GB2105122A - A DC voltage converter - Google Patents

Abstract

The converter includes in one rail a transistor TR2 (or a thyristor, FET or GT0 device), a choke Land a diode D1 in series between a D.C. source and an output. A blocking diode D2 is connected between the return rail and the junction between the transistor TR2 and choke L. A second transistor TR1 (or a thyristor, FET or GTO device) is connected between the return rail and the junction between the choke L and diode D1. A capacitor C is connected across the circuit output. The switching of the transistors is controlled by respective drive circuits. in one mode the converter steps-up the input voltage by maintaining transistor TR2 continuously conductive and repetitively turning transistor TR1 on and off. In another mode the converter steps-down the input voltage by maintaining transistor TR1 continuously non-conductive and repetitively turning transistor TR2 on and off. <IMAGE>

Description

SPECIFICATION A voltage control device This invention relates to a voltage control device and particularly but not exclusively to a device adapted for selectively raising and lowering a D.C. battery voltage.

It is known to control a D.C. voltage from, for example a battery, by using a chopper circuit.

Chopper circuits for raising a D.C. output are known, as are chopper circuits for reducing the D.C. output.

The present invention seeks to provide a device for both raising and lowering the output voltage selectively with a simpler circuit incorporating a smaller number of components than the known systems.

According to the present invention there is provided a voltage control device comprising a circuit having two rails adapted to be connected to a D.C.

source, the circuit including in one rail a first switching device in series with a choke and a first diode; a reverse blocking diode connected between the other rail and the junction between the switching device and the choke; a second switching device connected between the junction between the choke and the first diode and said other rail and a capacitor connected in parallel across the output terminals of the circuit, the switching devices being, in use, selectively switched on and off by control means to control the voltage output.

Preferably, the switching devices comprise transistors, the bases of which are selectively fired to render the transistors conductive. Alternative forms of solid-state switching devices may be used such as thyristors (SCR's), Mosfets and gate-turn-off devices.

A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows two conventional circuits for respectively raising and lowering an output derived from a D.C. source, Figure 2 shows a circuit in accordance with the present invention, and Figure 3 shows a practical arrangement of the circuit illustrated in Figure 2.

Referring now to Figure 1, there is shown, in effect, two circuits suitable for the voltage control of a D.C.

current derived from D.C. source. The left hand side, section A, illustrates the step up circuit while the right hand side section B, to the right of the dotted line shows a step down circuit.

The step up circuit A is connected to a source of D.C. and comprises a choke L1 in series with a diode D1, a capacitor C1 connected across the output terminals and a transistor TR1 the collector of which is connected to the junction between the choke L1 and diode Do, the emitter being connected to return rail 3 between the capacitor C1 and the source.

The step down circuit B consists of a transistor TR2 the collector of which is connected to its own D.C.

source and the emitter of which is connected to a choke L2. A reverse blocking diode D2 is connected between the return rail 2 and the junction between the emitter of the transistor TR2 and the choke L2. As with the step up section A, a capacitor, in this case bearing reference C2, is connected across the output terminals.

In operation the two circuits work as follows.

- When the transistor TR1 is turned on by the applica tion of a control signal to its base, current flowing through the choke L1 increases, but when the transistor is turned off and rendered non conductive, the current flowing through the choke L1 is diverted through the diode D1 into the capacitor C1. Thus, the energy in the choke L1 is transferred to the capacitor. The level of voltage generated across the capacitor C1 is controlled by the duty cycle of the transistor TR1 as determined by external control means (not shown).

The operation of the step down section B is similar. When transistor TR2 is turned on current flows through the transistor TR2, the choke 12 and into the load across the output terminals which is in parallel to the capacitor C2 which carries the magne tising current. When the transistor TR2 is rendered non-conductive by the control means, current com bines to flow through the choke L2, the load and the diode D2. The output is thus proportional to the input voltage multiplied by a factor determined by the turn-on cycle of the transistor TR2.

An embodiment of the present invention is shown in Figure 2 in which, in effect, the step down section B is closer to the source than the step-up section A.

The circuit has the advantage that only one choke and one capacitor is required compared with the two of each required in the known arrangement de scribed in Figure 1.

As shown in Figure 2, a transistor TR2, choke Land a first diode D1 are connected in series between a D.C. source and an output. A reverse blocking diode D2 is connected between the return rail and the junction between the emitter of the transistor TR2 and the choke L. A second transistor TR1 has its emitter connected to the return rail at a point thereon further from the source than the connection of the diode D2, the collector of the transistor TR1 being connected to the junction between the choke Land the diode Do. A capacitor C is connected in parallel across the output terminals.

The preferred method of operation of the circuit for stepping up the output is to turn on transistor TR2 to render it continuously conductive and control the turn-on cycle of the transistor TR1 to obtain the required output. In this way the components L, D1, TR1 and C perform, essentially, the same function as the components of section A of the known arrange ment described in Figure 1. Similarly, stepping down the output may be accomplished by controlling the turn-on cycle of transistor TR2 such that the compo nents TR2, D2, and C perform, essentially, the same function as the components of section B of the known arrangement described in Figure 1. In this mode of operation the transistor TRi is kept turned off.

It is also possible to turn the transistors TR1 and TR2 on and off synchronously giving full step up and step down performance depending upon their joint duty cycle but this results in increased transistor, diode and choke current and hence heavier duty components would be required.

Figure 3 shows a typical working arrangement of the circuit shown in Figure 2 including the control means for switching the transistors TR1 and TR2.

The base of transistor TR2 is connected to the output of a drive circuit DC1, the base of the second transistorTR1 being driven by a second drive circuit DC2. The switching of the transistors is controlled through a comparator COM which compares the output voltage with a reference voltage. The output of the comparator COM controls a pulse width modulator PWM which in turn drives the drive circuits. The drive circuits are interconnected by logic gating. The output of the drive circuit DC2 is applied to the primary winding of the transformer Tri, the voltage thereby impressed on the secondary winding driving the base of transistor TR1.

Claims (6)

1. A voltage control device comprising a circuit having two rails adapted to be connected to a D.C.

source, the circuit including in one rail a first switching device in series with a choke and a first diode; a reverse blocking diode connected between the other rail and the junction between the switching device and the choke; a second switching device connected between the junction between the choke and the first diode and said other rail, and a capacitor connected in parallel across the output terminals of the circuit, the switching devices being, in use, selectively switched on and off by control means to control the voltage output.

2. A device as claimed in claim 1 wherein the switching devices are solid state switching devices.

3. A device as claimed in claim 1 or 2 wherein the switching devices comprise transistors, the bases of which are selectively fired to render the transistors conductive.

4. A device as claimed in claim 1, 2 or 3 wherein said control means includes a pulse width mod ulated controller.

5. A device as claimed in any one of claims 1 to 4 wherein the first switching device is controlled by a transistorised drive circuit powered by a D.C. source and the second switching device is controlled by a pulse width modulated controller, the output of which controls the second switching device through a drive circuit and a transformer.

6. A voltage control device substantially as de scribed herein with reference to and as illustrated in Figures 2 and 3 of the accompanying drawings.