Interpretation of various IGBT drive circuits and IGBT protection methods

The function of the drive circuit is to amplify the pulse output by the microcontroller to drive the IGBT. To ensure the reliable operation of the IGBT, the drive circuit plays a vital role. The basic requirements for the IGBT drive circuit are as follows:

(1) Provide appropriate forward and reverse output voltages to enable the IGBT to be turned on and off reliably.

(2) Provide sufficiently large transient power or instantaneous current so that the IGBT can quickly establish a gate-controlled electric field and turn on.

(3) Keep input and output delays as short as possible to improve work efficiency.

(4) Sufficiently high input and output electrical isolation performance to insulate the signal circuit from the gate drive circuit.

(5) It has sensitive over-current protection capability.

The first drive circuit EXB841/840

The working principle of EXB841 is shown in Figure 1. When a current of 10mA flows through the 14th and 15th pins of EXB841 for 1us, the IGBT is turned on normally, VCE drops to about 3V, and the voltage of pin 6 is clamped at about 8V. Since the VS1 voltage regulation value is 13V, it will not be broken down, V3 is not turned on, the potential of point E is about 20V, and the diode VD is cut off, which does not affect the normal operation of V4 and V5.

When no current flows through pins 14 and 15, V1 and V2 are turned on. The conduction of V2 turns off V4 and turns on V5. The gate charge of the IGBT is rapidly discharged through V5, and the potential of pin 3 drops to 0V. The gate-emitter of the IGBT is subjected to a negative bias of about 5V, and the IGBT is reliably turned off. At the same time, the rapid rise of VCE makes pin 6 "floating". The discharge of C2 makes the potential of point B 0V, so VS1 is still not turned on, the subsequent circuit does not operate, and the IGBT is turned off normally.

If overcurrent occurs, the V CE of the IGBT is too large, causing VD2 to be cut off, VS1 to break down, V3 to be turned on, C4 to discharge through R7, and the potential at point D to drop, thereby reducing the voltage UGE between the gate and the emitter of the IGBT, completing the slow shutdown and protecting the IGBT. From the process of EXB841 realizing overcurrent protection, it can be seen that the main basis for EXB841 to determine overcurrent is the voltage of pin 6, which is not only related to VCE, but also to the conduction voltage Vd of diode VD2.

The typical wiring method is shown in Figure 2. Please pay attention to the following points when using it:

a. The round-trip wiring of the IGBT gate-emitter drive circuit should not be too long (generally less than 1m), and twisted pair wiring should be used to prevent interference.

b. Since the IGBT collector generates a large voltage spike pulse, increasing the IGBT gate series resistance RG is beneficial to its safe operation. However, the gate resistance RG cannot be too large or too small. If RG is increased, the turn-on and turn-off time will be extended, resulting in increased turn-on energy consumption; on the contrary, if RG is too small, di/dt will increase, which is prone to mis-turn-on.

c. The capacitor C in the figure is used to absorb the supply voltage change caused by the power supply connection impedance. It is not the power supply filter capacitor of the power supply. The general value is 47 F.

d. Pin 6 is the over-current protection sampling signal connection terminal, connected to the IGBT collector through a fast recovery diode.

e. 14 and 15 are connected to the driving signal. Generally, pin 14 is connected to the ground of the pulse forming part, and pin 15 is connected to the positive end of the input signal. The input current of pin 15 should generally be less than 20mA, so a current limiting resistor is added in front of pin 15.

f. In order to ensure reliable shutdown and conduction, add a Zener diode to the gate emitter.

The second M57959L/M57962L thick film drive circuit

The M57959L/M57962L thick film drive circuit adopts dual power supply (+15V, -10V), the output negative bias voltage is -10V, the input and output levels are compatible with TTL levels, equipped with short circuit/overload protection and closed short circuit protection functions, and also has delay protection characteristics. It is suitable for driving 1200V/100A, 600V/200A and 1200V/400A, 600V/600A and below IGBTs. When driving small and medium power IGBTs, the M57959L/M57962L has excellent driving effect and various performances, but when it works at high frequency, the front and rear edges of the pulse become poor, that is, the maximum transmission width of the signal is limited. And the thick film adopts the printed circuit board design inside, the heat dissipation is not very good, and it is easy to burn the internal components due to overheating.

The M57959L integrated IGBT driver chip of Mitsubishi Corporation of Japan can be used as a 600V/200A or 1200V/100A IGBT driver. Its maximum frequency is also 40KHz, and it uses dual power supply (+15V and -15V) with an output current peak of ±2A. The M57959L has the following features:

(1) Optocouplers are used to achieve electrical isolation. Optocouplers are fast type and suitable for high-frequency switching operation of about 20KHz. The primary side of the optocoupler has a current-limiting resistor in series, so a 5V voltage can be directly added to the input side.

(2) If dual power drive technology is used, the output negative gate voltage is relatively high, and the power supply voltage limit is +18V/-15V, generally +15V/-10V.

(3) The signal transmission delay time is short. The low level-high level transmission delay and the high level-low level transmission delay are both less than 1.5μs.

(4) With overcurrent protection function. M57962L determines whether the IGBT is overcurrent by detecting the saturation voltage drop of the IGBT. Once overcurrent occurs, M57962L will soft-shut down the IGBT and output an overcurrent fault signal.

(5) The internal structure of M57959 is shown in the figure. The driving part of this circuit is similar to that of the EXB series, but the overcurrent protection is different. The overcurrent detection still uses voltage sampling. The circuit features the use of gate voltage slow drop to achieve IGBT soft shutdown.

Overvoltage and high current shock during shutdown are avoided; in addition, during the shutdown process, the state of the input control signal loses its effect, that is, the protection shutdown is completed in a closed state. When the protection starts, a fault signal is immediately sent out to cut off the control signal, including other active devices in the circuit.

The third 2SD315A integrated driver module

The integrated drive module is powered by a single +15V power supply and has an internal overcurrent protection circuit. Its biggest features are safety, intelligence and ease of use. 2SD315A can output a large peak current (maximum instantaneous output current can reach ±15A), has a strong driving capability and a high isolation voltage capability (4000V). 2SD315A has two drive output channels, suitable for driving two single tubes or a half-bridge dual-unit high-power IGBT module with a driving level of 1200V/1700V or above. When used as a half-bridge driver, the dead time can be easily set.

2SD315A is mainly composed of three functional modules, namely LDI (Logic To Driver Interface), IGD (Intelligent Gate Driver) and DC/DC converter with insulated input and output. When the external PWM signal is input, it is encoded by LDI. To ensure that the signal is not interfered by external conditions, the processed signal needs to be electrically isolated by a high-frequency isolation transformer before entering IGD. The signal received from the other side of the isolation transformer is first decoded in the IGD unit, and the decoded PWM signal is amplified (±15V/±15A) to drive the external high-power IGBT. When the overcurrent and short-circuit protection circuit in the intelligent gate driver unit IGD detects that the IGBT has an overcurrent and short-circuit fault, the blocking time logic circuit and the state confirmation circuit generate the corresponding response time and blocking time, and encode the state signal at this time to the logic control unit LDI. The LDI unit decodes the transmitted IGBT working state signal so that it can be processed in the control loop. To prevent the two output drive signals of 2SD315A from interfering with each other, a DC/DC converter provides isolated power supplies.

2SD315 use precautions:

a. Working mode

The mode selection terminal MOD of the driver module is connected to an external +15V power supply, and the input pins RC1 and RC2 are grounded, which is a direct working mode. The logic control level uses +15V, and the signal input pins InA and InB are connected together to receive the pulse signal from the microcontroller. The SO1 and SO2 pins of 2SD315A output the working status of the channel. When MOD is grounded, MOD is grounded. Usually the half-bridge mode drives a bridge arm on a DC bus. In order to avoid the upper and lower bridge arms being directly connected, the dead time must be set. During the dead time, the two tubes are turned off at the same time. Therefore, the RC 1 and RC2 terminals must be connected to an external RC network according to the requirements to generate the dead time. The dead time can generally be from 100n to several ms. The RC 1 and RC2 shown in the figure are connected to a 10k resistor and a 100pF capacitor respectively, so the dead time generated is about 500ns.

b. Port VL/Reset

This terminal is used to define the input InA and InB with Schmitt properties, so that the input is turned on at 2/3 VL and serves as a shutdown signal at 1/3 VL. When the PWM signal is TTL level, the terminal connection is shown in Figure 3-5. When the input InA and InB signals are 15V, the terminal should be connected to the ++15V power supply through a resistor of about 1K, so that the turn-on and turn-off voltages should be lov and 5V respectively. In addition, the input UL/Reset terminal has another function: if it is grounded, the error information in the logic drive interface unit l.DI001 is cleared.

c. Gate output terminal

The gate output Gx terminal is connected to the gate of the power semiconductor. When the SCALE driver is powered by 15V, the gate output is ±15V. The negative gate voltage is generated inside the driver. Using the circuit structure shown in Figure 3-6 can achieve different turn-on and turn-off speeds, increasing the flexibility of user use.

d. Layout and routing

The driver should be placed as close as possible to the power semiconductor, so that the lead from the driver to the power transistor will be as short as possible. Generally speaking, the driver connection should not be longer than 10 cm. At the same time, it is generally required that the leads to the collector and emitter use twisted wires, and a pair of Zener diodes (15~18V) can be connected between the gate and emitter of the IGBT to protect the IGBT from breakdown.

The mode selection terminal MOD of the driver module is connected to an external +15V power supply, and the input pins RC1 and RC2 are grounded, which is a direct working mode. The logic control level uses +15V, and the signal input pins InA and InB are connected together to receive the pulse signal from the microcontroller for synchronous control. The two pins SO1 and SO2 of 2SD315A are connected to external transistors and optocouplers to output the working status of the two output channels to the microcontroller. The output terminal structure is an open collector output, which can be applied to various level logics through an external pull-up resistor. Add light-emitting diodes between pins SO1, SO2 and the power supply, and between VisoX and LSX for fault indication. Under normal circumstances, the outputs of SO1 and SO2 are both high level. After power-on, D3 and D4 light up first, and then go out after a few seconds, while D8 and D15 light up.

When a fault signal is detected, the output levels of SO1 and SO2 are pulled down to the ground, that is, D3 and D4 light up, and D8 and D15 flash. 2SD315A determines whether the circuit is short-circuited and overcurrent by monitoring UCE (sat). When overcurrent is detected in one or two circuits, the detection circuit will feed back the abnormal state to the driver module. A fault signal will be generated inside the driver module and latched. The latching time is 1s. During this period, the driver module will no longer output signals, but will shut down the two groups of IGBTs in time for protection. At the same time, the high level of the status output pins SO1 and SO2 is pulled down, the optocoupler TLP521 is turned on, and the two status signals are sent to the microcontroller through the OR gate 74LS32. In order to prevent the generation of a high back electromotive force on the collector of the IGBT due to too fast a shutdown speed, the circuit structure shown in Figure 3.11 is used at the gate output end to achieve different opening and closing speeds. The gate resistance is 3.4Ω when turned on and 6.8Ω when turned off. The diode is a fast recovery type, which reduces the turn-off speed to a safe level. This is a thumbnail. Click to enlarge. Press CTRL and scroll the mouse wheel to zoom freely.

IGBT short circuit failure mechanism

Several consequences of IGBT load short circuit

(1) Exceeding the thermal limit: The intrinsic temperature limit of the semiconductor is 250°C. When the junction temperature exceeds the intrinsic temperature, the device will lose its blocking ability. When the IGBT load is short-circuited, the junction temperature rises due to the short-circuit current. Once it exceeds its thermal limit, the gate-level protection will also fail accordingly.

(2) Current holding effect: Under normal operating current, the IGBT has no current holding phenomenon due to its small sheet resistance Rs. However, under short-circuit condition, due to the large short-circuit current, when the voltage drop on Rs is higher than 0.7V, J1 is forward biased, resulting in current holding, and the gate level loses voltage control.

(3) Turn-off overvoltage: In order to suppress short-circuit current, when a fault occurs, the control circuit immediately removes the positive gate voltage, turns off the IGBT, and the short-circuit current decreases accordingly. Since the short-circuit current is large, the current drop rate during the turn-off is very high, and a very high voltage will be induced in the wiring inductance. In particular, this induced voltage on the package lead inductance inside the device is difficult to suppress, which will cause the device to fail due to overcurrent turning into a turn-off overvoltage.

IGBT overcurrent protection method

(1) Decompression method: refers to reducing the gate voltage when a fault occurs. Since the short-circuit current is proportional to the external positive gate voltage Ug1, the positive gate voltage can be reduced when a fault occurs.

(2) Pulse cut-off method: Since the Uce voltage increases when there is overcurrent, we use the collector voltage detection method to determine whether there is overcurrent. If there is overcurrent, the trigger pulse is cut off. At the same time, the soft shutdown method is used as much as possible to alleviate the drop rate of the short-circuit current and avoid overvoltage that causes damage to the IGBT.