Why does IGBT desaturation occur?

Why does IGBT Module desaturation occur?
This starts with the planar structure of the IGBT. IGBT and MOSFET have similar device structures. The drain D in the MOS is equivalent to the collector C of the IGBT, and the source S of the MOS is equivalent to the emitter E of the IGBT. Desaturation occurs in both. The figure below shows a simplified MOS cross-sectional view to illustrate the cause of desaturation. A positive voltage VGS greater than the threshold is applied to the gate, and a conductive channel will be formed under the gate oxide layer. At this time, if a positive voltage VDS is applied to the drain D, the electrons in the source will continue to flow under the action of the electric field. The ground flows from the drain D to the source S, so that a current is formed. At this time, the current increases linearly with the increase of the DS voltage. With the increase of the DS voltage, the voltage difference between the gate and the silicon surface is too small to maintain the strong inversion of the silicon surface, the channel appears pinched off, and the current no longer increases proportionally with the increase of the DS voltage.
 
Desaturation state So in the application, what is the harm of desaturation phenomenon?
In practical applications, desaturation generally occurs when the device is short-circuited. At this time, the CE voltage rises to the bus voltage, and the current is generally 4 times the rated current. The power increases abnormally, the junction temperature rises sharply, and the device is not turned off in time. The device may be burned. Most IGBTs have a certain short-circuit withstand capability, generally within 10us, please refer to the product specifications for details.
 
What should I do if I encounter desaturation?
1. The simplest method: monitor the current. After the current exceeds a certain threshold, the driver reports an error and turns off the IGBT. This method is essentially over-current detection and cannot distinguish between over-current and desaturation.
2. Check the CE voltage of the device. The CE voltage is very low when the IGBT is working normally, generally between 1V and 3V, and the CE voltage will rise rapidly after the IGBT is desaturated. The desaturation detection circuit sets a threshold voltage in advance, such as 9V. When the voltage between CE exceeds 9V, the drive chip reports an error and turns off the IGBT.
 
At present, most driver chip manufacturers have introduced chips with integrated desaturation detection function, which can be realized by simple peripheral circuits. The following is a typical desaturation detection circuit. The driver chip has a built-in 500uA constant current source. The voltage drop is very low when the device is working normally, and 500uA current flows through the power device; when the device desaturates, the voltage drop rises sharply, the diode DDESAT is cut off, and the 500uA current can only charge the capacitor CDESAT. When the voltage on CDESAT exceeds 9V, the internal comparator of the chip flips, and the logic circuit reports an error.
 
What should I pay attention to when desaturation is turned off?
Because the device current is very large in the desaturation state, if the IGBT is turned off without restriction, a large di/dt will be generated, and a high CE voltage will be induced on the parasitic inductance of the loop. If this voltage is high If the rated voltage of the device is lower, the IGBT may be damaged. We have two methods to control the voltage spike during turn-off.
 
1. Soft turn-off: When the driver detects a short circuit, the soft turn-off function does not directly convert the gate voltage of the IGBT into 0V or the corresponding negative voltage, but uses a relatively large resistance to shoot the gate through it. The inter-capacitance Cge is discharged, thereby reducing the turn-off speed of the IGBT and avoiding overvoltage. Once the gate voltage drops to a certain value (such as 2V), the large resistance will be replaced by a small resistance, which can ensure that the gate emitter capacitor Cge is discharged quickly and completely.
 
2. Active clamping: The basic principle of active clamping is shown in Figure a. As long as the potential at the collector exceeds a certain specific voltage threshold UCE, and this voltage is higher than the avalanche voltage of the diode VD1, the current I1 flows After D1, D2, RG and T2, if the current on RG is greater than the threshold voltage Vth of the IGBT, the IGBT is turned on again, the turn-off process is slowed down, and the collector voltage is suppressed. This method has higher requirements on the TVS tube and will increase the capacitance between the gate and the collector, so in practice, a push-pull front stage that feeds the signal back to the gate amplifier is often used.

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