AN4192 datasheet(25/47 Pages) STMICROELECTRONICS

25 / 47 page

AN4192

Low-side FET selection

Doc ID 023820 Rev 1

25/47

removed, the voltage drop across the LS diode becomes equal to the steady-state reverse

biased value and, after its turn-on, the HS FET provides all the load current.

Figure 26.

Single-phase synchronous buck converter

Figure 27.

Reverse recovery charge waveforms

The “reverse recovery” charge process causes power dissipation, which depends on the

charge amount to be recovered (Qrr), the switching frequency and the converter input

voltage:

Equation 5

In standard technical literature, the “reverse recovery losses” are added to the HS losses: in

fact, as previously explained, during the reverse recovery process, the LS body diode

transfers energy to the HS FET, which, in other words, must provide not only the load current

but also the diode reverse recovery current.

The body-drain diode optimization plays a crucial role not only for the efficiency

improvement but also for the LS switching performance. In fact, reducing Qrr (the excess

charge stored during deadtime), the LS body diode transfers less energy (smaller reverse

AM16470V1

AM16471V1

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AN4192 Datasheet(PDF) 25 Page - STMicroelectronics