RELATED APPLICATIONS

The present application is based on, and claims priority from, German Application No. 10 2018 115 802.5 filed Jun. 29, 2018, the disclosure of which is hereby incorporated by reference herein in its entirety.

This invention relates to a device for discharging an intermediate circuit capacitor, this device comprising a power converter for supplying electricity to an electrical machine powering a vehicle, this power converter having the intermediate circuit capacitor and, connected in parallel with the intermediate circuit capacitor, a discharge device with a series circuit of a controllable switching element and a load resistor, and a high-voltage battery feeding the power converter on the input side.

The invention additionally relates to a process for discharging an intermediate circuit capacitor and a process for producing a device for discharging an intermediate circuit capacitor.

Intermediate circuit capacitors provide high transient currents when a power converter is operated. If a fault occurs or in the case of an accident of a vehicle having the power converter, the intermediate circuit capacitor should be quickly discharged, by means of a discharge device, to a protective extra-low voltage, to ensure electrical safety. The charge stored in the intermediate circuit capacitor must be converted into heat, at least until the protective extra-low voltage has been reached. A controller of the vehicle signals that the intermediate circuit capacitor should be discharged and simultaneously controls an interrupter, which connects a high-voltage battery with the intermediate circuit capacitor, to interrupt this connection. Typically, it is not known, on the part of the power converter, whether this interruption was successful. If this is not the case, controlling the discharge device to discharge the intermediate circuit capacitor through a load resistor can lead to its destruction, since the high-voltage battery is providing a full load current through the load resistor.

The document EP 2 284 982 A1 discloses a discharge circuit for an intermediate circuit capacitor that is used in a power conversion device, this discharge circuit comprising a resistor that discharges the capacitor; a switch that is connected in series with the resistor and that either allows a discharge current to pass from the capacitor to the resistor or interrupts this discharge current; a measuring circuit that measures a terminal voltage of the capacitor; and a control circuit that controls the switch to conduct or block the discharge current. After the control circuit controls the switch to conduct and the discharging of the capacitor through the resistor has begun, the control circuit controls the switch to block and to end the discharge through the resistor if the capacitor's terminal voltage measured by the measuring circuit deviates from specified voltage decrease characteristics.

Such a device does allow the discharge to be stopped if the intermediate circuit capacitor is connected with the high-voltage battery. However it requires a substantial expense for components: first the measuring circuit and second the control circuit, which must perform computationally intensive comparisons of the measured terminal voltage with the voltage decrease characteristics. Typically, this can only be realized by a microcontroller which must, being a safety critical component, meet high integrity requirements. This leads to substantial expense when designing and producing such a device.

Consequently, the invention has the goal of indicating a possible process for discharging an intermediate circuit capacitor, this process being less expensive to implement and preventing damage to the intermediate circuit capacitor, even if a direct voltage source is not disconnected from the intermediate circuit capacitor.

To achieve this goal, the invention proposes a device of the type mentioned at the beginning in which the power converter has control equipment that is set up to control the switching element, depending on a signal state at an input of the control equipment, to perform a fixed predefined number of switching operations, in which the switching element conducts, with a fixed predefined duration and a fixed predefined delay between two successive switching operations, the load resistor and its heat transfer to the power converter being set up so that the load resistor is not damaged if a maximum constant voltage of the high-voltage battery is applied to it during a respective switching operation and it cools for the or a respective delay.

This invention is based on the idea of having the switching element control the discharge device, without any checking of whether the intermediate circuit capacitor has really been disconnected from the high-voltage battery, to discharge the intermediate circuit capacitor, and structurally designing the power converter, including the load resistor, so that even in the most unfavorable case of a load current flowing from the high-voltage battery, damage of the intermediate circuit capacitor is avoided. The fixed predefined duration is expediently selected in such a way that if the capacitor is charged with the maximum constant voltage of the high-voltage battery, the capacitor is discharged to a protective extra-low voltage if the intermediate circuit capacitor is disconnected from the high-voltage battery. Then, the following switching operations only lead to small further load currents. By contrast, if the high-voltage battery is not disconnected from the intermediate circuit capacitor, then further attempts to discharge the intermediate circuit capacitor are made by the further switching operations, since the disconnection still could have occurred in the meantime. The power converter is structurally designed so that although the load resistor does heat up when the maximum constant voltage is applied, it cools back down during the subsequent delays, so the fixed predefined number of switching operations does not damage the load resistor.

This advantageously makes it possible to realize a safe discharge strategy without any monitoring of the voltage drop across the intermediate circuit capacitor. In particular, it is possible to do without expensive measuring devices and a complex evaluation of the intermediate circuit voltage, which would be subject to high integrity requirements. The reason why is that controlling the switching element by the control equipment in this way is so simple to implement that it is possible to do without a microcontroller.

Preferably, the fixed predefined number of switching operations is at least two, preferably at least five. According to empirical tests, this number represents a sensible compromise between the temperature and current stability of the load resistor and the probability that the high-voltage battery is disconnected from the intermediate circuit capacitor close in time to these switching operations. It is expedient that the fixed predefined duration of a switching operation be at least one second and/or no more than three seconds. The delay can be at least four seconds and/or no more than seven seconds.

According to an advantageous further development of the inventive device, the control equipment is further set up to control the switching element after the last of the switching operations and after passage of a fixed predefined second delay, which is longer than the first delay, to perform multiple further switching operations, the second delay being provided between two successive further switching operations. Typically, the load resistor almost reaches its thermal load limit after the last of the first switching operations. After that, more intensive cooling is required for the duration of the second delay. Then, in case the intermediate circuit capacitor had not yet disconnected from the high-voltage battery after the first switching operations, the attempt is still made, by the further switching operations, to discharge the intermediate circuit capacitor because a disconnection has occurred in the meantime. However, longer cooling phases are used for this, which are realized by the second delay. The second delay can be at least 10 seconds, especially at least 25 seconds, and/or no more than 50 seconds, especially no more than 35 seconds.

Furthermore, the control equipment is preferably set up to control the discharge device to perform the further switching operations until a deactivation signal is received at the input. Consequently, no fixed number of further switching operations is provided. Instead, these further switching operations are performed until there is a deactivation signal or until no voltage, in particular low voltage, is supplied to the control equipment.

As was already indicated at the beginning, the inventive device preferably provides that the intermediate circuit capacitor and the high-voltage battery are connected with one another through an electrical connection having an interrupter, and a controller is provided that is operable independently of the first control equipment and is set up to provide a change in the signal state as an input signal requesting the discharge of the intermediate circuit capacitor to both the control equipment and also to the interrupter.

In addition, the invention relates to a process for discharging an intermediate circuit capacitor in a power converter of a vehicle, this power converter being used for supplying electricity to an electrical machine powering the vehicle, this power converter having the intermediate circuit capacitor and, connected in parallel with the intermediate circuit capacitor, a discharge device with a series circuit of a controllable switching element and a load resistor, the switching element being controlled, depending on a signal state, to perform a fixed predefined number of switching operations, in which the switching element conducts, with a fixed predefined duration and a fixed predefined delay between two successive switching operations, the load resistor and its heat transfer to the power converter being set up so that the load resistor is not damaged if a maximum constant voltage of a high-voltage battery feeding the power converter is applied to it during a respective switching operation and it cools for the or a respective delay.

Finally, the invention relates to a process for producing a device for discharging an intermediate circuit capacitor, this process comprising the following steps:providing a high-voltage battery for feeding a power converter and providing an intermediate circuit capacitor for the power converter;providing control equipment for the power converter, this control equipment being set up to control the switching element, depending on a signal state at an input of the control equipment, to perform a fixed predefined number of switching operations, in which the switching element conducts, with a fixed predefined duration and a fixed predefined delay between two successive switching operations;providing the discharge device to be controlled by the control equipment with a series circuit of a controllable switching element and a load resistor, the load resistor being selected and integrated into the power converter in such a way that the load resistor is not damaged if a maximum constant voltage of the high-voltage battery is applied to it for the fixed predefined duration to discharge the intermediate circuit capacitor, followed by cooling for the or a respective delay;connecting the high-voltage battery with the discharge device and the intermediate circuit capacitor; andconnecting the control equipment with the discharge device.

All embodiments of the inventive device can be analogously applied to the inventive processes, so that these inventive processes can also achieve the previously mentioned advantages.

