Solution Finder

TOP271VG Start-up Problem at Low Line Input Voltage

Posted by: aliaykan on Sat, 08/28/2021


I have designed 85-265VAC input, 24V/5A output flyback converter with TOP271VG. Design is working well and stable from 110VAC to 265VAC input at full load.

Converter is going to restart mode under 100VAC and restarts above 104VAC input. I have tried input capacitor up to 360uf and there is no difference.

How can I improve the input voltage regulation down to 85VAC. 


Best regards,

Design Results.pdf


Transformer construction.pdf


Attachment Size
Design Results.pdf 287.54 KB
Schematic_2.PNG 49.21 KB
Transformer construction.pdf 88.59 KB


Submitted by PI-Wrench on Tue, 08/31/2021

First off, the efficiency estimate of 99% is overly optimistic for a flyback design like this. An efficiency of 87-88% would be more attainable. This will impact the transformer design, possibly requiring a larger core size. The design will also likely require a higher KI value for the TOPSwitch current limit.  The design as currently implemented is probably entering autorestart at low line due to reaching primary current limit, or perhaps by saturating the current transformer design. You could check this by using PIXLS, entering the practical efficiency estimate mentioned, and adding the current transformer parameters and KI setting. This will flag any problem areas.