GB2132436A - Darlington transistor switch - Google Patents

Abstract

An electrical power switch comprising two transistors (1 and 2) in Darlington connection and a transformer (4) of which the primary (5) is in series with the collector/emitter circuit of the second transistor (2) and the secondary (6) is connected in series with the collector/emitter circuit 9 the first transistor in such a way that when current flows in the primary a current is induced in the secondary in a sense which accords with the current already flowing therein, and the voltage that appears across the secondary is in a sense which reduces the collector/emitter voltage of the second transistor, thus also reducing power loss. The winding ratio is chosen so that when current flows the second transistor is always in saturation. Further embodiments show two of these transistor pairs connected in series (Fig. 3), and parallel (Fig. 4), being switched on and off alternately such that the magnetic flux is reset in each secondary when the associated pair is off. Also there is an embodiment wherein the magnetic flux is reset by load current (Fig. 5) which free-wheels when the switch is off. <IMAGE>

Description

SPECIFICATION Darlington transistor switch This invention relates to electrical switches and switching systems. The invention particularly relates to switches and switching systems comprising bi-polar transistors.

The use of bi-polar transistor in a switching system for high currents necessitates the generation of sufficient base current for the main output transistor to ensure that the voltage appearing across the transistor is low when the transistor is conductive. It is well-known that in order to saturate a transistor, that is to say to reduce the voltage across the transistor to an extemely low value, the base current must increase as the collector current increases. If jBsat is the ratio of collector current to base current to achieve a given collector-emitter voltage when the transistor is saturated, for transistors of high current handling capacity sssat is generally in the order of 5 or 6.It is possible to yenerate base current by means of a separate low voltage power supply and an appropriate network arranged to maintain a constant /3sat between the instantaneous collector current and the base current. However, such an expedient is complex and unless the base drive source has over capacity, namely is capable of delivering much more current than is normally required, the main output transistor might come out of saturation during a transient peak current. If this should happen, there may be excessive dissipation in the output transistor and possibly destruction of it.

It is preferable therefore to constitute a switch by means of a composite transistor in a Darlington connection. In the basic form of such a connection, the emitter current of a first transistor provides the base current for a second transistor and the collectors of the two transistors are connected to a common point. It is also possible to connect more than two transistors in cascade in this manner so as to minimise the base drive current for the input transistor in the cascade connection. The present invention is applicable to Darlington connections having either two or more than two transistors in cascade but particularly concerns the penultimate and ultimate transistors in such a connection.

In a Darlington connected composite transistor the gain under saturated conditions of the composite transistor is the product of the gain of each individual transistor in the chain. Thus for a two-stage Darlington connection, a gain of 25 is readily available at a high current level.

An important disadvantage of the use of a Darlington connection for the switching of high currents is, for reasons explained in more detail hereinafter, the substantial collector-emitter voltage under saturated conditions of the combination. This cannot normally fall below the sum of the collector-emitter voltage, under saturated conditions, of the first or penultimate transistor and the base-emitter voltage, under saturated conditions, of the second or output transistor. Although in high voltage systems the additional voltage drop due to the base-emitter voltage of the second or output transistor may be negligible, in low voltage systems this baseemitter voltage may represent a significant fraction of the available supply voltage.

Furthermore, in high current systems it may represent more than half the power loss within the output transistor. If this component of power loss can be eliminated or substantially reduced, the system may be made substantially more efficient and easier to cool.

It is accordingly one purpose of the present invention to preserve the simplicity and high gain of a switching system employing the Darlington connection yet substantially to reduce loss arising from the magnitude of the base-emitter voltage of the output transistor when that transistor is saturated.

It has been proposed to provide, for a Darlington connection of transistors, a transformer of which the primary winding is in series with the emitterjcollector circuit of the drive transistor and of which the secondary is connected by way of a resistor to the emitter of the second, or output transistor. Such an arrangement is disclosed in British Patent Specification No. 1553815, for the ostensible purpose of inducing, when the current in the collector of the drive transistor falls, reverse bias current in the base-emitter junction of the output transistor whereby to promote rapid switching of the output transistor to its on state and thereby to minimise the power dissipated by the output transistor during switch-off. Such an arrangement does not limit power dissipation during the conduction phase of the output transistor.

It has also been seperately proposed to provide an inductor in series with the collector of the output transistor of a Darlington pair of transistors and to connect a tap on the inductor to the emitter of the first, or driver, transistor of the Darlington pair. Whereas in such an arrangement, as suggested by British Specification No.

1132191 it may be possible to reduce the emitter/collector voltage of the main or output transistor, the current carried by the driver, or first, transistor is undefined. A high gain driver may divert a substantial portion of the load current via itself and the base of the main device, the resultant saturation of the main transistor having little or no effect on the magnitude of this current.

Summary of the invention In one preferred form of the invention, an electrical switch comprises a first and a second bi-polar transistor in Darlington connection, and includes a transformer which responds to current flow through the collector of the second transistor to provide a regenerative feedback voltage to the collector of the first transistor. As will be apparent from the specific description which follows, the aforementioned voltage overcomes the baseemitter voltage of the second transistor and thus substantially reduces or eliminates the aforementioned component of loss and permits the switch to operate more efficiently.

Thus in a particular form of the invention, an electrical switch comprises a composite Darlington transistor, of which the emitter of a first bi-polar transistor is connected to the base of a second bi-polar transistor and the collector of the first transistor is connected to the collector of the second transistor, and a transformer of which the primary winding is disposed in series with the collector/emitter circuit of the second transistor and of which the secondary is disposed to produce in the collector/emitter circuit of the first transistor a voltage which reduces the baseemitter voltage of the second transistor. In most configurations employing a switch according to the invention, the magnetic flux in the transformer will require resetting.One advantageous further development of the basic circuit disclosed herein is an electrical switch arrangement comprising two switches each with a transformer as previously described; the transformers share a common magnetic circuit and the arrangement is such that the flow of respective load currents through the second or output transistors produce magnetic fluxes in opposite senses in the said common magnetic circuit. Thus the conduction of one switch inherently provides resetting of the magnetic flux in the magnetic circuit of the transformer for the other switch and a separate means for resetting the magnetic flux is unnecessary. One circuit which can usefully employ switches according to the invention is an inverter in which switches as aforesaid are arranged to provide current flow in opposite directions through a common load.In such an inverter there may therefore be a winding, in series with the load, constituting a common primary for the transformers associated with the switches. However, the switches may also be useful in a circuit in which the switches are disposed in parallel to provide unidirectional current flow through a common load. In such an arrangement the primary windings of the transformers for the two stitches may be disposed electrically in parallel and be wound in opposite senses on a common core.

Another advantageous development of the basic circuit employs a magnetic circuit which is driven into saturation by the conduction of at least one output transistor in a Darlington pair but which does not require any separate switch arrangement for resetting the magnetic flux in the magnetic circuit of the transformer or transformers.According to this aspect of the invention, an electrical switch arrangement comprises at least one Darlington pair of transistors constituted by a driver transistor and an output transistor and a transformer of which the primary winding is disposed to carry load current of the output transistor and of which a secondary winding provides regenerative feed back to the collector of the driver transistor of the Darlington pair, there being provided in series with a load and the said output transistor a further winding which shares a common magnetic core or circuit with the aforesaid transformer and which is arranged to provide magnetic flux in a sense opposite to the magnetic flux produced by load current flowing through the primary of the aforesaid transformer, there being provided a free-wheeling loop for current to flow through the load and the further winding when the said output transistor is non-conductive. Such an arrangement may include more than one Darlington pair each with its transformer sharing the same magnetic core; it is preferable that the Darlington switches be controlled so that only one is conductive at any particular time. As will be explained in more detail hereinafter, the magnetic flux in the common core can be reset by the freewheeling load current and such an arrangement provides the maximum on time for which the transformer overdrive action in the Darlington pair is maintained; the core is or can be driven from one saturation limit to the other and optimum use may be made of the magnetic characteristics of the material of the core.

Brief description of the drawings There follows a more detailed description of the present invention with reference to the accompanying drawings, in which: Figure 1 illustrates a Darlington connection of transistors; Figure 2 illustrates a basic form of the invention; Figure 3 illustrates an inverter including two switches according to the invention; Figure 4 illustrates another circuit according to the invention; and Figure 5 illustrates a further development of the basic circuit of the invention.

Detailed description of the invention Figure 1 of the accompanying drawings illustrates a basic form of a composite transistor comprising two bi-polar transistors in Darlington connection. The first or penultimate transistor 1, usually termed drive transistor, is connected so that its emitter current corresponds to the base current of the second, or output, transistor 2. The collectors of the two transistors are connected together to a common point 3. The connection shown in Figure 1 is not the only form of Darlington connection for which the present invention is suitable. For example, the second or output transistor 2 may be constituted by a plurality of transistors arranged in parallel, fed from a single input transistor corresponding to the transistor 1 in Figure 1. Moreover, there may be a cascade of more than two transistors; in this example the transistor 2 would normally be constituted by the last transistor in the Darlington connection. One advantage of the Darlington connection as shown in Figure 1 is the multiplication of the gains of the individual stages.

However, an important disadvantage is the increase in the collector/emitter voltage of the transistor 2. When the two transistors are saturated, it is clear that the potential difference between the point 3 and the emitter of the transistor 2 cannot be less than the sum of the collector/emitter voltage of the transistor 1 and the base-emitter voltage of the transistor 2. As previously indicated, the component represented by the base-emitter voltage of the transistor 2 may represent a significant fraction of the supply voltage and the power loss due to this component may represent a very significant part, perhaps 60% to 70%, of the total power loss within the transistor 2.

Figure 2 illustrates a Darlington connection as shown in Figure 1 but including a transformer 4 which provides regenerative feedback. By way of example, the primary winding 5 of this transformer is in series with the collector/emitter circuit of the output transistor 2 whereas the secondary winding 6 of the transformer 4 is in series with the collector circuit of the input transistor 1. In particular, the winding 6 is connected between the collectors of the transistors 1 and 2. Preferably the transformer has a core 7 with a square B-H characteristic and preferably low hysteresis loss.

When the transistor 1 is rendered conductive by an input signal current to the base8 of the transistor 1, current will start to flow in both transistors 1 and 2. The collector current of transistor 2 flows through the primary winding 5 of the transformer 4 and induces a secondary current, of which the magnitude is governed by the ratio of the turns of windings 5 and 6, in a direction which aids the conduction of the transistor 2. This current is driven by a voltage appearing across the secondary 6 in a sense which, for the conductivities shown, raises the collector of the transistor 1 and thereby raises the base potential of the transistor 2 even though its collector potential is depressed relative to the emitter thereof. Thus the required drive voltage to overcome the base-emitter voltage of the second transistor is generated by the transformer.

The transformer 4 constitutes a current transformer which determines the saturated gain psat of the transistor 2; the gain is given by the ratio N between the number of turns of the primary winding to the number of turns of the secondary winding. To ensure maximum efficiency, it is essential that magnetic saturation of the core 7 of the transformer 4 should not occur during a period of conduction of the transistors. If magnetic saturation does occur, the transformer would be unable to generate the required secondary voltage. When the core saturates, the circuit then behaves as an ordinary Darlington connection, as shown in Figure 1.

However, for some purposes the loss of efficiency may be tolerable and it is accordingly not absolutely essential for the present invention that the saturation of the magnetic core of the transformer should be avoided.

If the switch, as would be usual, is required to perform repeated switching cycles, the magnetic flux in the transformer should be reset after each period of conduction of the switch. The means for resetting might comprise a third winding which is energised alternately with the conduction periods of the transistors 1 and 2, but in many circumstances the use of a separate means for resetting is unnecessary. Examples of circuits in which more than one switch is used and the switches provide resetting of the flux in the transformer are shown in Figures 3 and 4.

Figure 3 illustrates the invention employed in a half bridge inverter. Other forms of inverter, whether single or polyphase, can be constituted in a similar manner. In the inverter shown in Figure 3, there are effectively two switches of the kind shown in Figure 2. The second switch, of which the components are denoted by the postscript "a" is arranged with its output transistor 2a in series with the output transistor 2 of the first switch, the switches being connected between a positive rail 9 and a negative rail 10. The primary winding 5 is common to the two switches and is connected to a point between the emitter of the transistor 2a and the junction point 3 of the first switch and is also connected in series with a load 11 of which the other side is connected to earth 12.

In the inverter shown in Figure 3, the two Darlington switches conduct currents which are in opposite directions through the load 11. Input drive signals may be applied to the bases 8 and 8a of the respective input transistors 1 and 1 a.

Each Darlington switch operates in the manner already described with reference to Figure 2.

Owing to the reversal of the direction of the load current, when each Darlington switch is conductive, the current flow in the primary winding 5 effectively resets the magnetic flux for proper operation of the other Darlington switch.

Figure 3 also shows that the load and the primary winding can be connected to the emitter rather than to the collector of the output transistor in the Darlington connection.

Figure 4 illustrates another circuit in which two Darlington switches are employed. In this circuit the switches constituted by the transistors 1, 2 and 1 a, 2a respectively, are in parallel and the primary windings 5 and 5a of the respective transformers are constituted by single turns on a common core 7. Each primary winding 5, 5a is in series with the common load 11. A free-wheeling diode 13 is connected around the load 11. The circuit shown in Figure 4 could form one arm of a multi-phase inverter but could be used on its own in order to provide uni-directional current flow through the load 11.

Whenever the load current is required to flow in the circuit shown in Figure 4, the Darlington switches are turned on alternately, the periods of conduction of the two switches being preferably, though not essentially, equal. Each Darlington switch operates in the manner described with reference to Figure 2 and the conduction of each Darlington switch automatically resets the flux of the transformer for the other Darlington switch.

Figure 5 illustrates another circuit which combines the transformer overdrive for a Darlington pair of transistors with the use of load current to reset the overdrive flux.

It will be understood that when bi-polar transistors are used to control high currents the lowest voltage drop in the on state of the transistor is achieved when the transistor is driven into its saturated state by the injection of a large base current sufficient to depress the collector/emitter voltage to its lowest level. If a collector current of, for example, 500 amps is to be controlled then a base current of about 100 amps is required. This is difficult and expensive to provide and would cause a high power loss external to the transistor, making the overall system inefficient.

If a Darlington configuration is used, the base drive required to control 500 amps may be of the order of 5 to 8 amps but the voltage of the Darlington switch in the conductive state rises to the sum of the collector/emitter voltage of the driver transistor and the base/emitter voltage of the output transistors; accordingly the power loss in the on state is two to three times that of a single transistor. The power loss in a Darlington switch can be reduced to the order of that in a single transistor, while the current gain is preserved, using the overdrive described previously but in some circumstances it may become difficult or expensive to reset the flux in the transformer core if the transistor switch is required to operate at low frequencies, for example at less than 1 kilohertz, and with a high mark to space ratio.In particular, such difficulty arises unless the switch is operated at a high frequency with a maximum of 50% mark to space ratio or the transformer core is very large.

Thus the circuit shown in Figure 5 by way of example is suitable particularly for comparatively low frequency operation though is not limited thereto.

In the circuit shown in Figure 5, the transistors 1 and 2 are arranged in Darlington connection.

The primary winding 5 of a transformer is in series with the collector of the transistor 2 and the secondary of this transformer is connected between junction 3, to which the primary 5 is connected, and the collector of the driver transistor 1. A second Darlington switch is constituted by the driver transistor 1 a and the output transistor 2a which are associated with the transformer having a primary 5a and a secondary 6a similar to the primary 5 and secondary 6. It is desirable that the driver transistors 1 and 1 a be operated such that the two Darlington switches are never conductive at the same time but part from this constraint the Darlington switches may be turned on and off independently by appropriate signals on the respective input lines 8 and 8a.

Figure 5 illustrates the two Darlington switches connected in a half bridge configuration driving a single load 11. However, the same configuration may be adopted in various inverter and converter circuits, both single-phase and multi-phase.

Moreover, the same scheme may be applied to a single Darlington switch which may be used, for example, as a DC chopper. For convenience the operation of the circuit will be described with reference to only one Darlington switch, as constituted by the transistors 1 and 2.

All the transformer windings shown in Figure 5 are on a common core. These windings include a further winding 20 which is connected between the common junct;on 3 and the load 11. The polarity of this winding is such that it produces flux in the common magnetic core opposite to that produced by normal load current through the primary windings 5 and 5a.

It is convenient to consider an example wherein the transistor 1 a and 2a are normally non-conductive and transistors 1 and 2 are being turned on and off under the control of signals applied to the line 8 so as to draw current out of the load over a comparatively long period, for example over one half cycle. When the transistors 1 and 2 are off (non-conductive) the load current can free-wheel through free-wheeling diode 21 connected from the winding 20 to the positive rail 9. A further free-wheeling diode 22 is connected between the winding 20 and the negative rail 10 in this particular embodiment.

If, in the conditions stated, the off period is short compared to the time constant of the load the load current will remain almost constant. The uni-directional current flow through the winding 30 will saturate the core of the transformer in one direction when none of the other windings 5, 6, 5a and 6a are carrying current.

When the transistors 1 and 2 are rendered conductive the load current is diverted from the diode 21 to flow through the windings 5 and 6 in appropriate proportions. In this particular example the winding 20 has one turn, the winding 5 has two turns and the winding 6 has six turns. The flow of load current through windings 5 and 6 is in a ratio of approximately 6:1 and in a direction that produces magnetic flux opposing that produced by the winding 20. In fact the magnetic flux produced by the winding 5 will exceed the magnetic flux produced by the winding factor of approximately 2 (if the effect of the winding 6 is ignored) so that the transformer core will be driven eventually to saturation in the opposite direction to that caused by the action of winding 20 on its own. The time taken to reach saturation in this direction is dependent upon the volts required across the secondary winding 6 and the material of the core.

This arrangement provides the maximum on time for which the transformer overdrive action, described earlier herein, is maintained and since the core is being driven from one saturation limit to the other optimum use may be made of the material of the core.

When the transistors 1 is turned off, its collector swings positive until either a diode 25, which is optional, fitted between the collector of transistor 1 and the positive rail 9, conducts so as to clip the voltage across winding 6 or, if the diode 25 is not provided, the collector of the transistor 1 swings positive until the transformer action between the winding 5 and the winding 6a causes the conduction of a diode 23, which is connected between the collector of transistor 1 a and the negative rail 10. The conduction of this diode will limit the voltage across the winding 6 to the supply voltage between the rails 9 and 10.

When the transistor 2 turns off, the diode 21 starts to conduct and the voltage on the collector supply rail. At the same time the voltage across the winding 6 reverses since the winding 20 is the only winding which carries current. The voltage on the collector of the transistor 2 will continue to rise until a diode 24, which is connected between the collector of transistor 1 and the negative rail 10, conducts to clamp the negative fall of the voltage across the winding 6.

Thus the voltage across the winding 6 will equal the supply voltage at the collector of transistor 2 is, by transformer action, above the voltage of the positive supply rail by the product of the supply voltage and the turns ratio of the windings 5 and 6.

While transistors 1 and 2 are off, the whole of the supply voltage, less the voltage drop across one diode, is maintained across the winding 6 until the core flux is reset, that is to say the core saturates again in the direction caused by the load current. In this way a high voltage is used to reset the core in a short time without any requirement for a reset switching transistor. The maximum mark to space ratio during the conductive period is given by the expression (VsVd)/V6 (where V5 is the supply voltage, Vd is the voltage drop across one diode and V6 is the voltage across the winding 6). The voltage V6 is typically two or three volts.

Claims (9)

Claims

1. An electrical switch comprising a first and second bi-polar transistor in Darlington connection and a transformer which responds to current flow through the collector of the second transistor to provide a regenerative feedback voltage to the collector of the first transistor.

2. An electrical switch comprising a composite Darlington transistor of which the emitter of a first bi-polar transistor is connected to the base of the second bi-polar transistor and the collector of the first transistor is connected to the connector of the second transistor, and a transformer disposed to respond to current flow through the collector/emitter circuit of the second transistor to produce in the collector/emitter circuit of the first transistor a voltage which reduces the baseemitter voltage of the second transistor.

3. An electrical switch according to claim 1 or claim 2 in which the transformer includes a primary winding in series with the collector/emitter circuit of the second transistor and a secondary winding connected between the collectors of the first and second transistors.

4. An electrical switch according to any foregoing claim, including means for resetting the magnetic flux in the transformer.

5. An electrical switch arrangement comprising two switches according to any one of claims 1 to 3, the transformers sharing a common magnetic circuit and the arrangement being such that conduction of the second transistors produces respective magnetic fluxes in opposite senses in the said magnetic circuit.

6. An electrical switch arrangement according to claim 5 in which the primary winding of one transformer constitutes the primary winding of the other transformer.

7. An electrical switch arrangement according to claim 5 in which the switches are disposed in parallel to provide uni-directional current flow through a common load.

8. An electrical switch arrangement according to claim 7 in which the primary windings are wound in opposite senses on a common core.

9. An electrical switch arrangement comprising at least one electrical switch consisting of a first and a second bi-polar transistor in Darlington connection, a transformer of which the primary winding is in series with the second transistor, so as to respond to current flow through the collector of that transistor, and of which a secondary winding is disposed to provide regenerative feedback to the collector of the first transistor, a load, a further winding which is disposed on a common core with the windings of the transformer and is arranged to provide in response to load current magnetic flux in a sense opposite to that produced by the said primary winding, and a free-wheeling loop including the load and the further winding.

1 0. An electrical switch according to any of claims 1 to 4 or an electrical switch arrangement according to any of claims 5 to 9, in which the transformer has a magnetic core with a square B-H characteristic.