WO1989000360A1

WO1989000360A1 - Programmable switching transistor circuit

Info

Publication number: WO1989000360A1
Application number: PCT/AU1988/000235
Authority: WO
Inventors: Beresford Clair Joseph
Original assignee: Beresford Clair Joseph
Priority date: 1987-07-03
Filing date: 1988-07-01
Publication date: 1989-01-12

Abstract

A programmable switching transistor circuit has at least one pair of semi-conductor devices (12, 14) of opposite types, e.g., pnp and npn types, interconnected with the gate terminal of the first device (12) connected to the anode terminal of the second device (14). The cathode terminal of the first device (12) is connected to a control terminal (G). A voltage dependant switch (15), such as a zenner diode, is connected between the control terminal (G) and the gate terminal of the second device whereby the circuit becomes voltage controlled for "on" and "off" switching in accordance with external circuit parameters and the voltage value of the switch (15).

Description

PROGRAM AB E SWITCHING TRANSISTOR CIRCUIT Field of the Invention

This invention relates to a programmable, or controllable, switching transistor circuit and relates particularly to such a circuit which may be used to control power delivered to a motor, a power dimming circuit, welding equipment, switching regulators, overload protectors, chokes for fluorescent lights and the like. Background of the Invention It is known to use silicon-controlled rectifier devices (SCR), also known as Thyristors, as motor controllers, dimmer controllers, voltage and current regulators and the like. Germanium-controlled rectifier devices (GCR) operate in a similar fashion and have similar functions. However, in certain application, because the SCR (or GCR) does not switch off until the anode-cathode current is reduced to a value less than the holding current, difficulties can arise in some switching operations which are desirably voltage controlled. Many forms of voltage controlled switching circuits have been devised for specific applications. Thus, it is well known to use zenner diodes for switching transis¬ tors between specific voltages which are a function of the zenner diode voltage. However, a problem with zenner diode switching of transistors is that as the zenner voltage increases to a level where the transistor commences to conduct, that voltage level is insufficient to allow full conduction. Therefore, for proper zenner switching, it is necessary that the voltage applied across the zenner diode tries to rise sufficiently rapidly to permit enough current flow to enable proper operation of the transistor in the switching circuit.

It is therefore desirable to provide a switching circuit which avoids, or at least alleviates disadvantages of previously known switching circuits.

It is also desirable to provide a voltage switching circuit which has a variety of applications ranging from a simple oscillator circuit to motor control. It is also desirable to provide a programmable switching transistor circuit arrangement where a simple, variable impedence, such as a variable resistor, or potentiometer, may be used to control the switching parameters.

Summary of the Invention

According to one aspect of the invention there is provided a programmable switching transistor circuit comprising at least two semi-conductor devices of opposite types, wherein the base terminal of one device is connected to an anode terminal of a second device and a cathode terminal of the said one device is connected to a control terminal, and a voltage dependant switch means is connected between the control terminal and a base terminal of the second device.

In its preferred configuration, the programmable switching transistor circuit of the invention behaves similarly to an SCR but is programmable, or controllable, to switch on and off at predetermined voltage levels. A pair of switching circuits may be connected back-to-back, or as a complimentary pair, for full wave use of ac or for a non-polar dc inverter, or the like. When a pair of such circuits are used as a complimentary pair, the circuit arrangement behaves in a similar manner to a TRIAC, but the circuit is fully programmable or controllable to switch ON and OFF at predetermined voltage levels.

If desirable, the switching transistor circuit of the invention may be used inside a diode bridge for a full wave control.

In use as an oscillator, the anode and cathode terminals^', and the control terminal are connected through a resistor or impedance network on one side of the load, and the voltage across the zenner diode is controlled to vary the frequency of oscillation. Generally, the lower the voltage of the zenner the higher is the possible oscillator frequency. Use of the circuit as in a self-oscillating mode enables the circuit to be used as a dc ballast, an inverter or converter or for an arc welder or other high frequency application.

In order that the invention is more readily under¬ stood, embodiments thereof will now be described with reference to the accompanying drawings. Description of the Drawings

Figure 1 depicts schematically a conventional silicon-controlled rectifier,

Figure 2 illustrates the programmable switching transistor circuit of the present invention in its simplest form,

Figure 3A illustrates one possible switching circuit configuration,

Figure 3B is a modified circuit of that illustrated in Figure 3a,

Figure 4 illustrates a simple oscillator circuit. Figure 5 illustrates another circuit configuration with two switching circuits of the present invention connected in parallel, Figure 6 illustrates another oscillator circuit similar to that of Figure 5,

Figure 7A illustrates a battery charger control circuit utilizing the switching circuit of the present invention, and Figure 7B is a modified circuit of that illustrated in Figure 7A. Description of the Preferred Embodiments

Referring to the drawings, in Figure 1 there is illustrated schematically the arrangement of an SCR which can be considered the equivalent of a pnp type transistor 12 connected to an npn type transistor 14. With no positive ^" trigger applied to the gate terminal G, both transistors are non-conducting. Upon application of a small positive trigger voltage to the base of the transistor 14 through the terminal G, the transistor 14 begins to conduct, and as the collector current ■ in transistor 14 is equal to current flowing to the base of transistor 12, that transistor 12 also begins to conduct. Both transistors will continue to conduct after termination of the trigger voltage until anode current is reduced to a value less than the holding current for the transistors used. As previously indicated, it is often desirable to turn off a SCR before the anode current has been reduced, and the present invention has been devised in order to facilitate this operation.

As shown in Figure 2, the simplest form of programmable switching transistor circuit of the invention is the schematic representation of the SCR with the addition of a zenner diode, or other voltage dependant switching device.

In Figure 2, the zenner diode is in the gate circuit, between the base of transistor 14 and the collector of transistor 12.

With this arrangement, the value of the zenner diode determines the switching voltage at which the transistor 14 switches OFF to provide a voltage controlled circuit which can be turned OFF while the current is above the holding current value.

In applications with reactive loads, such as motor control and the like, the voltage controlled switching circuit avoids difficulties which otherwise arise due to phase differences causing power surging, particularly at low power levels. Further, gate temperature compensa¬ tion, which is often necessary with conventional devices, is generally unnecessary.

Figure 3A illustrates one operative configuration of the invention where the circuit is used to control a load current, such as a motor controller. The transistors 12 and 14 are connected with the emitter of the transistor 12 connected to one side of the load 18 and the emitter of transistor 14 connected through the potentiometer 16 and resistor 17 to the other side of the load 18. The value of the zenner diode 15 and the gate voltage as determined by the. potentiometer control the ON and OFF switching voltages for the load. Alternatively, when used as a motor control to control the speed of a dc motor by chopping a full wave or half-wave rectified input, the motor speed is controlled by the potentiometer 16 which effectively varies the width of the wave form of the motor supply current. When the potentiometer is high, a substantially wide or full width wave form cause the motor to run at full speed. When the potentiometer is low, the width of the wave form is restricted thus reducing motor speed. Figure 3B illustrates a way whereby the control range may be further extended.

Referring to Figure 4, when the programmable switching circuit of the invention is utilized as an oscillator, a circuit configuration similar to that shown in Figure 3 is contemplated. In Figure 4, the transistors

12 and 14 are connected in the same way as in Figure 3 except that the emitter of transistor 14 is connected through the potentiometer 16 and resistor 17 to the emitter of transistor 12. In operation of the circuit of Figure 4, it will be seen that when the circuit is turned on, the voltage at point B drops below the zenner voltage so that both the transistors 12 and 14 are turned OFF. Supply voltage then appears through the load and, when sufficiently high, fires the zenner which turns ON the transistors 12 and

14. With both transistors conducting, and a voltage drop again occurring across the load, the voltage at point

B again drops below the zenner voltage at which point the transistors are both turned OFF. It will be seen that the zenner diode 15 turns off only when the voltage across transistor 14 is greater than the zenner voltage.

The resistor 17 may only be required in some circuit configurations to protect the base of the transistor 14 in case one or other of the transistors does not turn on quickly enough. However, in most applica¬ tions, this resistance may be omitted and the circuit operated only with the potentiometer.

In the oscillator configuration, the frequency can be set by any reactive component in the gate circuit. Such reactive component may be inductive or capacitive or a combination thereof, and including resistance components, the variation of which can be used to change or adjust the oscillation frequency.

If the programmable switching transistor circuit of the present invention is to be utilized in an ac circuit, a complimentary pair of switching transistors may be utilized or the circuit of the invention connected in a back-to-back or parallel configur§tion, or the circuit may be connected inside a bridge rectifier. In the latter case in which a circuit is located within a bridge rectifier, the arrangement approaches a basic TRIAC circuit. When a complimentary pairs circuit is similarly configured it does behave as a TRIAC.

Figure 5 illustrates a self-oscillating programm¬ able switching transistor circuit comprising parallel connected, reversed transistor pairs TR1 , TR2 and TR3 and TR4. Both transistors TR1 and TR4 are pnp transistors while the transistors TR2 and TR3 are both npn type transistors. Because the second pair of transistors TR3 and TR4 conduct in the reverse direction, the zenner diode 15a is connected in the reverse way as compared to tήe zenner diode 15. The potentiometers 31 and 32 are a two-ganged pot reversely connected. The resistors 33 and 34 provide protection for the respective transistors TR1 and TR4. The back-to-back configuration of Figure 5 operates in an ac circuit configuration. The diodes 21 and 22 are provided to prevent the transistors TR1 and TR3, respectively, firing both through the potentiometer as well as through the respective zenner diodes. This diode block can also be used to advantage in the other circuits illustrated. The circuit of Figure 5 can be used in any self-oscillating mode and is particularly useful for- an arc welder or other high frequency application. The circuit is particularly useful in an arc welder or a ballast for fluorescent lights to either substantially reduce or completely obviate the need for highly inductive loads.

When the circuit of Figure 5 is configured as shown in Figure 6, it can be used as a controllable switching circuit in an ac circuit, such as a motor control circuit. The circuit operates in a similar manner to that described for the circuit of Figure 3.

It will be appreciated that the load may be connected on either the anode or cathode side of the circuit of the invention in all cases.

The circuits may be driven by external oscillator or external pulse generator or may be run as a self-oscillating circuit and modulated by such external circuitry, such as an adjustable pulse-width modulator to give a variable power supply. Whatever is used to modulate the device, a square wave output will result. Thus, any modulation will be converted to a square wave output. The device may therefore be used to modulate any ac waveform.

Referring to Figure 7, there is illustrated a battery charger control circuit incorporating a programm¬ able switching transistor circuit of the present invention and utilizing two potentiometers 24 and 26. When a particular zenner diode 15 is selected the control of either of the potentiometers controls the on and off switching of the transistors. The potentiometer 24 .is used to adjust the high value of switching voltage and the potentiometer 26 adjusts the voltage differential, thus setting the low voltage level. The circuit operates as a differential switch to switch on and off between the two -chosen voltages and actuates a relay coil 28 which controls battery charging operation. Used this way, it is advantageous for TR2 to be of Darlington type. Preferably, a timer is incorporated into the circuit to prevent undesirable oscillation in the event of a battery failure. Thus, when the charger is switched ON at low battery voltage, and OFF at a higher voltage, a deteriorating battery may exhibit a voltage drop below the "turn on" voltage when load is switched in and rise above the "turn off" voltage as soon as the charger is witched ON thereby causing the charger to switch OFF again. The oscillation resulting may destroy other circuit components, such as a supply transformer or the like. A timer in the charger circuit will act to hold the charger in an ON state for a predetermined period after the battery voltage exceeds the "turn off" voltage, e.g. a period of five seconds, which interrupts a potential oscillation. Modification of the circuit of Figure 7A enables single pot variable power. P16 R15 act as variable voltage divider so that turn off is zenner volts below turn on.

It will be appreciated that the zenner diodes shown in the circuits described above may be replaced with any suitable voltage dependant switch, such as a diac, Schottky diode or the like.

The switching circuits of the present invention are particularly suitable for use in arc welders, including MIG and TIG welders, to reduce choke and transformer sizes. The circuits may also be used as ballasts in fluorescent light circuits, for inverters, converters, miniaturization of transformers, various switches and regulators including voltage dependant switches, current dependant switches and overload protectors, simple light dimmer circuits and motor control circuits, and the like. The circuits of the present invention may utilize SCR's, GCR's, valves, and field effect transistors (FET, MOSFET, etc.). Varia¬ tions will be self evident to the person skilled in the art. Another application for the circuit of the invention is as a chopped mode regulator, i.e., the device may be used as a chopped mode dc supply whereby the power supply in a circuit is adjustable. All the circuits indicated and described above can be made of opposite polarity by reversing the roles of the pnp and npn transistors together with reversal of the zenner diodes, Schottky diode- or other voltage dependant switch device.

Claims

Claims .

1. A programmable switching transistor circuit comprising at least two semi-conductor devices of opposite types interconnected with the gate terminal of a first device connected to an anode terminal of a second device and a cathode terminal of the said first device connected to a control terminal, and a voltage dependant switch means is connected between the control terminal and a base terminal of the second device whereby the circuit switches "ON" at or above a predetermined voltage and switches "OFF" below the predetermined voltage.

2. A circuit according to claim 1 wherein the voltage dependant switch means comprises a zenner diode, a Schottky diode or a diac.

3. A circuit according to claim 1 or claim 2 wherein the said first device is a pnp device and said second device is an npn device.

4. A circuit according to claim 1 or claim 2 wherein said first device is an npn device and the second device is a pnp device.

5. A circuit according to any one of the preceding claims wherein a reactive^' circuit component is connected in the base circuit of the second device, said reactive component determining the voltage at which the circuit is turned off.

6. A circuit according to any one of the preceding claims wherein a further pair of semi-conductor devices is connected together in a manner similar to the first and second devices, and the two pair are connected in back-to-back configuration.

7. A circuit according to claim 6 wherein the base of the ^"second device is connected to a potentiometer between the emitters of the first and second devices, and the base of a fourth device is similarly connected to a potentiometer between the emitters of the third and fourth devices.

8. A circuit according to any one of the preceding claims wherein a potentiometer connected to the emitter of the second device and to a load provides a control signal to the base terminal of the second device which switches the circuit ON when a voltage appears across the load, the load voltage then falling below the switch off voltage of the voltage dependant switch means, whereby the circuit oscillates at a frequency which is a function of the potentiometer setting.

9. A circuit substantially as hereinbefore described with reference to the accompanying drawings.