Power Sources and Supplies: World Class Designs

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261Conduction and Switching Losses

MOSFET when it is OFF (in our given application). Later, we will show how to minimize

this error

, 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 ...