In 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:
Yes, 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.
So 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:
(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:
Before 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!