Hi Teoh,

Is the waveform taken during start-up full load? You may check and measure the length of skip time to see if it's an autorestart protection being triggered (around 1.5 sec skip time).
Also, did it happen after you alter the value of your feedback resistor R15? R15 should be chosen so that the current through the FB pin of LNK419 IC is within the CC Control Region of the FEEDBACK Pin Characteristic curve which could be found in the LinkSwitch-PH datasheet. If not chosen properly, then the autorestart protection maybe prematurely triggered.

You may refer to the PI Expert Online tool in sizing the value of your FB resistor and transformer inductance values. Below is the link.

https://led-driver.power.com/design-support/pi-expert/

Regards

Hi Sir,

Ya. I put the full load which reach around 46V at output. The blank waveform measured is around 3.28ms. If it is 1.5s, we would see the LED blink? What is the criteria for auto restart can be observed?

I indeed alter R15 value. Original is 110K (Based on the simulation software) but output current can go to 1.2A. Suppose the design is for 600mA. I also wind the transformer based on the inductance and turns given. Not sure where goes wrong.

Regards,
Teoh

Hi Teoh,

Auto-restart could be triggered when the FB pin current falls below the CC Control Region threshold of the FEEDBACK Pin Characteristic curve which could be found on the LNK419 datasheet. On the other hand, cycle skipping could result during over-current protection, which seems to be what's happening to your circuit based on the drain current waveform. You might have to redesign your transformer if you want a higher output power.

Regards.