Output Regulation Issue with LED Driver AL1665

[Johnson R]

Hello Everyone,

I recently designed a non-PFC based flyback SMPS with AL1665 for a power of 150W. The schematic from the application note : https://www.diodes.com/assets/Evaluation-Boards/AL1665-AL5822EV1-Flyback-User-Guide.pdf
is followed.

I have designed it for the universal AC voltage range from 90V rms to 300V rms. Output voltage is desired at 24V.
From the design sheet of the AL1665, the minimum operating frequency was to be 66KHz.

The issues that I am facing are
1) The switching frequency is observed to be at around 16KHz instead of 66KHz
2) Output Voltage is not regulated at 24V at no load. Instead the output voltage just keeps increasing till I turn OFF the input(see attachment - Pink is the rectified HV input and blue is the unregulated output)
3) When the output is loaded with just 50mA, the observation that is made in #2 is more faster

I have checked the FB pin, CS pin, NTC pin and the ADIM pin and all these have spike noises during switching which even inspite of the bypass capacitors are not suppressed(see attachment - Pink is the rectified HV input and blue is the VCC and yellow is the FB voltage)

I had made a design for the same power with other controller UC2843 and there I had did not face this kind of issue of unregulated output.

Kindly let me know what am I missing with this design.

[mtwieg]

AL1665 datasheet it has a minimum on-time of 1us typical. Therefore if there is no load, Vout will rise slowly without any ability to regulate itself.

The datasheet also states there is an output OVP function, with the OVP threshold depending on how the FB pin is connected. Can you show your full schematic?

[Johnson R]

Yes, but I see that the OVP is not triggering -> Rather the Output is rising beyond the rated value of the output capacitors. As I mentioned, I tried with some burden resistance of ~ 50mA but that actually accelerated the voltage rise.

Turns ratio between Pri to Sec is 6.5 and Pri to Aux is 8.5

I have attached the schematic for your reference.