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UCC28810 - Single stage Flyback LED Driver - Triac misfire

Prodigy 120 points

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I have designed a led driver based on the UCC28810EVM-001. The schematic is the same with a few modifications for Output voltage=24V and the current is 3.5A. The triac decoder from the secondary side was removed.

Without any triac connected, the power supply works perfect in both modes: CC and CV. Delivers 80W with good efficiency and amazing PFC.

This is a great solution for LED drivers, I wish that TI could see the opportunity here and provide more support. Thank you Lisa Dinwoodie for posting the mathcad calculation - really helpful!.

My problem starts when I connect the triac dimmer (Leading edge) into the circuit.
Without dimming - the triac misfire on low loads (around 0.5-1A output current). Once I start dimming and pass a certain angle, the triac acts normal on low end.

In example: If I connect a 3A load on the output the dimmer works perfect on all range. If I connect a 700mA load on the output the triac misfire on high end (no dimming) and acts normally when the angle exceeds 90 deg.

I have tried several dimmers but the behavior is the same.

Any suggestions? ideas?

  • I have found the problem.

    The controller averages the current per entire triac conduction angle. For the same output power, on large conduction angles the input current will be small and could reach levels below the triac hold current. This will result in triac misfire. For small conduction angles the current will increase above the triac hold current and the power supply will work properly.

    The hold current for the off the shelf dimmers is around 50mA or lower for some models.

    Looks like a bleeder is necessary to be added to the power supply.

    Any sugestions?

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