Why RFB for LNK500 SMPS GOT DAMAGED ?

Posted by: Tony Tong on 10/20/2015

Hi PI Experts,

we got 2 PCBA from production dept that the 5V output does not work ( only equals 3V ) , after further analysis, we found RFB(10k, R5 in attached schematic ) got damaged in open mode, after repalacing it with good one, the 5V output becomes OK. I do not know why R5 got damaged ? Is there any problem with the schematic design ? ( Resistor Type : Yageo MFR-25FRF52-10K)

Files

Schematic.jpg (136.67 KB)

Comments

Hi,

Resistor R5 might have got damaged even before population on to the board, either because of handling or faulty component. The value of R5 is selected to give a feedback current into the CONTROL pin of approximately 2.3 mA at the peak output power point of the supply. The actual value depends on the VOR selected during design. Any 0.25 W resistor is suitable.

Regards,

PI-NANO

Thanks very much for your feedbacks , actually i have the same concern as yous at the beginning, then our PCBA supplier showed us the PCBA function test result which shows the PCBA works well before shipped out from their plant .......

two further points regarding the subject to confirm with you :

a) if RFB is in open mode, how will LNK500 work, what duty cycle will it be ? will it enter into auto-start mode or just work at certain duty cycle ?

b) Do you think the Hi-pot test on production line ( L/N-GND, 1500VAC, 1MIN) will possibly damage the RFB ?

I also attach some measured waveform, with RFB open , I found the voltage across RFB is about 100V , and it seems the IC stops for some time and then starts at a very low duty cycle.....is it auto-start mode ?

Thanks very much in advance !

Best Regards!

Tony Tong

Attachment	Size
Measured Waveform.pdf (330.47 KB)	330.47 KB

Hi,

Or else it might have got damaged during transportation/handling.

Yes, the part enters into auto-restart mode when R5 is open.

Auto-restart operation

When a fault condition, such as an output short circuit or open loop, prevents flow of an external current into the CONTROL pin, the capacitor C6 discharges towards 4.7 V. At 4.7 V, autorestart is activated, which turns the MOSFET off and puts the control circuitry in a low current fault protection mode. In auto-restart, LinkSwitch periodically restarts the power supply so that normal power supply operation can be restored when the fault is removed.

Regards,

PI-NANO