WO1998036342A1

WO1998036342A1 - Heat protection

Info

Publication number: WO1998036342A1
Application number: PCT/DE1998/000402
Authority: WO
Inventors: Martin Feldtkeller
Original assignee: Siemens Aktiengesellschaft
Priority date: 1997-02-12
Filing date: 1998-02-12
Publication date: 1998-08-20
Also published as: EP0960366A1; EP0960366B1; DE59801762D1; DE19705338C1

Abstract

The invention relates to a heat protection circuit comprising a first bipolar transistor (T1) whose emitter terminal (E) is connected to a terminal (2) for a reference potential whose collector terminal (C) is connected to a first power source (J1) and whose base terminal is connected to the tap (P) of a voltage divider (R1, R2) which is connected by a first terminal (K1) to the terminal (2) of the reference potential. A temperature-controlled switching signal (SS) can be picked off the collector terminal (C), wherein a second bipolar transistor (T2) is provided and whose emitter terminal (E) is connected to the terminal (2) for a reference potential, whose collector terminal (C) is connected to a second power supply (J2) and whose base terminal (B) is connected to a second terminal (K2) of the voltage divider (R1, R2).

Description

description

Thermal protection

The invention relates to a thermal protection circuit with a first bipolar transistor whose emitter connection is connected to a terminal for reference potential, whose collector connection is connected to a first current source and whose base connection with a tap of a voltage divider, which is connected to the terminal for reference potential with a first terminal is connected.

The purpose of thermal protection circuits of this type, which are used, for example, in integrated power circuits, is to switch off circuit components with high power loss when a predetermined temperature threshold is exceeded, in order to protect the overall circuit, usually an IC, from being destroyed in the absence of cooling. For this purpose, a temperature-dependent switching signal is necessary, which has a value at temperatures above the specified temperature threshold, which is clearly distinguishable from values of the switching signal at temperatures below the specified temperature threshold. In such known thermal protection circuits, the strong dependence of the collector current on the temperature is used in bipolar transistors connected in the emitter circuit to generate the switching signal. For a given collector current, the base-emitter voltage of a bipolar transistor operated in an emitter circuit decreases per Kelvin temperature increase by a certain value, which is approximately 2 millivolts per Kelvin temperature increase in silicon-based bipolar transistors. Since the collector current is exponentially sis emitter voltage, if the transistor is in the linear modulation range, there is an exponential dependency of the collector current on the temperature, so that the collector current increases exponentially for a given base emitter voltage and temperature increase. If the current supplied by the current source connected to the collector connection is no longer sufficient to keep the bipolar transistor in the linear modulation range as the temperature rises, the transistor saturates and the collector potential drops rapidly compared to values present at lower temperatures, which makes a clearly distinguishable one Switching signal results. In the case of already known thermal protection circuits which use such temperature dependencies of bipolar transistors in an emitter circuit, a reference voltage source which is as precise as possible and independent of temperature is required for setting the base emitter voltage and thus the transistor operating point. Bandgap circuits can be used, for example, to generate such a reference voltage, as described in Botti / Stefani, "Smart-Power ICsX Springer-Verlag, 1996, page 424 ff. Or in EP 0 618 658 AI from SGS Thomson.

A disadvantage of such circuits is the strong dependency of the temperature accuracy of the protective circuit on the accuracy of the reference voltage source and the considerable circuit complexity.

The object of the invention is to develop the thermal protection circuit mentioned at the outset in such a way that a complex circuit for generating a reference voltage can be dispensed with, so that the disadvantages mentioned above do not arise in particular, and the protection circuit can be implemented with only a few components and thus in a space-saving manner leaves.

This goal is achieved for the thermal protection circuit mentioned in the introduction in that a second bipolar transistor is provided, the emitter connection of which is connected to the terminal for reference potential, the collector connection of which is connected to a second current source and the base connection of which is connected to a second terminal of the voltage divider. The base-emitter voltage of the first bipolar transistor thus results from the base-emitter voltage of the second transistor via the voltage divider. Since the base-emitter voltage of the second transistor decreases with a predetermined collector current, which is given by the second current source, with increasing temperature, the base-emitter voltage of the first transistor also decreases. The operating points of the first transistor can be set, inter alia, via the divider ratio of the voltage divider so that the collector current of the first transistor, which is necessary to keep the first transistor in the linear modulation range, increases with increasing temperature. If this collector current exceeds the current supplied by the first current source, the first transistor saturates, as a result of which the collector potential and the value of the switching signal which can be tapped at the collector connection decrease. The present thermal protection circuit contains only a few components and can be implemented in a space-saving manner, particularly when the current sources are designed using MOS technology.

Advantageous embodiments of the invention are the subject of the dependent claims.

The first transistor advantageously has an emitter area which is larger by a factor m than the emitter area of the second transistor. If the base-emitter voltage is identical, the collector current of the first transistor is m times the collector current of the second transistor, which gives a further possibility for a given first and second current source to set the temperature threshold at which the switching signal drops. To set the operating points of the second transistor, a third current source is provided, which is connected to the base connection and thus simultaneously to the second terminal of the voltage divider.

The second current source, which supplies the collector current of the second transistor, and the first current source, which supplies the collector current of the first transistor, preferably form a current mirror such that the maximum possible collector current of the first transistor is dependent on the collector current of the second transistor.

According to one embodiment of a thermal protection circuit according to the invention, the current mirror consists of a third and fourth transistor, the emitter connections of which are each connected to a first terminal of a supply voltage source, the collector connection of the third transistor also being connected to the collector connection of the first transistor and the collector connection of the fourth transistor is connected to the collector terminal of the second transistor. Furthermore, the base connection of the third transistor is connected to the base connection of the fourth transistor, as a result of which both bases are at a common potential.

The operating point of the second transistor is preferably set by means of a fifth and sixth transistor, the base of the sixth transistor being connected to the collector connection of the second transistor and the emitter connection of the sixth transistor being connected to the base connection of the second transistor. The collector terminal of the sixth transistor is connected to the collector terminal of the fifth transistor, the emitter of which is connected to the first terminal of the supply voltage source. The collector connection and the base connection of the fifth transistor are connected to one another and preferably to the base of the third and fourth transistor. The emitter area of the fourth transistor is preferably larger by a factor n than the emitter area of the third transistor, so that in the described connection of the third and fourth transistor to the current mirror, the collector current supplied by the third transistor corresponds to the maximum collector current flowing through the first transistor corresponds to the nth part of the collector current flowing through the fourth transistor, which corresponds to the amount of the collector current of the second transistor when the base current of the sixth transistor is neglected.

At least one of the transistors that form the current mirror and / or the third current source is advantageously designed as a MOS transistor, for example as a MOS-FET. This embodiment enables a particularly space-saving implementation of the current mirror and / or the third current source. In order to make it easy to distinguish the values of the switching signal before and after the temperature threshold is exceeded, a hysteresis circuit is also provided which amplifies the drop in the values of the switching signal after the temperature threshold has been exceeded.

The thermal protection circuit according to the invention is explained in more detail below using exemplary embodiments. Show it:

FIG. 1 first exemplary embodiment of a thermal protection circuit according to the invention,

FIG. 2 shows a second exemplary embodiment of a thermal protection circuit according to the invention with the first, second and third current sources using bipolar technology,

FIG. 3 third exemplary embodiment of a thermal protection circuit according to the invention with the implementation of the most, second and third power source in MOS technology,

FIG. 4 another embodiment of a thermal protection circuit according to the invention with hysteresis circuit,

FIG. 5 mode of operation of a thermal protection circuit according to the invention according to the second exemplary embodiment, specifying selected currents and voltages at selected temperatures,

FIG. 6 dependence of the collector current on the temperature and the base-emitter voltage in the bipolar transistors used in FIG. 5.

In the figures, unless otherwise stated, the same reference numerals, the same components with the same meaning.

Figure 1 shows an embodiment of a thermal protection circuit according to the invention with a first and second transistor T1, T2, a first, second and third current source J1, J2, J3 and a voltage divider consisting of a first and second resistor R1, R2. In the exemplary embodiment shown, the base B of the first transistor T1 is connected to a center tap P of the voltage divider, which is connected to a terminal 2 with a terminal 2 for reference potential. The base B of the second transistor T2 is connected to a second terminal K2 of the voltage divider. Both the emitter terminal E of the first

Transistor Tl and the emitter terminal E of the second

Transistor T2 is connected to terminal 2 for reference potential, so that the following relationship results for the base-emitter voltage U _BE] _ of the first transistor T1, depending on the base-emitter voltage U _BE 2 of the second transistor T2:

U _BE1 = R2 / (R1 + R2) ^• U _BE2 = a • U _BE2 , where a <1 denotes the voltage divider ratio of the voltage divider.

The collector terminal C of the first transistor, at which a temperature-dependent switching signal SS can be tapped, is connected to a first current source Jl, which determines the maximum collector current flowing through the first transistor Tl. The collector terminal C of the second transistor T2 is connected to a second current source J2, which determines the maximum collector current flowing through the second transistor T2. To set the operating point or the base-emitter voltage of the second transistor T2, a third current source J3 is provided, which is connected to the base terminal B of the second transistor or the second terminal K2 of the voltage divider.

FIG. 2 shows an exemplary embodiment of a thermal protection circuit according to the invention, the current sources J1, J2, J3 shown in FIG. 1 being implemented in bipolar technology and the first and second current sources Jl, J2 being additionally formed by a current mirror. Figure 2 shows a third and fourth transistor T3, T4, which form a current mirror. The collector terminal C of the third transistor T3 is connected to the collector terminal C of the first transistor Tl, the collector terminal C of the fourth transistor T4 is connected to the collector terminal of the second transistor T2. Both the emitter terminal E of the third transistor T3 and the emitter terminal E of the fourth transistor T4 are connected to a first terminal 1 of a supply voltage source. The base B of the third transistor T3 is connected to the base B of the fourth transistor T4, which are thus at a common potential which is determined by the collector-emitter voltage or the base-emitter voltage of a fifth transistor T5, the emitter terminal E of which the first terminal 1 of the supply voltage source and its collector connection C so is probably connected to its own base connection B and to the base connection B of the third and fourth transistors T3, T4. The collector terminal C of the fifth transistor T5 is further connected to the collector terminal C of a sixth transistor T6, whose emitter terminal E is connected to the base B of the second transistor T2 and the second terminal K2 of the voltage divider. The base terminal B of the sixth transistor T6 is connected to the collector terminal C of the second transistor T2. In the circuit described, the operating point of the second transistor T2 is set via the fourth, fifth and sixth transistor T4, T5, T6, the amount of the collector current flowing through the fourth transistor T4 being equal to the collector current flowing through the second transistor T2, with neglect - The base current of the sixth transistor T6 corresponds to. Since both the base terminal B and the emitter terminal E of the third and fourth transistors T3, T4 are at the same potential due to the circuitry, the collector current flowing through the third transistor T3 corresponds to the collector current flowing through the fourth transistor T4 and that flowing through the fourth transistor T4, respectively The collector current is n times the collector current flowing through the third transistor T3 if the fourth transistor T4 is selected such that its emitter area is n times the emitter area of the third transistor T3. A third resistor R3 connected in parallel with the collector-emitter path of the sixth transistor T6 contributes to accelerating the operating point setting of the second transistor T2.

FIG. 3 shows a further exemplary embodiment of a thermal protection circuit according to the invention, the bipolar transistors T3, T4, T5, T6 shown in FIG. 2, which form the current sources, being formed by a first, second, third and fourth MOS-FET, M1, M2, M3, M4 are replaced. The drain terminal D of the first MOS-FET Ml is connected to the collector terminal C of the first transistor T2, the drain terminal D of the second MOS-FET M2 is connected to the collector terminal C of the second ten transistor Tl connected. The source connections S of the first, second and third MOS-FET Ml, M2, M3 are each connected to the first terminal 1 of the supply voltage source, the drain connection D of the third MOS-FET M3 being connected to the drain connection D of the fourth MOS-FET M4 and wherein the drain terminal D of the third MOS-FET M3 is connected to both the gate terminal G of the first, the gate terminal G of the second and the gate terminal G of the third MOS-FET. The gate terminal G of the fourth MOS-FET M4 is connected to the collector terminal C of the second transistor T2, the source terminal S of the fourth MOS-FET M4 is connected to the base terminal B of the second transistor T2 and the second terminal K2 of the voltage divider. In the exemplary embodiment of a thermal protection circuit shown in FIG. 3, the third resistor shown in FIG. 2 is replaced by a fifth MOS-FET M5, the gate connection G of which is connected to the terminal 2 for reference potential. By using transistors in MOS technology, the thermal protection circuit shown in FIG. 3 is less space-consuming to implement than the thermal protection circuit shown in FIG.

FIG. 4 shows the thermal protection circuit shown in FIG. 3, which is additionally expanded by a hysteresis circuit consisting of a sixth, seventh and eighth MOSFET M6, M7, M8. The source terminal S of the sixth MOS-FET M6 is connected to the first terminal 1 of the supply voltage source, the gate terminal G of the sixth MOS-FET is connected to the gate terminals G of the first, second and third MOS-FET M1, M2, M3. The source connections S of the seventh and eighth MOS-FET M7, M8 are each connected to the drain terminal D of the sixth MOS-FET M6, the drain terminal D of the seventh MOS-FET is connected to the terminal 2 for reference potential, and the drain terminal D of the eighth MOS- FET M8 is connected to the base terminal B of the first transistor T1. The gate connections G of the seventh and eighth MOSFETs M7, M8 are connected to the collector connection C of the second transistor T2 and connected to the collector terminal C of the first transistor Tl. The task of the described hysteresis circuit is to amplify the decrease in the collector potential when a predetermined temperature threshold, above which the collector potential of the first transistor T1 drops, in order to make the switching signal more clearly distinguishable from switching signals at lower temperatures. The temperature threshold at which a significant drop is tentials of the first transistor Tl is carried out of the Kollekorpo- ^{'characterized} denotes Ge, that the collector current, which is necessary in order to keep the first transistor Tl in the linear region by the second MOS-FET M2 can no longer be provided. The drain potential of the second MOS-FET M2 and thus the gate potential of the eighth MOS-FET M8 therefore drops compared to the drain potential of the sixth MOS-FET M6. The eighth MOS-FET M8 thus becomes conductive and a current flows through the sixth MOS-FET M6, the eighth MOS-FET M8 and the second resistor R2 of the voltage divider in the direction of the terminal 2 for reference potential. As a result of the current additionally flowing through the second resistor R2, the base-emitter voltage present at the first transistor T1 increases, as a result of which the collector current which is necessary to keep the first transistor T1 in the linear modulation range increases further, which further decreases of the collector potential of the first transistor T1.

FIG. 6 shows the dependence of a collector current I - on the base-emitter voltage U _BE and the temperature T of a bipolar transistor selected as an example, on the basis of which the functioning of a thermal protection circuit according to the invention according to the second exemplary embodiment shown in FIG. 2 is to be explained. The thermal protection circuit shown in FIG. 2 is shown in FIG. 5, neglecting the third resistance, specifying selected currents and voltages at temperatures of T = 350K, T = 400K and T = 450K. The current and voltage values for different temperatures are among each other standing, whereby the values are given from top to bottom with increasing temperature. The following explanation of the mode of operation takes place with the neglect of all base flows. The circuit was dimensioned for a temperature T = 400K. Assuming that the bipolar transistors used meet the characteristics shown in Figure 6 and by selecting the two resistors Rl, R2 of the voltage divider to Rl = 1, 6kΩ and R2 = 8.4kΩ, a working point of the results at a temperature of T = 400K second transistor T2, which is characterized by a base-emitter voltage of 500 mV and a collector current of 100 μA. This operating point is set via the fourth, fifth and sixth transistor T4, T5, T6. If the base current of the sixth transistor T6 is neglected, the collector current of the fourth transistor T4 is likewise 100 μA. According to the voltage divider ratio a = 0.84 of the voltage divider used, the base-emitter voltage of the first transistor is 420 mV. In the example shown, the emitter area of the first transistor is 5 times the emitter area of the second transistor T2, so that the collector current flowing through the first transistor T1 is 5 times the value of 10 μA that can be read from the characteristic curves for a base emitter voltage of 420 mV is. The emitter area of the fourth transistor T4 is 2 times the emitter area of the third and fifth transistor T3, T5, so that the collector current of the fourth transistor is 2 times the collector current of the third and fifth transistor T3, T5, which are each 50μA .

When the temperature is reduced to T = 350 K, in the present circuit there is an operating point for the second transistor T2, which is characterized by a base-emitter voltage of 607 mV and a collector current of 120 μA. The base-emitter voltage of the first transistor T1 necessarily results from the base-emitter voltage of the second transistor T2 and the voltage divider of 510 mV. The characteristic curve belonging to a temperature of 350K results in a such base-emitter voltage a collector current of 5μA, which, however, due to the use of a transistor with an emitter area increased by a factor of 5, is 5 times the collector current readable from the characteristic curve and thus 25μA. The collector current of the third transistor T3 is due to the interconnection of the third and fourth transistors T3, T4 to form a current mirror and the double emitter area of the fourth transistor T4 compared to the third transistor T3 is half the collector current of the fourth transistor T4 and thus 60μA. The collector current of the third transistor T3 specified with 60μA is to be regarded as the maximum possible collector current. Since at the operating point of T = 350K of the first transistor T1, no collector current can flow through it, which significantly exceeds the specified 25μA, only a collector current of 25μA also flows through the third transistor T3, which may cause this transistor to saturate. The first transistor Tl remains in the linear modulation range when the temperature drops to T = 350K, the collector potential and thus the switching signal SS therefore do not change significantly.

When the temperature rises to T = 450K, the second transistor T2 has an operating point which is characterized by a base-emitter voltage of 392mV and a collector current of 80μA. According to the division ratio a = 0.84, the first transistor therefore has a base emitter voltage of 329mV and a collector current of 80μA, which, according to the enlarged emitter area, is 5 times the value of 16μA that can be read from the characteristic curve. The maximum collector current flowing through the third transistor T3 is, according to the emitter area ratio of the third and fourth transistor T3, T4, half of the collector current flowing through the fourth transistor T4 of 80μA, namely 40μA. The maximum collector current of 40μA supplied by the third transistor T3 is lower than that for a base emitter voltage of 329mV with 5 times the emitter area. appropriate collector current of 80μA. The collector current supplied by the third transistor T3 is not sufficient to keep the first transistor T1 in the linear modulation range at the given base-emitter voltage of 329 mV. The first transistor T1 thus goes into saturation and the collector potential and thus the switching signal SS drops rapidly compared to collector potential values in the linear modulation range. This fact is applied emitter voltage against normal transistor characteristics, in which the collector current as a function of the ^'collector, significantly. In the linear control range there is only a slight dependency of the collector current on the collector-emitter voltage or on the collector potential, while in the saturation range there is a strong dependency of the collector current on the collector potential or the collector-emitter voltage.

Instead of the bipolar transistors used for the first and second transistors in the exemplary embodiments, it is also possible to use MOS transistors, wherein the voltage divider must be dimensioned accordingly.

Claims

claims

1. Thermal protection circuit with a first bipolar transistor (Tl), the emitter connection (E) of which is connected to a terminal (2) for reference potential, the collector connection (C) of which is connected to a first current source (Jl) and the base connection of which is tapped (P) a voltage divider (Rl, R2), which is connected to the terminal (2) for reference potential with a first terminal (Kl), a temperature-dependent switching signal (SS) being tapped at the collector terminal (C), characterized in that a second bipolar transistor (T2) is provided, the emitter connection (E) of which is connected to the terminal (2) for reference potential, the collector connection (C) of which is connected to a second current source (J2) and the base connection (B ) is connected to a second terminal (K2) of the voltage divider (Rl, R2).

2. Thermal protection circuit according to claim 1, characterized in that the first transistor (Tl) has an iter area which is larger by a factor m than the emitter area of the second transistor (T2).

3. Thermal protection circuit according to claim 1 or 2, characterized in that the second terminal (K2) and the base terminal (B) of the second transistor (T2) are connected to a third current source (J3).

4. Thermal protection circuit according to one of the preceding claims, characterized in that the first current source (Jl) and the second current source (J2) are formed by a current mirror.

5. Thermal protection circuit according to claim 4, characterized in that the current mirror consists of a third and fourth transistor (T3, T4), the emitter connections (E) are each connected to a first terminal (1) of a supply voltage source that the collector connection (C) of the third transistor (T3) with the collector terminal (C) of the first transistor (Tl) and the collector terminal (C) of the fourth transistor (T4) with the collector terminal (C) of the second ^' transistor (T2) and that the base terminal (B) of the third transistor (T3) is connected to the base terminal (B) of the fourth transistor (T4).

6. Thermal protection circuit according to one of claims 3 to 5, characterized in that the third current source has a fifth transistor (T5) and a sixth transistor (T6), the emitter terminal (E) of the fifth transistor (T5), the collector terminal ( C) and base terminal (B) are connected to each other as well as to the collector terminal (C) of the sixth transistor, to the first terminal (1) of the supply voltage source and that the base terminal (B) of the sixth transistor (T6) to the collector terminal (C) of the second transistor is connected.

Thermal protection circuit according to Claims 5 and 6, characterized in that the base connections of the third, fourth and fifth transistor (T3, T4, T5) are connected to one another.

8. Thermal protection circuit according to one of claims 5 to 7, characterized in that the emitter area of the fourth transistor (T4) is larger by a factor n than the emitter area of the third transistor (T3).

9. Thermal protection circuit according to one of claims 5, 7 or 8, characterized in that at least one of the transistors (T3, T4) which form the current mirror is a MOS transistor.

10. Thermal protection circuit according to one of claims 6, 7 or 8, characterized in that at least one of the transistors (T5, T6) which form the third current source is a MOS transistor.

11. Thermal protection circuit according to one of the preceding claims, characterized in that it has a hysteresis circuit.

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