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I have an MPPT charge controller that has recently started behaving strangely and upon investigation I found a very hot mosfet and a transformer (below) that has clearly seen more heat than it should have.

photo

The manufacturer isn't particularly helpful in providing information about this. I've been able to figure out what's going on, but not why this happened in the first place nor what is required to repair it.

From what I could determine, the particular circuit is a step-up converter that generates 15V from the original 5V. I have no idea what values are used, it is hard to decipher. The final 15V regulator is a 78l15, so I know the voltage on that end is below 35V and the current will be below 0.1A.

circuit

The input signal is a 5V 40khz square wave generated by a PIC microcontroller.

I suspect, though I'm not 100% sure yet, that this 15V is used to drive the main mosfet. This particular controller switches in the positive line (as is the proper way to do it), so a higher-than-input voltage is required. I think this is it. That's why it needs to be isolated.

My question: What would be the simplest way to replace either the whole step-up converter with another -- with the proviso that it MUST be isolated -- or to somehow replace that transformer with something off the shelf?

I do not presently have the equipment to measure inductors. I do own an oscilloscope but lack a function generator. This makes it hard to really get into it further, which is why I would also strongly consider another off-the-shelf solution to just replace the whole converter, possibly with something that's even more efficient.

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    \$\begingroup\$ Buy one of these cheap 15 buck transistor tester thingies on ebay. Cheap, inaccurate, but they give at least some ballpark figures for very little money. Anyways I think you should first figure out why things are failing before you replace the parts that got damaged by the failure. \$\endgroup\$ – PlasmaHH Aug 4 '15 at 10:21
  • \$\begingroup\$ I tried the ebay route. Problem is none of the ones I found are isolated. This was the first idea I had too. I agree about figuring out why it failed. That tiny 78l15 on the other side is fine though, so the problem appears to be primary side. The PIC in this device was replaced recently, and my only guess at this point is that perhaps the new firmware on this one uses a different frequency to drive that mosfet. The mosfet also appears fine, though it is possible that it fails 'on', though this is not what I saw on the scope. I saw a 'very laboured' square wave if that makes sense. \$\endgroup\$ – izak Aug 4 '15 at 10:28
  • \$\begingroup\$ So as it goes with these things, the moment you ask, you tend to be more thorough with your investigation. I found that the Zener on the output is shorted out. I removed it from the board, and the number on it says 4750. Now a 1N4750 just happens to be a 27V 1W Zener, which fits this picture perfectly. This doesn't really answer my question, but it might just be that I no longer need an answer to this question. I don't really want to post this as the answer when it isn't an answer. \$\endgroup\$ – izak Aug 4 '15 at 11:04
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    \$\begingroup\$ All of these from Digikey .... \$\endgroup\$ – Russell McMahon Aug 4 '15 at 11:05
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    \$\begingroup\$ .... should meet your need - sorted by ascending price and starting at $9.50/1 in stock . Here is the datasheet for the cheapest one. 4.5V - 9V in, 15V @ 67 mA out. Isolated 1000V. Note that a 67 mA supply will supply a reservoir cap provide eg 1A++ at 1% duty cycle which is ample for FET gate driving. \$\endgroup\$ – Russell McMahon Aug 4 '15 at 11:08
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If you are 100% sure about the schematic you provide I would estimate the transformer to be a certain type of 1:4 or 1:5 job, depending on the smoothing capacitor and current drain.

If then, again you are 100% sure about the schematic not doing anything fiddly with the generated 40kHz on any other point (another winding? The incoming "+5V"?) this is an exceptional candidate for replacing with an isolated DC/DC module.

A very good reason to build this like they did is that in mass production the components may be cheap compared to a 1W5 avg / 3W peak DC/DC module.

Another very good reason is needing the resulting 40kHz wave at another point, or secretly tapping off another voltage somewhere. So you do need to make absolutely sure you don't interrupt anything but the 5V to 15V conversion.

The safest thing to do is leave the 15V regulator in place and replace the remaining circuit by a DC/DC unit that makes (one way or another) at least 18V, since we do not know the ripple requirements on the output. If it's just a gate voltage (and you can absolutely verify that it is) 100mVpp on 15V won't be much of an issue and you can go for 15V DC/DC blocks. In the latter case you may need to add an extra capacitance on the output (with or without Schottky and/or small resistor) to make the module's regulation less susceptible to the MOSFET's peak gate current.

If you have a floating bench top supply that does a nice 15V and/or 18V I strongly suggest you try it with that at a conservative current limit shortly to see if that works okay, since you can know and control the parameters with those and what happens upon a mistake with a DC/DC module is always a bit guess work.

Another thing to be very weary of is that the PIC might turn the auxiliary DC/DC off by stopping the signal and you may have to account for that in any of the possible fixes, since then it may assume its "off-ness" at other points in control loops. So certainly consider that in any further research.

As to the "laboured" square, that's possibly explained with the parallel capacitor, though I would say you'd still want some kind of a protection zener to prevent the MOSFET from getting into dangerous waters. If there is none of that at all, that is your first clue they might have not spent enough time on the DC/DC design, IMHO.

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  • \$\begingroup\$ Thank you for your detailed answer. I will replace the shorted Zener as soon as it arrives from the supplier (I had to order) and if that fails I will certainly apply your advice. \$\endgroup\$ – izak Aug 4 '15 at 14:59
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    \$\begingroup\$ @izak As pointed out before; be very cautious just replacing one component. You do not know whether the zener broke because of the transformer, or the zener broke the transformer. Even if you were sure, the transformer melted, which means its design insulation level is no longer there. \$\endgroup\$ – Asmyldof Aug 4 '15 at 21:56
  • \$\begingroup\$ You're right. But there is nothing I can do until my spare parts arrive. Sadly in my country availability and speedy delivery isn't something you can count on. It is also why sadly the numerous possible replacements from digikey listed above is nice and all... but will likely take weeks to get here. \$\endgroup\$ – izak Aug 6 '15 at 7:03
  • \$\begingroup\$ Finally got around to this again. Asmyldof is correct, just replacing the Zener didn't do it, the transformer was also damaged. The rest of the circuit works as I expected, this 15V supply goes to pin 8 (Vcc) on the 3120 (Gate Drive Optocoupler) and from there to the main mosfet gate. I put a 20V supply into that circuit and it operates correctly, draws about 5mA. It is never switched off by the PIC. I could, if all else fails, replace it with a muRata CMR0515 which is available from my local supplier. \$\endgroup\$ – izak Aug 13 '15 at 8:37
  • \$\begingroup\$ @izak I would say, you are best off removing all components to do with power-conversion, then do a quick measurement if no secondary leads to primary and then add your own fresh capacitor to the DC/DC module. Many things can have caused the Zener failure and more than half of them will also damage caps and other such things. \$\endgroup\$ – Asmyldof Aug 13 '15 at 8:39

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