Method for detecting faults in semiconductor switches

Abstract

The invention relates to a method for checking for faults in semiconductor switches, wherein the semiconductor switches are actuated with a PWM signal having a variable pulse duty factor. In order to reliably and cost-effectively detect faults in the semiconductor switches, it is provided that when the semiconductor switches are operated with a pulse duty factor of 100% or 0%, a current measurement of the entire system is evaluated, and when the semiconductor switches are operated with a pulse duty factor between 0% and 100%, a voltage pulse generated at the semiconductor switches is evaluated.

Description

Technical Field

[0001] The invention relates to a method for checking for faults in semiconductor switches, wherein the semiconductor switches are driven with a PWM signal having a variable pulse duty factor. Background Art

[0002] In order to detect faults, such as short circuits or open circuits, of semiconductor switches (e.g. IGBTs, MOSFETs or bipolar transistors), current or voltage measurements are performed on the semiconductor switch or on the entire system in which the switch is integrated. In current measurements, the branch currents of the semiconductor switch or the total current of the entire system are monitored. In voltage measurements, the voltage drop across the semiconductor switch is monitored.

[0003] It is often necessary to control semiconductor switches with PWM signals having a variable duty cycle in order to ensure load / power regulation at different ambient variables, such as the supply voltage. Fault diagnosis of semiconductor switches with the aid of PWM signals with a variable duty cycle reaches its limits in the following two situations:

[0004] If the pulse duty factor set is very high, for example 98%, then no significant difference from a permanent short circuit results in the current through the semiconductor switch and in the voltage across the semiconductor switch.

[0005] The same applies to very small duty cycles, for example 2%. In this case, no difference from permanent disconnection can be obtained in the opposite case.

[0006] Furthermore, in pure current measurements, a strong dependence on environmental variables, such as the supply voltage, results.

[0007] DE 10 2014 214 156 A1 discloses a method for checking the functional capacity of a first semiconductor switch of a cut-off path of a load current circuit for supplying power to at least one valve coil (L) of a brake system. During operation, the first semiconductor switch is operated in the conducting state and current regulation of the load current circuit is achieved by means of at least one second semiconductor switch actuated in a pulse width modulated manner. During operation of the motor vehicle brake system, the first semiconductor switch is actuated in such a way that it switches to the blocked state when it is functionally capable, and after a predetermined duration of the blocked state, the first semiconductor switch is actuated in such a way that it switches back to the conducting state.

[0008] DE10 2010 042 292A discloses a method for functional testing of an electrical semiconductor switch, wherein an ignition pulse is applied to a control interface of the electrical semiconductor switch by means of a control unit at the ignition time, wherein the current time when the current intensity flowing through the semiconductor switch leaves the zero value is detected, and the time difference between the ignition time and the current flow time is analyzed and evaluated for the functional testing of the semiconductor switch. Summary of the invention

[0009] The object of the present invention is to provide a method of the type mentioned at the outset, with which the presence of a fault in a semiconductor switch can be determined reliably and cost-effectively.

[0010] The present invention therefore proposes a method for checking faults of semiconductor switches, wherein the semiconductor switch is controlled by a PWM signal with a variable duty cycle, wherein when the semiconductor switch is operated with a duty cycle of 100% or 0%, a measured current of the entire system is evaluated, and when the semiconductor switch is operated with a duty cycle between 0% and 100%, a voltage pulse generated at the semiconductor switch is evaluated.

[0011] The invention is therefore limited to a substantially reliable diagnosis of semiconductor switches in PWM operation. If the switch is permanently active or inactive (duty factor 100% or 0%), the measured current of the entire system is evaluated.

[0012] The method according to the invention is independent of most ambient conditions, such as the supply voltage or the load, since it only evaluates the alternating voltage level at the semiconductor.

[0013] It is also possible to detect faults such as high-resistance short circuits, which cannot be detected by pure voltage and current measurements.

[0014] The method according to the invention is independent of most environmental conditions, such as supply voltage or load, since it only evaluates the alternating voltage level on the semiconductor. It is also possible to detect faults, such as high-resistance short circuits, which are therefore not detectable by pure voltage and current measurements.

[0015] For simple but reliable ascertainment of semiconductor malfunctions, the voltage pulses generated at the semiconductor switch are counted for evaluation, wherein a short circuit or an open circuit of the semiconductor switch is inferred from the evaluation if the semiconductor switch generates no voltage pulses.

[0016] In order to ensure reliable and conclusive fault detection at a duty cycle of 100% or 0%, the evaluation of the voltage pulses generated at the semiconductor switch is switched off and the presence of a fault is detected by measuring the current of the entire system.

[0017] The invention also provides a device for carrying out the method according to the invention, wherein the voltage pulses generated at the semiconductor switch are applied from the semiconductor switch via a capacitor to an A / D converter, which applies a digital signal to a counter input of a control unit for evaluation. Only short-term current and voltage pulses are generated in the diagnostic circuit via the capacitor. As a result, the circuit on which the device is based can be realized with a small number of cost-effective components.

[0018] By means of diode interconnections (OR connection), several semiconductor switches can be connected to an A / D converter consisting of two transistors by means of the described diagnostic method. If the PWM activation of the semiconductors to be diagnosed is phase-shifted, all semiconductors can thus be monitored via the A / D converter and the Timer input on the controller. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention is explained in detail below with the aid of the accompanying drawings; in the accompanying drawings:

[0020] Figure 1 A circuit diagram showing a first specific embodiment of the device according to the invention for carrying out the method according to the invention for checking for faults of semiconductor switches, and

[0021] Figure 2 Schematically shows Figure 1 A variant of a device for detecting faults in a circuit having two semiconductor switches. DETAILED DESCRIPTION

[0022] Figure 1 The device shown in is based on a circuit for detecting faults of semiconductor switches, wherein the semiconductor switches are driven with PWM signals having a variable duty cycle. The semiconductor switch S3 is connected to a positive voltage HV+ via a load resistor HC3 with its collector on the one hand and to a ground terminal GND_HC on the other hand. The PWM signal is applied to the input of the semiconductor switch.

[0023] The voltage pulses generated during PWM switching are filtered out from the collector of the semiconductor switch by capacitor C4, since these are only needed for the diagnosis of the switch. This wiring provides another advantage: the remaining circuit is not permanently connected to the supply potential, but only acts in a manner that loads individual pulses. This allows the number of components to be significantly reduced, since no permanent losses occur.

[0024] The capacitor is connected to the ground terminal GND_HC through a voltage divider including resistors R10 and R7. The voltage divider divides the generated voltage pulse into smaller values ​​and reduces the pulse current through C4.

[0025] The pulse penetrating capacitor C4 is passed as a positive pulse to the subsequent signal processing via diode D1. At this location, further diagnostic branches (not shown) can be connected with their cathodes to a disjunction / OR operation (disjunction) (OR connection) (provided these diagnostic branches have a phase shift).

[0026] The Zener diode D9 connected after the OR connection protects the subsequent signal processing components from overvoltage. The component includes two transistors Q1 and Q2, which are connected to form a simple A / D converter via resistors R18, R11 or R6, R9, respectively, and the digital signal "Timer" is generated from the positive pulse at the cathode of the diode D9, which is fed to the counter input of the controller (not shown). The semiconductor switch is diagnosed at the software level on the controller, where the incoming pulses "Timer" are counted. In the event of a fault in the semiconductor switch (possibly a short circuit or an open circuit), no pulses are generated. As a result, the software can recognize the fault and take appropriate measures.

[0027] At a duty cycle of 0% and 100%, this software-level diagnosis is preferably switched off and the fault diagnosis is performed via the total current of the circuit.

[0028] Only short-term current pulses and voltage pulses are generated in the diagnostic circuit by capacitor C4, so that the circuit can be realized with a small number of cost-effective components.

[0029] Figure 1 The working principle of the circuit is Figure 2 The case where two semiconductor switches are tested for faults simultaneously is shown in FIG.

Claims

1. A method for detecting a fault in a semiconductor switch, wherein: The semiconductor switch is controlled by a PWM signal with a variable duty cycle, characterized in that when the semiconductor switch is operated at a duty cycle of 100% or 0%, a measured current of the entire system is evaluated, and when the semiconductor switch is operated at a duty cycle greater than 0% and less than 100%, a voltage pulse generated at the semiconductor switch is evaluated, wherein the voltage pulses generated at the semiconductor switch are counted for evaluation, wherein when the semiconductor switch does not generate a voltage pulse, a short circuit or an open circuit of the semiconductor switch is inferred from the evaluation.

2. The method according to claim 1, wherein: In the case of a duty cycle of 100% or 0%, the evaluation is switched off and a fault is ascertained by means of a measured current of the entire system.

3. A device for performing the method according to claim 1 or 2, wherein: The voltage pulse generated at the semiconductor switch (S3) is tapped from the semiconductor switch via a capacitor (C4) and applied to an A / D converter which applies a digital signal to a counter input of a control unit for evaluation.

4. The device according to claim 3, wherein: The voltage pulse tapped off by the capacitor is converted by means of a diode into a positive pulse which is applied to the A / D converter.

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