Patent Application
20220029526
The present invention relates to a device and a method for discharging a DC link capacitor according to the independent claims.
A DC link capacitor in a power electronics drive train should be able to be discharged in a controlled manner in the event of an error for safety reasons. A permanent passive discharge by high amperage resistors or a redundant active discharge circuit is used for this. The active discharge circuit can comprise, e.g., numerous resistors, which absorb the energy from the DC link capacitor, and/or a high voltage semiconductor switch, which combines the load resistors with the DC link capacitor as needed. This technology can be used for systems of up to 400V.
For 800V systems, there is the problem that the high voltage semiconductor switches and the load resistors are significantly larger, because the DC link capacitor contains four times the energy at the same capacitance.
Based on this, the present invention results in an improved device and an improved method for discharging a DC link capacitor according to the independent claims. Advantageous embodiments can be derived from the dependent claims and the following description.
A device for discharging a DC link capacitor is presented, wherein the device comprises the following features:
A discharge unit can be a unit or element that enables current to flow in response to a control signal to discharge the DC link capacitor in a discharge process. By way of example, the discharge unit can be a semiconductor component, in particular for power electronics. A control unit can be a unit, in particular an electronic unit, that generates the control voltage in accordance with a predefined rule or circuit topology. The control voltage can be generated numerically or through circuitry. By way of example, the discharge unit can be a component or element in an inverter.
The approach proposed herein is based on the knowledge that a discharge unit can exhibit different connection behaviors, such that when the control input on the discharge unit is supplied with a variable control voltage, an operating point of the discharge unit is activated, also resulting in a reliable discharging of the DC link capacitor. The approach proposed herein has the advantage of being able to reliably discharge the DC link capacitor in different scenarios in response to the control signal or control voltage using technically simple and inexpensive means.
The approach presented herein therefore offers a solution to the problem of how the redundant active discharge according to one embodiment can be obtained by appropriately controlling semiconductor switches serving as part of a discharge unit, e.g. as part of the inverter. Corresponding load resistors and an associated high voltage load resistor are not necessary for this. The power semiconductor(s) forming an embodiment of the discharge unit is/are operated in a linear range for this, for example, and a defined resistance and discharge current for discharging the DC link capacitor are set in this manner.
According to one aspect of the approach presented herein, a particularly simple and functional control process for power semiconductors serving as an embodiment of the discharge unit can therefore be implemented in order to carry out a controlled active discharge of the DC link capacitor. A novel control method for the discharge unit, e.g. in the form of a power semiconductor, is therefore proposed according to one embodiment of the approach presented herein, in order to be able to integrate the function of a redundant, active discharge of the DC link capacitor in the control thereof.
According to a particularly advantageous embodiment, the control unit can be designed to vary the control voltage from a low voltage level to a high voltage level. In this manner, the discharge unit can advantageously be controlled such that the DC link capacitor is discharged as quickly and reliably as possible via the discharge unit.
According to another embodiment, the control unit can also be designed to vary the control voltage evenly, linearly, and/or monotonically, in particular strictly monotonically. As a result, it can be ensured that the discharge unit is activated long enough in an optimal voltage range for the control voltage that the DC link capacitor can be reliably and quickly discharged. At the same time, such a control voltage can be easily and efficiently provided.
An embodiment of the approach proposed herein in which the control unit contains an RC link for determining a voltage level of the control voltage is particularly advantageous. Such an embodiment can be obtained easily in terms of the circuitry.
According to another embodiment of the approach proposed herein, the control unit can be designed to cause a voltage jump in the control voltage during or at the start of the discharge process, and/or cause a voltage jump in the control voltage after completion of the discharge process. Such an embodiment has the advantage of controlling the discharge unit such that there is no intentional discharge of the DC link capacitor prior to the desired start of the discharge process, and/or the discharge unit can be quickly brought to a state in which the DC link capacitor can be recharged after completion of the discharge process.
In this context, an embodiment of the approach presented herein is particularly advantageous in which the control unit is configured to set the control voltage to 0 volts at the start of the discharge process, in particular starting from a minimum value at the control input on the discharge unit prior to starting the discharge process, and/or wherein the control unit is designed to set the control voltage to a minimum value after completion of the discharge process, in particular based on a maximum value at the control input on the discharge unit upon completion of the discharge process. In this manner, the discharge unit can be controlled such that the discharging of the DC link capacitor takes place with the greatest reliability at a desired discharge time or during a discharge time interval, whereas discharging the DC link capacitor at other times or time intervals can be reliably prevented.
According to another embodiment of the approach proposed herein, the discharge unit can be designed as a semiconductor switch, in particular a power semiconductor switch. In this manner, discharging the DC link capacitor can be quickly and easily activated. At the same time, this semiconductor switch be part of an inverter in the DC link, such that components already used in the DC link can be used for an additional function, and additional, separate components can be eliminated, resulting in a very inexpensive implementation of the approach presented herein. Particularly advantageously, the semiconductor switch can also be operated in a linear (characteristic) range, such that the technical functions of the semiconductor switch can be used as efficiently as possible for the discharge process for the DC link, e.g. for converting the electrical energy stored in the DC link capacitor into thermal energy.
The discharge unit can also be a transistor according to another embodiment of the approach presented herein, in particular a MOSFET transistor or an IGBT. Such an embodiment has the advantage of a particularly quick and reliable activation of the discharge unit for discharging the DC link capacitor, wherein one component in the DC link, for example, can also be used as the discharge unit, thus reducing production costs for implementing the approach presented herein.
In another embodiment of the approach presented herein, the control unit can also be designed to determine the control voltage based on the temperature of the discharge unit or a component in the discharge unit. Such an embodiment offers the advantage of activating the appropriate control voltage at an optimal operating point for the discharge unit as quickly as possible, such that the DC link capacitor can be discharged as quickly as possible.
The approach proposed herein can be implemented particularly quickly and economically if a circuit topology is used in which the control unit has at least two resistors, wherein one of the resistors can be connected in parallel to the other resistor, or coupled or can be coupled to the control input on the discharge unit, wherein the control unit has a capacitor that is or can be interconnected between the control input on the discharge unit and a contact on the DC link capacitor, in particular wherein the capacitor has a second switch for a parallel connection of the capacitor between the control input and the contact on DC link capacitor. Such an embodiment of the approach presented herein has the advantage of being able to provide the desired change in the control voltage during the discharge process, or to initiate the discharge process with technically simple means.
An embodiment of the approach proposed herein can be particularly efficiently used in a DC link for transmitting electricity from an energy source to an actuator, wherein the DC link contains a DC link capacitor and a device according to one of the variations presented herein coupled to the DC link capacitor, in particular wherein the device (also) uses at least one component that is also used by an inverter connected to the DC link capacitor. The component used by the inverter can then be used as the discharge unit for the device. Such an embodiment offers the advantage of efficiently, quickly, and reliably discharging the DC link capacitor with the device.
An embodiment of the approach presented herein in the form of a method for discharging a DC link capacitor by means of one of the variations of a device presented herein is also advantageous, wherein the method comprises the following step:
Advantages of the approach presented herein can also be obtained quickly and efficiently with such an embodiment.
An embodiment of the approach presented herein in the form of a control unit is also advantageous, which is configured to execute and/or control the step in a variation of the method presented herein in a corresponding unit.
A control unit can be an electric device that processes electric signals, e.g. sensor signals, and outputs control signals on the basis thereof. The control unit can have one or more hardware and/or software interfaces. A hardware interface can be part of an integrated circuit, for example, in which the functions of the device are implemented. The interfaces can also be integrated circuits or at least partially comprised of discrete components. A software interface can be one of numerous software modules, e.g. on a microcontroller.
A computer program comprising program code is also advantageous, which can be stored on a machine-readable medium, e.g. a solid state memory, a hard disk, or an optical memory, and is used to execute the method according to any of the embodiments described above, when the program us run on a computer or control unit.
The invention shall be explained in greater detail by way of example, based on the attached drawings. Therein:
The same or similar reference symbols are used in the following description of preferred exemplary embodiments of the present invention for the elements having similar functions in the various figures, wherein there shall be no repetition of the descriptions of these elements.
There is a DC link capacitor 140 for preventing or smoothing out fluctuations in the voltage UB in the power supply system 115 when the load to the drive motor 120 fluctuates. This DC link capacitor 140 is usually configured to receive large amounts of energy, in order to absorb these fluctuations in the voltage UB in the power supply system 115. If, however, the electrical system in the vehicle 100 malfunctions, e.g. due to a short circuit or an electrical defect, is may be necessary, for safety purposes, to discharge the DC link capacitor 140 as quickly as possible, in order to minimize the risk of the vehicle 100 catching on fire, or an electrical shock to the occupants of the vehicle 100 caused by the high voltage still contained in the DC link capacitor 140. A protective circuit is usually used for this, such as that represented by the device 105 for discharging the DC link capacitor 140 presented herein.
The device 105 for discharging the DC link capacitor 140 contains a discharge unit 145 and a control unit 150. The discharge unit 145 can be interconnected between terminal clamps 155 on the DC link capacitor 140, wherein the discharging of the DC link capacitor 140 can be controlled by the discharge unit 145 by means of a control voltage applied to a control input 160. The control unit 150 is configured to provide the control voltage to the control input 160 on the discharge unit 145, wherein the control unit 150 provides the control voltage such that the control voltage is varied during the discharge process of, or for discharging (i.e. at the start of the discharging), the DC link capacitor 140.
To initiate discharging the DC link capacitor 140, the corresponding control voltage Uge can be generated in the control unit 150 in response to a malfunction detected by an error detection unit 165 and transmitted to the control unit 150 by means of an error signal 170, e.g. a defect in the electrical system in the vehicle 100, and sent to the control input 160 on the discharge unit 145, as shall be described in greater detail below.
If a power semiconductor is then used as the discharge unit 145, which is, e.g. part of the inverter 130 or a bridge circuit in the inverter 130, there may be difficulties in obtaining a controlled activation of this power semiconductor for ensuring that it only conducts a very small current (a few hundred milliamperes) instead of its nominal current (a few hundred amperes). For this, the gate voltage Uge for this power semiconductor (i.e. the voltage between the gate and the source connection for the power semiconductor used as the discharge unit 145), which sets the current flow I in the power semiconductor, should be set to a specific constant value (Uge.konst). Because the gate voltage Uge necessary for a desired, controlled, low discharge current depends on numerous parameters such as temperature and production tolerances, active discharge by applying a previously defined gate voltage Uge is not possible.
In other words, in the exemplary embodiment shown in
A novel control method according to an exemplary embodiment is proposed to address this problem of obtaining the parameter-dependent gate voltage for a constant and controlled discharge current. In this case, the discharge unit 145, e.g. in the form of a semiconductor, is not controlled with a constant gate voltage, but instead with a variable control voltage, e.g. a gate voltage ramp.
The use of such a variable gate voltage Uge as the control voltage at the control input 160, e.g. in the form of the gate voltage ramp according to the approach presented herein, allows the control voltage to be increased over time with a fixed gradient, such that all of the relevant gate voltages Uge are eventually obtained. This control results in the gate voltage Uge at the semiconductor functioning as the discharge unit 145 at some point opening the electron channel and the optimal discharge current IC flowing through the discharge unit 145 in the form of the semiconductor, independently of its temperature and other parameters, such that the DC link capacitor 145 can be discharged.
As the ramp inclines, it is possible to set how fast the electron channel in the semiconductor functioning as the discharge unit 145 is to opened, and therefore how quickly the discharge process should be carried out for the DC link capacitor 140.
The control circuit or control unit 150 for the semiconductor (functioning as a discharge unit 145) is supplemented with four additional components, specifically a first switch S1, a second switch S2, a capacitor CAD, and a resistor RAD, wherein the two switches S1 and S2, for example, can be closed or opened depending on the error signal 170, by a switch control unit 510. A semiconductor or power semiconductor (e.g. a MOSFET power transistor) is used here as the discharge unit 145, which can also be part of the inverter 130, e.g. a bridge circuit in the inverter 130 for converting the DC voltage UB to AC voltage for operating the drive motor 120.
In normal switching operation (i.e. without errors), the first switch S1 is closed, and the second switch S2 is open. Because the resistor RAD is selected such that its resistance is much greater (e.g. by a factor of 10) than that of the gate resistor Rg, the switching behavior of the semiconductor functioning as the discharge unit 145 is not affected by the parallel connection of the resistors RAD and Rg. The capacitor CAD is inactive when the second switch S2 is open. If the DC link, or the DC link capacitor is to be discharged, this is indicated by the error signal 170, and the control unit 410 activates a new voltage source, in order to switch the control voltage Us to a positive control voltage, wherein the first switch S1 is opened, and the second switch S2 is closed. Because the capacitance of the capacitor CAD is advantageously much greater (e.g. by a factor of 10) than the gate-source capacitance of the semiconductor functioning as the discharge unit 145, the voltage of the gate-source capacitor is immediately adjusted to the voltage of the capacitor CAD (e.g. through a typical jump from −5V to 0V). The rest of the curve for the gate voltage Uge is determined by the charging of the RC time link comprising the resistor RAD and the capacitor CAD, by means of which the desired gate voltage ramp Uge is obtained, e.g. corresponding to the graph in
At time t=0, the discharge process is activated, at which point the gate voltage (curve 600) jumps at time 0 seconds to 0V. The gate voltage ramp subsequently increases. At 610, the channel in the semiconductor functioning as the discharge unit 145 begins to open, and a controlled discharge current (curve 620) flows, with a maximum amperage of 1A. The voltage of the DC link Uce (line 630) decreases through the discharge current IC within a discharge interval td of 0.7 seconds from 800V to 0V. A power loss ploss of 500 W is obtained at an energy loss eloss of 200 J.
One important aspect of the approach presented herein could be that the control of the discharge unit (in the form of a semiconductor here) with a variable control voltage, e.g. a gate voltage ramp, can be used to trigger an active discharging of the DC link capacitor 140. The variable control voltage or ramp can be generated by different variables, e.g. a powered RC link or a defined current source. The great advantage of such an exemplary embodiment is that all of the different types of discharge units, e.g. advantageous types of semiconductors (Si-IGBTs, Si-MOSFETs, and SiC-MOSFETS) can be used in numerous relevant voltages (650V, 1200V, 1700V) to obtain a redundant, control-integrated, active discharge circuit corresponding to the concept proposed herein.
The exemplary embodiments described above and shown in the figures are selected merely by way of example. Different exemplary embodiments can be combined with one another in their entirety or with respect to individual features. One exemplary embodiment can also be supplemented by the features of another exemplary embodiment.
Furthermore, steps can be repeated in the method or carried out in an order other than that in the description.
If an exemplary embodiment comprises an “and/or” conjunction between a first feature and a second feature, this can be read to mean that the exemplary embodiment according to one embodiment comprises both the first feature and the second feature, and comprises either just the first feature or just the second feature according to another embodiment.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/080146 | 11/5/2019 | WO | 00 |
