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MPPT Solar Charger Design

MPPT Solar Charger Design

(OP)

I am designing a MPPT solar charge controller (buck mode) to charge batteries from 500 Watt solar panel. As per my calculations i need 50uH inductor with current rating of 50A for 12V battery.

My problem here is getting an inductor for 50A rating as it will be big and bulky.

The solution i plan to use is divide the buck circuit into 4 sub-circuit with 4 individual inductor rated for 12.5A.
And finally parallel the output.

But some people suggested that it might cause uneven loading of power and might result in uneven heating of mosfet.

I need some advice in this part, as to whether the idea of sub-circuits will work or do i need to take some precautions during the design.

Thanks
Regards
Gokkul

RE: MPPT Solar Charger Design

(OP)

@IRstuff

Thanks for pointing that out :). But this is a design that a friend of mine plans to manufacture, so i need to get the design done.

Thanks
Regards
Gokkul

RE: MPPT Solar Charger Design

(OP)
Sure it would be worthwhile, but i get some new ideas from the internet. Some are suggesting interleaved pattern of inductors.Currently doing some online research on that.

If nothing works out i guess i will have to reverse engineer someone else design.

Thanks
Regards
Gokkul

RE: MPPT Solar Charger Design

Switch-Mode power, even for what appears to be a simple solution, can quickly get surprisingly complex when you begin to add some constraints (like physical inductor size or interleaving multiple-parallel buck regulators). This is true even for those who spend their career in the field.

Another option would be to buy several pre-made buck-mode boards of around 12 Amps each to check out your concept to quickly make sure it will work. A quick check at Linear Tech showed they had a LTC3867 demo design with a 38V max input and 14V output. NOTE - The LTC3867 demo is just the quickest and first thing I found. I'm not saying base your final design on whatever chip is used on the demo board, just first use the demo boards to check your concept or approach to the problem.

RE: MPPT Solar Charger Design

I would not worry about the concern that "some people" have expressed for interleaving. If you have one master control loop which outputs a control signal to each individual supply which is current mode controlled, the power from each one will balance. There are many papers describing interleaving and many PWM ICs made specifically for interleaving.

RE: MPPT Solar Charger Design

(OP)
I did find some ICs and documents on interleaving buck mode supply, i think i will give it a shot. Thanks for all your feedback.

Thanks
Regards
Gokkul

RE: MPPT Solar Charger Design

Since you are designing a unit for commercial production, I submit that it would be irresponsible _not_ to acquire an exemplar of every similar device on the market and reverse engineer each and every one, so as to make sure that your product is an improvement, or at least develop sales talking points about the competition, and to make sure that you're not infringing someone else's patents.

Mike Halloran
Pembroke Pines, FL, USA

RE: MPPT Solar Charger Design

I designed a 3-legged buck converter @ 100A/60V output. Worked fine. A few tips:

  • Don't try to wind the inductors on the same core. They're unlikely to share correctly. Keep them separate and ideally not physically lined up.
  • Your output waveform will be much smoother if you interleave the duty cycle of each leg. But otherwise it doesn't matter much. It does mean you need individual MOSFETs and gate drivers, but that's probably a good idea anyway - then you don't have to worry about the legs sharing perfectly. I had the 3 legs overlapped 33% - gave me one third the ripple.
  • Be very wary of saturating your inductor - inductor manufacturers can misunderstand the DC component of a buck converter waveform, and will not necessarily size the inductor correctly. If you saturate it the inductance plummets and the current sky-rockets. Do your own calcs and measurements.
  • Do your calcs and measurements.

Quote (Gokkul55)

But some people suggested that it might cause uneven loading of power and might result in uneven heating of mosfet.

Note sure what they're talking about. Just do your calcs.

RE: MPPT Solar Charger Design

(OP)

Quote (LiteYear)

I designed a 3-legged buck converter @ 100A/60V output.

When you mean 3 legged you mean that you used 3 buck converter in interleaved pattern, so current on each inductor is approx 33A ?

Quote (LiteYear)

Keep them separate and ideally not physically lined up.

Keep them separate means individual inductors on individual core right? What does "not physically lined up" mean?

Will keep in mind all the mentioned points.

I am currently planning to implement interleaved converter design with separate driver circuit, mosfet and inductor.

Thanks
Regards
Gokkul

RE: MPPT Solar Charger Design

Yep, 33A per leg. Effectively 3 buck converters in parallel with a common supply and a common output capacitor.

Not physically lined up literally means not positioned directly side-by-side. If possible, put them at right angles to each other, or physically separate them. Otherwise any magnetic flux not contained within the core can be coupled into the neighbouring core and give you undesirable results.

RE: MPPT Solar Charger Design

(OP)

Quote (LiteYear)

Not physically lined up literally means not positioned directly side-by-side. If possible, put them at right angles to each other, or physically separate them. Otherwise any magnetic flux not contained within the core can be coupled into the neighbouring core and give you undesirable results.

Ahh nice tip, thanks :).

Thanks
Regards
Gokkul

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