Switching Power Supplies A - Z

Chapter 11

available. The freewheeling path is provided by the catch diode (not shown in

Figure 11-2). But for this diode to become “available” (conduct) it must get

forward-biased. Which means that the voltage across the mosfet has to rise fully,

before the current through it even starts to diminish. But for the mosfet voltage to

rise up, all the parasitic and non-parasitic capacitances preventing it from doing so

must get charged up too. We know that, for example, one of these capacitors is the

drain-to-source capacitance (the C

OSS

of the mosfet). Another such capacitor that we

can now identify is the parasitic mounting capacitance to earth. Therefore, ...