GB2394369A

GB2394369A - Bi-directional DC-DC converter

Info

Publication number: GB2394369A
Application number: GB0215371A
Authority: GB
Inventors: Guy Richard Mundy; Technology Limited Perdix
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-07-03
Filing date: 2002-07-03
Publication date: 2004-04-21
Anticipated expiration: 2022-07-03
Also published as: GB2394369B; GB0215371D0

Abstract

A bi-directional converter having a transformer T1 with primary and secondary windings 1 and 2, and at least one pair of coupled primary choke windings 3 and 4, wound around a core to comprise a second transformer T2. The primary transformer windings 1 are driven in a full H-bridge manner by a switching circuit comprising MOSFETs Q1-4, each pair of MOSFETs being supplied separately from a current source consisting of one of the primary chokes 3 or 4. The primary chokes are coupled magnetically as a forward converter, such that when the MOSFETs are switched cyclically, one pair drive one of the forward coupled chokes so as to double the voltage supplied to the other pair of MOSFETs and hence to the transformer T1. The transformer secondary side 2 may be configured similarly with MOSFETs Q5-8 and transformer T3, the transformers T1-3 also optionally being wound onto the same core 8. When the converter is operated in reverse the secondary side is then considered to be the primary side and vice versa. Bi-directional voltage gain and control is hereby achieved with the benefit of increased power density and efficiency from lower voltage sources under current control, also being applicable to unidirectional use.

Description

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.

Claims

CLAIMS What is claimed is:

1. A converter for converting an input voltage to an output voltage, the converter comprising: a voltage input means for receiving an input voltage, the voltage input means having an input voltage source connection and an input voltage return connection for connection to said input voltage; a voltage output means for outputting an output voltage, the voltage output means having an output voltage sink connection and an output voltage return connection for connection to an output voltage sink; a first transformer assembly comprising a primary winding and a secondary winding, the primary winding being coupled to the voltage input means for receiving the input voltage therefrom and the secondary winding being coupled to the output voltage means for producing the output voltage, and means for inductively coupling the respective primary and secondary windings to each other; a voltage input means comprising a second transformer and four switch means; the second transformer further comprising a first and a second winding and a means for inductively coupling said first and second windings, the first winding of the second transformer being coupled at one end to the input voltage source and at the other end to the first switch, the second winding of the second transformer being coupled at one end to the input voltage source and at the other end to the second switch, the first and second windings of the second transformer being orientated to each other such that a current drawn from the input voltage source in one winding will induce a current to be drawn from the same voltage source in the other winding, both currents being directed simultaneously toward the first and second switches respectively; the four switches means being arranged in a full bridge configuration, couple the second transformer to the first transformer, such that the first and second switch means couple the respective first and second windings of the second transformer to opposite ends of the primary winding of the first transformer and the third and fourth switch means couple opposite ends of the primary of the first transformer to the input voltage return; said means for inductive coupling, in either first or second transformer or both, comprising a core assembly, the primary and secondary windings, or first and second windings, being the four switch means being operable in a continuous four phase cycle in which the switch means are cyclically switched on and off such that, in a first phase of the cycle, for a predetermined period, the first, second and third switches are on and the fourth switch off, in the second phase all switches are off, in the third phase the first, second and fourth switches are on for a predetermined time with the third switch being off, and in the fourth phase all switches are off, whereby opposite ends of the primary winding of the first transformer are alternately connected to either the second transformer and hence indirectly to the input voltage source, or to the input voltage return, thereby applying an

alternating voltage to the primary winding of the first transformer and inducing an alternating voltage in the secondary winding of the first transformer and hence, by the rectification action of the output voltage means, an output voltage is generated from the input voltage source.

2. The converter defined in claim 1 wherein the output voltage is proportional to the ratio of the on and off periods of the four switch means, such ratio being continuously variable and selectable to provide the required output voltage.

3. The converter defined in claim 2 wherein a second secondary winding is wound on the first transformer, inductively coupled to the primary winding as for the first secondary winding, and also coupled to the voltage output means.

4. The converter defined in claim 3 wherein additional secondary windings are also wound onto the first transformer, inductively coupled to the primary winding as for the first secondary winding and coupled by additional voltage output means to additional output voltage sinks.

5. The converter defined in claim 2 wherein the voltage output means is a rectifier coupling the secondary winding of the first transformer to the output voltage sink and return.

6. The converter defined in claim 3 wherein the voltage output means is a rectifier coupling both the secondary windings of the first transformer to the output voltage sink and return.

7. The converter defined in claim 4 wherein the voltage output means are rectifiers coupling the respective secondary windings of the first transformer to the respective output voltage sinks and returns.

8. The converter defined in claim 2 wherein the voltage output means comprises output switch means and control means, the output switch means to coupling the secondary winding of the first transformer to the output voltage sink and return, and the control means to control the action of said output switch means.

9. The converter defined in claim 3 wherein the voltage output means comprises output switch means and control means, the output switch means to coupling the secondary windings of the first transformer to the output voltage sink and return, and the control means to control the action of said output switch means.

10. The converter defined in claim 8 wherein, the output voltage sink connected to the voltage output means is replaced by a input voltage source, the input voltage source connected to the input voltage means is replaced by an output voltage sink and the control means of the voltage output means operated such that the output switch means supply an alternating voltage to the secondary winding of the first transformer, derived from the input voltage source now connected to the voltage output means, whereby an output voltage is generated from an input voltage with concurrent transfer of power in the opposite direction to the converter defined in claim 8.

11. The converter defined in claim 9 wherein, the output voltage sink connected to the voltage output means is replaced by a input voltage source, the input voltage source connected to the input voltage means is replaced by an output voltage sink and the control means of the voltage output means operated such that the output switch means supply an alternating voltage to the secondary windings of the first transformer, derived from the input voltage source now connected to the voltage output means, whereby an output voltage is generated from an input voltage with concurrent transfer of power in the opposite direction to the converter defined in claim 9.

12. The converter defined in claim 8 wherein, the input voltage source connected to the voltage input means may behave as a source or sink of current, or power, and the output voltage sink connected to the voltage output means may behave as a source or sink of current, or power, and under the control of the voltage input means and of the voltage output means, current or power may be transferred bidirectionally through the converter as required on a continuous basis.

13. The converter defined in claim 9 wherein, the input voltage source connected to the voltage input means may behave as a source or sink of current, or power, and the output voltage sink connected to the voltage output means may behave as a source or sink of current, or power, and under the control of the voltage input means and of the voltage output means, current or power may be transferred bidirectionally through the converter as required on a continuous basis.

14. The converter defined in claim 2 wherein the voltage output means is a duplicate of the voltage input means, comprising a third transformer, switch means and control means such that current and power may be transferred bidirectionally through the converter.

15. The converter as defined in any preceding claim wherein the cores of the transformers are the same core, the windings of first, second or third transformer being wound on that same core.

16. A converter substantially as herein described above and illustrated in the accompanying drawings.