Tutorial: Mitigating parasitic turn-on effect in IGBT-output drives to improve drive performance
Bottom IGBT parasitic turn-on due to Miller capacitor
One of the common problems faced in a majority of industrial motor applications is the cross-conduction phenomenon caused by parasitic Miller capacitance in the insulated gate bipolar transistor (IGBT) output transistors. The following tutorial examines the technical and economic trade-offs of using four different techniques to mitigate the affects of a parasitic turn-on due to a Miller capacitor.
1,200 V IGBTs are commonly used in a majority of 3-phase motor inverter applications. These industrial products not only require safety insulation and noise isolation, but also control and special protection functions to ensure reliable operation. In a typical industrial motor application, the Miller capacitor causes a dV/dt shoot-through during IGBT switching. This effect is noticeable in single supply gate drivers (0 to +15V). Due to this gate-collector coupling, a high dV/dt transient created during IGBT turn-off can induce a parasitic turn-on effect that is potentially dangerous. This effect will lead to an IGBT shoot-through across both IGBTs, which could damage them.
When turning on the upper IGBT, a voltage change dVCE/dt occurs across the lower IGBT. Current flow through the parasitic Miller capacitor the upper IGBT, the gate resistor and the internal driver gate resistor creates a voltage drop across the gate resistor. If this voltage exceeds the IGBT gate threshold voltage, a parasitic turn-on occurs. Designers should be aware that rising IGBT chip temperature leads to a slight reduction of gate threshold voltage, usually in the range of millivolts per °C. A lower IGBT voltage threshold translates to lesser driver voltage required to turn-on the IGBT. As a result, IGBT shoot-through would be more frequent and a parasitic turn-on can easily affect the IGBT.
There are three classical solutions to the above problem; the first is to vary the gate resistor, second is to add a capacitor between gate and emitter, and third is to use a negative gate drive. A fourth approach that can prove to be simple and effective is the active clamp technique.
Separate gate resistors for turn-on and turn-off
Adding a gate-on resistor influences the voltage and current change during IGBT turn-on. Increasing this resistor reduces the voltage and current changes, but also increases switching losses.
Separate on and off gate resistor
Additional capacitor between gate and emitter
Negative supply voltage
Active Miller clamping using additional transistor
Parasitic turn-on can be prevented by reducing the gate-off resistor. The smaller gate resistor will also reduce switching loss during IGBT turn-off. However, the trade off of switching off faster is a higher over-shoot and oscillation during turn-off due to stray inductances. Higher over-shoot voltage and oscillation is a negative behavior as it could make the required for IGBT maximum voltage rating higher. As a result, some design optimization between lower parasitic Miller voltage, switching losses, over-shoot voltage and voltage oscillation of both on and off gate resistors would be required. Hence, this is not an ideal solution.
Additional gate emitter capacitor
Adding a capacitor between the gate and emitter will influence the switching behavior of the IGBT. Its job is to take up additional charge originating from the Miller capacitance. The gate charge necessary to reach the threshold voltage, however, increases. This increases the required driver power and the IGBT exhibits higher switching losses for the same gate resistor.
Negative power supply
The usage of negative gate voltage to safely turn-off and block an IGBT is typically used in applications with nominal current above 100 A. Due to cost, negative gate voltage is often not utilized in IGBT applications below 100 A. The addition of a negative supply voltage increases design complexity.
Active Miller clamp
Another measure to prevent unwanted IGBT turn-on is proposed by shorting the gate to emitter path. This can be achieved by an additional transistor between the gate and emitter. This 'switch' shorts the gate-emitter region after a certain gate-emitter voltage is reached. The currents through the Miller capacitance are shunted by the transistor instead of flowing through the output driver pin. This technique is called active Miller clamp.
Unlike the earlier gate resistor and capacitor solutions, the Miller clamp will pull the gate voltage to a low value until a fixed gate threshold is reached. Hence this can be viewed as a general solution that will work at different operating conditions compared to the fixed gate and capacitor design. The only drawback is the addition of the transistor and passive components that would increase design size.
The gate-resistor and gate-emitter capacitor solutions are typically used in smaller power application (IGBT rating less than 25 A) and for designs that are more cost sensitive. The negative supply and active Miller clamp are more suitable for medium to higher power applications, such as industrial motor control, uninterruptible power supplies and industrial inverters, where protection and safety outweigh cost.
The active Miller solution is a lower cost alternative to adding a negative voltage supply. However, for applications with nominal IGBT current above 120 A, a gate driver with dual power supply (negative supply voltage included) can be used, as cost sensitivity is significantly reduced. Also, the requirement for a higher current Miller clamp transistor for larger IGBT rating should be taken into consideration.
In recent years, integrated IGBT gate drivers have included the active Miller clamp solution along with de-saturation protection and under-voltage lock-out. This approach has helped to reduce design complexity and product size for many power designers and industrial/consumer manufacturers.
1. Avago Gate Optocoupler Datasheet ACPL-332J / ACPL-331J
2. Active Miller Clamp , Avago Application Note AN5315
3. Semikron Application Manual , Chapter 3.5 Driver
4. Semikron Application Manual , Chapter 1 Power Semiconductor Basics
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