IGBT Driver Calculation - Server application notes

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Application Note AN-7004Parasitic and Low-Signal CapacitancesC ies, C oes, C res = f(V CE)CapacitancesDesignationC GEGate-emitter capacitanceC CEC GCCollector-emitter capacitanceGate-collector capacitance(Miller capacitance)Low-SignalCapacitancesC ies = C GE + C GCC res = C GCC oes = C GC + C CEDesignationInput capacitanceReverse transfer capacitanceOutput capacitanceVGE = 0Vf = 1MHzThe following table shows simplified the gate chargewaveforms V GE= f(t), I G=f(t), V CE=f(t), and I C=f(t) duringturn-on of the IGBT. The turn-on process can be dividedinto three stages. These are charging of the gate-emittercapacitance, charging of the gate-collector capacitanceand charging of the gate-emitter capacitance until fullIGBT saturation.To calculate the switching behaviour and the driver, theinput capacitances may only be applied to a certainextent. A more practical way of determining the driveroutput power is to use the gate charge characteristicgiven in the IGBT data sheets. This characteristic showsthe gate-emitter voltage V GE over the gate charge Q G.The gate charge increases in line with the current ratingof IGBT modules. The gate charge is also dependent onthe DC-Link voltage, albeit to a lesser extent. At higheroperation voltages the gate charge increases due to thelarger influence of the Miller capacitance. In mostapplications this effect is negligible.Simplified Gate Charge WaveformsGate Charge CharacteristicV GE(on)V GE(off)Q GE Q GC Q GEt0 switching interval: The gate current I G charges the input capacitance C GE and the gate-emitter voltage V GE rises toV GE(th). Depending on the gate resistor, several amperes may be running in this state. As V GE is stillbelow V GE(th), no collector current flows during this period and V CE is maintained at V CC level.2 / 8 2007-10-31 – Rev00 © by SEMIKRON
Application Note AN-7004t1 switching interval: As soon as V GE passes V GE(th), the IGBT turn-on process starts. I C begins to increase to reach the fullload current I C(load), which is valid for an ideal free-wheeling diode (shown in the simplified waveform).For a real free-wheeling diode, I C exceeds I C(load). This is because a reverse recovery current, whichflows in reverse direction, is added to I C(load). Since the free-wheeling diode is still conducting currentat the beginning of section t2, the collector-emitter voltage V CE will not drop. V GE reaches the plateauvoltage V GE(pl).t2 switching interval: V GE maintains V GE(pl). When the free-wheeling diode is turned-off, V CE starts to drop rapidly anddv CE/dt is high.t3 switching interval: While V CE is decreasing to reach on-state value V CEsat, the Miller capacitance C GC increases as thevoltage decreases and is charged by I G. V GE still remains on a plateau, which is V GE(pl) level.t4 switching interval: At the beginning of section t4, the IGBT is fully turned-on. The charge conducted to C GE induces anexponential increase in V GE up to the gate control voltage V GE(on). I G ends with an exponential fade outand V CE reaches V CEsat level.During turn-off the processes described are running in reverse direction. The charge has to be removed from the gate.Measuring the Gate ChargeA simplified test circuit that can be used to measure thegate charge is shown in the following table. The gate issupplied by a constant gate current. Furthermore, a pulseconstant collector current is applied. The constant gatecurrent causes the measured waveform V GE = f(t) to beequivalent to V GE = f(Q G) due to Q G = I G × t.The document IEC 60747-9, Ed.2: SemiconductorDevices – Discrete Devices – Part 9: Insulated-GateBipolar Transistors (IGBTs) describes the gate chargetest method.Basis Test Circuit for Gate Charge Measurement Waveform V GE = f(t) ⇔ V GE = f(Q G)Determining the Gate ChargeThe gate charge per pulse needed to drive the IGBT canbe determined using the gate charge characteristicdiagram, which shows gate-emitter voltage over gatecharge. The total gate charge can be read out by takinginto account the amplitude of the applied gate voltage,i.e. from turn-on gate voltage V G(on) to turn-off gatevoltage V G(off). The SEMIKRON IGBT data sheet showsthe gate charge curve in the positive and negativequadrant.If the gate charge curve is given in the positive quadrantonly, the gate charge amplitude can be read out byextrapolation, as shown in the following table. The brightgreen represents the area of a diagram given in the IGBTdata sheet. A parallel adjustment of the bright green areaalong the gate charge curve into the negative quadrantup to V G(off) allows for the amplitude of the gate charge tobe determined.© by SEMIKRON2007-10-31 – Rev003 / 8
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IGBT Driver Calculation - Server application notes

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