Switching Regulator Conversion to MPPC Operation

[bcroe]

If I understand correctly, MPPC is a method of regulating solar panel
array voltage so as to maintain a constant array voltage over varying
loading. The voltage would be chosen to be about the MPPT voltage,
which varies little with current, more with temperature. It works because
the array efficiency varies little when close to the actual MPPT voltage.

The method I see proposed is to take a switching reg and reconnect
or swamp out the original feedback, from the output voltage, to the
input voltage. The use of temp compensation will be difficult if the
array is not close to the switching reg circuitry.

I have considered this approach, but not constructed any. Bruce Roe

[jflorey2]

The method I see proposed is to take a switching reg and reconnect
or swamp out the original feedback, from the output voltage, to the
input voltage.

That would result in a regulator that either output full array voltage (if the input voltage was under the setpoint) or zero output voltage (if the array was over the setpoint.) Not too useful.

You'd need to do a lot more work than that. Specifically, you'd need a linear UVLO, which means a direct (not inverted) term with plenty of gain going into the regulator.

[bcroe]

That would result in a regulator that either output full array voltage (if the input voltage was under the setpoint) or zero output voltage (if the array was over the setpoint.) Not too useful.

You'd need to do a lot more work than that. Specifically, you'd need a linear UVLO, which means a direct (not inverted) term with plenty of gain going into the regulator.

No gain, phase, or damping specified here, just the objectives. Some kind of PID loop
would be required. Mine would involve Op Amps, and Counters if Integral was used.
Bruce Roe

[jflorey2]

No gain, phase, or damping specified here, just the objectives. Some kind of PID loop
would be required. Mine would involve Op Amps, and Counters if Integral was used.
Bruce Roe

Fun fact - the old Blue Sky MPPT regulators (which sorta sucked) used a similar scheme. They would open circuit once a second, measure the OC voltage, then set the UVLO to about 10% below that. That way it tracked changing voltage due to temperature, but of course had efficiency and EMI problems.

[PNPmacnab]

"They would open circuit once a second, measure the OC voltage, then set the UVLO to about 10% below that."

Not exactly, you set the power point voltage closer to 20% and the time interval would be significantly longer. I've done this with microprocessors and highly recommend it for beginner programmers. It is pretty effective except for extreme shadows which cause bypass diodes to function. The open circuit voltage tracks temp and it only needs to be done every half hour.

There is some waste in MPPT searching and some controllers et a little crazy with sudden load changes. Panel voltage just shouldn't move minute to minute. Current can change quite dramatically in conditions you would never notice.

The circuit I provided as a LCB is simple and quite effective. On a motor it has to work over a wide range and has some minor issues. With a battery, changing the output only a half volt dramatically changes the charge current. With Li batteries, this simple buck conversion is ideal, just set maximum battery voltage. Current limiting when appropriate.

The TL431 can be found in older wall warts and switching supplies of almost any old electronics (I buy them for 3 cents). Find anything with three pins that says 431. With not having to fake switcher chip when using a battery, the pull up resistor can probably be 15-20K. The sense pin of the switcher only has to be raised a .001volt.

The low voltage inhibit can be done with almost anything. op amp, comparator, FET, zeners with transistor or opto isolator. This is one I built to charge my battery over winter from a 60V array used for water heating. It converts down to 14.5V (it says 20V and then I use a $4 PWM charge controller to do the niceties. This converter is only $4.50 shipped and this is a nice setup for someone starting out. It lets you use one or two cheap 60 cell grid tie panels in series until graduating to a more expensive MPPT controller. I hate people wasting their money on 12V panels. 10_DOLLAR_SYS.JPG