Multiphase flyback transformer design - more power by using lower power parts - Page 1
Forum section: electronics / projects

author: daqq
author_rank: Super Contributor
timestamp: August 05, 2015, 11:53:56 am
content: Hi guys,For a design I need a 48VDC -> 350-400 VDC @ 1 Amp avg. Is it possible to use several 120W transformers in the configuration in the attachement, add a diode for each transformer, filter it by a great lot of caps and get good results? The load is pretty much constant.The feedback would be handled by an ADC with galvanic isolation that would feed the controller on the other side with data about the current output. The controller would be running a simple regulator (I've had success with a PI like regulator for a similar situation) that would turn on the transformers, so they would not overlap (more even load for the main power supply) and regulate the pulse width and wheter it is even used.The reason for this is that you can get off the shelf 100W ferrite core transformers readily for peanuts, the control electronics should be relatively simple for such a low power per unit and the extra heat would be divided well. Also, for a lower power version, you'd just not assemble 4, but, say 2.Thanks,David

author: langwadt
author_rank: Super Contributor
timestamp: August 05, 2015, 12:08:39 pm
content: multiphase is common for high current buck and it should work just as well for boost

author: Circlotron
author_rank: Super Contributor
timestamp: August 05, 2015, 12:38:32 pm
content: ^^ Not only that, flyback converters are fundamentally a current source output so they will parallel and current share very well.

author: nctnico
author_rank: Super Contributor
timestamp: August 05, 2015, 01:39:08 pm
content: I'd go for a forward converter design. At these power levels a flyback starts to dissipate a lot of energy because of the energy stored in the leakage inductance (which is send back into the primare side).

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2015, 04:52:55 pm
content: A forward converter has different properties that the flyback:The flyback is somehing like a constant current source and can provide a rather wide voltage range, depending on the load and PWM setting.The forward converter gives a rather constant output voltage set by the winding ratio and input voltage. So it's more like a transformer but can not regulate by itself. There are additions to give it some regulation via an external inductor. So It's a good choice at a rather constant ratio.The transformers for flyback and forward converter are also different.

author: poorchava
author_rank: Super Contributor
timestamp: August 05, 2015, 05:39:47 pm
content: Flyback is also a very noisy topology. A 70-100W flyback is already hard to push through EMC compliance tests, and much less 4 of those in parallel (unless of course this is a one-off or a private project)Sent from my HTC One M8s using Tapatalk

author: daqq
author_rank: Super Contributor
timestamp: August 05, 2015, 06:32:50 pm
content: Thanks guys for the info!QuoteIn this specific application I can shield the hell out of it and filter it with everything. Also, this is going to feed caps for a pretty nasty pulse generator, so noise from the power supply should not be the major concern. But thanks! I'll keep an eye out for that.QuoteI've never heard of those, thanks! They look interesting, I'll study up on them. The flyback design caught my eye because it seems simple and you can get whole transformers for it from most distributors for a very low price.QuoteThanks, I've simulated the whole system in theory, but simulation will only get me so far

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2015, 07:07:42 pm
content: Speaking of leakage, that's the beauty of multiphase: they act in parallel, so each stage is literally only 100W or whatever, and if you can build one that works at that level, you can scale it up indefinitely (as long as you have a multiphase controller that can be configured for enough outputs).Notice the source resistors (or some means to sense inductor current) aren't shown; those are required for a multiphase circuit (absolutely necessary in CCM, recommended otherwise), and in any case, a current mode controller is the safest type, especially for flyback.For one single output, forward converters have some advantage, but that's somewhat negated by the multiphase approach -- the main drawbacks to a flyback topology (stray inductance, high ripple current on both sides) go away, and you keep one of the big advantages of flyback, it's better for high voltages (if this is needed).Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2015, 07:35:31 pm
content: For capacitor charging one needs a changing voltage ratio, which much better fits the flyback. Just the control must also handle low output voltages (e.g startup) - current control helps here a lot.If you find a suitable controller for multiphase flyback, I would very much prefer that over a pure software solution.Leakage likely refers to magnetic leakage in the transformer. So the transformer should be made for a flyback. Often you have an extra winding recovering some of the energy from magnetic leakage.

author: langwadt
author_rank: Super Contributor
timestamp: August 05, 2015, 11:31:47 pm
content: first hit on google:http://cds.linear.com/docs/en/design-note/dn453f.pdf