GB2257854A

GB2257854A - Drive circuits

Info

Publication number: GB2257854A
Application number: GB9115657A
Authority: GB
Inventors: Michael Lamoth
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1991-07-16
Filing date: 1991-07-16
Publication date: 1993-01-20
Anticipated expiration: 2011-07-16
Also published as: GB9115657D0; GB2257854B

Abstract

A drive circuit for an inductive load (L1) comprises: a driver transistor (M1); capacitance element (C1) and a feedback circuit (10) responsive to the output voltage and output current of the driver transistor for controlling the rate of charge and discharge of the capacitance element (C1) whereby to control the switching transfer function of the driver transistor, a switch element (C1) providing a fast discharge path for the capacitance element (C1), and a control circuit (12, 14, Q2 - Q4) for switch element (C1) which is responsive to the output current or voltage of driver transistor (M1) for providing at the capacitance element (C1) a voltage which simulates the control (gate) voltage of the driver transistor and for switching off switch element (C1) at a point at which the simulated control voltage corresponds to a predetermined operating region of the transistor, whereupon the feedback circuit (10) resumes control of the capacitance element (10). When transistor M1 is to be switched off, M3 is turned on to bias Q1 on via M2, Q4, Q3, Q2. The invention enables a reduction in the switch off time of transistor M1. The circuit may be used in automotive applications. <IMAGE>

Description

INDUCTIVE LOAD DRIVER CIRCUIT Field of the Invention This invention relates to a driver circuit for an inductive load, especially a solenoid coil for a relay in automotive applications.

Background of the Invention In certain automotive applications, for example producing control signals for gear changing in automotive transmissions, it is essential that very clean switching waveforms are produced with no transients giving rise to false switching conditions or rf radiation, which may interfere with other circuits in the automobile.

In our copending application (SC00160EG) we disclose and claim an arrangement for controlling not only rate of change of output voltage with time (dt) for a solenoid driver circuit, but in addition, in order to better suppress rf radiation, etc, to control rate of change of outPut current di with time (t) t thus copending application

discloses and claims a driver circuit for an inductive load comprising:: a driving means having a first terminal coupled to the inductive load, a second terminal and a control terminal, said driving means supplying a current signal to the inductive load; feedback means for controlling the current supplied to the inductive load, said feedback means comprising current sensing means coupled to said second terminal for sensing the current signal supplied to the inductive load and for providing a voltage signal at an output thereof representative of the sensed current signal and a capacitor coupled to the output of the current servicing means and the control terminal of the driving means said capacitor in response to the voltage at the output of the current sensing means controlling the voltage on the control terminal, said driver circuit characterised in that said feedback circuit further comprises voltage combining means coupled to the output of the current sensing means and the second terminal, for combining a voltage sign representative of the voltage signal at the second terminal with the voltage signal at the output of the current sensing means, whereby said feedback means controls the current signal and voltage signal supplied to the inductive load.

However, although such arrangement controls in a more di effective manner than heretofore dtt there remains problems in ensuring that the driver transistor is switched off as quickly as possible.

Summary of the Invention It is an object of the invention to provide an arrangement for fast switching off of a driver transistor but which does not produce switching transients.

The present invention provides a drive circuit for an inductive load (L1) comprising: a driver transistor (M1); capacitance means (C1) and a feedback circuit responsive to the output voltage and output current of the driver transistor for controlling the rate of charge and discharge of the capacitance means (C1) whereby to control the switching transfer function of the driver transistor, characterised by a switch means (Q1) providing a fast discharge path for the capacitance means (C1), and control means (12, 14, Q2 -Q4) for switch means (Q1) which is responsive to the output current or voltage of driver transistor (M1) for providing at the capacitance means (C1) a voltage which simulates the control (gate) voltage of the driver transistor and for switching off switch means (Q1) at a point at which the simulated control voltage corresponds to a predetermining operating region of the transistor, whereupon the feedback circuit resumes control of the capacitance means.

Brief Description of the Drawinas A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein: Figure 1 is a circuit diagram of a known arrangement for controlling rate of change of voltage and rate of change of current in a driver transistor arrangement for an inductive coil; Figure 2 is an arrangement for controlling rate of change of voltage and rate of change of current in a driver transistor for an inductive coil according to the invention described and claimed in our copending application

Figure 3 is a circuit diagram of an arrangement for controlling the switching "offg' characteristics for a driver transistor for an inductive load in accordance with the invention; Figure 4 are graphs indicating switching characteristics for the circuit of Figure 3; and Figure 5 is a graph indicating the relation between gate voltage of the driver transistor and the output voltage of the preceding buffer stage.

Description of the Preferred Embodiment Referring now to Figure 1, this discloses a known arrangement for controlling the switching transfer function of a driver transistor M1 for an inductive load L1, the arrangement comprising a sense resistor Rs in the main current path of the driver transistor M1. The voltage developed across resistor R5 is amplified in amplifier Al and feedback via capacitor C2 to the gate of driver transistor M1. This provides an arrangement for controlling the switching of the transistor in dependence on the rate of change of current flow in the transistor.In addition a feedback resistor R1 and a feedback capacitor C1 coupled to the output voltage node of driver transistor M1 provide a feedback circuit for controlling the switching characteristics of transistor M1 independence on the rate of change of output voltage.

Referring now to Figure 2, this shows an alternative arrangement for controlling the switching characteristics of transistor M1 which is the subject of our copending application

Essentially, this arrangement provides for controlling the switching characteristics of the transistor in dependence on the rate of change of current and rate of change of output voltage of the transistor but wherein a single feedback capacitor C1 is provided, feedback signals from the output voltage node and the current sense resistor Rs being summed in a summing arrangement R4, R5, B1 and B2.

Referring now to Figure 3, this shows an arrangement for decreasing the discharge time of capacitor C1 whereby to reduce the switch "off" time of driver transistor M1. In Figure 3 similar parts to those of Figure 2 are indicated by the same reference numeral. A feedback circuit 10 corresponds to the part of the circuit indicated in Figure 2 by the same reference numeral. The voltage developed across sense resistor R5 is converted to a current value in a voltage current converter 12 and this current is supplied via a current mirror arrangement 14 to a circuit comprising a transistor Q4 configured as a load, a transistor M2 configured as a load and which has similar matching electrical characteristics to that of transistor M1 and a switching FET transistor M3 which is controlled by a gate voltage signal V2. The current developed through Q4 controls the base of a further transistor Q3 which is coupled via a load transistor Q2 and a current source I1 to the base of transistor Q1.

It will be appreciated that transistor M1 is a known type of TMOS and denoted by the Motorola part number MTP30NO8M. Referring to Figure 4, the lower graph denotes the desired switching "off" time of transistor M1. The upper graph shows the waveforms of input signal V2 applied to transistor M3 the gate voltage Vg at the gate of transistor M1 and the output voltage Q of transistor M1. When transistor M1 is "on" as shown in the time period 80 to 100 microseconds, Vg has a value of around 8 volts.When the TMOS transistor M1 is to be switched "off", transistor M3 is turned "on" whereby to bias "on" transistor Q1 with a voltage appearing at its emitter and hence at one node of capacitor C1 having a value which simulates the gate voltage of transistor M1. This is because transistor M2 has similar electrical characteristics to transistor M1 and its gate voltage is transmitted through the transistors Q4, Q3, Q2, Q1 to the node of capacitors C1. When transistor M3 is switched "on", this turns the transistor Q1 into a conductive state to permit capacitor C1 to discharge at a fast rate through transistor Q1. This is shown in Figure 4 and it will be seen that the gate voltage Vg drops to a threshold value Vga of about 2.5 volts within a short period of time, less than 5 microseconds. At this threshold gate voltage the TMOS transistor M1 reaches its active region (also called ohmic or non-saturation region). At this point, transistors Q1 is switched "off" and the gate voltage of transistor M1 is controlled by feedback circuit 10 via the buffer circuit.

Referring to Figure 5 this shows the voltage output value of the buffer amplifier and the voltage at the gate of the TMOS transistor M1 has a function of the voltage and current through a transistor M1.

Claims

1. A drive circuit for an inductive load (L1) comprising: a driver transistor (M1); capacitance means (C1) and a feedback circuit (10) responsive to the output voltage and output current of the driver transistor for controlling the rate of charge and discharge of the capacitance means (C1) whereby to control the switching transfer function of the driver transistor, characterised bv a switch means (Q1) providing a fast discharge path for the capacitance means (C1), and control means (12, 14, Q2 -Q4) for switch means (Q1) which is responsive to the output current or voltage of driver transistor (M1) for providing at the capacitance means (C1) a voltage which simulates the control (gate) voltage of the driver transistor and for switching off switch means (Q1) at a point at which the simulated control voltage corresponds to a predetermining operating region of the transistor, whereupon the feedback circuit (10) resumes control of the capacitance means (10).

2. A circuit as claimed in claim 1 characterised in that the control means for switch means (Q1) comprises a resistor (Rs) for sensing the output current of transistor (M1), a circuit (12) for connecting the voltage across resistor (Rs) to a current value, and providing this current value to a circuit (M3, M2; Qq) which includes a matching transistor (M2) having similar electrical characteristics to driver transistor (Q1) whereby to generate said simulated voltage corresponding to the gate control voltage of transistor (M1).

3. A circuit as claimed in claim 2 characterised by a current mirror circuit (14) for providing said current value to matching transistor (M2).

4. A circuit as claimed in claim 2 or 3 characterised by a transistor (Q3) responsive to the simulated voltage for a providing a control signal to switch means (Q1)

5. A circuit claimed in claim 4 characterised in that switch means (Q1) comprises a bipolar transistor whose base is coupled to a base for matching transistor (Q2) configured as a bias circuit including the main current path of transistor (Q3) and current source (I1).

6. A circuit as claimed in any preceding claim wherein said transistor (M1) has a main current path and a subsidiary current path in which a current sense resistor (Rs) is located.

7. A circuit as claimed in any preceding claim including a driver circuit as claimed in any preceding claim of copending application