Temperature determination of a half bridge

Abstract

Method (300) for temperature determination in a half-bridge (125) for voltage control on an inductive load (120), wherein the half-bridge (125) has a first and a second field-effect transistor (130, 135), the method (300) comprising the following steps: - switching off (310) the switched-on first field-effect transistor (130) while the second field-effect transistor (135) is switched off; - switching on (315) the second field effect transistor (135); - temporarily switching off (320) the second field effect transistor (135); - simultaneously determining (330) a voltage applied between the source and drain of the second field-effect transistor (135) and (335) a current flowing between the half-bridge (125) and the inductive load (120); and - determining (340) a temperature of the second field effect transistor (135) based on the determined voltage and the determined current.

Description

[0001] The present invention relates to the determination of a temperature in a half-bridge for voltage control. In particular, the invention relates to the determination of the temperature in a field-effect transistor encompassed by the half-bridge.

[0002] A half-bridge comprising a first and a second field-effect transistor (FET) can be used to supply a voltage to a load. The half-bridge is connected to a bridge voltage, with the first FET acting as a current valve between a high potential and the load, and the second FET acting as a current valve between the load and a low potential of the bridge voltage. The FETs can be alternately opened and closed at a frequency, with a duty cycle of opening and closing times determining the voltage applied to the load. The load can be inductive or include an inductive component.

[0003] In certain applications, a large current can flow through the load and thus through the current valves of the half-bridge. To prevent overload or to control the half-bridge depending on the operating temperature, it is necessary to determine the temperature of one of the FETs. For this purpose, a dedicated temperature sensor can be integrated into the FET, or an external temperature sensor can be thermally coupled to a cooling surface of the FET.

[0004] DE 10 2014 100 122 B3 proposes injecting a current into an off-state FET and determining its temperature based on a voltage drop across a diode formed in the FET. However, this technique is only applicable when no current flows through the diode. This is not possible for an inverter in normal, continuous operation.

[0005] US 2020 / 0 112 245 A1 describes a method for determining the temperature in a half-bridge arranged in an array of three half-bridges for driving an inductive load.

[0006] Such an arrangement is also disclosed in US 2022 / 0 018 720 A1, where an electrical machine, namely a motor, is controlled as the inductive load.

[0007] US 2004 / 0 208 227 A1 describes a method for determining the temperature of a MOSFET by waiting for the decay of a reverse current.

[0008] DE 10 2020 116 424 A1 describes a method for determining a junction temperature, in which the temperature determination is either carried out continuously or repeated regularly.

[0009] DE 103 51 843 A1 describes a method for determining the temperature of a power semiconductor in a half-bridge; the temperature determination is carried out when a load current exceeds a specified value.

[0010] DE 10 2017 205 625 A1 describes a method for temperature determination in a half-bridge for voltage control on an inductive load, wherein the half-bridge has a first and a second field-effect transistor, the method comprising the following steps: - switching off the switched-on first field-effect transistor while the second field-effect transistor is switched off; - Switching on the second field effect transistor; - determining a voltage applied between the source and drain of the second field-effect transistor and a current flowing between the half-bridge and the inductive load; and - Determining a temperature of the second field effect transistor based on the determined voltage and the determined current.

[0011] An object underlying the present invention is to provide an improved technique for determining the temperature of a FET in a half-bridge. The invention achieves this object by means of the subject matter of independent claim 1. Subclaims specify preferred embodiments.

[0012] A half-bridge is configured to control the voltage across an inductive load and comprises a first and a second field-effect transistor (FET).

[0013] The present invention provides a method for temperature determination in the half-bridge, comprising the steps of - switching off the switched-on first field-effect transistor while the second field-effect transistor is switched off; - switching on the second field effect transistor; - temporarily switching off the second field-effect transistor; - simultaneously determining a voltage applied between the source and drain of the second field-effect transistor and a current flowing between the half-bridge and the inductive load; and - determining a temperature of the second field effect transistor based on the determined voltage and the determined current.

[0014] When the first FET is turned off, the energy stored in the inductive load at that time must flow away in the form of a current. This can be achieved by diverting the current through a diode formed between the source and drain of the second FET. Due to its design, the FET's diode is unavoidable and is also called parasitic. Since its breakdown voltage is significantly greater than the voltage drop across the closed second FET, the latter is preferentially closed, thus bypassing the diode and reducing thermal losses. The current then flows "backwards," i.e., opposite to the usual current direction, through the second FET; this is also referred to as a "reverse-closed" second FET.

[0015] According to the invention, it is proposed to determine the temperature of the second FET on the basis of a current flowing from the inductive load through the parasitic diode after the first FET has been turned off. More specifically, the reverse closing of the FET can be interrupted in order to create a voltage drop across the diode. The magnitude of the current flowing through the diode can be determined using a current sensor, which is already present in many applications. For example, field-oriented control of an electrical machine may require the determination of the currents through several phases of the machine, with a voltage to each phase being controlled by means of an associated half-bridge. The method can be carried out on a half-bridge that continuously controls the voltage across the inductive load, for example in the manner of a sinusoid.

[0016] A conventional FET can be used cost-effectively to implement the method. If the method is implemented with multiple half-bridges, an FET module comprising multiple FETs can also be used. The temperature of the first FET of the half-bridge can be determined in the same way as that of the second. The temperature can be determined quickly and accurately and can, for example, allow observation of FET aging, determination of an indication of impending overload, or adaptation of the control system to the specific temperature. A specific temperature can correspond well to the actual temperature of the FET and follow it quickly. A safety function for protecting the half-bridge, an intermediate circuit, or the load can be expanded, checked for plausibility, or supported by monitoring the FET temperature. The safety function can, in particular, comprise switching off the load, in which case all FETs can be opened.

[0017] Preferably, after turning off the second FET and before determining the voltage, a waiting period is allowed to allow the diode formed between the source and drain of the second FET to fully close. The speed at which the diode conducts may be known, and this waiting period can be determined based on the voltage applied to the load. Alternatively, a maximum time required until the diode is fully conducting can be set.

[0018] To determine the temperature with sufficient accuracy, it is preferable that the current flowing between the half-bridge and the load be determined at the same time as the voltage is determined. In other words, it should be ensured that the time of determining the current does not deviate from the time of determining the voltage by more than a certain amount of time. Furthermore, the accuracy of the current determination should be sufficiently high.

[0019] In a preferred embodiment, the current can be determined by means of a Hall sensor on a conductor leading from the half-bridge to the load.

[0020] The temperature is preferably determined based on a temperature-dependent relationship between current and voltage at a diode formed between the source and drain of the second FET, namely a temperature-related diode characteristic of a diode of a field-effect transistor. This temperature-related characteristic of the diode can be determined before the method claimed in claim 1 is carried out. This diode characteristic can be determined for each individual diode or determined on a representative diode of an FET and transferred to other diodes on identical FETs.

[0021] The present invention also provides a further method, namely a method for voltage control across an inductive load by means of a half-bridge having two field-effect transistors that are alternately switched on to adjust a voltage across the load, wherein the temperature of the second field-effect transistor is determined by means of one of the methods described above.

[0022] This provides an inventive extension of a conventional method for controlling the voltage across the inductive load, which includes the method of claim 1. The temperatures of one or both FETs of the half-bridge can be determined while the voltage across the load is continuously controlled.

[0023] Preferably, the temperature is determined at a specific frequency. For example, a temperature can be determined every approximately 10-100 ms during normal operation. The FET can be loaded more heavily by a large current than by a smaller one. In another embodiment, the frequency is therefore determined as a function of the current flowing through the inductive load. If the current increases, the frequency can also increase. In one embodiment, a number of frequencies are provided, each of which is assigned a specific range of the current flowing through the inductive load. In another embodiment, the frequency can be determined continuously with respect to the current.

[0024] According to yet another aspect of the present invention, a control device for controlling a voltage across an inductive load by means of a half-bridge with a first and a second FET comprises a voltage sensor for determining a voltage between the source and drain of one of the FETs; a current sensor for determining a current flowing between the half-bridge and the load; and a processing device configured to carry out a method described herein. The processing device can carry out the method according to claim 1, preferably also embedded in the further method according to claim 8.

[0025] The processing device can be configured to carry out one of the methods described herein in whole or in part. For this purpose, the processing device can comprise a programmable microcomputer or microcontroller, and the method can be in the form of a computer program product with program code means. The computer program product can also be stored on a computer-readable data carrier. Features or advantages of the method can be transferred to the device, or vice versa.

[0026] According to yet another aspect of the present invention, an inverter for a multi-phase electrical machine comprises a half-bridge for each of the phases and a control device as described herein for controlling one of the half-bridges. The inverter may, in particular, comprise three half-bridges for controlling three phases of an electrical machine.

[0027] The invention will be described in more detail below with reference to the accompanying figures, in which: Fig. 1 an inverter; Fig. 2 a flow diagram of a process; Fig. 3 a flowchart of another method; Fig. 4 first time courses; and Fig. 5 second temporal courses.

[0028] Fig. 1 shows an inverter 100 configured, by way of example, to control an electric machine 105 based on current from an intermediate circuit 110. The electric machine 105 can be provided on board a vehicle and, in particular, configured to drive it. The intermediate circuit 110 includes an optional intermediate circuit capacitor 115 and can provide energy from an electrical energy storage device on board the vehicle.

[0029] The electric machine 105 comprises, for example, three phases, each representing an inductive load. To control a voltage across a phase, the inverter 100 comprises a half-bridge 125, so that a total of three half-bridges 125 are provided. The half-bridges 125 each comprise a first FET 130 and a second FET 135.

[0030] The first FET 130 is also called a high-side FET and in this case includes a drain (D) terminal connected to a high potential of the intermediate circuit 110, a source (S) terminal connected to the load 120, and a gate (G) terminal for control. The second FET 135 is also called a low-side FET and in this case includes a drain (D) terminal connected to the load 120, a source (S) terminal connected to a low potential of the intermediate circuit 110, and a gate (G) terminal for control. A current flow between source and drain can be controlled by providing a suitable control voltage at the gate. The FETs 130, 135 each include a diode 140, which in this case runs from source to drain and is attributable to the design of field-effect transistors.

[0031] A current sensor 145 is assigned to a half-bridge 125 to determine a current flowing between the half-bridge 125 and an associated load 120. Furthermore, a voltage sensor 150 is assigned to a FET 130, 135 to determine a voltage applied to the DS path or diode 140, respectively.

[0032] The half-bridges 125 can be controlled by a control device 155 connected to the gate terminals of the FETs 130, 135, the current sensors 145, and the voltage sensors 150. The control device 155 typically only controls one of the FETs 130, 135 of a half-bridge 125 at a time, so that a voltage is established at each phase of the machine. The voltages at the phases are preferably substantially sinusoidal and phase-shifted by 120°.

[0033] Fig. 2 shows a flowchart of a method 200 that may be performed by the controller 155. The method 200 relates to only one of the half-bridges 125 and illustrates a common way to control a voltage applied to the associated load 120.

[0034] In a step 205, the first FET 130 is switched on and the second FET 135 is switched off, so that the voltage at the load 120 increases. The increase is limited by the inductance of the load 120. In a step 210, both FETs 130, 135 are switched off, before in a step 215 the first FET 130 remains switched off and the second FET 135 is switched on, so that the voltage at the inductive load 120 drops. Step 210 is inserted to prevent both FETs 130, 135 from being switched on at the same time, thus causing a bridge short circuit across the intermediate circuit 110. Subsequently, in a step 220, both FETs 130, 135 are switched off again before the process can continue with step 205.

[0035] The voltage established at load 120 is determined by a ratio of times during which method 200 is in steps 205 and 215. One run of method 200 typically requires a constant time, and in one example, approximately 120,000 runs per second can occur. It is proposed to refine method 200 for determining a temperature of one of FETs 130, 135. For this purpose, additional steps can be provided, particularly at a transition between steps 205 and 210 or between steps 215 and 220.

[0036] Fig. 3 shows a flowchart of a method 300, which can be used individually or as part of a method 200 to determine a temperature at a FET 130, 135. The following assumes, by way of example, a determination of the temperature of the second FET 135, i.e., a transition of the method 200 from step 205 to step 210.

[0037] In a step 305, the first FET 130 is turned on and the second FET 135 is turned off. Then, in a step 310, the first FET 130 is turned off. To dissipate energy stored in the load 120, in a first variant of the method 300, the second FET 135 can be closed backward, i.e., turned on, in a step 315.

[0038] If the first FET 130 is completely switched off, the second FET 135 can be switched off again in a step 320, so that the energy from the load 120 flows through the diode 140 of the second FET 135 into the intermediate circuit 110. Optionally, in a step 325, it is possible to wait until the diode 140 completely allows the current to flow. Then, in a step 330, the voltage across the diode 140 or between the drain and source of the second FET 135 can be determined. If possible, at the same time, the current flowing through the diode 140 can be determined in a step 335. Since the first FET 130 is completely switched off at this time, this current corresponds to the current flowing between the half-bridge 125 and the load 120, which can be determined using the current sensor 145. Based on the determined voltage and current, the temperature of the second FET 135 can be determined in a step 340.A characteristic curve can be used that allows the temperature to be determined based on a specific current and voltage. The characteristic curve can be determined in advance.

[0039] Once the voltage and current are determined, the second FET 135 can be closed backward again in a step 345, ie turned on, to short-circuit the diode 140 and to divert the current from the load 120 into the intermediate circuit 110 with greater efficiency.

[0040] In a second variant of the method 300, which is Fig. 3 is indicated by dashed lines, a branch can be made from step 310 to step 325. Thus, the current drawn from load 120 is immediately diverted through diode 140 without temporarily turning on second FET 135. After measurements have been completed, second FET 135 can be turned on in reverse in step 345 as described.

[0041] Method 300 can be used in a corresponding manner to determine the temperature of the first FET 130, preferably during the transition between steps 215 and 220 of method 200. Typically, the temperature of an FET 130, 135 is not determined during each run of method 200, but rather, for example, every approximately 100 or approximately 10 ms. The frequency of a temperature determination can depend on a current flowing through load 120.

[0042] Fig. Figure 4 shows a representation of the temporal waveforms of voltages and currents across a half-bridge 125. A time waveform is shown horizontally. The waveforms correspond to the first variant of the method 300 described above.

[0043] A first trace 405 relates to a voltage between the source and drain of the second FET 135. A second trace 410 shows a current flowing through the diode 140 of the second FET 135. A third trace 415 shows a current through the second FET 135.

[0044] Initially, the second FET 135 is off, and the voltage 405 across the second FET 135 is high. When the first FET 130 is turned off, the second FET is reverse-conducted, and the voltage 405 drops, while the current 415 flowing through the second FET 135 increases. A current 410 initially flowing through the diode 140 is reduced by the conducting second FET 135.

[0045] Then, the second FET 135 is blocked, i.e., turned off. The current 410 previously flowing through it is quickly taken over by the diode 140, and the current 410 through the diode 140 increases slightly. Once all signals have settled, the current and voltage can be measured at a time t1. The second FET 135 is then turned back on, so that it takes over the current 410 that was just flowing through the diode 140. The gradient of the current 410 before and after the measurement, or of the diode current 410 during the measurement, is due to the inductive effect of the load 120.

[0046] Fig. 5 shows a further representation of gradients according to Fig.4, but assuming method 300 in the second variant. Turning off the first FET 130 causes diode 140 to turn on, increasing its current. At the same time, the voltage across it decreases. Once diode 140 is fully open, the current and voltage are measured at time t1. The second transistor 135 is then reverse-opened, taking over the current 410 previously flowing through diode 140. Voltage 405 drops even further. Reference symbol 100 inverters 105 electric machine 110 intermediate circuit 115 DC link capacitor 120 inductive load 125 Half Bridge 130 first FET (high side) 135 second FET (low side) 140 diodes 145 Current sensor 150 voltage sensor 155 Control device 200 procedures 205 FET1 closed, FET2 open 210 FET1 open, FET2 open 215 FET1 open, FET2 closed 220 FET1 open, FET2 open 300 procedures 305 FET1 closed, FET2 open 310 Open FET1 315 FET2 reverse close 320 FET2 open 325 waiting 330 Determine DS voltage 335 Determine current 340 Determine temperature 345 FET2 reverse close 405 Voltage SD on the second FET 410 Current through diode 140 of the second FET 415 Current through second FET

Claims

[1] Method (300) for determining the temperature in a half-bridge (125) for voltage control on an inductive load (120), wherein the half-bridge (125) has a first and a second field-effect transistor (130, 135), the method (300) comprising the following steps: - switching off (310) the switched-on first field-effect transistor (130) while the second field-effect transistor (135) is switched off; - switching on (315) the second field effect transistor (135); - temporarily switching off (320) the second field effect transistor (135); - simultaneously determining (330) a voltage applied between the source and drain of the second field-effect transistor (135) and (335) a current flowing between the half-bridge (125) and the inductive load (120); and - determining (340) a temperature of the second field effect transistor (135) based on the determined voltage and the determined current. [2] Method (300) according to claim 1, wherein after switching off (320) the second field effect transistor (135) and before determining (330) the voltage, a waiting time is waited (325) in order to allow a diode (140) formed between the source and drain of the second field effect transistor (135) to close completely, wherein a maximum required time until the diode is completely switched on is set as the waiting time. [3] Method (300) according to one of the preceding claims, wherein the method (300) comprises the further step: - Switching on (345) the second field effect transistor (135). [4] Method (300) according to one of the preceding claims, wherein the switching on (315, 345) of the second field effect transistor (135) takes place in the reverse direction in order to bridge a diode (140) comprised by the second field effect transistor (135). [5] Method (300) according to one of the preceding claims, wherein the determination (335) of the current takes place at the time of determining (330) the voltage or at a time that does not deviate from the time of determining (330) the voltage by more than a certain time. [6] Method (300) according to one of the preceding claims, wherein the current is determined by means of a Hall sensor on a conductor leading from the half-bridge (125) to the load (120). [7] Method (300) according to one of the preceding claims, wherein the temperature is determined on the basis of a temperature-dependent relationship between current and voltage, namely a temperature-related diode characteristic of a diode of a field-effect transistor, at the diode (140) formed between the source and drain of the second field-effect transistor (135). [8] Method (200) for voltage control at an inductive load (120) by means of a half-bridge (125) with two field-effect transistors (130, 135) which are switched on alternately in order to set a voltage at the load (120), wherein a temperature of one of the field-effect transistors (130, 135) is determined by means of a method (300) according to one of the preceding claims. [9] The method (200) of claim 8, wherein the temperature is determined at a certain frequency. [10] The method (200) of claim 9, wherein the frequency is determined as a function of a current flowing through the inductive load (120). [11] Control device (155) for controlling a voltage across an inductive load (120) by means of a half-bridge (125) with a first (130) and a second field-effect transistor (135), the control device (155) comprising: - a voltage sensor (150) for determining a voltage between source and drain on one of the field effect transistors (130, 135); - a current sensor (145) for determining a current flowing between the half-bridge (125) and the load (120) and - a processing device (155) configured to carry out a method (200, 300) according to any one of the preceding claims. [12] Inverter (100) for a multi-phase electrical machine (105), comprising - one half bridge (125) for each of the phases and - a control device (155) according to claim 11.

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