IGBT are widely used in the filed of power electrinics nowadays,IGBT and IGBT driver take an essential part in the converter system. As the reliability of coverter syetem are determined by the selection of the driver and the calculation of output power, the shortage power of driver or a wrong selection may result in damage to driver,we should pay attention to it.The following calculation about IGBT driver output supplied for your reference.
The switching characteristic of IGBT are determined by inner and external resistance of gate charge.Figure 1 shows the gate capacitance distrubtion of IGBT,CGE for Gate-Emmitter capacitor,CCE for Collector-Emitter capacitor ,CGC for Gate-Collector capacitor or Miller Capacitor.Gate input capacitance be shown by Cies CGE and CGC ,it is the important value for calculating the output power of IGBT driver .The capacitor was not affected by the temperature but closely related with the voltage of Gate-Emitter .The capacitance value by IGBT datasheet is not uesful in actual application because it was get from the bridge.In testing circuit,the voltage added to the collector always are 25V (some factorys adjust it for 10V).In this case,the measuring capacitor are bigger than VCE=600V(Figure 2).As the gate measuring voltage rather than the threshold voltage is low(VGE=0V ), the Miller effect are not included in and the actual gate capacitance are much bigger than the capacitance, the capacitance Cies by IGBT datasheet are just being for reference
The determination of the IGBT gate charge:
Gate charge ( IGBT gate total charges under the gate voltage difference )is most important for designing a driver .We can do the following calculation with it.
Gate Drive Energy E = QG • UGE = QG • [ VG(on) - VG(off) ]
Gate Drive Power PG = E • fSW = QG • [ VG(on) - VG(off) ] • fSW
Total Driver Power P = PG + PS(the power consumption of driver)
Average Output Current IoutAV = PG / ΔUGE = QG • fSW
Maximal Switching Frequency fSW max. = IoutAV(mA) / QG(μC)
Peak Current IG MAX = ΔUGE / RG min = [ VG(on) - VG(off) ] / RG min
And RG min = RG extern + RG intern
fsw max. : Maximal switching frequency
IoutAV : Average current of signal-channel
QG : IGBT gate total charges under gate voltage difference
RG extern : The external gate resistance of IGBT
RG intern : Internal gate resistance of IGBT chip
In fact in many cases, we can't find the gate charge in datasheet and there is also no description about it in the rising process of gate voltage.You should test the turn-on energy E to calculate QG according to the method by IEC 60747-9-200 - Semiconductor
devices - Discrete devices - Part 9: Insulated-gate bipolar transistors (IGBTs).
E = ∫IG • ΔUGE • dt = QG • ΔUGE
The method is simple but complicated ,actually we can calculate gate charge Cies with the value of input capatiance by the IGBT datasheet :
If the Cie by the IGBT datasheet is VCE = 25 V, VGE = 0 V, f= 1 MHz，then we can get
Cin=4.5Cies，
Gate charge QG ≈ ΔUGE • Cies • 4.5 = [ VG(on) - VG(off) ] • Cies • 4.5
Cies :The input capacitance of IGBT(Cies can be found in IGBT dataheet)
If the Cie by the IGBT datasheet is VCE = 10 V, VGE = 0 V, f= 1 MHz，then we can get Cin=2.2Cies，
Gate charge QG ≈ ΔUGE • Cies • 2.2 = [ VG(on) - VG(off) ] • Cies • 2.2
Cies :The input capacitance of IGBT(Cies can be found in IGBT dataheet)
If the gate charge curve in positive quadrant being given in IGBT datasheet, then the negative quadrant caculated by the Cies will be more accurate.
Gate charge QG ≈ QG(on) + ΔUGE • Cies • 4.5 = QG(on) + [ 0 - VG(off) ] • Cies • 4.5
The condition for Cies: VCE = 25 V, VGE = 0 V, f= 1
Gate charge QG ≈ QG(on) + ΔUGE • Cies • 2.2 = QG(on) + [ 0 - VG(off) ] • Cies • 2.2
The condition for Cies: VCE = 10 V, VGE = 0 V, f= 1
When choosing IGBT driver for all applications , the following shoule be taken into consideration:
•The maximal output gate capacitance should be enough for gate charge and discharge to IGBT.
•The output peak current IoutPEAK of the driver should be bigger than or equal to maxium peak current.
•The maximal output gate capacitance should be enough for gate charge and discharge to IGBT.
The maxium output charge per pulse given in POWER-SEM datasheet should be taken into consideration while selecting the driver.
In addition,the isolation voltage Visol IO and dv/dt also should be considered |
Application
