DEVICE AND METHOD FOR DISCHARGING AN INTERMEDIATE CIRCUIT CAPACITOR AND METHOD FOR MAKING A DEVICE FOR DISCHARGING AN INTERMEDIATE CIRCUIT CAPACITOR

Description

[0001] The present invention relates to a device for discharging an intermediate circuit capacitor, comprising a power converter for electrically supplying an electrical machine driving a vehicle, which has the intermediate circuit capacitor and a discharge device connected in parallel to the intermediate circuit capacitor with a series circuit comprising a controllable switching element and a load resistor, and a high-voltage battery feeding the power converter on the input side.

[0002] In addition, the invention relates to a method for discharging an intermediate circuit capacitor and a method for producing a device for discharging an intermediate circuit capacitor.

[0003] DC link capacitors are used to provide transiently high currents during the operation of a power converter. In the event of a fault or accident involving a vehicle equipped with the power converter, the DC link capacitor must be quickly discharged to a protective extra-low voltage using a discharge device to ensure electrical safety. The charge stored in the DC link capacitor must be converted into heat at least until the protective extra-low voltage is reached. A vehicle control unit signals that the DC link capacitor needs to be discharged, and the control unit simultaneously activates a disconnecting device connecting a high-voltage battery to the DC link capacitor to disconnect this connection. Typically, the power converter does not know whether this disconnection 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 a load current through the load resistor is provided entirely by the high-voltage battery.

[0004] Document EP 2 405 565 A1 discloses a power converter system, comprising: an inverter circuit unit that converts a direct current provided by a direct voltage source into an alternating current, wherein the direct current is provided to the inverter unit via a contactor that allows or interrupts the direct current; a capacitor that smoothes the direct current; a discharge circuit unit that is connected in parallel to the capacitor and has a discharge resistor and a switching element for the discharge resistor connected in series therewith; a discharge control circuit that outputs a control signal for the switching element for discharging. The control signal comprises a first test pulse or a first chain of test pulses followed by an evaluation pause in which the voltage difference achieved during the test phase is compared with a threshold value. If the voltage difference is above the threshold, a long discharge pulse ora long discharge pulse chain is switched on. Otherwise, the discharge is aborted.

[0005] Document DE 11 2012 005937 T5 discloses an electric vehicle with a main battery having an output voltage of 300 V. The main battery is connected to an inverter that converts energy from the main battery into an alternating voltage for driving a motor. A capacitor is connected upstream of the inverter. A discharge circuit is also provided, which is connected in parallel with the capacitor and comprises a series circuit of a semiconductor switch, a resistor, and a PTC thermistor. When a discharge condition is present, a system main relay connected downstream of the battery is opened, and a discharge controller closes the semiconductor switch. The discharge controller opens the semiconductor switch after waiting a predetermined period of time. The period of time is set to a time in which the capacitor's capacitance can be expected to be discharged.Characteristics of the resistor and the PTC thermistor are chosen such that discharge of the capacitor is stopped while the temperature of the PTC thermistor is less than or equal to the Curie temperature.

[0006] Document US 2013 / 0033914 A1 discloses a power converter system comprising: an inverter circuit connected to a DC voltage supply via an openable and closable contactor; a voltage smoothing capacitor connected in parallel with an input of the inverter circuit; a discharge unit connected in parallel with the voltage smoothing capacitor, including a resistor for discharging the voltage smoothing capacitor and a discharge switching element connected in series with the resistor; and a discharge control section that turns on the discharge switching element when the contactor is opened and causes the discharge circuit to discharge.

[0007] Document EP 2 284 982 A1 discloses a discharge circuit for an intermediate circuit capacitor used in a power conversion device, comprising a resistor that discharges the capacitor, a switch connected in series with the resistor and either allowing or interrupting a discharge current from the capacitor to the resistor, a measuring circuit that measures a terminal voltage of the capacitor, and a control circuit that controls the switch to conduct or block. After the control circuit controls the switch to conduct and begins discharging the capacitor through the resistor, it controls the switch to block and stops discharging through the resistor when the terminal voltage of the capacitor measured by the measuring circuit deviates from a predetermined voltage-discharge characteristic.

[0008] While such a device allows for termination of the discharge process when the intermediate circuit capacitor is connected to the high-voltage battery, it requires considerable component complexity, as it requires both the measuring circuit and the control circuit, which must perform complex comparison operations between the measured terminal voltage and the voltage reduction characteristic. This is typically only feasible with a microcontroller, which, as a safety-critical component, must meet high integrity requirements. This results in considerable design and manufacturing complexity for such a device.

[0009] The invention is therefore based on the object of providing a possibility for discharging an intermediate circuit capacitor which can be implemented with less effort and which prevents damage to the intermediate circuit capacitor even if a direct voltage source is not separated from the intermediate circuit capacitor.

[0010] To achieve this object, the invention proposes a device according to claim 1.

[0011] The present invention is based on the idea of ​​controlling the switching element of the discharge device to discharge the intermediate circuit capacitor without any check as to whether the intermediate circuit capacitor is actually disconnected from the high-voltage battery, and of structurally designing the power converter, including the load resistor, so that damage to the intermediate circuit capacitor is avoided even in the worst-case scenario of a load current flowing from the high-voltage battery. According to the invention, the specified duration is selected such that when the capacitor is charged with the maximum continuous voltage of the high-voltage battery, it is discharged to a protective extra-low voltage when the intermediate circuit capacitor is disconnected from the high-voltage battery. The subsequent switching operations then result in only small, additional load currents.However, if the high-voltage battery is not disconnected from the DC link capacitor, further switching operations will attempt to discharge the DC link capacitor, as the disconnection may have occurred in the meantime. Since the converter is structurally designed so that the load resistor heats up while the maximum continuous voltage is applied, but cools down again during the subsequent waiting periods, the load resistor is not damaged during the fixed number of switching operations.

[0012] This advantageously allows a safe discharge strategy to be implemented without any monitoring of the voltage drop across the DC link capacitor. In particular, complex measuring devices and complex evaluation of the DC link voltage, which is subject to high integrity requirements, can be dispensed with. The control of the switching element by the control device is so easy to implement that a microcontroller is unnecessary.

[0013] According to the invention, the fixed number of switching operations is at least two, preferably at least five. According to empirical tests, this number represents a reasonable compromise between the temperature and current resistance of the load resistor and the probability of prompt disconnection of the high-voltage battery from the intermediate circuit capacitor. The fixed duration of a switching operation is expediently at least one second and / or at most three seconds. The waiting time can be at least four seconds and / or at most seven seconds.

[0014] According to the invention, the control device is further configured to control the switching element several times to carry out further switching operations after the last of the switching operations and after the expiry of a fixed second waiting time which is longer than the first waiting time, wherein the second waiting time is provided between each two successive further switching operations. Typically, the load resistance 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 waiting time. Subsequently, in the event that the intermediate circuit capacitor has not yet been disconnected from the high-voltage battery after the first switching operations, an attempt is made by the further switching operations to still discharge the intermediate circuit capacitor as a result of a separation that has occurred in the meantime.However, the longer cooling phases provided by the second waiting period are used for this purpose. The second waiting period can be at least 10 seconds, in particular at least 25 seconds, and / or at most 50 seconds, in particular at most 35 seconds.

[0015] Furthermore, the control device is preferably configured to control the discharge device to perform further switching operations until a deactivation signal is received at the input. Therefore, no fixed number of further switching operations is provided. Rather, these are performed until a deactivation signal is present or a power supply, in particular a low-voltage supply, of the control device fails.

[0016] As already indicated at the outset, it is preferably provided in the device according to the invention that the intermediate circuit capacitor and the high-voltage battery are connected to one another via an electrical connection having a separating device and a control unit is provided which can be operated independently of the first control device and is designed to provide a change in the signal state as an input signal requesting the discharge of the intermediate circuit capacitor to both the control device and the separating device.

[0017] The object underlying the invention is further achieved by a method for discharging an intermediate circuit capacitor according to claim 7.

[0018] Finally, the object underlying the invention is achieved by a method for producing a device for discharging an intermediate circuit capacitor according to claim 8.

[0019] All statements regarding the device according to the invention can be transferred analogously to the methods according to the invention, so that the aforementioned advantages can also be achieved with these.

[0020] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. These are schematic representations and show: Fig. 1 shows a block diagram of an embodiment of the device according to the invention for discharging an intermediate circuit capacitor; and Fig. 2 shows time profiles of a control signal for a switching element of a discharge device and a temperature of a load resistor of the discharge device.

[0021] Fig. 1 is a schematic diagram of an embodiment of a device 1, which has a power converter 2, a high-voltage battery 3, a disconnecting device 4 connecting the high-voltage battery 3 on the input side to the power converter 2, and a control unit 5. The device 1 is, in particular, part of a high-voltage on-board electrical system of an electrically driven vehicle, in particular an electric vehicle or a hybrid vehicle, wherein the power converter 2 is configured to convert a direct voltage of, for example, 500 volts provided by the high-voltage battery 3 into a multi-phase alternating voltage for supplying an electric machine 6 driving the vehicle.

[0022] The power converter 2 comprises an intermediate circuit capacitor 7 and a discharge device 8 connected in parallel with it with a series circuit consisting of a controllable switching element 9 and a load resistor 10. In addition, the power converter 2 comprises a power unit 11 with a plurality of power switching elements connected to form half-bridges, for example IGBTs, which can be controlled in such a way that they convert the direct voltage into the multi-phase alternating voltage.

[0023] The power converter 2 comprises a control device 13, which is configured to control the switching element 9 of the discharge device 8 depending on a signal state at an input 14 of the control device 13. For this purpose, the control device 13 provides a control signal 16 for the switching element 9 at an output 15.

[0024] The control device 13 receives an input signal 17 at input 14 from the control unit 5, which is designed as the higher-level control unit of the vehicle. The control unit 5 outputs the input signal 17 as a deactivation signal, upon receipt of which at input 14, the control device 13 suppresses activation of the switching element 9 for discharging the intermediate circuit capacitor 7. If the control unit 5 requests a discharge of the intermediate circuit capacitor 7, for example due to a malfunction, an accident, or when the vehicle is parked, or if the input signal 17 at input 15 is lost, for example due to a communication error between the control devices 5, 13, the control device 13 activates the switching element 9 to conduct, as described in detail below, whereby the load resistor 10 is connected in parallel with the intermediate circuit capacitor 7.The control unit 5 also provides the input signal 17 to the isolating device 4, which disconnects the intermediate capacitor 7 from the high-voltage battery 3 when discharging is requested.

[0025] The primary function of the device 1 is therefore to disconnect the intermediate circuit capacitor 7 from the high-voltage battery 3 when requested by the control unit 5 and to switch the switching element 9 on, so that the electrical energy stored in the intermediate circuit capacitor 7 is converted into heat at least until a protective extra-low voltage, for example of 60 volts, is reached.

[0026] However, cases are conceivable in which a change in the signal state at input 14 initiates a switching operation by switching element 9, but the high-voltage battery 3 is not disconnected from the intermediate circuit capacitor 7 by means of the isolating device 4. This includes, for example, cases in which the communication between the control unit 5 and the control device 13 is disrupted, so that the control device 13 specifies a discharge of the intermediate circuit capacitor 7 even though the control unit 5 has not requested such a discharge. Furthermore, it is conceivable that the isolating device 4 does not disconnect as intended despite a request from the control unit 5, for example, because mechanical switches of the isolating device 4 are "stuck" in the closed state.

[0027] Fig. 2 shows temporal courses of the signal state denoted by a of the control signal 16 and a temperature ϑ of the load resistor 10.

[0028] At a time t 0 , the signal state of the input signal 17 at the input 14 of the control device 13 changes. In response to this change in the signal state, the control device 13 controls the switching element 9 to carry out a fixed number of five switching operations 18a to 18e with a fixed duration 19 and a fixed first waiting time 20 between two consecutive switching operations 18a to 18e. If the high-voltage battery 3 is successfully separated from the intermediate circuit capacitor 7 by means of the isolating element 4, the intermediate circuit capacitor 7 is discharged from its instantaneous voltage to the protective extra-low voltage within the duration 19 of, in this case, two seconds. In the process, the purely qualitatively represented temperature profile 21 occurs at the load resistor 10. The following switching operations 18b to 18e only contribute to a further discharge of the intermediate circuit capacitor 7 below the protective extra-low voltage.

[0029] However, if the intermediate circuit capacitor 7 remains connected to the high-voltage battery 3, a load current flowing through the load resistor 10 during the switching operation 18a is essentially provided by the high-voltage battery 3, which can provide its maximum continuous voltage depending on the operating state. This leads to a significant heating of the load resistor 10, which is described by the temperature curve 22. The load resistor 10 clearly heats up considerably and cools down slightly during the waiting time 20. If there is no subsequent separation by the isolating device 4, the load resistor 10 continues to heat up during the switching operations 18b to 18e until it reaches a maximum temperature 23 after the switching operation 18e, which it survives without damage.The load resistor 10 and its heat transfer to the power converter 2 are therefore configured such that the load resistor 10 is not damaged when the maximum continuous voltage of the high-voltage battery 3 is applied during a respective switching operation 18a to 18e and during cooling during the waiting times 20. If the isolating device 4 separates the high-voltage battery 3 from the intermediate circuit capacitor 7 between time t 0 and the start of the switching operation 18e, the intermediate circuit capacitor 7 is discharged to the protective extra-low voltage during one of the switching operations 18b to 18e without the maximum temperature 23 being reached subsequently.

[0030] The control device 13 is further configured to control the switching element 9 several times to perform further switching operations 18f, 18g after the last switching operation 18e and after a second waiting time 24, which is longer than the first waiting time 20 and in this case amounts to 30 seconds, has elapsed, with the second waiting time 24 being provided between each two consecutive further switching operations 18f, 18g. The further switching operations 18f, 18g are carried out until the control device 13 receives a deactivation signal at the input 14.The provision of the further switching operations 18f, 18g is based on the consideration that after the first five switching operations 18a to 18e, the probability of a separation of the high-voltage battery 3 from the intermediate circuit capacitor 7 is relatively small, so that the further switching operations 18f, 18g are only carried out if it is ensured by observing the second waiting time 24 that the temperature 23 is not reached even after an indefinite number of further switching operations 18f, 18g. Nevertheless, the switching operations 18f, 18g continue to attempt to discharge the intermediate circuit capacitor 7 in case the isolating device 4 should still achieve a separation.

[0031] The further switching operations 18f, 18g are therefore carried out until the deactivation signal is received at the input 14, but at the latest until a low-voltage power supply 25 of the control device 13 is no longer available.

[0032] The device 1 can therefore be manufactured by a method in which, in a first step, the high-voltage battery 3 for supplying the power converter 2 and the intermediate circuit capacitor 7 for the power converter 2 are provided. In a subsequent manufacturing step, the control device 13 is provided, which is configured to carry out the previously described switching operations 18a to 18g. Subsequently, the discharge device 8 to be controlled by the control device 13 is provided, wherein the load resistor 10 is selected and integrated into the power converter 2 such that the load resistor 10 is not damaged when a maximum continuous voltage is applied to the high-voltage battery 3 for the specified duration 19 and subsequent cooling during a respective waiting period 20, 24.The high-voltage battery 3 is then connected to the discharge device 8 and the intermediate circuit capacitor 7 via the isolating device 4 and the control device 13 is connected to the discharge device 8.

Claims

1. Device (1) for discharging an intermediate circuit capacitor (7), comprising a converter (2) for electrical supply of an electric machine (6) driving a vehicle, which has the intermediate circuit capacitor (7) and a discharge device (8) connected in parallel to the intermediate circuit capacitor (7) with a series connection of a controllable switching element (9) and a load resistor (10), and a high-voltage battery (3) feeding the converter (2) on the input side, wherein the converter (2) has a control device (13) which is configured to control the switching element (9) - depending on a signal state at an input (14) of the control device (13) to perform a fixed predetermined number of switching operations (18a-18e), in which the switching element (9) conducts, with a fixed predetermined duration (19) and a fixed predetermined first waiting time (20) between two consecutive switching operations (18a-18e) and - after the last of the fixed predetermined number of switching operations (18a-18e) and expiration of a fixed predetermined second waiting time (24), which is longer than the first waiting time (20), to repeatedly perform further switching operations (18f, 18g) wherein the fixed predetermined number of switching operations (18a-18e) is at least two, wherein the duration is selected such that when the intermediate circuit capacitor (7) is charged with a maximum continuous voltage of the high-voltage battery (3), it is discharged to a protective low voltage when the intermediate circuit capacitor (7) is disconnected from the high-voltage battery (3), wherein between two consecutive further switching operations (18f, 18g) the second waiting time (24) is provided in each case, wherein the load resistor (10) and its heat transfer to the converter (2) are configured such that the load resistor (10) is not damaged when the maximum continuous voltage of the high-voltage battery (3) is applied during each of the fixed predetermined number of switching operations (18a-18e) and cooling during the or a respective waiting time (20).

2. Device according to claim 1, wherein the fixed predetermined number of switching operations (18a-18e) is at least five.

3. Device according to claim 1 or 2, wherein the fixed predetermined duration (19) of a switching operation (18a-18e) is at least one second and / or at most three seconds, and / or the waiting time is at least four seconds and / or at most seven seconds.

4. Device according to any of the preceding claims, wherein the second waiting time (24) is at least ten seconds, in particular at least twenty-five seconds, and / or at most fifty seconds, in particular at most thirty-five seconds.

5. Device according to any of the preceding claims, wherein the control device (13) is further configured to control the discharge device (8) to perform the further switching operations (18f, 18g) until receiving a deactivation signal at the input (14).

6. Device according to any of the preceding claims, wherein the intermediate circuit capacitor (7) and the high-voltage battery (3) are connected to each other via an electrical connection having a disconnection device (4), and a control unit (5) is provided which is operable independently of the first control device (13) and is configured to provide a change of the signal state as an input signal (17) requesting the discharge of the intermediate circuit capacitor (7) to both the control device (13) and the disconnection device (4).

7. Method for discharging an intermediate circuit capacitor (7) in a converter (2) of a vehicle, which is used for electrical supply of an electric machine (6) driving the vehicle and has the intermediate circuit capacitor (7) and a discharge device (8) connected in parallel to the intermediate circuit capacitor (7) with a series connection of a controllable switching element (9) and a load resistor (10), wherein the switching element (9) is controlled - depending on a signal state to perform a fixed predetermined number of switching operations (18a-18e), in which the switching element (9) conducts, with a fixed predetermined duration (19) and a fixed predetermined first waiting time (20) between two consecutive switching operations (18a-18e) and - after the last of the fixed predetermined number of switching operations (18a-18e) and expiration of a fixed predetermined second waiting time (24), which is longer than the first waiting time (20), to repeatedly perform further switching operations (18f, 18g) wherein the fixed predetermined number of switching operations (18a-18e) is at least two, wherein the duration is selected such that when the intermediate circuit capacitor (7) is charged with a maximum continuous voltage of the high-voltage battery (3), it is discharged to a protective low voltage when the intermediate circuit capacitor (7) is disconnected from the high-voltage battery (3), wherein between two consecutive further switching operations (18f, 18g) the second waiting time (24) is provided in each case, wherein the load resistor (10) and its heat transfer to the converter (2) are configured such that the load resistor (10) is not damaged when the maximum continuous voltage of a high-voltage battery (3) feeding the converter (2) is applied during each of the fixed predetermined number of switching operations (18a-18e) and cooling during the or a respective waiting time (20).

8. Method for manufacturing a device (1) for discharging an intermediate circuit capacitor (7), comprising the following steps: - Providing a high-voltage battery (3) for feeding a converter (2) and an intermediate circuit capacitor (7) for the converter (2); - Providing a control device (13) for the converter (2), which is configured to control a switching element (9) of a discharge device (8) depending on a signal state at an input (14) of the control device (13) to perform a fixed predetermined number of switching operations (18a-18e), in which the switching element (9) conducts, with a fixed predetermined duration (19) and a fixed predetermined first waiting time (20) between two consecutive switching operations (18a-18e) as well as after the last of the fixed predetermined number of switching operations (18a-18e) and expiration of a fixed predetermined second waiting time (24), which is longer than the first waiting time (20), to repeatedly perform further switching operations (18f, 18g), wherein - the fixed predetermined number of switching operations (18a-18e) is at least two, - wherein the duration is selected such that when the intermediate circuit capacitor (7) is charged with a maximum continuous voltage of the high-voltage battery (3), it is discharged to a protective low voltage when the intermediate circuit capacitor (7) is disconnected from the high-voltage battery (3), wherein - between two consecutive further switching operations (18f, 18g) the second waiting time (24) is provided in each case; - Providing the discharge device (8) to be controlled by the control device (13) with a series connection of the controllable switching element (9) and a load resistor (10), - wherein the load resistor (10) is selected and integrated into the converter (2) such that the load resistor (10) is not damaged when the maximum continuous voltage of the high-voltage battery (3) is applied for the fixed duration (19) to discharge the intermediate circuit capacitor (7) and subsequent cooling during the or a respective waiting time (20); and - Connecting the control device (13) to the discharge device (8).