Reviving a Failed Inverter

AIMS 180W InverterIn my last post I discussed the building of a solar-powered “reverse UPS” called the Doom Box (a name given for, really, no good reason). To briefly recap, its purpose was to capture, store, and convert solar energy into AC power. This AC output was provided by either a direct connection to the mains or, when solar power was available, an AIMS 180W pure sine wave inverter (specifically model number PWRI18012S, shown in the image at right).

Well after about six months of daily operation (with an average power output of 35W), my inverter bit the dust. To be precise, its 12VDC input suddenly turned into a dead short. Now, if you short out a big lead-acid battery, bad things can happen (burning insulation, melted wires, explosions). This is why I always (mostly) put fuses in series with my batteries. So because of these fuses, when this inverter failed, it wasn’t all that exciting. All I heard was a small pop from the fuse and that was it. Thinking that this was just a temporary problem, I replaced the fuse, removed the AC load, re-enabled the inverter, and pop – another fuse blown.

As a first test, I got my multimeter into the system and checked that the inverter’s input was in fact internally shorted. The next step was to remove it from the system. If you’ve seen the pictures from my last post, you’ll know this was no easy task. However, after disentangling it from the rest of the components, I popped it open and had a look:

AIMS 180W InverterAIMS 180W InverterYes, that assembly really is as sloppy as it looks. It appears that components were just shoved in at random. Seriously, it’s amazing that this thing ever worked. The two TO-220 package ICs (which are linear regulators) in the middle of the device are actually insulated from the toroidal inductor by a thin piece of paper. So much for build quality.

Well enough ranting – the question was, can this be fixed? I’d already likely voided my warranty by cutting off the cigarette-lighter plug, so returning the unit would be difficult. I started to poke around looking for anything that had obviously failed (e.g. burst capacitors – these are fairly common culprits). It turns out both of the input MOSFETs had shorted out. I’m not sure what caused this, but I suspect they were fairly low-quality components to begin with, and repeated thermal cycling led to failure.

AIMS 180W Inverter - Replaced MOSFETsSo to make a long story short, I replaced the failed MOSFETs (indicated by the red arrow in the image above) with similar, but higher-rated devices from Fairchild Semiconductor (FDP8860). By higher-rated I mean that these new transistors were rated for greater currents (80A) and had lower on-resistance (less than 3 mΩ). As a result the new MOSFETs should actually be more efficient and thus run cooler. So my messy little AIMS inverter is now up and running once more.

The Long Story: How to Identify Failed MOSFETS

You might be wondering, just how did I know which transistors had failed? Well many times, when a transistor fails and shorts out, it rapidly becomes very hot – so hot that its case can actually melt, crack, burst, glow red, or all of the above, as seen here:

Failed IC

(Note that the IC in the 14-DIP package shown above is not a MOSFET, but a dual MOSFET gate driver – I may discuss its failure in a future post.) Now, sometimes the damage to a failed device is much less obvious. I’ve seen a number of failed MOSFETs that appear to have one or two small droplets of water on their surface after failure. It’s not actually water, but some melted component of the package.

However, if you’re taking appropriate precautions and fusing your power source, it’s likely that your failed device will shown no physical signs of failure whatsoever. This is because after the device shorts, the fuse immediately blows and prevents catastrophic heating. This was the case with my inverter: the MOSFETs I replaced did not appear damaged in any way. So how did I find them, out of the multitude of devices crammed into that tiny case? Simple, actually – I measured the drain-source resistance on each device:

MOSFET SymbolBefore I continue, a little background is in order. A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) works because voltage applied between the gate and source (Vgs) creates a small electric field inside the device. This electric field causes charge carriers to be pulled into the region between the drain and source. As a result, the resistance between drain and source drops as Vgs increases. So, if there is no voltage applied between the gate and source, and if the gate has been discharged (this happens over time, but can be accelerated by a short to the source), the resistance between drain and source should be infinite (or at least in the megaohm range).

So one way to verify that a MOSFET is working properly is to touch the positive probe of your multimeter to the drain pin (typically the middle pin on a TO-220 package) and the negative lead to the source pin (typically the rightmost pin). The meter should register at least several tens of kΩ, if not MΩ. If you’re getting a low resistance, try shorting the gate and source pins to remove and residual gate charge. If the resistance is still low (typically less than 100 ohms), the MOSFET is likely dead shorted. Of course, if the MOSFET is still in a circuit, your meter may be measuring the resistance of some other component. One thing you might try with the TO-220 package (pictured above) is to clip only the source lead before you test the device. Then you should be able to safely test the device. If it seems to be working, just dab a bit of solder over the cut lead.

One last thing you may wish to verify is the internal body diode (seen in the schematic symbol above) between the source and drain. Many multimeters have settings to measure the voltage drop across diodes. However, if your meter does not have this function, a simple resistance check will do. Our earlier test of resistance between the drain and source should have indicated very high resistance. However, if you reverse the polarity of your meter’s leads (positive → source and negative → drain), you should now read a very low resistance due to the body diode (if all is well).

Well, that’s my preferred technique for testing MOSFETs – if you have further suggestions or tips please feel free to comment! Thanks!

ΩΩ
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8 Responses to Reviving a Failed Inverter

  1. Jorge Pinto says:

    Good for you. Going uprated mosfet is not always going to buy more efficiency. A larger mosfet (more current, and/or higher VDS breakdown) will incur in more losses switching the gate. Moreover, if the gate driver circuit did not have design margin, it becomes a strong possibility that losses around switching times (while the channel is being formed, Rds is not high, but it’s also not miliohms) become much higher than the losses during full conduction.

    Anyway, I just ripped off one of your pics to put on a ppt about mosfets.

    • Mike says:

      Thanks Jorge, you make a very good point! It’s been so long now, I don’t remember whether or not I computed switching losses when I performed this repair. However, in this case, I’m not too worried. If I just make a rough approximation and assume that we lose the energy stored in the gate capacitance (9200pF) at a rate equivalent to the switching frequency (and I’ll guess that’s 20kHz, but I don’t know), that’s a loss of just (20000Hz)*(1/2)*(9200E-12F)*(12V)^2 = 13mW. I don’t know the exact topology of this inverter, but since there are two FETs at the input, double that for a loss of 26mW. Now as you say, the increased gate capacitance also causes a delay in the formation of the conduction channel. The math for that number is going to be a little tougher since we need to integrate the power consumption in the FET as Rds is changing. Determining the rate of change of the gate voltage (and therefore Rds) is something I can’t do since I have no clue how much current the driver can pump into the gate. At any rate, my gut feeling is that this loss isn’t going to be too large. However, I do remember that these new FETs cut down Rds by a factor of 10, reducing conduction losses at full power from (0.03)*(15^2) = 6.75W to (0.003)*(15^2) = 0.675W. That ~6W savings is pretty good in my opinion.

      Anyway, I’m glad you found my pics useful, and thanks again for the comment! Feel free to check out one of my more recent posts which discusses MOSFET losses in a little greater detail: http://www.nlvocables.com/blog/?p=188

  2. Roberto Alonso says:

    I have an AIMS 5 KW inverter. One of the mosfet ground itself to the aluminum case and burned. I try typing the PN on the web but I get chinese page and they don’t tell you jack. I try to cross reference the PN of 1RFP260N to digi-key but can’t find it.
    Does anyone know what this PN be in Fairchild or any other electronic manufacturer.
    I paid $450 for my inverter and last me 2 years.
    Any help will be greatly appreciated.

  3. Pete says:

    You are referencing 1RFP260N. It should be IRFP260N (International Rectifier). You should be able to source this MOSFET datasheet and suppliers as you search now.

  4. sunshinekhan says:

    Was just wondering what would happen if 110V DC is applied to a regular 12V inverter such as yours?

  5. sunshinekhan says:

    Sorry for the double post!
    But I had to clarify that my inverter is not a pure sine wave inverter, it is a regular run of the mill modified sine wave inverter.

    • Mike says:

      Hmm, I’m not sure what you mean… If you’re wondering about applying 110VAC to the output of the inverter, perhaps as a way to tie it back into the utility grid, I believe the result would be sparks and smoke. A pure sine wave inverter like the one above might be able to match the incoming AC waveform, but it is not designed to match phase or precise voltage levels. The problem is even worse with a non-sine inverter, as these basically just produce square waves. And in general, when you connect two voltage sources directly together, unless they’re producing exactly the same waveform, they’re going to exchange a great deal of current. So you’d likely fry the inverter.

      And just to cover all the bases, if you were wondering what would happen if you applied 110V DC/AC to the 12VDC input of an inverter, again, I’m pretty sure the result would be sparks and smoke. At minimum a blown fuse. I highly doubt the designers of any 12VDC inverter planned for it to receive a 110V input…

  6. Zubair Ahmed says:

    sir,
    i am intrested in ferromagnetic core winding turns… i want to wind one for my self and i am unable to understand how to wind it..need help.

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