USRE42425E1 - Dimmer having a power supply monitoring circuit - Google Patents

Abstract

A two-wire dimmer for control of a lighting load from an alternating-current (AC) power source includes a semiconductor switch, a power supply, and a control circuit. The power supply includes an energy storage input capacitor that is able to charge only when the semiconductor switch is non-conductive. The control circuit continuously monitors the voltage on the input capacitor and automatically decreases the maximum allowable conduction time of the semiconductor switch when the voltage falls to a level that will not guarantee proper operation of the power supply. The dimmer of the present invention is able to provide the maximum possible conduction time of the semiconductor switch at high end (i.e., maximum light intensity) while simultaneously ensuring sufficient charging time for proper operation of the power supply, and hence, the dimmer.

Description

FIELD OF THE INVENTION

The present invention relates to a two-wire load control device, specifically a two-wire dimmer for electronic low-voltage (ELV) lighting loads.

BACKGROUND OF THE INVENTION

Low-voltage lighting, such as electronic low-voltage (ELV) and magnetic low-voltage (MLV) lighting, is becoming very popular. Low-voltage lamps allow for excellent, precise sources of illumination, extended lamp life, higher efficiencies than incandescent lamps, and unique lighting fixtures, such as track lighting. To power an electronic low-voltage lamp, an ELV transformer is required to reduce a line voltage (typically 120 V_ACor 240 V_AC) to a low-voltage level (such as 12 volts or 24 volts) to power the ELV lamp.

Many prior art two-wire dimmers exist for control of ELV lighting loads. A conventional two-wire dimmer has two connections: a “hot” connection to an alternating-current (AC) power supply and a “dimmed hot” connection to the lighting load. Standard dimmers use one or more semiconductor switches, such as triacs or field effect transistors (FETs), to control the current delivered to the lighting load and thus control the intensity of the light. The semiconductor switches are typically coupled between the hot and dimmed hot connections of the dimmer.

Since an ELV transformer is normally characterized by a large capacitance across the primary winding, the ELV lighting load is typically dimmed using reverse phase-control dimming (often called “trailing-edge” dimming), in which the dimmer includes two FETs in anti-serial connection. One FET conducts during the first, positive half-cycle of the AC waveform and the other FET conducts during the second, negative half-cycle of the AC waveform. The FETs are alternately turned on at the beginning of each half-cycle of the AC power supply and then turned off at some time during the half-cycle depending upon the desired intensity of the lamp. To execute reverse phase-control dimming, many ELV dimmers include a microprocessor to control the switching of the FETs.

In order to provide a direct-current (DC) voltage to power the microprocessor and other low-voltage circuitry, the dimmer includes a power supply, such as a cat-ear power supply. A cat-ear power supply draws current only near the zero-crossings of the AC waveforms and derives its name from the shape of the waveform of the current that it draws from the AC supply. The power supply must draw current through the connected ELV lighting load. The FETs must both be turned off (non-conducting) at the times when the power supply is charging. So, the FETs cannot be turned on for the entire length of a half-cycle, even when the maximum voltage across the load is desired.

To ensure that the power supply is able to draw enough current to maintain its output voltage at all times, the FETs are turned off at the end of each half-cycle for at least a minimum off-time. The proper operation of the ELV dimmer is constrained by a number of worst-case operating conditions, such as high current draw by the low-voltage circuitry, worst-case line voltage input (i.e. when the AC power supply voltage is lower than normal), and worst-case load conditions (such as the number and the wattage of the lamps, the types of ELV transformers, and variations in the operating characteristics of the ELV transformers). By considering these worst-case conditions, the minimum off-time is determined by calculating the off-time that will guarantee that the power supply will charge fully for even the worst-case conditions. The resulting off-time generally ends up being a large portion of each half-cycle and constrains the maximum light level of the attached load.

However, the worst-case condition is not normally encountered in practice, and under typical conditions, the FETs could normally be turned off for a shorter amount of time at the end of each half-cycle, thus conducting current to the load for a greater amount of time resulting in a higher intensity of the load that is closer to the intensity achieved when only a standard wall switch is connected in series with the load. Prior art dimmers have held the minimum off-time constant under all conditions, and thus, have suffered from a small dimming range.

Thus, there exists a need for an ELV dimmer that includes a power supply and has an increased dimming range. More specifically, there exists a need for an ELV dimmer that includes a power supply and is able to drive an ELV lighting load above the maximum dimming level of prior art ELV dimmers without compromising the operation of the power supply.

SUMMARY OF THE INVENTION

According to the present invention, a two-wire dimmer for control of a lighting load from a source of AC voltage includes a semiconductor switch, a power supply, and a control circuit. The semiconductor switch is operable to be coupled between the source of AC voltage and the lighting load and has a conducting state and a non-conducting state. The power supply has an input that receives an input voltage and is operable to draw current from the source of AC voltage during the non-conducting state of the semiconductor switch. The control circuit is operable to control the semiconductor switch into the conducting state for an on-time each half-cycle of the AC voltage and is coupled to the input of the power supply for monitoring the input voltage of the power supply. The control circuit is operable to decrease the on-time when the input voltage of the power supply falls below a first predetermined level. Further, the control circuit is operable to increase the on-time when the input voltage rises above a second predetermined level greater than the first level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of the two-wire dimmer of the present invention;

FIG. 2a is a waveform of the dimmed hot voltage of the dimmer ofFIG. 1;

FIG. 2b is a waveform of the voltage across the dimmer ofFIG. 1;

FIG. 3 is a flowchart of the process implemented by a control circuit of the dimmer ofFIG. 1;

FIG. 4 shows voltage waveforms of the dimmer ofFIG. 1 during a first part of the process ofFIG. 3; and

FIG. 5 shows voltage waveforms of the dimmer ofFIG. 1 during a second part of the process ofFIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description of the preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purposes of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred, in which like numerals represent similar parts throughout the several views of the drawings, it being understood, however, that the invention is not limited to the specific methods and instrumentalities disclosed.

FIG. 1 shows the two-wire dimmer100 of the present invention, which is connected in series between anAC power supply102 and anELV lighting load104. Thedimmer100 has two connections: a HOT connection106 to theAC power supply102 and a DIMMEDHOT connection108 to thelighting load104. Since ELV loads operate at a low-voltage level (such as 12 volts or 24 volts), a step-down transformer XFMR is required for theELV lamp104A. The ELV transformer XFMR is typically characterized by a large capacitance C_ELVacross the primary winding.

To control the AC voltage delivered to theELV load104, two field-effect transistors (FETs)110,112 are provided in anti-serial connection between the HOT terminal106 and theDIMMED HOT terminal108. Thefirst FET110 conducts during the positive half-cycle of the AC waveform and thesecond FET112 conducts during the negative half-cycle of the AC waveform. ELV lighting loads are dimmed using reverse-phase control dimming, in which the FETs are alternately turned on at the beginning of each half-cycle of the AC power supply and then turned off at some time during the half-cycle depending upon the desired intensity of the lamp. The conduction state of theFETs110,112 is determined by acontrol circuit114 that interfaces to the FETs through agate drive circuit116. To execute reverse-phase control dimming, thecontrol circuit114 includes a microprocessor to control the switching of theFETs110,112.

The ELV dimmer also includes a plurality ofbuttons118 for input from a user, and a plurality of light emitting diodes (LEDs)120 for feedback to the user. Thecontrol circuit114 determines the appropriate dimming level of theELV lamp104A from the input from thebuttons118.

A zero-cross circuit122 provides a control signal to thecontrol circuit114 that identifies the zero-crossings of the AC supply voltage. A zero-crossing is defined as the time at which the AC supply voltage equals zero at the beginning of each half-cycle. The zero-cross circuit122 receives the AC supply voltage through diode D1 in the positive half-cycle and through diode D2 in the negative half-cycle. Thecontrol circuit114 determines when to turn off the FETs each half-cycle by timing from each zero-crossing of the AC supply voltage.

In order to provide a DC voltage (V_CC) to power the microprocessor of thecontrol circuit114 and the other low-voltage circuitry, thedimmer100 includes apower supply124. Thepower supply124 is only able to charge when theFETs110,112 are both turned off (non-conducting) and there is a voltage potential across the dimmer. Since there are only two connections on a two-wire dimmer, the power supply must draw a leakage current through the connectedELV lighting load104. For example, during the positive half-cycle, current flows from theAC supply102 through diode D1 to thepower supply124 and then, via circuit common, out through the body diode of thesecond FET112 and through theload104 back to the AC supply. Thepower supply124 may be implemented as a “cat-ear” power supply, which only draws current near the zero-crossings of the AC waveform, or as a standard switch-mode power supply.

In a typical two-wire dimmer, thepower supply124 is implemented as a “cat-ear” power supply, which only draws current near the zero-crossings of the AC waveforms. Thepower supply124 has an input capacitor C1 and an output capacitor C2. The output capacitor C2 holds the output of the power supply Vcc at a constant DC voltage to provide power for thecontrol circuit114. The input of thepower supply124 is coupled to the Hot and Dimmed Hot terminals through the two diodes D1, D2, such that the input capacitor C1 charges during both the positive and negative half-cycles.

The dimmer100 also includes a voltage divider that comprises two resistors R1, R2 and is coupled between the input of thepower supply124 and circuit common. The voltage divider produces a sense voltage V_Sat the junction of the two resistors. The sense voltage V_Sis provided to thecontrol circuit114 such that the control circuit is able to monitor the voltage level at the input of thepower supply124. The microprocessor in thecontrol circuit114 preferably includes an analog-to-digital converter (ADC) for sampling the value of the sense voltage V_S. The resistors R1, R2 are preferably sized to ensure that the maximum voltage at the pin of the microprocessor of thecontrol circuit114 does not exceed the power supply output V_CC. For example, if the input voltage to the waveform is 240 V_RMSand the power supply output V_CCis 3.3 V_DC, then the values of R1 and R2 can be sized to 450 kΩ and 3 kΩ, respectively, in order to ensure that the magnitude of the sense voltage is less than 3.3 V_DC. Alternatively, the voltage divider could be coupled between the output voltage (or another operating voltage) of thepower supply124 and circuit common to provide a signal to thecontrol circuit114 that is representative of the present operating conditions of the power supply.

According to the present invention, thecontrol circuit114 monitors the sense voltage V_Sand decreases the conduction times of theFETs110,112 when the sense voltage V_Sdrops below a first predetermined voltage threshold V₁. Further, thecontrol circuit114 increases the conduction times of theFETs110,112 when the sense voltage then rises above a second predetermined voltage threshold V₂, greater than the first threshold. In a preferred embodiment of the present invention (when used with an input voltage of 240 V_RMS), the first and second voltage thresholds V₁and V₂are set to 0.67 V_DCand 0.8 V_DC, respectively, which correspond to voltages of 100 V_DCand 120 V_DCat the input of thepower supply124. Alternatively, if the microprocessor does not include an ADC, the dimmer100 could include a hardware comparison circuit, including one or more comparator integrated circuits, to compare the sense voltage with the first and second voltage thresholds and then provide a logic signal to the microprocessor.

FIG. 2a shows examples of a dimmedhot voltage210 measured from the DimmedHot terminal108 of the dimmer100 to neutral (i.e. the voltage across the lighting load104). The dashed line represents theAC voltage220 measured across theAC power supply102. The period of theAC voltage220 is split into two equal half-cycles having periods T_H. The dimmedhot voltage210 has a value equal to theAC voltage220 during the time t_ONwhen one of the FETs is conducting. Conversely, the dimmedhot voltage210 has a value equal to zero during the time t_OFFwhen neither FET is conducting. Thecontrol circuit114 is able to control the intensity of the load by controlling the on-time t_ON. The longer the FETs conduct during each half-cycle, the greater the intensity of thelighting load104 will be.

FIG. 2b shows an example of thedimmer voltage230 measured from the Hot terminal106 to the DimmedHot terminal108 of the dimmer (i.e. the voltage across the dimmer) Thepower supply124 is only able to charge during the off-time t_OFFbecause the off-time is the only time during each half-cycle when there is a voltage potential across the FETs and thus across thepower supply124. Conversely, when the FETs are conducting during the on-time t_ON, the FETs form a low impedance path through the dimmer100 and the input capacitor C1 of thepower supply124 is unable to charge.

With prior art ELV dimmers, a maximum off-timet_OFF-MAX-WCneeded to charge the power supply during worst-case conditions was used to determine the maximum on-time t_ON-MAX-WCof the dimmer. The worst-case conditions may include a low-line AC input voltage or a high current drawn from the power supply by the microprocessor and other low-voltage components. However, the dimmer is not always operating with the worst-case conditions and it may be possible to increase the on-time above the maximum on-time t_ON-MAX-WCin order to provide a greater light output of thelighting load104 at high-end.

The dimmer100 of the present invention has a maximum on-time limit, t_ON-MAX-LIMITthat is greater than the worst-case on-time t_ON-MAX-WC. The maximum on-time limit t_ON-MAX-LIMITof the dimmer100 is determined from the appropriate off-time required to charge the input capacitor C1 of thepower supply124 during normal operating conditions. The dimmer100 also has a dynamic maximum on-time, t_ON-MAX, that thecontrol circuit114 is operable to control from one half-cycle to the next. The dynamic maximum on-time t_ON-MAXcannot exceed the maximum on-time limit t_ON-MAX-LIMIT, but can be decreased below the limit in order to increase the off-time of the FETs to allow the input capacitor C1 of thepower supply124 more time to charge. By driving the on-time of the FETs above the worst-case on-time t_ON-MAX-WC, the dimmer100 of the present invention is able to achieve a greater light output of theconnected lighting load104 than prior art ELV dimmers. However, when the on-time of the FETs is greater than the worst-case on-time t_ON-MAX-WC, there is a danger of the input capacitor C1 not having enough time to charge in during the off-time of the half-cycle.

By monitoring the input of thepower supply124, thecontrol circuit114 of the dimmer100 of the present invention is able to determine when the input voltage has dropped to a level that is inappropriate for continued charging of the input capacitor C1. For example, if the sense voltage V_Sfalls below a first voltage threshold V₁, then the capacitor C1 needs a greater time to properly charge and the on-time is decreased. On the other had, if the sense voltage V_Sremains above the first voltage threshold V₁, the input capacitor C1 is able to properly charge each half-cycle.

FIG. 3 shows a flowchart of the process for monitoring the sense voltage V_Sand determining whether to change the on-time t_ONin response to the value of the sense voltage V_S. The process ofFIG. 3 runs each half-cycle of the AC voltage. The on-time t_ONis changed in response to the maximum on-time t_ON-MAXbeing decreased or increased if the maximum on-time is less than a desired on-time, t_ON-DESIRED, of the dimmer. The desired on-time t_ON-DESIREDis determined by thecontrol circuit114 from the inputs provided by thebuttons118. The maximum on-time t_ON-MAXis only changed if the sense voltage V_Sis below the first voltage threshold V₁or if the sense voltage V_Sis above the second voltage threshold V₂and the maximum on-time t_ON-MAXhas not returned to the maximum on-time limit, t_ON-MAX-LIMIT, of the dimmer100.

The flowchart ofFIG. 3 begins atstep310 at the beginning of each half-cycle. First, atstep312, the sense voltage V_Sis sampled once immediately after the FETs are turned off. If the sampled sense voltage V_Sis less than the first voltage threshold V₁atstep314 and the maximum on-time t_ON-MAXis greater than the present on-time t_ONatstep316, the dimmer has detected that the sense voltage has dropped below the first voltage threshold V₁. Then, the maximum on-time t_ON-MAXis set to the present on-time t_ONatstep318 and the maximum on-time t_ON-MAXis decreased by a first predetermined time increment t₁atstep320. The first predetermined time increment t₁preferably corresponds to 1% of the dimming range. If the maximum on-time t_ON-MAXis less than the present on-time t_ONatstep318, the maximum on-time t_ON-MAXis decreased by a first predetermined time increment t₁atstep320.

Atstep322, a determination is made as to whether the maximum on-time t_ON-MAXis less than the desired on-time t_ON-DESIRED. If so, the on-time t_ONis set to the present value of the maximum on-time t_ON-MAXatstep324. Since the sense voltage is only sampled after the FETs are turned off (at step312), the change to the on-time t_ONatstep320 will affect the on-time of the dimmed hot voltage during the next half-cycle. The process then exits atstep326 for the current half-cycle to begin again at the beginning of the next half-cycle. If the maximum on-time t_ON-MAXis greater than the desired on-time t_ON-DESIREDatstep322, then the dimmer has returned to normal operating conditions. The desired on-time t_ON-DESIREDis used as the on-time atstep328 and the process exits atstep326.

If the sense voltage V_Sis greater than the first voltage threshold V₁atstep314 and the sense voltage is less than the second voltage threshold V₂atstep330, then the maximum on-time t_ON-MAXand thus the on-time t_ONare not changed. If the sense voltage V_Sis greater than the second voltage threshold V₂atstep330, the process moves to step332 where a determination is made as to whether the present maximum on-time t_ON-MAXis less than the maximum on-time limit t_ON-MAX-LIMIT. If not, the maximum on-time t_ON-MAXhas returned to the limit and the maximum on-time t_ON-MAXand the on-time t_ONare not changed. However, if the present maximum on-time t_ON-MAXis greater than the maximum on-time limit t_ON-MAX-LIMITatstep332, then the maximum on-time t_ON-MAXis increased by a second predetermined time increment t₂for the next half-cycle atstep334. The second predetermined time increment t₂preferably corresponds to 0.5% of the dimming range.

FIG. 4 shows the voltage waveforms of the dimmer100 operating with in accordance with the present invention as the voltage at the input of thepower supply124 is falling. The upper waveform shows the dimmed hot voltage, which is across theELV load104. In the first few line cycles (a), (b), (c), and (d), the dimmed hot voltage is zero for only a small off-time at the end of each half-cycle. The lower waveform shows the sense voltage V_S, which is a scaled version of the voltage at the input of thepower supply124. During the off-time each half-cycle, the input capacitor C1 of thepower supply124 charges and the sense voltage rises. During the first few cycles (a), (b), (c), the sense voltage remains above the first voltage threshold V₁.

During the fourth half-cycle (d), the sense voltage falls below the first voltage threshold V₁. Thecontrol circuit114 decreases the on-time of the dimmed hot voltage during the next half-cycle (e) by the first time increment t₁. Thus, the input capacitor C1 has more time to charge during the off-time of the next half-cycle (e).

However, during the half-cycle (e), the sense voltage once again falls below the first voltage threshold V₁. So, thecontrol circuit114 decreases the on-time of the dimmed hot voltage during the next half-cycle (f) by the first time increment t₁. The cycle repeats again until the sense voltage does not fall below the first voltage threshold V₁during the half-cycle (g). Now, the on-time of the dimmed hot waveform is held constant through the next half-cycles (h), (i).

FIG. 5 shows the voltage waveforms of the dimmer100 after a low-voltage condition has been detected and the sense voltage V_Sis rising. The on-time of the first few half-cycles (j), (k), (l), (m), (n) of the dimmed hot waveform (the upper waveform ofFIG. 5) is the decreased on-time (that was determined from the description ofFIG. 4). Now, the voltage at the input of thepower supply124, and thus the sense voltage V_S, is rising (as shown in the lower waveform ofFIG. 5).

During half-cycle (n), the sense voltage remains above the second voltage threshold V₂. Therefore, thecontrol circuit114 increases the maximum on-time of the dimmed hot waveform during the next half-cycle (o) by the second time increment t₂. While the sense voltage continues to remain above the second voltage threshold V₂, thecontrol circuit114 continues increasing the maximum on-time each half-cycle by the second time interval t₂until the maximum on-time is equal to the original maximum on-time.

The dimmer100 of the present invention has been described such that thecontrol circuit114 is operable to change the maximum on-time t_ON-MAXfrom one half-cycle to the next. However, it may be preferable to only change the maximum on-time t_ON-MAXfrom one line-cycle to the next. Many dimmers are operable to drive multiple types of lighting loads. Some lighting loads, such as magnetic low-voltage (MLV) lighting loads, are susceptible to asymmetries that produce a DC component in the voltage across the load. For example, the magnetic low-voltage transformers required for MLV lighting may saturate and overheat when the load voltage has a DC component. When the on-time is changed from one half-cycle to the next, the voltage across the lighting load with be asymmetric and a DC component will be present in the voltage. On the other hand, when the on-time is only changed from one line-cycle to the next, the load voltage will remain symmetric and the problem of saturating or overheating the MLV transformer will be avoided.

While the dimmer100 of the present invention was described primarily in regards to control of ELV loads, the dimmer may be used to control other load types, for example, incandescent or MLV loads.

Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Claims (11)

1. A two-wire dimmer for control of a lighting load from a source of AC voltage, comprising:

a semiconductor switch operable to be connected between said lighting load and said source of AC voltage; said semiconductor switch having a conducting state and a non-conducting state;

a power supply operable to draw current from said source of AC voltage when said semiconductor switch is in said non-conducting state; and

a control circuit operable to control said semiconductor switch into said conducting state for an on-time each half-cycle of said AC voltage; said control circuit coupled to an input of said power supply for monitoring a voltage of said power supply;

wherein said control circuit is operable to decrease said on-time when said input voltage of said power supply falls below a first predetermined voltage threshold.

2. The two-wire dimmer ofclaim 1, wherein said power supply has an input for receipt of an input voltage and said control circuit is coupled to said input of said power supply for monitoring said input voltage of said power supply.

3. The two-wire dimmer ofclaim 2, wherein said control circuit is operable to decrease said on-time by a first predetermined time interval.

4. The two-wire dimmer ofclaim 3, wherein said control circuit is operable to increase said on-time when said input voltage of said power supply remains above a second predetermined voltage threshold.

5. The two-wire dimmer ofclaim 4, wherein said control circuit is operable to increase said on-time by a second predetermined time interval.

6. The two-wire dimmer ofclaim 5, further comprising a resistive voltage divider for providing a scaled-down representation of said input voltage of said power supply to said control circuit.

7. A method for controlling the intensity of a lighting load from a source of AC voltage in a dimmer comprising a semiconductor switch coupled between said lighting load and said source, a control circuit for controlling said semiconductor switch, and a power supply for powering said control circuit; of ensuring that a power supply for powering low-voltage circuitry of a dimmer is able to properly charge, the dimmer adapted to be coupled between a source of AC voltage and a lighting load, said method comprising the steps of:

scaling down a voltage of said power supply to produceproducing a sense voltage in response to a voltage of said power supply;

measuring said sense voltage;

comparing the said sense voltage to a first predetermined voltage threshold; and

changing a conduction time of said semiconductor switch dimmer based on the result of the step of comparing, such that said power supply has enough time to charge.

8. The method of claim7 11, wherein said voltage of said power supply comprises an input voltage of said power supply.

9. The method of claim8 11, wherein changing said conduction time comprises decreasing said conduction time by a first predetermined time when said sense voltage falls below said first predetermined voltage threshold.

10. The method ofclaim 9, further comprising the steps of:

comparing said sense voltage to a second predetermined voltage threshold;

increasing said conduction time by a second predetermined time when said sense voltage rises above said second predetermined voltage threshold.

11. The method of claim 7, wherein the step of producing a sense voltage in response to a voltage of said power supply further comprises scaling down said voltage of said power supply to produce said sense voltage.

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