Control device and method for discharging a DC link capacitor, power converter and vehicle

Abstract

The present invention relates to a control device (1) for discharging a DC link capacitor (2) by means of a discharge device (3), the discharge device comprising a load resistor (5) and a switching component (6) connected in series with the load resistor (5), the control device further comprising a generator unit (12) and a control unit (11), the generator unit being configured to generate a pulse width modulation drive signal for controlling the switching component using a set duty cycle, and the control unit being configured to stabilize the duty cycle using the desired discharge current flowing through the load resistor (5) over a time average.

Description

Technical Field

[0001] The invention relates to a control device for discharging a DC link capacitor by means of a discharge device, wherein the discharge device comprises a load resistor and a switching component connected in series with the load resistor.

[0002] Furthermore, the invention relates to a power converter, a vehicle and a method for discharging a DC link capacitor. Background Art

[0003] The DC link capacitor can store a DC voltage, which is converted into a multi-phase AC current via a power converter for powering an electric motor, for example. When a discharge event occurs, such as when a vehicle including the power converter fails or an accident occurs, when the electric motor is disconnected, the safe operation goal of the system is to quickly discharge the DC link capacitor to a safe voltage value (non-hazardous voltage) to ensure electrical safety. When a discharge device including a load resistor and a switching component connected in series with the load resistor is used, the switching component is closed when a discharge event occurs to convert the energy stored in the DC link capacitor into heat energy.

[0004] According to German patent publication number DE102013224884A1, such a device for discharging a DC link capacitor in a voltage converter comprises an electrical load and a discharge regulator. The discharge regulator is designed to discharge the DC link capacitor by applying a default discharge current to the electrical load. The discharge regulator comprises an operational amplifier, which uses the discharge current as a discharge current for discharging the DC link capacitor and thereby actuates a power transistor.

[0005] In this way, a substantially stable power consumption of the load resistor can be achieved. However, the power transistor or switching component is operated in the working region, and the aforementioned operating characteristics cause a considerable portion of the stored energy to be converted into heat energy during operation. It can be seen that the effect of uniform power consumption on the load resistor is achieved by the variable power consumption on the switching circuit. Therefore, the switching component must be designed considering the condition of high power consumption, thereby increasing the complexity of the component design. Summary of the invention

[0006] It is therefore an object of the present invention to provide a low complexity alternative for discharging a DC link capacitor.

[0007] The present invention achieves this object by a control device of the aforementioned type, which comprises a generator unit and a control unit, wherein the generator unit is configured to generate a pulse width modulated actuation signal for the switching component with a set working cycle, and the control unit is configured to set the working cycle according to the expected discharge current flowing through the load resistor within a time average value.

[0008] The present invention is based on the design consideration of operating the switching element only in the switching mode, and actuating the switching element by a pulse width modulation driving signal. In which, the clocked current flowing through the load resistor is preset based on the working cycle. In this case, on the one hand, in terms of the power loss of the switching element, the switching element in this design is preferably designed to operate only in the switching mode, not in the working mode, and on the other hand, the load resistor can be designed based on the power loss caused thereon in the time average value.

[0009] The expression "time average" is to be understood in the context of the invention as meaning a moving average or weighted average over several periods of the pulse width modulated actuation signal. In this way, the value of the aforementioned average is ideally used only for clocking compensation in the form of pulse width modulation. In this operation, the period duration of the actuation signal is advantageously selected to be lower than the thermal time constant of the load resistor, if necessary additionally taking into account the cooling conditions at the load resistor. Due to the thermal capacitance equivalence involved, the load resistor averages the thermal power dissipation caused by the current.

[0010] Preferably, the control unit is arranged to set the duty cycle in dependence on the discharge current caused by the load resistor resulting from a substantially constant power consumption over said time average. The load resistor thus only has to allow a maximum power consumption corresponding to the constant discharge capacity. Compared to the continuous current flowing through the load resistor, the load resistor thus need not be designed with considerations of how to cope with high energy and occasional peak power or peak current. It can therefore be chosen to be implemented with a smaller and lower cost solution. Due to the thermal capacity of the load resistor, the current during the on-time of the actuation signal can flow without damaging the complex resistor, since the on-time duration is smaller than the thermal time constant of the load resistor. The power consumption can be determined based on the maximum energy level stored in the DC link capacitor and the predetermined discharge time.

[0011] In the control device according to the invention, it can be particularly advantageously provided that the control unit is arranged to set the duty cycle as a function of voltage information describing the DC link voltage drop across the DC link capacitor. In the process, a larger discharge current, i.e. an actuation signal with a larger duty cycle, is generally pre-set with a reduced DC link voltage.

[0012] According to a first preferred embodiment of the present invention, the control unit comprises a target value setting unit, which is configured to set the discharge current target value according to the voltage information. The discharge current target value is preferably selected to utilize the voltage drop across the load resistor, and the quadratic power consumption of the load resistor is compensated (P=U 2 / R). When the discharge current target value corresponds to the quotient of the load resistance power consumption and the DC link voltage, the discharge current target value is optimally compensated.

[0013] The target value determination unit can be configured to set the discharge current target value according to the inverse, in particular negative value, of the DC link voltage. The use of a negative value approximates the inverse in the form of a reciprocal value of the DC link voltage, which allows the relevant circuit to be implemented with a simpler circuit. In other words, the negative value reflects the waveform curve of the DC link voltage. The target value setting unit is further advantageously configured to set the discharge current target value as the sum of the inverse and an offset.

[0014] According to a second preferred embodiment, the control unit is configured such that the duty cycle is calculated as a ratio of the product of the resistance value of the load resistor and the expected power consumption of the load resistor to the square value of the DC link voltage. In this case, the compensation of the square of the DC link voltage is generated by calculation. Further, the duty cycle can be directly calculated.

[0015] In an example of the control device of the present invention, it can be further configured as follows: the control unit is configured to set the working cycle according to the current information, which describes the time average value of the current flowing through the load resistor. The control device adjusts (conditions) the clocked current flowing through the load resistor by setting the current information. To achieve this purpose, the control device can include a low pass unit, which is configured to generate current information from a current measurement, which describes the instantaneous current flowing through the load resistor. In other words, the low pass unit adjusts the current measurement that is clocked by the pulse width modulated actuation signal. The low pass unit can be a resistor-capacitor circuit (RC circuit), a holding component, or another smoothing circuit or an averaging circuit.

[0016] In order to regulate the discharge process, the control device can include a controller, which is configured to set the duty cycle according to the deviation of the time average value of the current compared to the discharge current target value. The controller may be a controller with a proportional unit (Pbehavior), a proportional-integral unit (PI behavior) and a proportional-integral-differential unit (PID behavior). In this process, if the current flowing in the time average value has a tendency to decrease while the discharge current target value remains the same, especially in the case of a voltage drop, it is usually compensated by a correspondingly larger duty cycle. The aforementioned operation takes into account the current flowing through the load resistor, so that in particular the discharge current can be controlled to a large extent independently of the changes caused by the resistance value of the load resistor. This advantageously reduces the accuracy requirements for the load resistor.

[0017] In an example of a control device according to the invention, the control unit is preferably designed as an analog circuit. For the purpose of application to a vehicle, the advantage of the aforementioned disclosure is that standard components can be applied to the realization of the control device, thereby minimizing the complexity of the verification of the Automotive Safety Integrity Level (ASIL). In an alternative embodiment, the control unit can be designed as a microprocessor. This is particularly applicable when calculating the discharge current according to the aforementioned second embodiment, in which the determination of the discharge current target value and the current information is particularly omitted. However, it is conceivable that the application of the aforementioned calculation is implemented in the form of an analog circuit.

[0018] The invention additionally relates to a power converter comprising a DC link capacitor, a discharge device, a load resistor, a switching component connected in series with the load resistor, and a control device according to the invention for actuating the discharge device, by means of which the DC link capacitor can be discharged.

[0019] The invention further relates to a vehicle, in particular an electric vehicle or a hybrid vehicle, comprising an electric motor for driving the vehicle and a power converter according to the invention, arranged to provide electric power to the electric motor.

[0020] Finally, the present invention relates to a method for discharging a DC link capacitor using a discharge device, the discharge device comprising a load resistor and a switching component connected in series with the load resistor, wherein a pulse width modulated actuation signal for the switching component is generated according to a set working cycle, and the working cycle is set according to a discharge current flowing through the load resistor within a time average value.

[0021] All embodiments for a control device according to the invention can be applied analogously to a power converter according to the invention, a vehicle according to the invention and a device according to the invention, so that the aforementioned advantages can also be achieved thereby. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Further advantages and details of the invention become apparent from the following description of exemplary embodiments and the accompanying drawings. The accompanying drawings are schematic. In the accompanying drawings:

[0023] Figure 1 Shown is a circuit diagram of a first exemplary embodiment of a control device according to the present invention,

[0024] Figure 2 A block diagram of a second exemplary embodiment of a control device is shown, and

[0025] Figure 3 Shown is a block schematic diagram of an exemplary embodiment of a vehicle according to the invention, which vehicle includes a power converter according to an exemplary embodiment of the invention. DETAILED DESCRIPTION

[0026] Figure 1 Shown is a circuit diagram of a first exemplary embodiment of a control device 1 for discharging a DC link capacitor 2 via a discharge device 3 .

[0027] Specifically, the DC link capacitor 2 is powered by a DC voltage source 4 and can be connected in parallel with a power unit of a power inverter to smooth the waveform of the instantaneous current. When a discharge event occurs, the discharge device 3 has the performance of reducing the voltage across the DC support capacitor 2 to less than 60 volts within 2 seconds. To achieve this purpose, the discharge device 3 includes a load resistor 5 and a switching component 6 connected in series with the load resistor 5. For example, the switching component 6 takes the form of an electronic switch, such as an insulated gate bipolar transistor (IGBT) or a power metal-oxide-semiconductor field-effect transistor (MOSFET). For discharge, the DC link capacitor 2 is usually disconnected from the DC voltage source 4 by means of a one-pole or two-pole switching device 7.

[0028] The discharge process of the DC link capacitor 2 is controlled by the control device 1. To achieve this purpose, the control end 8 of the discharge device 3 or the switching component 6 is connected to the output end 10 of the control device 1 through the actuation unit 9. The actuation unit 9 amplifies the actuation signal at the output end 10 by turning on or off the switching component 6; when the switching component 6 is turned on, the current 19 discharges the DC link capacitor 2 via the load resistor 5.

[0029] The control device 1 comprises a control unit 11 and a generator 12 which is arranged to generate a pulse width modulated actuation signal at an output 10 for switching components, using a duty cycle set via the control unit 11 .

[0030] The control unit 11 is configured to set the duty cycle in operation according to the discharge current caused by the substantially constant power consumption generated by the load resistor within the time average value. This enables the load resistor 5 to have a substantially constant electrical load and thermal load. At the same time, the switching component 6 is also operated only in a conducting or blocking mode and is thus manipulated outside the active or amplifying region. In this way, the power consumption of the switching component 6 is minimized compared to the in-phase control of the current 19 flowing through the load resistor 5, so that it can be designed to meet significantly reduced power requirements.

[0031] The control unit 11 comprises a target value setting unit 24, which is configured to set a discharge current target value according to voltage information describing a DC circuit voltage 13, which is a voltage drop across the DC link capacitor. To achieve this purpose, the control device 1 comprises a first input terminal 14, through which the control device 1 receives voltage information. In the present embodiment, the first input terminal 14 is connected to a voltage detection device 15 for this purpose. In an exemplary implementation, the voltage detection device 15 is designed as a voltage divider, which is connected in parallel with the DC link capacitor 2, and at a voltage dividing point 18 of the voltage detection device 15, a resistor 16 and a resistor 17 of the voltage divider divide the DC link voltage 13 into a voltage proportional to the DC link voltage 13. In the process, as a practice, the resistor component 16 can be implemented as a resistor with a resistance value of 500 kilo-ohms (kΩ), and the resistor component 17 can be implemented as a resistor with a resistance value of 820 ohms (Ω). For example, the resistor elements 16 and 17 are each composed of one resistor element or a plurality of resistor elements connected to each other.

[0032] In order to set the current target value, the target value setting unit 24 receives the voltage information provided at the first input terminal 14. The target value setting unit 24 includes an operational amplifier 25, which inverts the DC link voltage 13 described by the voltage information. The specific working method is that after finding the negative value of the DC link voltage 13, a fixed offset is added to the inverse of the DC link voltage 13 obtained. To achieve the above purpose, the negative input terminal of the operational amplifier 25 is connected to two resistors 26 and 27, and the positive input terminal of the operational amplifier 25 is connected to a constant voltage source 28, so as to form an inverting amplifier using the above combination. For example, the resistor 26 and the resistor 27 both have a resistance value of 20 kilo-ohms. In this case, the discharge of the DC link capacitor 2 is achieved by the following method: the current 19 flowing through the load resistor 5 is selected to compensate the quadratic dependence of the power consumption on the load resistor 5 to a large extent.

[0033] The control unit 11 is further configured to set the duty cycle according to current information describing the time average of the current 19 flowing through the load resistor 5. In order to set the current information, the control device 1 receives the current value via the second input terminal 20, and the control device 1 is connected to the current detection device 21, and the current detection device 21 is connected in series with the discharge device 3. In this embodiment, the current detection device 21 is formed by a shunt resistor 22. Since the current measurement value describes the pulse control of the current 19 achieved by the switch of the switching component 6, a low-pass unit 23, for example in the form of a resistor-capacitor circuit (RC circuit) is connected downstream of the second input terminal 20. The above circuit system controls the current value according to the current information proportional to the weighted average of the current 19 or the short-term moving of the current 19.

[0034] In order to adjust the duty cycle, the control unit 11 comprises a controller 29, which is arranged to set the duty cycle as a function of the deviation from the discharge current target value within the time average value of the current 19. In the exemplary embodiment, the controller 29 implements the determination of the deviation in the form of a proportional-integral controller (PI controller, PI control behavior). For this purpose, the controller 29 comprises an operational amplifier 30, which is provided with two impedances 31, 32 at its negative input and is fed with the output signal of the target value setting unit 24 at its positive output. Figure 1The application of the impedances 31 and 32 in FIG. 2 is purely exemplary. For example, the impedance 31 is 20 kilo-ohms, and the impedance 32 is formed by a resistor having a resistance value of 100 kilo-ohms and a capacitor having a capacitance value of 1 nanofarad (nF) in parallel. In an alternative embodiment, the controller 29 can be replaced by a proportional controller (Pcontrol behavior) or a proportional-integral-differential controller (PID control behavior).

[0035] In another alternative embodiment, the output signal of the target value setting unit 24 and the current information are initially provided to a subtractor unit (not shown) of the controller 29, for example, a differential amplifier and an operational amplifier are connected to each other as a subtractor unit, and the output signal of the subtractor unit is provided to the negative input terminal of the operational amplifier 30. In this process, the positive input terminal of the operational amplifier 30 is connected to the ground potential.

[0036] The output signal of the controller 29 is provided to the generator unit 12. The generator unit 12 comprises a comparator 33 having a positive input terminal receiving the output signal of the controller 29 and a negative input terminal connected to a signal generator 34. The signal generator 34 generates, for example, a sawtooth or triangular voltage for generating a pulse width modulated output signal at the output terminal 10.

[0037] The control device 1 additionally comprises a deactivation unit 35, by which the output of the activation signal at the output 10 can be deactivated in response to a deactivation signal at a third input 36 of the control device 1. To achieve the above, the deactivation unit 35 comprises a transistor circuit 37. In response to the deactivation signal, the output 10 is switched to ground potential via a resistor by the transistor circuit 37 so that the switching element 6 of the discharge device 3 is permanently switched off and no discharge of the DC circuit capacitor 2 occurs, in particular during normal operation. When the transistor circuit 37 is switched off, the activation signal is allowed to pass through the output 10.

[0038] In this exemplary embodiment, the control unit 11 and the generator unit 12 are both designed as analog circuits. In another alternative implementation, the control unit 11 and / or the generator unit 12 can also be designed as a microcontroller.

[0039] Figure 2 A block diagram of a second embodiment of a control device 1 is shown, which corresponds to the above-described exemplary embodiment, except for the changes described below, in which the same or similar components are marked with the same reference numerals.

[0040] Here, the control unit 11 is arranged to calculate the duty cycle directly from the ratio of the product of the resistance value of the load resistor 5 and its desired power consumption to the square value of the DC link voltage 13. To achieve this, the control unit 11 comprises a quadrator unit 38, which squares the DC link voltage 13 obtained at the first input terminal 14 based on the voltage information. The output signal of the quadrator unit 38 is fed to a divider unit 39, which receives a signal describing the product of the load resistor 5 and the desired power consumption of the load resistor 5 from a memory unit 40 and divides the signal by the output signal of the quadrator unit 28. The output signal of the divider unit 39 then describes the duty cycle, which enables a substantially constant power consumption of the load resistor 5 and is fed to the generator unit 12. It is expressed by the formula, where R is the resistance value, the desired power consumption is P, and the duty cycle is D:

[0041]

[0042] In order to simplify the calculation of the duty cycle, in the second embodiment, the control unit 11 and the generator unit 12 are designed as microcontrollers, wherein the orthogonal unit 38 and the divider unit 39 are implemented by the microcontroller and appropriate software routines. In an alternative implementation, the orthogonal unit 38 and the divider unit 39 can also be implemented by suitable analog computing circuits.

[0043] Figure 3 1 is a block diagram of an exemplary embodiment of a vehicle 40 including an exemplary embodiment of a power converter 41. The power supply 41 is configured to provide power to an electric motor 42 to actuate the vehicle 40. Figure 1 and 2 The same reference numerals are used to indicate the same or similar components.

[0044] The power converter 41 comprises a DC link capacitor 2 and a discharge device 3, through which the DC link capacitor 2 can be discharged, and a control device 1 according to any of the aforementioned exemplary embodiments, which can actuate the discharge device 3. To achieve this purpose, the DC link capacitor 2 is powered by a DC voltage source 4 in the form of a high-voltage battery, which is connected to the DC voltage side of the power converter 41 via a switching device 7. The power converter 41 further comprises an actuation unit 9, a voltage detection device 15 and a power detection device 21. For the purpose of clarity, the components of the power converter 41 are not individually labeled here. Figure 3 middle.

[0045] The DC link capacitor 2 smoothes the instantaneous current generated by the power supply unit 43 in the power converter 41. The power supply unit 43 converts the DC voltage provided by the DC voltage source 4 into a polyphase power supply for the electric motor 42. In one embodiment, the polyphase power supply is a three-phase or six-phase AC current. The power supply unit 43 includes three or six half-bridge circuits, which are connected in parallel to the DC link capacitor 2 and each consists of two power semiconductor switching components connected in series.

[0046] The control unit 1 receives a fault signal from a control module 44 of the vehicle 40. The control module 44 is arranged to detect a discharge event, such as an accident, an insulation fault, or other shut-off event of the vehicle 40 and / or the electric motor 42. If a discharge event occurs, or if the fault signal does not occur for other reasons, a change in the signal state of the fault signal at the input 36 or the absence of the fault signal triggers the discharge of the DC link capacitor 2 by the discharge device 3.

Claims

1. A control device (1) for discharging a DC link capacitor (2) via a discharge device (3), the discharge device comprising a load resistor (5) and a switching component (6) connected in series with the load resistor (5), characterized in that: The control device further comprises a generator unit (12) and a control unit, wherein the generator unit is configured to generate a pulse width modulation drive signal for controlling the switching component (6) using a set working cycle, and the control unit (11) is configured to set the working cycle according to an expected discharge current flowing through the load resistor (5) within a time average value, wherein the control unit (11) is configured to set the working cycle using a power consumption caused by the discharge current on the load resistor (5), wherein the power consumption is constant in real value within the time average value, The control unit (11) is configured to set a duty cycle based on voltage information describing a DC link voltage drop across the DC link capacitor (2); The control unit (11) comprises a target value setting unit (24), which is configured to set a discharge current target value according to the voltage information. 2 . The control device according to claim 1 , wherein the target value setting unit is configured to determine the discharge current target value according to a reciprocal of a DC link voltage. 3 .

3. The control device according to claim 2, wherein the target value setting unit (24) is configured to stabilize the discharge current target value according to a sum of the reciprocal and a compensation.

4. The control device according to claim 3, wherein the control unit (11) is configured to calculate the duty cycle according to a ratio of a product of a resistance value of the load resistor and a desired power consumption of the load resistor to a square value of the DC link voltage (13).

5. The control device according to any of the preceding claims, wherein the control unit (11) is configured to set the duty cycle as a function of current information describing a time average of a current (19) flowing through the load resistor (5).

6. A control device according to any one of claims 1 to 3 and claim 5, wherein the control unit (11) comprises a controller (29), which is configured to set the working cycle according to the deviation between the time average value of the current (19) and the discharge current target value.

7. The control device according to claim 5 or 6, comprising a low-pass unit (23) which is arranged to generate current information based on a current calculation value describing the instantaneous current (19).

8. The control device (1) according to any one of the preceding claims, wherein the control unit is designed as an analog circuit, a microprocessor or a combination thereof.

9. A power converter (41), comprising a DC link capacitor (2), a discharge device (3), a load resistor (5), a switching component (6) connected in series with the load resistor (5), and a control device (1) according to any one of claims 1 to 8 for actuating the discharge device (3), by means of which the DC link capacitor (2) is discharged.

10. A vehicle (40) comprising an electric motor (42) for actuating the vehicle (40) and a power converter (41) according to claim 9, the power converter being arranged to provide electric power to the electric motor (42).

11. A method for discharging a DC link capacitor (2) using a discharge device (3), the discharge device comprising a load resistor (5) and a switching component (6) connected in series with the load resistor (5), the method comprising the following steps: The working cycle is set by using the power consumption caused by the discharge current on the load resistor (5), wherein the power consumption is constant in real value within a time average value; generating a pulse width modulated actuation signal for the switching component according to a set duty cycle; setting a discharge current target value and a duty cycle according to voltage information describing a DC link voltage drop across the DC link capacitor (2); and The working cycle is set according to the discharge current flowing through the load resistor (5) within the time average value.

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