A temperature sensing circuit

Abstract: A temperature sensing circuit (100) for sensing the temperature of an active semiconductor device, in this example a power MOSFET (11), of a semiconductor body (10). The circuit has a first temperature sensing device (R1,R2) provided on the semiconductor body (10) at a first position (P₁) adjacent a periphery (12) of the active semiconductor device (11), a second temperature sensing device (R3,R4) provided on the semiconductor body (10) at a second position (P₂) further from the periphery (12) of the active semiconductor device (11) than the first position (P₁) and an arrangement, for example a comparator (Q₁,Q₂,Q₃,Q₄,Q₅,Q₈) responsive to the first and second temperature sensing devices (R1,R2,R3,R4) for providing a control signal to switch off the active semiconductor device (11) when the difference in the temperature sensed by the first (r1,R2) and second (R3,R4) sensing devices reaches a predetermined value.

Description: This invention relates to a temperature sensing circuit for sensing the temperature of an active semiconductor device of a semiconductor body, for example for sensing the temperature of a power MOSFET. Various different forms of circuits for sensing the temperature of active semiconductor devices such as power MOSFETs have been proposed. Thus, for example, EP-A-341482 proposes providing a bipolar transistor in thermal contact with the active semiconductor device and using the increase with temperature of the current through the bipolar transistor to control the switching of a MOSFET which provides a signal indicative of an undesired temperature rise. EP-A-224274 adopts a different approach in providing a temperature sensitive polycrystalline silicon diode on top of the active semiconductor device. The use of such temperature sensing circuits is of particular interest in the field of so-called 'smart-power' devices in which control functions provided by low voltage logic devices are integrated with power semiconductor devices such as power MOSFETs to provide intelligent power switches for use in high-volume applications such as automotive control systems. It is desirable in such devices for the power device to be switched off or shut down if it overheats because, for example, of a short-circuit. However, in order to provide high current handling capability power semiconductor devices are generally rather large area devices and in certain overload conditions, for example if a low resistance, say about 100mΩ (milli-ohm), load is connected between ground (earth) and the power semiconductor device, the temperature of the centre of the power semiconductor device can rise rapidly whereas the temperature of the semiconductor body surrounding the power semiconductor device may rise considerably more slowly. In such circumstances, the provision of a temperature sensitive device in thermal contact with the power semiconductor device may not provide a sufficiently accurate or rapid response to the change in temperature of the power semiconductor device. Also, where the temperature sensitive element is provided on top of the active semiconductor device, the accuracy of the temperature sensing may be adversely affected especially if the thermal contact to the active semiconductor device is not particularly good. Instead of sensing the temperature at a single point, it is known from published Japanese Patent Application JP-A-58-53730 (Patent Abstracts of Japan, Vol 7, No 138, p-204) to sense the temperature at at least two points on a temperature gradient. The temperature sensing circuit of JP-A-58-53730 comprises a first temperature sensing device in the vicinity of the active semiconductor device, a second temperature sensing device separated from the active semiconductor device, and comparator means responsive to the first and second temperature sensing devices for providing a signal indicative of the difference in the temperatures sensed by the first and second sensing devices. As shown in the drawing in JP-A-58-53730, the temperature sensing devices may be respective bipolar transistors with a common emitter connection and with a respective reference voltage applied to their base. Respective load resistors are connected to the collectors of these respective transistors, and the differences in voltage at the respective nodes of the collector and load resistor are compared in a comparator to provide the output signal indicative of the detected heat output of the active device. It is an aim of the present invention to provide a sensitive temperature sensing circuit of this type which allows rapid rises in the temperature gradient of an active semiconductor device such as a power MOSFET, to be detected and to be used to provide a signal for switching off the active semiconductor device before it overheats significantly. According to the present invention there is provided a temperature sensing circuit for sensing the temperature of an active semiconductor device of a semiconductor body, and comprising a first temperature sensing device provided on the semiconductor body at a first position adjacent the active semiconductor device, a second temperature sensing device provided on the semiconductor body at a second position remote from the periphery of the active semiconductor device, and comparator means responsive to the first and second temperature sensing devices for providing a signal according to the difference in the temperatures sensed by the first and second sensing devices, characterised in that the first temperature sensing device comprises first and second temperature sensing elements, and the second temperature sensing device comprises third and fourth temperature sensing elements, the temperature sensing elements have a resistance which varies with temperature and are connected in a Wheatstone bridge with the first and third elements and the second and fourth elements, respectively, connected in series between first and second supply lines and with the second and third elements connected to the first supply line, and in that the comparator means has a first input from a first junction between the first and third elements, has a second input from a second junction between the second and fourth elements, and is part of a control circuit means providing a control signal for switching off the active semiconductor device, the comparator means thereby serving to compare a first signal representing the voltage at the first junction and a second signal representing the voltage at the second junction so as to generate an output signal for providing the control signal to switch off the active semiconductor device when the difference between the first and second voltage signals represents a temperature difference of a predetermined value. The temperature sensing circuit may further comprise a transistor having its main current path connected between the first junction and the first element and another transistor having its main current path connected between the second junction and the fourth element with the gates of the transistors being connected to one another and to one of the first and second junctions. The incorporation of the current-mirror connected transistors should enable a larger differential gain which results in a lower accuracy requirement for the comparing means. Also, because this arrangement should have a higher sensitivity, it may be particularly useful where the elements have a smaller temperature coefficient of resistance or a higher noise margin. In one example, a circuit in accordance with the invention may also comprise means for generating a proportional-to-absolute-temperature (PTAT) voltage and means for deriving a first current source for the first and third elements and a second current source for the second and fourth elements from the PTAT voltage. The PTAT voltage generating means may comprise first and second semiconductor junction devices having respective outputs across which is generated the PTAT voltage and the current source deriving means may comprise a differential amplifier for which each semiconductor junction device provides an input signal to a respective one of the two inputs of the differential amplifier, a resistor connected in series between one of the semiconductor junction devices and an output transistor of the differential amplifier and a current mirror arrangement comprising first and second transistors for mirroring the current through the resistor, the first transistor being connected in series with the first and third elements for forming the first current source and the second transistor being connected in series with the second and fourth elements for forming the second current source. Such an arrangement should assist the performance of the circuit with respect to absolute temperature so that the predetermined value is, as desired, determined by the difference in the temperatures sensed and is not significantly affected by their actual values. Conveniently, means are provided for preventing the active semiconductor device from being switched on again until the temperature difference has fallen to a second predetermined value below the predetermined value. Such an arrangement introduces a hysteresis into the circuit and thus prevents the circuit from repeatedly switching the active semiconductor device on and off for small changes in temperature. 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 diagram for one embodiment of a differential temperature sensing circuit in accordance with the invention; Figure 2 illustrates graphically a typical temperature profile from the centre to the periphery of a power MOSFET; Figure 3 is a plan view of a semiconductor body illustrating very schematically the relative locations of the active semiconductor device and the first and second temperature sensing devices; Figure 4 illustrates a circuit diagram for a modified version of the differential temperature sensing circuit shown in Figure 1; Figure 5 is a simplified schematic diagram for illustrating the principle of a further form of differential temperature sensing circuit in accordance with the invention; and Figure 6 is a circuit diagram of a second embodiment of a differential temperature sensing circuit in accordance with the invention based on the principle illustrated in Figure 5. Referring now to the drawings, for example Figures 1 and 3, there is illustrated a temperature sensing circuit 100 for sensing the temperature of an active semiconductor device, in this example a power MOSFET 11, of a semiconductor body 10. The circuit comprises a first temperature sensing device R1,R2 provided on the semiconductor body 10 at a first position P 1 adjacent the active semiconductor device 11, a second temperature sensing device R3,R4 provided on the semiconductor body 10 at a second position P 2 remote from the periphery 12 of the active semiconductor device 11 and means Q 1 ,Q 2 ,Q 3 ,Q 4 ,Q 5 ,Q 8 responsive to the first and second temperature sensing devices R1,R2,R3,R4 for providing a control signal OT to switch off the active semiconductor device 11 when the difference in the temperatures sensed by the first and second sensing devices exceeds a predetermined value. In the example shown in Figure 1, the temperature sensing circuit has first and second power supply lines 1 and 2. The first power supply line 1 is the positive and the second power supply line 2 the negative supply line. In this example, the first and second temperature sensing devices are provided as four temperature sensitive elements in the form of resistors R1 to R4 which provide a Wheatstone bridge connected between the power supply lines 1 and 2. Thus, the first temperature sensing device is formed by first and second resistors R1 and R2 provided in opposite arms of the Wheatstone bridge whilst the second temperature sensing device is formed by third and fourth resistors R3 and R4. The third and first resistors R3 and R1 are connected in series between the power supply lines 1 and 2 as are the second and fourth resistors R2 and R4 so that, as shown in Figure 1, the second and third resistors R2 and R3 are connected to the first or positive supply line 1 whilst the first and fourth resistors R1 and R4 are connected to the second or negative supply line 1. As will be explained in greater detail below with reference to Figure 3, the first and second resistors R1 and R2 are located at a first position P 1 adjacent the periphery 12 of the active device 11 and so constitute hot resistors, that is they form resistors which experience the temperature changes at the periphery 12 of the active device 11, whilst the third and fourth resistors R3 and R4 are positioned further away from the periphery 12 of the active device 11 and so are 'cold' resistors which do not experience the temperature changes at the periphery 12 of the active device 11. In this example, the resistors R1 and R3 have positive temperature coefficients, that is their resistance increases with temperature. The outputs from the Wheatstone bridge are taken from a first junction 4 between the third and first resistors R3 and R1 and a second junction 5 between the second and fourth resistors R2 and R4 and are supplied as respective inputs to a comparator 6 which provides a control signal to switch off the active semiconductor device 11 when the voltage at the first junction 4 exceeds the voltage at the second junction 5. In this example, the resistors R1 to R4 are selected such that when all four resistors R1 to R4 are at the same temperature the 'hot' first and second resistors R1 and R2 are of lower resistance than the third and fourth resistors R3 and R4. Thus, when there is no temperature difference between the first and second temperature sensitive devices R1,R2 and R3,R4, the voltage at the second junction 5 is higher than the voltage at the first junction 4 and the Wheatstone bridge does not reach a balance (that is the point at which the voltages at the first and second junctions 4 and 5 are equal) until the temperature of the first and second resistors R1 and R2 has increased sufficiently for their resistances to equal the resistances of the resistors R3 and R4. By selecting the relative values of the resistors R1,R2 and R3,R4, the temperature gradient between the first and second temperature sensing devices R1,R2 and R3,R4 at which the comparator 6 provides a control signal to switch off the active device 11 can be selected. The respective values of the resistors R1 and R2 and the resistors R3 and R4 will be dependent upon many variables such as the characteristics of the resistors and other elements of the circuit and the respective distances of the resistors from the periphery 12 of the active semiconductor device 11. In this example, the resistance of the resistors R1 and R2 increases with temperature in such a manner that a balance is reached when there is a temperature difference of, for the values given above, 25°C between the first and second temperature sensing devices. Any temperature difference exceeding this value will cause the active semiconductor device to be switched off. In the circuit shown in Figure 1, the comparator 6 comprises a differential pair of matched transistors Q1 and Q2 which form the input transistors of the comparator 6. In this example, the transistors Q1 and Q2 are n-channel MOSFETs with their gates G1 and G2 connected to the first and second junctions 4 and 5, respectively. The comparator 6 also includes two p-channel matched MOSFETs Q3 and Q4 having their source electrodes connected to the first supply line 1. The gate electrodes of the MOSFETs Q3 and Q4 are connected whilst the gate electrode of MOSFET Q3 is also connected to its drain. The MOSFETs Q3 and Q4 thus provide a current mirror arrangement and with their drain electrodes connected to respective ones of the drain electrodes of the MOSFETs Q1 and Q2 provide an active load for the differential pair Q1 and Q2. The sources of the MOSFETs Q1 and Q2 are connected to the drain of an n-channel MOSFET Q5. The source of the MOSFET Q5 is connected to the second or negative supply line 2 whilst its gate is connected to the gate of a gate-drain connected n-channel MOSFET Q9 which is connected in series with a conventional current source I which may, in this example, be a resistor (typically with a resistance of 200 Kilo-ohms) connected between the supply lines 1 and 2 because, in this example, the voltages on the supply lines 1 and 2 are fixed. Alternatively, a suitable conventional external current source may be connected between the supply lines 1 and 2. The current source I forces current into MOSFET Q9 and this current is mirrored in MOSFET Q5 which thus acts as a current source of a value determined by the external current source I for the differential amplifier comparator 6. Other suitable structures may, of course, be used to form the comparator 6. The output 7 of the comparator 6 is supplied to the gate of a p-channel MOSFET Q8 which has its source connected to the first supply line 1 and its drain connected via an n-channel MOSFET Q7 to the second supply line 2. The gate of the n-channel MOSFET Q7 is, like that of MOSFET Q5, connected to the external current source I and thus similarly acts as a current source of a value determined by the external current source I. The MOSFETs Q9,Q5 and Q7 are integrated together so as to be similar. It should be understood that, as used herein, the term 'similar' means that the devices, for example transistors, are integrated together so as to be matched or else so as to have a known ratio of effective junction areas so that the current I provided by the current source through the MOSFET Q9 is mirrored in the MOSFETs Q5 and Q7 with the currents through Q5 and Q7 being either equal to (where the transistors are matched) or a known multiple of the current through the MOSFET Q9. With appropriate modification of component values, it may be possible for the MOSFETs Q1 and Q2 and Q3 and Q4 forming the comparator 6 to have known ratios of effective junction areas rather than be precisely matched. The drain of the p-channel MOSFET Q8 provides, via line 8, the control signal OT for controlling the operation of the active semiconductor device 11 (Figure 3). In addition the drain of p-channel MOSFET Q8 is connected to a NOT gate 9 in the form of an inverter which drives the gate of an n-channel enhancement mode MOSFET Q11. The n-channel MOSFET Q11 is connected in series with a resistor R5 and the series circuit of the n-channel MOSFET Q11 and resistor R5 is connected in parallel with the fourth temperature sensitive element R4. Resistor R5, MOSFET Q11 and inverter or NOT gate 9 provide a hysteresis circuit 20. Thus, when the signal on the output 7 of the comparator 6 is high providing a low output signal OT from the MOSFET Q8 for turning off the active device 11, the NOT gate 9 provides a high input signal to switch on the MOSFET Q11, so connecting the resistor R5 in parallel with the fourth temperature sensing element R4. This reduces the effective resistance of the fourth temperature sensing element R4 so that the temperature of the first and second heat sensitive elements R1 and R2 has to drop to a second predetermined value, typically 20°C (degrees Celsius), below the predetermined value before the output of the comparator 6 again goes low to provide a high signal OT to switch on the active device 11. This hysteresis circuit 20 prevents the temperature sensing circuit 100 hunting or cycling, that is continually causing the active semiconductor device to switch on and off, with small temperature changes. Figure 2 illustrates graphically the change in temperature from the centre to the edge of a typical active semiconductor device such as a power MOSFET with a source-drain voltage of 21 volts and a power of dissipation of 300 watts when for example an overload of small resistance (for example about 100mΩ) is applied between the power MOSFET and ground (earth). Under such conditions where a relatively small resistance overload is so applied, the temperature at the centre of the power MOSFET will rise rapidly whereas the temperature in the bulk or remainder of the semiconductor body will rise only slowly. Accordingly, as illustrated in Figure 2, a steep temperature gradient occurs adjacent the periphery 12 of the power MOSFET. By locating the first temperature sensing device, which in this case comprises the two heat sensitive elements R1 and R2, at a first position P 1 adjacent, generally just outside, the periphery 12 of the power MOSFET and the second temperature sensing device, which in this case comprises the two temperature sensitive elements R3 and R4, at a second position P 2 further from the periphery 12 of the active device 11 where the temperature is only changing slowly, if at all, with time as illustrated in Figure 2, the temperature gradient at the periphery of the power MOSFET can be detected and when it reaches a certain value the power MOSFET can be switched off by the circuit described with reference to Figure 1. As indicated above, the temperature sensing circuit 100 is integrated in or on the same semiconductor body as the active device, in this example a power MOSFET. Figure 3 indicates very schematically a plan view of the top surface of the semiconductor body 10 to show the relative locations of the periphery 12 of the power MOSFET 11 and the first and second temperature sensitive devices R1 to R4. The power MOSFET 11 will generally be a vertical MOSFET manufactured using conventional so-called DMOSFET technology and the various MOSFETs Q1 to Q11 (not shown in Figure 3) will be manufactured at the same time as the power DMOSFET 11 using a known CMOS (complementary MOSFET) technology compatible with the DMOS technology. The temperature sensitive elements R1 to R4 and the resistor R5 may be formed using any suitable conventional technology, for example the resistors R1 to R4 and R5 may be formed as diffused regions or from a doped polycrystalline silicon layer. The use of diffused regions to form the resistors R1 to R4 is preferred as they should provide better thermal contact to the power MOSFET and should thus provide a more accurate detection of temperature. Typically, where the power MOSFET is a vertical DMOSFET having a surface area of 10mm 2 and consisting of, for example, 10,000 to 20,000 parallel cells, then the 'hot' temperature sensitive elements R1 and R2 may be positioned at a first position P 1 which is 125µm distant from the MOSFET periphery 12 whilst the 'cold' temperature sensitive elements R3 and R4 may be positioned at a second position P 2 1125µm distant from the periphery 12 of the power MOSFET 11 and the resistances of the temperature sensitive elements R1 to R4 may be selected such that the comparator 6 provides a high output signal resulting in a low signal OT to switch off the power MOSFET 11, when the temperature gradient between the first and second positions P 1 to P 2 reaches 25°C. In this example, the resistors R1 and R2 may have a resistance which is, for example, 0.9 times that of the resistors R3 and R4. It should be understood that the reference to the periphery 12 of the power MOSFET means the last row of cells and that the above measurements are taken between the periphery 12 and the centre of the resistors forming the temperture sensitive elements R1 to R4. In operation of the temperature sensing circuit 100 shown in Figure 1 when there is no temperature difference between the resistors R1 to R4, then the voltage at the second junction 5 will be higher than that at the first junction 4 because, as discussed above, of the ratio of the resistances, so that the comparator 6 provides a low output on output line 7. The MOSFET Q8 is thus on and the output signal OT is high. As the temperature of the resistors R1 and R2 increases so does their resistance and at a temperature selected by the ratio of the ratio of the resistors the bridge will come into balance. As the temperature of the resistors R1 and R2 increases further the resistance of the temperature sensitive elements R1 and R2 will be greater than the resistance of the temperature sensitive elements R3 and R4, so causing the voltage at the first junction 4 to exceed that at the second junction 5. Typically, this will be arranged to occur when there is a large, typically 25°C, temperature difference between the first and second positions P 1 and P 2 . In such circumstances, the comparator 6 produces a high output signal on output line 7 to the gate of p-channel MOSFET Q8 so that the MOSFET Q8 is off and the circuit 100 provides a low output signal OT for switching off the active device in this case the power MOSFET 11 shown schematically in Figure 3. As mentioned above, at this time, the NOT gate 9 provides a high signal to switch on the MOSFET Q11 so connecting the resistor R5 in parallel with the fourth temperature sensing element R4. As the power MOSFET 11 has now been switched off, the temperature difference between the first and second positions P 1 and P 2 will begin to decrease and when this temperature difference drops sufficiently, to reach the second lower predetermined value determined by the resistor R5, the voltage at the second junction 5 will rise above that at the first junction 4 causing the comparator 6 to provide a low output signal to the gate of the MOSFET Q8, so switching on the MOSFET Q8 and providing a high output signal OT to switch on the active device 11. At the same time, the inverter or NOT gate 9 provides a low signal switching off the MOSFET Q11 and so disconnecting the resistor R5 so that the comparator 6 will again provide a high output signal for switching off the active device 11 when the first higher predetermined temperature difference is reached. Figure 4 illustrates the circuit diagram of a modified version 100' of the differential temperature sensing circuit shown in Figure 1. In the circuit shown in Figure 4, a first n-channel MOSFET Q12 is connected in series between the resistors R2 and R4 so that its source is connected to resistor R4 and its drain to resistor R2. A second n-channel MOSFET Q13 matched or similar to MOSFET Q12 is connected between the resistors R3 and R1 with its source connected to the resistor R1 and its drain connected to the resistor R3. The gates of the two MOSFETs Q12 and Q13 are connected together at a node 13 and the gate of the MOSFET Q12 is connected to its drain at node 4'. The node 4' is also connected to one input of a comparator 6' while a node 5' between the resistor R3 and the drain of MOSFET Q13 is connected to the other input of the comparator 6'. The comparator 6' may have any suitable form and may be similar to the comparator 6 shown in Figure 1. Assuming, in the interests of simplicity, that the gain of the MOSFETs Q12 and Q13 is very high so that their gate-source voltage Vgs is nearly constant. When the resistors R1 to R4 are all at the same temperature R3 = KR2 and R4 = KR1 with K being, for example, about 1.1. If the current through resistor R2, MOSFET Q12 and resistor R4 is defined as I' the current I'' flowing through resistor R1 is approximately (R4/R1)I' = KI'. The voltage across resistor R2 is I'R2. The voltage across resistor R3 is I"R3 = KI'R3 = K 2 I'R2. The difference in voltage between the nodes 4' and 5' is therefore: ΔV = I'R2(1-K 2 ) As the temperature of the hot resistors R2 and R1 increases the value of K will decrease until K = 1 when the temperature difference between the hot resistors R2 and R1 and the cold resistors R3 and R4 reaches the critical value. At this point the difference in voltage ▵V = 0. As the temperature of the resistors R1 and R2 increases further the resistance of the resistors R1 and R2 will exceed that of the resistors R3 and R4 and the comparator 6' will provide a signal for switching off the active semiconductor device 11. As mentioned above, the comparator 6' may be similar to the comparator 6 shown in Figure 1, may be connected to a similar output stage (MOSFETs Q8 and Q7) and may be provided with a similar hysteresis circuit 20 to that shown in Figure 1. The incorporation of the current-mirror connected MOSFETs Q12 and Q13 enables a larger differential gain which results in a lower accuracy requirement for the comparator 6. Because the circuit 100 of Figure 4 should have a higher sensitivity than the circuit 100' of Figure 1, it may be useful where the resistors R1 to R4 have a smaller temperature coefficient of resistance or a higher noise margin which characteristics will primarily be determined by the process used to form the resistors. Although the temperature sensing circuit shown in Figure 1 enables the active device 11 to be switched off when a certain differential temperature is detected under given ambient temperature conditions, the resistances of the resistors R1,R2,R3 and R4 vary with ambient temperature so that the differential temperature at which the Wheatstone bridge formed by the four resistors R1,R2,R3,R4 is brought into balance may vary depending upon the ambient temperature, that is upon the temperature of the cold resistors R3 and R4. Figure 5 illustrates schematically the basic components of one example of a temperature sensing circuit 100a designed to mitigate the effects of a change in ambient temperature. As illustrated in Figure 5, the first temperature sensing device of the temperature sensing circuit 100a again comprises two resistors R11 and R14 which are located adjacent the periphery 12 of the active device 11 (that is in a position similar to the resistors R1 and R2 shown in Figure 3). In this example, the second temperature sensing device comprises two resistors R13 and R16 which are located away from the periphery 12 of the active device 11 in a position similar to that of the resistors R3 and R4 shown in Figure 3. Two further 'cold' resistors R12 and R15 are similarly located away from the periphery 12 of the active device 11. The resistor R13 is connected to the power supply line 1 and in series with the resistor R11 which is connected via the resistor R15 to the power supply line 2. The resistors R12,R14 and R16 are similarly connected in series with the resistor R14 connected to the power supply line 1 via the resistor R12 and to the power supply line 2 via the resistor R16. A first current source 30 derived from a proportional-to-absolute teperature (PTAT) voltage source (not shown in Figure 5) is connected to a junction 21 between the resistors R11 and R15 to draw a current I 1 through the series-connected resistors R13,R11 and R15 and a second similar current source 31 is connected to a junction 22 between the resistors R12 and R14 to, in this example, draw the same current current I 1 through the series-connected resistors R12,R14 and R16. The circuit may be arranged, depending for example, on the respective vlaues of the resistors, so that the two current sources 30 and 31 do not draw the same currents. A comparator 40 has a first positive input 41 connected to a junction 23 between the resistors R13 and R11 and a second negative input 42 connected to a junction 24 between the resistors R14 and R16. The comparator 40 may be of any suitable type and may, for example, be similar to the comparator 6 shown in Figure 1. The comparator 40 provides an output signal indicating the presence of a given differential temperature on output line 43. Although not shown in Figure 4, a hysteresis circuit similar to the circuit 20 shown in Figure 1 may be provided to inhibit hunting or cycling of the temperature sensing circuit 100a. Figure 6 illustrates one example of the detailed implementation of the circuit 100a shown in Figure 5. The circuit 100a shown in Figure 6 uses a differential amplifier 52 to provide the first and second current sources 30 and 31. However, it will of course be appreciated by those skilled in the art that various such current sources are known from the prior art, and that that to be described represents just one possible arrangement. The differential amplifier 52 comprises two p-channel MOSFETS P1 and P2 in a long-tailed pair configuration, with their common source current I SS being supplied from the power supply line 1 via a further p-channel transistor P3. The drains of the p-channel pair P1,P2 are connected to the other power supply line 2 via an n-channel current mirror active load formed by a diode-connected MOSFET N1 and a MOSFET N2, respectively. Two n-channel MOSFETs N3A and N3B have their gates connected to the junction 54 of the drains of the MOSFETs P2 and N2, and form first and second output transistors of the differential amplifier 52, respectively. Although not shown in Figure 6, as will be appreciated by the person skilled in the art, a start-up circuit may be required to raise the voltage at node 54, initially. The start-up circuit may be of any suitable conventional form. The drain of the first output transistor N3A is connected at a node 58 to the emitter of a first n-p-n bipolar transistor QA, whose collector is connected to the power supply line 1. The drain of the second output transistor N3B is connected via a further resistor R3 to the emitter of a second n-p-n bipolar transistor QB whose collector is also connected to the power supply line 1. The second bipolar transistor QB has an effective emitter area which is a known factor J times larger than that of the first bipolar transistor QA. The bipolar transistors QA (x1) and QB (xJ) both have their bases connected to a bias rail 66, which is connected via a diode-connected p-channel MOSFET P6 to the power supply line 1. The node 58 is connected to the gate of MOSFET P2 which forms the inverting input (-) of the differential amplifier 52. The node 62 is connected to the gate of MOSFET P1 which forms the non-inverting input (+) of the amplifier 52. In operation, the diode-connected p-channel MOSFET P6 serves to hold the bias rail 66 at a voltage V BB which is just a few volts below that of the power supply line 1. This provides enough "headroom" for the operation of the differential amplifier 52. The output transistors N3A and N3B of the differential amplifier 52 draw currents I 3A and I 3B through the first and second bipolar transistors QA (x1) and QB (xJ) respectively. If the output transistors N3A and N3B are matched, then the currents I 3A and I 3B must be equal. Therefore the current density in the first bipolar transistor QA (x1) is J times larger than that in the second bipolar transistor QB (xJ). It can therefore be predicted that there will be a difference V AB between the base-emitter voltage drops across the two bipolar transistors QA and QB, given by the equation: V AB = kT q ln (J), where k is Boltzmann's constant (1.38 x 10 -23 joule.kelvin -1 ), T is absolute temperature in kelvin, q is the electronic charge (1.6 x 10 -19 coulomb) and ln(J) is the natural logarithm of the factor J. Since the bases of the bipolar transistors QA and QB are both connected to the bias rail 66 (V BB ), the PTAT voltage difference V AB appears between their emitters, as shown. At the same time, the feedback from the output transistors N3A and N3B to the inputs (-) and (+) of the differential amplifier 52 ensures that the voltages at the nodes 58 (-) and 62 (+) are equal. This in turn means that the PTAT voltage signal V AB is impressed across the resistor R3, so that: I 3B = V AB /R3. A third n-channel output MOSFET N4 also has its gate connected to the same node 54 as that of the first and second output transistors N3A and N3B, and therefore generates a current I 4 proportional to I 3A and I 3B . This current I 4 is fed to the diode-connected input MOSFET P5 of a p-channel current mirror whose output transistor P3 supplies the bias current I SS for the differential amplifier 52. With appropriate scaling of the geometries of the MOSFETs N3A, N3B, N4, P5 and P3, this feedback biasing arrangement can eliminate the systematic offset errors which would otherwise occur when the differential amplifier 52 drives a non-infinite impedance load as it does in the present circuit. This biasing technique is described in detail in GB-A-2222497 (PHB33483). That application also describes an arrangement which may be necessary for avoiding undesirable latch-up conditions, but is not shown in Figure 6 for the sake of clarity. Thus far the circuit described above and shown in Figure 6 is similar to that shown in Figure 3 of EP-A-369530 (PHB33507) and for convenience the same reference numerals have been used to indicate the same components. However, in other respects, the circuit 100a of Figure 6 differs from that of Figure 3 of EP-A-369530. Thus, as shown in Figure 6, the node 54 is also connected to the gates of fourth and fifth output n channel MOSFETs N5 and N6, respectively, which have their sources connected to the power supply line 2. The drain of the fourth output transistor N5 is connected at node 55 to the connected gates of a pair of p-channel MOSFETs P6 and P7 which have their sources conected to the power supply line 1. The drain of the p-channel MOSFET P6 is connected to its gate so as to form a current mirror arrangement. The drain of MOSFET P7 is connected to the junction 21 between the resistors R11 and R15. The drain of the transistor N6 is connected to the junction 22 between the resistors R12 and R14. The MOSFET N5, with the associated current mirror arrangement P6 and P7, and the MOSFET N6, like MOSFET N4, mirror the current in the transistors N3A and N3B and thus draw a current I 1 proportional to I 3B and I 3A through their respective series-connected resistors R13,R11 and R15 and R12,R14 and R16. The current I 1 can be adjusted by a suitable choice of the resistance of the resistor R3. Although it is preferred that the MOSFETs N5 and N6 be matched so that they draw the same current I 1 , the MOSFETs N5 and N6 may merely be similar with the relative values of the currents flowing through the MOSFETs N5 and N6 being determined by their relative geometries. If desired negative versions of the currents I 3A and I 3B through the MOSFETs N 3A and N 3B may be obtained by appropriate use of current mirrors. It should of course be appreciated that all of the resistors R3,R13,R11,R15,R12,R14 and R16 should respond in the same way to changes in temperature and thus should be fabricated using the same process. As in the example described above with reference to Figure 1, the resistors may be, for example, diffused resistors or doped polycrystalline silicon resistors. In order to produce an output signal from the comparator 40 for causing the active device 11 to be switched off, then the voltage different ▵V between the negative and positive input lines 42 and 41 of the comparator should be zero, that is the bridge should be in balance. Defining Rx as the value of a resistor at 25 degrees Celsius and R'x as the resistance at some other temperature T so that R'x = Rx (1 + tc (T-298)) where T is the temperature in degrees Kelvin and tc is the temperature coefficient of resistance and letting Rc equal the resistance of the cold resistors R13 and R16, Rh equal the resistance of the hot resistors R14 and R11 and Rs equal the resistance of the resistors R15 and R12 then(Rc + Rh + Rs) ▵V = V (Rc - Rh - Rs) - 2RsR'cI 1 (298), , where I 1 (298) is the value of the current I 1 at 25 degrees Celsius (298 degrees Kelvin), then, when the whole semiconductor body 10 is at 25 degrees Celsius, from above: I 1 (T) = I 3B = VAB R'3 = kT ln (J) qR'3 let k ln (J) q = A The circuit should provide a signal to switch off the active device 11 when the overall temperature of the circuit reaches a certain maximum allowable temperature Tc, for example 423 degrees Kelvin. No local heating in the active device 11 is permitted under these circumstances and ▵V = 0 so that V (Rc - Rh - Rs) = 2ATcRsRc R3 Accordingly the critical temperature Tc is selected by selecting the values of the resistors Rc, Rh, Rs. The other situation of interest is when the chip is at a normal ambient temperature, for example 25 degrees Celsius, but a high transient power dissipation occurs in the active device 11 and the temperature of the hot resistors R14 and R11 is raised. Under these circumstances, the circuit should provide a signal to switch off the active device 11 when the differential temperature between the 'hot' resistors R11 and R14 and the 'cold' resistors R13 and R16 reaches a critical value ΔTc. Again ΔV is zero and V(Rc - Rs - Rh (1+tcΔTc)) = 2ATRsRc R3 From these equations, the values of the resistors can be calculated so that the behaviour of the circuit can be optimised so that the predetermined value is not adversely affected by the actual temperatures sensed but remains approximately constant, that is is substantially independent of the absolute or actual temperature sensed. In some circumstances, it may be desirable for the circuit to become more sensitive (that is for the differential value to decrease) with increase in the actual sensed temperatures. The above description assumes that the two hot resistors R11 and R14 have the same resistance as the two cold resistors R13 and R16. However, advantages may be gained in terms of the temperature dependency by making the bridge asymmetric so that the respective pairs of resistors do not necessarily have the same resistance. The use of the Wheatstone bridge arrangement described above, amplifies the difference signal being detected because the increase in temperature and thus, in the example described, resistance of the first heat sensitive device acts to increase the voltage at the first junction whereas the similar increase in temperature experienced by the second heat sensitive device acts to reduce the voltage at the second junction. It has been assumed in the example described above that the heat sensitive elements have positive temperature coefficients, that is that their resistance increases with temperature, however it is possible that heat sensitive elements with negative temperature coefficients could be used by for example modifying the circuit shown in Figure 1 appropriately so that a low output 7 from the comparator 6 switches off the active device or by reversing the position within the Wheatstone bridge of the elements R1 and R3 and the elements R2 and R4 so that a reduction in the resistance of elements R1 and R2 caused by a temperature increase results in the voltage at the first junction 4 exceeding the voltage at the second junction 5. Also, although in the example described above, the heat sensitive elements are provided as resistors other devices having temperature dependent resistances, for example diodes (or base-collector connected bipolar transistors functioning as diodes) could be used. The temperature sensing circuit 100 described above thus enables a steep temperature gradient to be detected and to be used to switch off the active device before it overheats significantly. This form of temperature sensing circuit is more sensitive than a single sensor and may be realised more easily and accurately than a temperature sensing circuit using a single temperature sensing element. Although in the examples described above, the temperature sensing elements are generally provided within the semiconductor body they could be provided on top of the semiconductor body by, for example, the use of polycrystalline silicon or other suitable material structures provided on top of the semiconductor body. In such a case, it may even be possible for the 'hot' temperature sensing element, for example, resistors R1 and R2 in Figures 1 to 4, resistors R11 and R14 in Figures 5 and 6, to be provided directly on top of or even in a small area defined within the active semiconductor device 11 rather than adjacent the periphery of the active semiconductor device 11. In addition, where appropriate, the MOSFETs used in the circuit 100 shown in Figure 1, and comparable components in Figures 4 to 6, may be replaced by other suitable components, for example bipolar transistors, and the active device 11 may be a power device other than a power MOSFET, for example a power bipolar transistor or thyristor or an insulated gate bipolar transistor. The active device could also be a lateral rather than a vertical device. It will be appreciated that whereas the embodiments described herein contain enhancement mode MOSFETs of particular conductivity types, the inventive principle can equally be applied to circuits in which devices have the opposite conductivity type and signals have the opposite polarity, and to circuits using bipolar devices. It will also be appreciated that where a particular form of circuit has been shown for performing a function which is well-known per se (for example a differential amplifier or a current mirror), there may be many other circuits for performing the same function which can be used in place of those described. Although as described above, where it is desired to monitor the temperature of, for example, a so-called "smart-power" device the circuit 100 is integrated with the active device, it may be possible for only the first or hot temperature sensing device to be integrated so as to be in close thermal contact with the thermal source. The other parts of the circuit could thus be provided separately. From reading the present disclosure, other modifications and variations will be apparent to persons skilled in the art. Such modifications and variations may involve other features which are already known in the semiconductor art and which may be used instead of or in addition to features already described herein.

Claims: A temperature sensing circuit for sensing the temperature of an active semiconductor device of a semiconductor body, and comprising a first temperature sensing device provided on the semiconductor body at a first position adjacent the active semiconductor device, a second temperature sensing device provided on the semiconductor body at a second position remote from the periphery of the active semiconductor device, and comparator means responsive to the first and second temperature sensing devices for providing a signal according to the difference in the temperatures sensed by the first and second sensing devices, characterised in that the first temperature sensing device comprises first and second temperature sensing elements, and the second temperature sensing device comprises third and fourth temperature sensing elements, the temperature sensing elements have a resistance which varies with temperature and are connected in a Wheatstone bridge with the first and third elements and the second and fourth elements, respectively, connected in series between first and second supply lines and with the second and third elements connected to the first supply line, and in that the comparator means has a first input from a first junction between the first and third elements, has a second input from a second junction between the second and fourth elements, and is part of a control circuit means providing a control signal for switching off the active semiconductor device, the comparator means thereby serving to compare a first signal representing the voltage at the first junction and a second signal representing the voltage at the second junction so as to generate an output signal for providing the control signal to switch off the active semiconductor device when the difference between the first and second voltage signals represents a temperature difference of a predetermined value. A temperature sensing circuit according to Claim 1, further comprising a transistor having its main current path connected between the first junction and the first element and another transistor having its main current path connected between the second junction and the fourth element with the gates of the transistors being connected to one another and to one of the first and second junctions. A temperature sensing circuit according to Claim 1 or 2, further comprising means for generating a proportional-to-absolute temperature (PTAT) voltage and means for deriving a first current source for the first and third elements and a second current source for the second and fourth elements from the PTAT voltage. A temperature sensing circuit according to Claim 3, wherein the PTAT voltage generating means comprising first and second semiconductor junction devices having respective outputs across which is generated the PTAT voltage and the current source deriving means comprises a differential amplifier for which each semiconductor junction device provides an input signal to a respective one of the two inputs of the differential amplifier, a resistor connected in series between one of the semiconductor junction devices and an output transistor of the differential amplifier and a current mirror arrangement comprising first and second transistors for mirroring the current through the resistor, the first transistor being connected in series with the first and third elements for forming the first current source and the second transistor being connected in series with the second and fourth elements for forming the second current source. A temperature sensing circuit according to any one of the preceding claims, wherein the comparing means comprises first and second transistors having control gates connected to the first and second junctions, respectively, a current mirror arrangement connected between the first supply line and one main electrode of each of the first and second transistors and current source means connected to the other main electrode of each of the first and second transistors, the control signal being supplied via an output connection from the one main electrode of the second transistor. A temperature sensing circuit according to any one of the preceding claims, wherein a resistance in series with switch means is provided across the fourth temperature sensitive element with the switch means being controlled by the output signal of the comparing means to connect the resistance in parallel with the fourth temperature sensitive element when the temperature difference exceeds the predetermined value for preventing the active semiconductor device from being switched on again until the temperature difference has fallen to a second predetermined value below the predetermined value. A temperature sensing circuit according to any one of the preceding claims, wherein the resistance values of the first, second, third and fourth temperature sensing elements are equal when the said difference in the temperatures sensed reaches the predetermined value.

Inventors: EILLEY EDWARD STRETTON, KELLY BRENDAN PATRICK, KORTELING AART GERRIT, LOWIS ROYCE, MOODY PAUL TIMOTHY, EILLEY, EDWARD STRETTON, KELLY, BRENDAN PATRICK, KORTELING, AART GERRIT, LOWIS, ROYCE, MOODY, PAUL TIMOTHY, KELLY, BRENDAN PATRICK, PHILIPS COMPONENTS LTD., LOWIS, ROYCE, PHILIPS COMPONENTS LTD., MOODY, PAUL TIMOTHY, PHILIPS COMPONENTS LTD.
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS UK LTD, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS ELECTRONICS UK LIMITED
Application year: 1991
Main classification: G01K3
IPC codes: H01L29, G01K3, G01K7, H03K17