LNK-XT resistor burn ! Why

Posted by: cosebe on 08/14/2017

Hi,

I design a power supply since 2007 with LNK362P.
I have some problem with resistor R3.

Sometime, this resistor burn, and only this resistor. If I change it, it works again.
I had to place a 3W resistor to workarounds this problem.

Now I have to make some modification on this board.
I follow an application note in 2007, but I cannot find it in the new website
Can you please give me some information :
Do you have an idea why this resistor burn ?
Can you give me some modification I can do to have a better work of this power supply.
I need to keep the transformer because of my stock !
I cannot change a lot of things because of the place.

Regards,

Files

power supply (129.19 KB)

Comments

Hi cosebe,

Thank you for using Power Integrations' part.

Regarding your question, basically the resistor is getting burned due to power dissipation. It is not advisable to have a high resistance in series with the line since it will dissipate alot of power loss suffering your efficiency. Instead of resistor, we usually put a choke ideally 1mH to meet EMI compliance requirement. May I know the reason why you use such a high value resistor for R3? If you reduced it to 10R or less, it will definitely address your problem and will improve your efficiency without affecting your regulation. Hopefully it help.

Thanks Mallora,

You can imagine I already think about this, but since many years, I test some things :
1. When I power up the power supply and use it at the maximum (ie at max power of the board 0.5W) with a R3 resistor of 100 ohms 1/4 watt, the resistor is not hot. For me, it is normal because 0.5W / 65% = 0.77W
0.77 / 220V = 3.5mA (This is really approxita calcul).
But R3 should have a means current of 3.5mA so a power dissipation of 350mW. Not a problem of power dissipation.
2. When R3 "burn", it is not completely black. I can see only a black dot on it. Looks like R3 broke because of over voltage. R3 is 500V !
3. Why I use a 100 ohms resistor. Because on the application note AN-40 page 7 (AN of the LNK-XT), it is suggest to use this schematic for <1W power supply.

I need some light.
Regards

Hi Cosebe,

I see, your application is below 1W power supply. In that case you are correct, it doesn't dissipate much on normal operating condition. The failure is due to inrush current caused by transient voltage like surge or start-up. During start-up when C5 is fully discharged, there is an inrush current to charge the capacitor which can damage the R3. Same thing when there is voltage surge, some of the energy is dissipated by the R3. It is not recommended to use metal film or carbon film resistor for R3 as it doesn't have good tolerance for surge/inrush current. The recommended use is a wire wound resistor. Thank you.