261Conduction and Switching Losses
MOSFET when it is OFF (in our given application). Later, we will show how to minimize
, by the use of a certain “ scaling factor. ”
8.6 Gate Threshold Voltage
The “ perfect MOSFET ” we talked about earlier ( Figure 8.1 ) started conducting the
moment we raised the gate voltage above ground (i.e., source). But an actual MOSFET
has a certain gate threshold voltage Vt . This is typically 1 to 3 V for logic-level
MOSFETs, and about 3 to 5 V for high-voltage MOSFETs. So basically, we have to
exceed the stated threshold voltage to get the MOSFET to conduct at all ( “ conduction ”
deﬁned typically as a current in excess of 1 mA).
Because Vt is not zero, the deﬁnition of transconductance also needs to be modiﬁed
Note that, in our analysis, we are making another simplifying assumption—that the
transconductance too is a constant.
Finally, with all this background information, we can start looking closely at what
actually happens during the turn-on and turn-off transitions.
8.7 The Turn-on Transition
We have divided this interval into four subintervals as detailed individually in Figures 8.7
through 8.10 . For quick reference and ease of understanding, the relevant explanations
and comments for each subinterval are also provided within their respective ﬁgures.
Brieﬂy, the interval t 1 is just the time to get to the threshold Vt . During this time, we
just have a simple RC charging circuit. In t 2 also, the exponential rise continues, but
this time, the drain current starts ramping up. But for all practical purposes, the gate
doesn ’ t “ know ” anything has changed, because the transconductance is fully responsible
for the drain current (and further, there is no change in the drain voltage). But in t 3, the
diode is allowed to stop conducting (since all the inductor current has by now shifted
over into the switch). So now the drain voltage swings. But in doing so, it injects a
current through Cgd . Note that this capacitance, despite being usually rather small, has
probably the greatest effect on the crossover time—because of the fact that it directly
injects current from a high switching voltage node (drain) on to the gate. Just prior to the
interval t 3, Cgd has a relatively high voltage across it. But when the switch is fully ON,
the voltage across Cgd must decrease to its new ﬁnal low value. Therefore, during t 3, Cgd