GB2278025A

GB2278025A - Circuit arrangement for switching a current-consuming apparatus on and off

Info

Publication number: GB2278025A
Application number: GB9408879A
Authority: GB
Inventors: Wolfram Breitling; Rolf Falliano; Ulrich Geisler
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-05-14
Filing date: 1994-05-05
Publication date: 1994-11-16
Anticipated expiration: 2014-05-05
Also published as: FR2705508A1; GB9408879D0; GB2278025B; DE4316185A1; FR2705508B1

Description

V - 2278025 Circuit arrangement for switching a current-consuming

aDDaratus on and off

Prior art

The invention proceeds from a circuit arrangement for switching a currentconsuming apparatus on and off, according to the preamble of the main claim. A circuit arrangement of the type described is known from DE-PS 33 38 764, in which the voltage drop at a switching transistor is supplied to a monitoring circuit. In dependence upon the overshooting of a preset threshold of the voltage drop at the switching transistor and in dependence upon time conditions, the monitoring circuit, if need be, effects cut-off of the switching transistor control signal. The control circuit supplies the control signal always at the greatest possible level to the control input of the switching transistor so that the transistor, while the current-consuming apparatus is switched on, switches through as completely as possible. power loss arising at the switching transistor is at-its lowest in said operating state. The voltage drop at the switching transistor is a measure of the current flowing through the transistor and hence of the current flowing through the load. The known circuit arrangement therefore enables the preselection of a maximum permissible current flow through the current-consuming apparatus. Defects which have arisen in the load circuit may be detected. A load defect which manifests itself in the form of a sudden short circuit in the current-consuming apparatus, or an immediate shorting of the circuit arrangement upon connection of a currentconsuming apparatus may lead to destruction of the switching transistor because there is a departure from the safe 1 operating range, which is evident from the characteristics field of the transistor and in which the voltage lying across the transistor is to be regarded as a function of the current flowing through the transistor. The known circuit arrangement is not short-circuit-proof.

The object of the invention is to indicate a circuit arrangement for switching a current-consuming apparatus on and off which, on the one hand, enables a current limiting or a cut-off of the current flowing through the current-consuming apparatus and, on the other hand, is short-circuitproof.

Said object is achieved by the features indicated in the main claim.

Advantages of the invention The circuit arrangement according to the invention has the advantage that the current flowing through the switching transistor is already limited by the purposeful limiting of the switching transistor control signal to a maximum permissible value. Said measure allows the current to be fixed at a maximum permissible value which still lies within the safe operating range of the switching transistor specified by the manufacturer. A possibly temporarily occurring overcurrent, which would exist up to the response of the monitoring circuit, may no longer occur and destroy the transistor.

Advantageous developments and improvements of the circuit arrangement according to the invention are indicated in the sub-claims.

In a simple refinement, the monitoring circuit cuts off the at least one control signal source. In another advantageous refinement, it is provided that the monitoring circuit limits or totally suppresses the control signal already generated by 2 the control signal source. Said refinement offers advantages particularly in the case of a circuit comprising microprocessor-based control. In said refinement, the microprocessor need not constantly monitor for the presence of a cut-off condition. Furthermore, there is no need for an immediate reaction, e.g. cut-off of the control power source.

An advantageous development provides the use of a timing element, which delays the activation circuit or the reaction to a detected overcurrent for a preset time. With said measure, it is possible temporarily to permit a higher current which may occur, for example, as a making current in certain current-consuming apparatuses.

Another advantageous development provides that the switching transistor control signal, under the control of another timing element, is temporarily raised in.order to permit a higher maximum current. With said measure, an increased makingcurrent demand of a current-consuming apparatus is likewise permitted.

In an advantageous refinement, it is provided that the monitoring circuit contains a bipolar transistor which is preferably thermally coupled to the switching transistor. The thermal coupling provides a compensation of temperature influences upon the preset electrical-values. An unintentional raising of the threshold of the monitoring circuit or a lowering of the voltage lying across the switching transistor despite a current increase may be eliminated.

Further advantageous developments and improvements of the circuit arrangement according to the invention are indicated in the following description in conjunction with the drawings.

3 Drawings Figures 1 to 3 show different refinements of a circuit arrangement according to the invention for switching a current-consuming apparatus on and off.

Figure 1 shows a current-consuming apparatus 10 which may be switched on and off by means of a switching transistor 11. The transistor 11 connects the current-consuming apparatus 10 to a power source, not shown in detail in Figure 1, which is connected between a first connection 12 of the currentconsuming apparatus 10 and a circuit earth 13. The transistor 11 connects a second connection 14 of the current-consuming apparatus 10 to the circuit earth 13. The second connection 14 of the current-consuming apparatus 10 is connected by a resistor 15 to a base terminal of a transistor 16, whose emitter is connected to circuit earth 13 and whose collector is connected both to an operating stage 17 and to a resistor 18. The operating stage 17 is preferably disposed inside a microprocessor 19, which has a power supply connection 20 to which the resistor 18 is connected. The operating stage 17 supplies a signal to a first timing element 21, which supplies a first cut-off signal 22 to a first control power source 23 and a second cut-off signal 24 to a second control power source 25. The first control power source 23 produces a first output signal 26 and the second control power source 25 produces a second output signal 17. The output signals 26, 27 are combined via resistors 28, 29 into the control signal 30 of the transistor 11. The control signal 30 is supplied to a control input 31 of the transistor 11. The control input 31 is connected by a resistor 32 and by a capacitor 33 to circuit earth 13.

The dashed line connecting the transistors 11, 16 in Figure 1 symbolizes a thermal coupling of the two transistors 11, 16.

4 m The first control power source 23 is activated by an activation signal 34, which is further supplied to a third timing element 35 which supplies a signal to the second control power source 25.

In Figure 2, parts which are identical to the parts shown in Figure I are provided with the same reference numerals. In Figure 2, the activation signal 34 is supplied to an inputoutput port 36, which is contained in the microprocessor 19 and produces the first output signal 26. The inputoutput port 36 may, by means of the activation signal 35, be switched over from providing the first output signal 26 to receiving an input signal 37 which likewise arises at the resistor 28. The input-output port 36 is further connected to a detection arrangement 38. The second timing element 35 activated by the activation signal 34 supplies a signal to an output stage 39, which is contained in the microprocessor 19 and passes on an operating signal 40 to the emitter terminal of the transistor 16. The collector terminal of the transistor 16 is connected to the control input 31 of the transistor 11. The base of the transistor 16 is connected by a limiter diode 41 to circuit earth 13.

In Figure 3, parts which are identical to parts shown in Figure 1 are provided with the same reference numerals. In Figure 3, the collector terminal of the transistor 16 is connected to a third timing element 42, which contains a resistor-capacitor combination 43, 44 and a further transistor 45. The emitter terminal of the further transistor 45 is connected to the collector terminal of the transistor 16. The collector terminal of the further transistor is connected to the control input 31 of the transistor 11. The base terminal of the further transistor 45 is connected to the resistorcapacitor combination 43, 44, which is connected between the circuit earth 13 and the connection between the input-output port 36 and the resistor 28.

The circuit arrangement shown in Figure 1 operates as follows.

The switching transistor 11 is provided for switching the currentconsuming apparatus 10 on and off. The command to switch on or off is contained in the activation signal 34. The activation signal 34 causes the first control power source 23 to produce the first output signal 26, which is supplied via the resistors 28, 32, wired up as a voltage divider, to the control input 31 of the transistor 11. In Figures 1 to 3, as an example of a switching transistor, a MOS field-effect transistor is shown, whose control input 31 is activated by a voltage. Instead of a MOS fieldeffect transistor, it is possible to provide, as switching transistor 11, a bipolar transistor which admittedly may likewise be activated by a preset voltage but which is preferably activated by a preset loaddependent current. -

The MOS field-effect transistor 11 shown in Figures 1 to 3 is activated by a voltage preset by the resistors 28, 32 acting as a voltage divider, with it being assumed that the first control power source 23 operates as a voltage source with a known output voltage. The preset voltage at the control input 31 of the transistor 11, taking the characteristic of the transistor 11 into account, allows a maximum possible current to flow in the break gap of the transistor 11 lying between the second connection of the current-consuming apparatus 10 and the circuit earth 13. Limiting the maximum possible current in the break gap of the transistor 11 enables safe operation of the transistor 11 even in the event of a short circuit, without any risk of the transistor 11 departing from the safe operating range defined by the curve of maximum permissible power loss in the characteristic field. The maximum permissible current is to be preset after consulting the characteristic specified by the manufacturer for the safe operating range given a known operating voltage of the currentconsuming apparatus 10. The circuit arrangement according to the invention reliably prevents destruction of

6 j the transistor 11 in the event of accidental shorting of the apparatus 10 or a defect in the apparatus 10 equivalent to a short circuit, as well as in the event of accidental connection of the transistor 11 directly to the operating voltage source (not shown). In the absence of adequate cooling of the transistor 11, however, it should be ensured that the transistor 11 is disconnected before thermal overload occurs.

Disconnection is effected by a monitoring circuit, which monitors the voltage lying across the break gap of the transistor 11 and compares it to a preset threshold value. In Figure 1, the monitoring circuit comprises the resistors 15, 18, the transistor 16 and the operating stage 17. The voltage arising across the break gap of the transistor 11 is a measure of the current flowing through the break gap. The threshold value is to be fixed in such a way that an overcurrent detected by the monitoring circuit still lies below the maximum permissible current. Only in such a case may the monitoring circuit respond. In the embodiment shown in Figure 1,, the transistor 16 acts as a voltage amplifier, which supplies an output signal to the operating stage 17 which cuts off the first control power source 23. The operating stage 17 is preferably disposed inside the microprocessor 19. In said refinement, the operating stage is an input-output port of the microprocessor 19 which is operated as an input. The voltage amplified by the transistor 16 has to be raised to a level at which the microprocessor port may detect a change of level. The operating threshold, in said refinement, is therefore dependent both on the voltage amplification by the transistor 16 and on the operating threshold of the microprocessor port.

A temporarily increased operating current of the currentconsuming apparatus 10 during the switching-on process, during which the increased current may lie below the maximum Possible current flow through the break gap of the transistor 11 but above the threshold preset by the monitoring circuit, enables 7 suppression of the first cut-off signal 22 for the first control power source 23. A simple realization of said cut-off is achieved by the first timing element 21 which, after the apparatus 10 is switched on, suppresses the first cut-off signal 22 for the time preset by the timing element 21. An increase of the current is possible by increasing the control signal 30 to a value which permits a higher maximum current flow in the break gap of the transistor 11. A simple realization is provided by the temporary connection to system by the second timing element 35 of the second control Power source 25, which produces the second output signal 27. For the time preset by the second timing element 35, the resistor 29 is connected in parallel to the resistor 28. The voltage at the voltage divider, which from now on contains the resistors 28, 291 32, increases so that the maximum permissible current in the break gap of the transistor 11 is also increased. If in said refinement a time delay already provided for the response of the monitoring circuit is not in itself sufficient, it is possible, in said refinement also, to provide the first timing element 21 which after a time delay passes on both the first cut-off signal 22 and the second cutoff signal 24 intended for the. second control power source 24.

The thermal coupling between transistor 11 and transistor 16 shown by the dashed line in Figure 1 offers the advantage that the short-circuit strength of the circuit arrangement according to the invention is maintained even at an increased temperature. As temperatures increase, so too does the resistance of the break gap of the transistor 11 and hence the voltage drop over the break gap given constant current. At the same time, the amplification of the transistor 16 increases so that, already at lower current values, the threshold of the monitoring circuit is reached.

If, in the circuit arrangement shown in Figure 1, the microprocessor 19 is used, continuous processor activity for monitoring the operating stage 17 is required to enable the 8 fastest possible reaction to the occurrence af an overcurrent situation. In the construction of the circuit arrangement according to the invention shown in Figure-2, the monitoring circuit cuts off the control signal 30 without processor activity. In said embodiment, the monitoring circuit contains the resistor 15, the transistor 16, the limiter diode 41 and the output stage 39, which is contained in the microprocessor 19 and produces the operating signal 40. In the event of an overcurrent, the transistor 16 limits the control signal 30 or cuts it off completely by shorting towards circuit earth 13. A simple refinement therefore provides that the emitter terminal of the transistor 16 is connected directly to circuit earth 13. In the refinement shown in Figure 2, in which the emitter terminal is connected to the output stage 39 of the microprocessor 19, an increased current of the currentconsuming apparatus 10, e.g. during the switching-on process, may be taken into account. The output stage 39 activated by the second timing element 35 connects the emitter terminal of the transistor 16 internally in the microprocessor 19 to circuit earth 13 only after the time preset by the second timing element 35. The circuit configuration shown by way of example in Figure 2 requires the limiter diode 41, which limits the base voltage of the transistor 16 to a value at which a continuous switching-through of the transistor 16 in view of the voltage across the voltage divider 28, 32 does not occur. The limiter diode 41 therefore displaces the response voltage of the monitoring circuit.

A diagnosis of the circuit arrangement shown in Figure 2 is enabled by the input-output port 36, which is disposed in the microprocessor 19 and may be switched over from operation as a control power source to operation as an input circuit. In its function as a control power source, the input-output port 36 produces the first output signal 26. In its function as an input port, the input signal 37 is read in. when the monitoring circuit has responded, the input signal falls below a lower operating level of the input circuit. Said state is 9 detected by the detection arrangement 38. An operational requirement is the capacitor 33 which is connected in parallel to the resistor 32. The capacitor 33 is to be dimensioned in such a way that the voltage across the resistor 32, and hence the voltage of the control signal 30, decreases during the read-in process at most to a value which is not yet evaluated by the input-output port 36 operating as an input circuit as a response signal of the monitoring circuit.

The mode of operation of the circuit arrangement according to the invention shown in Figure 3 substantially corresponds to that of the circuit arrangement shown in Figure 2. Compared to the previously described circuit, the microprocessor 19 is further relieved by dispensing with the second timing element 35 and the output stage 39. The second timing element 35 is replaced by the third timing element 42, which contains the resistor-capacitor combination 33, 44 and the further transistor 45. The third timing element 42 prevents an immediate response of the monitoring circuit when the currentconsuming apparatus 10 is switched on. During the switchingon process, during which the first output-signal 26 assumes a voltage level preset by the input-output port 36, the still discharged capacitor 44 prevents a switching-through of the further transistor 45 and hence a reduction or cut-off of the control signal 30 by the transistor 16. The time of the third timing element 42 is dependent on the value of the resistor 43, the capacitance of the capacitor 44 and the voltage of the first output signal 26. In the circuit arrangement according to the invention shown in Figure 3, only one connection to the microprocessor 19 is required, by means of which the currentconsuming apparatus 10 is switched on and off and by means of which the diagnosis as to whether the monitoring circuit has responded is effected.

A particularly advantageous development of the device according to the invention provides for complete integration in a microprocessor module. only the switching transistor 11 S remains as a single external component. The microprocessor 19 then contains two connections, of which one is to be connected to the second connection 14 of the current-consuming apparatus 10 and the other to the control input 31 of the switching transistor 11.

11

Claims

Claims

Circuit arrangement for operating a current-consuming apparatus, having a transistor whose break gap is connectable in series to the currentconsuming apparatus, having a monitoring circuit which reduces or completely cuts off a control voltage of the transistor in dependence upon a voltage which may be picked off at the break gap of the transistor, and having a circuit for activating the transistor, characterized in that a control signal (30) produced by the control circuit (23, 25, 28, 29, 32) for the transistor (11) is fixed at a value which limits a current flowing through the break gap of the transistor (11) to a given maximum value.

2. Circuit arrangement.according to claim 1, characterized in that the monitoring circuit (15. 16, 18) cuts off a control power source (23, 25) contained in the control circuit (23, 25, 28, 29, 32).

3. Circuit arrangement according to claim 1, characterized in that the monitoring circuit (15, 16) limits or totally suppresses the control signal (30).

4. Circuit arrangement according to one of the preceding claims, characterized in that a timing element (21, 35, 42) is provided for activating the monitoring circuit (15, 16, 17, 18).

12 Circuit arrangement according to one of the preceding claims, characterized in that a timing element (35).is provided which, during the time preset by the timing element (35), enables the preselection of an increased control signal (30) corresponding to a higher maximum current.

9.

6. Circuit arrangement according to one of the preceding claims, characterized in that the monitoring circuit (15, 16, 17, 18) contains a bipolar transistor (16) for voltage amplification.

7. Circuit arrangement according to claim 6, characterized in that the bipolar transistor (16) is thermally coupled to the transistor (11).

Circuit arrangement according to claim 11 characterized in that the control power source (23, 25) is an inputoutput port of a microprocessor (19), which supplies an output signal (26, 27) to a voltage divider (28, 29, 32).

Circuit arrangement according to claim 8, characterized in that the voltage divider (28, 29, 32) contains a capacitor (33) for at least temporary storage of the control signal (30).

10. Circuit arrangement according to claim 8 and 5, characterized in that a first and a second control power source (23, 25) are provided in the microprocessor (19) and that, upon switching-on of the current-consuming apparatus (10), the second control power source (25) is caused by the timing element (35) for a preset time to produce a second output signal (25).

11. Circuit arrangement according to claim 8, characterized in that the microprocessor (10) contains an input-output 13 port (36) which is operable as a control power source and as an input stage.

12. Circuit arrangement according to claim 11, characterized in that a timing element-(42, 43, 44, 45) is provided for suppressing the response of the monitoring circuit (15, 16) upon switching-on of the currentconsuming apparatus (10), a resistor-capacitor combination (43, 44) contained in the timing element (42) being-connected to the connection between the input-output port-(36) of the microprocessor (19) and the resistor (28) of the voltage divider (28, 42).

13. Circuit arrangement according to one of claims 1 to 7, characterized by integration in a microprocessor (19), with the switching transistor (11) being provided as an external element.

14. A circuit arrangement substantially as herein described with reference to the accompanying drawings 14 I