Buck (in CCM) equation adjust for voltage sag

ACharnley said:

Hi all,
I've a voltage-mode buck controlled by a MCU and it's a large enough inductor etc to ensure it almost always operates in CCM. I set the duty by knowing what input voltage I want to drive to (it's MPPT) and the output is fixed at 5V.

duty = output / input * 100
At high loads and when the output voltage is slower there is a sag in the actual output voltage, I figure due to losses in the FET and inductor.

Should I make the 5V dynamic based on an actual read of the output rather than the expected? Say it's 4.75V, that would slightly decrease the duty which would make the output drop further. I think I've semi answered my own question but thought I'd seek some clarity!

Cheers, A.

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ACharnley said:

This is what I'd expect, up the duty to maintain the voltage level under load, however doing so would reduce the input voltage - and I'm tracking it for MPPT purposes.

My thinking for doing the opposite would worsen the voltage level but continue tracking the correct input voltage.

ie. 5/30 > 4.7/30
But I also think this smells - I read 4.7v at the output because of losses in the fets/inductor at higher current. Perhaps it's correct to use the fixed voltage output regardless?

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