Switching waveforms of MOSFET
Switching Waveforms of MOSFET:
- To turn the MOSFET on, the gate drive voltage changes from zero to Vgg. The gate source voltage which was initially zero starts rising towards Vgg with a time constant τ1 = Rg (CGS CGD1).
- Note that during this period the drain voltage vDS is clamped to the supply voltage VD through the free wheeling diode DF. Therefore, CGS and CGD can be assumed to be connected in parallel effectively. A part of the total gate current ig charges CGS while the other part discharges CGD.
- Till vGS reaches vGS (th) no drain current flows. This time period is called turn on delay time (td(ON)). Note that td(ON) can be controlled by controlling Rg. Beyond td(ON) iD increases linearly with vGS and in a further time tri (current rise time) reaches Io.
- At this point the complete load current has been transferred to the MOSFET from the free wheeling diode DF. iD does not increase beyond this point. Since in the active region iD and vGS are linearly related, vGS also becomes clamped at the value vGSIo. The gate current ig now discharges CGD and the drain voltage starts falling.
- The fall of vDS occurs in two distinct intervals. When the MOSFET is in the active region (vDS > (vGS – vGS (th)) CGD = CGD1.Since CGD1 << CGD2, vDS falls rapidly. This fast fall time of vDS is marked tfv1 in Fig . However, once in the ohmic region, CGD = CGD2 >> CGD1. Therefore, rate of fall of vDS slows down considerably (tfv2). Once vDS reaches its on state value (rDS(ON) Io) vGS becomes unclamped and increases towards Vgg with a time constant τ2 = Rg (CGS CGD2). Note that all switching periods can be reduced by increasing Vgg or / and decreasing Rg. The total turn on time is tON = td(ON) tri tfv1 tfv2.
- To turn the MOSFET OFF, Vgg is reduced to zero triggering the exact reverse process of turn on to take place. The total turn off time toff = td(off) trv1 trv2 tfi.