Exploring a faulty LED driver (with schematic)

This is the LED driver out of one of the floodlights at my work bench. It started blinking out briefly, so I swapped it out before it went into full-on disco mode as they often do.

The most common failure of these drivers is the degradation of the output capacitor due to the stress of operating at high frequency and being used in a hot enclosure. With switch mode power supplies the secondary capacitor has to have a very low ESR (Equivalent Series Resistance) to deal with the ripple of current flowing in and out of it 20,000 times per second or more. As the capacitors age they develop a higher ESR which can accelerate the aging faster. That sometimes shows as pressure build up from the failing electrolyte causing the top of the capacitor to bulge or split.

When the capacitors ESR is high enough it can't do its job and the primary side feedback circuit can interpret that as a fault and cause the control circuit to shut down before attempting to start again.

In most power supplies like this there is a bootstrap circuit where the control chip's supply capacitor is trickle charged via high value resistors at power up until it reaches a threshold voltage that the control chip can start running. When the power supply starts successfully it provides its own power via an extra winding that is also used for feedback to monitor the load.

If the circuit fails to provide that power due to a fault on the secondary side then the control circuit capacitor discharges to a threshold where the control chip cuts off again. The cycle then repeats. That's what causes the slight delay before the light starts and also the pulsing effect when there is a fault.

A quick fix for many of these types of circuit is to routinely replace the main capacitors, ensuring that the bootstrap and output capacitor are low ESR types. The main input capacitor can be a traditional electrolytic as it only operates with a 100/120Hz ripple.
Another common failure is the secondary side diode, which is usually a high speed schottky type.

If the current sense resistors close to the control chip or switching transistor are visibly burned then it often hints at a failure of the chip or transistor. That often has knock-on effects of rectifier damage and sometimes transformer damage too.


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