GB2217938A - Current sensing circuit - Google Patents

Abstract

A current-sensing circuit senses an output current (IA) of a power semiconductor device (10) which device has a major electrode (14) for carrying the output current (IA) and a minor electrode (16) for carrying a sensing current (IB). The circuit comprises means for impressing on the minor electrode (16) a voltage equal to that on the major electrode (14) and means for monitoring the total current (IB) flowing through the minor electrode. The voltage impressing means has a first input (26) coupled via a first impedance (Z1) to the major electrode (14) and a second input (34) coupled via a second impedance (Z2) to the minor electrode (16), the first and second impedances being matched. The circuit comprises means (N3, N4) for maintaining equal currents (I1, I2) in the first and second impedances so that the differences between the voltages at the first and second imputs (26, 34) is substantially equal to the difference between the voltages (VA, VB) at the major and minor electrodes, while the absolute values of those voltages are translated to levels suitable for the first and second inputs of the voltage impressing means. The circuit is suitable for inclusion in an intelligent power switch integrated circuit. Output HC could be used to actuate an automatic shut down or external warning. Output LC could be used for detecting filament breakage in a loop load 6. <IMAGE>

Description

Description CURRENT SENSING CIRCUIT The invention relates to a current-sensing circuit for sensing an output current of a power semiconductor device which device has a major electrode for carrying the output current and a minor electrode for carrying a sensing current, the circuit comprising means for impressing on the minor electrode a voltage equal to that on the major electrode and means for monitoring the current flowing through the minor electrode.

Such a power semiconductor device is disclosed in EP-A1-O 139 998. The device contains a major current-carrying section which includes a large number of basic semiconductor eLements, and a minor current-carrying section which includes a much smaller number of such elements, or only one basic element.

The basic semiconductor elements may comprise diodes, bipolar and/or field-effect transistors or thyristors. Alternatively, each section may contain only one basic semiconductor element, each of which however has a different construction and/or geometry. The two sections will have common first main electrodes but separate second main electrodes which form the major and minor electrodes of the device described.

A current-sensing circuit as set forth in the opening paragraph is disclosed in EP-A1-O 227 149 (PHN11587), wherein the voltage impressing means comprises a differential amplifier or comparator which has an output stage suitable for controlling the current flowing in the minor electrode. The inputs of the differential amplifier are connected to the major and minor electrodes so that by controlling the current flowing through the minor electrode the voltage on the minor electrode is kept equal to that on the major electrode by negative feedback. This ensures that the current in the minor current-carrying section is an accurately proportional representation of the current flowing in the major current-carrying section.A particular differential amplifier circuit and a particular current sensing circuit as set forth in the opening paragraph including such an amplifier are disclosed in copending UK Patent Application number 8713387 (PHB33365).

When the power semiconductor device is used as a "high-side" switching device (for example in common-collector configuration in the case of a bipolar power transistor or in common-drain configuration in the case of an MOS power transistor) then the voltages at the major and minor electrodes may be too close to the supply voltage to be applied directly to the inputs of a differential amplifier. In the circuit disclosed in UK patent application No. 8713387 (PHB33365), a pair of potential dividers is used to drop the voltages to a lower level. Unfortunately, the accuracy of this technique is impaired by the difficulty of matching the ratios of the two dividers and the fact that offset errors in the ampLifier caused by input transistor mismatch for example are magnified by the scaling action of the dividers.

A further source of inaccuracy in the known circuit is that the small, relatively constant currents drawn from the major and minor electrodes to operate the potential dividers cause an error in the measured ratio between the output current and the sensing current, particularly when the sensing current is small.

It is an object of the present invention to enable the provision of a current sensing circuit as described in which some or all of the above disadvantages of the known circuit are mitigated.

The invention provides a current-sensing circuit for sensing an output current of a power semiconductor device which device has a major electrode for carrying the output current and a minor electrode for carrying a sensing current, the circuit comprising means for impressing on the minor electrode a voltage equal to that on the major electrode and means for monitoring the total current flowing through the minor electrode, wherein the voltage impressing means has a first input coupled via a first impedance to the major electrode and a second input coupled via a second impedance to the minor electrode, the first and second impedances being matched, the circuit comprising means for maintaining equal currents in the first and second impedances so that the difference between the voltages at the first and second inputs is substantially equal to the difference between the voltages at the major and minor electrodes, while the absolute values of those voltages are translated to values suitable for the first and second inputs of the voltage impressing means. Because the two voltages being compared are translated to the suitable values without attenuation, offset errors in the differential amplifier are not scaled up when fed back to the electrodes of the power device. Therefore the resultant error between the voltages at those electrodes is less than that produced by the known circuit.

The voltage impressing means may include means for dividing the total current flowing through the minor electrode so that respective predetermined fractions of the total current flow through the second impedance and one or more inputs of the monitoring means. Because the current taken for biasing the voltage translating impedances is a known fraction of the total current flowing through the minor electrode, it can be accounted for in the scaling fraction which relates the current(s) received by the monitoring means to the actual output current flowing in the major electrode. This enables a further improvement in accuracy over the known circuit to be obtained.

The dividing means may be combined with an output stage of the voltage impressing means. Combining functions reduces the number of components and thus releases chip-area for other functions, which is particularly valuable when the circuit integrated together with a large power device. Combining functions also tends to reduce the accumulation of errors in signals which are passed between the electrodes of the power device, the voltage impressing means and the monitoring means.

The combined dividing means and output stage may comprise at least two similar transistors having first main electrodes arranged for connection, together with the second matched impedance, to the minor electrode of the power semiconductor device and having control electrodes connected together and driven by a signal representative of the difference between the voltages at the inputs of the voltage impressing means, wherein a second main electrode of at least one of the similar--transistors is connected to an input of the monitoring means and a second main electrode of a further one of the similar transistors is connected to the input of a current mirror circuit, the current mirror circuit having a first output connected to the first matched impedance and a second output connected to the second matched impedance.The term "similar transistors" refers to transistors (of any particular type) fabricated together so that under identical bias conditions the currents in the devices are identical or are related by a fixed ratio defined by the relative geometries of the transistors. It is known that current can be divided in predetermined proportions between current paths comprising transistors which are all similar.

This is a variation of the well-known current-mirror principle. In this embodiment of a current sensing circuit, however, the action of the further one of the similar transistors together with the current mirror circuit ensures that the current in the matched impedances is equal to a known fraction of the total current flowing through the minor electrode, even though the second matched impedance is not similar to the similar transistors. This has advantages when the nature of the first and second matched impedances is dictated by the function they have to perform in enabling the other impedances in the current sensing circuit to be tailored to their function, for example to be fabricated as lower voltage devices.

The monitoring means may comprise means for comparing a predetermined fraction of the total current flowing through the minor electrode with a reference current and for indicating which current is the greater. Since the predetermined fraction of the total current flowing through the minor electrode is a scaled-down replica of the output current flowing through the major electrode of the power device, a threshold value of the output current can be detected by comparison with a reference current which is equal to the threshold value scaled down by the same amount.

The monitoring means may, for example, indicate that a maximum safe operating current for the power device has been exceeded.

Alternatively, the monitoring means indicates that a minimum output current has not been reached, this may indicate an open-circuit fault in a load connected to the major electrode, for example a blown lamp or fuse.

The first and second matched impedances may comprise rectifying means so that the first impedance serves to protect the first input of the voltage impressing means against damage by voltages applied by an external circuit to the major electrode of the power semiconductor device when that device is turned off.

In some applications the external connections of the circuit may receive voltages outside the range that can safely be applied to the components of the voltage impressing means. For example, the electrical supply of a motor vehicle is generally subject to voltage transients far in excess of the nominal battery voltage.

If the current sensing circuit is part of a so-called intelligent power switch integrated circuit for use in a motor vehicle it is desirable that it is protected against all conditions which will forseeably be imposed upon the terminals. Forming the first and second matched impedances as rectifying means provides some protection against transients and extreme values of the input signals.

The rectifying means may comprise diode-connected transistors.

The rectifying means may be constructed so as to have breakdown voltages significantly higher than those of devices within the voltage impressing means. Such a current sensing circuit may be suitable for connection to a power semiconductor device connected so as to draw the output current from a relatively positive, relatively high-voltage supply terminal wherein the voltage impressing means is constructed to operate from a low-voltage supply which is regulated with reference to the voltage on the relatively positive supply terminal. Using a high-voltage-referenced supply for the low-voltage components of an intelligent power switch integrated circuit can reduce the need for high-voltage isolation between those components and the substrate when the substrate form the positive supply terminal for the power switch.This construction is disclosed in copending application number 8713385 (PHB 33363).

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawing which shows a power semiconductor arrangement and current sensing circuit constructed in accordance with the present invention.

The drawing shows a power semiconductor arrangement 10 comprising major and minor portions 10A and 10B respectively. The major and minor portions have a common first electrode 12 connected to a supply terminal 13 and respective major and minor electrodes 14 and 16 connected to terminals 15 and 17 respectively. The terminal 15 forms an output of an intelligent power switch circuit and is connected via a load 6 to ground at 8.

In the embodiment shown, the power semiconductor arrangement 10 is a cellular, n-channel vertical power MOSFET device. The device may typically contain hundreds or even thousands of cells, all having a common drain electrode 12. The majority of the cells are connected in parallel to form the major portion 10A of the arrangement, with their sources 14 connected to terminal 15. One cell (or possibly a few cells) forms the minor portion 10B, having its source electrode 16 connected to terminal 17. The gate electrodes of both portions are connected to a common control terminal 18.

As mentioned already in the introductory part of this specification, power MOSFETs are not the only semiconductor arrangements amenable to current sensing in accordance with the invention, and are shown here merely to provide an example of the application of such a current sensing circuit. N-channel, vertical MOSFETs are particularly useful power devices in many applications which make use of their compactness, low on-resistance and insulated gate input.

The arrangement 10 is connected to a current sensing circuit having inputs formed by the terminals 15 and 17. The circuit is also connected to a first low-voltage supply terminal 20 and a second low-voltage supply terminal 22. The input 15 of the current sensing circuit is connected via a first impedance Z1 to a first input 26 of a differential amplifier circuit 28. The circuit 28 includes n-MOS transistors N1 and N2 in a long-tail pair configuration with their sources connected via a current source 30 to the supply terminal 22. Two p-MOS transistors P1 and P2 which form a current mirror circuit 32 are connected between the supply terminal-20 and the drains of the transistors N1 and N2 respectively, to complete the differential amplifier circuit 28.

Transistor P1 forms the input path of the current mirror circuit 32. The gate of transistor N1 forms the first input 26 of the circuit 28 and the gate of transistor N2 forms a second input 34 of the circuit 28. The connection between transistors P2 and N2 forms an output 36 of the differential amplifier circuit 28.

Unless otherwise stated, it is to be assumed that all of the p-MOS and n-MOS transistors N1,P1 etc. are enhancement-mode types.

The differential amplifier circuit 28 is thus conventional and does not make use of the invention disclosed in UK patent application No. 8713387 (PHB33365). However, the transistors N1 and N2 and the current source 30 could be replaced by a set of matched depletion-mode devices in accordance with that other invention without affecting the rest of the current sensing circuit.

The input 17 of the current sensing circuit, which is connected to the minor electrode 16 of the power arrangement 10, is connected via a second impedance Z2 to the second input 34 of the differential amplifier circuit 28. The impedances Z1 and Z2 are constructed to be matched to one another as accurately as possible. In the embodiment shown, each impedance is formed by a diode-connected, high-voltage n-MOS transistor. These devices can be integrated conveniently with the remainder of the circuit and afford protection to the low-voltage components N1 etc. as described hereinafter. Two high-value resistors R1 and R2 are connected between the supply terminal 20 and the two inputs 26 and 34 of the circuit 28 respectively.The resistors are of such high value that they do not affect the normal operation of the current sensing circuit but play a part in the protection just mentioned, to be described hereinafter. In an alternative embodiment, the resistors R1 and R2 are replaced by breakdown (zener) diodes, as described hereinafter.

The inputs 26 and 34 of the circuit 28 are also connected, via respective n-MOS transistors N3 and N4, to the supply terminal 22.

The transistors N3 and N4 are matched and form identical output branches of a current mirror circuit whose input branch is formed by a further n-MOS transistor N5. Thus the transistors N3 and N4 form means for maintaining equal currents in the first and second matched impedances Z1 and Z2. The aspect ratios (W/L)N3 to (W/L)N5 of the transistors N3 to N5 define the relationship between the respective currents I1, 12, I3 flowing in those transistors so that I1 = 12 = X.I3.

Three similar p-MOS transistors P3, P4 and P5 have their sources connected to the input 17 of the current sensing circuit and have their gates connected to the output 36 of the differential amplifier circuit 28. The drain of transistor P3 carrying a current 13) is connected to the drain of n-MOS transistor N5, the input transistor of the current mirror circuit N3,N4,N5. The drain of transistor P4 carrying a current 14) is connected at a node 38 to a first reference current source 40 (ILC) and to the input of a threshold detecting inverter circuit 42 which drives a first logic output 44 (LC) of the current-sensing circuit.The drain of transistor P5 (carrying a current I5) is connected at a node 46 to a second reference current source 48 (IHC) and to the input of a second threshold detecting inverter circuit 50. The output of the circuit 50 drives a second logic output 54 (HC) via a further inverter 52. The threshold detecting circuits 42 and 50 can be conventional CMOS Schmitt trigger circuits, and the inverter 52 can be a conventional CMOS inverter.

In a typical application, such as an intelligent power switch in a motor vehicle, the semiconductor arrangement 10 may replace a conventional dashboard switch or a relay, for controlling a headlamp, for example. In such an application, terminal 13 is connected to the battery supply rail (+12 volts, for example) of the vehicle, and ground connection 8 becomes the chassis earth of the vehicle. The polarities of the components shown are those suitable for a negative earth system. Because terminal 15 is connected to the positive side of the load 6, which has a permanent return connection to ground at 8, this configuration is known in the art as a "high-side" switch. The current sensing circuit shown is designed to be suitable for inclusion in an intelligent high-side switch integrated circuit for use in a motor vehicle, and will be described in that application as a practical example.

Other fields of application will be apparent to those skilled in the art.

In operation, supply terminal 13 is connected to the battery supply rail, of the motor vehicle, at a voltage of +VBATT and the chassis earth 8 is at zero volts (OV). To protect against the relatively high voltages which often occur in vehicle electrical systems, the devices Z1 and Z2, marked with an asterisk (*), are constructed as high voltage devices. Other parts of the circuit, such as the differential amplifier circuit 28, transistors N3-N5, P3-P5 etc., may be constructed using conventional complementary MOS (CMOS) devices, because of the provision of a low-voltage supply at 20,22.Supply terminal 20 is connected to battery positive (VBATT) and supply terminal 22 is held by a regulator, not shown, at a supply voltage VLOW, which is below VBATT by a fixed amount suitable for the operation of the CMOS circuitry. The voltage of the low voltage supply (VBATT - VLOW) may be 5 to 12 volts, depending on the design of the chosen regulator. By operating the low voltage circuitry relative to VBATT, and not relative to ground (OV), the circuit is more suitable for integration with the power semiconductor arrangement 10 itself, avoiding the need for high voltage isolation between the substrate and the low voltage circuitry. This technique is disclosed more fully in co-pending UK Patent Application 8713385 (PHB33363).

In operation, to energise the load 6, the power arrangement 10 is turned on by applying a voltage VG.somewhat higher than VBATT to the gate of the arrangement 10 via the control terminal 18. Such a voltage can be generated on-chip by means of an oscillator and charge pump circuit, as is well-known in the art. When the arrangement 10 is turned-on, both sections 10A and 10B conduct and the voltages VA and VB on terminals 15 and 17 respectively rise almost to VBATT. The equal currents I1 and 12 flowing in the matched impedances Z1 and Z2 respectively cause equal voltage drops across those impedances.The voltages at the inputs 26 and 34 of the differential amplifier circuit 28 are thus equal in difference to the voltages VA and VB but are lower in absolute terms by a few volts. This voltage translation is necessary to provide the necessary "headroom" for the amplifier 28 to operate correctly. It should be appreciated that because this voltage translation is achieved without the use of potential dividers, the important difference signal (VA - VB) is applied to the amplifier 28 without attenuation and with high accuracy.

The output 36 of the differential amplifier 28 drives the gates of p-channel transistors P3 to P5, which form, together with n-channel transistors N4 and N5 and impedance Z2, a circuit 56 (dotted box) which combines the functions of an output stage for the differential amplifier 28 and a current dividing circuit. This circuit 56 incorporates the function of the single current-controlling transistor 25 in EP-A1-0 227 149, in that it enables the differential amplifier 28 to control the total current Ig flowing through the mirror current carrying section 10B of the power arrangement 10. Thus the voltage VB is maintained equal to VA by negative feedback. Consequently the current IB is an accurate representation of the current IA flowing in the major section 10A of the arrangement 10.

In the circuit 56, by virtue of the current mirror principle, the currents 13, 14 and Ig are maintained in fixed proportions to one another in accordance with the formula: 13 : 14 : I5 = (W/L)p3 : (W/L)p4 :(W/L)p5 As has already been mentioned, the current 12 flowing through impedance Z2 is also related by a fixed ratio X to current 13 and so to all the currents 13 to 15 and to the total current flowing into the input 17 of the current sensing circuit, namely IB.

It should be appreciated that this division of the current 1B into the four smaller currents I2, I3, 14 and is effected in accordance with known ratios, even though one of the current paths includes an impedance Z2 which is totally unrelated to the impedances of the similar transistors P3, P4 and P5. The provision of the output transistor N3, ((W/L)N3 =(W/L)N4) enables the provision of the current I1 = I2 = X.I3 which is required to ensure equal voltage drops across the matched impedances Z1 and Z2.

The intelligent power switch integrated circuit which includes the circuit shown may have a nominal current rating INOM of several amperes. In that context, a current I1 of several microamps can be considered negligible, and so the current 1LOAD delivered to the load 6 can be considered equal to IA. However, if one considers that the number of cells in the power arrangement may be of the order of ten thousand (10,000) so that 18 = IA/10,000, then it is not necessarily the case that current I2 = I1 is a negligible fraction of 1B This is particularly the case when IA is much lower than 1NOM' less than 1 amp, say.In the known circuit, it is a potential source of error that the current required to bias the potential dividers was not necessarily negligible in relation to the sensing current. The circuit shown in the drawing however eliminates this source of error, because the current I2 is a known fraction of Ig and can be accounted for in scaling calculations.

Thus the currents 14 and Ig are accurate fractions of the current Ig and hence of the load current 1LOAD The currents 14 and Ig could be measured in any appropriate manner to arrive at a measure of 1LOAD' but in this example are applied to two complementary threshold detecting arrangements which operate as follows. If the current 14 exceeds the small reference current ILC, then the threshold detecting circuit (Schmitt trigger) 42 is triggered and output 44 carries a logic signal LC = 'O'. Otherwise LC = '1'.Similarly, if current 15 exceeds the reference current IHCt then the-Schmitt trigger 50 is triggered and output 54 carries a logic signal HC = '1'. Otherwise HC = 'O'. Therefore the provision of the threshold detecting circuits with or without an additional inverter such as inverter 52 allows a logic '1' to indicate either an excess or a deficiency of load current respectively.

As a numerical example, assume that nominal load current INOM = 7 amps and IA : 1B = 10,000. Further, assume also that the aspect ratios (W/L) of the various transistors are such that: (W/L)p3 = 4.(W/L)p4 = 4.(W/L)p5 and (W/L)N3 =(W/L)N4 = CW/L)N5/4, i.e. X = 1/4.

This gives: I3 = 4.IB/7 and I1 = 12 = 14 = Ig = 1B17 = ILoAD/70'000- Therefore, the signal HC = '1' will indicate a high-current condition where the load current is more than twice the nominal current, (ILOAD > 2.INoMt i.e. ILOAD > 14 amps) if the reference current IHC is made equal to 14/70,000 = 0.0002 amps or 200 microamps. The signal HC could thus be used to activate an overload protection mechanism within the intelligent power integrated circuit to cause an automatic shut-down, and/or perhaps an external warning signal.

Similarly, the signal LC = '1' could indicate a low-current condition, for example where ILOAD is less than five per cent of INOM (i.e. 1LOAD < 350mA), if the reference current ILC were made equal to 0.35/70,000 = 5 microamps. Such an indication could be used for example for detecting filament breakage in a lamp, or some other open-circuit fault.

Compared with the known circuit, in which smaller representative currents were formed by scaling down the current through the minor section using cascaded current mirrors, the circuit shown generates the currents 14 and Ig in a single step.

This avoids the accumulation of scaling errors that can occur when using several current mirrors in cascade.

It will be appreciated that a greater or lesser number of subcurrents could be divided from the current 1B by providing a greater or lesser number of similar p-channel transistors in the circuit 56. Thus a larger number of thresholds could be detected, or one sub-current could be used for an analogue indicating function, for example, driving a microammeter. Subcurrents of opposite-polarity could be generated, if required, by providing additional output transistors in the n-channel current mirror circuit N3-N5.

Since the impedances Z1 and Z2 are constructed as high-voltage rectifying devices, they afford protection to the circuit against voltages outside the range VLOW to VBATT, that may be applied to the terminal 15 when the power arrangement 10 is turned off, or when there is a short-circuited load fault, for example. In these circumstances, VLOAD goes to OV, and rectifier Z1 becomes reverse-biased. Resistor R1 is provided to supply the reverse leakage current for Z1, so that the voltage at 26 is held within the range VLOW to VBATT and no harm can come to the low voltage devices N1 etc. of the current sensing circuit.An identical resistor R2 is provided at the input 34 purely to maintain optimum matching on both sides of the differential amplifier.

The small currents that will flow through R1 and R2 will in principle introduce an error in the division of current Ig into the calculated fractions I2 to 15. If R1, R2 are linear resistances, they must be made high enough that this error is negligible, while still low enough to supply the necessary leakage current. Alternatively, by using non-linear devices, however, it is possible that R1 and R2 can be made virtually infinite in value until a certain threshold, beyond the normal operating value but above VLOW, is reached. This can be achieved for example by replacing R1 and R2 with a pair of breakdown diodes, having their cathodes connected to supply terminal 20 and having a breakdown voltage approximately the same as VBATT - VLOW.

Clearly those skilled in the art will recognise that other sets of possible conditions may require different means of protection. For example if the low-voltage circuitry were made to operate with reference to OV instead of VBATT, as is in fact conventional, then different devices may need to be made high voltage devices. Also it may be preferable in other circumstances to use signals of the opposite polarity, and/or components of the opposite conductivity type to those described.

In summary, with an intelligent power switch circuit such as that described above it is possible to provide not only the simple power switch function, but also fault detection and protection against costly damage. These functions, and others, may conveniently be integrated upon a single chip. Such a power switch may also be integrated upon a single chip. Such a power switch may also be provided with on-chip temperature protection, surge protection and other functions as desired.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design and use of current sensing circuits, power semiconductor device and component parts thereof, and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation of one or more of those features which would be obvious to persons skilLed in the art, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims (12)

1. A current-sensing circuit for sensing an output current of a power semiconductor device which device has a major electrode for carrying the output current and a minor electrode for carrying a sensing current, the circuit comprising means for impressing on the minor electrode a voltage equal to that on the major electrode and means for monitoring the total current flowing through the minor electrode, wherein the voltage impressing means has a first input coupled via a first impedance to the major electrode and a second input coupled via a second impedance to the minor electrode, the first and second impedances being matched, the circuit comprising means for maintaining equal currents in the first and second impedances so that the difference between the voltages at the first and second inputs is substantially equal to the difference between the voltages at the major and minor electrodes, while the absolute values of those voltages are translated to levels suitable for the first and second inputs of the voltage impressing means.

2. A current sensing circuit as claimed in Claim 1 wherein the voltage impressing means includes means for dividing the total current flowing through the minor electrode so that respective predetermined fractions of the total current flow through the second impedance and one or more inputs of the monitoring means.

3. A current sensing circuit as claimed in Claim 2 wherein the dividing means is combined with an output stage of the voltage impressing means.

4. A current sensing circuit as claimed in Claim 3 wherein the combined dividing means and output stage comprise at least two similar transistors having first main electrodes arranged for connection together with the second matched impedance to the minor electrode of the power semiconductor device and having control electrodes connected together and driven in response to a difference between the voltages at the inputs of the voltage impressing means, a second main electrode of at least one of the similar transistors being connected to an input of the monitoring means while a second main electrode of a further one of the similar transistors is connected to the input of a current mirror circuit having a first output connected to the first matched impedance and a second output connected to the second matched impedance.

5. A current sensing circuit as claimed in any preceding claim wherein the monitoring means comprises means for comparing a predetermined fraction of the total current flowing through the minor electrode with a reference current and for indicating which current is the greater.

6. A current sensing circuit as claimed in any preceding claim wherein the first and second matched impedances comprise rectifying means so that the first impedance serves to protect the first input of the voltage impressing means against damage by voltages applied by an external circuit to the major electrode of the power semiconductor device when that device is turned off.

7. A current sensing circuit as claimed in Claim 6 wherein the rectifying means comprise diode-connected transistors.

8. A current sensing circuit as claimed in Claim 6 or Claim 7 wherein the rectifying means are constructed so as to have breakdown voltages significantly higher than those of devices within the voltage impressing means.

9. A current sensing circuit as claimed in Claim 8 suitable for connection to a power semiconductor device connected so as to draw the output current from a relatively positive, relatively high-voltage supply terminal wherein the voltage impressing means is constructed to operate from a low-voltage supply which is regulated with reference to the voltage on the relatively positive supply terminal.

10. A current sensing circuit substantially as described herein with reference to the accompanying drawing.

11. An intelligent power switch integrated circuit comprising a power semiconductor device having a major electrode for connection to a load and a minor electrode, the integrated circuit further comprising a current sensing circuit as claimed in any preceding claim having its first and second inputs connected to the major and minor electrodes of the power semiconductor device respectively.

12. Any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation of one or more of those features which would be obvious to persons skilled in the art, whether or not it relates to the same invention as presently claimed in any preceding claim.