LnkSwitch PL minimum input capacitance

Posted by: Osnirok on 09/19/2011

For proper operation, the link switch PL must detect line voltage zero crossing, for dimmable or non dimmable versions, then, this limits the maximum capacitance for the maximum value of 19V at zero crossing at the DC bus. But, what about the minimum input capacitance necessary for proper operation?

And now, an important information: In testing a design, I verified that with 100nF it burns the LNK458 immediatly at startup. With 200nF it works very well. I´m using Lnk458, Cout 2000uF, Rsense= 0,44ohms; transformer EE19,primary inductance 500uH, Np 117 turns, Ns 11 turns, input filter common mode inductor.

look image schematic...

Any help ????

Comments

Minimum allowable input capacitance for LNK458 for normal operation is around 47nF.

You might have encountered start-up failure when using 100nF due to the surge voltage that was created by the inrush current between the EMI choke and the input capacitance. Worst case scenario is at 90 degrees phase angle of AC line.

It is recommended to suppress the voltage across the bulk input by limiting the inrush current by adding a series resistor in the AC input.

If efficiency is the major concern for adding a series resistor, then use a clamping circuit across the bulk using a TVS or RCD clamp.

Please monitor the bulk voltage and drain voltage during the evaluation.

Best regards.

At the Ac input I have a 275V varistor ( RV1 in the schematic). There is no fuse now in testing... And I burned several Ics... :)

I think the cheaper solution is to place a fusible resistor at the ac input, before RV1, and use just one 47nF or 100nF bulk capacitor... Now this design is not dimmable.

The fusistor will work like a damper an act as a fuse in case o failure... I saw in this forum that fusistor is cheaper than fusible and act as a damper.
Based in your experience, What do you think ??????? or a TVS across de bulk to dont have problems at all...

Thanks, I will monitor the bulk voltage and drain voltage too.

You are right. Using 200nF of bulk capacitor, during start-up, at 115 VAC, 300V pk in bulk capacitor, some times I can see a surge voltage of 450V, ( 300V pk + 150V of spyke) I placed a series resistor of 33ohms in series with AC mains and the spyke is now only 25V. I dont tried 100nF + 33ohms because I see that I have other problems too. The drain voltage is too high with a big spyke, I will post a new thread with this subject.