Abstract: This paper introduces the driving principle and circuit characteristics of the EXB84l insulated gate field-effect transistor (IGBT) driver in the design of an active power filter. The gate drive characteristics and gate series resistance of the IGBT are discussed, and effective methods for overcurrent protection and overvoltage absorption are provided. Prototype trial operation proves that this design scheme is reliable and effective. Keywords: Active power filter; IGBT; EXB84l; drive protection Introduction The insulated gate field-effect transistor (IGBT), as a composite device, integrates the voltage drive and high switching frequency of a MOSFET with the low loss and high power characteristics of a power transistor. It has wide applications in motor control, switching power supplies, converters, and many other fields requiring high speed and low loss. This paper discusses the characteristics of IGBTs used in active power filters and the design of its proprietary EXB84l driver, and proposes a drive circuit with comprehensive protection functions. In the active power filter design, four IGBTs are used as switches, and four EXB84l drivers are used to form the drive circuit, the principle of which is shown in Figure 1. In the experiment, based on the relationship between the compensation current and the command current, a digital signal processor (DSP) controls the high and low levels of the PWM pin, and the drive circuit controls the on/off state of the IGBT. The drive circuit also monitors for overcurrent faults, and the DSP handles these faults by blocking control signals or shutting down the system. 2. Drive Circuit Design 2.1 Drive Circuit Power Supply The drive circuit requires four isolated DC power supplies to power the four IGBT drive circuits. 220V/22V transformers are used to rectify the four AC power supplies, and capacitors and a 78L24 voltage regulator are used to stabilize the output to four 24V DC voltages, as shown in Figure 2. 2.2 Gate Voltage IGBTs are typically driven by gate voltage, which places special requirements on the gate drive circuit. The rise and fall rates of the gate drive voltage pulse must be sufficiently large. During turn-on, a steeply rising gate voltage UGE allows the IGBT to turn on quickly and reduces conduction losses. During turn-off, the gate drive circuit must provide the IGBT with a steeply falling turn-off voltage and apply an appropriate reverse negative bias between the gate and emitter to enable rapid turn-off and reduce turn-off losses. After the IGBT turns on, the gate drive voltage and current must have sufficient width to ensure that the IGBT does not exit the saturation region and suffer damage during instantaneous overload. The recommended gate drive voltage is 15 V ± 1.5 V, which ensures the IGBT is fully saturated and conducts, minimizing conduction losses. Applying a turn-off negative bias can suppress mis-turn-on of the IGBT when du/dt occurs between the emitter and emitter, and also reduce turn-off losses. 2.3 Gate Resistor R1 The selection of the gate resistor R1 has varying degrees of influence on the on-state voltage, switching time, switching losses, and short-circuit withstand capability. When the gate resistance increases, the turn-on and turn-off times of the IGBT increase, thus increasing the turn-on and turn-off losses. When the gate resistance decreases, it leads to an increase in di/dt, which can cause the IGBT to mis-turn on. Therefore, the value of R1 should be selected according to the current capacity, voltage rating, and switching frequency of the IGBT. The value of R1 can be calculated using the following formula: IC is the collector current of the IGBT. As shown in Figure 3, R1 is generally taken to be tens of ohms, and R2 is 30 Ω. Since the IGBT is a voltage-controlled device, when a high voltage is applied between the collector and emitter, it is easily affected by external interference, causing the gate-emitter voltage to exceed a certain value, which can cause the device to mis-turn on. To prevent this phenomenon, a resistor R6 connected in parallel between the gate and emitter can play a certain role. Generally, the resistance of R6 is 1,000 to 5,000 times that of R2, and it should be connected in parallel at the closest point between the gate and emitter. The capacitors C1 and C2 in the circuit are used to suppress the supply voltage changes caused by the power supply connection impedance, not for power supply filtering. 2.4 EXB841 Driving Circuit In the experiment, the author used the EXB841 dedicated IGBT driving module, which has a maximum operating frequency of 40 kHz. The input signal is internally optocoupled and isolated, with an optocoupler driving current of 10 mA and a maximum delay of approximately 1 μs. The operating temperature range is -10℃ to +85℃, and the supply voltage is +20 V to +25 V. The author extended the functionality of the EXB841, as shown in Figure 3, which illustrates the driving circuit. The diode connected to pin 6 of the EXB841 detects the saturation voltage drop of the IGBT, providing overcurrent protection. The overcurrent protection signal on pin 4 is output with a 10 μs delay. When the IGBT experiences overcurrent, if UCE is greater than 7.5V, the internal overcurrent protection circuit activates, softly shutting off the IGBT. Typically, UCE is 3.5 V when the IGBT is carrying its rated current. When UCE = 7.5 V, the IGBT experiences overcurrent, approximately 3 to 5 times the rated current. However, since the protection threshold is not reached, the protection circuit does not function. Prolonged operation in this state will damage the IGBT. To reliably protect the IGBT, the overcurrent protection threshold should be lowered. This can be achieved by connecting a Zener diode in reverse series between D1 and the IGBT collector, or by connecting several fast recovery diodes of the same specifications as D1 in series. As shown in Figure 3, the protection threshold is reduced to 4.2V by connecting an IN4728 3.3V Zener diode in reverse series. When an IGBT overcurrent is detected, pin 5 goes low, the TPL521 optocoupler output goes low, the control signal input is blocked by an AND gate, and the output of the 4-input NAND gate goes low, triggering the power drive protection interrupt and completing the corresponding protection process. 2.5 Isolation between the control and drive sections: The control circuit is a low-voltage section and is highly susceptible to interference; the drive circuit is directly connected to the external circuit and is a strong source of interference; to achieve electromagnetic compatibility of the entire device, the control circuit must be isolated from the drive section. To avoid interference from the common power supply to the control circuit, the control circuit and drive circuit should be powered separately. The EXB84l's power supply voltage is +20V, while the typical control circuit's power supply voltage is 5V. Therefore, the DC-DC micropower module shown in Figure 4 can be used for power isolation. The A2405D micropower module is used to achieve power isolation. 3. Protection of IGBT and Drive Circuit 3.1 Overvoltage Protection of IGBT Instantaneous overvoltage between the collector and emitter of the IGBT can damage it. The author uses a clamping absorption circuit to suppress instantaneous overvoltage. When the IGBT is turned on, due to the action of the diode, the charge of the capacitor will not be discharged, and the capacitor voltage remains the power supply voltage. When the IGBT is turned off, the load current still flows through the IGBT until the voltage between the collector and emitter of the IGBT reaches the power supply voltage, at which point the freewheeling diode turns on. By applying this circuit, the energy in the stray inductance can be transferred to the absorption capacitor through the diode, while the collector potential of the IGBT is clamped to the capacitor voltage, thus suppressing the spike voltage of the IGBT collector. The capacitance of the absorption capacitor can be selected according to formula (2): where L is the lead inductance; i is the current when the IGBT is turned off; ΔU is the voltage overshoot on the absorption capacitor. When the voltage of the capacitor in the absorption circuit is higher than the voltage on the DC side capacitor, energy is returned to the DC side capacitor through the resistor until it is equal to the voltage of the DC side capacitor. When the IGBT is turned off, the line inductance generates a very high peak voltage at the collector and emitter terminals. After the clamping absorption circuit is added, UCE is clamped to the capacitor voltage. When UCE is higher than the capacitor voltage, the energy of the line inductance is transferred to the absorption capacitor. After the peak voltage passes, the voltage of the absorption capacitor that is higher than the main capacitor will become equal to the main capacitor due to the energy return. This suppresses the peak voltage between the collector and emitter. The larger the absorption capacitor, the better the absorption effect. Since most of the overshoot energy on the absorption capacitor is returned to the DC side capacitor, the power consumption of the resistor is reduced. 3.2 Eliminating du/dt between the IGBT collector and gate. Figure 5 shows the schematic diagram of the connection circuit between the EXB841 and the IGBT gate-emitter. When V4 in the drive circuit is on, the IGBT is in the normal conducting state. When V5 is on, a -5V voltage is provided across the IGBT gate-emitter through the Zener diode VZ2, causing the IGBT to turn off. At this time, V5 is in the critical conducting state, and the Zener diode VZ2 is in the reverse bias state. However, due to the influence of the distributed capacitance between the collector and gate, when the du/dt between the collector and gate increases, the current formed by the distributed capacitance flows through it. Therefore, to overcome the du/dt between the collector and gate and ensure that the Zener diode does not overvoltage, it is necessary to avoid the IGBT from being mis-biased. There are two methods to overcome du/dt: one is to use a twisted-pair shielded cable for the connection between the drive circuit output and the IGBT gate-emitter, with the shielding layer grounded; the other is to use a fast absorption circuit to absorb overvoltage. 3.3 Expansion of EXB841 Overcurrent Protection Function The EXB841 itself has overcurrent protection. Its protection principle utilizes the approximately linear relationship between the IGBT's collector saturation voltage drop and collector current. When the IGBT is operating normally, pin 6 of the EXB841 is clamped at 8V, and the internal protection does not activate. When the IGBT exits saturation due to overcurrent, the voltage between the IGBT's collector and emitter rises significantly. The fast diode connected to pin 6 of the EXB841 is cut off, pin 6 of the EXB841 is left floating, the internal protection activates, and the output drive voltage gradually decreases, achieving soft turn-off of the IGBT. In practical applications, relying solely on pin 6 of the EXB841 to detect the IGBT collector voltage for overcurrent protection is insufficient to effectively protect the IGBT. Therefore, it is necessary to add a Hall current sensor to the main circuit to detect overcurrent in the circuit, as shown in Figure 6. After an overcurrent occurs, the detection circuit detects the current. If the signal still exists after an 8μs delay, the drive signal is blocked to turn off the IGBT. In the diagram, if the Hall current sensor detects an overcurrent signal in the main circuit, the PNP transistor will conduct, while the NPN transistor will be cut off, and pin 6 of the EXB841 will be left floating. When there is no overcurrent signal, the PNP transistor will not conduct, and the NPN transistor will conduct. In this case, the circuit is equivalent to the circuit before the expansion. 4. Conclusion This design uses the above circuit to drive and protect IGBTs. This drive circuit is an improvement and refinement based on a typical drive circuit, and an isolation section and overcurrent protection extension section were designed independently. This drive circuit is relatively simple and practical, providing comprehensive protection for the driven IGBTs, with low output impedance and strong anti-interference performance. Prototypes using this IGBT module active filter have been tested, demonstrating that the hardware works in harmony, providing stable and precise control, and mass production of boards has already begun.