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Solar MPPT - Freewheel diodes, buck regulators etc

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RayJones:
I've been playing around with a PIC controlled buck regulator for the purposes of a Max Peak Power tracker for solar panels.

The original design I started with was based upon Tim Nolan's where he used a synchronous rectifier (Mosfet) in place of the back swing diode. (www.timnolan.com)

For sure this improves efficiency tremendously due to the much reduced voltage drop, BUT.

The BUT comes about when you have a day where the sun quickly goes behind clouds and the panel's output drops quickly.
I've determined what happens is the synchronous buck regulator suddenly becomes a synchronous boost regulator in the reverse direction >:(
ie the battery can supply current in the reverse direction through the inductor to ground via the synchronous lower mosfet.
When that switches off, the top mosfet transfers the voltage kick in the reverse direction back to the panel's input (albeit with a blocking diode) - Ala the classic boost regulator topology.

After blowing yet another set of mosfets with god knows what current reversals and voltage spikes going on today, I'm now considering reliability over efficiency and using a true back swing diode in the usual location.
A true diode will simply not allow the reversed inductor currents to flow.

So there then becomes two obvious choices:

Fast recovery, or Schottky?

It looks like the Fast Recovery diodes have a much higher Vf, but are the switching losses more pronounced if you use a Schottky?

I'm interested to hear from others with perhaps more experience in switching regulator design, and even more so a discussion on MPPTs would be great....

This power electronics is spectacular at times  :o

Simon:
yes I've been wanting to design one of these for a while but am still messing about with another project.

I'm no expert on diodes and switching regulators, I started by tring to use a switcher on my panels but figured i was getting no increase in efficiency due to the regs dropout but i was using a single chip reg. I think the proper IC controllers with an external mosfet (my chip used a BJT hence the losses ?) is going to be more efficient

You may be able to get arouind your issues by putting some sort of control in the MCU conntrol system ? I can see the sort of trouble your having as I'm doing a project to control a car dynamo and need to know if the dynamo is outputting enough to charge the battery instead of the dynamo becoming a motor and being run by the battery, in my case I sense the current and cut off power under a certain current value, in your case you could actually sense the direction of the current and refuse to let it to work if the current starts to go the wrong way ? or have an ADC input to the MCU so it can check the voltage and switchoff when it goes low, you could also have a battery and panel ADC input and check that the difference bettwen the two is great enough for it to work ?

mikeselectricstuff:
I wonder if a compromise would be to have a schottky diode, and a MOSFET that only turns on when you know it's OK...

Another thing to bear in mind is that sync rectification only has major efficiency benefits when the Vf of a diode is significant compared to the output voltage - for something like a 12V supply, a big chunky schottky diode is only going to be a few % less efficient.
Choice of diode type will depend on frequency, which again depends on lots of other factors, but it generaly boils down to switching losses versus static losses and inductor/cap size. if you are not pushed for space, there is probably little need to have the frequency particularly high, so a Schottky is probably going to give the best efficiency. Shottkys aren't exactly slow.

There are some more complex buck-boost architectures like sepic which can deal with inputs higher or lower then the output, and as the source can potentially be floated, an inverting topology may be worth looking at to deal with the input range issue.

Zero999:
You could connect two MOSFETs back-to-back, it would mean a higher voltage drop and gate capacitance but still might work out more efficient than a Schottky diode.

RayJones:
Yeah that's a good idea to check the current direction, but I'm pretty sure the high side current sensor I'm currently using only works in one direction.

I first tried low side sensing of panel and battery, but found I was having all sorts of trouble getting consistent readings with increasing currents due to the "ground" wandering about.
The new design using high side sensors works a treat, and I now have a dirty big fat single ground across the bottom of the power control stage.

There is already a schottky diode across the lower mosfet, this takes care of the initial back swing as the FET drive is deliberately non overlapping - for obvious punch through reasons (which is perhaps what is taking out my mosfets anyway when the sun rapidly goes away...)

My initial check was for low solar panel volts to stop the switcher, but that was when I found the boost converter action never allowed the "panel" volts to fall!

I'll have a closer look at the battery current sense, and see about making it bipolar somehow.
Actually even just a comparator looking for -ve flow would work as a yes/no control into the micro or even, shock horror, direct into the FET driver.

One thing you must be careful of with mosfets is their parasitic diode. I'm not sure what is meant by back to back mosfets, but the didoe may become another issue.

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