Bias Capacitor problem with LNK419

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Hello,

 I have two problem with non-isolated flyback LNK419 driver.

 

1) Some of my driver failure because of Bias Capacitor 10uF 25V( X7R Samsung Original). It happen even after 2 months.

When I change this capacitor it's start again.
It's take 2-3 Watt power with no output at failure time. I checked each & every components but all are working fine.

This failed capacitor shows correct capacitance with LCR meter.

 

2) Current increses upto 12% with temperature rise in driver.

 

Driver details are below.

Input : 220V 50hz
Output :  58Volt,  350mA, 20Watt , PF 0.98

Efficiency :  88%

 

I have attached circuit & PCB layout. 

 

AttachmentSize
PCB Diagram.png59.59 KB
24watt.png98.99 KB

Hi,

For item 1, the problem is the capacitor quality. This is the first time we hear a failure on BP capacitor.  The IC has the internal regulator of 6.2V so therefore no way the capacitor will be stressed.   The 2 to 3W input power is maybe due to the shorted capacitance on the BP pin. To confirm, can you measure the impedance of the capacitor before desoldering from the board if it happens again.  

 

For item 2. There are so many variations to factor in for 12% increase in output current. The Load, feedback and reference variations.   If you want a tighter regulation then do direct current sensing.

 

 As for the layout, can you move the output capacitor close to output diode.  This will improve your EMI.  This will have a better loaction for the feedback circuit also.

 

 

Regards,

Pi-Jono

 

Hello,

 thanks for your answer.

I have checked that my bias capacitor is  100% original samsung. But I had found that I provided ground via 4.7uF 400V surge protactor capacitor. Is this valid reason for failure of it ?

Anyway I have change the pcb layout as per your instruction. Also added 100nF cap to voltage pin.

I have attached new pcb layout. Please let me know if any changes required ?

Thanks. 

Hi,

Might be, you need to be careful in soldering SMD capacitor.  They are very sensitive to soldering that it stresses the film inside the capacitor. 

 

As for the layout, you did a significant improvement. If I were you I will avoid positioning the component in 45 degree angle. This will add process time in manufacturability unless it is avoidable.  Put extra clearance between output cap and the output diode, this will help us to extend the life of the output capacitor. 

 

 

 

Regards,

PI-Jono 

 

 

Thanks for your suggestion. You help me a lot.

 

Our output capactior hast big lifespan (10000 hours at 105C). Still I have put some clearance between diode & capacitor.
Also have change 45Degree components.

 

Thanks again. 

Great! 

 

You are welcome.