AN3994 datasheet(45/53 Pages) STMICROELECTRONICS

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AN3994

MOSFET critical parameters in high switching environments

Doc ID 022380 Rev 1

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The impact of the parasitic inductance becomes heavier as the current increases, as clearly

shown by the trend of Eon and Eoff in Figure 50 and 63.

The turn-on is impacted more by the parasitic Ls rather than the Eoff, as the di/dt starts at

the very beginning of the turn-on phase, just after the VGS has reached the threshold. In this

situation the dI/dt and the VGS are severely slowed down and the plateau region is greatly

widened even if low RG values are used. So, as evident from Figure 60, the main

contribution over the IV cross is provided by the di/dt portion.

As for turn-off, the first part of the IV cross during the plateau region is not affected by the Ls,

and can be sped up by adopting a proper RG value. Additionally, the voltage across the

switch rises in the last portion of the plateau region, due to the particular output capacitance

dependence with voltage; so, this first contribution on the total energy OFF is negligible and

independent of Ls. As voltage across the switch reaches the clamp value, the ID drops down:

the Ls acts during this phase, but its impact is not heavy, as clear from the waveforms in

Figure 54

Figure 60.

STW77N65M5 Eon@40 A, 2.2 Ω with

non-optimizedcommonpath

Figure 61.

STW77N65M5 Eon@40 A, 2.2 Ω with

non-optimizedcommonpath

Figure 62.

STW77N65M5 Eon@40 A, 2.2 Ω

with optimized common path

Figure 63.

STW77N65M5 Eon@40 A, 2.2 Ω

with optimized common path

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