2. The transient switching characteristics of a MOSFET employed in a switching-m
ID: 1715250 • Letter: 2
Question
2. The transient switching characteristics of a MOSFET employed in a switching-mode power electronics converter (Buck converter) are as follows: R DS on) 20m2, tador) 100ns, t 30ns, tfi 20ns, t day 50ns trw 25ns and t MJ 15ns. Assume that input voltage of the converter Vine 12V, output voltage Vauta 9V, and switching frequency f 75kHz. Also, assume that the converter supplies 3A load a) Calculate the average switching losses in the MOSFET b) Calculate the average conduction losses in the MosFET Suppose the MOSFET in problem 2 is replaced with a diode that has a forward voltage drop of VE 0.7V. Compute the conduction losses in the diode and compare it with the average switching and conduction losses in the MOSFET. What is the conclusion?Explanation / Answer
Problem 3)
When MOSFET is replaced by a diode having forward voltage drop of Vf=0.7V.
Pconduction1 =Vf *Iout* (1-Vout/Vin)
=0.7* 3* (1-9/12)
= 2.1*0.25 =0.525 W [answer]
Pswitchloss1 is negligible here
For MOSFET,
Pconduction2 =(Iout)2*Rd(on) * (Vout/Vin) = 32 x 0.02*(9/12) = 0.135W [answer]
Pswitchloss=(Iout x Vin)(ton+toff)fs/2
=(3*12)(100+50)*10-9*75*103/2 =0.2025 W [ANSWER]
On comparing these values ,we can conclude that MOSFET power less is very less compared to that of diode.At the same time diodes have switching losses can be neglible if schottky diode is assumed.
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