Gate drive device

Abstract

The present invention provides a gate drive device for switching elements. [Solution] This is a gate drive device for a half-bridge circuit in which first and second GaN-HEMT switching elements are connected in series. The gate drive device can switch the gate voltage of the first switching element between an on-voltage value, an off-voltage value, and a specific voltage value. The on-voltage value is higher than the gate threshold voltage of the first switching element. The off-voltage value is lower than the gate threshold voltage. The specific voltage value is lower than the gate threshold voltage and higher than the off-voltage value. When the load is normal, the gate drive device switches the gate voltage between the on-voltage value and the off-voltage value at a predetermined period. When an abnormality occurs in the load, the gate voltage is changed to the off-voltage value. After a predetermined time has elapsed, the gate voltage is changed from the off-voltage value to the specific voltage value. The state in which the gate voltage is at the specific voltage value is maintained for a longer period than the predetermined period.

Description

Technical Field

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[0001] The technology disclosed in this specification relates to a gate driving device for a switching element.

Background Art

[0002] A high electron mobility transistor (HEMT) using gallium nitride (GaN) has excellent high-frequency characteristics, low operating resistance, and high breakdown voltage. However, GaN-HEMT does not have a body diode like a MOSFET as an element structure. Therefore, when a freewheeling diode is not used separately, a device for reducing losses during the freewheeling operation is required. Patent Document 1 discloses a gate driving device for a half-bridge circuit including a GaN-HEMT. In this gate driving device, during the dead time period in the normal switching operation, the gate voltage of the GaN-HEMT through which the freewheeling current flows is set to a bias voltage lower than the gate threshold voltage. This can reduce the reverse voltage, thereby reducing losses.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0005] According to the configuration described in claim 1, a forced shutdown can be achieved by setting the gate voltage to an off voltage value in response to an abnormality in the load. This causes the first switching element to perform a freewheeling operation. Then, after a predetermined time has elapsed since the gate voltage was changed to the off voltage value, the gate voltage is changed to a specific voltage value. The specific voltage value is lower than the gate threshold voltage and higher than the off voltage value. Because the specific voltage value is lower than the gate threshold voltage, the off state of the first switching element can be maintained. Furthermore, because the specific voltage value is higher than the off voltage value, losses during freewheeling operation can be reduced compared to when the off voltage value is used. This makes it possible to ensure the protection performance of the first switching element during long-duration freewheeling operation in the event of a forced shutdown.

[0006] Furthermore, the types of abnormalities that occur in the load can vary. For example, they may include overcurrent, overvoltage, overheating, and other such abnormalities. [Brief explanation of the drawing]

[0007] [Figure 1] This is a diagram of power conversion system 1. [Figure 2] This is an operation waveform diagram illustrating the forced shutdown operation in the comparative example. [Figure 3] This is an operation waveform diagram illustrating the forced shutdown operation in this embodiment. [Modes for carrying out the invention] [Examples]

[0008] <Outline configuration of power conversion system 1> Figure 1 shows the power conversion system 1 of this embodiment. The power conversion system 1 in Figure 1 schematically shows the basic structure used in various power conversion systems such as inverters and DC-DC converters. In other words, the power conversion system 1 in Figure 1 can be applied to various power conversion devices. In the following, the case in which the power conversion system 1 functions as an inverter will be used as an example. The applications of the power conversion system 1 may vary, and may include, for example, a charger or a motor drive device.

[0009] The power conversion system 1 mainly comprises a gate drive device 10, a half-bridge circuit 20, and a load 30. The half-bridge circuit 20 includes a first switching element 21 that constitutes the upper arm and a second switching element 22 that constitutes the lower arm. The first switching element 21 and the second switching element 22 are high electron-mobility transistors (GaN-HEMTs) using gallium nitride. The first switching element 21 and the second switching element 22 have a gate threshold voltage Vth. Furthermore, the first switching element 21 and the second switching element 22 are elements that do not have a reverse-parallel connected freewheeling diode. The first switching element 21 and the second switching element 22 have identical characteristics. Therefore, in this specification, only one of them may be described, and the description of the other may be omitted.

[0010] The drain of the first switching element 21 is connected to the high-potential DC power line 23. The source of the first switching element 21 is connected to the drain of the second switching element 22 via node N1. The source of the second switching element 22 is connected to the low-potential DC power line 24. The DC power lines 23 and 24 are supplied with power voltage from a DC power source (e.g., a battery) not shown.

[0011] A load 30 is connected to node N1. This means the half-bridge circuit 20 is electrically connected to the load 30. In Figure 1, the load 30 is represented as a virtual inductance. The load 30 can be various devices, such as a motor. The output current (load current IL) of the half-bridge circuit 20 is supplied to the load 30.

[0012] The gate drive device 10 is a device that controls the operation of the half-bridge circuit 4. The gate drive device 10 may include, for example, a microcomputer. The gate voltage V1 output from the gate drive device 10 is input to the gate of the first switching element 21. The gate voltage V2 output from the gate drive device 10 is input to the gate of the second switching element 22. The gate drive device 10 is configured to switch the gate voltages V1 and V2 between an on-voltage value Vgon, an off-voltage value Vgoff, and a specific voltage value Vg_S. The on-voltage value Vgon is a voltage value higher than the gate threshold voltage Vth. The off-voltage value Vgoff is a voltage value lower than the gate threshold voltage Vth. The specific voltage value Vg_S is a voltage value lower than the gate threshold voltage Vth and higher than the off-voltage value Vgoff.

[0013] Furthermore, the gate drive unit 10 receives a detection signal Sd from a current detection unit (not shown). The detection signal Sd is a signal indicating the current value of the load current IL. Based on the detection signal Sd, the gate drive unit 10 performs PWM control on the first switching element 21 and the second switching element 22 so that the load current IL matches the desired target current.

[0014] <Challenges> The problem will be explained using the operation waveform diagram in Figure 2. Figure 2 is a diagram illustrating the forced shutdown operation of a comparative example. In Figure 2, the operation of the first switching element 21 is explained as an example. Furthermore, the case where the normal operation period NP is from time t1 to time t5 and the abnormal operation period AP0 is from time t5 to time t8 is explained.

[0015] The normal operating period NP (times t1 to t5) is the period when the load is normal. During this period, the gate drive device alternately switches the gate voltage V1 of the first switching element 21 between the on voltage value Vgon and the off voltage value Vgoff at a predetermined period SP.

[0016] The abnormal operation period AP0 (times t5 to t8) is the period during which an abnormality occurs in the load. In the example in Figure 2, consider the case where a short-circuit fault occurs in the load at time t5. In this case, the load current IL rises with a steep slope and reaches the current upper limit Imax at time t6 (see region R1). As a result, the gate drive unit 10 recognizes that an overcurrent abnormality has occurred. Therefore, the gate drive unit 10 changes the gate voltage V1 to the off voltage value Vgoff (see arrow Y1). This results in a forced shutdown.

[0017] In response to a forced shutdown, a return current operation is performed. Specifically, as shown in Figure 1, a return current RC flows from the load 30 through the first switching element 21 to the DC power line 23. This return current RC causes a loss L0 in the first switching element 21. This loss discharges the stored energy of the load 30. Therefore, the load current IL decreases over time and becomes zero at time t8. Thus, the return current operation during a forced shutdown ends at time t8. Also, the loss L0 increases over time (see the lower graph in Figure 2).

[0018] Here, the time during which the freewheel operation continues during a forced shutdown is defined as the freewheel operation time RT0. The freewheel operation time RT0 is significantly longer than the freewheel operation time during the dead time period in the normal operating period NP. This is because, as mentioned above, it is necessary to discharge the stored energy of the load. For example, when the switching frequency is 100 kHz, the freewheel operation time during the normal operating period NP is on the order of microseconds. On the other hand, the freewheel operation time RT0 during a forced shutdown is on the order of milliseconds.

[0019] Due to the loss L0, the first switching element 21 generates heat. The longer the reflux operation time, the greater the loss L0, and thus the greater the amount of heat generated. During forced shutdown, the reflux operation time RT0 becomes extremely long, resulting in an extremely large loss L0 and an extremely large amount of heat generated. On the other hand, the GaN element that constitutes the first switching element 21 has a lower breakdown tolerance compared to Si elements and SiC elements. Also, the GaN element has a lower thermal conductivity compared to the SiC element. Therefore, there is a risk that the first switching element 21 will be damaged.

[0020] <Solutions and Effects> The solution to the problem will be described using the operation waveform diagram of FIG. 3. FIG. 3 is a diagram for explaining the forced shutdown operation of this embodiment. The same reference numerals are given to the common contents in FIGS. 2 and 3 to omit the description. Also, since the operation content during the normal operation period NP is the same in FIGS. 2 and FIG. 3, the description is omitted.

[0021] At time t6, the gate driver 10 changes the gate voltage V1 to the off voltage value Vgoff (see arrow Y1). This causes forced shutdown.

[0022] At time t7a after a predetermined time ST has elapsed from time t6, the gate driver 10 changes the gate voltage V1 from the off voltage value Vgoff to a specific voltage value Vg_S (see region R2). Then, the gate driver 10 maintains the gate voltage V1 at the specific voltage value Vg_S until time t8a when the reflux operation during forced shutdown ends. In other words, the gate driver 10 maintains the state where the gate voltage V1 is the specific voltage value Vg_S for a time longer than the predetermined period SP.

[0023] GaN-HEMTs exhibit a gate voltage dependency, meaning that the reverse voltage decreases as the gate voltage increases. In other words, the loss during freewheeling operation of a GaN-HEMT can be reduced as the gate voltage increases. Furthermore, the specific voltage value Vg_S is higher than the off-voltage value Vgoff. Therefore, the loss L1 (Figure 3) in this embodiment using the specific voltage value Vg_S can be reduced compared to the loss L0 (Figure 2) in the comparative example using the off-voltage value Vgoff.

[0024] The loss L1 in this embodiment is smaller than the loss L0 in the comparative example. Therefore, the slope of decrease in load current IL is smaller in this embodiment (Figure 3, slope S1) than in the comparative example (Figure 2, slope S0). As a result, the recirculation operation time during forced shutdown can be made longer in this embodiment (Figure 3, RT1) than in the comparative example (Figure 2, RT0). Compared to the comparative example, this embodiment can increase the time required to complete the discharge of the load's stored energy. In other words, the same amount of energy can be released over a longer period of time. Because the heat dissipation effect can be improved, it is possible to suppress short-term localized heat generation. Because destruction due to heat generation can be prevented, the element breakdown withstand capability of the first switching element 21 can be improved.

[0025] Furthermore, the specific voltage value Vg_S is lower than the gate threshold voltage Vth. This allows the first switching element 21 to be kept in the off state during the abnormal operation period AP1. While maintaining the off state of the first switching element, it is possible to reduce losses during freewheeling operation.

[0026] The value of the specific voltage Vg_S can be determined as appropriate. Preferably, the specific voltage Vg_S is the highest possible voltage value within a range lower than the gate threshold voltage Vth. This is because increasing the specific voltage Vg_S reduces losses during freewheeling operation.

[0027] Furthermore, the length of the predetermined time ST can be determined as appropriate. For example, immediately after the first switching element 21 is turned off (time t6), the gate voltage V1 fluctuates. The predetermined time ST may be set to the length of time until this fluctuation in gate voltage V1 converges to a value that does not pose a risk of the first switching element 21 misfiring. This makes it possible to prevent the first switching element 21 from misfiring after a forced shutdown.

[0028] Although embodiments have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. The technical elements described in this specification or drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technologies illustrated in this specification or drawings achieve multiple objectives simultaneously, and achieving even one of these objectives constitutes technical usefulness in itself.

[0029] <Variation> The technology described herein is also applicable to controlling the gate voltage V2 of the second switching element 22.

[0030] The power conversion system 1 described herein is applicable to various circuits. For example, it may be applied to a three-phase inverter circuit equipped with three half-bridge circuits 20 as shown in Figure 1. [Explanation of Symbols]

[0031] 1: Power conversion system 10: Gate drive device 20: Half-bridge circuit 21: First switching element 22: Second switching element 30: Load V1, V2: Gate voltage Vgon: On voltage value Vgoff: Off voltage value Vg_S: Specific voltage value SP: Predetermined period ST: Predetermined time

Claims

[Claim 1] A gate drive device for a half-bridge circuit having a configuration in which a first GaN-HEMT switching element and a second GaN-HEMT switching element are connected in series, The aforementioned half-bridge circuit is configured to be electrically connectable to a load, The gate drive device is configured to switch the gate voltage of the first switching element between an on voltage value, an off voltage value, and a specific voltage value. The aforementioned on-voltage value is a voltage value higher than the gate threshold voltage of the first switching element. The off-voltage value is a voltage value lower than the gate threshold voltage. The aforementioned specific voltage value is a voltage value that is lower than the gate threshold voltage and higher than the off voltage value. The gate drive device, If the load is normal, the gate voltage of the first switching element is alternately switched between the on voltage value and the off voltage value at a predetermined period. In response to an abnormality occurring in the load, the gate voltage is changed to the off-voltage value. After a predetermined time has elapsed since the gate voltage was changed to the off-voltage value in response to an abnormality, the gate voltage is changed from the off-voltage value to the specific voltage value, and the state in which the gate voltage is the specific voltage value is maintained for a longer period than the predetermined cycle. Gate drive device.

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