IGBT Driver Calculation

Posted on 25 July 2010

Simplifies Gate Charge Waveforms

 

This article provides information on the determination of driver output performance for switching IGBTs. The information given in this application note contains tips only and does not constitute complete design rules; the information is not exhaustive. The responsibility for proper design remains with the user.

 

 

Introduction

One key component of every power electronic system is – besides the power modules themselves – the IGBT driver, which forms the vital interface between the power transistor and the controller. For this reason, the choice of driver and thus the calculation of the right driver output
power are closely linked with the degree of reliability of a converter solution. Insufficient driver power or the wrong choice of driver may result in module and driver malfunction.

Gate Charge Curve

The switching behaviour (turn-on and turn-off) of an IGBT module is determined by its structural, internal capacitances (charges) and the internal  and outer resistances. When calculating the output power requirements for an IGBT driver circuit, the key parameter is the gate charge. This gate charge is characterised by the equivalent input capacitances CGC and CGE.

IGBT Capacitances

IGBT Capacitances

The following table explains the designation of the capacitances. In IGBT data sheets these capacitances are specified as voltage-dependent low-signal capacitances of IGBTs in the “off” state. The capacitances are independent of temperature, but dependent on the collector-emitter voltage, as shown in the following curve. This dependency is substantially higher at a very low collector-emitter voltage.

Parasitic and Low-Signal Capacitances

Capacitances

Designation

CGE Gate-emitter capacitance
CCE Collector-emitter capacitance
CGC Gate-collector capacitance
(Miller capacitance)

 

Low-Signal
Capacitances
Designation

 C_{ies} = C_{GE} + C_{GC}

Input capacitance

 C_{res} = C_{GC}

Reverse transfer capacitance

 C_{oes} = C_{GC} + C_{CE}

Output capacitance

 

Cies, Coes, Cres = f(VCE)

Cies, Coes, Cres = f(VCE)

The figures below show simplified the gate charge waveforms VGE= f(t), IG=f(t), VCE=f(t), and IC=f(t) during turn-on of the IGBT. The turn-on process can be divided into three stages. These are charging of the gate-emitter capacitance, charging of the gate-collector capacitance and charging of the gate-emitter capacitance until full IGBT saturation.

To calculate the switching behaviour and the driver, the input capacitances may only be applied to a certain extent. A more practical way of determining the driver output power is to use the gate charge characteristic given in the IGBT data sheets. This characteristic shows the  gate-emitter voltage VGE over the gate charge QG. The gate charge increases in line with the current rating of IGBT modules. The gate charge is also dependent on the DC-Link voltage, albeit to a lesser extent. At higher operation voltages the gate charge increases due to the larger influence of the Miller capacitance. In most applications this effect is negligible.

Simplifies Gate Charge Waveforms

Simplified Gate Charge Waveforms

Gate Charge Characteristic

Gate Charge Characteristic

t0 switching interval: The gate current IG charges the input capacitance CGE and the gate-emitter voltage VGE rises to VGE(th). Depending on the gate resistor, several amperes may be running in this state. As VGE is still below VGE(th), no collector current flows during this period and VCE is maintained at VCC level.

t1 switching interval: As soon as VGE passes VGE(th), the IGBT turn-on process starts. IC begins to increase to reach the full load current IC(load), which is valid for an ideal free-wheeling diode (shown in the simplified waveform). For a real free-wheeling diode, IC exceeds IC(load). This is because a reverse recovery current, which flows in reverse direction, is added to IC(load). Since the free-wheeling diode is still conducting current at the beginning of section t2, the collector-emitter voltage VCE will not drop. VGE reaches the plateau voltage VGE(pl).

t2 switching interval: VGE maintains VGE(pl). When the free-wheeling diode is turned-off, VCE starts to drop rapidly and dvCE/dt is high.

t3 switching interval: While VCE is decreasing to reach on-state value VCEsat, the Miller capacitance CGC increases as the voltage decreases and is charged by IG. VGE still remains on a plateau, which is VGE(pl) level.

t4 switching interval: At the beginning of section t4, the IGBT is fully turned-on. The charge conducted to CGE induces an exponential increase in VGE up to the gate control voltage VGE(on). IG ends with an exponential fade out and VCE reaches VCEsat level.

During turn-off the processes described are running in reverse direction. The charge has to be removed from the gate.

Measuring the Gate Charge

A simplified test circuit that can be used to measure the gate charge is shown in the following table. The gate is supplied by a constant gate current. Furthermore, a pulse constant collector current is applied. The constant gate current causes the measured waveform VGE = f(t) to be equivalent to VGE = f(QG) due to QG = IG x t. The document IEC 60747-9, Ed.2: Semiconductor Devices – Discrete Devices – Part 9: Insulated-Gate Bipolar Transistors (IGBTs) describes the gate charge test method.

Basis Test Circuit for Gate Charge Measurement

Basis Test Circuit for Gate Charge Measurement

Waveform VGE = f(t)  VGE = f(QG)

Waveform VGE = f(t)VGE = f(QG)

Determining the Gate Charge

The gate charge per pulse needed to drive the IGBT can be determined using the gate charge characteristic diagram, which shows gate-emitter voltage over gate charge. The total gate charge can be read out by taking into account the amplitude of the applied gate voltage, i.e. from turn-on gate voltage VG(on) to turn-off gate voltage VG(off).  If the gate charge curve is given in the positive quadrant only, the gate charge amplitude can be read out by extrapolation, as shown in the following table. The bright green represents the area of a diagram given in the IGBT data sheet. A parallel adjustment of the bright green area along the gate charge curve into the negative quadrant up to VG(off) allows for the amplitude of the gate charge to be determined.

Gate charge characteristic

Gate charge characteristic

Extrapolation Method

Extrapolation Method

Another method for determining the gate charge uses the input capacitance Cies and a special factor instead of the gate charge curve. The value for Cies is given in the IGBT data sheet. The necessary gate charge or the charging energy per pulse must be available at the right time. This can only be achieved by using low-impedance, low-inductance output capacitors at the driver output stage. The size of the capacitors is indicated by the calculated value QG. The gate charge is the basic parameter used to determine driver output power and gate current.

Gate Charge Calculation with Cies Method

The gate charge can be expressed as

Q_G = C_G \times (V_[{G8on)} - V_{G(off)}

where

 C_G = k_c \times C_{ies}

the gate capacitance factor kc can be roughly calculated as

 k_c = \frac {Q_{G(ds)}}{C_{ies} \times (V_{G(on)} - V_{G(off)})}

where QG(ds) is the value specified in the IGBT data sheet, and VG(on) as well as VG(off) are the gate voltages applied to QG(ds).

Thus, the alternative gate charge calculation is as follows:

 Q_G = k_c \times C_{ies} \times (V_{G(on)} - V_{G(off)})

Please note: This method is not entirely accurate and should only be used if nogate charge curve is available.

Driver Output Power

The individual power of each internal supply needed to drive the IGBT can be found as a function of the intended switching frequency and the energy which has to be used to charge and discharge the IGBT.

Calculation of Driver Output Power per Channel

Power can be expressed as

P_{GD(out)} = E \times f_{sw}

Substituting

 E = Q_G \times (V_{G(on)} - V_{G(off)})

the driver output power per channel is :

P_{GD(out)} = Q_G \times (V_{G(on)} - V_{G(off)}) \times f_{sw}

Rough calculation of driver power usinf the Cies method:

P_{GD(out)} = Q_G \times (V_{G(on)} - V_{G(off)})^2 \times f_{sw}

 

Gate Current

One of the key requirements for IGBT driver circuits is that enough current be supplied to charge and discharge the input capacitances of the IGBT and thus to switch the IGBT on and off. This gate current can be calculated using the equations for IGBT input capacitance charging. The gate current calculated is the minimum average output current IoutAVG of the driver output stage per channel.

IGBT Capacitances

IGBT Capacitances

Calculation of Gate Current Charge can be calculated as follows:

 Q_G = \int idt

With

 Q_{GE} = i_{GE} \times t_{sw},

 

 Q_{GC} = i_{GC} \times t_{sw}

the total charge is:

 Q_G = Q_{GE} + Q_{GC} = (i_{GE} + i_{GC}) \times t_{sw}

Substituting

 t= \frac{1}{f_{sw}}

the average gate current is:

I_G = I_{GE} + I_{GC} = Q_G \times f_{sw}

Roughly the calculation of average gate current by using the Cies method:

 I_G = k_c + C_{ies} \times (V_{G(on)} - V_{G(off)}) \times f_{sw}

Peak Gate Current

The IGBT switching time is controlled by charging and discharging the gate of the IGBT. If the gate peak current is increased, the turn-on and turn-off time will be shorter and the switching losses reduced. This obviously has an impact on other switching parameters such as overvoltage stress, which have to be watched. The gate charge currents can be controlled by the gate resistors RG(on) and RG(off). The theoretical peak current value IGPEAK can be calculated using the equation below. The IGBT module's internal gate resistor RG(int) must be taken into account when calculating the peak gate current. In practice, stray inductance reduces the peak value below the possible theoretical value.

Peak gate current

Peak Gate Current

Calculation

Peak gate current can be calculated as follows:

 I_{GPEAK} = \frac{V_{G(on)} - V_{G(off)}}{R_G + R_{G(int)}}

In the data sheet of an IGBT driver, a maximum peak current is given, as are the minimum values for the gate resistors. If both these maximum and minimum ratings are exceeded, the driver output may be destroyed as a result.

Selection Suitable IGBT Driver

When selecting the suitable IGBT driver for the individual application, the following details have to be considered:

  • The driver must be able to provide the necessary gate current (output current / output power). The maximum average output current of the driver must be higher than the calculated value.
  • The maximum peak gate current of the driver must be equal to or higher than maximum calculated peak gate current.
  • The output capacitors of the driver must be able to deliver the gate charge needed to charge and discharge the gate of the IGBT. In the data sheet of SEMIKRON drivers the maximum charge per pulse is given. This value must be duly considered when selecting a suitable driver.

Other parameters worth mentioning in the context of IGBT driver selection are insulation voltage and dv/dt  capability.

DriverSel – The Easy IGBT Driver Calculation Method

DriverSel facilitates IGBT driver calculation and the selection of a suitable driver, regardless of the application. This software tool takes into consideration the aforementioned characteristics and equations, and calculates suitable IGBT drivers on the basis of the IGBT module selected, the number of paralleled modules, gate resistor, switching frequency and collector-emitter voltage. This tool can be used for driver calculation  and selection for any brand and IGBT package, as well as to calculate the necessary gate charge and average current.

Terms and Symbols used

Letter Symbol Term
CCE Collector-emitter capacitance
CG Effective gate capacitance
CGC Gate-collector capacitance
CGE Gate-emitter capacitance
Cies Input capacitance IGBT
Coes Output capacitance IGBT
Cres Reverse transfer capacitance IGBT
dvCE/dt Rate of rise and fall of collector-emitter voltage
E Electrical energy
fsw Switching frequency
IC Collector current
ICpuls Pulse constant collector current
IG Gate current
IGM Peak gate current
IoutAVG Average output current of the driver
kC Gate capacitance factor
PGD(out) Driver output power
QG Gate charge
QGC Gate-collector charge
QGE Gate-emitter charge
RG Gate resistor
RG(int) IGBT module internal gate resistor
RG(off) Turn-off gate resistor
RG(on) Turn-on gate resistor
t Time
tsw Switching time
VCC Collector-emitter supply voltage
VCE Collector-emitter voltage
VCEsat Collector-emitter saturation voltage
VG Gate voltage (output driver)
VG(off) Turn-off gate voltage (output driver)
VG(on) Turn-on gate voltage (output driver)
VGE Gate-emitter voltage
VGE(pl) Plateau gate-emitter during switching
VGE(th) Gate-emitter threshold voltage

 

References

[1] www.SEMIKRON.com
[2] Application Manual Power Modules, SEMIKRON International
[3] M. Hermwille, "Plug and Play IGBT Driver Cores for Converters", Power Electronics Europe Issue 2, pp. 10-12, 2006
[4] M. Hermwille, "Gate Resistor – Principle and Application", Application Note AN-7003, SEMIKRON
[5] P. Bhosale, M. Hermwille, "Connection of Gate Drivers to IGBT and Controller", Application Note AN- 7002, SEMIKRON
[6] IEC 60747-9, Ed.2: Semiconductor Devices – Discrete Devices – Part 9: Insulated-Gate Bipolar Transistors (IGBTs)

 

For more information, please read:

Criteria for Successful Selection of IGBT and MOSFET Modules

Comparing the Incomparable - Understanding and Comparing IGBT Module Datasheets

Connection of Gate Drivers to IGBT and Controller

Gate Resistors – Principles and Applications

 

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One Response

  1. avatar Radke says:

    Nice articel but the explantaion about the slope Gtae-Volatge vs Gate-charge dV/dQ is missing.
    Why the slope is chaing if the gate-emitter voltge Vge chnage is passsing the 0V (change from nagative to positive).
    Ans why the slope is differnt after passing the miller plateau.

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