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Low frequency vs high frequency charging


Solarphile

Question

When you go into the settings of the microcare MPPT there is one setting which is called Operation Mode. You get the options of Low frequency (34mhz) and high frequency (77mhz) By default it is set to 34mhz. 

I asked the guys at Microcare which is the better option and they don’t know. 

Does anyone know what this setting does, or which is the better option? 

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41 minutes ago, Solaris said:

You get the options of Low frequency (34mhz) and high frequency (77mhz) By default it is set to 34mhz.

That's new....

The older unit I disassembled some years ago had just one frequency, and it was 40Khz. You sure you got that right... the setting says MEGA-Hz?

I've never heard of a DC/DC converter running that high up. I believe 250Khz is about the limit at this power level, though in smaller power supplies it might go up to 4Mhz.

A buck converter works a bit like an irrigation holding dam. Think of a farm where you have canal water (typical in many parts of the country). The flow of the canal is not enough to run your pivot point properly, so what you do is you catch up the canal water into a holding dam, and then when you have enough water you start the irrigation. This then turns into an on/off pattern, waiting for the holding dam to refill and waiting for it to empty.

You can use a smaller holding dam if you repeat the on/off pattern faster (aka frequency). Similarly, in a buck converter, you can use a smaller core in the inductor (basically a big round toroidal thing with a wire wound around it) if you run at a higher frequency... BUT... at higher frequencies skin effect kicks in and reduces how much current the inductor can handle.

So my gut feeling... assuming it's Khz and not Mhz, is that you might eke out a bit more efficiency at the higher frequency IF you are not running at the MPPTs full capacity. That's my guess.

Edited to add:

Truth be told though, this makes very little sense to me. Generally you want to make the magnetics smaller (the core in the inductor), but if you make it too small, you have heat issues, so you try to hit a balance. Once you decide on the inductance and the capacitance on the output (and these are things you choose... they are not variable), you've pretty much decided the frequency too. I can see no benefit in changing the frequency.

Unless.... unless it does more than that. Perhaps it has two inductors and at the "higher" frequency it actually runs interleaved, so each inductor still runs at the lower frequency, but because they are alternately switched the output ripple is much less. But then again... why bother with making it an option?

Unless... (third guess), it has to do with EMI? Select the other one if the default causes noise issues?

OK... I'm done :-)

Edited by plonkster
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