Safe active discharge circuit for inverters in vehicles

Abstract

An active discharge circuit for an electric vehicle inverter, the active discharge circuit being intended to be connected in parallel with a DC link capacitor connected between a positive line and a negative line of a DC power link, wherein the circuit comprises a dissipation current source, a switch connected in series with the current source between the DC lines, and a controller connected to the switch and arranged to apply an activation signal in response to a control signal, the activation signal placing the switch in an on-state, wherein the current source is configured to draw a discharge current and dissipate any energy stored in the DC link capacitor when the switch is in the on-state. As long as the switch is closed by the activation signal, the current source draws a constant current and dissipates power, and the voltage across the DC link capacitor decreases linearly.

Description

Technical Field

[0001] The present invention relates to a safe active discharge circuit arranged in parallel with a DC link capacitor connected between the positive and negative lines of a DC power link. Background Art

[0002] Inverters used in vehicles (and in many other applications) receive a high-voltage input and provide alternating current to drive, for example, AC machinery. The inverter's switches must operate at high frequencies and require a stable and reliable DC input voltage. To this end, a capacitor (called a DC link capacitor) is typically connected between the positive and negative lines to absorb the ripple caused by switching in the inverter. Thus, the DC link capacitor ensures a stable and reliable voltage across the inverter.

[0003] The high-voltage input voltage is received via a DC link, which is essentially two power lines connected to a DC power source (e.g., a high-voltage battery) via mechanical circuit breakers (relays). These circuit breakers enable the inverter to quickly disconnect from the battery in the event of a shutdown, such as due to a key-off, power failure, or vehicle crash. However, the DC link capacitors remain charged, and for safety reasons, this charge needs to be discharged.

[0004] The traditional, straightforward solution is to hardwire a passive resistor across the capacitor. The resistance is high enough to prevent excessive power loss during normal operation. Therefore, discharging the capacitor through this resistor takes a relatively long time, typically on the order of several minutes. Under normal circumstances, such as key-off operation, this is generally not a problem. However, in certain situations, such as crashes, on-site safety regulations require a faster discharge time, for example, around five seconds.

[0005] To provide such rapid discharge, a much lower discharge resistor can be connected across the capacitor in series with a switch. The switch is wired to disconnect the resistor whenever it receives a disable command from the control unit. If a fault occurs, the disable command is no longer present, and the switch connects the discharge resistor, causing the capacitor to discharge rapidly. This type of switch-controlled discharge resistor is called an active discharge resistor.

[0006] One challenge with this type of active discharge resistor is handling situations where the capacitor remains continuously charged. For example, if a circuit breaker fails to open, the battery may still be connected, or, while the vehicle is in motion, the AC motor may generate back electromotive force that couples to the capacitor. This continuous voltage supply forces the active discharge resistor to discharge the continuous energy supply. Consequently, conventional active discharge resistors must have sufficient power capacity and thermal rating for continuous operation, significantly increasing their cost.

[0007] US2017 / 0355267 proposes a solution to this problem, in which a timing circuit is introduced to further control the switching of an active discharge resistor. According to this solution, if voltage is still present across the capacitor, the timing circuit disconnects the discharge resistor after a predetermined period of time. However, this solution is relatively complex and expensive. EP 3468 019 provides another example.

[0008] An improved active discharge circuit is needed to avoid the above disadvantages. Summary of the Invention

[0009] According to a first aspect of the invention, this and other objects are achieved by an active discharge circuit for an electric vehicle inverter, the active discharge circuit being intended to be connected in parallel with a DC link capacitor connected between a positive and negative line of a DC power link and being configured to discharge the DC link capacitor in less than 7 seconds, wherein the circuit comprises a dissipation current source, a switch connected in series with the current source between the DC lines, and a controller connected to the switch and arranged to apply an activation signal in response to a control signal from a vehicle control system, the activation signal placing the switch in an on-state, wherein the current source is configured to draw a discharge current and dissipate any energy stored in the DC link capacitor when the switch is in the on-state.

[0010] With this design, the energy in the capacitor is dissipated by the current source rather than by the passive resistor. As long as the switch is closed by the activation signal, the current source draws a constant current and dissipates power, and the voltage across the DC link capacitor decreases linearly.

[0011] The active discharge circuit using a dissipative current source according to the present invention has improved performance at similar power ratings, or, in other words, requires a lower power rating to meet performance requirements. Here, performance involves discharging a given high voltage to below a given limit within a given time.

[0012] As an example, a typical prior art active discharge resistor may have a power rating of 24W, while the active component of the present invention may have a power rating of only 7W as a performance comparison.

[0013] In a preferred embodiment, the current source is connected in series with one or several additional active components. Each active component will provide additional power dissipation capability. This has the advantage of increasing thermal mass at a low cost (many small components instead of one large component).

[0014] The active components may be transistors connected in a source-drain chain. The transistors will operate in the linear (non-saturated) region, meaning that each transistor has current flowing through it and a voltage across it. The transistors may be, for example, field-effect transistors (FETs), insulated-gate bipolar transistors (IGBTs), or bipolar junction transistors (BJTs).

[0015] The circuit may further comprise a set of resistors connected in series across the DC link capacitor to divide the voltage across the DC link capacitor into a set of intermediate voltages, each of which is connected to the gate of one of the (field effect) transistors, so that each transistor is turned on whenever there is voltage across the DC link capacitor.

[0016] In one embodiment, the dissipation current source includes a transistor and a voltage regulator connected between the gate of the field-effect transistor and the negative DC line. As long as the bias current flows through the voltage regulator, a constant voltage is applied to the gate of the transistor. This causes a constant current (discharge current) to flow through the transistor when the switch is turned on.

[0017] Here as well, the transistor may be, for example, a field effect transistor (FET), an insulated gate bipolar transistor (IGBT), or a bipolar junction transistor (BJT). The voltage regulator may be implemented by a Zener diode, a transient voltage suppressor diode, or a voltage reference IC.

[0018] Preferably, the drain of the transistor is connected to the positive line without any intermediate resistive load. This means that the discharge current does not cause any resistive dissipation, ie all dissipation is provided by the active circuit (ie the current source and any additional active components).

[0019] In some embodiments, a predefined "idle" current is also allowed to flow through the dissipative element (transistor) when the switch is in the non-conducting state. This idle current is significantly less than the discharge current. For example, the current can be less than 1 mA, such as less than 0.5 mA or less than 0.1 mA, while the discharge current can be approximately 5-50 mA. This idle current can be used to power the controller, thereby providing a safe, independent power source while the link capacitor is charging.

[0020] In one embodiment, the switch comprises a field effect transistor having a drain connected to the set of active components, a source connected to ground (via a resistor), and a gate connected to receive an activation signal.

[0021] The controller can be configured to apply a steady activation signal, thereby controlling the current source to draw a constant discharge current, causing the voltage across the DC link capacitor to decrease linearly. Alternatively, the controller can be configured to apply an intermittent activation signal, thereby allowing for a non-constant discharge current. For example, the current source can be controlled to draw an increasing current to dissipate constant power, causing the energy discharge of the DC link capacitor to be linear. Alternatively, the current source can be controlled to draw a decreasing current, causing the voltage across the DC link capacitor to decrease exponentially. This effectively corresponds to the performance of a passive discharge resistor, which can be advantageous in situations where discharge should be synchronized with other discharge processes.

[0022] The control signals can be communicated over a bidirectional serial communication bus. Such a serial communication bus is often already present in the vehicle and provides a simple method of accessing the controller. To ensure the required security, communication can be provided over a "black channel," for example involving a specific (safety) communication protocol.

[0023] Optionally, the control signal is a (unidirectional) discharge request signal, and the controller is configured to apply the activation signal when the discharge request signal is absent. Such a signal is always present during normal vehicle operation, but in the event of a fault, the discharge request signal is no longer provided and active discharge should be activated.

[0024] In one embodiment, the active discharge circuit includes circuitry for pulsing a discharge request signal to generate a pulsed discharge signal, and a controller is configured to verify the pulsed discharge signal and provide an activation signal if verification is unsuccessful. For example, the controller can be configured to verify the pulse width, pattern, and / or pulse repetition frequency of the pulsed discharge signal. This is done to ensure signal integrity and detect potential faults in the input circuit.

[0025] Note that this pulsing of the control signal in order to make it more reliable may be considered to represent another inventive concept which is also beneficial in other cases than the active discharge circuit according to the first aspect of the invention described above. In fact, in any case where no control signal is used for a system fault, the modulation (pulsing or other modulation) of the control signal using the power supply inherent in the system will enable verification of the complete signal path of the control signal, i.e. all components between the control signal input and the controller. For example, an interface between a high voltage domain and a low voltage domain, such as an optocoupler, may fail, producing a constant "high" level. By pulsing the control signal, such a fault is immediately detected. Furthermore, in the case where the control signal remains "high" even during a system fault, the system fault will typically also result in a loss of power to the low voltage circuit and therefore an interruption in the modulation. Therefore, the system fault will still be detected.

[0026] The switch can be connected to the negative line via a resistor, and the controller can be connected to detect the voltage across this resistor. The detected voltage indicates the current flowing through the resistor and can be used for a simple functional test. To ensure functional availability, a short (millisecond-scale) activation signal can be applied while measuring the resulting current. The controller can thus verify the correct operation of the active discharge circuit.

[0027] According to one embodiment, the active discharge circuit may further include a voltmeter connected to detect a link voltage between the DC lines (i.e., across the link capacitor), and the controller may then be connected to receive an indication of the link voltage from the voltmeter and determine whether the link voltage is dropping correctly, and when it is determined that the link voltage is not dropping correctly, cause the switch to enter a non-conducting state.

[0028] This allows the controller to immediately disable the active discharge process, preventing the voltage from decreasing as intended in the event of a fault condition. This could occur, for example, if the DC power source is not properly disconnected from the inverter for some reason. By disabling active discharge, thermal events (overheating, etc.) in the discharge circuit can be avoided. The active discharge circuit according to this embodiment of the present invention can comply with relevant safety regulations for electric vehicles, such as ISO 26262 and IEC 13849. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will now be described in more detail with reference to the accompanying drawings, which show currently preferred embodiments of the invention.

[0030] Figure 1 is a schematic block diagram of an active discharge circuit connected across a DC link capacitor of an electric vehicle inverter according to an embodiment of the present invention.

[0031] Figure 2 yes Figure 1 A more detailed circuit diagram of an embodiment of an active discharge circuit is shown in FIG.

[0032] Figure 3 is a circuit diagram of a discharge signal processing circuit according to an embodiment of the present invention. DETAILED DESCRIPTION

[0033] Figure 1 An inverter circuit 1 is shown connected to a DC power source 2 via a DC link comprising a positive line 3 and a negative line 4. A DC link capacitor 5 is connected in parallel with the inverter 1, and a high resistance passive discharge resistor 6 is connected in parallel with the link capacitor to ensure discharge of the capacitor in the event of an inverter fault or power loss.

[0034] The inverter is connected here to provide AC voltage to the electric motor in the electric vehicle. The electric motor may form part of the vehicle's traction system, but may alternatively be a motor for some other device such as power regeneration, an air compressor, a water pump, etc. For example, the voltage across lines 3 and 4 of the DC link is 800V.

[0035] An active discharge circuit 10 according to an embodiment of the present invention is also connected in parallel with the link capacitor 5. Here, the active discharge circuit 10 comprises a current source 11 connected in series with a switch 12 across the link capacitor 5. A controller 13 is connected to control the switch 12. Furthermore, in the embodiment shown, an additional dissipation element 14 is connected in series with the current source 11.

[0036] The active discharge circuit 10 may also include a voltmeter 15 connected to measure the voltage across the DC link and provide a signal indicative of the voltage to the controller 13 .

[0037] The controller 13 is connected to receive control signals 20 from the vehicle controller 16 via a suitable interface 19. For example, the interface 19 may provide isolation between the high voltage and low voltage domains of the system. For this purpose, the interface 19 may include an optocoupler.

[0038] The control signals 20 may be communicated over a bidirectional serial communication link, such as a CAN bus. Such serial communication is then preferably configured as a safety-critical "black channel", eg provided with a communication protocol capable of detecting any fault conditions.

[0039] Alternatively, the control signal 20 is a unidirectional communication of a (binary) discharge signal. In this case, it may be useful to provide the interface 19 with circuitry for increasing the reliability of such a simple control signal. Figure 3 Discuss this in detail.

[0040] In use, the controller 13 receives a control signal 20 from the vehicle controller 16 and, in response thereto, provides an activation signal to the switch 12, thereby placing the switch in a closed (conducting) state. Typically, the discharge signal is always present (high) unless there is a fault condition or damage, in which case the discharge signal is absent (low). Therefore, in the absence of a discharge signal, the controller 13 provides an activation signal to the switch 12.

[0041] When the switch 12 is closed, a constant and predefined current will be drawn by the current source 11 and energy will be dissipated in the current source 11 and in any additional dissipative elements 14 .

[0042] Figure 2 Shown Figure 1A more detailed embodiment of the active discharge circuit is shown in FIG. 1 . In this case, the switch 12 is implemented by a field effect transistor 21, the source of which is connected (via resistor 18) to the negative DC line 4 and the drain of which is connected to the current source 11. The gate of the transistor 21 is connected to receive an activation signal from the controller 13.

[0043] The current source 11 here comprises a field effect transistor 24 having its source connected (via a resistor 25 ) to the drain of the transistor 21 and its gate connected to the cathode of a Zener diode 26 , the anode of which is connected to the negative power line 4 .

[0044] Here, the dissipation element 14 is implemented by a set of (one or more) field effect transistors 27 connected source-to-drain between the drain of the transistor 24 and the positive power line 3. In addition, the circuit comprises a set of resistors 28 connected in series between the positive line 3 and the cathode of the diode 26. Each resistor 28 is connected between the gates of adjacent transistors 27 so as to form a string of interconnected resistors 28 and transistors 27.

[0045] In use, the voltage across the DC link will be split by resistors 28 into a set of intermediate voltages, one across each resistor 28. Each transistor gate will be subjected to one of these intermediate voltages, thereby maintaining each transistor 27 in a conducting state.

[0046] In addition, there will be a small current i 偏置 Flows through the series resistor 28 and ultimately through the Zener diode 26. This current will act as a bias current to maintain a constant voltage (e.g., approximately 15V) across the Zener diode 26. This constant voltage will be applied to the gate of the transistor 24, thereby defining a specific operating state of the transistor 24. The bias current will gradually decrease as the charge of the link capacitor 5 is discharged. However, as long as the voltage across the capacitor 5 is large enough, the voltage across the Zener diode 26 will remain substantially constant. For example, if there are six resistors 28 (e.g., Figure 2 As shown), each resistor has a resistance of 270kOhm, then for a capacitor voltage of 800V, the bias current is approximately 0.5mA.

[0047] When the transistor 21 receives the activation signal from the controller 13, the transistor 21 will enter the on state, allowing the current i 放电 The magnitude of this discharge current will be determined by the state of transistor 24 , which is determined by the voltage across Zener diode 26 and the voltage developed across resistor 25 .

[0048] As current flows through the set of transistors 27, energy will be dissipated in each transistor 27 (and also in transistor 24) so ​​as to gradually discharge capacitor 5. As long as the activation signal is present, the discharge current through transistors 27, 24 will be constant and the voltage drop across capacitor 5 will be linear.

[0049] The voltage across Zener diode 26—and therefore the current drawn by transistor 24—will remain essentially constant until the voltage across the capacitor falls below 60V, which is the regulatory requirement. In practice, the discharge circuit will continue to function below 60V, but then with a slightly lower discharge current, because the voltage across Zener diode 26 will be smaller when the bias current is very low. Eventually, the remaining voltage across link capacitor 5 will be too low to keep transistors 24, 27 in their conductive state, and discharge circuit 10 will be disabled.

[0050] In an alternative embodiment, the current sources 11, 24 are configured to draw a greater discharge current than the dissipative elements 14, 28 can withstand at the full capacitor voltage (e.g., 800V). The controller 13 is then configured to provide an intermittent (pulsed) activation signal, starting with a relatively low duty cycle and then continuously increasing the duty cycle (ultimately reaching a permanently on state). With sufficient duty cycle control, the current sources 11, 24 can be controlled to draw an increasing average current so that the dissipated power is constant. The energy discharge of the DC link capacitor 5 will then be linear (and the voltage drop exponential).

[0051] In yet another embodiment, controller 13 is again configured to provide an intermittent (pulsed) activation signal, this time with a decreasing duty cycle. With sufficient duty cycle control, current sources 11, 24 can be controlled to draw decreasing average currents, causing the voltage across the DC link capacitor to decrease exponentially. This behavior is similar to that of a conventional discharge resistor and can be advantageous if the discharge is aligned with the discharge of other capacitors.

[0052] During discharge, controller 13 can be configured to continuously monitor the voltage across the DC link using voltmeter 15 to verify that the voltage is decreasing as expected. If the voltage is detected to be not decreasing, this is an indication of a fault condition, such as if power source 2 is still connected to the DC link. In this case, continued activation of active discharge circuit 10 could cause a thermal event in dissipation component 14 (transistor 27), potentially damaging the component or even creating a fire hazard. Therefore, controller 13 is preferably configured to deactivate switch 12 if it determines that the voltage across the DC link is not decreasing as expected.

[0053] Also refer to Figure 1 and Figure 2, resistor 18 can be connected between switch 12 (transistor 21) and negative line 4. The voltage across resistor 18 can then be provided to controller 13 and used as a current detector. This can be used to implement a simple functional test. Controller 13 can be configured to provide a short (millisecond-scale) activation signal to switch 12 and then verify that the detected current is as expected.

[0054] In either of the above situations, i.e., if the voltage does not decrease as expected or if the functional test fails, the active discharge circuit can be disabled. In this case, the high-resistance discharge resistor 6 will provide a "safe state," ensuring that the link capacitor 5 will be discharged (albeit slowly). When the controller 13 communicates with the vehicle controller 16 via the bidirectional communication link (see above), the controller can also communicate to the vehicle controller 16 that the active discharge circuit has been disabled.

[0055] By implementing the above safety functions, the active discharge circuit can be designed to comply with relevant safety regulations, such as ISO26262 and IEC 13849.

[0056] Figure 3 An example of the interface 19 is shown where the communication 20 is a discharge signal from the vehicle controller 16. The interface 19 here includes an optical switch 33, which includes an LED 34 and a phototransistor 35. The phototransistor 35 is connected between the operating voltages and to the negative line 4 via a resistor 36. The anode of the LED 34 is connected to the discharge signal 20, and the cathode of the LED 34 is connected to the drain of a transistor 38. The transistor 38 has a source connected to ground and a gate connected to a pulse signal 40 from a pulse generator 39.

[0057] In use, when both the discharge signal 20 and the pulse signal 40 are active, the phototransistor 35 will provide a pulse signal output 37. If the discharge signal 20 or the pulse signal 40 are not present, there will be no pulse output 37.

[0058] Turning to the controller 13, it Figure 3 , which includes processing circuitry 41 for verifying that the output from phototransistor 35 is a pulsed signal. Circuitry 41 can be configured to verify voltage level, pulse frequency, pulse duration, pulse pattern, or a combination thereof. Processing circuitry 41 will provide an activation signal output if and only if the pulsed discharge signal cannot be verified. In other words, if discharge signal 20 is absent, or if power to pulse generator 39 is terminated, controller 13 will activate switch 12 to discharge capacitor 5.

[0059] Reference again Figure 1 and Figure 2In the embodiment shown, the voltage on the source of transistor 24 is used to drive controller 13. Specifically, the voltage is applied to voltage regulator circuit 17, and the output (V 操作 ) is used to power the controller 13. In the example shown, the voltage across the Zener diode 26 is approximately 15V, and the voltage between the gate and source of the transistor 24 (V gs ) is about 3V, so that the voltage V on the source of transistor 21 驱动 This is approximately 12 V. The output from the voltage regulator 17 may typically be 2-5 V.

[0060] When the voltage is used to power the controller, a small reactive current will flow through transistor 27. This reactive current will cause losses and should therefore not be larger than necessary. Specifically, the reactive current should be significantly smaller than the discharge current. Typically, the reactive current will be at most one-fifth of the discharge current, and can be as little as one-tenth.

[0061] As an example, for a relatively small link capacitor 5, a discharge power of 4 W may be sufficient, which corresponds to a discharge current of 5 mA (=4 W / 800 V) for a link voltage of 800 V. If the reactive current is 1 mA, this will result in a loss of 0.8 W (=800 V×1 mA), which is acceptable.

[0062] Note that switch 12 (in Figure 2 The transistor 21 in FIG. 1 may alternatively be implemented by suitable circuitry in the controller 13. For example, the controller 13 may be configured to connect the operating voltage from the voltage regulator to ground via a suitable resistor (not shown).

[0063] Aspects of the invention may be embodied in one or more of the following embodiments.

[0064] Item 1). An active discharge circuit (10) for an electric vehicle inverter (1), the active discharge circuit being intended to be connected in parallel with a DC link capacitor (5) connected between a positive line (3) and a negative line (4) of a DC power link and being configured to discharge the DC link capacitor (5) in less than seven seconds, wherein the active discharge circuit comprises:

[0065] dissipation current source (11; 24),

[0066] a switch (12; 21) connected in series with the current source between the positive line and the negative line, and

[0067] a controller (13) connected to the switch and arranged to apply an activation signal according to a control signal (20) from a vehicle control system (16), the activation signal placing the switch in a conducting state,

[0068] wherein the current source is configured to draw a discharge current and dissipate any energy stored in the DC link capacitor when the switch is in the on-state,

[0069] Such that, when the switch is in the on-state, the voltage across the DC link capacitor will decrease linearly.

[0070] Item 2). The active discharge circuit according to item 1) further comprises a set of active components (14; 27) connected in series between the positive line and the current source.

[0071] Item 3). The active discharge circuit according to item 2), wherein the active component is a source-to-drain connected transistor (27).

[0072] Item 4). The active discharge circuit according to item 3) further includes a set of resistors (28) connected in parallel with the DC link capacitor to divide the voltage across the DC link capacitor into a set of intermediate voltages, each of which is connected to the gate of one of the transistors (27).

[0073] Item 5). Active discharge circuit according to one of the preceding items, wherein the current source (11) comprises a transistor (24) and a voltage regulator (26) connected between the gate of the transistor (24) and the negative line.

[0074] Item 6). The active discharge circuit according to item 5), wherein the drain of the transistor (24) is connected to the positive line (3) without any intermediate resistive load.

[0075] Item 7). Active discharge circuit according to one of the preceding items, wherein when the switch (12; 21) is in the non-conducting state, a reactive current is allowed to flow through the current source, the reactive current being significantly smaller than the discharge current.

[0076] Item 8). The active discharge circuit according to item 6), wherein the reactive current (i reactive ) is at most one fifth of the discharge current, and preferably at most one tenth of the discharge current.

[0077] Item 9). The active discharge circuit according to item 7) or 8), wherein the reactive current is used to power the controller (13).

[0078] Item 10). An active discharge circuit according to any one of the preceding items, wherein the switch comprises a transistor (21) having a drain connected to the current source, a source connected to the negative line, and a gate connected to receive the activation signal.

[0079] Item 11). An active discharge circuit according to any one of the preceding items, wherein the controller (13) is configured to apply a stable activation signal to control the current source to draw a constant discharge current so that the voltage drop across the DC link capacitor is linear.

[0080] Item 12). An active discharge circuit according to any one of items 1) to 9), wherein the controller (13) is configured to apply an intermittent activation signal with an increasing duty cycle, thereby controlling the current source to draw an increasing average current to dissipate constant power, so that the voltage drop across the DC link capacitor increases exponentially.

[0081] Item 13). An active discharge circuit according to any one of items 1) to 10), wherein the controller (13) is configured to provide an intermittent activation signal with a continuously decreasing duty cycle, thereby controlling the current source to draw a continuously decreasing average current, so that the voltage across the DC link capacitor decreases exponentially.

[0082] Item 14). Active discharge circuit according to one of the preceding items, wherein the control signal (20) is communicated on a bidirectional serial communication bus.

[0083] Item 15). The active discharge circuit according to one of items 1) to 13), wherein the control signal (20) is a discharge request signal, and the controller (13) is configured to apply the activation signal when the discharge request signal is absent.

[0084] Item 16). The active discharge circuit according to Item 15) further includes a circuit (38, 39) for pulsing the discharge request signal (20) to generate a pulsed discharge signal (37), and wherein the controller is configured to verify the pulsed discharge signal (37) and provide the activation signal when the verification is unsuccessful.

[0085] Item 17). An active discharge circuit according to any of the preceding items, wherein the switch (12; 21) is connected to the negative line (4) via a resistor (18), and the controller (13) is connected to detect the voltage across the resistor (18).

[0086] Item 18). An active discharge circuit according to any of the preceding items, further comprising a voltmeter (15) connected to detect a link voltage between the positive line (3) and the negative line (4), and wherein the controller (13) is connected to receive an indication of the link voltage from the voltmeter and to determine whether the link voltage is dropping correctly, and when it is determined that the link voltage is not dropping correctly, to cause the switch (12) to enter a non-conducting state.

[0087] Those skilled in the art will recognize that the present invention is by no means limited to the preferred embodiments described above. On the contrary, numerous modifications and variations are possible within the scope of the appended claims. For example, other types of transistors may be used in place of the field-effect transistors shown. Furthermore, the voltage and current levels shown are exemplary only.

Claims

1. An active discharge circuit (10) for an electric vehicle inverter (1), the active discharge circuit being intended to be connected in parallel with a DC link capacitor (5) connected between a positive line (3) and a negative line (4) of a DC power link and being configured to discharge the DC link capacitor (5) in less than seven seconds, wherein: The active discharge circuit comprises: Dissipation current source (11), a switch (12; 21) connected in series with the dissipation current source between the positive line and the negative line, and a controller (13) connected to the switch and arranged to apply an activation signal according to a control signal (20) from a vehicle control system (16), the activation signal placing the switch in a conducting state, wherein the dissipation current source is configured to draw a discharge current and dissipate any energy stored in the DC link capacitor when the switch is in the on-state, Such that, when the switch is in the on-state, the voltage across the DC link capacitor decreases linearly, wherein the dissipation current source (11) comprises a transistor (24) and a voltage regulator (26) connected between the gate of the transistor (24) and the negative line, the source of the transistor (24) being connected to the drain of the switch (12; 21) and the drain of the transistor (24) being connected to the positive line (3), and When the switch (12; 21) is in a non-conducting state, a reactive current is allowed to flow through the transistor (24), the reactive current being significantly smaller than the discharge current, and the reactive current being used to power the controller (13).

2. The active discharge circuit according to claim 1, further comprising a set of active components (14; 27) connected in series between the positive line and the dissipation current source.

3. The active discharge circuit according to claim 2, wherein: The active component is a source-drain connected transistor (27).

4. The active discharge circuit of claim 3, further comprising a set of resistors (28) connected in parallel with the DC link capacitor to split the voltage across the DC link capacitor into a set of intermediate voltages, each intermediate voltage being connected to the gate of one of the transistors (27).

5. An active discharge circuit according to any one of the preceding claims, wherein The drain of the transistor (24) is connected to the positive line (3) without any intermediate resistive load.

6. The active discharge circuit according to any one of claims 1 to 4, wherein: The reactive current (i 无功 ) is at most one fifth of the discharge current.

7. The active discharge circuit according to claim 6, wherein: The reactive current (i 无功 ) is at most one tenth of the discharge current.

8. The active discharge circuit according to any one of claims 1 to 4, wherein: The switch includes a transistor (21) having a drain connected to the dissipation current source, a source connected to the negative line, and a gate connected to receive the activation signal.

9. The active discharge circuit according to any one of claims 1 to 4, wherein: The controller (13) is configured to apply a stable activation signal to control the dissipation current source to draw a constant discharge current so that the voltage drop across the DC link capacitor is linear.

10. The active discharge circuit according to any one of claims 1 to 4, wherein: The controller (13) is configured to apply an intermittent activation signal with an increasing duty cycle, thereby controlling the dissipation current source to draw an increasing average current to dissipate constant power, so that the voltage drop across the DC link capacitor increases exponentially.

11. The active discharge circuit according to any one of claims 1 to 4, wherein: The controller (13) is configured to provide an intermittent activation signal with a decreasing duty cycle, thereby controlling the dissipation current source to draw a decreasing average current, so that the voltage across the DC link capacitor decreases exponentially.

12. The active discharge circuit according to any one of claims 1 to 4, wherein: The control signal (20) is communicated on a bidirectional serial communication bus.

13. The active discharge circuit according to any one of claims 1 to 4, wherein: The control signal (20) is a discharge request signal, and the controller (13) is configured to apply the activation signal in the absence of the discharge request signal.

14. The active discharge circuit of claim 13, further comprising circuitry (38, 39) for pulsing the discharge request signal (20) to generate a pulsed discharge signal (37), and wherein The controller is configured to verify the pulse discharge signal (37) and provide the activation signal if the verification is unsuccessful.

15. The active discharge circuit according to any one of claims 1 to 4, wherein: The switch (12; 21) is connected to the negative line (4) via a resistor (18), and the controller (13) is connected to detect the voltage across the resistor (18).

16. The active discharge circuit according to any one of claims 1-4, further comprising a voltmeter (15) connected to detect a link voltage between the positive line (3) and the negative line (4), and wherein, The controller (13) is connected to receive an indication of the link voltage from the voltmeter and determine whether the link voltage is dropping correctly and, when it is determined that the link voltage is not dropping correctly, cause the switch (12) to enter a non-conducting state.

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