IGBT Driver Calculation - Server application notes

Application Note AN-7004Gate Charge Characteristic (e.g. SKM400GB126D)Extrapolation MethodV G(on)V G(off)Q GAnother method for determining the gate charge uses theinput capacitance C ies and a special factor instead of thegate charge curve. The value for C ies is given in the IGBTdata sheet.The necessary gate charge or the charging energy perpulse must be available at the right time. This can onlybe achieved by using low-impedance, low-inductanceoutput capacitors at the driver output stage. The size ofthe capacitors is indicated by the calculated value Q G.The gate charge is the basic parameter used todetermine driver output power and gate current.Gate Charge Calculation with C ies MethodThe gate charge can be expressed asPlease note:This method is not entirely accurate and should be onlyused if no gate charge curve is available.QG= CG× (VG(on)− VG(off))where C = k × CGCiesThe gate capacitance factor k c can be roughly calculated askC=CiesQ× (VG(ds)G(on)− VG(off )where Q G(ds) is the value specified in the IGBT data sheet, andV G(on) as well as V G(off) are the gate voltages applied to Q G(ds).Thus, the alternative gate charge calculation is as follows:QG= kC× Cies× (VG(on)− VG(off)))4 / 8 2007-10-31 – Rev00 © by SEMIKRON