Issue R1 and VR4 Failure in DER-622-Based Power Supply

Hello,

I’m working on a power supply design based on the DER-622 reference circuit using the LinkSwitch-TN2 (LNK3202D). My application is a 2-wire (no neutral) system supplying a load of approximately 20–50W.

Recently, both R1 (20kΩ/1206) and VR4 (43 V Zener diode) were damaged (burned) during operation.

I understand that:

• R1 is intended to limit inrush and standby current.
• VR4 is supposed to bypass R1 during startup or when higher current is needed.

Symptoms:

• R1 was visibly burned.
• VR4 failed (likely shorted or open).
• The circuit stopped functioning entirely.

Possible suspects:

• bulk capacitors may have shorted.
• LNK3202D may have internally failed.
• High inrush current or a downstream short may have caused R1 and VR4 to overload.
• AC input surge or relay malfunction.

Request for help:

• What is the best way to prevent R1 and VR4 from being damaged under such conditions?
• Are there recommended modifications to R1, VR4, or protection circuitry for handling 50W loads reliably?
• Should I lower the R1 value or increase Zener power rating?

Any insights or practical suggestions from experience are highly appreciated.

Thanks in advance!