GB2166608A

GB2166608A - DC/AC converter

Info

Publication number: GB2166608A
Application number: GB08524291A
Authority: GB
Inventors: Wolfgang Brandstatter
Original assignee: Schrack Elektronik AG
Current assignee: Schrack Elektronik AG
Priority date: 1984-10-05
Filing date: 1985-10-02
Publication date: 1986-05-08
Also published as: ATA317384A; GB8524291D0; FR2571562A1; GB2166608B; DE3534570A1; FR2571562B1; AT383917B

Abstract

A DC/AC converter comprising a converter transformer and two controllable switching devices operating in overlapping commutation mode, an alternating-voltage source being arranged between the gate electrodes of the two controllable switching devices and a shunt being connected between the gate electrode and the reference electrode of each controllable switching device. <IMAGE>

Description

SPECIFICATION DC/AC Converter The invention relates to a DC/AC converter comprising a converter transformer and two controllable switching devices operating in overlapping commutation mode.

In known circuit arrangements of this type, the gating currents in switching devices controlled by gating current have been limited by a resistor. To achieve that, nevertheless, the switching devices rapidly switch on, this current-limiting resistor has been bridged by a capacitor (speed-up capacitor).

To switch off the switching devices, in known circuit arrangements an auxiliary voltage is needed which is derived from the gating current. This is associated with additional losses in the drive circuit.

In addition, in the case of known circuit arrangements for DC/AC converters comprising a convertertransfomrer operating in overlapping commutation mode, this mode of operation is achieved by means of one each drive source for each of the two switching devices and overlapping driving of the two switching devices. This necessitates not only an increased constructional expenditure but also increased losses due to the greater gating energy required if the two switching devices are separately driven.

For these reasons, the invention has the objective of specifying a circuit for a DC/AC converter operating in overlapping commutation mode, which circuit permits both switching devices to be jointly driven with little expenditure and provides the possibility of overlapping operation.

In a circuit arrangement of the type initially mentioned, this objective is achieved in accordance with the invention by the fact that an alternatingvoltage source is arranged between the gate electrodes of the two controllable switching devices and that a shunt is connected between the gate electrode and reference electrode of each controllable switch device.

These measures particularly result in the advantage that the anti-serial circuit of the two controlling paths of the controllable switching devices ensures that at least one of the two switching devices is in each case conducting, in which arranngement the shunt establishes specific potential ratios for the switching-off condition for the switching device which happens to be not conducting. Which of the two switching devices happens to be conducting can be determined by the polarity of the gating voltage which is common to both switching devices.

According to the invention, an embodiment of a DC/AC converter can consist in the shunt being formed in each case by a resistor. As a result, a switching characteristic which is largely defined in terms of time is achieved particularly in the case of switching devices displaying capacitive behaviour of the controlled path (MOS FET).

According to the invention, another embodiment of a DC/AC converter can consist in the shunt being formed in each case by a series circuit consisting of a diode and a resistor, and in which arrangement the diodes are connected in such a direction that in each case the series circuit arranged between the gate electrode and reference electrode of a controllable switching device which has been switched to its conducting state does not load the alternating-voltage source. Particularly in connection with bipolar transistors used as gatable switching devices, the stored charge fed to the transistor which is starting up can by this means be kept equal to the charge to be removed of the transistor just stopping. In this manner, an optimum switching-on and switching-off of bipolar transistors can be achieved.According to the invention, further simplification of the drive of a DC/AC converter can be achieved in that in each case a diode is connected with one of its electrodes to a gate electrode of a controllable switching device and other diode electrodes are connected to each other and led via a common resistor to the reference potential.

In the text which follows, the invention is described by way of an example, referring to the drawings, in which: Figure 1 shows the greatly simplified circuit diagram of a DC/AC converter according to the invention, and Figures 2 and 3 show DC/AC converters provided with a particularly advantageous development of the circuit arrangement according to the invention for driving the two controllable switching devices.

Figure 1 shows the basic circuit diagram of a DC/ AC converter according to the invention which is preferably constructed as a current-controlled pushpull converter. The converter comprises a'converter transformer 1 which has a centre-tapped primary winding 2 and at least one secondary winding 3. The tap of the primary winding 2 is connected via a choke 4 to a supply terminal A. The converter is fed with energy by a voltage source 5 arranged between the supply terminal A and the reference terminal B.

In this arrangement, the voltage source 5 can be formed, for example, by a direct-voltage source or an alternating-voltage source followed by a rectifier.

It is also possible to supply the DC/AC converter from a current source between terminals A and B. In this case, the choke 4 can be omitted. If the DC/AC converter is supplied from a switched current source, the choke 4 preferably represents a part of this circuit (bucking-type converter circuit).

The alternating output circuit voltage of the DC/AC converter can be picked up at the output terminals C, D of the secondary winding. An alternating current load can be directly connected to these terminals; a direct-current load is preferably connected via an interposed rectifier circuit.

The two ends of the primary winding 2 of the converter transformer 1 are in each case connected via the main path between the main electrodes K, E of one controllable switching device 6 and 7 each of the reference terminal B. The gate inputs F, G of the two controllable switching devices 6,7 are connected to each other via an alternating-voltage source 8. In addition, the gate input of the controllable switching devices 6,7 is in each case connected via a shunt 9, 10 to the reference terminal B.

For the description of the operation of a circuit arrangement according to the invention according to Figure 1, it is assumed that the terminal F of the alternating-voltage source 8 happens to be carrying a positive potential and if it is assumed that conventional controllable switching devices such as, for example, bipolar switching transistors, gate turn-off thyristors, MOS field-effect power transistors and similar, have a high-resistance controlled path in the switched-off condition, the control circuit is closed via the controlled path of the switching device 7, the connection between the main electrode E of this switching device and the shunt 9, the shunt 9 of the terminals G of the alternating-voltage source.As a function of the implementation of the switching devices, that is to say depending on whether the controlled path has a high or a low resistance in the switched-on condition, a corresponding current will additionally flow through the shunt 10. In the case of switch elements having statically high-resistance controlled paths (such as, for example, MOS fieldeffect power transistors), the total gating current supplied by the alternating-voltage source 8 is limited by the two shunts being connected in series and if bipolar transistors are used, which, when switched on, display a characteristic of very low resistance of the controlled path formed by the base-emitter junction, the current is limited by shunt 9.

In accordance with the current flowing through shunt 9, a negative voltage will be present at the controlled path of the switching device 6 which -reliably keeps this switching device in the switchedoff condition. The current flowing through the controlled path of the switching device 7, or the voltage drop across the shunt 10, respectively, keeps the switching device 7 in the switched-on condition.

The commutation process takes a similarform, independently of the practical design of the switching device 6 and 7. But is is particularly advantageous if the switching devices, in the switched-on condition of operation, have either a low-resistance controlled path (for example bipolar transistors) or, in the case of a high-resistance controlled path, the latter has a capacitive character (MOS FET). The commutation of the current conduction between the two switching devices is initiated by reversing the polarity of the alternatingvoltage source 8.During the switching process, the gating circuit leads from the terminal G, now positive, of the alternating-voltage source 8 via the controlled path of the switching device 6 to its main electrode; the gating circuit is closed via the main electrode of the switching device 7, its controlled path and the terminal F of the alternating-voltage source.

The current flowing in this circuit is used as switching-on current for the switching device 6, it flows in the opposite direction in the controlled path of the switching device 7 and causes this switching device to be switched off as soon as the electrical charge stored in the controlled path has been removed.

In the conventionaliy valid assumption that the switching-on process of a switching device is more rapid than the switching-off, the overlapping of the alternating current flow of the two switching devices can be achieved in this manner, independently of the types of switching devices used in practice.

Figure 2 shows another circuit according to the invention in which the shunts 9 and 10 consist of one series circuit each of a resistive resistor 11 and 13 and diode 12 and 14, respectively. This achieves that the shunt in parallel with the switching device 6 or 7 just switched on remains without current due to the fact that the diode 12 or 14 is then connected in the cut-off direction. This reduces the controlling power supplied by the alternating-voltage source 8.

In this arrangement, a positive potential in comparison with the reference electrode B must be present at the terminal G of the alternating-voitage source, for example so that the switching device 6 is switched on. In this context, the gating circuit is formed by the control led path of the switching device 6, the shunt resistance 13 and the diode 14 which is connected in the conducting direction. In this arrangement, the diode 12 is biased in the cutoff direction. If the switching device 7 is conducting, the gating circuit is formed by the controlled path of the switching device 7, the shunt resitance 11 and the diode 12.

Since in each case only one of the shunts is conducting current, and thus only one of the shunt resistances 11 or 13, the function of the resistance 11 and 13 can befulfilled by a common resistance 15, as indicated in Figure 3, if the anodes of the two diodes 12 and 14 are connected and the resistance 15 is arranged between the junction and the reference terminal B.

Figure 3 shows a DC/AC converter circuit using bipolar transistors 16, 17 as switching devices.

The alternating-voltage source 8 consists of a drive transformer 18 the secondary winding of which is connected between the base connections of transistors 16 and 17. The primary winding of the drive transformer 18 is connected to a rectangularwave generator 19 as the gating generator.

The shunts between the base connections of the transistors 16 and 17 and the reference terminal B are in each case formed by a series diode 12 and 14 of the cathode of which is connected to the base connection fo the transistor 16 and 17, respectively, and by a common resistor 15 which is arranged between the reference terminal B and the junction of the joined anodes of the diodes 12,14.

As a result of this circuit arrangement, a particularly advantageous switching characteristic of the bipolar transistors is achieved.

Assuming that the gating voltage Us happens to have the polarity specified by the voltage arrow, the base current for the transistor 16 flows at the positive terminal of the secondary winding of the drive transformer 18 into the base of the transistor 16 and from the emitter via the resistor 15 and diode 14 to the negative terminal of the secondary winding of the drive transformer. The continuous base current is then determined by the value of the resistor 15,the voltage drop across this resistor being equal to the gating voltage Us reduced by the base-emitter voltage of the transistor 16 and the forward voltage of the diode 14.

At the base-emitter path of transistor 17, the gating voltage Us is present reduced by the base emitter voltage drop of transistor 16, as a negative voltage which reliably keeps the transistor 17 in the cut-off mode. This condition represents one of the two possible steady-state conditions in which in each case one transistor is conductive and the second one is cut off.

The following operational characteristic of bipolar transistors is determining for the switching-over process: If a bipolar transistor, which is cutoff, is to be switched on, the losses in this switching-on process are particularly low if the base current is increased for a brief period to a multiple of the value which is required for switching the transistor to be completely conductive. During this process, the base zone is flooded with charge carriers and this overdriven condition is subsequently maintained by the continuous base current flowing.

To switch off a conducting bipolar transistor, first the charge stored in the base zone must be removed by a negative base charge-removing current. The transistor can cut off only after removal of this charge and the base-emitter path and the collectoremitter path become high-resistance paths.

The process of commutation from the resistor 16, still conducting, to the previous cut-off transistor 17 is initiated by changing the polarity of the gating voltage Us. Since the base-emitter path of the conducting transistor 16 is a low-resistance path, the transistor 17 is supplied with a base current pulse the amplitude of which is limited only by the series circuit of the now low-resistance base-emitter paths of the conducting transistors 16 and 17 and the internal resistance, transformed to the secondary side, of the gating generator 19.

The two diodes 12 and 14 are without current and the charge of the transistor 16 is rapidly removed by the large charge-removing current flowing.

Approximately the same amount of charge is being fed to the base zone of transistor 17. As soon as the base charge of the transistor 16 is completely removed, this transistor cuts off and its base-emitter path becomes a high-resistance path. The base circuit for the transistor 17, now conducting, is then closed via the resistor 15 and diode 12. The resistor 15 limits the continuous base current for the transistor 17 to a relatively low value which completes the process of commutation. The duration of time required for the removal of the base storage charge represents the time of overlapping of the curentflow of the two transistors. These commutation processes are repeated at regualrtime intervals, predetermined by the gating generator 19, for using this circuit as a DC/AC converter. The gating generator 19 can be, for example, the componentTDB 0555 by Thomson.

Claims

1. DC/AC converter comprising a converter transformer and two controllable switching devices operating in overlapping commutation mode, characterised in that between the gate electrodes of the two controllable switching devices an alternating-voltage source is arranged and that a shunt is connected between the gate electrode and the reference electrode of each controllable switch device.

2. DC/AC converter according to Claim 1, characterised in that the shunt is in each case formed by a resistor.

3. DC/AC converter according to Claim 1 or 2, characterised in that the shunt is in each case formed by a series circuit of a diode and a resistor, in which arrangement the diodes are connected in such a direction that in each case the series circuit arranged between the gate electrode and the reference electrode of a controllable switching device which is switched to its conducting state does not load the alternating-voltage source.

4. DC/AC converter according to Claim 3, characterised in that in each case a diode is connected with one of its electrodes to a gate electrode of a controllable switching device and the other diode electrodes are connected to each other and led via a common resistor to the reference potential.

5. A DC/AC converter susbstantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.