AN3994 datasheet(37/53 Pages) STMICROELECTRONICS

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AN3994

MOSFET critical parameters in high switching environments

Doc ID 022380 Rev 1

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Both parasitic inductances (drain and common source) impact on the VGS(t) and IG(t)

waveforms, as the voltage across the Ls is directly subtracted (or added in the case of a

turn-off event) from the input VGM, while the voltage induced across the LD changes the

voltage value of the Cds capacitance.

The effect of both inductances on the VGS(t), ID(t) and VDS depends on the relative values of

LD and Ls, rather than RG and the internal MOSFET capacitances Cgd and Cgs.

Under high current levels and low Ls values, the MOSFET experiences high voltage

overshoot during turn-off. Additionally, the voltage across the LD during the positive di/dt at

turn-on is subtracted from the VDS voltage across the switch and this is evident as a

“missing step” in the voltage waveform.

Figure 44.

Turn-on of STW77N65M5@400 V, 13 A, 25 °C

Figure 44

shows the turn-on of the STW77N65M5 working in the STEVAL-ISF001V1 3 kW

PFC, where the parasitic source inductance has been dramatically cut thanks to the 4-pin

solution (explained in Section 7.2). Nevertheless, the parasitic drain component is still

present, causing the typical “missing step” in the voltage waveform.

On the contrary, as the Ls value becomes significant if compared to the LD value, the driving

loop circuit has a relevant impact on the switching behavior, slowing down the ID current.

This implies an energy loss which worsens both at turn-on and at turn-off.

In the following sections, the impact of common source inductance and the main switching

loop inductance is analyzed by making the hypothesis of neglecting one of the two

contributors.

7.2

Common source inductance

The source inductance plays the most important role in influencing the switching

performances of a Power MOSFET, especially when high current levels need to be

commutated.

When dealing with source inductance, it is necessary to split it between an internal

component (the source bond wire inside the MOSFET package) and external components

including the inductance wiring between the source pin and the common ground of the PCB.

AM10807v1

“missing step” at turn-on

due to high parasitic L

AN3994 Datasheet(PDF) 37 Page - STMicroelectronics