9020 Datasheet, PDF(9/25 Page) Fairchild Semiconductor

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9020 Datasheet, PDF (9/25 Pages) Fairchild Semiconductor – IGBT Basic II

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B. RG

a. Effect on turn-on

Series resistance (RG), which is connected to the gate, is a parameter that has a significant

effect on switching waveform. When RG decreases, diC/dt and dvCE/dt increases in both turn-

on and turn-off, and switching loss becomes smaller. There are some important significant

advantages when RG is smaller, which included improved dv/dt noise immunity. Dangerous

surge voltage on the IGBT gate caused by Miller effect or dv/dt coupled noise, which means

induced turn-on can be avoided. When one side of IGBT of the half-bridge turns on, then the

freewheeling diode (FWD) of the IGBT on the opposite side recovers reverse voltage and

there is dv/dt. This dv/dt can instantly conduct IGBT on the other side. As the parasitic capaci-

tance Ccg between the gate and the collector on the other side is charged by dvCE/dt at the

IGBT on the other side, this current then flows to the gate to reduce voltage. This leads the

gate voltage to exceed the threshold voltage momentarily, then the IGBT is in conduction. This

is called dv/dt shoot through. This causes unnecessary loss, but reducing RG decreases the

amount of reduction in voltage at the ends. Increasing the value of VGG- is also effective as it

reduces the possibility of IGBTâs gate voltage to rise above VGE(th). Despite these advantages,

the minimum value of RG is limited, and it is limited by the FWD recovery characteristics of the

IGBT on the opposite side. di/dt and dv/dt stress of FWD on the opposite side change with the

value of RG under hard switching inductive load. If diC/dt is large there can be oscillation, and

when diC/dt increases, dvCE/dt also increases. As diC/dt becomes greater, the greater the pos-

sibility of dv/dt shoot through on the IGBT on the opposite side. Under this situation, one must

assume the turn-on switching loss, and increase RG to reduce FWD stress.

b. Effect on turn-off

Although RG is small, it has the same effect as increasing VGG-. As RG becomes larger, turn-

off fall time increases and switching loss rises. However, the effect is generally less than dur-

ing turn-on. Since IC current is divided into MOSFET and PNP transistor, only the current from

MOSFET can be controlled during turn-off. From the perspective of dv/dt noise immunity, dv/dt

malfunction from the IGBT on the opposite side can be reduced during turn-on if the value of

RG is large. On the contrary, from the off state, the possibility of dv/dt shoot through during

turn-on from IGBT increases. As such, if different values of turn-on RG and turn-off RG are

used, the values can be adjusted accordingly in consideration of the two cases.

C. Gate drive power requirement

The proper gate voltage does not mean the proper operation of the IGBT. When IGBT turns on

and off, then the gate is either charged or discharged and the current IG flows out of or into the

gate. The value of the current should be enough to charge and discharge in order to properly

turn on or turn off the IGBT. The waveform is shown in Fig. 5.

Fig. 5. IGBT VGE, IG waveform

Rev. A, April 2002

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