DE102023201509A1 - Safe shutdown of an inverter - Google Patents

Abstract

An output stage (120) for an inverter, in particular for controlling an electrical machine (110), comprises a half-bridge with a current valve (215) designed as a JFET. The current valve (215) is designed to be controlled by a driver (125). The output stage (120) further comprises a safety circuit (300) for actively opening the current valve (215) in the event of a lack of supply voltage to the driver (125) and a separating device (400) for isolating the current valve (215) from the driver (125).

Description

The present invention relates to an electrical inverter, in particular for controlling an electrical machine. More specifically, the invention relates to the shutdown of the inverter in the event of a loss of a supply voltage required for the controlled operation of the machine.

A motor vehicle comprises an electric machine for propulsion, which can be operated from an electrical energy storage device using an inverter. To do this, the inverter provides the machine with three alternating currents, each 120° out of phase. A torque provided by the machine or a speed assumed by it can be controlled by influencing the alternating currents.

The inverter usually comprises an output stage with three half-bridges. A half-bridge comprises two current valves connected in series between connections of the energy storage device, between which a connection of the electrical machine is connected. A driver opens and closes the current valves alternately with a variable duty cycle in order to provide a predetermined alternating current. Semiconductors are usually used as current valves, for example IGBT or FET. To avoid a short circuit, it is essential to ensure that both current valves are not closed at the same time.

Correct control of the current valves cannot be guaranteed if the driver's supply voltage is too low. If current valves are used that are closed without control (normally-on), for example JFET, they must be actively opened by means of a separate circuit in the event of a voltage error when the supply voltage fails.

It has been proposed to protect a normally-on current valve by means of a capacitor that is charged during normal operation and provides energy to close the current valve in the event of a voltage fault. However, the capacitor can be quickly discharged again via the driver so that the shutdown is not successful or is only short-lived.

One object of the invention is to provide an improved technique for safely switching off an inverter with a current valve that is closed without control. The invention solves this problem by means of the subject matter of the independent claims. Subclaims give preferred embodiments.

An output stage for an inverter, in particular for controlling an electrical machine, comprises a half-bridge with a current valve that is closed without control, for example in the form of a JFET. The current valve is designed to be controlled by a driver.

According to a first aspect of the present invention, an output stage for an inverter, in particular for controlling an electrical machine, comprises a half-bridge with a current valve designed as a JFET. The current valve is designed to be controlled by a driver. The output stage further comprises a safety circuit for actively opening the current valve in the event of a lack of supply voltage to the driver and a separating device for isolating the current valve from the driver.

The isolation circuit can be used to disconnect the capacitor from the driver in the event of a voltage fault, thereby ensuring that the current valve can be kept closed for a longer period of time. For example, in a case where the supply voltage is low in magnitude but not zero, a leakage current from the capacitor can be prevented from flowing through the driver.

The output stage usually comprises two current valves connected in series to form a half-bridge, to which a voltage can be provided by alternately controlling the current valves. To provide different voltages, the output stage can comprise several half-bridges, each with two current valves. A typical electrical machine comprises three phases, for which individual voltages must be provided.

It may be sufficient to equip only one of the current valves of a half-bridge with the described safety circuit in order to prevent a short circuit through the output stage in the event of a voltage fault. Optionally, several independent safety circuits can also be set up. A separate isolating device can be provided for each current valve that is protected by a safety circuit.

By appropriately dimensioning the capacitor, a time can be determined for which the current valve can be kept safely open. This time can be adapted to another time within which another safety measure can take effect, for example, a separation of the operating voltage from the output stage by means of a protective device that can include a relay or a semiconductor arrangement. The capacitor In the event of a voltage fault, when the driver supply voltage is too small, the capacitor is essentially only discharged by a small leakage current through the current valve, so that it only needs to have a small capacitance to keep the current valve open for a sufficiently long time. In a typical application on board a motor vehicle, a capacitance of just a few 100 µF can be enough to keep the current valve open for several seconds. The capacitor is usually only charged to a few tens of volts.

The driver can be set up to actively open the flow valve using a switch-off signal or to actively close it using a switch-on signal; the isolating device is preferably set up to isolate the switch-on signal from the flow valve. The switch-off signal can be connected to the flow valve without change. The switch-on signal can be represented by a predetermined first voltage and the switch-off signal by a predetermined second voltage, which can each occur at a control connection of the flow valve. Separate or a common connection can be provided for both signals.

The isolating device preferably comprises a semiconductor switch, preferably a FET (field effect transistor), to control the forwarding or isolation of a signal from the driver to the current valve. The semiconductor switch can be designed to be self-locking, so that it separates the current valve from the driver without its own control signal. For this purpose, the semiconductor switch can comprise an enhancement type, in particular with an N-channel, in order to be more compatible with a control voltage provided by the fuse circuit.

The semiconductor switch can be controlled by means of a voltage provided by the safety circuit. A Z-diode can be provided to open the semiconductor switch when the voltage provided reaches a predetermined value. The minimum decoupling threshold of the capacitor can be set via a breakdown voltage of the Z-diode depending on the negative control voltage used in the safety circuit, so that a sufficient level for the safe off state of the current valve or the semiconductor switch is not undercut.

Further preferably, a resistor is provided in series with the Z-diode, with a control voltage for the semiconductor switch being dropped across this resistor. The resistor can in particular be connected between a gate and a source terminal of the semiconductor switch. The drain terminal of the semiconductor switch can be connected to the switch-on signal and its source terminal to the gate terminal of the current valve.

The safety circuit preferably comprises a further JFET, the drain terminal of which is connected to the gate terminal of the current valve by means of a capacitor. A signal can be applied to the gate terminal of the further JFET, which closes the further JFET when the supply voltage falls below a predetermined value. A diode and a resistor can be connected in series between the drain terminal and the source terminal of the further JFET in order to charge the capacitor when there is no voltage error on the supply voltage. The Z-diode can lead from the gate terminal of the semiconductor switch to the drain terminal of the further JFET. Optionally, a resistor (pull-down) is provided between the gate terminal and the source terminal of the further JFET in order to prevent the further JFET from opening unintentionally.

The driver can control the current valve in different ways. In a first variant, the driver provides a signal that is converted into a switch-on signal for the current valve by means of a first diode and into a switch-off signal for the current valve by means of a second diode that is antiparallel to the first. For example, the driver can be constructed using CMOS technology. The diodes and optional resistors connected in series can be included in the driver or located outside of it. In this variant, it can be sufficient to isolate or separate the switch-on signal from the current valve using the semiconductor switch in the event of a voltage error.

In a second variant, the driver provides a switch-on signal and an independent switch-off signal, and the switch-off signal is decoupled from the current valve by means of a diode. In the event of a voltage error, the switch-on signal can be isolated from the current valve by means of the semiconductor switch, as in the first variant. Such a structure is also known as an open collector and can allow the switch-on speed and switch-off speed of the current valve to be controlled independently of one another.

According to a further aspect of the invention, an inverter for controlling an electrical machine comprises an output stage as described herein. The output stage may comprise one or more half-bridges and a half-bridge may comprise one or more current valves. Optionally, the driver for controlling current valves is also from the output stage. The inverter can also include an intermediate circuit capacitor for decoupling a direct voltage at which a half-bridge can be operated. The inverter can also include a cooling system with a fluid channel in which a fluid can be moved in order to transport heat away from a current valve, a half-bridge or the intermediate circuit capacitor.

The inverter is designed to provide one or more alternating voltages based on a direct voltage, which can be provided in particular by a battery or another portable energy source. The direct voltage for use on board a motor vehicle is usually around 400 V, around 800 V or around 900 V, but other voltages are also possible. The alternating voltages are further preferably coordinated with one another in such a way that they can be used to control an electrical machine, for example a permanently excited synchronous machine. In one embodiment, the inverter is designed to provide the alternating voltages depending on a control signal. The control signal can include a torque to be provided by the machine and/or a speed to be assumed by the machine. In other words, the driver can carry out a vector-oriented control or regulation of the electrical machine. The electrical machine is further preferably designed to drive a motor vehicle.

According to yet another aspect of the present invention, an electric axle drive for a motor vehicle comprises an inverter as described herein. The axle drive can comprise a fixed or switchable transmission in order to transmit a rotary motion of the electric machine to a drive wheel of the motor vehicle. The axle drive can be designed to be driven by a further drive machine, in particular an internal combustion engine. Torques of the electric machine and the further drive machine can be coupled to one another accordingly.

According to yet another aspect of the present invention, a motor vehicle comprises an electric axle drive as described herein. The motor vehicle preferably comprises a passenger car, but can also comprise a motorcycle, a truck or a bus.

• 1 ein Kraftfahrzeug;
• 2 einen Schaltplan eines Wechselrichters;
• 3 einen Schaltplan einer Sicherungsschaltung;
• 4 einen Schaltplan einer Trennschaltung;
• 5 einen Schaltplan einer Trennschaltung in einer weiteren Ausführungsform; und
• 6 Verläufe von Spannungen an einem Wechselrichter

darstellt.The invention will now be described in more detail with reference to the accompanying figures, in which:

• 1 a motor vehicle;
• 2 a circuit diagram of an inverter;
• 3 a circuit diagram of a fuse circuit;
• 4 a circuit diagram of an isolating circuit;
• 5 a circuit diagram of a separating circuit in another embodiment; and
• 6 Voltage curves on an inverter

represents.

1 shows an electric axle drive 100 on board a motor vehicle 105. The axle drive 100 comprises an electric drive machine 110 and an inverter 115, which comprises an output stage 120 and a driver 125. An electrical energy storage device 130 is provided on board the motor vehicle 105, which can in particular comprise an accumulator.

Not in 1 An optional transmission between the electric drive motor 110 and a drive wheel of the motor vehicle 105 is shown. Optionally, a further drive motor is provided on board the motor vehicle 105, which can also be coupled to a drive wheel.

The inverter 115 is preferably designed to generate a plurality of phase-shifted alternating voltages on the basis of a direct voltage provided by the energy storage device 130 and to provide them to the drive machine 110. The provision preferably takes place as a function of a request signal that can be provided by a driver of the motor vehicle 105.

2 shows a circuit diagram of an inverter 115 in an exemplary embodiment. The inverter 115 is connected to an energy storage device 130 and comprises, for example, an intermediate circuit capacitor (ZKK) 205, three half-bridges 210, each with a first (“high-side”, shown above) and a second (“low-side”, shown below) current valve 215, and the driver 125. The current valves 215 are each designed as a JFET, preferably in SiC technology. Two current valves 215 of a half-bridge 210 can be included in a control module 225.

By way of example, a gate connection G, a drain connection D and a source connection S are designated on one of the current valves 215 in accordance with the usual conventions. Two current valves 215 of a half-bridge 210 are connected in series by connecting the source connection of the high-side current valve 215 to the drain connection of the low-side current valve 215. The connection between the two connections is brought out in order to be connected to a phase of the electric machine 110. to be connected. The current is guided here, for example, through a resistor 220, so that a voltage drops across it that is proportional to the current that flows through this phase of the electrical machine 110. This voltage can be evaluated by the driver 125. Other methods of measuring the current are also possible.

The drain connection of the high-side current valve 215 is connected to a high potential and the source connection of the low-side current valve 215 is connected to a low potential of the energy storage device 130. The ZKK 205 is connected in parallel to the energy storage device 130.

The driver 125 provides control voltages to the total of six current valves 215 in order to generate a predetermined voltage at each phase of the electrical machine 110. Since a current valve 215 is closed without control, it must be actively opened by providing a suitable control voltage. The driver 125 is operated with a supply voltage that can be provided from the energy storage device 130 or another source. If the supply voltage fails or falls below a predetermined minimum, the driver 125 can no longer work correctly and in particular cannot prevent both current valves 215 of a half-bridge 210 from being closed at the same time, which can cause a short circuit across the energy storage device 130. A protective device 230 is usually provided to separate the energy storage device 130 from the inverter 115, but this can require a certain response time. To prevent a short circuit, it is proposed to provide a safety circuit on a current valve 215 of a half-bridge 210, which opens the current valve 215 when the supply voltage drops or fails and keeps it open for a predetermined time.

3 shows a safety circuit 300 for protecting a current valve 215. The gate terminal of the current valve 215 is connected to the driver 125. A capacitance 305 of the JFET acts between the gate terminal and the source terminal of the current valve 215. A capacitor 310 and a diode 315 are connected in parallel to the capacitance 305, with a cathode of the diode facing the capacitor 310. Optionally, a resistor 320 is connected in parallel to the capacitor 310.

Another JFET 325 is connected in parallel to the diode 310, with a drain connection being connected to the cathode and a source connection being connected to the anode of the diode 310. The gate connection of the other JFET 325 is connected, for example, to an interface 330. The source connection of the other JFET 325 is also connected to a high potential of a voltage supply 335, which provides a supply voltage for the driver 125.

If the gate connection of the additional JFET 325 is connected to a suitable potential, the JFET 325 opens and the capacitor 310 is prevented from discharging when the level of the control 125 changes. The diode 315 allows the capacitor 310 to be charged via the control by the driver 125 and prevents the capacitor 310 from discharging when the additional JFET 325 is closed. If the signal from the gate connection is removed or connected to a complementary signal, the JFET 325 closes so that the charged capacitor 310 is located on the current valve 215 between the gate connection and the source connection. The capacitor voltage opens the current valve 215 and prevents current flow through the current valve 215 from the drain connection to the source connection.

In one embodiment, the interface 330 is connected to a source of an appropriate shutdown signal, for example to a control terminal of a control module 225 that includes the flow valve 215. The signal may become active if the supply voltage drops below a predetermined threshold. The signal may have a hysteresis so that it is not deactivated again until the supply voltage exceeds a second threshold that is higher than the first threshold. A resistor 340 between the interface 330 and the source terminal of the further JFET 325 may ensure that the JFET 325 is closed when no signal is present at the interface 330.

An optional circuit that is 3 can generate a suitable signal at the interface 330. For this purpose, a Z-diode 345 and a resistor 350 are connected in series with the supply voltage of the power supply 335. The anode of the Z-diode 345 is at the negative potential of the power supply 335. The connection between the cathode of the Z-diode 345 and the resistor 350 is connected to the interface 330.

If there is no voltage error on the supply voltage, the negative supply voltage drops across the resistor 350 and the Z-diode 345. The Z-diode 345 has a suitable breakdown voltage, so that the voltage across the resistor 350 is sufficiently high in this case to keep the other JFET 325 safely open. For example, the supply voltage can be approximately -20 V. The Z-diode 345 has an exemplary breakdown voltage of approximately 10 V. Thus, the voltage drops across the resistor The voltage drop from the output 350 is also 10 V and the control voltage of the other JFET 325 is -10 V, so that the other JFET 325 is open. The current valve 215 can be freely controlled by the driver 125.

If the supply voltage now drops, only the voltage that drops across the resistor 350 is initially reduced, since the Zener diode 345 continues to block a predetermined voltage of 10 V, which drops across the Zener diode 345. If the voltage at the resistor 350 falls below the threshold voltage of the other JFET 325, the capacitor 310 is switched to the gate connection of the current valve 215, so that the current valve 215 is opened. Closing of the current valve 215 can be prevented until the capacitor 310 is discharged far enough.

The resistor 320 can cause the current valve 215 to close very slowly from the safely switched off state by discharging the capacitor 310 in a controlled manner. This preferably takes place at a time when the energy storage device 130 is already safely separated from the inverter 115 by means of a device such as the protective device 245. By gradually closing the current valve 215, the ZKK 245 can be discharged, whereby the energy stored in it can be converted into heat at the current valve 215.

If the supply voltage increases again, the additional JFET 325 can open again so that the safe shutdown of the current valve 215 is terminated and the current valve 215 can be freely controlled by the driver 125.

4 shows a circuit diagram with a current valve 215 of an inverter 115, a safety circuit 300 and an isolating circuit 400. The driver 125 is shown as an equivalent circuit with a first voltage source 410 and a second voltage source 415, the voltages provided by which are combined to a common connection by means of a first switch 420 and a second switch 425, respectively. Such an embodiment can preferably be implemented using CMOS technology. The switches 420, 425 are controlled alternately by the driver 125 in order to open and close the current valve 215, respectively.

A first diode 430 in the forward direction and, antiparallel thereto, a second diode 435 in the reverse direction can be provided in order to provide two separate signals from the combined signal, one of which comprises a switch-on signal and the other a switch-off signal for the current valve 215. Preferably, the switch-on signal and the switch-off signal each pass through a resistor 440. The diodes 430, 435 and the resistors 440 can be included in the driver 125.

In the present embodiment, the switch-off signal is fed directly to the gate terminal of the current valve 215. The switch-on signal can also be connected to the gate terminal of the current valve 215 or isolated from it by means of a semiconductor switch 445. The semiconductor switch 445 is preferably designed as a field effect transistor or as a bipolar transistor. The semiconductor switch 445 closes when a sufficiently high voltage drops across a resistor 450 between its source terminal and its gate terminal, thus enabling the current valve 215 to be controlled via the switch-on signal. The gate terminal of the semiconductor switch 445 is connected to the fuse circuit 300 by means of an optional Z-diode 455.

The safety circuit 300 responds when the supply voltage of the voltage supply 335 of the driver 125 falls below a predetermined threshold value and then connects the terminal of the Z-diode 455 to the source terminal of the current valve 215. If the voltage across the capacitor 310, which is parallel to the gate capacitance 305 of the switch 215, falls below a value that can be set by the Z-diode 455 due to self-discharge or the discharge via the driver in the path of the switch 420, the semiconductor switch 445 blocks, so that discharging of the capacitor 310 via the switch-on signal is prevented.

5 shows a circuit diagram according to 4 , however, the driver 125 has a different structure and the isolating circuit 400 is adapted to it. The voltage sources 410, 415 of the driver 125 are not connected to one another here, but are led separately from one another to the corresponding resistors 440, whereby the diodes 430, 435 are omitted. Such an embodiment is known as an open collector. In this case, the isolating circuit 400 can additionally comprise a diode 505, which is arranged in the blocking direction in the signal path of the switch-off signal. The diode 505 is located here between the gate connection of the current valve 215 and the resistor 440, which leads to the second voltage source 415 via the second switch 425.

6 shows simulated voltage curves on an inverter 115. A time is plotted in a horizontal direction and a voltage in a vertical direction. Scales in both directions are to be regarded as exemplary. It should be noted that the voltages shown are essentially negative, cf. Circuit diagrams of the 3 to 5 . A first representation 605 takes into account the shutdown device 300 with the isolating circuit 400, a second representation 610 assumes a shutdown device 300 without isolating circuit 400.

A first curve 615 represents the supply voltage of the power supply 335 of the driver 125. Without a voltage error, this is approximately -20 V. At a time of approximately 0.55 ms, a voltage error occurs and the supply voltage rises to approximately 0 V.

A second curve 620 relates to a control signal at the gate connection of the current valve 215. Without a voltage error, a square wave voltage is present here to periodically open and close the current valve 215. After the voltage error occurs, no control signal is provided.

A third curve 625 shows a capacitor voltage applied to the capacitor 310. If the shutdown device 400 is provided, the capacitor voltage increases slightly from approximately -20 V to approximately -15 V after the voltage error occurs, but can then be kept practically constant. The current valve 215 is held in the open state by the capacitor voltage.

Without the shutdown device 400, the capacitor voltage can only remain at a sufficiently low level for a short time after the voltage error occurs and ensure the opening of the current valve 215. Less than 0.05 ms after the voltage error occurs, the capacitor voltage has reached a value close to 0 V, so that the current valve 215 can no longer be kept open with energy from the capacitor 310.

Reference symbols

100

Axle drive

105

Motor vehicle

110

electric drive machine

115

Inverter

120

Power amplifier

125

driver

130

Energy storage

205

Intermediate circuit capacitor ZKK

210

Half bridge

215

Flow valve

220

Resistance, current sensor

225

Control module

230

Protective device

D

Drain connection

G

Gate connection

S

Source connection

300

Safety circuit

305

GS capacity

310

capacitor

315

diode

320

Resistance

325

further JFET

330

interface

335

Power supply

340

Resistance

345

Z-diode

350

Resistance

400

Isolating circuit

410

first voltage source

415

second voltage source

420

first switch

425

second switch

430

first diode

435

second diode

440

Resistance

445

Semiconductor switch, FET

450

Resistance

455

Z-diode

505

Diodes

605

first representation

610

second representation

615

first curve: supply voltage

620

second course: control signal current valve

625

third curve: capacitor voltage

Claims (10)

Output stage (120) for an inverter, in particular for controlling an electrical machine (110), wherein the output stage (120) comprises a half-bridge with a current valve (215) designed as a JFET; wherein the current valve (215) is designed to be controlled by a driver (125); wherein the output stage (120) further comprises a safety circuit (300) for actively opening the current valve (215) in the event of a lack of supply supply voltage of the driver (125) and a separating device (400) for isolating the current valve (215) from the driver (125). Power amplifier (120) after Claim 1 , wherein the driver (125) is configured to actively open the flow valve (215) by means of a switch-off signal or to actively close it by means of a switch-on signal; wherein the isolating device (400) is configured to isolate the second signal from the flow valve (215). Power amplifier (120) after Claim 1 or 2 wherein the isolation device (400) comprises a semiconductor switch (445) to control forwarding or isolation of a signal from the driver (125) to the flow valve (215). Power amplifier (120) after Claim 3 , wherein the semiconductor switch (445) is controlled by means of a voltage provided by the fuse circuit (300); wherein a Zener diode (455) is provided to open the semiconductor switch (445) when the voltage reaches a predetermined value. Power amplifier (120) after Claim 4 , further comprising a resistor (450) in series with the Zener diode (455), wherein a control voltage for the semiconductor switch (445) drops across this resistor (450). Output stage (120) according to one of the preceding claims, wherein the driver (125) provides a signal which is converted by means of a first diode into a switch-on signal and by means of an anti-parallel second diode into a switch-off signal for the current valve. Power amplifier according to one of the Claims 1 until 5 , wherein the driver provides a switch-on signal and an independent switch-off signal and the switch-off signal is decoupled from the current valve by means of a diode. Inverter (115) for controlling an electrical machine (110), comprising an output stage (120) according to one of the preceding claims. Electric axle drive (100) for a motor vehicle (105), comprising an inverter (115) according to Claim 8 . Motor vehicle (105) comprising an electric axle drive (100) according to Claim 9 .

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