MPPT Boost Converter - Output Voltage

Question

I am working on design of a MPPT for charging batteries via solar panels. After doing some reasearch, I am a little confused as to how the boost converter can output a constant voltage, as well as limit the current flowing from the solar panel.

I get that you can use a PWM signal to basically output a constant voltage from a varying input voltage. However I am failing to understand how you can control the current coming from the solar panel when you do so, and how using a PWM signal on the boost converter can maximize the power point.

Can someone please explain in depth how a boost converter controlled via PWM allows you to not only output a constant voltage to the batteries, but also perform the function of an MPPT?

Am I missing something obvious that otherwise controls the current from the cells?

Here are some of the resources I have been reading if they are any help: http://ww1.microchip.com/downloads/en/AppNotes/01211A.pdf http://ww1.microchip.com/downloads/en/AppNotes/00001521A.pdf

Answer 1

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.