LinkSwitch-TN2 (LNK3206) startup

Posted by: ATokes on 01/21/2019

Hi,

Based on the AN-70 application note, I'm trying to implement a buck converter using the LNK3206 IC. I'm trying to implement the High-Side Buck – Optocoupler Feedback topology:

My schematics looks like:

Unfortunately, the TN2 keeps restarting for some reason:
- the output rises to about 25 V in 18.4 ms
- at this point the TN2 enters the auto restart mode
- after ~1.5 seconds the process re-starts

I tested the feedback circuit separately and that seems to work as expected:

It should cause the LNK3206 to switch at about 12.07V

What happens, I think, is that at startup the output voltage overshoots. This causes the feedback current to exceed 670 µA and this activates the over-voltage protection feature, causing the TN2 to restart.

Can you help me with some ideas on how to prevent this?

Thanks,
Attila

Comments

Hi ATokes,

Thank you for using PI product on your design. With regards to your inquiry, we would like to confirm the actual value you use for the BP/M capacitor C11. Based from your schematic the value is only 1nF. There are 2 values for BP/M capacitor for the IC to function correctly. 100nF is for standard current limit and 1uF for reduced current limit. Please try changing C11 to this value then check again. Thank you.

Best Regards,
PI-SevenofNine

Hi PI-SevenofNine,

The BP/M capacitor should be the one placed between the BP/M and S pins right?

In my case the BP/M capacitor is the C3 capacitor with a value of 0.1uF (100nF). I think tried with 1.0uF too, but the behaviour was the same.

The C11 is a filtering capacitor for the opto-coupler output.

Thanks,
Attila

Hi ATokes,

Yes, I'm talking about the BP/M to S capacitor. Can you attached your schematic again? I can no longer see your attachment. Thank you.

Best Regards,
PI-SevenofNine

Hi,

Here is the schematic:

The C3 is the capacitor placed between BP/M and S. It has a value of 0.1uF / 100nF.

Thanks,
Attila

Hi Attila,

Can you try adding a resistor in series with the transistor side of the optocoupler. You can also try putting an additional preload across the output.. Thank you.

Best Regards,
PI-SevenOfNine

Hi Attila,

Another thing what is the Zener test current of the Zener (Dz1) you use? Thank you.

Best Regards,
PI-SvenofNine