Method for reverse operation of a flyback converter with active voltage limiting circuit

Abstract

The invention relates to a method (100) for operating a flyback converter (250) for charging a DC link capacitor (210), comprising the steps of: Specifying a first duty factor (T1) for controlling the first circuit breaker (252) and specifying a second duty factor (T2) for controlling a second circuit breaker (261) of an overvoltage protection circuit.

Description

[0001] Description

[0002] title

[0003] Method for reverse operation of a flyback converter with active voltage limiting circuit

[0004] The invention relates to a method for operating a flyback converter for charging an intermediate circuit capacitor, as well as to the flyback converter for charging the intermediate circuit capacitor. Furthermore, the invention relates to a drive train with a corresponding device and a vehicle with a drive train, as well as to a computer program and a computer-readable storage medium.

[0005] State of the art

[0006] Methods and devices for charging an intermediate circuit capacitor in a high-voltage network are known from the prior art. Preferably, vehicles with an electric drive train comprise an intermediate circuit capacitor in a high-voltage network, which is arranged between an energy source, preferably a direct current and / or high-voltage energy source, and the power switches of a pulse-controlled inverter. The intermediate circuit capacitor is preferably arranged within the pulse-controlled inverter on the direct current side. The energy source, preferably a traction battery, serves to supply an electrical machine with electrical energy. For this purpose, the electrical energy is converted by means of the pulse-controlled inverter. The direct current of the energy source is converted into an alternating current to supply a multi-phase electrical machine.When the vehicle is switched off or parked, the energy source is disconnected from the high-voltage network using at least one switch, and the high-voltage network is discharged to ensure that the high-voltage network does not pose a danger, even if conductive parts are touched. When the vehicle is restarted, the energy source must be conductively connected to the high-voltage network. Due to the voltage difference between the discharged high-voltage network and the energy source, switching it on using at least one switch would result in unacceptably high compensating currents. To reduce the voltage differences before switching on the energy source, the intermediate circuit capacitor in the high-voltage network is precharged or charged to a voltage that approximately corresponds to the voltage of the energy source.When the at least one switch is subsequently switched on or closed, there are no longer any significant voltage differences, meaning that no relevant compensating currents occur. The charging, recharging or pre-charging of the intermediate circuit capacitor is usually carried out by means of an additional circuit that provides a parallel current path to the at least one switch and also comprises a switch and a resistor, pre-charging resistor or series resistor. For charging, the switch of the parallel current path is closed. The resulting compensating current is limited to permissible values ​​by means of the resistor. Once the charging process is complete, the at least one switch between the high-voltage network and the energy source is closed. The drive train is then ready for use again. The parallel current path with switch and pre-charging resistor requires additional components and space and leads to additional weight and costs.Therefore, there is a need for alternative solutions that enable charging of the intermediate circuit capacitor without the parallel current path. DE 10 2020 206987 A1 discloses a method for charging an intermediate circuit capacitor in a vehicle's high-voltage network. For this purpose, a flyback converter between a low-voltage network and a high-voltage network can be used. This is preferably a bidirectional flyback converter, which usually supplies the low-voltage network from the high-voltage network during ferry operation (buck operation or step-down converter operation) and, when a vehicle is started and the direction of the energy flow is reversed, supplies the high-voltage network from the low-voltage network (boost operation or step-up converter operation), thereby charging an intermediate circuit capacitor in the high-voltage network.By specifying a duty cycle for a switching element on the low-voltage side of the flyback converter, electrical energy can be transferred from the low-voltage side to the high-voltage side. When the switching element is closed, electrical energy is stored in the transformer winding. Once the switching element is opened, this energy is transferred to the high-voltage side, where it is rectified via a passive diode, enabling the charging of an intermediate circuit capacitor. Once the current in the high-voltage network has subsided, the switching element on the low-voltage side is closed again - and the process begins again. Control of any switching elements on the high-voltage side of the flyback converter is not necessary during boost converter operation.To prevent overvoltages on the high-voltage side of the flyback converter, overvoltage protection circuits or active clamping circuits are used, preferably for buck operation or step-down converter operation of the flyback converter to supply the low-voltage network from the high-voltage network. Overvoltage protection circuits comprise a capacitor for absorbing excess energy and a second switching element. It has been observed that during boost converter operation, as described above, the supply voltage of the gate driver of the second switching element, which is preferably passive in boost converter operation, continues to rise over time and exceeds permissible values. This can lead to failure of the gate driver of the second switching element and thus of the entire flyback converter.Therefore, there is a need for methods and devices for operating a flyback converter for charging an intermediate circuit capacitor, in which an increase in a supply voltage of the gate driver of a second switching element on the high-voltage side of the flyback converter is minimized.

[0007] Disclosure of the invention

[0008] A method is provided for operating a flyback converter for charging an intermediate circuit capacitor. The flyback converter can be connected to a low-voltage network on the input side and to a high-voltage network on the output side. The high-voltage network comprises the intermediate circuit capacitor to be charged. During operation, the flyback converter is preferably connected to the low-voltage network on the input side and to the high-voltage network on the output side. In this context, "connected" or "connectable" preferably means electrically conductively or galvanically connected, coupled, or connected together. The flyback converter comprises, on the input side between the input terminals, a series circuit comprising a first power switch and a primary winding of a transformer, and, on the output side between the output terminals, a series circuit comprising a secondary winding of the transformer, a leakage inductance of the transformer, and a first diode connected in the reverse direction.The first diode is preferably designed as a body diode of a third power switch 268. A power switch is preferably a switching element or semiconductor power switch, preferably based on IGBT, MOSFET, silicon, SiC, or GaN technology. On the output side, a first capacitor is connected between the output terminals. On the input side, a second capacitor is preferably connected between the input terminals. To ensure non-destructive operation of the flyback converter, preferably for the buck converter operation of a bidirectional flyback converter, an active clamping circuit or overvoltage protection circuit is provided on the output side of the flyback converter. The overvoltage protection circuit preferably comprises a series connection of a third capacitor and a second power switch, wherein the series connection is connected in parallel to the secondary winding and the stray inductance of the transformer.The method comprises the steps of: specifying a first duty cycle for controlling the first circuit breaker to charge the intermediate circuit capacitor and specifying a second duty cycle for controlling the second circuit breaker.

[0009] Preferably by means of a control device, a duty cycle for controlling the first power switch is specified, so that a charging current is established on the output side of the flyback converter to the intermediate circuit capacitor. In principle, for this boost converter operation of the flyback converter for charging the intermediate circuit capacitor, controlling the switching elements located on the high-voltage side of the flyback converter is not necessary, since the charging current is conducted via the first diode. According to the invention, a second duty cycle for controlling the second power switch is specified, preferably by means of a control device. This acts as a load on the gate driver of the second power switch and thereby prevents the supply voltage at the gate driver of the second power switch from increasing. The second power switch is preferably located on the output side or high-voltage side of the flyback converter.When the first circuit breaker is controlled with a first duty cycle to charge the intermediate circuit capacitor, a small current always flows, preferably from the high-voltage network, towards the supply voltage connection of the gate driver of the second circuit breaker. As long as the second circuit breaker remains open, the supply voltage increases due to the current flow and can reach inadmissible values. To limit the supply voltage, a load is required on the gate driver of the second circuit breaker to reduce the unwanted energy flow. Each time the second circuit breaker is switched on, the gate driver requires energy that comes from the supply voltage and thus prevents it from rising. Switching on due to the specification of the second duty cycle therefore acts as a load on the gate driver of the second circuit breaker and counteracts a voltage increase.

[0010] To charge the intermediate circuit capacitor, the flyback converter is operated as a boost converter. Preferably, a lower input voltage, for example a vehicle's on-board network voltage of approximately 12 - 14 volts or 48 volts, is continuously boosted, thus charging the intermediate circuit capacitor until an output voltage corresponding to an energy source connected to the high-voltage network, for example 200, 400, 800 or even 1000 volts, is applied to the intermediate circuit capacitor. For this purpose, the first power switch on the input side is controlled with a predetermined duty cycle. Preferably, the precharging or charging of the intermediate circuit capacitor to the desired high-voltage voltage should take place as quickly as possible. To avoid excessive currents and / or voltages at the first power switch, the operation of the flyback converter for charging the intermediate circuit capacitor is divided into at least two phases.First, a current-controlled phase is carried out, followed by a voltage-controlled phase. Preferably, in the current-controlled phase, the duty cycle for controlling the first power breaker is determined as a function of the determined voltage in accordance with a predetermined maximum target charging current value, and the first power breaker is controlled accordingly. If the determined voltage rises above a predeterminable first voltage value, for example 90% of the achievable high-voltage voltage, due to the operation of the flyback converter for charging the intermediate circuit capacitor, the current-controlled operation is ended and the voltage-controlled phase is carried out. Preferably, in the voltage-controlled phase, the duty cycle for controlling the first power breaker is determined as a function of the determined voltage in accordance with the predetermined target charging voltage value, and the first power breaker is controlled accordingly.If the measured voltage rises above a predefined second voltage value due to the operation of the flyback converter for charging the DC link capacitor, which, for example, corresponds to the high-voltage target, the voltage-controlled operation is terminated. This advantageously enables rapid charging of a DC link capacitor while limiting the maximum currents and voltages at the first circuit breaker.

[0011] Preferably, the capacitance of the overvoltage protection circuit is specifically charged by controlling the first circuit breaker using a predeterminable duty cycle for a predeterminable period of time. The duty cycle is specified such that excessive currents and voltages never occur at the first circuit breaker. Accordingly, the duty cycle is selected to be very small, for example, 1 to 10% of the maximum duty cycle selected when providing the target charging current. The time period is selected such that the third capacitor is charged reliably. After the third capacitor has been charged, the intermediate circuit capacitor can be charged quickly without excessive currents and voltages occurring at the first circuit breaker. Advantageously, a method is provided which enables rapid, non-destructive charging of the intermediate circuit capacitor despite the overvoltage protection circuit.

[0012] In another embodiment, the first duty cycle and the second duty cycle are specified synchronously with each other.

[0013] The first duty cycle and the second duty cycle are specified synchronously with one another. Preferably, the duty cycles are different, at least temporarily. By specifying a synchronous second duty cycle, the gate driver of the second power switch is sufficiently loaded. Preferably, the second duty cycle is specified depending on the operating strategy of the first and / or third power switch or depending on the duty cycle of the first and / or third power switch, preferably in the same PWM grid. This advantageously prevents an increase in the supply voltage at the gate driver of the second power switch. In another embodiment, the values ​​of the first duty cycle and the second duty cycle are the same.

[0014] The values ​​of the first duty cycle and the second duty cycle match. By specifying a second duty cycle that matches the first duty cycle, the gate driver of the second power switch is sufficiently loaded, which advantageously prevents an increase in the supply voltage at the gate driver of the second power switch. This is a particularly simple specification for the second duty cycle.

[0015] In another embodiment, the method comprises the following step: specifying a target charging current value or target voltage charging value for charging the intermediate circuit capacitor and specifying a first duty cycle for controlling the first power switch as a function of the specified target charging current value or target voltage charging value.

[0016] A target charging current value or a target charging voltage value is specified, preferably by means of a control device. Preferably, a charging voltage or charging current that is approximately equal to or corresponding to the target charging voltage value or target charging current value results on the output side of the intermediate circuit capacitor to be charged.

[0017] Furthermore, the invention relates to a computer program comprising instructions which, when the program is executed by a control device, cause the control device to carry out the steps of the described method.

[0018] Furthermore, the invention relates to a computer-readable storage medium comprising instructions which, when executed by a control device, cause the control device to carry out the steps of the described method.

[0019] The invention further relates to a flyback converter for charging an intermediate circuit capacitor in a high-voltage network. The flyback converter can be coupled to a low-voltage network on the input side and to a high-voltage network on the output side. The flyback converter comprises a control device configured to carry out a method as described above. A flyback converter for charging an intermediate circuit capacitor in a high-voltage network is provided. The flyback converter comprises a control device configured to carry out the described method. For this purpose, the control device preferably comprises configured and controlled inputs and outputs that enable control of the first and second power switches as described.

[0020] Advantageously, a flyback converter is provided for rapidly charging an intermediate circuit capacitor, in which an overvoltage of the supply voltage at the gate driver of the second power switch is prevented.

[0021] The invention further relates to a drive train with a described flyback converter and preferably with power electronics and / or an electric drive. Such a drive train serves, for example, to drive an electric vehicle. The method and the flyback converter enable operation of the drive train without an excessive supply voltage at the gate driver of the second power switch.

[0022] The invention further relates to a vehicle with a described drive train. Advantageously, a vehicle is thus provided that includes a flyback converter that enables safe charging of the intermediate circuit capacitor.

[0023] It is understood that the features, properties and advantages of the method according to the invention apply or are applicable accordingly to the device or the drive train and the vehicle and vice versa.

[0024] Further features and advantages of embodiments of the invention will become apparent from the following description with reference to the accompanying drawings.

[0025] Brief description of the drawing In the following, the invention will be explained in more detail with the aid of some figures, which show:

[0026] Figure 1 shows a first schematic representation of a flyback converter for charging an intermediate circuit capacitor,

[0027] Figure 2 shows a second schematic representation of a flyback converter for charging an intermediate circuit capacitor,

[0028] Figure 3 shows a schematic representation of a vehicle with a drive train,

[0029] Figure 4 shows a schematic flow diagram for a method for charging an intermediate circuit capacitor.

[0030] Embodiments of the invention

[0031] Figure 1 shows a first schematic representation of a flyback converter 250 or a possible circuit topology for charging an intermediate circuit capacitor 210. The intermediate circuit capacitor 210 is preferably connected to a high-voltage network 205 via the output terminals 242_p, 242_n for charging. Figure 1 shows a flyback converter 250 configured to execute a method for operating the flyback converter 250 for charging the intermediate circuit capacitor 210 in the high-voltage network 205. The flyback converter 250, also called a flyback converter, is connectable, preferably connected, on the input side to a low-voltage network 295 and on the output side to a high-voltage network 205. The high-voltage network 205 includes the intermediate circuit capacitor 210. The flyback converter 250 includes, on the input side, a series circuit consisting of a first power switch 252 and a primary winding 254 of a transformer 260 between the input terminals 244_p, 244_n.Preferably, the input terminal 224_p is the positive input terminal. Preferably, the positive input terminal 224_p is configured to be connected to a positive terminal pole of a low-voltage source of a connectable low-voltage network 295. Preferably, the input terminal 224_n is the negative input terminal. Preferably, the negative input terminal 224_n is configured to be connected to a negative terminal pole of the low-voltage source of a connectable low-voltage network 295. On the output side, the flyback converter comprises, between the output terminals 242_p, 242_n, a series circuit of a secondary winding 256 of the transformer 260, the leakage inductance 265 of the transformer, and a reverse-biased first diode 258, wherein on the output side, a first capacitor 262 is connected between the output terminals 242_p, 242_n. Preferably, the output terminal 242_p is the positive output terminal.The positive output terminal 242_p is preferably configured to be connected to a positive terminal pole of a high-voltage source of the connectable high-voltage network 205. The output terminal 242_n is preferably the negative output terminal. The negative output terminal 242_n is preferably configured to be connected to a negative terminal pole of the high-voltage source of the connectable high-voltage network 205. The primary winding 254 and the secondary winding 256 of the transformer 260 are preferably wound in opposite directions to define the current direction on the primary and secondary sides. A second capacitor 264 is preferably connected on the input side between the input terminals 244_p, 244_n to smooth the voltage between the input terminals, even during reverse operation of the flyback converter. The first diode 258 is preferably configured as a body diode of a third power switch 268.The flyback converter can preferably transfer electrical energy from the output side to the input side in buck converter operation by controlling the third power switch 268. Preferably, the third power switch 268 converts the DC voltage of the high-voltage network 205 into an AC voltage so that the transformer 260 can transmit it. Merely for clarity, the stray inductance 265 of the transformer 260 between the secondary winding 256 of the transformer 260 and the first diode 258 is shown as an additional component in the equivalent circuit diagram in Figure 1. Equivalent to this is a secondary winding 256 with a parasitic stray inductance, which includes the stray inductance 265.The flyback converter 250 comprises an active clamping circuit, or overvoltage protection circuit, which is designed as a series connection of a third capacitor 263 and a second power switch 261. The series connection of the active clamping circuit is connected in parallel to the secondary winding 256 of the transformer 260 and its stray inductance 265. The active clamping circuit uses the third capacitor 263 to reduce voltage peaks occurring at the third power switch 268 during switching operations of the third power switch 268. To charge the intermediate circuit capacitor 210 connected to the flyback converter 250, preferably in boost converter operation, a first duty cycle T1 is specified for controlling the first power switch 252 and a second duty cycle T2 is specified for controlling the second power switch 261. A gate driver 270 of the second power switch is supplied with a supply voltage VDDA.By means of the temporally overlapping control of the first power switch 252 and the second power switch 261 via the gate driver 270, an increase in the supply voltage VDDA at the gate driver 270 of the second power switch 261 is prevented. A voltage, preferably the voltage Ux, results between the output terminals 242_p, 242_n, which, for example, characterizes the voltage at a connected intermediate circuit capacitor 210. To charge the intermediate circuit capacitor 210, a current Ix flows through the flyback converter 250 and via the positive output terminal 242_p into the intermediate circuit capacitor 210.

[0032] Figure 2 shows a second schematic representation of a flyback converter 250 for charging an intermediate circuit capacitor 210. The high-voltage network 205 is connectable to a power source 220 or the high-voltage source by means of at least one switch 230. The high-voltage network 205 is coupled to a low-voltage network 295 by means of the DC-DC converter or flyback converter 250. The flyback converter 250 comprises an inductive coupling or a transformer and thus galvanically separates the high-voltage network 205 from the low-voltage network 295. The low-voltage network 295 is preferably supplied with electrical energy from the high-voltage network 205. For this purpose, the flyback converter 250 is preferably controlled in reverse and in the buck converter operating mode. The low-voltage network 295 preferably comprises a low-voltage energy source 222 or the low-voltage source, for example a battery or an accumulator, and further consumers (not shown), for example control devices.A control device 255 is designed and / or configured to specify a first duty cycle T1 for controlling the first power switch 252 and a second duty cycle T2 for controlling the second power switch 261. In parallel with the control of the first power switch 252, the second power switch 261 is controlled by means of a second duty cycle via the gate driver 270. This prevents an increase in the supply voltage VDDA at the gate driver 270 of the second power switch 261. Preferably, the control device 255 is designed and / or configured to determine a voltage Ux, to specify a target charging current value l_L, to specify a corresponding first duty cycle T1 for controlling the first power switch 252, and to specify a target charging voltage value U_L.The control device 255 preferably comprises a current regulator or a voltage regulator for determining the corresponding first duty cycle T1 as a function of the target charging current value or the target charging voltage value. In this illustration, the control device 255 is shown with the current regulator outside the flyback converter 250, although the control device 255 and the current regulator can also be arranged within the flyback converter 250.

[0033] Figure 3 shows a schematically illustrated vehicle 400 with a drive train 300. The illustration shows, by way of example, a vehicle 400 that can be used equally on land, on water, and in the air. The drive train 300 comprises the flyback converter 250 and preferably power electronics, a pulse-controlled inverter 272. The drive train preferably further comprises an energy source 220, the intermediate circuit capacitor 210, an electric machine 280, and / or a low-voltage energy source 222. The pulse-controlled inverter 272 preferably serves to supply the electric machine 280 with electrical energy from the energy source 220. The pulse-controlled inverter 272 preferably comprises the intermediate circuit capacitor 210, so that during operation the flyback converter 250 is connected to the pulse-controlled inverter 272.

[0034] Figure 4 shows a schematically illustrated flowchart for a method 100 for operating a flyback converter 250 for charging an intermediate circuit capacitor 210. The method begins with step 105. Preferably, in step 135, a target charging current value l_L or target voltage charging value U_L for charging the intermediate circuit capacitor 210 is specified. In step 140, a first duty cycle T1 for controlling the first power switch 252 is specified, and in step 150, a second duty cycle T2 for controlling the second power switch 261 is specified. The method 100 for operating a flyback converter 250 is preferably terminated after a determined voltage Ux exceeds a predeterminable first voltage value U1. The method ends with step 175.

[0035] Procedure.

Claims

Claims 1. Method (100) for operating a flyback converter (250) for charging an intermediate circuit capacitor (210), wherein the flyback converter (250) is connectable on the input side to a low-voltage network (295) and on the output side to a high-voltage network (205), wherein the high-voltage network (205) comprises the intermediate circuit capacitor (210), wherein the flyback converter (250) comprises on the input side between the input terminals (244_p, 244_n) a series circuit of a first power switch (252) and a primary winding (254) of a transformer (260) and on the output side between the output terminals (242_p, 242_n) a series circuit of a secondary winding (256) of the transformer (260), a stray inductance (265) of the transformer (260), and a first diode connected in the reverse direction (258), wherein on the output side a first capacitor (262) is connected between the output terminals (242_p, 242_n), wherein the flyback converter (250) comprises an overvoltage protection circuit,which is designed as a series circuit of a third capacitor (263) and a second power switch (261) and is connected in parallel to the secondary winding (256) and the stray inductance (265) of the transformer (260), comprising the steps of: Presetting (140) a first duty cycle (T1) for controlling the first power switch (252) to charge the intermediate circuit capacitor (210), characterized by the step, Specifying (150) a second duty cycle (T2) for controlling the second circuit breaker (261).

2. Method according to claim 1, wherein the first duty cycle (T1) and the second duty cycle (T2) are specified synchronously with one another.

3. Method according to one of the preceding claims, wherein the values of the first duty cycle (T1) and the second duty cycle (T2) match.

4. Method according to one of the preceding claims, comprising the step: specifying (135) a target charging current value (l_L) or target voltage charging value (U_L) for charging the intermediate circuit capacitor (210) and specifying a first duty cycle (T1) for controlling the first power switch (252) as a function of the specified target charging current value (l_L) or target voltage charging value (U_L).

5. A computer program comprising instructions which, when the program is executed by a control device (255), cause the control device (255) to carry out the method / steps of the method (100) according to claims 1 to 4.

6. A computer-readable storage medium comprising instructions which, when executed by a control device (255), cause the control device (255) to carry out the method / steps of the method (100) according to claims 1 to 4.

7. Flyback converter (250) for charging an intermediate circuit capacitor (210) in a high-voltage network (205), wherein the flyback converter (250) can be coupled on the input side to a low-voltage network (295) and on the output side to a high-voltage network (205), wherein the flyback converter (250) comprises a control device (255) and is designed to carry out a method according to one of claims 1 to 4.

8. Drive train (300) with a flyback converter (250) according to claim 7.

9. Vehicle (400) having a drive train (300) according to claim 8.

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