Boost converter for PV

I think you have to give up the idea that an MPPT charger outputs a constant voltage.

It outputs a controlled voltage that

(a) is safe for charging the batteries
(b) results in a safe charging current for the batteries
(c) gives the highest current to the batteries that the solar cells can provide at the moment.

In practice this means measuring the input voltage and current, calculating the power, and adjusting the output voltage to maximise power (goal c) while also considering the output voltage and current AND the state of charge of the batteries to meet goals (a) and (b).

State (b) probably means you need a bigger battery with a higher charging rate - or it is close to fully charged (and only permits trickle charging) and state (a) means the battery is fully charged. The rules for these depend on the battery technology. Lead acid batteries have characteristics allowing safe MPPT charging; lithium batteries may have stricter charging regimes... In either case these rules give you some leeway to charge at different rates, and the MPPT charger uses that.

Normally an MPPT controller will be in state (c) - the MPPT state - until the batteries are close to fully charged. And here it is controlling the charging current to the batteries - by adjusting the charge voltage.

The main issue is the PV panel voltage drops the MPP voltage with solar power and on a 19.4Voc PV panel that Vmpp is now dropping from 14.5V@100%sun to 13.6V@50%sun to 10.9V@10%sun. So now you need a boost. So there is much more work to do to define how to regulate the PWM with Voc, Vmpp and Vbat & SoC.

It turns out the panel threshold voltage (1%current) also drops with %sun. So using a solar sensor you can bias the regulator voltage to77% of this Voc(1%A) and that will be your Vmpp reference voltage from a small solar cell or similar diffused response photo diode.

This means you need your regulator to control both the supply side voltage and the demand side current to match at various currents and voltages to achieve this optimal power transfer, so as the demand does not exceed the optimal supply and the supply does not exceed the float voltage of the battery.

This is one solution using load line analysis. However, I prefer to use a photo sensor to prevent hunting and instability for determining the MPP cheaply from a solar sensor.

ESR of PV is low and fixed at all PV voltages for %sun> 50. At no load I rises sharply then reaches a curved slope then rises sharply again below Vmpp at low voltage. 0.5 to 25 Ohm range in this 50W example.

If you wish to harvest the meagre power below 20% sun, your regulator losses must be less than the gain of a few Watts using a boost regulator with a battery above the Vmpp.

This SMPS can be any type that accepts current and voltage controls to match the load line with your buck-boost arrangement. But gains in power must exceed fixed losses.

Since load is much lower ESR than the source, you will want a big cap on the PV with a much lower ESR than the PV for stability and noise.