AN1049 datasheet(12/24 Pages) STMICROELECTRONICS

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Unlike the two prior contributions, gate drive losses are not wasted inside the MOSFET (except

for a very small amount), but in the output stage of the L5991 and on the series gate resistor. The

current IGD is seen as an additional current consumption that is added to the quiescent current of the

L5991 (see equation 25).

The appendix "Light load losses evaluation" helps estimate the above mentioned contributions.

When selecting the MOSFET, the parameters to look at are the voltage rating V(BR)DSS, the on-state re-

sistance RDS(on), (this only as to full load considerations), the total gate charge Qg and the parasitic ca-

pacitances Ciss, Crss and Coss.

The RDS(on) should be "just what needed": low enough to reduce resistive losses at full load but not too

low since Qg, Ciss, Crss and Coss build up as RDS(on) decreases. It must not sound surprising to give up

some efficiency points at high load in favor of an improvement at light load if that is worth it.

The voltage rating of the device should be the lowest possible. In fact, for a given RDS(on), the lower the

V(BR)DSS is, the lower the total gate charge Qg and the parasitic capacitance. A 110 V application should

use a 400V device, a 220V or wide-range application a 600V device. The transformer plays a significant

role as to this point (see "Transformer").

VAC1

VDS

TFW

TOFF

TON

TDEAD

VAC2

TRES

Figure 9. Drain voltage waveform (DCM operation)

Ippk

VDS

0.9 Ippk

0.1 Ippk

Vin + VR

Vspike

Figure 10. Current and Voltage waveform at MOSFET turn-off.

AN1049 APPLICATION NOTE

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