The function of the driver circuit is to amplify the power of the pulses output by the microcontroller to drive the IGBT. The driver circuit plays a crucial role in ensuring the reliable operation of the IGBT. The basic requirements for the IGBT driver circuit are as follows:
(1) Provide appropriate forward and reverse output voltages to enable reliable IGBT turn-on and turn-off.
(2) Provide sufficient transient power or instantaneous current so that the IGBT can quickly establish a gate control electric field and turn on.
(3) Minimize input and output delay time to improve work efficiency.
(4) Sufficiently high input-output electrical isolation performance to insulate the signal circuit from the gate drive circuit.
(5) It has sensitive overcurrent protection capability.
The first type of drive circuit EXB841/840
The working principle of EXB841 is shown in Figure 1. When a current of 10mA flows through pins 14 and 15 of EXB841 for 1us, the IGBT is turned on normally, VCE drops to about 3V, and the voltage at pin 6 is clamped at about 8V. Since the voltage regulation value of VS1 is 13V, it will not be broken down. V3 is not conducting, and the potential at point E is about 20V. 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 causes V4 to be turned off and V5 to be turned on. The charge on the IGBT gate is rapidly discharged through V5, and the potential at pin 3 drops to 0V. This allows the IGBT to withstand a negative bias voltage of about 5V between the gate and emitter, ensuring reliable IGBT turn-off. At the same time, the rapid rise of VCE causes pin 6 to be "floating". The discharge of C2 makes the potential at point B 0V, so VS1 is still not turned on, the subsequent circuit does not operate, and the IGBT is turned off normally.
If an overcurrent occurs, an excessively high VCE of the IGBT will cause VD2 to cut off, resulting in VS1 breakdown, V3 conduction, and C4 discharge through R7. This causes the potential at point D to drop, thereby reducing the gate-emitter voltage UGE of the IGBT, completing a slow turn-off and protecting the IGBT. As seen from the overcurrent protection process implemented by the EXB841, the main basis for the EXB841 to determine overcurrent is the voltage at pin 6. The voltage at pin 6 is related not only to VCE but also to the forward voltage Vd of diode VD2.
A typical wiring method is shown in Figure 2. Please note the following points when using it:
a. The back-and-forth 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. Because the IGBT collector generates large voltage spikes, increasing the IGBT gate series resistance RG is beneficial for its safe operation. However, the gate resistance RG cannot be too large or too small. If RG increases, the turn-on and turn-off times will be prolonged, resulting in increased turn-on energy consumption; conversely, if RG is too small, di/dt will increase, which can easily lead to false turn-on.
c. The capacitor C in the diagram is used to absorb the supply voltage changes caused by the power supply connection impedance. It is not a power supply filter capacitor and its value is generally 47F.
d. The overcurrent protection sampling signal connection terminal at pin 6 is connected to the IGBT collector via a fast recovery diode.
pins e, 14, and 15 are connected to drive signals. Generally, pin 14 is connected to the ground of the pulse forming section, and pin 15 is connected to the positive terminal of the input signal. The input current at pin 15 should generally be less than 20mA, so a current-limiting resistor is added before pin 15.
f. To ensure reliable turn-off and turn-on, a Zener diode is added to the gate-emitter junction.
The second type of M57959L/M57962L thick film drive circuit
The M57959L/M57962L thick-film driver circuit uses a dual power supply (+15V, -10V), with an output negative bias of -10V. Its input and output levels are compatible with TTL levels, and it features short-circuit/overload protection and enclosed short-circuit protection, along with delay protection. It is suitable for driving IGBTs of 1200V/100A, 600V/200A and 1200V/400A, 600V/600A and below, respectively. When driving small to medium power IGBTs, the M57959L/M57962L exhibits excellent driving performance and overall performance. However, when operating at high frequencies, its pulse leading and trailing edges deteriorate, limiting the maximum signal transmission width. Furthermore, the thick-film design uses a printed circuit board, which results in poor heat dissipation and a risk of overheating and burning out internal components.
The Mitsubishi M57959L integrated IGBT driver chip from Japan can be used as a 600V/200A or 1200V/100A IGBT driver. Its maximum frequency reaches 40kHz, and it uses a dual power supply (+15V and -15V) with a peak output current of ±2A. The M57959L has the following characteristics:
(1) Electrical isolation is achieved by using optocouplers. Optocouplers are fast and suitable for high-frequency switching operation at around 20KHz. The primary side of the optocoupler has a current-limiting resistor connected in series, so that 5V voltage can be directly applied to the input side.
(2) If dual power supply drive technology is used, the output negative gate voltage ratio is relatively high, and the limit value of the power supply voltage is +18V/-15V, generally +15V/-10V.
(3) The signal transmission delay time is short, and the transmission delay time from low level to high level and from high level to low level is less than 1.5μs.
(4) It has overcurrent protection function. M57962L determines whether IGBT is overcurrent by detecting the saturation voltage drop of IGBT. Once overcurrent occurs, M57962L will softly 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. Overcurrent detection still uses voltage sampling. The circuit feature is the use of gate voltage sag to achieve IGBT soft turn-off.
This avoids overvoltage and high current surges during shutdown; furthermore, the input control signal becomes ineffective during shutdown, meaning the protection shutdown is completed in a closed state. When protection begins, a fault signal is immediately sent out to cut off control signals, including those from other active devices in the circuit.
The third type is the 2SD315A integrated driver module.
The integrated driver module operates on a single +15V power supply and features internal overcurrent protection circuitry. Its key advantages are safety, intelligence, and ease of use. The 2SD315A can output a large peak current (maximum instantaneous output current up to ±15A), possessing strong drive capability and high isolation voltage capability (4000V). The 2SD315A has two drive output channels, suitable for driving dual-unit high-power IGBT modules with two single transistors or one half-bridge at 1200V/1700V and above. When used as a half-bridge driver, the dead time can be easily set.
The 2SD315A consists of three main functional modules: LDI (Logic-to-Driver Interface), IGD (Intelligent Gate Driver), and a DC/DC converter with input and output isolation. When an external PWM signal is input, it is encoded by the LDI. To ensure the signal is not affected by external interference, the processed signal is electrically isolated by a high-frequency isolation transformer before entering the 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 an overcurrent or short-circuit fault in the IGBT, the blocking time logic circuit and status confirmation circuit generate corresponding response and blocking times, and encode the status signal at this time, sending it to the logic control unit (LDI). The LDI unit decodes the received IGBT operating status signal, processing it within the control loop. To prevent the two output drive signals of the 2SD315A from interfering with each other, a DC/DC converter provides isolated power supplies.
Precautions for using 2SD315:
a. Work Mode
The driver module's mode selection terminal MOD is connected to an external +15V power supply, and the input pins RC1 and RC2 are grounded, enabling direct operation. The logic control level uses +15V, and the signal input pins InA and InB are connected together to receive pulse signals from the microcontroller. The SO1 and SO2 pins of the 2SD315A are shown in the diagram, indicating their operating status. When MOD is grounded, it is effectively grounded. Typically, in half-bridge mode, one arm of a DC bus is driven. To prevent shoot-through between the upper and lower arms, a dead time must be set, during which both transistors are simultaneously turned off. Therefore, RC1 and RC2 terminals must be connected to an external RC network to generate the dead time, which can range from 100ns to several milliseconds. As shown in the diagram, RC1 and RC2 are connected to a 10kΩ resistor and a 100pF capacitor, respectively, resulting in a dead time of approximately 500ns.
b. Port VL/Reset
This terminal is used to define the Schmitt trigger inputs InA and InB, which are turned on at 2/3VL and turned off at 1/3VL. When the PWM signal is TTL level, the connection of this terminal is shown in Figure 3-5. When the input signals InA and InB are 15V, this 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 1VL 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 internally by the driver. Using the circuit structure shown in Figure 3-6, different turn-on and turn-off speeds can be achieved, increasing user flexibility.
d. Layout and wiring
The driver should be placed as close as possible to the power semiconductor to minimize the length of the leads from the driver to the power transistor. Generally, the driver connection should not exceed 10 cm. Additionally, stranded wires are typically used for the leads to the collector and emitter. Furthermore, a Zener diode (15-18V) can be connected between the gate and emitter of the IGBT to protect it from breakdown.
The driver module's mode selection pin MOD is connected to an external +15V power supply, and the input pins RC1 and RC2 are grounded for direct operation. The logic control level uses +15V, and the signal input pins InA and InB are connected together to receive pulse signals from the microcontroller for synchronous control. The SO1 and SO2 pins of the 2SD315A are connected to external transistors and optocouplers to output the operating status of the two output channels to the microcontroller. Both outputs are open-collector outputs, which can be adapted to various logic levels by adding external pull-up resistors. LEDs are added between pins SO1 and SO2 and the power supply, and between VisoX and LSX, for fault indication. Under normal conditions, both SO1 and SO2 outputs are high. After power-on, D3 and D4 light up first, then turn off after a few seconds, while D8 and D15 light up simultaneously.
When a fault signal is detected, the output levels of SO1 and SO2 are pulled low to ground, meaning D3 and D4 light up, while D8 and D15 flash. The 2SD315A determines whether the circuit is short-circuited or overcurrent by monitoring UCE (sat). When one or two overcurrent events are detected, the detection circuit feeds back the abnormal status to the driver module. The driver module generates a fault signal and latches it for 1 second. During this time, the driver module stops outputting signals and instead turns off both IGBTs for protection. Simultaneously, the high levels of the status output pins SO1 and SO2 are pulled low, the optocoupler TLP521 conducts, and the two status signals are sent to the microcontroller via an OR gate 74LS32. To prevent a high back EMF from being generated at the IGBT collector due to excessively fast turn-off speed, a circuit structure as shown in Figure 3.11 is used at the gate output to achieve different turn-on and turn-off 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. Hold down 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 a 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 protection will also fail.
(2) Current holding effect: Under normal operating current, IGBT does not have current holding phenomenon because the thin layer resistance Rs is very small. However, under short circuit conditions, due to the large short circuit current, when the voltage drop across Rs is higher than 0.7V, J1 is forward biased, resulting in current holding, and the gate loses voltage control.
(3) Turn-off overvoltage: To suppress short-circuit current, when a fault occurs, the control circuit immediately removes the positive gate voltage, turning off the IGBT, and the short-circuit current decreases accordingly. Because the short-circuit current is large, the current drop rate during turn-off is very high, inducing a very high voltage in the wiring inductor. This induced voltage, especially on the inductor of the device's internal package leads, is difficult to suppress, and will cause the device to fail due to overcurrent turning into turn-off overvoltage.
IGBT Overcurrent Protection Method
(1) Pressure reduction method: This refers to reducing the gate voltage when a fault occurs. Since the short-circuit current is proportional to the applied positive gate voltage Ug1, the positive gate voltage can be reduced when a fault occurs.
(2) Trigger pulse cutoff method: Since the Uce voltage rises during overcurrent, we use the method of detecting the collector voltage to determine whether there is an overcurrent. If there is an overcurrent, the trigger pulse is cut off. At the same time, we try to use a soft turn-off method to alleviate the rate of decrease of the short-circuit current and avoid overvoltage that could damage the IGBT.
