Offline 1W Flyback won't regulate in no load. - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: December 26, 2017, 09:30:25 am
content: Hello,The following is an offline 15V,1W Flyback.When on no load, its output rises up above 15V.This is in spite of the Current sense pin being raised above 1V by the PNP circuit shown.Do you know why this happens?LT1243 datasheethttp://cds.linear.com/docs/en/datasheet/1241fa.pdfLTspice sim and pdf schem attached.

author: oldway
author_rank: Super Contributor
timestamp: December 26, 2017, 09:48:16 am
content: An offline flyback need a minimum load , it can't work with no load.

author: chris_leyson
author_rank: Super Contributor
timestamp: December 26, 2017, 11:16:55 am
content: Flybacks need a minimum load. It's a fundanental property of the flyback topology, where is the energy going to go that was stored during the primary ON time ?. With no load some of it will supply the losses in the secondary, some will be lost in the primary side switch and snubber, some will power the controller through an aux winding. Some controllers will drop their switching frequency and then enter pulse skipping mode to fix the no load condition but more often than not you still need a minimum load.

author: ocset
author_rank: Super Contributor
timestamp: December 26, 2017, 11:40:32 am
content: Chris_LeysonThankyou , I understand what you mean, when operating properly, in proper control and switching at F(sw), it needs a min load…however, (and sorry i should have explained this earlier) we are just trying to wangle (by whatever napharious means) a zero to 1W, 200-265VAC Flyback that can have a very rough 15V-ish rail...but we want it to stay below 18V at all costs.Blueskull:Thankyou, your fix has worked well,but according to the LT1243 datasheet, the circuit I already had should have given skip-cycle operation anyway?...after all, I had taken the CS pin above 1V. (As you know , the LT1243 is very identical to the UCC28C4X series from ti.com). How did you know that the CS pin had to be taken above 2.2V to give skip cycle?...the LT1243 datasheet did not state this?

author: chris_leyson
author_rank: Super Contributor
timestamp: December 26, 2017, 12:14:05 pm
content: It's not always easy keeping the no load volts in check. A lot of off line flyback controllers will drop their switching frequency and then go into pulse skipping mode but that will give you a lot of no load ripple. A single transistor 17V shunt regulator would be a cheap fix.

author: ocset
author_rank: Super Contributor
timestamp: December 26, 2017, 12:18:06 pm
content: QuoteThanks, i am a dunce with a capital D!, ..i had been reading the datasheet of the almost identical UCC28C43 and mentally superimposing it...woopsYes the rail is very rough.....and the bias rail also looks the same.....do you think the attached rough rail voltage will be alright to feed into the Vdd pin of the PWM controller directly?

author: ocset
author_rank: Super Contributor
timestamp: December 26, 2017, 12:27:22 pm
content: If only there was an offline current mode PWM controller, like the UCC28C43, but with in-built skip cycle, and with a start/stop Vdd hysteresis of about 5V or so?The NCP12XX range are good, (and have nice inbuilt skip cycle) but they have a HV startup pin with a 500V abs. max. rating, and with our mains transients we have to deal with , that would be too delicate for us.The TNY284 etc are good, (& have inbuilt skip cycle) but the inbuilt FETs only have a 725V abs max rating…again too low for us.UCC28C43 datasheethttp://www.ti.com/lit/ds/symlink/ucc28c43.pdfAlso, the lack of output overload, or output short circuit protection of the UCC28C43 makes it not so useful.–It’s nice to have this inbuilt, eg a fault timer which shuts the chip down if the chip hasn’t been in regulation for more than say 800ms or so.Do you know of any suitable pwm controllers as such?