GB2124050A - Semiconductor switching circuits - Google Patents

Abstract

Speed of switching of semiconductor devices, such as transistor 10, is improved by providing base drive which is a function of load current by means of current transformer 14; switch 17 reverses the polarity of the base drive and causes a switching transition of transistor 10, extracting excess charge from the base. In a modified arrangement (Fig. 3), an auxiliary transformer (T5) is connected in series with the main transformer (T1) the outputs of the two transformers being summed to provide the base drive current. The auxiliary transformer is arranged to saturate early in the switching cycle, thus providing a boost to the base drive at the commencement of each cycle, when base drive current would otherwise be low. <IMAGE>

Description

SPECIFICATION Semiconductor switching circuits The present invention relates to semiconductor switching circuits, and in particular to means for optimising the characteristics, especially the switching speed, of semiconductor devices in these circuits.

The ideal characteristics of a switch are that it should exhibit zero resistance in the "on" state and infinite resistance in the "off" state: furthermore, transitions between these states should be performed in zero time.

In semiconductor switching circuits, bipolar transistors fall short of the ideal in that they exhibit (i) a finite turn-on time, (ii) saturation loss when "on", and (iii) a finite turn-off time.

The last effect is further exacerbated by the additional factor that a bipolar transistor does not begin to turn off instantly at the cessation of its drive pulse, but has an undesirable delay, known as the storage time, during which base charge is removed prior to turn off.

The speed of turn-on can be improved by flooding the base of the transistor with current in excess of the normal requirement, and similarly saturation loss can be minimised by supplying the base with a greater current than would appear necessary form the large signal gain factor of the transistor. In one arrangement the base is supplied with a fixed fraction of the collector current. However, the excess current increases the surplus stored charge in the transistor and considerably worsens the turn-off problem, since more charge needs to be removed prior to turn-off.

The present invention provides a semiconductor switching circuit comprising a transformer having a first winding connected in the switch current path of a semiconductor switch and a second winding arranged to provide drive current to a control terminal of the semiconductor switch such that the drive current is a function of the switch current, the circuit further including polarity reversing means responsive to control signals for controlling the semiconductor switch to reverse the direction of the drive current provided by the second winding of the transformer, thereby to accelerate removal of stored charge from the semiconductor switch and increase the switching speed.

A single transformer can be utilised for the above purpose, but in the preferred arrangments, two or more transformers are included to advantage.

A further aspect of the invention, which may preferably but not necessarily be combined with the polarity reversing feature, is a semiconductor switching circuit comprising main and auxiliary transformers having first windings connected in series in the switch current path of a semiconductor switch, and second windings whose outputs are summed to provide a drive signal for the semiconductor switch, the auxiliary transformer being arraged to saturate early in the switching cycle of the semiconductor switch whereby its output boosts the drive signal at the commencement of each cycle.

This effect of providing a boosted drive signal early in the switching cycle considerably speeds up the turn-on transition.

The boost transformer core remains saturated until reset by the alternative half cycle of the drive.

Features and advantages of the present invention will become apparent from the following description of embodiments thereof, when taken with the accompanying drawings, in which: Figure 1 is a circuit diagram of a basic embodiment of the invention; Figure 2 is a circuit diagram of an improved embodiment of the invention; and Figure 3 is a detailed circuit diagram of an inverter including drive circuits in accordance with the Fig. 2 embodiment.

Referring to Fig. 1, a semi-conductor switch in the form of a bipolar transistor 10 is connected in series with a load 11 so as to switch current from a D.C. power source 1 2 through the load. Also connected in the load current path is a current transformer 1 4 having one winding 1 5 actually in the load current path and a second winding 16, shown as a tapped winding.The centre tap of winding 1 6 is connected to that terminal of power source 1 2 which is connected to that terminal of power source 1 2 which is connected to the emitter of transistor 10, whereas the opposite ends of the winding 1 6 are connected through respective contacts of a two-way switch 17, so as to be alternately applied to the base of the transistor 1 0. Switch 1 7 is schematically shown by way of broken line 1 8 to be externally controlled and in practice this switch will comprise a suitable semi-conductor arrangement such as two transistors switchable in antiphase (see below) in response to switching drive signals.

In operation, assuming a starting pulse has been applied to the base, transistor 10 will start to conduct and load current will begin to flow through the winding 1 5 of the transformer 1 5. Assuming that switch 1 7 is appropriately set, a base current which is a proportion of the load current (the proportion depending on the turns ratio between winding 1 5 and the appropriate part of winding 16) will flow into the base and this has the effect of reducing saturation loss by ensuring that the transistor always has a sufficient base drive for maximum conduction.When a transition in conduction is required, the switch 1 7 is changed over, the instantaneous effect of this being to cause a current to flow in the reverse direction, e.g. out of the base in the circuit as shown, which reverse current is effectively the load current transformed to a desired level and extracted from the base as a turn-off current. This process occurs only after the switch 1 7 has been changed to the transistor off position, and therefore any collector current still flowing is a result of charge stored within the transistor. It will be seen that as soon as the stored charge reaches a minimum value, the collector current will begin to decrease and thus so will the base current.The base drive reverse current pulse shape, in particular its width, is therefore precisely tailored to the exact requirements of the actual transistor 10 in circuit, irrespective of type or batch variations, without any adjustment needing to be made. A further advantage is that, because the reverse current drive is proportional, as the collector current begins to fall so does the base current; consequently there is no possibility of inducing what is known as "current tailing" due to premature emitter current cut-off. It will be apparent that the combination of centre-tapped winding 1 6 and switch 1 7 is illustrative of just one way of achieving the required polarity reversal; any other similar arrangement can be used which provides the required effect.

Fig. 2 shows a practical realisation of the switch drive circuit. As before, the transistor 10 is connected in series with the load 11 and a transformer 24 (specifically its winding 25) across a power source 1 2. However, in this case the polarity reversal mechanism is achieved in a rather different manner. The transformer 24 has a second winding 26 connected via a diode 27 to a tap on one winding 28 of a further transformer 29. The ends of this winding are connected via respective switching transistors 30, 31 to the negative side of power source 1 2. A second winding 32 on transformer 29 provides base drive to the transistor 10. Thus, the two transformers are connected in cascade. Complementary drive Q, Q is provided for the transistors 30, 31 respectively.A starting resistor 33 is connected between the positive side of power source 12, and the winding 28.

The operation of this circuit is similar to that of Fig. 1 but in this case two cascade-connected transformers 24, 29 are provided instead of one. The advantage of this is that by suitable choice of turns the ratio, the working current of transistors 30, 31 can be chosen.

Also, starting current can conveniently be fed via resistor 33. Finally, if necessary, the transistors 30,31 and windings 26, 28 can be operated from an isolated supply, or referenced to a different potential level as will be described below.

A drive pulse provided at Q turns transistor 30 on, current flows through windings 28 and provides a corresponding base drive current for transistor 10 in winding 32; turn-on of transistor 10 causes a load current to flow through winding 25 which then provides the proportional drive current via winding 26, diode 27 and winding 28 through to winding 32. A turn-off command removes the pulse at Q and provides a complementary pulse at Q; transistor 30 is turned off and transistor 31 is turned on. As described with reference to Fig.

1, this causes the direction of base current in winding 32 to reverse, forcing the transistor 10 to turn off rapidly by virtue of enhanced extraction of base charge. Again, the base drive reverse current pulse is exactly tailored to the turn-off requirements of the transistor 10.

When the load 11 is demanding a relatively low current, there may be insufficient feedback via transformers 25, 29 to maintain adequate base drive; the value of resistor 33 is chosen to provide a minimum but sufficient current (via winding 28 when transistor 30 is on) to maintain switching of transistor 1 0.

Fig. 3 shows an inverter circuit, known as the half-bridge inverter, including drive circuits for the switching transistors similar to those previously described with relation to Figs. 1 and 2. The basic operation of the halfbridge inverter is well known and documented (e.g. U.S. Patent Nos. 4,164,014 and 4,199,807) and will not be described in detail. Suffice to say that switching transistors TR1, TR2 alternately switch current from the power source via transformers T1, T5 and the load, to the junction between capacitors C1, C2 connected in series across the power source. Both transistors TR 1, TR2 are switched by drive circuits in accordance with the invention and therefore only the drive circuit with respect to transistor TR1 will be described, that for transistor TR2 being identical.

Transistor TR1 switches load current from the power source via transformers T1,T5 through the load. Secondary windings of the transformers provide outputs which pass via respective diodes D2, D4, are summed and applied to a tap of a winding on a transformer T3. The winding portions divided by the tap (1, 2 and 3,4) are switched in complementary manner by transistors TR3, TR4 in similar fashion to that described with reference to Fig. 2. A transformer T4 receives switching command signals for both switching transistors TR1 and TR2, and the appropriate signals are then supplied directly to the base of transistor TR4 and indirectly, via an inverting transistor TR5, to the base of transistor TR3 so as to generate the complementary drive.

The output winding of T3 includes a tap connected via a diode D6 and inductor L2 to the base of the switching transistor TR 1; the other ends of the output winding are connected directly to the emitter, and to the collector via a diode D8. This base drive arrangement is more complex than that shown in Fiq. 2 and Drovides a clamoina action which further improves turn-off performance.

In outline, the switching operation of transistor TR1 is identical to that described for Fig. 2. Proportional drive is fed via transformers T1, T5 and diodes D2, D4 to transformer T3 and thence to the base of transistor TR1 during conduction of the transistor. The turn-off signal (via transformer 14) reverses the polarity of drive current in the output winding of transformer T3 and extracts base charge from transistor TR 1 accelerating the turn-off time, the reverse base current lasting only until the collector current has diminished.

Detail differences between the drive circuits of Figs. 2 and 3 include the provision in the latter of two transformers T1, T5 in the load current path whose outputs are summed, and the clamped base drive arrangement.

Referring firstly to the provision of two transformers T1, T5 this arrangement produces a further improvement in switching speed of the transistor. Transformer T1 is the main transformer and operates as previously described to provide the proportional base drive. Transformer T5 is an auxiliary transformer, typically a miniature ring core current transformer which is designed (e.g. by suitable dimension of core cross-section) to saturate within some fraction of the half periodic time of the inverter. The auxiliary transformer T5 thus provides a boost in the drive current very early on in each half cycle of operation when the current from the main transformer T1 is still very low. The turn-on time is therefore considerably reduced, since the base of transistor TR1 receives a substantial inital drive current.This is particularly advantageous in the illustrated inverter circuit, since at turn-on, the power transistors TR1, TR2 have to switch current from snubbing capacitors C3, C4.

Turn-off performance of transistor TR1 is further improved by the clamped base drive arrangement provided by the tapped output winding of transformer T3 and diode D8 together with resistor R1. Assuming that winding 7-8 of the transformer T3 has the same number of turns as winding 6-5, the total voltage across the complete winding 8-5 will be 2VBE (base-emitter voltage). If the collector of transistor TR attempts to fall below a voltage equal to 2VBE-VD (VD being the forward voltage drop across diode D8) the diode D8 will conduct. Because the transformer T3 provides essentially a constant current source, when diode D8 conducts, it diverts current away from the base of transistor TR 1. The effect of this is to stabilise the collector voltage of transistor TR 1 at some voltage above the saturation voltage which would normally be achieved with the designed forced gain.

Claims (8)

1. A semiconductor switching circuit comprising a transformer having a first winding connected in the switch current path of a semiconductor switch and a second winding arranged to provide drive current to a control terminal of the semiconductor switch such that the drive current is a function of the switch current, the circuit further including polarity reversing means responsive to control signals for controlling the semiconductor switch to reverse the direction of the drive current provided by the second winding of the transformer, thereby to accelerate removal of stored charge from the semiconductor switch and increase the switching speed.

2. A semiconductor switching circuit according to claim 1, wherein the polarity reversing means comprises a switching arrangement for reversing connections between the second winding of the transformer and the semiconductor switch terminal.

3. A semiconductor switching circuit according to claim 1 or 2, wherein the polarity reversing means includes a second transformer having a centre-tapped winding connected to the second winding of the firstmentioned transformer, and an output winding providing drive current to the semiconductor switch control terminal, first and second portions of the centre-tapped winding on opposite sides of the tap being alternatively switched to provide the polarity reversal.

4. A semiconductor switching circuit according to claim 3, further including a starting resistor connected between the centre-tapped winding of the second transformer and a source of current, thereby to provide a starting current to the circuit.

5. A semiconductor switching circuit according to any preceding claim, including an auxiliary transformer having a first winding connected in series in the switch current path and with the first winding of the first- mentioned transformer, and a second winding whose output is summed with that of the second winding of the first- mentioned transformer to provide the drive current to the control terminal of the semiconductor switch, the auxiliary transformer being arranged to saturate early in the switching cycle of the semiconductor switch whereby its output boosts the drive current at the commencement of each cycle.

6. A semiconductor switching circuit comprising main and auxiliary transformers having first windings connected in series in the switch current path of a semiconductor switch, and second windings whose outputs are summed to provide a drive signal for the semiconductor switch, the auxiliary transformer being arranged to saturate early in the switching cycle of the semiconductor switch whereby its output boosts the drive signal at the commencement of each cycle.

7. An inverter arranged to alternate the direction of current flow from a power supply through a load, including two semiconductor switching circuits according to any preceding claim.

8. A semiconductor switching circuit, or an inverter including same, substantially as herein described with reference to and as illustrated in the accompanying drawings.