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How to suppress IGBT collector overvoltage peak

Update : November 15, 2022

When IGBT is turned off, the collector current Ic rapidly decreases to 0, and the sharply varying di/dt flows through the stray inductance of the system, resulting in the induced voltage ΔV. The superposition of ΔV on the busbar voltage causes IGBT to undergo higher voltage stress than usual. Even a short voltage spike may cause permanent damage to the IGBT.


di/dt is related to the characteristics of IGBT chip and the current of the device when it is turned off. When the device is turned off in a short circuit or overcurrent state, the collector voltage overshoot will be particularly large, may exceed the rating, thus damaging the IGBT.


Therefore, how to suppress the voltage peak during shutdown is a topic worth discussing.

Electrodeovervoltage.png


Calculation formula from collector overshoot voltage:


V=Ls*di/dt


We can see that there are two ways to reduce voltage overshoot:



  1. Reduce the system's stray inductance

  2. The current is reduced, thus reducing the current change rate di/dt

  3. The drive is slower, which reduces the current change rate di/dt



To reduce the system noise, is a system level problem, we will talk about this separately.


However, reducing the current change rate di/dt will increase the turn-off loss. How to solve this contradiction?


In this paper, some methods to reduce the rate of current change and suppress voltage overshoot are discussed from the point of view of drive design.


To reduce the rate of change of current, one of the first ways that many people think of is to increase the gate resistance, but this method is not always useful, especially for the FS+trench stop technique. A slight increase in the gate resistance may even increase di/dt, which is only reduced when the gate resistance is very large. Blindly increasing the gate shut-off resistance will significantly increase the shut-off loss, so this method is not advisable.


Electrodeovervoltage2.png

The switching waveform of IGBT4 changed the gate resistance, and the collector overvoltage did not change significantly




So besides increasing the gate resistance, what else can we do to reduce di/dt? From the driving point of view, there are three approaches:


1 Two levels are off


The idea of two-level shutdown is to slow down the shutdown speed and reduce di/dt in the process of shutdown, so as to reduce the overvoltage to a reasonable value. When the IGBT is turned off, the gate voltage is not lowered directly to 0V or negative, but for a short period of time, the gate voltage is first lowered to UTLTO, which is lower than the normal on-off voltage but higher than the Miller platform voltage. And then down from UTLTO to 0V or negative voltage. Generally speaking, the UTLTO can choose between 9 and 14V, and the voltage and duration length of the UTLTO can be adjusted.


The two-level off function can be integrated into the IGBT driver chip, such as the 1ED020I12-FT. The voltage and duration of a two-level off are usually achieved using a capacitive CTLTO or a combination of capacitors and resistors. When the capacitor is charged to a certain value, it triggers the driver's output signal UOUT. If the input signal Uin is shorter than the set tTLTO, the input signal is usually suppressed while the output signal remains unchanged.


Electrodeovervoltage3.jpg

The following figure shows the comparison of short-circuit current with or without TLTO function. FIG. a The short circuit is turned off without TLTO, while FIG. b shows the waveform with TLTO turned off. It is clear that the strong oscillations in the gate voltage and the empole-collector voltage are significantly reduced, and more importantly, the overvoltage generated is reduced. In this example, a peak voltage of 1125V appears in Figure A. In the measurement method shown in Figure b, the voltage is only 733V(in each example the DC bus voltage is 400V and a 400A/1.2kV IGBT is used).


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(a) There is no TLTO function


Electrodeovervoltage5.png


(b) TLTO function


The two-level off function can be integrated into the driver chip. The traditional IGBT driver IC with integrated two-level off function is shown below. TLSET pin is connected with a Schottky diode and a capacitor. Schottky diode is used to set the voltage of two level off. The capacitor is used to set the time for both levels to be turned off.


Electrodeovervoltage6.png

1ED020I12_BT/FT


The latest Infineon X3 Enhanced driver chip, 1ED38X1MX12M, does not require external capacitor resistance, only through digital configuration, you can set the level and duration of two level off, which can simplify the circuit design and BOM

Electrodeovervoltage7.png




2 Soft shutdown


Soft shutdown can ensure the safe shutdown of short circuit. If the driver detects a short circuit, instead of pulling the IGBT's gate voltage down to 0V or negative with a standard turn-off resistor, the soft turn-off function releases the gate current with a relatively high impedance that delays the discharge of the gate capacitance and slows the IGBT's turn-off. Once the gate voltage is reduced to a certain value (e.g., 2V), the high impedance is shorted by a low impedance, which ensures a fast and complete discharge of the gate-emitter capacitor. The principle of soft shutdown is shown in the following figure.

The IGBT with soft turn-off function is driven by Infineon 1ED34X1MX12M and 1ED38X1MX12M. These two chips are configured with soft turn-off current through analog and digital methods respectively, and there are up to 16 soft turn-off current gears to choose from.

Electrodeovervoltage12.png

3 Active pliers


Also known as the emitter clamp, the following is a typical implementation of the active clamp:

Electrodeovervoltage15.jpg

The principle of active clamp is that during the shutdown process, the IGBT CE voltage spikes due to di/dt. Unidirectional TVS diode VD1 switches on and passes a current as long as the potential at the collector exceeds the avalanche voltage of diode VD1. Current I1 flows through VD1,VD2,RG, and VT2. If the voltage drop generated at the gate resistance Rg is higher than the threshold voltage Vth of the IGBT, the IGBT is turned on again, thereby reducing di/dt during the shutdown. Thus, in order to increase the gate voltage, sufficient current must be generated.


If the external gate voltage of the IGBT is 1ohm, the gate voltage is -15V, and the threshold voltage is 6V, in order to turn on the IGBT again, the current of the active clamp must be greater than 21A. Therefore, TVS diode VD1 and blocking diode VD2 must meet the demand of 21A pulse current. In addition, the TVS tube must be a high voltage diode, the common series model is 1.5KExxx


However, this circuit also has disadvantages, such as breakdown voltage is closely related to temperature, and the blocking diode junction capacitance is large, IGBT switch, the displacement current will be extra increased by du/dt.


Another, more concise approach is to feed the signal back to drive the push-pull circuit, as shown below:

Electrodeovervoltage16.jpg



Current I2 is passed by blocking diode VD5, resistor R2 and MOSFET VT8. The resistance R2 is much higher than the resistance of RG, so a partial outflow of the current I1 can generate enough voltage to turn VT5 on and off VT6. Once the VT5 is switched on, I1 no longer passes through the gate resistor RG, but instead charges the input capacitor CGE. All of these have the following advantages for the circuit:


1. Because the current through the diode is very low, you can use a cheaper TVS SMD diode.


2. The required space is determined only by the creepage distance and electrical clearance.


3. The circuit responds very fast.


The above are several commonly used collector voltage peak suppression methods. Active clamp requires high voltage diode, the additional cost is high; The two-level off can be integrated into the driver chip. The traditional scheme only needs to add a diode and capacitor to the driver chip, but the latest Infineon 1ED X3 digital chip can realize the parameter adjustment of two-level off without external devices. In addition, the 1ED X3 analog/digital chip is also integrated with the soft turn-off function, which can adjust the 16-grade soft turn-off current without peripheral devices.