flickering LED circuit

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the drvier oscillates btw 50V to 80V; plz validate the design and comment.

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20W PSU Design Using LNK416EG (1).doc123.5 KB

The schematic shows your output voltage is 42V, and there is an OVP circuit, (Q1, VR2, D6, etc), which will be active when there is higher output voltage.

Can u be specific what changes need to be done to control, have already changed the VR2 at 18v zener the oscillation stops whereas the output are de-rated to 50% ie., the output truncates to 28V with max output current 200mAmp.

The intent of the design is to get the rated output shown in the schematic.

Is the transformer design correct?

Appreciate your comments.
Elan

The transformer design looks all right, but the sum of upper Vpin resistors, R5 and R6, is 3.6 M, which could limit the output power.

I would trouble shooting the board as follows;

1) Remove,Q1 the OVP circuit,until there is no issue with output power.

2) Remove R7

3) Change R5 to 1.8 M

4) Short R17 and replace D2 with ultrafast diode, UF4007.

5) add a heatsink on U1.

If there is no poor layout issue, there should be no issue of getting output power.

After getting desired output power,

1) Fine tuning Vpin resistors, R5, R6, and R7 for improving line regulation as suggested in PIXL.
2) Connect Q1 and select VR2 for open load, OVP protection.

Thanks, will try this changes recommended and revert back.

Good luck !!!

Thanks, with your suggestion we could fix the problem.
How could I extend this design to have a PWM based dimming control. Plz let know
Elan

You can use LinkSwitch-PH for the PWM dimming as shown in the attached cicuit diagram.

Had reviewed the ckt sent, will explore, is there any standard PI application note for such PWM control, plz let know.
Rgds,
Elan

There is no Application Note for the PWM dimming only, but you might want to review DER263 engineering report, which is the "14 W PWM-Analog Dimmable LED driver using LNK406EG".

On pages 9 and 10 of the report, PWM dimming was explained in detail including circuit diagram.