Saturday, September 20, 2014

Failure is one thing. Not knowing why is another.

For the first time, I'm stymied.

I've made mistakes since I started this making thing. All of them have been learning opportunities. If you've bee following me on twitter, you'll have seen some "Tips du jour" - most of those are lessons that I've learned from figuring out what I did wrong.

Well, now I've got a situation I can't figure out.

I have the same circuit on two different boards. One one board, it works perfectly. On the other, it doesn't work at all.

The circuit in question is a boost converter based around the NCP1450-5 boost controller. Both circuits have the identical schematic. The board layouts differ, but  I've conferred with a colleague at work and he didn't see anything wrong with it either.

The symptom is that with no load, the output is 5 volts. But while circuit A can supply up to an amp of output with less than a tenth of a volt of sag, barely 100 mA of load on circuit B is sufficient for the voltage to sag down to the battery voltage. It's as if the controller "gives up" on trying.

My first thought was that the feedback sampling was being drawn from too close to the output of the regulator as opposed to near the load. But if you do the math, there isn't enough voltage drop on the traces of either design to account for anything.

I then thought that perhaps the output filter cap was too far away from the cathode of the diode. So I added a 68 µF tantalum cap tacked onto the end of the cathode and a short wire to ground. That didn't help.

The board that doesn't work also has a LiPoly battery charger on it, but the exact same circuit on a separate board has been used in concert with the working version of the booster board for a while now, so the mere presence of that circuit doesn't cause any trouble (and why would it anyway?).
The schematic

The board that doesn't work

The board that works.

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