Power losses in power electronic devices

Assuming ideal diode and no reverse recovery currents:

1. Io = iT + iD at all time
2. When turning the switch T on, the diode is reverse biased only after i_T rises to i_o. Until then, V_T = V_d.
3. When turning the switch off, the diode does not conduct until v_T rises to V_d. Until then 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_T, given by the shaded areas. If the turn-on and turn-off transients are not short compared to T_s, the power loss in the switching process, P_s, may become large compared to the loss during the ON time.

Note that Ps increase proportionately with f_s, while P_on does not, since T_on and T_s are proportional to each other.

Exercise-1

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

The MOSFET parameters are listed below:

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

1. What is the power dissipation in the MOSFET assuming a switching frequency fs = 10 kHz and a duty cycle D = 50%?
2. What is the maximum average power that can be dissipated in the MOSFET? Assume an ambient temperature of 250C.
3. 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

                        ΔT _ja = ΔT _j − ΔT _a = P (R _θjc + R _θcs + R _θsa)

              Or      ΔT _ja = P (R _θja)

The total thermal resistance is Rθja

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