Digital Dimming with LNK414 resulting in LED output flashes

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We are currently using the prescribed digital PWM dimming method to dim the LEDs. The problem that we are experiencing is that we find the occasional flicker in the LED light. With inspection the output voltage dropped considerably from time to time with no clear cause. A scope capture is attached. (Output sudden voltage drop - the green trace is the LED voltage whereas the red trace is the PWM signal).

 

 Another scope capture, duty cycle trip2, illustrates the voltages of some of the major sense pins of the Linkswitch. The green trace is again the output voltage, the blue trace is measured on the FB pin and the red trace is measured on the BP pin. The yellow trace is the line input voltage. The pwm signal is 100Hz at this stage to illustrate the problem better. The issue is however still present at 1kHz.

 

When we increased the bus capacitance to 3.3uF the problem is solved, however the power factor is consequently no longer acceptable as the client requires at least 0.7.

 

Any support would be much appreciated. 

 

Attachmentサイズ
Circuit_0.JPG87.14 KB
dutycycle_trip2.png150.96 KB

I'm suspecting that when the collector of U3 is pulled down to turn-on the device again, you might be pulling the BP below 5V causing it to shutoff the device. I can't clearly see the BP voltage on your attached waveform when the output begins to fall down, but if that is the case, you can try moving R27 from BP pin to anode of D10 and just adjust its value accordingly.