Sentinel 300P Battery Charger Repair

One day the backup generator at our local fire station was throwing alarms about a dead battery. The battery charger was this beautiful thing in a stainless box with CANBUS control and alarm outputs and…some error message about no AC power. The charger would turn on with a battery connected, but not with only AC power connected.

Turns out this beautiful thing costs about $1000, and has essentially zero parts or service support from the company.

As expected, the charger is basically a switch-mode power supply with a microcontroller directing current flow. Generally speaking with devices like this, the things that fail are a) big power-handling devices, and b) anything close enough to AC power to get fried.

With no schematic, the first thing I tried was to look for fried components, but no joy in this case.

When tracing schematics, a trick I like to use is to scan the solder side of the board (for single-sided) with a flatbed scanner, then flip the image in software so I’m looking down from above. Then I can draw the components on in their “correct” orientations. After that I can trace out the schematic.

I didn’t save any of those pictures, but I did save the pictures of my hand-drawn schematics. The board has tens if not hundreds of components on it, but most of them are after the isolation transformer — and roughly speaking that’s the side that works. So I focused on the high-voltage side.

As you know if you’ve gotten this far, a SMPS basically rectifies AC to a high-voltage DC rail, and then switches that at high frequencies through a transformer. In this case, the 150v rail was okay, but we weren’t getting any switching happening. So I started there.

In this first schematic Q3 and Q4 are big MOSFETS, and they’re acting as low-side switches grounding the transformer primary. U12 is a MOSFET driver connected to the gate of both. A quick lookup of the datasheet gave us the pinout, and so we can see where the power and drive should be coming in. With the drive being shorted to ground we already have a problem.

This next schematic shows the PWM controller chip that’s supposed to drive the FET driver. There’s a whole science to making switch-mode supplies run efficiently, but you don’t need to understand that to see what’s going on here. There’s some feedback from the transformer coil, there’s an optocoupler feeding signal from the logic (probably regulating the output), power, ground, and a bunch of passives. The chip gets power directly from the 150v rail, but also from “D31”.

Here’s the power supply for the chips above. There’s sort of a belt-and-suspenders approach here, with Q15 regulating power from the rail, but Q14 also providing power via a rectified winding from T1. These get “mixed” by D35 and D17. Give the high numbers on the left side, I could imagine their original jump start didn’t work, and they added the left side later. Anyway, if I ever figured out exactly why this was needed, I’ve forgotten, but note that the output voltages from Q15 are too low to drive U12.

In general I’m not one to throw parts at a problem, but we can make some educated guesses here. The FET driver is definitely toast, the PWM chip might or might not be, and Q15 is definitely suspect. The passives all look good. So the smart thing to do is replace every active in the area, including the big FETs because they see a fair amount of stress.

Total parts cost was about $40, including the big FETs. I did this SMD work with a regular iron, just by flowing solder over all of the legs and then heating them all at once. Hot air would’ve worked too.

After replacing these parts, the charger fired right up and is still working today.

So why did they die? Our area gets a lot of power disruptions, so a surge seems like the most likely culprit. To be honest, I was pretty impressed with the amount of filtering I found on the incoming AC lines (as it should be at this price), but apparently it wasn’t enough.

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