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# Question about gate resistor of IGBT

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#### stoyanoff

##### Full Member level 4
Hi, everyone! I have IGBTs with Qg=425nC. My driver has 1Amax output source current. I want to put small resistors on the IGBTs gates so the turn on and off time to be short. I made a few calculations and if I use 5ohms for gate resistors Ill have approximately 2us turn on time(ton=Qg*Rg/I). I want to ask is there a chance this small resistor to harm the driver? As far as I know when the driver output get high value the IGBT input capacity starts to charge, but if the charging current is too high isnt this dangerous? For example in my case if I have 5ohms resistance and 15Vcc this means Io=Vcc/R=5A
I cant find the output resistance of the driver but I dont think its high.
Thanks!

The purpose of gate resistors is generally to dampen ringing on the gate, control switching speed, and limit peak gate current. Adding more gate resistance shouldn't harm the gate driver, but will always slow down the switching action.

I think your math has some issues though, especially the ton=Qg*Rg/I one. It should be more like ton=Qg/I, assuming I is constant and independent of voltages (which is a very rough approximation). A more accurate equation is ton=Qgd*(Vdd-Vpt)/Rg, and toff=Qgd*Vpt/Rg, where Vpt is the plateau voltage of the IGBT and Qgd is the gate drain charge.

Yes! This is true, but I`m taking about the current peak! If the gate resistance is less, is there a chance the peak to become very high and this to harm the driver?

The question refers to the gate driver specification. Most integrated gate drivers are designed to drive capacitive loads without additional series resistors, but you should check the driver specification. The output current specification of the drivers usually describes an output current limitation by the output transistors, this means, it can be only achieved without larger gate resistors. Power dissipation of the driver with large gate capacitance and higher switching speed has to be checked however.

As mtwieg explained, gate resistors are demanded by application requirements in most cases. They may be e.g. required to limit inductive overvoltages that might damage the IGBT. In so far it's somehow theoretical to select gate resistors based on the driver specification. Only with good circuit layout you will be able to use the gate resistor values assumed in the IGBT datasheet safely.

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