Transconductance of FET needed for switching loss calculation.

[treez]

Hello,

What is the transconductance of this BUK7227 FET at 2A?...the graph of fig8, page 7 only shows transconductance down to 10A.

BUK7227 FET
**broken link removed**

I need the transconductance to calculate the switching loss in this FET as its used in a sepic converter.

 

[FvM]

I don't believe that it actually helps in loss estimation, but we would refer to ideal FET characteristic gm ~ √Id in case of doubt.

 

[treez]

ok thanks, by the way, its page 5 (bottom left ) of the following that tells of the need for gfs in switching los calculation..

**broken link removed**

 

[mtwieg]

Transconductance is a small signal parameter, and not really suitable for estimating behavior in a switching circuit. I've never seen a method based on transconductance. From what I can tell, that document is suggesting that you use gm to estimate the plateau Vgs, but that's a pretty dubious suggestion IMO, since, as your datasheet suggests, gm is not constant.

It's probably not possible to get exactly the data you want for your exact operating conditions. That's typical for these sort of things. You'll just have to choose an approximation, erring on the side of caution.

 

[treez]

Thanks, I am wondering if a better way to tell switching loss is simply to measure the time length of the drain voltage excursion, (for both turn on and off), and then just multiply that time by VDS(off)/2 * I(off) to get the energy in the turn on and turn off switching transitions, then simply multiply this by the switching frequency?

 

[FvM]

If you already have a circuit where you can measure actual switching times, you can measure switching losses directly. If you don't have the circuit, you most likely don't know the exact switching times and can't calculate losses.

 

[treez]

I can see the fall and rise time of vds. also I can scope the fet source sense resistor.....I can then do the calculation , I suppose this is what you mean?