GB2137834A - Control circuit for a motor - Google Patents

Abstract

A switch mode control circuit for a motor comprises 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. Drive circuits for applying base current to the base electrodes of the switching transistors each comprise 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.

Description

SPECIFICATION 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 freewheeling diodes. The current then drops 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 NPN 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 inductor 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 described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of one arm of a 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 mains transformer. The rectified output voltage is applied by way of a constant current source 1 2 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 1 6 which is connected to a line 1 8 which has a slight negative bias of one volt and which is also derived from the rectifier 1 0. 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 applied 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 and the motor is connected across the other diagonal. It is 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 rapidly switched on and off 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 1 6 is switched off, the full current output of the constant current source 1 2 flows through the base emitter path of the switching transistor 1 4 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 1 6 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 1 6. 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 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 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 emitter of a transistor 1 2a which in conjunction with the transistors 1 2b, 1 2c and 1 2d 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 across the resistors 56 is divided by a pair of resistors and applied to the base of the transistor 1 2d. This in turn operates the transistors 1 2b and 1 2c which control the base current through the transistor 12a.

When the current output is below a threshold level the transistor 1 2a is conductive but when the current output rises, the transistor 1 2b is switched off to prevent the flow of emitter-base current through the transistor 1 2a 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 1 2d.

If the transistor 1 6 is switched off, then the full current output of the constant current source 12 flows through the base-emitted junction of 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 1 6b and transistor 16a. The transistor 1 6a now presents a lower impedance 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 1 6a 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.

Claims (5)

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, 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.

2. A control circuit as claimed in claim 1, wherein the constant current source is a switch mode operated circuit comprising a power supply switch, an inductor 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.

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

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

5. A control circuit substantially as herein described with reference to and as illustrated in the accompanying drawings.