The driving circuit's primary function is to amplify the pulse signal from a microcontroller to effectively drive the IGBT. Ensuring the reliable operation of the IGBT is crucial, and this makes the driving circuit an essential component in power electronics systems. The key requirements for an IGBT driver circuit include:
1. Providing appropriate forward and reverse voltages to ensure the IGBT turns on and off reliably.
2. Delivering sufficient transient power or instantaneous current to enable the IGBT to quickly establish a gate-controlled electric field.
3. Minimizing input and output delay times to improve overall system efficiency.
4. Offering high isolation between the signal and gate drive circuits to prevent interference and ensure safety.
5. Including sensitive overcurrent protection to safeguard the IGBT from damage during abnormal conditions.
The first driver circuit is the EXB841/840. Its working principle is illustrated in Figure 1. When a current of 10 mA flows through pins 14 and 15 for 1 μs, the IGBT turns on, causing VCE to drop to approximately 3V. The voltage at pin 6 is clamped around 8V. Since the breakdown voltage of VS1 is 13V, it remains intact, preventing V3 from conducting. As a result, point E remains at about 20V, and diode VD is off, not affecting the normal operation of V4 and V5.
When no current flows through pins 14 and 15, V1 and V2 are activated. V2 turning on causes V4 to turn off, while V5 turns on, rapidly discharging the IGBT gate through V5. This causes the voltage at pin 3 to drop to 0V, applying a negative bias of around 5V to the IGBT gate, ensuring it turns off reliably. At the same time, the rapid rise in VCE causes pin 6 to "hang." The discharge of C2 brings the potential at point B to 0V, so VS1 does not conduct, and the rest of the circuit remains inactive, allowing the IGBT to turn off normally.
In case of an overcurrent situation, the VCE of the IGBT becomes too high, causing VD2 to cut off. This results in VS1 breaking down, V3 turning on, and C4 discharging through R7. The voltage at point D drops, reducing the UGE voltage across the IGBT’s gate, which leads to a slower turn-off and protects the IGBT. The overcurrent protection mechanism in EXB841 primarily relies on the voltage at pin 6, which is influenced by both VCE and the forward voltage of VD2.
Figure 2 shows the typical wiring configuration. Key considerations when using the EXB841 include:
a. The gate-emitter drive circuit should not be too long (generally less than 1 meter) and should use twisted-pair wiring to reduce interference.
b. To prevent voltage spikes from the IGBT collector, increasing the gate resistance RG can help with safe operation, but it must be chosen carefully. Too high or too low a value can increase switching losses or cause misfiring.
c. A capacitor C is used to absorb voltage fluctuations caused by the power supply line impedance, not as a filter capacitor. It is typically 47 μF.
d. Pin 6, which is used for overcurrent protection, should be connected to the IGBT collector via a fast recovery diode.
e. Pins 14 and 15 are connected to the drive signal. Usually, pin 14 is grounded, and pin 15 is connected to the positive end of the input signal. The input current at pin 15 should be limited to below 20 mA, so a current-limiting resistor is often added before it.
f. A Zener diode is added between the gate and emitter to ensure reliable turn-on and turn-off.
The second driver is the M57959L/M57962L thick-film driver circuit. Powered by a dual power supply (+15V and -10V), it provides a negative gate bias of -10V and is compatible with TTL-level signals. It includes short-circuit, overload, and time-delay protection, making it suitable for driving IGBTs up to 1200V/100A and 600V/200A. While it performs well for medium and small power IGBTs, its performance degrades at higher frequencies due to limitations in signal transmission width. Additionally, the internal PCB design may lead to overheating issues if not properly cooled.
Mitsubishi’s M57959L is specifically designed for 600V/200A or 1200V/100A IGBTs. It features a maximum frequency of 40kHz, a dual power supply (+15V and -15V), and a peak output current of ±2A. Key characteristics include:
1. Optical isolation using a fast optocoupler, suitable for high-frequency switching up to 20kHz.
2. Dual power supply allows for a higher negative gate voltage, typically +15V/-10V.
3. Short signal transmission delay, with both low-to-high and high-to-low delays below 1.5μs.
4. Overcurrent protection that detects the saturation voltage drop of the IGBT and initiates a soft shutdown.
5. Internal structure similar to the EXB series, but with a different overcurrent detection method based on voltage sampling and a gate voltage ramp-down for soft shutdown.
The third driver is the 2SD315A integrated driver module. It operates on a single +15V supply and integrates an overcurrent protection circuit. Known for its safety, intelligence, and ease of use, the 2SD315A can deliver large peak currents (up to ±15A) and offers high isolation voltage (4000V). It has two drive channels, making it ideal for driving single or half-bridge IGBT modules rated at 1200V or higher. When used as a half-bridge driver, it simplifies dead-time setting.
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