Changing output voltage of primary side PWM controller - ATX PSU - Page 1
Forum section: electronics / beginners

author: darko31
author_rank: Regular Contributor
timestamp: April 27, 2013, 06:56:24 pm
content: Hi guys,I've used couple of cheap Chinese PSU and changed the output voltages to serve as simple battery charger. Most of them have control IC on the secondary of the transformer like KA7500B.Recently I've opened up a really oldie of a ATX PSU, it's an Astec one, never heard of them, PCB manufacture date says 1996, it's still working, and it's probably supposed to be a quality one, it's full of Nichicon caps and it has proper filtering.Anyway, the PWM control circuit is AS2214F, Astec brandedhttp://www.alldatasheet.net/datasheet-pdf/pdf/86496/ETC/AS2214.htmland in description it says it's primary side PWM controller. The output 12V and other voltages are isolated from IC by a couple of opto-isolators.I've read a couple of datasheets of the bloody things, and I still don't understand how the output voltage is regulated. Where is the feedback loop?

author: IanB
author_rank: Super Contributor
timestamp: April 27, 2013, 08:10:40 pm
content: Quote from: darko31 on April 27, 2013, 06:56:24 pmThrough the opto-coupler(s)?The secondary voltages would be fed back to the primary side controller through the optical path and used to regulate the input to the transformer.

author: darko31
author_rank: Regular Contributor
timestamp: April 27, 2013, 09:08:41 pm
content: Well on the input or the LED of the optocouplers are connected output voltages. And the output or the phototransistor should be connected to the IC somehow.Oh, okay, optocouplers can be linear, it says so in the datasheet. I've always thought they are binary, either on or off. That makes sense, now of to find how exactly is opto connected to the IC.

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2013, 11:11:46 am
content: You will find a TL431 or equivalent on the secondary side that does the voltage comparison, and the optocoupler transfers this to the primary side as an error signal.

author: Alana
author_rank: Frequent Contributor
timestamp: April 28, 2013, 05:15:03 pm
content: Things may be tricky with HV side controller. For example look up application note for UC3842 familyhttp://www.onsemi.com/pub/Collateral/UC3842B-D.PDFController is powered by separate winding in main transformer and voltage from this winding is used as feedback. And changing this requires to work on mains side - tricky stuff for beginners. I'd recommend either leaving this one out [for safety reasons] or drawing/finding complete schematic and posting it here.

author: darko31
author_rank: Regular Contributor
timestamp: April 28, 2013, 05:46:09 pm
content: On the secondary side there is again Astec's IC, AS2350, how they call it, secondary side "housekeeping" circuit. It's in charge with the OVP, UVP, and other protections. Strangely there is another IC on the primary side, it's TOP210PFI, which could also be used for voltage control.http://www.datasheetcatalog.org/datasheets2/62/62233_1.pdfhttp://pdf1.alldatasheet.com/datasheet-pdf/view/34940/POWERINT/TOP210PFI.htmlIt's not easy working on it without at least function block diagram if not a proper schematic, I can't find any for this PSU, or anything like this, I'll try to trace it out.And yes, as always, I am using all the necessary precautions for working with mains.