Unexpected behaviour in multiple output flyback - Page 1
Forum section: electronics / projects

author: eecook
author_rank: Regular Contributor
timestamp: April 13, 2019, 06:31:15 pm
content: Hi All,I have a problem with a flyback converter I've designed. The specs are the following:Input Voltage: 100-250VAC 50/60HzOutput: 140V (40.5W), 12V (12W)Target efficiency > 85%Operation mode: quasi-resonantController: LM5023The voltage is regulated from the 12V output. The problem is the variability of the "unregulated" output, and how it varies:1. With no load on any rail, it will regulate at ~130V2. With the 12V output fully loaded the 140V rail climbs up to 160V (I would have imagined it to drop)3. With the 12V output with no load and the 140V rail fully loaded, it will regulate around ~125V4. With the power supply fully loaded on every rail, the 140V output goes down to ~125VThe 12V supply is tightly regulated and hardly varies on any load condition.Both rails have bleeding resistors (1M for 12V and 2M for 140V).Please forgive the lack of details, I can provide any more information if necessary.Basically what troubles me is item #2. It doesn't seem intuitive to me and I haven't been able to find any appnote or tutorial covering this.I have also tried to do a "hybrid" feedback from the 2 outputs to the "comparator" (the voltage reference). I still need to try just closing the loop on the 140V rail, but the 12V one would have to be stable with a 10% margin.Any comments will be appreciated.Cheers.

author: radix
author_rank: Contributor
timestamp: April 13, 2019, 07:53:29 pm
content: Item #2 is caused by winding losses in the 12 V winding. The duty cycle has to go up to compensate the winding losses and all the other losses (core etc.), while the unloaded 140 V output doesn't see the 12 V winding's losses. So the output voltage rises.I think that items #3 and #4 mean that the 140 V winding has very little winding losses (thick wire or a relatively low current), but that's just a guess.

author: Zero999
author_rank: Super Contributor
timestamp: April 13, 2019, 09:58:44 pm
content: The previous poster is right: loading the regulated winding, causes the duty cycle to rise, which pushes the voltages up on the other windings.I'm not sure about the drop to 125V, when all rails are fully loaded. Perhaps it could be because the losses in the primary will be higher with all windings loaded and more I2R losses?If this is a problem, then how about regulating the 140V? If the higher voltage on the 12V rail is an issue, then up the design voltage to 15V and add a buck converter to the output.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2019, 04:05:36 am
content: That's quite good for an unregulated output. Is your 140V load not tolerant of that much change?I've seen more like 40% regulation, on naively wound transformers. So that's not too bad.The solution is to:- Sausage-effect some of the error into the 12V rail (assuming they are common ground). Use a divider resistor from each supply down to the TL431 error amp (if this supply is isolated in the usual way), so that both are jointly regulated with some weighing factor determined by the ratio of resistors.- Reduce the leakage between secondaries. Design the transformer with transmission line transformer methodology. Get the impedance down by coupling windings more closely together and using wider, flatter wire (or multifilar in parallel, or foil). If the resulting increase in isolation capacitance is troublesome, consider using a larger core (more Ae) with less gap to use less winding length.- Split it into two independent converters. Annoying, but easier to design the transformer, and no worries about cross regulation with isolated outputs. (Note that, if the outputs are isolated, it's not enough to put an error amp on each channel -- that handles the excess voltage on each channel, but does nothing about the shortfall of voltage.)Tim