Bi-Directional DC-DC Converter The present invention relates to a bi-
directional converter, typically for battery backup but also for other uses requiring hi- and unidirectional power transfer and regulation, where the input voltage is boosted to give an overall voltage gain greater than the main transformer voltage gain hence achieving bi- directional power transfer and bi-directional voltage gain with improved performance and power density.
Background of the Invention
DC-DC converters as commonly used and widely available for electrical and electronic applications are not inherently reversible, being designed for unidirectional use. The majority use a transformer for voltage conversion and isolation, which whilst reversible in its own right, only shows voltage gain in one direction. In an application aimed at battery backup systems for example, where the same converter is being used for both charge and discharge control, the converter whilst being functionally reversible also requires to have a voltage gain greater than the transformer turns ratio in both directions.
It is the subject of the invention to give voltage boost to the transformer such that the overall converter is bi-directional in its voltage gain, but is not limited to this, being utilisable in any application requiring bi-directional power conversion and control, and also as a method for improved unidirectional power conversion.
Transformers used in DC-DC converters are operated in one of two ways, namely forward and flyback modes. In the forward converter, when the input power is applied to the transformer, power is produced simultaneously in the secondary circuit and transferred to the load according to the equation Vs/Vp= Ns/Np (1) where Vp is the voltage on the primary, Np are the number of primary turns and Vs is the voltage on the secondary, Ns are the number of secondary turns This is never achieved in practice as the transformer has at least a resistive loss, also, in the DC-DC converter the input circuit has a voltage loss due to the switching elements and the secondary circuit losses due to the rectifier elements, thus: Vp = Vi -Vsw and Vs = Vo +Vd (2,3) where Vi is the input voltage and Vsw is the loss on the switching elements where Vo is the output voltage and Vd are the rectifier losses thus combining these equations: Vo/Vi Ns/Np (4) In order to produce the required output voltage additional secondary turns are added: Vo/Vi = x.Ns/Np where x>1 are the proportional additional turns required (5) The use of such a transformer in reverse where Ns and Np, and Vi and Vo are swapped leads to the following: Vo/Vi << Ns/Np (6) since x>1 having been fixed at manufacture, is now on the primary winding and not on the secondary. The flyback circuit mentioned above draws power from the secondary winding only when the power is removed from the primary winding and as is well known to those practiced in the art, such that: Vo/Vi >> Ns/Np
Such bi-directional voltage gain defined in the above analysis is inherent in the flyback circuit operation and has been widely exploited for e.g. load regulation, this exemplified in prior art (US pat. 6,384, 491).
Such behaviour whilst simple and effective has the deficiency that the energy from the primary circuit has to be stored as magnetic energy in the transformer before release. This imposes a limit on the power handling capability to usually 150W in the case of the flyback convertor due to the size of the transformer core.
As is well known, higher power applications use forward type converters since little energy need be stored in the magnetic core, the forward and push-pull converters exceeding 200W, and bridge converters having multikilowatt performance. These do not show however bi-directional voltage gain as described above and it is to this end that the present invention is addressed, namely producing a voltage gain in the primary circuit such that a forward-type converter, namely a full-bridge converter, can achieve bi-directional voltage gain with the benefits of the power handling capability and density that the full-bridge converter has.
The essential feature of the current invention is the addition of an input transformer, arranged to be a forward converter and to feed it's output to the main transformer in a manner that uses existing circuit components efficiently. In the present invention the method of boosting the input voltage of the full-bridge converter will be described giving the following attributes: Voltage gain in both directions; improved power throughput and efficiency from lower voltage sources, only a single additional component over a full bridge converter; lower voltage noise on the output when used in reverse and a current fed topology giving more effective voltage control.
The Drawings Figure 1 shows a schematic of the input circuit of the present invention with an example output circuit.
Figure 2 shows an alternative output circuit using a single transformer secondary winding and a bridge circuit to attain bi-directional power transfer.
Figure 3 shows an alternative output circuit using a centre tapped transformer secondary and push-pull circuit for bi-directional use.
Figure 4 shows an alternative output circuit using the subject of this invention for both primary and secondary circuit, together with an integrated magnetic structure for the three transformer structures.
Detailed description
The present invention in its first and simplest embodiment is shown graphically in Figure 1. Only the components being the essence of the invention are shown, those of control and component protection i.e. Pulse width modulators, feedback circuits, drivers, snubbers, suppressors etc. are omitted for clarity as they are well known to those skilled in the art.
Figure l also shows the circuit attached to a battery Vs. load Vl, and a rectifier circuit comprising diodes D1,2,3,4, these being shown as an example of one possible use but not limited to that use, thus: A transformer T1 is shown connected to the voltage input Vs via a second transformer T2 and by four switches Q1,2,3,4 Usually MOSFETs, arranged in the manner generally called a full bridge. This input circuit enables the driving of the primary 1 of T1 with an alternating voltage derived from Vs. The secondary winding 2 of T1 is shown, as an example, coupled to
the output by four diodes Dl,2,3,4, thus providing rectification of the alternating voltage induced in winding 2 for output to the load Vl.
As is known to those skilled in the art, in a conventional full bridge converter the switches Q1 and Q4 or Q2 and Q3 would be alternately switched in a cyclical fashion, thus connecting the ends of the winding 1 alternately to the input voltage supply and return, the voltage Vx.
In the current invention the transformer T2is interposed between the supply Vs and the four switches Q1,2,3,4 such that one winding of T2, namely 3, supplies switch Q1 and the other winding 4, switch Q2. When the four switches are operated in a cyclical fashion a higher voltage than Vs will be supplied to T1 due to the action of T2 as follows: In the first phase of the cycle, for example with Q1 and Q4 being switched on, Q2is also switched on, a current is thus drawn toward the dot end of the winding 4 of T2 directly from the supply Vs and to it's return at Q4. As is well known this will induce a voltage across the winding 3 of T2 away from the dot end, the dot markings indicating the polarity of windings 3 and 4, such that the voltage presented via Q1 to the dot end of winding 1 is the sum of the supply voltage Vs and the voltage across 3. The windings of T2 are of equal number of turns thus the voltage supplied to T1 by Qlis, in an ideal circuit, double that of Vs. whilst the current supplied to T1 is half that that flows from Vs. being split equally between windings 3 and 4. The law of conservation of energy is thus applied, with equal power being supplied to T1 as drawn from Vs. albeit at double the voltage and half the current.
In the second phase of the conversion cycle all switches are off, whilst in the third phase the situation of phase one is reversed with switches Q1,2 and 3 being switched on. In a manner similar to the first phase, current drawn through winding 3 induces a voltage in winding 4 thus again doubling the supply voltage Vs that is presented to winding 1 of T1, this time via Q2. In the fourth phase again all switches are off.
As is well known to those skilled in the art, the variation of the periods of the four phases of the cycle gives control of the power presented to T1 for rectification and output, being pulse width modulation and will not be further dealt with here.
Thus in the circuit in figure 1, a voltage is output that is double that which would be output in the absence of transformer T2, thus attaining one of the goals of the current invention. An additional benefit is that the transformer T1 is now fed by a current source namely T2, and as is widely appreciated by those skilled in the art, current fed circuits have better stability and simpler control structures than voltage fed circuits.
In it's second embodiment Figure 2 extends the circuit of figure I by replacing the four rectifiers Dl,2,3,4 with switches Q5,6,7,8 of, for example, the MOSFET type. These have the property of an intrinsic body diode and as is well known act as rectifiers as well as switches.
The circuit of figure 2 may again be operated in an identical fashion to that of figure 1, transfering power from Vs to Vl at twice the voltage on T1 as would have been without T2.
Additionally and alternatively, with the output circuit as shown in figure 2, appropriate control circuitry (not shown for clarity), and Vl now being a voltage source, switching the MOSFETS QS,6,7,8 in an analogous manner to Q1,2,3,4 previously described for the conventional full bridge converter, an alternating voltage may be applied to the transformer winding 2 and hence by rectification by Q1,2,3,4 power may be transferred from Vl, charging the battery at Vs.
In the embodiment shown in figure 2 the transformer T2 now simply acts as a choke smoothing the voltage ripple as is well known to those skilled in the art. Thus in the circuit shown in figure 2, bi-directional power transfer has been attained and due to the voltage doubling by T2 in one direction only, bi-directional voltage gain has also been attained. To
those skilled in the art, defining the turns ratio for transformer T1 is a straight forward matter to define the exact required voltage conversions ratios under control of the switches Q1-8.
In it's third embodiment Figure 3 gives an alternative output circuit benefiting from fewer components, being a push-pull circuit with a centre tapped secondary winding 7, but also using the same input circuit, and, as will be appreciated to those skilled in the art, is also bi- directional in both power transfer and in voltage conversion.
In it's fourth embodiment, figure 4 demonstrates the use of the subject of this invention for both input and output circuits. The mode of action is as described in the first embodiment but with the integration of the three transformers into one magnetic structure. Such integrated magnetics are well known and will not be described in detail here other than to note that the output chokes now are driven in the manner of a transformer and give further voltage boost whichever way power is transferred, for example when current is drawn through winding 4 toward the dot end, a current is induced in windings 3 and 5 away from the dot end, a current flowing away from the dot end of winding 1 induces a current toward the dot end of winding 2, with the voltage induced across winding 5 serving to boost the current from winding 2 that flows through Q6.
As will be well known to those skilled in the art, such integration may be used in the second and third embodiments herein described and whose implementation is standard practice. Also, various modifications may be made within the scope of the present invention in the selection of practical components and addition of those required to form a practical circuit whilst all remaining within the spirit of the present invention.