WO2001011775A1

WO2001011775A1 - Protective circuit for an electrical circuit element

Info

Publication number: WO2001011775A1
Application number: PCT/DE2000/002662
Authority: WO
Inventors: Peter Gold
Original assignee: Siemens Aktiengesellschaft
Priority date: 1999-08-05
Filing date: 2000-08-04
Publication date: 2001-02-15
Also published as: DE19936857A1

Abstract

This invention relates to a protective circuit (7) for an electrical circuit element (3) which is mounted in series with an electrical consumer (2) and has a control inlet (5) for receiving a circuit signal in order to control the circuit which is to be protected (3), and a logical gate (8) which is connected onto the control input (5) and to the circuit which is to be protected (3) and on the output side to the circuit which is to be protected (3), thereby controlling said circuit (3) in accordance with the circuit signal received on the input side. The inventive protective circuit also comprises a voltage tap (9) on the output side of the electrical circuit which is to be protected (3). Said tap is connected to the input (E2) of the gate (8), to close down the circuit (3) in order to be protected when a short circuit or resistance failure of the electrical consumer (2) which is to be protected occurs, thereby preventing damage to the circuit (3) which is to be protected.

Description

description

Protection circuit for an electrical switching element

The invention relates to a protective circuit for an electrical switching element according to the preamble of claim 1.

Electrical consumers, such as the excitation coil of a fuel injection valve, are usually actuated by a switching element which is connected in series with the consumer, so that the load current of the consumer also flows through the switching element. If the consumer is short-circuited, the thermal power dissipated in the switching element rises suddenly, which can lead to destruction of the switching element without corresponding protective measures.

From the German patent application P 44 18 232, a protective circuit for such a switching element is known which, in the event of a short circuit in the consumer, destroys the

Switching element prevented. For this purpose, the protective circuit continuously measures the load current flowing through the switching element by means of a current measuring device and compares it with a predetermined upper limit value in order to detect a short circuit of the consumer in good time before the switching element is destroyed. When the predetermined limit value for the load current is exceeded, a logic circuit is then activated, which is connected upstream of the control input of the switching element to be protected and blocks the switching element, so that the short-circuit current is interrupted.

A disadvantage of the known protective circuit described above is the fact that in the event of a short circuit in the consumer before the predetermined limit value of the load current is exceeded, a considerable loss of energy occurs in the switching element to be protected. The limit value for the load current must therefore be set so low that the before the switching element is blocked, energy loss that occurs does not damage the switching element. On the other hand, the limit value for the load current must not be so low that a switch-off takes place even under heavy loads by the consumer.

Another disadvantage of the known protective circuit described at the outset is the fact that the time response of the switching element to be protected is impaired by the signal propagation time of the upstream logic circuit. The switching element to be protected therefore only reacts to the switching signal present at the input when it has passed through the logic circuit.

The invention is therefore based on the object of providing a protective circuit for an electrical switching element which, in the event of a short-circuit in the consumer, minimizes the energy loss which arises in the switching element without impairing the time response of the switching element.

The problem is solved, starting from the known protective circuit described at the outset, according to the preamble of claim 1, by the characterizing features of claim 1.

The invention includes the general technical teaching of tapping the voltage on the output side of the switching element to be protected instead of the load current flowing via the switching element and tracing it back to a logic gate which blocks the switching element to be protected in the event of a short circuit.

The term switching element is to be understood here and in the following in general terms and includes, within the scope of the invention, all electrical or electronic components whose resistance can be controlled, the switching elements being able to be implemented in a conventional discrete design or in an integrated form as part of an integrated circuit. ) cυ M r

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Range from 1 kΩ to 100 kΩ, while the capacitor offers values between 1 pF and 100 nF.

The connection of the voltage tap provided on the output side of the switching element to the logic gate for controlling the switching element is preferably carried out with the interposition of a protective resistor which enables operation at different supply voltages.

In a variant of the invention, a diagnostic line is also provided, which is connected to the voltage tap on the output side of the switching element to be protected, in order to be able to monitor the fault behavior of the consumer by means of an external diagnostic circuit.

In the preferred embodiment, the logic gate for controlling the switching element to be protected consists of an AND gate with an inverting output, a first input of the AND gate with the switching input of the entire circuit and a second input of the AND gate with that at the Output side of the switching element provided voltage tap is connected.

Other advantageous developments of the invention are characterized in the subclaims or are shown below together with the description of the preferred embodiment of the invention with reference to the figures. Show it:

1 as a preferred exemplary embodiment of the invention, a protective circuit for a power transistor for switching the excitation coil of a fuel injection valve,

Figure 2 shows a modified embodiment for a protective circuit according to the invention.

The block diagram shown in FIG. 1 shows an excitation coil 2 connected in parallel with a diode 1 for cυ cυ IV) M ¹ cn o Cπ O cπ o Cπ

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a capacitor Cl = 33 nF is also connected, which forms an RC element with the resistor Rl, which ensures a time-delayed response of the AND gate 8 to the switching signal at the control input 5.

The function of the circuit shown is explained below with reference to the above description of the structural circuit structure, it being initially assumed that the excitation coil 2 is not short-circuited.

In the idle state of the circuit, the switch 6 sets the control input 5 to ground, so that a LOW level is present at the first input E1 of the AND gate 8. In addition, the MOSFET switching transistor 3 blocks in the idle state, so that a high level is present at the second input E2 of the MOSFET switching transistor 3. At the inverting output A of the AND gate 8, a HIGH level thus appears in the idle state of the circuit, but this does not lead to the MOSFET switching transistor 3 being switched on, since the AND gate 8 has an open-drain output.

When the switch 6 is activated, the control input 5 assumes the potential U = + 5 V, so that the MOSFET switching transistor 3 switches on immediately, whereupon the potential of the second input E2 of the AND gate 8 assumes a LOW level almost immediately. At the same time, the capacitor C1 is charged via the resistor R1, so that the first input El changes from the LOW level to the HIGH level after a predetermined charging time. Accordingly, a HIGH level appears at the inverting output A of the AND gate 8 both in the charging phase and after the charging of the capacitor C1, but this has no effect on the circuit, since the output A of the AND gate 8 is an open drain output.

When the switch 6 is subsequently opened, the potential of the control input 5 drops again to ground, whereupon the MOSFET switching transistor 3 blocks almost immediately. This has to cυ cυ t NJ h- ¹ P »

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So the detection of short circuits of the excitation coil 2 by an external diagnostic circuit.

In addition, the circuit shown in FIG. 2 has, as a further special feature, a capacitor C2 = 100 pF, which connects the second input E2 of the AND gate 8 to ground and buffers voltage fluctuations. This prevents a short-term dip caused by electromagnetic interference from the voltage detected by the voltage tap 9 between the MOSFET switching transistor 3 and the excitation coil 2 not leading to a HIGH level at the second input E2 of the AND gate 8, since the MOSFET -Switching transistor 3 would then block.

The embodiment of the invention is not limited to the preferred exemplary embodiments specified above. Rather, a number of variants are conceivable which make use of the solution shown, even in the case of fundamentally different types.

Claims

claims

1. Protection circuit (7) for an electrical switching element (3), which is connected in series with an electrical consumer (2), with

a control input (5) for receiving a switching signal for controlling the switching element (3) to be protected and

a logic gate (8) which is connected on the input side to the control input (5) and on the output side to the switching element (3) to be protected in order to control the switching element (3) in dependence on the switching signal recorded on the input side, characterized by one on the output side of the Protective switching element (3) provided voltage tap (9) which is connected to an input (E2) of the logic gate (8) in order to block the switching element (3) to be protected in the event of a short circuit or a drop in resistance of the consumer (2) to be switched and thereby prevent damage to the switching element (3).

2. Protection circuit (7) according to claim 1, that the voltage tapping (9) for controlling the logic gate (8) between the switching element (3) to be protected and the consumer (2) to be switched is provided.

3. Protection circuit (7) according to claim 1 or 2, characterized in that the switching element to be protected (3) is connected on the input side substantially directly to the control input (5) in order to achieve a fast response of the switching element (3) to the switching signal.

4. Protection circuit (7) according to one of the preceding claims, characterized in that the logic gate (8) is connected on the input side via a time delay element (Rl, Cl) to the control input (5) in order to delay the gate (8) response. to reach the switching signal.

5. Protection circuit (7) according to claim 4, d a d u r c h g e k e n n z e i c h n e t that the time delay element consists of a resistor (Rl) and a capacitor (Cl).

6. Protection circuit (7) according to one of the preceding claims, characterized in that the voltage tap (9) provided on the output side of the switching element (3) is connected via a protective resistor (R2) to the input (E2) of the logic gate (8) is.

7. Protection circuit (7) according to one of the preceding claims, that the voltage tap (9) is connected on the output side to a diagnostic line (10) in order to be able to monitor the frequency of errors of the switched consumer (2) by an external diagnostic circuit.

8. Protection circuit (7) according to one of the preceding claims, characterized in that the logic gate (8) is an AND gate, with a first input (El) of the AND gate (8) with the control gear (9) and a second input (E2) of the AND gate (8) is connected to the voltage tap (9) provided on the output side of the switching element (3).

9. Protection circuit (7) according to claim 8, so that the second input of the AND gate (8) is connected to a capacitor (C2) in order to buffer brief voltage dips at the second input of the AND gate (8).

10. Protection circuit (7) according to claim 9, d a d u r c h g e k e n n z e i c h n e t that the AND gate (8) has an open-drain output.