Eight Questions on Design Techniques for Isolation Drive IGBT Power Devices

Power devices such as IGBT, Power MOSFET and Bipolar Power Transistor all need to be adequately protected to avoid damage caused by undervoltage, Miller effect, missing saturation, overload and short circuit. This article discusses the techniques of isolating and driving power devices such as IGBT through eight questions and answers participated by Avago.

　　1. How to avoid the Miller effect?

　　One of the problems faced when operating an IGBT is the parasitic capacitance of the Miller effect. This effect is evident in 0 to 15V type gate drivers (single supply drivers). The coupling between the gate collector and the electrode, during IGBT turn-off, high dV/dt transients can induce parasitic IGBT turn-on (gate collector voltage spikes), which is potentially dangerous.

　　When the IGBT in the upper half bridge turns on, a dVCE/dt voltage change occurs across the IGBT in the lower half bridge. Current will flow through the parasitic capacitance of the Miller, the gate resistor and the internal gate drive resistor. This will cause a voltage to be generated across the gate resistor. If this voltage exceeds the voltage of the IGBT gate threshold, it may cause the parasitic IGBT to turn on.

　　There are two traditional solutions. The first is to add a capacitor between the gate and emitter. The second solution is to use a negative gate drive. The first solution incurs efficiency losses. The second solution requires the additional expense of a negative supply voltage.

　　The solution is to shorten the gate-emitter path by using an additional transistor between the gate and emitter. After reaching a certain threshold, the transistor will short-circuit the gate-emitter region. This technique is called Active Miller Clamping and is available in our ACPL-3xxJ products. You can refer to Avago application note AN5314

　　2. What are the fault protection functions? Are they all integrated into the isolation driver?

　　Three fault protection functions are integrated into Avago's highly integrated gate driver ACPL-33xJ - UVLO (to prevent the IGBT from turning on when the VCC2 level is insufficient), DESAT (to protect the IGBT from overcurrent or short circuit), and Miller clamping (to prevent IGBT false triggering caused by parasitic Miller capacitance)

　　3. In what applications does the Miller effect need to be considered?

　　One of the problems faced when operating an IGBT is the parasitic capacitance of the Miller effect. This effect is evident in 0 to 15V type gate drivers (single supply drivers). The coupling between the gate collector and the electrode, during IGBT turn-off, high dV/dt transients can induce parasitic IGBT turn-on (gate collector voltage spikes), which is potentially dangerous.

　　When the IGBT in the upper half bridge turns on, a dVCE/dt voltage change occurs across the IGBT in the lower half bridge. Current will flow through the parasitic capacitance of the Miller, the gate resistor and the internal gate drive resistor. This will cause a voltage to be generated across the gate resistor. If this voltage exceeds the voltage of the IGBT gate threshold, it may cause the parasitic IGBT to turn on.

　　There are two traditional solutions. The first is to add a capacitor between the gate and emitter. The second solution is to use a negative gate drive. The first solution incurs efficiency losses. The second solution requires the additional expense of a negative supply voltage.

　　The solution is to shorten the gate-emitter path by using an additional transistor between the gate and emitter. After reaching a certain threshold, the transistor will short-circuit the gate-emitter region. This technology is called active Miller clamping and is available in our ACPL-3xxJ products.　　4. For 30~75A, 1200V IGBTs working on a 600V DC bus, can the five gate drive optocouplers with miller clamp protection, ACPL-33x and ACPL-H342, achieve high reliability drive with only a single power supply? Compared with traditional positive and negative power supplies, is the reliability higher or is it insufficient?

　　AvagoACPL-332J, ACPL-333J and ACPL-H342 gate driver optocouplers can output current of 2.5 A. These products are suitable for driving 1200V, 100A type IGBTs.

　　1) When using a negative power supply, there is no need to use a Miller clamp, but additional cost is required for the negative power supply.

　　2) If only a single supply is available, the designer can use the internal built-in active Miller clamp.

　　Both solutions are equally reliable. The Miller clamp pin should be connected to VEE when not in use.

　　5. How to better avoid undervoltage and lack of saturation?

　　AVAGO gate drive optocouplers have an undervoltage lockout (UVLO) protection function. When an IGBT fails, the voltage supplied by the gate drive optocoupler may be lower than the threshold. This lockout protection function ensures that the IGBT continues to be in a low resistance state.

　　Smart gate driver optocouplers, HCPL-316J and ACPL-33xJ, come with a DESAT detection function. When the voltage on the DESAT pin exceeds the internal reference voltage of about 7V while the IGBT is still in operation, about 5μs later, the Fault pin changes to a logic low state to notify the MCU/DSP.

　　At the same time, the 1X small transistor will be turned on, discharging the gate voltage of the IGBT through the RG resistor. Since this transistor is about 50 times smaller than the actual turn-off transistor, the IGBT gate voltage will be gradually discharged, resulting in the so-called soft shutdown.

　　6. The photovoltaic inverter is installed in a power plant, where the ambient temperature is quite harsh. What is the working ambient temperature range of the optocoupler?

　　The operating temperature range is -40°C to 105°C. This is sufficient for industrial applications. If customers require a higher operating temperature, the R2 Coupler can operate in an extended temperature range of up to 125°C.

　　7. How high is the insulation withstand voltage of the optocoupler?

　　Gate driver optocouplers are available in different packages. Each package has its own characteristics - such as different creepage distances and clearances to suit different applications. Different creepage distances and clearances correspond to different working insulation voltages, Viorm. The maximum Viorm ranges from 566V to 2262V.

　　8. What is the maximum output current of the optocoupler gate driver?

　　Depending on the device model selected, Avago's optocoupler gate driver maximum output current can reach 0.4A, 0.6A, 1.0A, 1.5A, 2.5A, 3.0A, 4.0A and 5.0A.