SMPS Switching losses in LTspice?

[treez]

Hello,

LTspice calculates a switching loss of 1.07W in the FET of the buckboost smps that I have uploaded here. "Buckboost SMPS _switching loss.asc".

How accurate is this to the real circuit? I mean, the switching Mosfet capacitances, especially Cgd, are voltage dependent, and this would need modelling to make the switching losses accurate. How accurate is it in LTspice?

 

[FvM]

The simulation is as accurate as the utilized models. You can use a manufacturer's model with precise nonlinear capacitance modelling as e.g. provided by ST. Simulation times will be probably further increased.

 

[treez]

So what you appear to be saying is that the mosfet models in the ltspice program aren't actually likely to accurately model the voltage dependency of the cgd capacitor?, and therefore, the switching losses for these ltspice fets isn't likely to be accurate?

(I got the switching loss in the fet by clicking for the power in the fet, and then subtracting the conduction loss and the drive loss)

 

[FvM]

You can check the implemented model parameters, review SPICE models from literature and compare capacitance curves. You can expect a voltage dependency, but in any case, the model doesn't take account for the "hyperabrupt" capacitance curve of power MOSFET.

If junction charge losses make a relevant part of total losses depends on your design criteria. If you tend to oversize transistors for low conduction losses, charge losses may be considerable, particularly for high voltage transistors. Generic switching losses are generated by transiting linear operation area and don't primarly depend on junction capacitance (respectively only indirectly by their effect on rise and fall time).

 

[treez]

surely the rise time of the vds in the miller region depends on the cgd capacitance...I=c*dv/dt, and cgd is definitely needed so we can calculate the length of the miller time...then energy = (Vds_OFF/2) * I * t

So I would have thought the cgd capacitance would have really needed to be accurate?