Flyback transformer Vs forward converter transformer - Page 1
Forum section: electronics / beginners

author: nitishsharmabit
author_rank: Newbie
timestamp: March 11, 2015, 09:05:06 pm
content: Hello Folks,I am new to the area of power supply designing. Can anybody tell me what is the difference between the transformer of flyback Vs forward converter,both design and working. I am confused about it.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2015, 09:44:49 pm
content: Inductance.Think of the flyback as coupled inductors. It must store energy. The inductance must be finite.The forward converter uses a transformer proper -- in the sense that it transforms the winding voltages, while drawing as little magnetizing current as possible. An ideal transformer has infinite inductance, stores zero energy and draws no (magnetizing) current.An energy storage inductor is still needed, which is why the forward converter needs a filter inductor as well.Tim

author: oldway
author_rank: Super Contributor
timestamp: March 11, 2015, 10:02:58 pm
content: Flyback transformer has a gap and forward converter transformer don't have.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2015, 10:16:27 pm
content: As a generalization no, but it's almost always the case that a flyback has a significant gap (since the gap stores energy) and the forward has little or none (usually, thin plastic film when it does -- just to improve DC balance, as you say).Flybacks with very oversized cores (usually happens at low power when size isn't a priority, or at high voltages or currents where extra winding area is required) tend towards thinner gaps, naturally. HV FBTs in CRT displays frequently use a moderately stiff plastic film (~0.2mm?) to set gap -- thinner than most, but still quite necessary.Note that low permeability cores serve the same purpose, so it doesn't need to be an explicit air gap. I've taken apart one such transformer (which I believe was used for the standby power in an ATX supply) which, by all appearances, used ungapped ferrite, but darned if the inductivity wasn't a whole lot smaller than every other (ungapped) core!Tim

author: Rog520
author_rank: Regular Contributor
timestamp: March 11, 2015, 10:22:02 pm
content: Since the transformer of the forward converter doesn't store energy (as opposed to the flyback's, which does), you can typically get by with a much smaller core for a forward converter, all other things being equal. In the case of the forward transformer, the core size is mostly dictated by how many turns you need to stuff on it. There are other advantages to forward (and bridge) converters over flybacks, but cost and simplicity usually favor the flyback...at least up to a certain power level.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2015, 10:41:17 pm
content: Quote from: blueskull on March 11, 2015, 10:22:15 pmHowso? The consequence of a small gap is low saturation current; any energy storage beyond that point would be leakage inductance, poorly coupled to the secondary if at all.CCM is popular with cheap powdered iron inductors; of course, those cheat by having a distributed air gap.Tim

author: Marco
author_rank: Super Contributor
timestamp: March 11, 2015, 11:08:43 pm
content: Assuming it won't saturate assumes you have unreasonably large sized core or unreasonably low permeability for most designs.