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Power Integrations Engineer Power Integrations ModeratorLNK306 Drain and Source Short ....
Dear Madam/Ser, Hi
I design a non isolated LED Driver with LNK306 based on the AN-37 and the PI software. Sometimes the drain and source of LNK has been short at power on and the inrush current caused by this, destroy the input resistor.
I use normal 2 watt resistor at the input instead of wire wound and I see at the forum that you recommend that use only the wire wound types. But in my case when the drain and source has been short, this problem happened. I try with higher resistors (56 and 150) and some MOV at the AC input but it doesn’t help.
Is the problem still with wire wound resistor? What should I do? I have gone to have a big problem with our products for this. Please reply me as soon as possible.
Cheers
N.Nahapetian
narbeh_rob
Dear Mr. Chekov, Hi
Thank you for your attention, I send the image of pcb and shematic. I hope it helps here I have a critical problem on this would you please reply me back as soon as possible.
Cheers
Thank you
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narbeh_rob | 10 posts Posted: Mar 10, 2010 | 10:01 AM In response to The design shematic and pcb image ....
Hello narbeh_rob
it looks like you posted the layout twice can you post the schematic again please.
Thanks
Cheers PI-Chekov
PI-Chekov | 174 posts Dear PI-Chekov, Hi
I am really glad for your attention....
I send the schematic, sorry for mistake...
Cheers, and waiting for your reply
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narbeh_rob | 10 posts Hello narbeh_rob
Based on the info you posted there is a problem somewhere in your design. To try to troubleshoot can you post your schematic, pcb layout image and inductor specification. Thanks.
Cheers PI-Chekov
PI-Chekov | 174 posts Hi I've reviewed the schematic and layout. Would you please check something for me.
1) schematic shows SW1 connected to R100 but in layout SW1 connects to another part of the circuit. I assume switch is really the unlabeled part next to D102, C104 and L101.
2) I think C100 and C101 are backwards. If I follw the trace labelled "AC P" it conencts to R100 and then D101 then the the negative of C100. Trace "GND" connects to +ve of C100, C101. If I apply a positive voltage between AC P and GND (one half AC input cycle) then a positive voltage appears on the -ve terminal of C100 (and the drain of the LNK-TN). If I apply a negative voltage between "AC P" and GND then D101 is reverse biased and blocks.
Bottom line I think you are applying a negative voltage to the drain and a large current flow through the device via D102 and then the fusible resistor opens.
You can check this by not fitting the LNK-TN part and applying say 5 V to the AC input terminals and then switching the polarity of the 5 V connection.
3) Make sure that D103 is an ultra fast diode and not a regulator rectrifier diode
Cheers PI-Chekov
PI-Chekov | 174 posts Dear Mr. Chekov Hi
First of all thank you for your consideration...
1) This board used for controlling the kitchen hood. So the others parts of SW1 is used to drive the motor of it. The only part which involved on LED power supply is the one shown in red rectangle at PCB (as you say near the D102, C104 and L101.
2) The print of PCB on Guide Layer is wrong for the C100 and C101. But all these components are assembled and soldered in correct position.
The problem which I said about it shows itself about 5% of the produced boards. When we tests the boards from about 100 controllers about 5 of them, the R100 blows up and damaged. When I check the LNK I saw that the drain and source is short. I don’t know maybe the damaging of the R100 cause this on LNK.
3) For the D103 I use 1N4007 as the application note say, do you think it should be the ultra fast one too?
We have some another product which at it power supply I use LNK306 too but because of higher current I use BYV26c instead of UF4007 but still for D103 I use 1N4007 and until now nothings go wrong.
Thank you and waiting for your reply
narbeh_rob | 10 posts Thanks - the additional information helps a lot but if you can send corrected PCB layout it will save time because now I have to make an educated guess as exactly how the components are configured.
I have two thoughts
1) The 700 V drain to source rating is being exceeded.
I would recommend connecting an oscilloscope probe across drain and source of the LNK-TN. Connect the scope probe ground to the drain (you must use an isolation transformer to supply the power to your supply) and the probe tip to source. Monitor the peak voltage voltage during AC cycling. Keep increasing the scope trigger level until you find the maximum voltage. Please post the worst case scope waveform.
2) A pulse of current flows out of the FB pin through the 10uF, 50 V cap during the reverse recovery time of D103 when AC is first applied. The 10uF 50 V cap is there for soft-start so I assume you need that but can you try either removing it temporarily or replacing D103 with a UF4007 and see if you still see the problem - a failure rate of 5% is extremely high so I suggest just test a single unit repeatedly until failure then make the changes and repeat on a second unit to see if there are no failurse then reconfigure the second unit back to the original configuration and confirm it then does fail.
Finally R103 and R104 - what is their purpose and where do they connect? I'm used to L1 and L2 referring to AC input phases is this is the case here?
Cheers PI-Chekov
PI-Chekov | 174 posts Dear Mr. PI-Chekov, Hi
1) I send the correct PCB Layout.
2) The R103 and R104 are current limiter for the led lamps.
3) We product 300 of this boards with 56 ohm/2W/ Wire Wound type resistor (instead of 150 ohm normal film resistors) if the result is ok I will inform you.
4) I will check the voltages as you said.
cheers ...
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narbeh_rob | 10 posts 
