AN3994 datasheet(49/53 Pages) STMICROELECTRONICS

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AN3994

MOSFET critical parameters in high switching environments

Doc ID 022380 Rev 1

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7.4

Minimizing common source inductance impact at turn-off:

negative VGMoff

Threshold voltage for the Power MOSFET has a negative temperature coefficient and this

aspect must be carefully considered especially at turn-off, when parasitic inductances

resonate with the output MOSFET capacitor and lead to oscillations in gate to source

voltage waveforms. If threshold voltage is lowered because of high operation temperature,

the risk of unwanted turn-on significantly increases.

In order to improve the MOSFET immunity against noise, a negative VGM during turn-off is

suggested.

The negative bias at turn-off also counterbalances the induced voltage VLs across the

common source inductance (which is negative if referred to Figure 73).

If the negative VGMoff is properly chosen to avoid any gate oxide damages, it contributes to

the reduction of the positive gate charge amount injected during the ID falling down and

consequently to the reduction of the negative Ls impact on energy losses.

Figure 73.

Basic driving stage of a Power MOSFET at turn-off with negative VGM

Figure 71.

STW77N65M5 Eon@40 A, 2.2 Ω

4-pin solution

Figure 72.

STW77N65M5 Eon@40 A, 2.2 Ω

4-pin solution

AN3994 Datasheet(PDF) 49 Page - STMicroelectronics