Assuming ideal diode and no reverse recovery currents:

*Io = iT + iD*at all time- When turning the switch T on, the diode is reverse biased only after
*i*rises to_{T}*i*Until then,_{o}.*V*._{T}= V_{d} - When turning the switch off, the diode does not conduct until
*v*rises to_{T}*V*. Until then_{d}*i*_{T}= I_{o}.

During transitions, the power loss in the switch is given by the product

*p = vT*⋅

*iT*, given by the shaded areas.

Thus,

and so on.

During transitions, the power loss in the switch is given by the product

*p = v*⋅

_{T}*i*, given by the shaded areas. If the turn-on and turn-off transients are not short compared to

_{T}*T*, the power loss in the switching process,

_{s}*P*, may become large compared to the loss during the ON time.

_{s}
Note that

*Ps*increase proportionately with*f*, while_{s}*P*does not, since_{on}*T*and_{on}*T*are proportional to each other._{s}**Exercise-1**

In the step-down converter circuit of Figure 8 below, Vd = 250 V and Io = 50 A.

The MOSFET parameters are listed b

**e**low:
BV

_{DSS}= 400 V, ID_{,max}= 80 A, VGS_{,th}= 5 V, r_{DS(on)}= 0.05 ohm,
T

_{j,max}= 175 0 C, R_{өj-a}= 0.5 0 C/W, t_{on}= t_{off}= 200 ns- What is the power dissipation in the MOSFET assuming a switching frequency fs = 10 kHz and a duty cycle D = 50%?
- What is the maximum average power that can be dissipated in the MOSFET? Assume an ambient temperature of 250C.
- The duty cycle D will vary from 20% to 90%. What is the maximum permissible switching frequency fs? Assume that the period 1/fs is large compared to the switching times of the MOSFET.

Note that junction temperature at the device (Tj), assuming a power dissipation of P is thus given by

Δ

*T*= Δ_{ja}*T*− Δ_{j}*T*=_{a}*P*(*R*_{θ}_{jc}*+**R*_{θ}_{cs}*+**R*_{θsa})*Or*Δ

*T*=

_{ja}*P (R*

_{θja})

The total thermal resistance is

*Rθja***IGCT/GCT**

next

**Protection of power electronic Devices**
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