I have a question regarding the calculation of a switching dissipated power of a MOSFET using only Vdd, Id, Vgs, Vth, Fsw and Qg as parameters.
Please tell me if the calculations are correct and/or the formula used.
$$Pd_{sw} = Vd * Id * 0.5 (tsw_{on} + tsw_{off}) * Fsw$$
where
$$tsw_{on} = Qg / Ig_{on} $$ and $$tsw_{off} = Qg / Ig_{off} $$
For example, for a IXFK48N50Q transistor, Vdd 400V, Id 10A, Fsw 100kHz and using Rg of 10Ω, I get Ig_on 1.49A and Ig_off 0.51A resulting in tsw_on and tsw_off to be 98ns, respectively 288ns.
This results in a Pd_sw of almost 80W.
I used PSPICE models from manufacturer and the same parameters as above I only get about 30W of total dissipated power, this means included static.
What I am doing wrong?
Best Answer
The switching times should be calculated by using the equations
tsw_on = (0.5*Qgs + Qgd) / Ig_on
and
tsw_off = (0.5*Qgs + Qgd) / Ig_off
This way, you will obtain shorter switching times, resulting in lower power dissipation.
There may be further errors in your calculations which I didn't review yet. When I am doing these calculations myself, I like to use Fairchild's Application note AN-6005.