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# Boost SMPS FET switching losses

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#### eem2am

##### Banned
irlr3110

Hello

I need to calculate what MOSFET switching losses will be in a 392KHz boost converter.

Vin = 9V
Vout ~ 31V
Iout = 1.3A

(it’s a LED driver)

here is schematic……………..

http://i29.tinypic.com/2a8j9c9.jpg

the FET above is FDS5690

I also need to be able to calculate the FET switching loss for this schematic but using the IRLR3110 FET.

FDS5690 N MOSFET
http://www.fairchildsemi.com/ds/FD/FDS5690.pdf

IRLR3110 N MOSFET
http://www.irf.com/product-info/datasheets/data/irlr3110zpbf.pdf

Google is giving a great many articles all throwing a different light on this subject –no unified theory.

Any ideas on which datasheet parameters mean most for this greatly appreciated.
LTC1871 datasheet suggests CRSS is most significant but many other sources state COSS.

Say if you have datasheets of two different FETs, how could you tell which one would give the lower switching losses –given that the drive circuitry was the same in each case?

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By the way…here is the LT Spice simulation .asc file. It gives FET dissipation –but I’ve no idea how accurate it really will be.

**broken link removed** for forum _130809.asc.html

smps boost

Hi,
Your total Mosfet loss will be as follows:

1. Conduction Loss = Pcond = 1.6*RdsON*(IMOSRMS^2) = 0.39 W
2. Gate Charge Loss = Pgate = Qg*Vgs*f = 0.013 W
3. Switching Loss = Psw = [0.5*Voff*Imos*(Tr+Tf)] / T = 0.38 W

So, Total Loss = Pcond+Pgate+Psw = approx. 0.8 W

Hope, this will be suffice for you. Thanks.

smps booster

thankyou, you are very good Tahmid, as simulator gave it as 993mW.

I am wondering how one should find "Tr" & "Tf"?

Also wondering what is "Imos" value.....Average or RMS

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