GB2032719A

GB2032719A - An electronic switch and method of operating it

Info

Publication number: GB2032719A
Application number: GB7932386A
Authority: GB
Original assignee: Ward and Goldstone Ltd
Current assignee: Volex PLC
Priority date: 1978-09-21
Filing date: 1979-09-18
Publication date: 1980-05-08

Abstract

A power FET is used as a voltage- controlled variable impedance to control the flow of current through a load. Inductive voltage surges which would otherwise occur when switching on an inductive load or filament lamp are avoided by including circuitry having a defined time constant to the gate, whereby the rate of change of output current in response to a step change in input voltage is limited to a safe value. The gate may be supplied with different voltages (not shown), to supply the load with different currents, for example to operate a filament lamp at different luminous intensities. <IMAGE>

Description

SPECIFICATION An electronic switch and method of operating it This invention relates to the switching of electrical loads in a vehicle. Hitherto electrical loads have been switched by mechanical means either electrically or manually operated, however, with the advent of electronic control of switching functions it is appropriate that electronic means using solid state devices be used. There are three main problems associated with the switching of loads. Firstly supplying power to an incandescent lamp such as a tungsten filament bulb causes problems because of the very low electrical resistance of the filament when at ambient temperature, because a very high surge current flows when connection is made to a voltage source.Secondly large back emfs may be produced when breaking the current in inductive circuits, and the back emf is a source of interference to electronics installed in the vehicle. Thirdly there is the problem of providing protection of the switch and circuitry against possible short circuit loads; hitherto this has been achieved by the use of fuses.

In the particular case of vehicle lamps, in the non-illuminating or off state, the lamp filament bulbs, and more particularly the filaments themselves are at ambient temperature.

Consequently, due to the resistance/temperature characteristic of the material of the filament on connection to the power supply, the initial resistance presented by the filament is very low resulting in a high surge of current.

The time taken for the filament to reach its full operating temperature depends on the value of the surge current which may be limited by the output resistance of the supply.

Typically for a 1 2V, 22W filament bulb the resistance at ambient temperature is 0.6S2 and a surge current of 20A would be expected from a 1 2V supply. The output resistance resulting from wires, connections and contacts can typically be 2 or thereabouts, and this results in a time delay in excess of 30 ms before full illumination is achieved due to the thermal capacity of the filament. The surge current resulting from the low ambient resistance of the filament in a given bulb will cause a large drop in voltage which is a very serious source of electrical noise and would be detrimental to the operation of electronics sharing a power supply with the bulb.Where two levels of illumination are required for different functions such as rear lights and stop lights, it is common practice to house two separately fed filaments, one rated at 6W and the other at 22W in one bulb.

According to one aspect of the present invention, there is provided, a circuit comprising a solid state power switch and a load, the solid state power switch having a smoothly varying input voltage/output current relationship characterised by impedance means connected to the input of the power switch for selectively varying the input voltage in order to vary the output current.

According to another aspect of the present invention, there is provided a circuit comprising a solid state power switch and an inductive load, the solid state power switch having a smoothly varying input voltage output current relationship characterised in that two impedances respectively having time constants to limit the rate of rise of the input voltage and to limit the rate of fail of the input voltage to the switch are connected to the input to the switch whereby to control the rate of rise and rate of fall of the output current to limit the magnitude of back emfs produced by changing current in the inductive load.

According to a further aspect of the present invention, there is provided a method of operating a filament lamp in which the lamp is supplied with a current just below a threshold value necessary to cause emission of light whereby, when the lamp is supplied with a current sufficient to cause emission of light, the current surge which would otherwise be produced on switching on the lamp is reduced.

A preferred embodiment of the invention may comprise any one or more of the following advantageous features: (a) The solid state power switch is a vertical-metal oxide-silicon (VMOS) power device.

(b) The VMOS device of (a) is p-channel.

(c) The input voltage/output current relationship is substantially linear.

(d) The load is a filament of a bulb.

(e) The bulb is the tail lamp bulb of a vehicle.

The invention also comprises a vehicle having a load such as a lamp, for example, controlled by a switching circuit as defined above.

In order that the invention may be more clearly understood, two embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a circuit comprising a pchannel vertical metal oxide semiconductor, Figure 2 shows the operating characteristic of a vertical metal oxide semiconductor of the type forming part of the circuit of Fig. 1, Figure 3 shows a circuit diagram of a power supply circuit for a rear/stop lamp for a vehicle, and Figure 4 shows the input voltage/drain current relationship for a power switch forming part of the circuit of Fig. 3.

Referring to Fig. 1, a load 1 is fed from a supply through terminals 3 and 7. This load is switched on and off by means of a switch 4 comprising a p-channel vertical metal oxide semiconductor device which has an operating characteristic as shown in Fig. 2. Such devices are complementary versions of n-channel VMOS devices produced, for example, by Siliconex Incorporated and described in their VMOS Power Fets Design Catalog.

The switch 4 is a current generator having a gate/source threshold turn on voltage of VT and a subsequent substantially linear relationship between the gate/source voltage (VGs, plotted on Fig. 2 as abscissa) and the drain current (ID plotted on Fig. 2 as ordinate). The gate electrode G of the switch 4 is connected through a parallel connection of a resistor R2 and diode D, in series with a further resistor R, to the terminal 2, the source electrode S is connected to the positive rail having a voltage VR and the drain electrode D is connected to earth via a diode D2 which is connected in parallel with the load. A capacitor C is connected between the- gate of the VMOS device and ground.

The current through the load 1 is determined by VGs according to the characteristic in Fig. 2, unless the device is saturated in which case the current in the load is determined by (V,-VDs sAT)/RL where VDS SAT is the saturation voltage between source S and drain D. The rate of rise of the output current is determined by the time constant R,C as the input resistance of the V.M.O.S. device 4 is almost infinite, whilst the rate of fall of the current is determined by the time constant (R2 + R,)C because diode D, is reverse biassed during turn off when the input voltage rises. The diode D2 connected across the load is to minimise further the back emf arising from reducing the current in the inductive loads.If the input voltage across terminals 2 and 3 rises, so that VGS lies within the threshold voltage, VT of the top rail voltage Vr then no current flows and the switch 4 is in the off state.

The above described arrangement provides a solid state switch which is capable of supplying current to incandescent filament bulbs, which is capable of turning off inductive loads without ecessive back emfs being generated, and which will provide protection against the advent of a short circuit load.

Fig. 3 shows a circuit specifically for a vehicle tail lamp. The circuit basically comprises input terminal means 31, switch means 32, a filament bulb 33 and resistor means 34. The input terminal means 31 consists of three separate inputs In1, ln2, ln3 any pf which may be activated by grounding the appropriate terminal. When not activated, a given input is at + 1 2V. The switch means 32 consists of a p-channel vertical-metal-oxide-silicon (VMOS) power device having a characteristic as shown in Fig. 4. This characteristic is the same as that shown in Fig. 2, but specific values relating to this particular embodiment have been shown on it. The device is complementary to the n-channel VMOS devices presently manufactured by Siliconex Ltd.As shown in Fig. 4 (and also in Fig. 2) electrical characteristics of the power switch are such that the output current is dependent upon the input voltage in an approximately linear manner, such that the current in the filament of the bulb 33 is determined either by the magnitude of the input voltage or by the resistance of the filament, whichever produces the least current. The drain current from the switch 32 is fed via the filament bulb 33 to ground. The input voltage V:n is defined as the gate voltage relative to the source voltage, and is determined by the resistor means 34 and the voltages present at the input means 31. The resistor means 34 comprises three resistors 3R1, 3R2 and 3R3. In Fig. 3 gate, source and drain electrodes of the switch 32 are respectively referenced G, S and D as in Fig. 1.

When the input ln3 is activated by grounding that particular input the resistors 3R, and 3R3 of the resistor 34 determine a value of the input voltage Vi,, of Vjn3, and a corresponding current 13 in the filament is produced as shown in Fig. 4. 13 is the maximum threshold current above which the filament of the bulb 33 begins to emit light.

When input ln2 is activated, the resistors 3R, and 3R2 of the resistor means 34 determine an input voltage Vjn2 which results in a current 12, producing a low level of illumination appropriate to a rear side light; see Fig. 4.

When input Inl is activated, the full 12V appears at the input Van1, which can generate a surge current I,. However, as the filament temperature rises and becomes more resistive the switch 32 saturates and the current settles to a value 14 which is determined by the resistance of the filament; see Fig. 4. 14 is appropriate to the maximum illumination from the filament and serves typically as a stop light for the vehicle.

The above described arrangement enables: (i) the time delay in achieving full illumination to be substantially reduced.

(ii) the current surge on switch on and the electrical noise associated with it to be substantially reduced.

(iii) one or more levels of light output from a given filament to be achieved. - It will be appreciated that the above described arrangement has been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims

1. A circuit comprising a solid state power switch and a load, the solid state power switch having a smoothly varying input voltage/output current relationship in which im pedance means are connected to the input of the power switch for selectively varying the input voltage in order to vary the output current.

2. A circuit comprising a solid state power switch and an inductive load, the solid state power switch having a smoothly varying input voltage output current relationship in which two impedances respectively having time constants to limit the rate of rise of the input voltage and to limit the rate of fall of the input voltage to the switch are connected to the input to the switch whereby to control the rate of rise and rate of fall of the output current to limit the magnitude of back emfs produced by changing current in the inductive load.

3. A circuit as claimed in Claim 1 or 2, in which the solid state power switch is a vertical metal oxide-silicon (VMOS) power device.

4. A circuit as claimed in Claim 3, in which the VMOS device is p-channel.

5. A circuit as claimed in any preceding claim, in which the input voltage/output current relationship is substantially linear.

6. A circuit as claimed in any preceding claim, in which the load is a filament of a bulb.

7. A circuit as claimed in Claim 6, in which the bulb is the tail brake lamp of a vehicle.

8. A vehicle incorporating a circuit as claimed in any preceding claim.

9. A method of operating a filament lamp in which the lamp is supplied with a current just below a threshold value necessary to cause emission of light whereby, when the lamp is supplied with a current sufficient to cause emission of light, the current surge which would otherwise be produced on switching on the lamp is reduced.

10. A circuit substantially as hereinbefore described with reference to Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.

11. A method of operating a filament lamp substantially as hereinbefore described with reference to Figs. 1 and 2 or Figs. 3 and 4 of the accompanying drawings.