Abstract: This paper discusses the gate drive characteristics, gate series resistance, and drive circuit of IGBT. Effective methods for slow gate voltage drop overcurrent protection and overvoltage absorption are proposed. Keywords: Switching power supply; Insulated gate bipolar transistor; Drive protection 1 Introduction IGBT is a composite device of MOSFET and bipolar transistor. It possesses the easy-to-drive characteristics of MOSFET and the advantages of high voltage and current capacity of power transistor. Its frequency characteristics are between those of MOSFET and power transistor, and it can operate normally in the frequency range of tens of kHz, thus dominating in high-frequency, medium-power applications. IGBT is a voltage-controlled device; when a DC voltage of tens of V is applied between its gate and emitter, only a leakage current in the μA range flows, consuming virtually no power. However, there is a large parasitic capacitance (thousands to tens of thousands of pF) between the gate and emitter of the IGBT. Several A of charging and discharging current is required at the rising and falling edges of the drive pulse voltage to meet the dynamic requirements of turn-on and turn-off, which necessitates that its drive circuit also output a certain peak current. As a high-power composite device, IGBT suffers from the problem of potential latch-up and damage during overcurrent. When overcurrent occurs, if the gate voltage is blocked at a normal speed, the excessively high rate of current change will cause overvoltage. Therefore, soft turn-off technology is required, making it essential to master the driving and protection characteristics of IGBTs. 2. Gate Characteristics The gate of an IGBT is electrically isolated from the emitter through a thin oxide film. Because this oxide film is very thin, its breakdown voltage is generally only 20-30V, making gate breakdown one of the common causes of IGBT failure. In applications, even if the gate drive voltage is ensured not to exceed the maximum rated gate voltage, the parasitic inductance of the gate interconnect and the capacitive coupling between the gate and collector can still generate oscillating voltages that damage the oxide layer. Therefore, stranded wires are usually used to transmit the drive signal to reduce parasitic inductance. Connecting a small resistor in series in the gate interconnect can also suppress oscillating voltages. Due to the distributed capacitances Cge and Cgc between the gate and emitter and between the gate and collector of the IGBT, as well as the distributed inductance Le in the emitter drive circuit, the influence of these distributed parameters makes the actual drive waveform of the IGBT not exactly the same as the ideal drive waveform, creating factors that are unfavorable to the IGBT's turn-on and turn-off. This can be verified using an inductive load circuit with a freewheeling diode (see Figure 1). [align=center] (a) Equivalent circuit (b) Turn-on waveform Figure 1 Equivalent circuit and turn-on waveform of IGBT switch[/align] At time t0, the gate drive voltage begins to rise. At this time, the main factors affecting the rise slope of the gate voltage uge are only Rg and Cge, and the gate voltage rises relatively quickly. At time t1, the gate threshold value of the IGBT is reached, and the collector current begins to rise. From this point onwards, there are two reasons that cause the uge waveform to deviate from its original trajectory. First, the induced voltage on the distributed inductance Le in the emitter circuit increases with the increase of the collector current ic, thereby weakening the gate drive voltage and reducing the rise rate of uge between the gate and emitter, slowing down the growth of the collector current. Second, another factor affecting the voltage of the gate drive circuit is the Miller effect of the gate-collector capacitance Cgc. At time t2, the collector current reaches its maximum value, causing the gate-collector capacitance Cgc to discharge. This increases the capacitive current of Cgc in the drive circuit, increasing the voltage drop across the internal impedance of the drive circuit and weakening the gate drive voltage. Clearly, the lower the impedance of the gate drive circuit, the weaker this effect. This effect persists until time t3, when uce drops to zero. Its influence also slows down the IGBT turn-on process. After time t3, ic reaches its steady-state value, and after the factors affecting the gate voltage uge disappear, uge ​​reaches its maximum value at a relatively fast rise rate. As shown in Figure 1, due to the presence of Le and Cgc, the rise rate of uge in the actual operation of the IGBT significantly reduces this effect that hinders the rise of the drive voltage, manifesting as an obstacle to the rise of the collector current and the turn-on process. To mitigate this effect, the internal resistance of Le, Cgc, and the gate drive circuit of the IGBT module should be minimized to achieve a faster turn-on speed. The waveform at IGBT turn-off is shown in Figure 2. At time t0, the gate drive voltage begins to decrease, reaching a level just high enough to maintain the normal operating current of the collector at time t1. The IGBT enters the linear operating region, and uce begins to rise. At this time, the Miller effect of the gate-collector capacitance Cgc dominates the rise of uce. Due to the coupling charging effect of Cgc, uge ​​remains basically unchanged during t1-t2. At time t2, uge ​​and ic begin to decrease at a rate determined by the inherent impedance between the gate and emitter. At time t3, both uge and ic drop to zero, and the turn-off ends. As shown in Figure 2, the presence of capacitance Cgc significantly prolongs the IGBT turn-off process. To reduce this effect, on the one hand, IGBT devices with smaller Cgc should be selected; on the other hand, the internal impedance of the drive circuit should be reduced to increase the charging current flowing into Cgc, thereby accelerating the rise of uce. [align=center] Figure 2 Waveform of IGBT turn-off[/align] In practical applications, the amplitude of IGBT uge also affects the saturation conduction voltage drop: as uge increases, the saturation conduction voltage decreases. Since the saturation conduction voltage is one of the main causes of IGBT heating, it must be minimized. Typically, the gate voltage (uge) is 15-18V. Excessive voltage can easily cause gate breakdown. 15V is generally used. Applying a certain negative bias voltage to the gate-emitter junction when the IGBT is turned off helps improve its anti-interference capability; typically, 5-10V is used. 3. The Influence of Gate Series Resistance on Gate Drive Waveform The rise and fall rates of the gate drive voltage have a significant impact on the IGBT's turn-on and turn-off processes. The MOS channel of the IGBT is directly controlled by the gate voltage, while the drain current of the MOSFET controls the gate current of the bipolar section. Therefore, the IGBT's turn-on characteristics are mainly determined by its MOSFET section, and thus, the IGBT's turn-on is greatly affected by the gate drive waveform. The IGBT's turn-off characteristics mainly depend on the recombination rate of minority carriers. Minority carrier recombination is affected by the MOSFET's turn-off, so the gate drive also affects the IGBT's turn-off. In high-frequency applications, the rise and fall rates of the drive voltage should be faster to increase the IGBT's switching speed and reduce losses. Under normal conditions, the faster the IGBT turns on, the lower the losses. However, during the turn-on process, if there is reverse recovery current from the freewheeling diode and discharge current from the absorption capacitor, the faster the turn-on, the larger the peak current that the IGBT will withstand, and the easier it is to damage the IGBT. In this case, the rise rate of the gate drive voltage should be reduced, i.e., the resistance of the gate series resistor should be increased to suppress the peak current. The cost is a larger turn-on loss. Using this technique, the peak current during the turn-on process can be controlled to any value. From the above analysis, it can be seen that the gate series resistance and the internal impedance of the drive circuit have a significant impact on the IGBT's turn-on process, but a smaller impact on the turn-off process. A smaller series resistance is beneficial for accelerating the turn-off rate and reducing turn-off losses, but too small a resistance will cause an excessively large di/dt, resulting in a large collector voltage spike. Therefore, the series resistance should be comprehensively considered according to specific design requirements. The gate resistance also affects the waveform of the drive pulse. Too small a resistance value will cause pulse oscillation, while too large a resistance value will cause delays and slowing of the pulse waveform's leading and trailing edges. The gate input capacitance Cge of the IGBT increases with its rated current capacity. To maintain the same leading and trailing edge rates of the drive pulse, a larger leading and trailing edge charging current should be provided for IGBT devices with large current capacity. Therefore, the resistance value of the gate series resistor should decrease as the IGBT current capacity increases. 4. IGBT Drive Circuit The IGBT drive circuit must have two functions: first, to achieve electrical isolation between the control circuit and the gate of the driven IGBT; and second, to provide a suitable gate drive pulse. Electrical isolation can be achieved using pulse transformers, differential transformers, and optocouplers. Figure 3 shows an IGBT drive circuit constructed using discrete components such as optocouplers. When a control signal is input, the optocoupler VLC is turned on, transistor V2 is turned off, and V3 is turned on, outputting a +15V drive voltage. When the input control signal is zero, VLC is turned off, V2 and V4 are turned on, and a -10V voltage is output. The +15V and -10V power supplies need to be close to the drive circuit. The leads from the drive circuit output terminal and power supply ground terminal to the IGBT gate and emitter should be twisted-pair cables, preferably no longer than 0.5m. [align=center]Figure 3 IGBT driver circuit composed of discrete components[/align] Figure 4 shows a driver composed of the integrated circuit TLP250. The TLP250 has a built-in optocoupler with an isolation voltage of up to 2500V, a rise and fall time of less than 0.5μs, and an output current of 0.5A, which can directly drive IGBTs up to 50A/1200V. With the addition of a push-pull amplifier transistor, it can drive IGBTs with larger current capacity. The driver composed of TLP250 is small in size and inexpensive, making it an ideal choice among IGBT drivers without overcurrent protection. [align=center]Figure 4 Driver composed of integrated circuit TLP250[/align] 5 IGBT Overcurrent Protection IGBT overcurrent protection circuits can be divided into two categories: one is low-multiplier (1.2~1.5 times) overload protection; the other is high-multiplier (up to 8~10 times) short-circuit protection. For overload protection, a rapid response is not necessary; centralized protection can be used. This involves detecting the total current at the input terminal or DC link. When this current exceeds a set value, the comparator flips, blocking all input pulses to the IGBT driver, causing the output current to drop to zero. This type of overload current protection requires a reset to restore normal operation after activation. IGBTs can withstand short-circuit currents for a very short time. The duration of short-circuit current withstand is related to the IGBT's on-state saturation voltage drop, increasing with the increase of the saturation voltage drop. For example, an IGBT with a saturation voltage drop less than 2V can withstand a short-circuit current for less than 5μs, while an IGBT with a saturation voltage drop of 3V can withstand a short-circuit current for up to 15μs, and over 30μs for 4-5V. This relationship exists because as the saturation voltage drop decreases, the IGBT's impedance also decreases, and the short-circuit current increases simultaneously. The power consumption during a short circuit increases with the square of the current, causing the short-circuit withstand time to decrease rapidly. Common protection measures include soft turn-off and gate voltage reduction. Soft turn-off refers to directly turning off the IGBT during overcurrent and short circuits. However, soft-switching has poor anti-interference capability; it shuts down immediately upon detecting an overcurrent signal, easily leading to malfunctions. To increase the anti-interference capability of the protection circuit, a delay can be added between the fault signal and the activation of the protection circuit. However, the fault current rises sharply during this delay, significantly increasing power loss and also increasing the device's di/dt. Therefore, often the protection circuit activates, but the device still fails. Lowering the gate voltage aims to immediately reduce the gate voltage upon detecting an overcurrent, while the device remains on. A fixed delay is applied after lowering the gate voltage, limiting the fault current to a smaller value during this delay. This reduces the device's power consumption during a fault, extends the device's short-circuit withstand time, and reduces the di/dt when the device turns off, which is highly beneficial for device protection. If the fault signal still exists after the delay, the device is shut down; if the fault signal disappears, the drive circuit can automatically resume normal operation, thus greatly enhancing the anti-interference capability. The above-mentioned gate voltage reduction method only considers the relationship between the gate voltage and the short-circuit current. In practice, the speed of gate voltage reduction is also an important factor, directly determining the di/dt of the fault current decrease. The slow gate voltage drop technique controls the rate of decrease of the fault current by limiting the speed at which the gate voltage drops, thereby suppressing the peak values ​​of dv/dt and uce of the device. Figure 5 shows the specific circuit for implementing the slow gate voltage drop. [align=center] Figure 5 Circuit for implementing the slow gate voltage drop[/align] During normal operation, the voltage at point a is clamped below the breakdown voltage of the Zener diode VZ1 due to the conduction of the fault detection diode VD1, and the transistor VT1 remains in the off state. V1 is normally turned on and off through the drive resistor Rg. Capacitor C2 provides a small delay for hard-switching applications, allowing uce a certain amount of time to drop from the high voltage to the on-state voltage drop when V1 is turned on, without triggering the protection circuit. When an overcurrent or short-circuit fault occurs in the circuit, the voltage uce on V1 rises, and the voltage at point a rises accordingly. When it reaches a certain value, VZ1 breaks down, VT1 turns on, the voltage at point b drops, and capacitor C1 charges through resistor R1. The capacitor voltage rises from zero. When the capacitor voltage rises to approximately 1.4V, transistor VT2 turns on, and the gate voltage uge decreases as the capacitor voltage rises. By adjusting the value of C1, the charging speed of the capacitor can be controlled, thereby controlling the rate at which uge decreases. When the capacitor voltage rises to the breakdown voltage of Zener diode VZ2, VZ2 breaks down, and uge is clamped at a fixed value, ending the slow gate voltage drop process. Simultaneously, the drive circuit outputs an overcurrent signal through the optocoupler. If the fault signal disappears during the delay, the voltage at point a decreases, VT1 returns to cutoff, C1 discharges through R2, the voltage at point d rises, VT2 also returns to cutoff, uge ​​rises, and the circuit returns to normal operation. 6. Overvoltage during IGBT switching When turning off an IGBT, its collector current drops at a high rate, especially under short-circuit fault conditions. Without soft turn-off measures, its critical current drop rate can reach several kA/μs. This extremely high current drop rate will induce a high overvoltage in the distributed inductance of the main circuit, causing the IGBT's current-voltage trajectory to exceed its safe operating area during turn-off, potentially leading to damage. Therefore, from a turn-off perspective, it is desirable to minimize the inductance and current drop rate of the main circuit. However, for IGBT turn-on, the inductance of the collector circuit helps suppress the reverse recovery current of the freewheeling diode and the peak current caused by capacitor charging and discharging, reducing turn-on losses and withstanding a higher turn-on current rise rate. Generally, the collector of an IGBT switching circuit does not require a series inductor; its turn-on losses can be controlled by improving the gate drive conditions. 7. IGBT Turn-off Buffer Absorption Circuit To effectively suppress IGBT turn-off overvoltage and reduce turn-off losses, a turn-off buffer absorption circuit is typically required in the IGBT main circuit. IGBT turn-off buffer absorption circuits are divided into charge-discharge type and discharge-blocking type. The charge-discharge type has two types: RC absorption and RCD absorption, as shown in Figure 6. [align=center] (a) RC type (b) RCD type Figure 6 Charge-discharge type IGBT buffer absorption circuit[/align] The RC absorption circuit causes an overshoot voltage because the charging current of capacitor C generates a voltage drop across resistor R. The RCD circuit overcomes the overshoot voltage by using a diode to bypass the charging current across the resistor. Figure 7 shows three types of discharge-blocking absorption circuits. In the discharge-blocking type buffer circuit, the discharge voltage of the absorption capacitor Cs is the power supply voltage. Before each turn-off, Cs only feeds back the overshoot portion of the previous turn-off voltage to the power supply, reducing the power consumption of the absorption circuit. Because the capacitor voltage rises from the power supply voltage when the IGBT is turned off, its overvoltage absorption capability is not as good as the RCD type charge-discharge type. [align=center] (a) LC type (b) RLCD type (c) RLCD type Figure 7 Three types of discharge-blocking absorption circuits[/align] In terms of overvoltage absorption capability, the discharge-blocking absorption effect is slightly worse, but the energy loss is smaller. The requirements for the buffer absorption circuit are: 1) Minimize the wiring inductance La of the main circuit; 2) The absorption capacitor should be a low-inductance absorption capacitor, and its lead should be as short as possible, preferably directly connected to the IGBT terminal; 3) The absorption diode should be a fast turn-on and fast soft recovery diode to avoid generating turn-on overvoltage and reverse recovery that causes large oscillation overvoltage. 8 Conclusion This article provides a detailed analysis of IGBT driving and protection technology, and concludes that several points should be noted during design: -- Due to the Miller effect of collector-gate parasitic capacitance, IGBTs can cause unexpected voltage spike damage, so the impedance of the gate circuit should be low enough to minimize its negative impact. -- The gate series resistance and the internal impedance of the driving circuit have a great influence on the turn-on process of the IGBT and the waveform of the driving pulse. Therefore, they should be considered comprehensively during design. —A slow-drop gate voltage technique should be used to control the rate of decrease of the fault current, thereby suppressing the peak values ​​of dv/dt and uce of the device and achieving the purpose of short-circuit protection. —When the operating current is large, in order to reduce the turn-off overvoltage, the wiring inductance of the main circuit should be minimized as much as possible, and the absorption capacitor should be of low inductance type.