Switching Power Supplies A - Z

Chapter 5

Vsat

If t=T1, Vd=0, and if t=T1+T2, Vd=Vin, so

Vgs is fixed at Vt+Io/g (Miller region). So current through Cgs is zero.

However, Drain voltage is changing. Thus that injects a charging current “Cgd*d(Vd)/dt” through Cgd into the gate.

But there is a current flowing out of the gate described by (Vsat−Vgs)/Rdrive. Therefore this must equal the Cgd charging current.

Equating terms, we get the d(Vd)/dt ,and so we get the equation for Vd.

During turn-off, the Drain current cannot change unless the switching node (and thus in effect Vd too) goes completely to Vin,

and thereby forward biases the diode (ignoring its forward drop), allowing it to start ...