EP0141828A1

EP0141828A1 - Control circuit for a motor

Info

Publication number: EP0141828A1
Application number: EP84901596A
Authority: EP
Inventors: Perran Vincent Leonard Newman
Original assignee: Prutec Ltd
Current assignee: Prutec Ltd
Priority date: 1983-03-31
Filing date: 1984-04-02
Publication date: 1985-05-22
Also published as: WO1984004007A1; GB2137834A

Abstract

Un circuit de commande à mode de commutation pour un moteur comprend des transistors de commutation du type de même conductivité disposés dans les quatre branches d'un circuit de pont qui est connecté au moteur et à une alimentation de tension. Des circuits d'attaque appliquant un courant de base aux électrodes de base des transistors de commutation comprennent chacun une source de courant constant (12) connectée à la base du transistor de commutation (14) et un transistor de by-pass (16) connecté pour être mis sous tension lorsqu'il y a interruption du courant passant par le transistor de commutation (14).A switching mode control circuit for a motor includes switching transistors of the same conductivity type arranged in the four branches of a bridge circuit which is connected to the motor and to a voltage supply. Driver circuits applying base current to the base electrodes of the switching transistors each include a constant current source (12) connected to the base of the switching transistor (14) and a bypass transistor (16) connected to be energized when the current flowing through the switching transistor (14) is interrupted.

Description

CONTROL CIRCUIT FOR A MOTOR^*

The present invention relates to a control circuit for controlling the current applied to a motor.

It is known to operate a motor in a so-called switch mode in order to determine the current flowing through the motor windings accurately at all times. Because of the inductive nature of the electrical load presented by the motor, the application of a voltage does not immediately result in the desired current flowing through the motor windings and instead the current builds up gradually to the desired level. In order to achieve rapid^' control, a higher voltage than necessary merely to sustain the desired current is applied -to the windings, with the result that the current rises more rapidly and tends to overshoot the desired current, upon exceeding the desired current, the applied voltage is. disconnected and the current in the windings is allowed to decay through free- heeling diodes. The current then αrops below the desired value and the voltage is again reconnected. This cycle is continuously repeated to maintain the current fluctuating about its desired value.

In the case of a motor which needs to be driven in both directions, it is known to connect the motor across the arms of a bridge of which each arm contains a switching transistor and a parallel connected free-wheeling diode. In this way, without requiring three voltage rails, the current through the motor windings may be reversed to cause movement in either direction.

The transistors used in the arms of the bridge are usually bipolar PN transistors which are capable of withstanding the substantial voltage applied to the motor windings. The present invention seeks to provide a circuit for driving such NPN transistors in the bridge circuit, which is tolerant to variations in the supply voltage and which consumes relatively little power.

According to the present invention, there is provided a switch mode control circuit for a motor comprising switching transistors of the same conductivity type arranged in the four arms of a bridge circuit which is connected to the motor and to a voltage supply and drive circuits for applying base current to the base electrodes of the switching transistors, the drive circuit for each switching transistor comprising a constant current source connected to the base of the switching transistor and a by-pass transistor -connected to be switched on when the current through the switching transistor is to be discontinued.

Preferably, the constant current source is a switch mode operated circuit comprising a power supply switch, an indutor connected in series with the base emitter path of the switching transistor and means for opening the power supply switch when the current through the inductor exceeds a predetermined value.

The power 'supply switch is conveniently a transistor.

Advantageously, the by-pass transistor is connected to the output of an optical coupler operative to receive signals for varying the current through the motor.

In the construction in accordance with the preferred embodiment, the drive current is insensitive to variations in the supply voltage over a wide range and because a constant current source driving a short circuit consumes substantially no power, the efficiency of the drive circuit is high, the circuit taking from the power supply only the base current necessary for the operation of the switching transistor.

The invention will now be decribed further, by way of example, with reference to the accompanying drawings, in which :

Figure 1 is a block diagram of one arm of a 5 circuit in accordance with the invention, and

Figure 2 is a detailed circuit diagram of circuit shown in Figure 1_

In Figure 1, a rectifier and filter 10 is connected to a low voltage AC supply such as the output winding of a

10 mains transformer. The rectified output voltage is applied by way of a constant current source 12 operating in switch mode to the base of the power switching

^'transistor 14. The output of the constant current source

12 is also applied to a by-pass transistor 16 which is

15 connected to a line 18 whicn has a slight negative bias of one volt and which is also derived from the rectifier 10. The by-pass transistor is switched on and off by means of an optical isolation circuit 20 which is in turn operated by a control signal for varying the power

20 .appliea to the motor windings.

The power switching transistor 14 is one of four arranged in a bridge circuit. A DC power supply for the motor is connected across one diagonal of the bridge ana the motor is connected across the other diagonal. It is

25 desired to control the current flowing through the motor such that the mean current can rapidly be changed and reversed if necessary. This is achieved by switch mode operation, as earlier described, in which the power switching transistors 14 are rapiαly switched on and off

30 in two of the arms of the bridge, depending on the desired direction of the current, the ratio of on to off time of the transistors determining the mean current flowing through the windings. The signal which determines whether the transistor 14 is to be switched^' on or off is the control signal applied to the optical isolation circuit 20.

The transistors in the bridge are bipolar and with present technology bipolar transistors capable of carrying the motor current are only available in NPN type. Consequently, the emitters of at least two of the transistors are connected to the motor winding and are at a floating potential. The control signal, on the other hand fluctuates relative to a fixed potential. The optical isolator 20 isolates the DC level of the control signal from that of the switching circuitry.

If the by-pass transistor 16 is switched off, the full current output of the constant current source 12 flows through the base emitter path of the s itching transistor 14 to turn the transistor 14 fully on. Because the source is of constant current, the current in the base-emitter path is substantially independent of the voltage at the base of the switching transistor • and of any variation of the input voltage. Thus, the collector current can be determined with accuracy while allowing considerable tolerance in most of the drive circuitry.

When it is desired to switch off the transistor 14, the by-pass transistor 16 is switched on and its ^* impedance is considerably lower than that of the base-emitter path of the switching transistor 14. Thus, substantially all of the output current of the constant current source 12. is diverted through this by-pass transistor 16. The by¬ passing is assisted by the fact that the by-pass transistor is connected to a line which is slightly negative with respect to the emitter of the switching transistor 14. Consequently,- substantially no current goes through the power switching transistor 14 which is thereby switched off. Under these circumstances, the power taken from the voltage supply is negligible since the constant current is flowing through an impedence of 5 nearly zero and therefore consuming no power.

In Figure 2, there is shown a more detailed diagram of the circuit in Figure 1. To assist in understanding the circuit, the active components in Figure 2 have been allocated the same reference numerals as the blocks in Figure 1 of which they form part.

The stepped down mains voltage is rectified by the rectifier 10 which is connected to a first smoothing capacitor 40. A diode 44 is connected in series with a resistor 42 across the smooth DC voltage so^' that a one 0 volt drop exists across the diode 44, this voltage being smoothed by a capacitor 46. This establishes a positive line 50 and two negative lines 52 and 54 with the potential of the line 54 one volt below that of the line 52. The positive voltage on line 50 is applied to the 5 emitter of a transistor 12a which in conjunction with the transistors 12b, 12c and 12 d constitutes the constant current source. The output current of the constant current source is taken through power resistors 56a and 56b connected in parallel. The potential drop U across the resistors 56 is divided y a pair of resistors and .applied to the base of the transistor 12d. This in turn operates the transistors 12b and 12c which control the base current through the transistor 12a.

When the current output is below a threshold level the 5 transistor 12a is conductive but when the current output rises, the transistor 12b is switched off to prevent the flow of emitter-base current through the transistor 12a which in turn switches off. This switch mode on/off operation occurs cyclically to maintain the current output at a fixed level determined by. the voltage divider connected to the base of the transistor 12d. If the transistor 16 is switched off, then the full current output of the constant current source 12 flows through the base-emitter junction o-f the power transistor 14 which then allows current to flow through the output terminals to the motor winding.

When it is desired to discontinue the current flowing through the transistor 14, the optical isolator 20 receives a control signal which switches on transistor 16c. This in turn switches on the transistor 16b and transistor 16a. The transistor 16a now presents a lower impedence for the constant current than the emitter-base path of the transistor 14 and substantially all of the current output of the constant current source 12 is diverted through the transistor 16a to switch off the power consumed by the motor winding.

All the circuit elements of the preferred embodiment of the invention are shown in Figure 2 for completeness but it is believed that the functions served by the elements not described in detail above will be clear to a person skilled in the art without further explanation. .

-^^T^*

O PI

Claims

1. A switch mode control circuit for a motor comprising switching transistors of the same conductivity type arranged in the four arms of a bridge circuit which is connected to the motor and to a voltage supply and drive circuits for applying base current to the base electrodes of the switching transistors, characterised in that the drive circuit for each switching transistor (14) comprises a constant current source (12) connected to the base of the switching transistor (14) and a by-pass transistor (16) connected to be switched on when the current through the switching transistor (14) is to be ^' discontinued.

2. A control circuit as claimed in claim 1, wherein the constant current source (12) is a switch mode operated circuit comprising a power supply switch (12a),. an inductor connected in series with the base emitter path of the switching transistor (14) and means for opening the power supply switch (I2a) when the current through the inductor exceeds a predetermined value.

3. A control circuit as claimed in claim 2, wherein the power supply switch is a transistor (12a).

4. A control circuit as claimed in any preceding claim, wherein the by-pass transistor (16) is connected to the output of an optical isolator (20) operative to receive signals for varying the current through the motor.

/ fc^: