AN3994 datasheet(13/53 Pages) STMICROELECTRONICS

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AN3994

MOSFET standard turn-on and turn-off analysis

Doc ID 022380 Rev 1

13/53

During phase “4” the voltage across the MOSFET has reached the ID*RDS(on)=VDS(on)

value, and the device has entered the ohmic region. The VGS increases up to the maximum

value VGM.

Figure 10.

Turn-off of a MOSFET in a clamped inductive load

Similarly, the turn-off event can be split into four time intervals, as shown in Figure 10.

Phase “1” is the time interval needed to discharge the input capacitance Ciss from its initial

value (+VGM) down to the plateau level. The gate current is supplied by both Cgs and Cgd

capacitors.

The IGoff current during the time interval (t'0 to t'1) follows the same exponential trend of the

turn-on during the (t0, t2) time interval according to Equation 9:

Equation 9

During phase “2” the gate voltage has reached a fixed value (the plateau level) and the VDS

rises from ID*RDS(on)=VDS(on) up to the final value, where it is clamped by the diode. As VGS

is constant in this time interval, the OFF gate current flowing through the RGtot is the

charging current of the Cgd that is charging from a negative starting voltage value (see

Figure 8

for reference) up to the VDS value, while the Cds capacitor is charging up to VDS.

IGoff current during this phase follows Equation 10:

Equation 10

Phase “3” is the phase of the diode turn-on: load current begins to be shared between the

MOSFET and the diode while the VGS decreases from the plateau down to the Vth value.

)

(

Gon

)

(

Goff

−

Gtot

)

(

Goff

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