Reg LNK302PD

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dear sir, i am working in LNK302PD.We need power supply for 13V/40mA.We just made circuit same as your resference design(0.5W Non isolated Buck boost converter using LNK302P(DER-92).During Power on RF2(Flameproof resistor)47E has burnt.We checked the circuit also.Why it has happen.what are the neccessay steps to take.kindly reply as soon as possible. Thanking you Regards P.Chellapandi Design-Engineer
I have heard of a similar problem with fusible resistors. In that case it turned out that a different kind of fusible/flameproof resistor was being used. The resistor you should use is the wire-wound kind. Wire wound resistors are more inductive which helps in this case.
We also noticed it with LNK304, it burned down lots of 10 ohm wire wound flame proof resistors along with 10 ohm normal resistors (the reference design showed 8.2 ohm). We increased it to 27 ohm, now it works and PI xls doesn't show any warning.

The inrush current causes a huge instantaneous dissipation in the resistor. For film type resistors this can cause the resistor to fail. I did test years ago cycling application of power on film resistor and found they would eventually fail even when using 2 W rated parts. The worst case is turn on at the peak of the AC at highest input voltage (say 265 VAC).

Wire wound don't suffer due to capability to handle this large transient dissipation - in my tests after weeks of cycling once per second there were no failures. This is why we mention that the resistor should be wire wound.

Cheers

PI-Chekov