Flyback "transformer" is NOT COUPLED INDUCTORS (summary)
The objective of this article is to correct two fundamental mistakes most POWER ELECTRONICS ENGINEERS make with following claims:
1. Flyback transformer is no different than transformer since after all the same symbol is used as for 60Hz line transformer!
2. Flyback transformer is really COUPLED INDUCTORS!
I am addressing the second claim here by claiming:
Flyback transformer is NOT COUPLED INDUCTORS. Here are the reasons
First drawing shows flyback converter with its “transformer” with common ground between primary and secondary side. As shown in blue color, the ground connection can be separated to result in galvanic isolation.
Second drawing shows the identical circuit with the magnetic coupling removed. Clearly this is NOT working circuit (see enclosed the comment below highlighted in yellow color).
Definition of the COUPLED INDUCTORS is that once the coupling is removed that converter still must operate. Phrased it differently, only a few switching converters qualify to allow coupling of their inductors, like Coupled Inductor Cuk converter, Coupled Inductor SEPIC converter, multiple outputs of forward converter, and a few others. I am afraid I should know that as I invented 40 years ago the general method I called Coupled Inductor and Integrated Magnetics Method!
Coupled Inductor equations model an AC transformer uses the primary and secondary self-inductances and the mutual inductance which represents the added voltage induced on primary due to the change of the secondary current. So much for the claim: current do not force the coupling. There are no “coupled-inductors” coupled by the common magnetic flux as also claimed.
In fact, one might be tempted to claim that the common magnetic flux in flyback “transformer” is described by mutual inductance. This is clearly false, the flux due to primary winding exists only during ON time of the switch and flux due to secondary winding exists only during the OFF time of the main switch, hence there is no coupling between those fluxes. Hence one correct comment made was: "flyback transformer" does not exist, it is a simple inductor where all the energy to be transferred is stored in GAP.
What are lesson to be learned?
The naming of Two-windings inductor (or alternatively double winding inductor!) in flyback converter as a “transformer” is NOT just a matter of semantics or being picky. It goes to the core of the understanding of magnetic components and how the power transfer is accomplished by either 100% energy storage or 100% energy transfer. When such “transformer” is further named COUPLED INDUCTORS, it shows that the powerful COUPLED INDUCTORS METHOD I pioneered 40 years ago is completely misunderstood. Ditto for the Coupled Inductor model of real transformer.
Misleading comments or PLAIN WRONG explanations do not advance POWER ELECTRONICS field but actually hinders progress. I believe that the second is the case here! My much more ambitious goal for all Electrical Engineers, not just 430,000 members of the IEEE, is to educate them how the everyday 50Hz and 60Hz transformer utility line transformers operate based on 1831 transformer invention of Faraday. How are we going to embark on using transformers now at 600kHz and drive them with square-wave instead of sinusoidal voltages if the basic transformer operation is NOT even understood.
at KSB Electronics Limited
2 年I don't know why you say that most power electronics engineers are mistaken in their understanding of flyback converter transformers. I wouldn't have thought this was the case or they would not be able to design these devices. I certainly understand them and I am not a specialist in this area. Its fairly straightforward and not difficult to understand. You are incorrect in saying that a flyback transformer is in principle different to a line transformer. The difference come in the way that it is operated, but both have windings around a common magnetic path, and that common magnetic path is vital to the operation of both. In the flyback case, the primary sets up the flux, storing energy in the magnetic path, and current flows in secondary winding(s) when the primary current is interrupted, thereby extracting stored energy. The common path is still vital. I can even make a circuit where I extract power from the same secondary winding both while the primary is conducting (forward mode) and while it is not (flyback mode). In the line transformer (or forward mode SMPSU transformer) the flux is minimal and both windings contribute and counteract each other, which is different to flyback, but I'm sure engineers understand that.
Ph.D., Senior Member of IEEE
5 年First it's hard to accept the definition provided for coupled inductors, which is associated with a converter. The concept of coupled inductors is easy to understand, which has nothing to do with a particular converter.? Second, a Flyback converter is dependent on magnetic coupling between the primary and secondary. As the second drawing shows, without coupling it just doesn't work.
CEO at TESLAco
8 年MY DOT-MARK CORRECTION FOR THE DRAWING Since no one has noticed so far, I hereby submit the DOT MARK correction for flyback "transformer" drawing: my obvious slip-up during drawing. The secondary winding dotted mark should be at the connection of secondary winding with the diode so that the output DC voltage could provide the negative voltage needed to insure AUTOMATIC volt-second balance ON INDUCTOR for both non-isolated and isolated versions. From current viewpoint, the secondary current must continue in the same direction (into the dot) as the primary current PRECISELY since it has a DC component in it!
Design Engineer
8 年As stated before, inductors are designed for energy storage by controlling the inductance, usually with an air gap to increase the integrated B*H product. In a transformer, the core is designed to minimize H and to maximize B. This is why transformers have, in general, no air gaps. In flyback conveerters the inductor is charged during the first half of the cycle and discharged during the second half. The continuity equation is energy in = energy out. With a "coupled inductor" the continuity equation is the same. The only difference is that the energy out is transferred by a second winding. Grounding seems to make no difference, just that the loops must be closed. It is still energy in = energy out. Of course "coupled inductors" look like transformers, but the design criteria are completely different. The inductor controls the B*H product while the transformer generally wants to minimize H.
Senior Researcher at Université Paul Sabatier Toulouse III
8 年This is difficult to understand. Are you referring to "flyback transformer", as found in CRT tube, with the purpose of generating HV pulse; or "flyback converter"; with the purpose of adding galvanic insulation between the primary power source or the load ?