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AN2738

Induced turn-on phenomenon

Doc ID 14589 Rev 2

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Induced turn-on phenomenon

One possible phenomenon to be analyzed is the induced turn-on that could occur on a

turned-off power IGBT (or MOSFET) when its companion on the same half-bridge is

switching on. This phenomenon is due to the current injected on the gate by the Miller

parasitic capacitance (CGC) which causes an undesired voltage increase on the gate of the

power IGBT (or MOSFET) which should be kept well turned off. Induced gate voltage

depends on the absolute value of the parasitic capacitance CGC, its relative ratio with CGE,

the value of the dVOUT/dt of the half-bridge and the value of the equivalent (turn-off)

resistance between the emitter (or the source) and the gate. Figure 32 illustrates this

phenomenon.

Figure 32.

Induced turn-on phenomenon - circuital description

Note:

Typically the induced turn-on phenomenon does not cause a complete turn-on of the power

MOSFET, so it is rare to have destructive cross-conduction on the half-bridge during

commutations. Nevertheless, weak conduction of the opened power switch could increase

the power dissipation of the power stage, increasing the overall temperature of the power

MOSFETs and reducing the efficiency. This is why this phenomenon deserves particular

attention also in terms of thermal performance of the power application.

Some strategies to reduce this phenomenon are:

Reducing resistance path between gate and emitter (source). Reducing as

much as possible the gate resistance in which the injected current flows, the

voltage drop on the gate becomes lower. The drawback is a possible increase of

the dVOUT/dt during turn-off of the power IGBT (or MOSFET) which commutates in

hard switching, and then an increase of electrical noise and EMI issues (as

explained in the previous paragraph, the power IGBT or MOSFET in hard-

switching is always the one which has the channel in the same direction as the

current). On the other hand, usually the dVOUT/dt during turn-off is lower than

during turn-on, because the plateau voltage is closer to the source voltage than

the supply voltage of the driver. This results in a lower gate current for the

discharge of the gate charge.

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