LNK362 Auto-Restart

Posted by: ingow on 01/27/2010

Hi,
my flyback-converter with optocoupler-feedback enters auto-restart mode when switched on at more than 250VAC input voltage supply and more than 100mA load. When load is less than 100mA switching on at 250VAC is no problem. It works fine at input voltages 80-240VAC and 0-250mA load.
Nominal Data: input 85-253VAC and output 5V/250mA
Do you have an idea why it does auto-restart at 250VAC ?

Comments

In order to start to dig into the problem I need you please post your schematic and transformer deign. You can post documents using PDF format. Thank you. PI-Worf

Hello PI-Worf,
thank you for your reply !
Here are the schmematics and the drawing of the transformer.
In the meantime I did a test with a transformer with more windings (11 instead of 8) on the secondary side. This helps a bit in increasing the output current from 100mA to 150mA.
Is this the right way to solve the problem ?
Regards,
ingow

So far I do not see anything wrong in your schematic. I would like to run some calculations to check your transformer design but I need the value for the primary inductance. I do not see this value in the transformer spec you posted. Do you have this value?
Thank you
PI-Worf

Hello PI-Worf,
the primary inductance calculated with PIXI-Designer is 1733uH. Measured with LC-Meter at 1600Hz: 2860uH
Regards,
ingow

Did your meter measure 2860 uH ? if this reading is right, it means you are very far from the calculated value (1733 uh)

One thing may be happening is that your transformer does not have the right inductance value for the primary winding. Can you capture the waveform of the DRAIN current just before the supply goes into auto-restart ?
Thank you
PI-Worf

Measuring the inductance with another LC-Meter shows the same reading. The gap of the transformer is a little smaller (100um) than the one calculated by PI-Expert (160um). This may cause the higher inductance.
I'm sorry, but I can not give you a screenshot of the drain current, because I don't have a current probe (yet).
Does the higher inductance lead to a higher drain current that causes the auto restart mechanism ?
Regards,
ingow

Adjust the primary inductance to 1733 uH as this is the value PI expert is calling for. Ignore the size of the gap, follow your LP value. PI-Worf

Hello PI-Worf,
reducing the number of turns for the primary winding prevents the regulator from entering auto-restart mode. Now the circuit works fine. Thanks for your help !
But I do not understand why a lower primary inductance (leading to a faster rising of the current) is o.k. whereas a higher inductance causes auto-restart. Can you explain, what happens inside the regulator ?
Regards,
ingow

I think you misunderstand me, Ignore the size of the gap you had before. Juts adjust the inductance (using the original number of turns ) by making the gap bigger or smaller. You have to use the turns your calculations is asking for.