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MPPT & Buck Duty - two approaches, which is correct?


Buck A or Buck B?

Thevenin creates an equivalent impedance (MPPT is where in=out impedance). As the source current is fixed I used this to derive a voltage at Vin. I then tracked this voltage using the method described at Buck A. This means the duty is variable, but I also thought because the load at VL is variable this is correct. One negative is there's no load until the input voltage meets the target, so slightly less ideal power factor.

Buck B uses the CCM equation (I can't find it right now but it's something like duty=V^2/R^2) but this isn't factoring that Load changes impedance.

I've simulated both in LTSpice and both methods come very close in performance, however I've probably done a bad job of simulating the Load (capacitor + resistor) when in real life it's feeding into a Boost and then on to outputs.

Trying to think it through logically Rthout is a fixed value. Given the current is fixed at 500mA then Vin should be Sine like, whereas Buck A is clamping it. But if I go with Buck B approach it doesn't consider the load, which changes and as I can see from LTSpice this also clamps Vin.

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