Customer brought their RCF sub in, was working ok then suddenly went into protect and no longer starts up and makes a click sound every other second or so.
Inside we see Digipro 1000s – a £300 replacement module unit. Sod that, let’s fix it… As long as it’s not firmware on that microcontroller! Unit configured as two Class-D channels in BTL mode driving a single woofer.
Part of the issue in diagnosing this is that the microcontroller both enables, and disables the Class D circuitry based on fault conditions (summed from both channels), so it was almost impossible to check each channel singularly in the fraction of a second available.
After disconnecting the failure signal to the controller, we could see one channel start up.
One side ok and PWM oscillating, the other not, activating the protection:
Could see it starting to oscillate then shut down almost immediately (less than a few ms, thankful for the DSO that can hold the trace at trigger as impossible to see otherwise). Typical for ‘overload/overcurrent’ protection.
Removed MOSFETs to test, all ok, put back in opposite channels, still the 1st channel that was a problem. No obvious shorts or anything on outputs, and LC filter coils reading ok.
Swapped over the Class D gate drivers ‘AEB DPI’ which are rebadged IRS20957S, in case there was a drive issue – no difference.
Then almost by chance, was going round just double checking values of components and found this little sod had gone high resistance and was pretty much open circuit instead of 4.7R on the high side gate (penny to show scale):
Replaced with new and all good, problem fixed!
Sometimes it’s the simplest of things but often the hardest to find! In this case being the high side driver with a floating ground made it more awkward to dynamically measure powered up and gave the indication it was receiving gate drive when probed directly on the gate (presumably capacitive coupling or inductive pickup to probe), and the fraction of a second before shutdown doesn’t help!
Anyway all’s well that ends well and customer can use it again for the fraction of the cost of a replacement!