. A ballast, or a power converter, is used within the fixture to start the lamp load and to convert the main ac power to an appropriate level of current limited ac power for use by the fluorescent lights. U.S Patent No. 4,484,108 issued on November 20, 1984, describes a "High Frequency Ballast-Ignition System for Discharge Lamps".

Several factors characterize the performance features of a light source driven by a ballast or a power converter. The efficiency of a ballast is related to the power factor

obtainable by the ballast. The power factor is understood to be the cosine of the angle of the current into the converter with respect to the applied ac voltage. The power factor for a conventional fluorescent light ballast is typically in the range of 0.92. The efficiency of a light source in many applications is measured as the ratio of lumens per watt.

Conventional ballasts, of the electromagnetic core and coil variety, have several disadvantages, but enjoy a continued popularity due to low cost. Their disadvantages include the facts that they are very heavy and are relatively expensive to operate. They are typically large geometry reactive devices having sufficient reactance to limit the current flow through a lamp load to a desired level. The service current to the lamp load heats the ballast. Conventional design practice allows the ballast designer to reduce the amount of copper in the windings to obtain a predetermined temperature rise in those wires closest to the core reactor. A temperature rise of 40° C is typically permissible. This design practice saves at the time of production but results in continued loss of heat from the unit once the unit is in service. The increase in cost of operation of the conventional ballast reflects an added burden on to the operating cost of other facilities such as air

conditioning.

Another problem with conventional ballasts is that the use of a dimmer control to reduce the applied voltage to a lampload will cause undesirable light flickering and erosion of the hot cathode. U.S. Patent No. 4,686,426 issued to Burke on August 11, 1990, describes a "Fluorescent Lamp Dimming Switch." As the applied voltage to the ballast is reduced even further,

conducted line emission noise may also become a problem.

Cold cathode systems (or cold light) systems may be

substitutes for state-of-the art fluorescent lights which use a heated filament or hot cathode, and cold cathode systems, heretofore, have not been a practical alternative for task lighting. A conventional ballast driven cold cathode system consists of 60 cycle ac high voltage magnetic component that steps the 60 Hz mains 120 Vac up to voltages of 10,000 volts or more but which has a low power factor. The prior art cold cathode light described in above U.S Patent Application

07/131,752 used a 24 volt direct current (dc) or ac but also was characterized by a low power factor. Additional disadvantages of cold cathode systems have been the need to have expensive capacitors, chokes, resistors and other semi-conductors to control voltage and current, and the fact that insufficient light levels have been produced for task areas.

Ballasts for light sources in general require special dimmer controls which create noise, can induce flickering, and can create excessive wear on the lamp load, such as a

fluorescent tube.

Electronic ballasts, introduced in the U.S. about 1979, operate at higher frequencies than conventional ballasts, and improve the power dissipation, humming, flickering and heat-generation problems associated with conventional ballasts.

Electronic ballasts are more expensive than the conventional ones and to date have had significant reliability problems.

Electronic ballasts are generally limited to a specific load and are designed to drive a limited bulb configuration (such as two four-foot hot cathode fluorescent lamps). Further, in a typical electronic circuit, a short-circuit in the output load will cause the input circuit to fail. Electronic ballasts employing integrated circuits generally are susceptible to failure either because of transient voltage spikes or because of the extra number of components added to protect the circuit from such spikes. The additional components also consume power and decrease the efficiency of the circuit.

None of the patents and patent applications above-cited are believed to provide a power converter with an isolation circuit such as that included in the invention. The isolation circuit, which is incorporated in the magnetic structure, protects the circuit from transient voltage spikes and obviates the need for inductors, capacitors, resistors, and other external protective devices. As a result of the fewer number of necessary

components, the efficiency of the unit is increased. Because the output winding (secondary) is separated from the main primary windings (which are incorporated in the magnetic

structure), the power converter will not fail due to changes in the load, such as a short-circuit or open circuit. Because of the invention's self-sensing winding structure, it can power a multitude of light sources.

DISCLOSURE OF THE INVENTION

It is an objective of the present invention to provide a unique power converter and method that converts low-voltage power into high voltage power.

It is a further objective to provide an oscillating circuit which provides asymmetrical switching of input power.

It is yet a further objective to provide a magnetic

structure to transmit the input power to the load. It is an additional objective to provide an isolation circuit to transmit input power from the oscillator circuit to the load.

It is another objective, in an alternative embodiment, to provide an input voltage circuit which induces power into the core of the magnetic structure to extend the converter's ability to operate at lower levels of input power.

According to the invention, a power converter using the above magnetic structure comprises in combination means for receiving a low voltage main input power supply to operate the power converter to supply power to a load-supplying output winding.

An oscillator circuit is disposed to receive the input power supply and includes a pair of switching transistors. Each of the switching transistors are connected to one of a pair of the first primary windings to establish an alternating current in the first secondary winding. The oscillator circuit includes a pair of sensing windings with one of each sensing winding connected to one gate of the transistor and coupled to one of the first primary windings to provide feedback voltage to the gates or bases of the transistors.

A continuous loop isolation circuit is coupled to the oscillator circuit via the first secondary winding and the load supplying output winding via the second primary winding to induct high frequency power into the load supplying output winding. In an alternative embodiment, an input voltage circuit is connected to the input power supply and is disposed to provide a voltage via a main input winding coupled to induct power into the load supplying output winding to even out the high frequency power.

A magnetic structure is provided that comprises in

combination, a pair of first primary windings each disposed to receive power via an oscillator circuit from one of a pair of transistors to establish alternating current in a first

secondary winding. A pair of sense windings are provided. Each sense winding couples a turns ratio related feedback voltage from one of the pair of first primary windings to provide regenerative voltage feedback to a gate of one of the pair of transistors. A second primary winding is connected via a continuous loop isolation circuit to the first secondary winding to induct power into a load supplying output winding.

The magnetic structure includes a first core which receives the pair of first primary windings and sense windings, a second core which receives the main input winding, and a third core which receives the load supplying output winding. The first core also receives the first secondary winding and the second core also receives the second primary winding.

In an alternative embodiment a main input winding is disposed to receive power from a main power source and to induct power into the load supplying output winding to even out power inducted by the second primary winding. As described above, the power converter using the magnetic structure may in turn be used in any gas filled lamp load, such as fluorescent lights, mercury vapor lights, low-pressure sodium lights or cold cathode lights. Gas filled lights may be structured into a cassette light adapted to be received into the housing portion of a light fixture. The cassette light comprises an open cassette, enclosure having a light diffuser closing the enclosure. A lightable tube(s) and the power converter are affixed within the enclosure to provide power to energize the lightable tube.

Fastening means attaches the cassette enclosure to the light fixture. The cassette enclosure and the diffuser may form a sealed unit which encases the power converter and the lightable tube(s), which in one embodiment is a cold cathode lightable tube.

As pointed out in greater detail below, the power

converter, which comprises an oscillator to convert low

frequency power into high frequency power via the unique

magnetic structure and the isolation circuit for a light source, provides many advantages. The power converter has a high power factor because of the unique magnetic structure and reduces noise normally associated with high power factor ballasts.

The magnetic structure and power converter are particularly suited to provide power to cold cathode light sources and readily adjust voltage and current levels to the varying voltage and resistance demands of such lights.

Alternatively the input voltage circuit in combination with the continuous loop isolation circuit enables the power

converter to operate with a dimmer to appropriately reduce power loads to the light source. The invention itself, together with further objects and attendant advantages, will best be

understood by reference to the following detailed description, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the power converter

including the magnetic structure;

FIG. 2 is a schematic diagram of the light source depicting the converter circuit, the magnetic structure and lamp load;

FIG. 3 is an electrical circuit diagram of the magnetic structure for the circuit of FIG. 2, and it depicts the physical arrangement of E cores in the preferred embodiment of the magnetic structure;

FIG. 4 is an electrical circuit diagram of an alternative embodiment of the light source converter circuit depicting the magnetic structure, lamp load, the oscillator circuit and an input voltage circuit connected to the input power supply.

FIG. 5 is a schematic diagram of the oscillator circuit suitable for use with the magnetic structure of FIG. 2 and FIG. 7; and 74;

FIG. 6 is an electrical circuit diagram of the magnetic structure of the circuit of Figure 4;

FIG. 7 is a schematic diagram of the magnetic structure illustrating the windings positioned on E cores;

FIG. 8(a) is an exploded view of the magnetic structure positioned on the cores, and FIG. 8(b) is an assembly of the magnetic structure;

FIG. 9(a) is a front view of a bobbin used for the load supplying winding of the magnetic structure, FIG. 9(b) is side elevation view of the bobbin; and FIG. 9(c) is an enlarged view of the cross-over slot of FIG. 9(a).

FIG. 10 is an electrical circuit diagram of the load supplying winding;

FIG. 11 is a front plan view of the bobbin having the load supplying windings positioned on the bobbin;

FIG. 12a is a graph of the voltage on capacitor C1 with respect to ground with the current through L1 superimposed on the graph to obtain phasing relationships; FIG 12b is an expanded view of insert region M in FIG 12a to clarify the amplitude and frequency of currents in L1;

FIG. 13a is a graph of the voltage at the drain of Q1 with respect to the source with the current through Q2 superimposed on the voltage waveform; FIG. 13b shows the voltage applied to the gate of Q1;

FIG. 14a is a graph of the voltage across the lamp load at 2ns/div driven by the circuit of FIG. 2; FIG. 14b is a graph of the lamp voltage with the high frequency lamp current

superimposed on the lamp voltage at 2ns/div;

FIG. 15 is a graph of the test light pattern in lumens of output for a group of four fluorescent lamps;

FIG. 16 is a graph of the test light pattern in lumens of output for one cold cathode fluorescent lamp;

FIG. 17a is a top plan view of the cassette light with the diffuser removed and FIG. 17b is a side view of FIG. 17a;

FIG. 18 is a cross-sectional view taken along line 18-18 of FIG. 17b with the diffuser shown assembled over the cassette enclosure;

FIG. 19 is a schematic diagram showing a plurality of cassette lights positioned in a room with the power converter of each cassette light connected to a master dimmer control; FIG. 20 is a perspective view of a cassette light, shown without a door frame used as a retrofit in a fluorescent light fixture;

FIG. 21 is a perspective view of the door frame for FIG. 7 and FIG. 22;

FIG. 22 is a perspective view of the interior of the fixed housing for receiving the parts of 20 and 21;

FIG. 23 is a partial sectional view of the cassette of FIG. 20 taken on line 23 - 23;

FIG. 24 shows a partial section of the retrofit cassette housing in relation to the housing.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a perspective view of an assembly representing the power converter 20 using the magnetic structure 22. The electrical components are assembled on a substrate or circuit board 23. Inductor L2 is omitted in the preferred embodiment circuit but is installed in an alternative embodiment.

In the preferred embodiment, an electrical circuit diagram of FIG. 2, a power converter 20, represented by a first phantom box 20, drives a light source 26 or load. An oscillator circuit 48 represented by a second phantom box 48, as shown in FIG. 2, includes an oscillator circuit. The magnetic structure 22 resides between first phantom box 20 and second phantom box 48.

A third phantom box 30 represents low voltage main input power supply or input power supply to operate the power

converter 20. In a typical application, the primary input voltage source at input terminals at A and B is conventional 60 Hz 120 Vac voltage from the main service of a public power source or from a commercial power company (not shown).

A number of components within third phantom box 30 are desirable for safety. A fuse 32 is used to protect the

electrical service to the power converter 20 from damage in the event of a converter component failure. A transzorbe 34 operates as a clamp to snub short term input transients on the mains that exceed the clamp breakdown limits of the transzorbe that is specified. A 7.0 uF, 250 Vac capacitor 36 is required in some markets. This component is intended to insure that the load does not deliver a net dc current back to the service. In the present power converter circuit, capacitor 36 is used for 220 - 277 Vac applications. It is not used in the power

converter circuit for 120 Vac applications. In 220 Vac

applications, capacitor 36 drops the voltage applied to the power converter to 120 Vac.

The input power supply 30 may supply alternating current ac or direct current dc. In the circuit of FIG. 2, the input power supply 30 includes a dimmer 38 suitable to reduce the input voltage. The dimmer 38 is a conventional ac phase control device capable of at least partially limiting the number of volt-seconds applied to the circuit on each half cycle in response to operation of a control on the dimmer to dim the light.

Although the circuit components within third phantom box 30 represent custom and practice, they may not be essential to practice the invention if the power to the lamp load need not be controlled.

In the embodiment of FIG. 2, full wave rectifier D1 - D4 in combination with capacitor C1 comprise a means 46 for receiving an input voltage from an input power supply 30 or primary voltage source and for converting the input voltage into a dc input voltage necessary for operating the power converter 20.

In the circuit of FIG. 2, the input voltage is applied through the input power supply 30 to first ac terminal 43 and second ac terminal 44. FIG. 12a is a graph of the dc input voltage, waveform (a), applied to the C1, 45 first terminal 46 by full wave rectifier D1 - D4. The value of C1 is

intentionally selected to only partially filter the voltage from the full wave rectifier D1 - D4 to keep the current, waveform (b), into inductor, 24 substantially in phase with the input voltage. The waveforms (a) and (b) in FIG 12a confirm that the dc input current from the full wave bridge is substantially in phase with the dc input voltage at C1 first terminal 46 thereby insuring a near unity powerfactor. Measurements performed on prototype circuits substantially equivalent to the circuit of FIG. 2 have achieved a power factor as high as .94 of unity.

As shown in FIG. 2, the components within phantom box 48 represent an oscillator circuit that is disposed to receive power from the input power supply 30 through a full wave

rectifier D1 - D4, and a pair of switching transistors such as FETs Q1,Q2. These FETs are typically large geometry, N-channel enhancement devices with low RdsON values and high drain to source breakdown voltages. Each FETs is connected to one of a pair of the first primary windings W3,W4. A symmetrical

switching circuitry in the oscillator circuit 48 establishes an alternating current in the first secondary winding W5. Each of a pair of sensing windings W1,W2 are connected to a gate 54, 56 of the transistors Q1,Q2 and coupled to one of the first primary windings W3,W4 to provide feedback voltage to the gates of transistors Q1, Q2 in response to voltage being applied to the primary windings.

FIG. 13a and FIG. 13b depict the relationships between voltages and currents in the oscillator circuit 48. The

invention power converter 20 of FIG. 2 and FIG. 4 have been found to operate most efficiently in the range of 70 KHz to 110 KHz. The preferred operating frequency for a plastic backed cassette was approximately 80 KHz and the preferred operating frequency for a metal backed cassette was approximately 90 KHz. The oscillator circuit of FIG. 2 operates at a frequency of approximately 92. KHz. Each of the switches Q1 and Q2 are turned on alternately for a 5 nano second. Capacitor C2 and the magnetizing inductance of the pair of primary windings W3 and W4 form a resonant tank circuit that is powered with current passing through L1, 24.

As each switch turns on, it substantially grounds one side of the winding W3 or W4. Current from inductor L1 drives the common tap 62 between the pair of primary windings maintaining voltage across the winding with an opposing end switched

substantially to ground. The flux density of the core is eventually exceeded and the voltage supported by the

substantially grounded primary collapses causing the voltage on each of the windings to collapse. With no substantial voltage on the gate winding, the switch that was on proceeds to turn off. The magnetizing current in the grounded primary just prior to turn off is supporting flux in the core. The flux in a core can not be changed instantly.

As the FET that was on switches to the off state, the current passing to the drain changes direction and develops a voltage on capacitor C2 with a polarity opposite that of the original polarity. As the voltage on C2 passes through zero and develops in the opposite sense, the transformer action of the primary couples a positive going voltage onto the gate of the FET that had been off driving it into conduction and thereby reinforcing the reversal of the direction of current in the capacitor C2.

The half-cycle on time of the switches QI and Q2 are a design function of the primary core area, the primary turns count and the applied voltage to the center tap 62 and C2. As the center tap voltage is increased, the time out time for a half cycle is reduced. Reducing the primary turns count or the core area also reduces the time out time. FIG. 13b shows that the voltage reflected to the gates of each of the FETs is a mirror of the voltage on the drains and the resulting voltages are quasi sinusoidal.

FIG. 14a is a graph of the voltage applied to a 70 watt cold cathode fluorescent lamp load. The waveform suggests that the circuit accommodates several abrupt mode changes as it adapts to changes in the voltage applied to the center tap resulting from the high ripple voltage on the input filter capacitor C1. FIG. 14b shows the relationship between the voltage across the lamp load and the current through the lamp load at one point in the half cycle. FIG. 3 depicts the circuit of the magnetic structure 22 of FIG. 2, and FIG. 3 shows the arrangement of first, second, third and fourth E cores 64, 66, 68 and 70 that can be used in making the component. Referring to the center of FIG 3a, the first secondary winding W5 and the second primary winding W6 connected in parallel comprise a continuous loop isolation circuit 60. The continuous loop isolation circuit 60 couples power from the pair of primary windings W3, W4 via the first secondary winding W5 to the load supplying output winding W8 via the second primary winding W6 to induct high frequency power into the load supplying output winding W8. The load supplying output winding or load winding W8 is disposed to supply power to the light source 26. The continuous loop isolation circuit 60 controls the impedance of the pair of the first primary windings W3 , W4.

FIG. 4 is a schematic of an alternative embodiment of the circuit of FIG. 2. The input voltage circuit 30 is connected to the ac input power supply at terminals A and B to provide a voltage via a main input winding W7 coupled to induct power into the load winding W8 to extend the starting capability and even running capabilities of the circuit to lower input voltages.

The addition of winding W7 to core 64 along with the addition of L2 to limit the current through W7 is the only difference between the circuits of FIG. 2 and FIG. 4. In the embodiment of FIG. 2, core 70 is used for the inductor L1. In the embodiment of FIG. 4, core 70 might be used for L1 or a separate discrete L1 might be used. Inductor L2 has been tested and found to work satisfactorily in the circuit of FIG. 4 as a discrete component.

The operation of the power converter 20 using alternating current ("ac") for a cold cathode light source is illustrated and described in connection with FIGS. 1, 2, 3, and 4. Since the magnetic structure 22 is an integral part of the power converter 20, and since it is fabricated as a separate assembled component, the magnetic structure connector terminals or the connector terminals between the magnetic structure and other components will also be described for clarity.

In addition, while the two operational embodiments describe a power converter 20 to be useful for driving a cold cathode light load 26 to provide a light source, the power converter 20 and magnetic structure 22 may be readily used in other

applications such as fluorescent lighting and high intensity discharge lamps, i.e., mercury vapor, metal halide, high

pressure sodium and low pressure sodium lamps. The embodiments of the invention power converter 20 should be considered for applications that require the conversion of low voltage from a low voltage source to a high frequency high voltage.

Turning to FIG. 4, the input power is routed to the light source 26, through the power converter 20 by way of a major current path or major path 70 and a minor current path or minor path 72. The major path 70 is through the oscillator circuit 48, and through diode bridge D1 - D4, through the continuous loop isolation circuit 60 and the load supplying output winding W8 powering the light source 26.

As shown in FIG. 4, in connection with the embodiment of the magnetic structure of FIG. 6, the minor path 72, is from the input terminal C through an input voltage circuit 66 comprising a path through inductor L2, to terminal Q, to main input winding W7, with a return from terminal R to a junction at the cathode of D1 and the anode of D4. A reduced level of mains power is coupled via winding W7 to second core 66, to third core 68 to the load winding W8, and then to the light source 26. Figure 6 shows how the first, second and third cores are positioned prior to assembly with the gaps specified in inches. The brackets are intended to indicate which cores the winding are positioned on. The fourth core is shown in phantom to indicate that the

inductor L1 can be formed on the magnetic structure by bonding it to the back of the first core. Winding W8 is shown coupled to drive the lamp load or light source 26. The terminal

connections from the magnetic structure of FIG. 6 to the

oscillator circuit 48 of FIG. 5 are indicated by the bracket at the left of FIG. 6 and the corresponding bracket and respective terms at the bottom of FIG. 5.

Referring now to FIG. 4, the applied voltage from the dimmer 38 is typically a phase controlled source that interrupts the applied voltage for a controlled phase interval beginning at each zero crossing of the input Vac. During these intervals of interruption, the voltage on capacitor C1 falls to a voltage approximating zero volts. The circuit intentionally uses a small value of capacitance for C1 to obtain increased power factor during rated operation. A high ripple voltage is normally present on C1 as shown in FIG. 12a which depicts operation with rated input voltage.

Once started, operation of the circuit becomes intermittent for input voltages below 30 VRMS. With an input voltage in this range, the lower peak values of the voltage on C1 reach levels approximating zero volts for periods of sufficient duration to allow the light source to flicker. Use of the main input winding in connection with inductor L2 provides additional stored energy during each mains power cycle sufficient to provide a low level of 60 Hz drive to the lamp load 26 via the second core, 66, and the third core 68 to maintain ionization in the lamp load for an additional extended range of reduced input voltage.

The preferred embodiment circuit of FIG. 2 has the

advantage of a lower cost due to a lower parts count. As the input voltage is reduced, the circuit of FIG. 2 allows

continuous flicker-free control of the light source 26 as the input voltage is reduced to a typical lower limit of 40 VRMS. By adding the main input winding and L2 to the topology of the circuit of FIG. 2, to obtain the circuit of FIG. 4, the range can reliably be extended to a lower level of 25 Vac.

In operation, the increased complexity and slightly higher cost of the power converter embodiment of FIG. 4 at lower input voltages from a dimmer, provides the effect of filtering or evening out the current or high frequency power passing through output winding W8 to the light source 26; this effect being hereinafter referred to as "to even out high frequencies" or "to even out high currents" in winding W8 to the light source 26. The magnetic structure 22 couples the major path 70 and the minor path 72 together in a manner to provide high frequency power to the load winding W8.

The light source 26 connected to the load winding W8 has a relatively high electrical resistance. The magnetic structure 22 has a typical resistance sufficient to limit the bulb current to less than 100 mA after ionization. The resistance of the light source 26 drops to set a current value in isolation winding 60 depending on the light source load, which is

dependant on factors such as the type of gas in the light source tube, the length of the tubing, the diameter of the tubing, and the temperature within the tube. The power converter 20 and magnetic structure have been used to drive cold cathode

fluorescent lamps with lengths ranging from seven (7) to forty (40) feet.

The major and minor paths 70, 72 of the input power supply 30 begin at terminals A and B, through the dimmer 38 to terminals C and D, with terminal C connected to a lead having a fuse SI and terminal D to a junction J1.

The major and minor paths 70, 72 divide at the junction J1 with the minor path 72 directed to the main input winding W7 and the major path 70 of the input power supply 30 directed to the oscillator circuit 48.

The major path 70 provides power to the full wave rectifier D1 - D4 which converts the ac main current to a dc voltage. The dc voltage is applied via inductor L1 to a common terminal between the primary winding W3, W4.

The dc voltage is also applied to a start circuit along a lead to resistor R1. The resistor R1 is in series with zener diode Z1. The zener diode is biased to its stable test voltage to establish a start reference voltage at junction J3 by current from the resistor R1. The voltage at J3 is applied via leads 53 to the second sense windings W2. The voltage on lead 52 is then applied via resistor R2 to sense winding W1. Resistor R2, has a value of 15 ohms, operates as a damping resistor in series with both sense windings.

The oscillator circuit 48 is made active and oscillatory by operation of the zener voltage at J3 as it raises the gates of QI and F3 into their active ranges via sense windings W1 and W2. Terminal connector V is connected by a lead L4 to a gate 54 of FET QI. Similarly, terminal connector Z is connected by a lead L5 to a gate 56 of FET Q2. Current does not flow in windings Wl and W2 because they are connected in series to each other and they are terminated at the gates of Q1 and Q2. The gates are dielectrically isolated from the circuit.

The gates 54, 56 of the FETS Q1 and Q2 receive opposite phased voltages from the sense windings W1 and W2. Once the voltage on these windings is biased up into the active range of the devices, the smallest imbalance or noise present in the circuit will produce a regenerative response driving the circuit into one of its two stable states.

FETS QI receives drain current via first primary winding W3. Current passes from the full wave rectifier D1 - D4 via junction J2 along a lead L7 to a inductor LI, 24. The current passes through L1, 24 along a lead L8 to a junction J7. The current divides at junction J7 into separate paths along two leads L11 and L10 to provide a current to the drains of FETS Q1, Q2. The FET Q1 current path is along a lead L11 to connector terminal T, through first primary winding W3, connector terminal S and along a lead to Junction J8 on one side of capacitor C2.

The FET Q2 current path is along a lead L10 to terminal connector U, through first primary winding W4, connector terminal Y and along a lead to Junction J9 on the other side of capacitor C2. The FET Q1 current flows along a lead L13 via another lead L14 to a drain 84, awaiting the FET Q1 gate "turn on" to enable current to flow to a source 80. Similarly, FET Q2 current flows along a lead L15 via another lead L16 to a drain 86 awaiting the FET Q2 gate "turn on" to enable current flow to a source 82. Both FET sources 80, 82 are connected to a lead L17 which in turn is connected to ground G2.

When FET Q1 is activated and begins to turn on, a voltage is developed across primary W3 that causes a voltage to be applied to the gate of Q2 to turn Q2 off. The voltage and current waveforms of FIGs. 13a and 13b clearly show that FETs Q1 and Q2 are on or off. The circuit has two stable states. The tank circuit formed by the primaries and the capacitor C2 rings at the resonant frequency but the switches Q1 and Q2 operate decisively within a very short part of each oscillator cycle to reverse their states. The point at which reversal of the states of the two switches occurs is controlled by the time required to saturate the core. As explained above, as the core reaches saturation, the conducting transistor or switch is turned off by the removal of voltage from its gate. The gate drive voltage is in a precise turns ratio relationship with the volts per turn on the primary. As the primary voltage collapses, the switch starts to turn off in response to removal of the gate drive voltage. The process becomes regenerative and the transformer reverses the polarity on each of its windings thereby turning the alternate switch on. The bias circuit of R1 and Z1 insures that the circuit will always start or return to a state centered at a point at which the circuit is most unstable, thereby insuring a return to the oscillating mode.

As shown by the indicating "dots" on FIGs. 2 and 4, the first primary windings W3, W4 are wound in opposite directions to control the sense windings alternately to a voltage below dc ground and switch the FETS Q1 and Q2. As FET Q1 and FET Q2 are switching, a sinusoidal wave form is inducted into the first secondary winding W5. The continuous loop isolation circuit 60 which comprises the first secondary winding W5 and a pair of leads L19, L20 are connected to the second primary winding W6. The second primary winding W6 inducts power into the load winding W8 and the light source 26.

Figure 4 shows that in this alternative embodiment, the minor path 72 receives ac main from one side of the input power supply 30, terminal A, along a lead, via a choke L2 of about 260 m.h., to a main input winding W7. The main input winding W7 inducts power into the load winding W8, but at a lesser level than the second primary winding W6. This occurs because the number of turns on the main input winding W7 are less than the number of turns on the second primary winding W6. The ac is then fed back into the full wave rectifier D1 - D4. The main input winding W7 allows portions of the current to be

eliminated, yet maintains a level output voltage to a light source 26. For example, the dimmer 38 can reduce the power and eliminate portions of the current to permit dimming of the light source 26 by permitting the voltage to be smoothed out across load winding W8. FIG. 4 shows that the magnetic structure 22 for use in the power converter 20 comprises in combination a pair of first primary windings W3, W4 each disposed to receive power via an oscillator circuit 48 from one of a pair of transistors Q1, Q2 to establish alternating current in a first secondary winding W5. A pair of sense windings W1, W2 are each disposed to sense power levels.

By way of example, voltage is coupled in a turns ratio relationship in accordance with conventional transformer turns ratio related voltage coupling rule from a primary winding to a respective secondary winding. Again by way of example, for an ideal transformer with perfect coupling and infinite magnetizing inductance, V1/V2 = N1/N2 = I2/I1. In the foregoing equation, Vn represents a winding voltage, Nn represents the turns count on the nth winding and In represents a current in the nth winding.

The pair of first primary windings W3, W4 provide feedback voltage or power level to the gates 54, 56 of one of the pair of transistors Q1, Q2. A second primary winding W6 is connected via a continuous loop isolation circuit 60 to the first

secondary winding W5 to induct power into a load supplying output winding W8.

The main input winding W7 is disposed to receive power from a main power source and induct power into the load supplying output winding W8 to even out power inducted by the second primary winding W6.

In a shorted output condition, i.e., where the lamp load shorted, as by a fault or bad lamp load, the continuous isolation circuit of windings W5 and W6 operating in cooperation with first core 64, second core 66 and third core 68 to function or to sense the reduced impedance of the load and to impose a limit on the current reflected to the primary.

In addition to the resistance of the output winding, the distributed gaps between the core segments, the resistance of winding W5, and the resistance of LI combine to protect the FETs from catastrophic instantaneous failure in response to a fault across the light source terminals. As the power converter 20 is presented with a fault at its output terminals at W8, and as the voltage at the terminal of W8 is reduced to zero, the reflected feedback voltage to the gates of Q1 and Q2 is reduced to a point at which current through the FETs is limited or the oscillator stalls in a substantially near open switch oscillating mode incapable of delivering destructive levels of current through the primaries to the drains of Q1 and Q2.

The power converter will continue to remain in a

protectively active state without experiencing failure until the fault is removed, after which normal operation will resume with the replacement of the fault load with a serviceable load.

As shown on FIGs. 7, 8(a) and 8, a first core 90 receives the pair of first primary windings W3,W4 and sense windings W1, W2, a first core 92 receives the main input winding W7, a third core 94 receives the load supplying output winding W8, and the second core 90 receives the first secondary winding W5 and the second core 90 receives the second primary winding W6. The first, second, and third cores 90, 92, 94 comprise an E section core with each E section core having a center common core 96 and two outer portions 98, 100. All of the windings W1-W8 are disposed on the center common core 96. A spacer gap 104 separates the second core 92 from the third core 94. A suitable wire for the first secondary winding W5 comprises Litz wire.

As illustrated in FIGs. 9(a), 9(b), 9(c), the load winding

W8 comprises a bobbin having a dielectric strength of at least

600 volts per mill, and preferably comprises DUPONT Rynite. The bobbin comprises three sections 110, 112, 114 having spacers

116, 118, 120, 122 therebetween. As illustrated in FIGS. 8 and

9, the first bobbin section 110 receiving substantially half of load winding W8 in a first direction, the second bobbin section

112 having at least one load winding W8 in a second direction, and the third bobbin section 114 receiving substantially the other half of the load winding W8 in the first direction. The first direction normally comprises a clockwise direction and the second direction also comprises a clockwise direction. The windings cross-over from the first bobbin section 110 to the second bobbin section 112 and from the second bobbin section 112 to the third bobbin section 114 through cross-over points 165.

Preferably, quadruple insulated wire, such as 36 A.S.W., is used for the load winding W8. This prevents breakdowns due to the Corona effect which removes insulation and causes arcing and ultimately replacement of the load winding W8.

Typical values for components for a power converter, including the oscillator circuit, magnetic structure, and input voltage circuit, for a cold cathode light source are illustrated in the following TABLE: TABLE

COMPONENT IDENTIFICATION VALUE

Oscillator Circuit 120 v

Inductor LI 200 mH

105 T, 10 × 36 Litz

Resistor R1 5 watt/22K ohms

Resistor R2 1/4 watt/15 ohm

Zener Z1 1/4 watt/3.9 v

Diode D1,D2,D3,D4 1.5 amp/200 v

Film Cap C3 5.0 mfd/200 v

Film Cap 4700 pf/2000 v

Core ETD 39 ferrite core

Seimens

Bobbin Dupont Rynite

Bobbin Connector 8 pin for ETD core

Windings W8 Quadruple Insulated

2 × 300 T + IT between windings. Wire is 1 × 36 quad insulation

Windings W3,W4 Litz wire/35 T

wire is 10 × 36 litz

Windings W1,W2 IT

wire is 1 × 26 AWG

Winding W5 8IT

wire is 10 × 36 litz Winding W6 98T

wire is 10 × 36

M.O.V. 2 amp/ 170 v

Fuse 3 amp/ 200 v

F.E.T. Q1 SSP4N70 F.E.T. Q2 SSP4N70

/ Heat Sinks H1,H2 5700 Aavid

Note:

**** winding **** W7 18T

**** Choke **** L2 250 M.H.

is only required for the embodiment of Figure 4.

Although the above parts list calls for an ETD 39 ferrite core, it should be understood that other core types will operate including RM, EC, PQ, EP, X, PCH, LP and POT cores; but at a slightly higher cost, and non-optimal form factor in some cases.

As pointed out above, and shown in FIGS. 4, 17a, 17b, 18, and 19, the power converter 20 using the magnetic structure 22 may be used in any light adapted to be received into a housing 128 for insertion into a ceiling structure, such as the cold cathode cassette light 130 shown in FIGs. 17a and 20. The cassette light 130 comprises an open cassette enclosure 132 having a light diffuser 134 closing the cassette enclosure. The light diffuser 134 bears against a peripheral flange 136 of the cassette enclosure 132. The cassette enclosure 132 must be formed of materials, such as steel, aluminum, and the like, which meet Underwriter Laboratories ("U.L.") or building code specifications. As shown in FIG. 17a, a lightable tube 140 and the power converter 20 are affixed within the cassette enclosure 132 to provide power to energize the lightable tube 140. The power converter 20 may be positioned in a raised protrusion 142 on one end of the cassette enclosure 132 which allows the rest of the cassette enclosure 132 to be relatively thin as compared to existing light fixtures. A suitable lightable tube 140 for use with the power converter 20 is a cold cathode tubing array, which can be prepared by various luminous glass bending

companies, such as Everbrite Electric Signs, Inc. of Harbor City, California. The array may be designed in various

configurations, such as one-piece-grid, two separate "S" shapes, separate "U" shapes or a parallel array of straight tubes. The ends of the lightable tube 140 are connected by wires 144, 146 to the power converter 20. Flexible fasteners 150, such as pop rivetted flexible fasteners, position the lightable tube 140 spaced-apart from the rear wall 152 of the housing 128.

As shown in FIG. 23 a reflector 154, such as polished aluminum, or a specular film affixed to a metal substrate may be secured to the back of the enclosure. Alternately, the backing of the cassette structure may be shaped in a contoured reflector design to enhance light output and distribution. Other suitable materials for the reflector 154 include a metallic or polished coating deposited on the back of the cassette enclosure 132, or a shaped light reflective sheet fastened to the back of the cassette enclosure 132.

Turning to FIG. 21 mechanical fastening means 156 attach the cassette enclosure 132 to the light fixture. The cassette enclosure 132 and the light diffuser 134 may form a sealed unit which encases the power converter 20 and the lightable tube 140, which in one embodiment is a cold cathode lightable tube 140.

As illustrated in FIG. 19, a plurality of the cassette lights 130 can be assembled in a building structure's ceiling 158 and controlled by one dimmer 38. The cassette enclosure 132 is sized to fit within commercial housings 128 of FIG. 22 formed into ceilings, such as suspended T-bar ceilings or the like, and is held in the ceiling structure by mechanical means 160, such as T-hooks or the like.

As pointed out in FIG. 2, the unique magnetic structure 22 including a power converter 20 having an oscillator circuit to provide high frequency power to a light source 26 provides many advantages. The power converter 20 has a high power factor because of the unique magnetic structure 22 and reduces noise normally associated with prior art ballasts having high power factors. The input voltage circuit 66 in combination with the continuous loop isolation circuit 60 enables the power converter 20 to operate with a dimmer 38 to appropriately reduce power loads to the light source 26. The magnetic structure 22 and power converter 20 are particularly suited to provide power to cold cathode light source 26 and readily adjust voltage and current levels to the varying voltage and resistance demands of such lights.

A comparative photometric test was performed by Lighting Sciences, Inc. of Scottsdale, Arizona, an independent lighting laboratory, to compare the lighting efficiency of the invention cassette to that of a standard fluorescent light source. The results of those tests are summarized below in examples 1 and 2,

EXAMPLE 1

A commercially available 2 × 4 fluorescent luminaire white painted interior, using a pattern 12 prismatic lens, and four F40T12/CW fluorescent lamps, with a scaled down lumen rating of 1000 lumen and RQM ballasts, was operated at 120 Vac, consumed 159.7 watts and produced an absolute light output of 6311 lumens. The lumens/watt ratio was thus a low 39.5. The test distance was 7.9 meters. The attached chart is a candle power summary. This chart should be read in connection with FIG. 15.

EXAMPLE 2

A prototype cassette light 2 x 4 luminaire metallized interior, with standard pattern 12 prismatic lens , and a cold cathode fluorescent lamp array, with a scaled down lumen rating of 1000 lumens was operated at 117 Vac consumed 91. 6 watts and produced an absolute light output of 5150 lumen. The

lumens/watt ratio was 56.2 , which is substantially higher than the ratio of 39. 5 that was obtained in the test of Example 1 above. The test distance was 7. 9 meters . The attached chart is a candle power summary and should be read in connection with FIG. 16.

Variations on the embodiments described above are possible. For example, the power converter using the continuous loop isolation circuit and magnetic structure could supply high voltage power to a hot cathode fluorescent light source.

Further, a single transistor may be used in the oscillator circuit 48 which can mirror the input power supply 30 in a manner to provide the sinusoidal wave form to the isolation circuit. The magnetic structure 22, while described for use to power a light source, may be used in any asymmetrical

switching oscillator circuit. The above described oscillator circuit 48 is optimized for use in a cold cathode light source.

The cassette light 130 in FIG. 17a and 18 may have the diffuser sealed to the cassette enclosure to form an integral unit. For example, silicone glue, ultra sonic welding or mechanical means may be used to seal the diffuser along its peripheral edge to a peripheral flange of the cassette

enclosure. The sealing inhibits dust or insects from entering the cassette enclosure and helps maintain the light's proper operating temperature.

Also, as illustrated in FIG. 17a - 24, the cassette lights 130 may be used as a retrofit to replace existing fluorescent light fixtures 168. As shown in FIGS. 22, wires to the

fluorescent light ballast 170 are cut and the ballasts 170 may remain within the fixture, or may be removed and safely

disposed. Typically, because the cassette light 130 is

relatively thin, the power converter 20 protrusion 142 of FIG. 18 avoids engagement with the existing fluorescent light

ballasts. FIG. 24 shows the thinness of the cassette light 130 positioned in a fluorescent light fixture 168 with the

fluorescent light ballast 170 spaced between the back of the fluorescent light fixture 168 the back of the cassette enclosure 132. Also, because the existing light fixture 168 already meets U.L. or code standards, the cassette enclosure may be

constructed with light weight materials, such as safety-approved plastics, aluminum or the like.

The dimensions shown in FIG. 17a are in inches and are presented by way of illustration only. The light fixture can be designed to many dimensions.

The cassette light 130 may be positioned into a door frame 172 of FIG. 21 which receives the cassette light 130 and is designed to take advantage of the original fastening scheme the fluorescent light fixture manufacturer employed. For example, as shown in FIG. 24, the old fluorescent light fixture 168 is supported by T-bars. T-slots in the fluorescent light fixture receive T-hooks fastened to the door frame and form a hinge to support the cassette light 130. The cassette enclosure may be hinged to an existing fixture in place of a conventional lens or louvre.

As shown in FIG. 23, the door frame 172 supports the cassette light 130 by the peripheral flange 126 and the

peripheral edge 174 of the diffuser 126. Light waves 180 emitting from the lightable tube are schematically illustrated as being directly emitted through the light diffuser 126 and indirectly off the reflector 154 on the back of the cassette enclosure 132 and through the diffuser 126.

FIG. 20 and FIG. 21 show the door frame 172 positioned to receive a cassette light in relation to the existing fluorescent light fixture 168 and FIG. 24 shows the combined unit being positioned into the existing fluorescent light fixture 168, which is supported by a T-bar ceiling 190. Once the cassette light 130 is fully supported, the power converter 20 is then connected to the wires which provide the main input power supply 30.

Of course, it should be understood that a wide range of changes and modifications can be made to the preferred

embodiment described above. By way of example, if the source voltage source available is dc, the power converter, with modifications to accommodate the higher average input voltage for a given peak amplitude, can be operated off the dc service with little or no change. The input low voltage source

comprising the D1 - D4 bridge can be left out of the circuit with the input A and B leads polarized to forward bias the diodes or the bridge.

In normal operation, the oscillator is powered from the high ripple dc source on C1 so operating off of a relatively clean dc source would be a small challenge. It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, which are intended to define the scope of this invention.

Claims

WE CLAIM:

1. A magnetic structure for use in a power converter comprising in combination:

a pair of first primary windings, each respective first primary winding being disposed to receive power via an

oscillator circuit from one of a pair of transistors to

establish alternating current in a first secondary winding;

a pair of sense windings, each sense winding being disposed to sense voltage in one of said pair of first primary windings and to provide feedback voltage to a gate of one of said pair of transistors; and

a second primary winding connected via a continuous loop isolation circuit to said first secondary winding to induct power into a load supplying output winding.

2. The magnetic structure of claim 1 further comprising: a first core receiving said pair of first primary windings, said pair of sense windings and said first secondary winding; a second core, said second core being characterized to receive said second primary winding; and,

a third core, said third core being characterized to receive said load supplying output winding.

3. The magnetic structure of claim 1 further comprising: a main input winding disposed to receive power from a main power source and induce power into said load supplying output winding to even out power inducted by said first primary

winding; a first core receiving said pair of first primary windings, said pair of sense windings and said first secondary winding; a second core receiving said second primary winding, and said main input winding; and,

a third core receiving said load supplying output winding.

4. The magnetic structure of claim 1, further comprising: a main input winding disposed to receive power from a main power source and induct power into said load supplying winding to even out power inducted by said second primary winding.

5. The magnetic structure of claim 2, wherein said first, second, and third cores comprise an E core section, each E core section having a center post including all of said windings being disposed thereon.

6. The magnetic structure of claim 2, wherein said first secondary winding comprises Litz wire.

7. The magnetic structure of claim 2, wherein said load supplying output winding further comprises:

a bobbin positioned on said third core, said bobbin being characterized to receive said load supplying output winding, said bobbin having a dielectric strength of at least 600 volts per mill.

8. The magnetic structure of claim 7, wherein said bobbin comprises a first, second and third section, said first and second section having a first spacer therebetween, and said second and third section having a second spacer therebetween, said first bobbin section receiving substantially half of said load winding in a first direction, said second bobbin section providing a predetermined separation between said first and third sections, said third bobbin section receiving

substantially the other half of said load winding in said first direction, the first and second half of said load winding being series connected by at least a partial turn via said second section.

9. The magnetic structure of claim 8, wherein said first direction is a clockwise direction, and wherein said second section receives at least said partial turn in a second

direction, said second direction being a clockwise direction.

10. A power converter for developing from an input power supply a high frequency power source comprising:

an oscillator circuit having asymmetrical switching to develop oscillatory power from the input power supply;

a continuous loop isolation circuit for applying said oscillatory power to a load; and

an input voltage circuit connected to the input power supply for supplying low frequency voltage to said load to even out the high frequency oscillatory power.

11. A power converter comprising in combination:

means for receiving a low voltage main input power supply to operate said power converter;

a load supplying output winding disposed to supply power to a load;

an oscillator circuit disposed to receive said input power supply and having a pair of switching transistors each connected to one of a pair of first primary windings to establish an alternating current in a first secondary winding; and

a continuous loop isolation circuit coupled to said

oscillator circuit via said first secondary winding and coupled to said load supplying output winding via a second primary winding disposed to induct high frequency power into said load supplying output winding.

12. The power converter of Claim 11, comprising:

an input voltage circuit connected to said input power supply and disposed to provide a voltage via a main input winding coupled to induce power into said load supplying output winding to even out said high frequency.

13. The power converter of claim 11, wherein said

oscillator circuit includes a pair of sensing windings, one of each connected to one gate of each transistor and coupled to one of said first primary windings, said sensing windings providing feedback to said transistors of said transistor output voltage.

14. The power converter of claim 11, wherein said

transistors comprise FET's.

15. The power converter of claim 11, wherein said input power supply comprises an ac main.

16. The power converter of claim 11, wherein a first core receives said oscillator primary and sensing windings, a second core receives said main input winding, a third core receives said load output winding, and said first core receives said first secondary winding and said second core receives said second primary winding.

17. The power converter of claim 16, wherein said first, second, and third cores comprise an E section core, each E section core having a center common core and two outer portions, all of said windings disposed on said center common core.

18. The power converter of claim 17, wherein it includes a spacer gap separating said second core section from said third core section.

19. The power converter of claim 16, wherein said first secondary winding comprises Litz wire.

20. The power converter of claim 16, wherein said load supplying winding comprises a bobbin having a dielectric strength of at least 600 volts per mill.

21. The power converter of claim 20, wherein said bobbin comprises DUPONT' Rynite.

22. The power converter of claim 20, wherein said bobbin comprises three sections having spacers therebetween, said first bobbin section receiving substantially half of said load winding in a first direction, said second bobbin section having at least one load winding in said first direction, and said third bobbin section receiving substantially the other half of said load winding in said first direction.

23. The power converter of claim 16, wherein said means for receiving said input power supply includes a dimmer suitable to reduce said input voltage.

24. The power converter of claim 12, wherein said input voltage circuit fills in a voltage input wave form as said dimmer eliminates portions of the voltage wave form, said input voltage circuit maintaining a power factor of .94 of unity.

25. The power converter of claim 14, where said input power supply comprises a D.C. main.

26. The power converter of claim 11, wherein said

continuous loop isolation circuit controls the impedance of said pair of first primary windings.

27. A method of converting low frequency input power into high frequency output power comprising:

providing a current path from said input power;

directing the current path through an asymmetrical

oscillator circuit to develop a high frequency oscillatory power from the input power; and

applying the high frequency oscillatory power to a load by inducting said high frequency oscillatory power into a

continuous loop isolation circuit which inducts said high frequency oscillatory power into said load.

28. The method of claim 27 further comprising:

providing a minor current path from said input power; and directing the minor current path from said input power into said load to even out said high frequency oscillator power by

supplying low frequency voltage to said load.

29. The method of claim 27, wherein said input power includes a voltage reduction device which adjustably reduces the input voltage.

30. A light source comprising:

a gas filled lamp load ;

a power converter coupled to receive an input voltage from a primary voltage source for outputing an output voltage and output current to drive said gas filled lamp load, said power converter having:

means for receiving voltage from the primary voltage source and for converting the input voltage into a dc input voltage; a load supplying output winding disposed to supply high frequency ac voltage to said lamp load;

an oscillator circuit disposed to receive said dc input voltage and having a pair of switching transistors each connected to one of a pair of first primary windings to establish a high frequency alternating current in a first secondary winding;

a continuous loop isolation circuit coupled to said oscillator circuit via said first secondary winding and coupled to said load supplying output winding via a second primary winding, said load supplying output winding being coupled to supply said high frequency ac voltage to said gas filled lamp load.

31. The light source of claim 30 wherein said gas filled lamp load comprises at least one hot cathode fluorescent lamp.

32. The light source of claim 30 wherein said gas filled lamp load comprises at least one metal halide lamp.

33. The light source of claim 30 wherein said gas filled lamp load comprises at least one low pressure sodium lamp.

34. The light source of claim 30 wherein said gas filled lamp load comprises at least one high pressure sodium lamp.

35. The light source of claim 30 wherein said gas filled lamp load comprises at least one mercury vapor lamp.

36. The light source of claim 30 wherein said gas filled lamp load comprises at least one cold cathode lamp.

37. The light source of claim 30 wherein power converter further comprises:

an input voltage circuit connected to said input power supply and disposed to provide a voltage via a main input winding coupled to induce power into said load supplying output winding to even out said high frequency.

38. The light source of claim 30 wherein power converter further comprises:

a pair of sense windings, each sense winding being disposed to sense voltage in one of said pair of first primary windings and to provide feedback voltage to a gate of one of said pair of transistors; said pair of first primary windings,

said sense windings, said first secondary winding being located on a first core;

said second primary winding being located on a second core and,

said load supplying output winding being located on a third core.

39. A magnetic structure for use in a power converter providing power to a load comprising:

a pair of first primary windings, each respective first primary winding being disposed to receive power via an

oscillator circuit from one of a pair of transistors;

a pair of sense windings, each sense winding being disposed to sense voltage in one of said pair of first primary windings to provide voltage feedback to one on said pair of transistors; and

means to limit the reflected load current to said first primary windings in response a short across the load, said means disposed between the load and said first primary windings.

40. The magnetic structure of Claim 39, wherein said means to limit said reflected load current comprises a continuous loop isolation circuit having a first secondary winding to receive power from said first primary windings and a first secondary winding to coupling circuit and voltage into a load winding.

41. A magnetic structure for use in a power converter providing power to a load comprising:

a pair of first primary windings, each respective first primary winding being disposed to receive power via a

oscillator circuit from one of a pair of transistors;

a pair of sense windings, each sense winding being disposed to sense voltage in one of said pair of first primary windings to provide feedback to one of said pair of transistors; and

means for limiting the rising voltage and current from an open load to said first primary windings, said means disposed between the load and said first primary windings.

42. The magnetic structure of Claim 41, wherein said means to limit said rising voltage comprises a continuous loop

isolation circuit having a first secondary winding to receive power from said first primary windings and a first secondary winding to coupling current and voltage into a load winding.

43. The light source of claim 37 wherein said power converter has an oscillation frequency greater than 32

kilohertz.

44. The light source of claim 37, wherein said cold cathode light source comprises a tube having a length of at least 200 centimeters.

45. The power converter of claim 10, wherein said

continuous loop isolation circuit includes means to adjust the current and voltage delivered from a first secondary winding to a second primary winding, supplying power to said load.

46. The power converter of claim 45 wherein said current and voltage adjusting means varying the number of turns on either of said first secondary windings or second primary winding.

47. A cassette light adapted to be received in a housing portion of a fixture comprising:

ann open cassette enclosure; and

a power converter having a continuous loop isolation circuit disposed to apply an oscillating power received from an oscillating circuit disposed to receive input power, said power converter positioned in the cassette enclosure and connected to a light source.

48. The cassette light of claim 47, wherein said enclosure includes a diffuser closing the enclosure, and means for fastening said enclosure to said housing.

49. The cassette light of claim 47, wherein said power converter includes a magnetic structure having a plurality of E-cores to receive a plurality of windings.

50. The cassette light of claim 47, wherein oscillating circuit includes at least a pair of field effect transistors.

51. The cassette light of claim 49, wherein at least one said windings includes quadruple insulated wire and a bobbin having a dielectric strength of about 600 volts/mil.