Flyback Transformer Winding Direction

Looking at the basic Flyback Converter Topology, the coupled inductor polarity should be opposite in order for the circuit to operate properly. However, I am confused on how to implement the correct polarity in a physical sense when winding the primary and secondary windings on the bobbin itself. I've tried looking at some other answers to this question online, but the answer given seems to change from source to source.

Therefore, I drew an image of what I believe is happening. The black rectangle represents the bobbin on which the windings are to be wound around. Lets say the red loop represents the primary winding and the blue loop represents the secondary winding. In the real world the two windings would be ontop one another, but for the sake of drawing a clear image, the windings have been separated. If we also assume that we are using some type of EE or E style core, the core will sit in the middle of the bobbin.

Ok, so if we are looking on the top of the bobbin, the red (primary) wire is wound in a counter-clockwise direction. If a current were to flow into the primary as shown by the red arrows in the picture, this would create a flux in the upward direction from the right hand rule. Then if the secondary winding (in Blue) was wound in the same direction relative to the primary then, from my understanding of Lenz's Law, then the induced current in the secondary would flow in such a way as to create a flux which opposes that of the primary as shown by the blue arrows. If we accept the dot convention, where the dot on the primary represents the current entering the primary winding and the dot on the secondary represents the current leaving the secondary winding, then the dots should be placed as shown in the drawing. Thus, coils that are wound in the same direction relative to one another would be in phase and the polarity dots are placed at the start of each winding. Therefore, for a Flyback circuit, like the one shown above, I would connect pin 1 of the primary to the input voltage source, pin 2 of the primary winding to the top of the switch, pin 3 of the secondary to ground, and pin 4 of the secondary to the anode of the diode, correct?

Similarly, If we consider the case in which the secondary is wound in the opposite direction than the primary, then the polarity dots would be opposite like the ones drawn in the picture below. In this case pins 1 and 2 of the primary would still be connected to the voltage source and switch respectively. However, pin 3 of the secondary would now connect to the anode of the diode and pin 4 would now connect to ground.

In summary, windings that are wound in the same direction relative to another winding are in-phase and the polarity dots are placed on the starting ends of the wire, and windings that are wound opposite to one another are out-of-phase, and the polarity dots are place on opposite ends of the wire (starting end on the primary, and the finishing end of the secondary). Since, I've read so many conflicting answers online, I've gone and confused myself it seems, so my question really is, am I correct in the way that I have explained how the polarity is marked given the winding direction of the primary and secondary relative to each other, especially for a Flyback, or am I missing something?

• Keep it simple - the windings are wound in the same direction and the dot represents the start of the winding. The schematic will then match the transformer. A common technique when winding is to use a bit of masking tape with the info written on it and stuck to the winding in question. Nov 19, 2023 at 8:04
• Voltage is induced on the secondary side, not current. Nov 19, 2023 at 18:33

3 Answers

I would connect pin 1 of the primary to the input voltage source, pin 2 of the primary winding to the top of the switch, pin 3 of the secondary to ground, and pin 4 of the secondary to the anode of the diode, correct?

Correct.

am I correct in the way that I have explained how the polarity is marked given the winding direction of the primary and secondary relative to each other

You are correct.

or am I missing something?

I think you shouldn't really talk about an induced current in relation to Lenz's law: -

then, from my understanding of Lenz's Law, then the induced current in the secondary would flow in such a way as to create a flux which opposes that of the primary as shown by the blue arrows.

Consider an ideal 1:1 transformer with perfect coupling between primary and secondary. We know that if the primary is driven with a voltage we get an induced voltage at the secondary terminals and, we know that if the two windings are identically wound, that the voltages are in phase like this (from my basic website): -

It then follows that if there is a load on the secondary, we get a secondary current like this: -

However, that secondary current does not oppose the original field that induced the secondary voltage because, if it did, then we would have a significant change to that voltage and, this just isn't true for any ideal or practical transformer.

What actually happens is that an additional primary current is taken. That current generates a magnetic field that entirely cancels the magnetic field from the secondary current. What remains (as always) is the original magnetic flux in the core (aka magnetization flux) as produced by the primary voltage: -

Also, current is not induced and Lenz never said that current was induced. It's all about conservation of energy.

In summary, windings that are wound in the same direction relative to another winding are in-phase and the polarity dots are placed on the starting ends of the wire

That's correct.

One thing to note here: Dots are generally used as the start point for simplicity but a dot normally doesn't have to indicate the start. It's just a reference point because it's meaningless on its own when there's no winding direction importance (e.g. for a half- or full-bridge converter with a centre-tapped secondary, the primary winding direction doesn't matter at all but half secondaries with respect to each other do). Think of it as something like ground. We use the ground as a reference point i.e. a "with respect to" point for voltages, for example.

Some young engineers confuse the winding direction with the winding "swipe" direction across the window (Apparently you are not one of them, that's good). For example, if you are winding the primary winding and you start from the left-side wall of the window, after reaching the right-side wall you continue winding from right to left. The second image is a good example of this situation. If you take (4) as the start (continuation of the primary, actually) and wind from the right to left (or bottom to top) you'll be winding in the same direction with the red winding.

If we consider the case in which the secondary is wound in the opposite direction than the primary, then the polarity dots would be opposite like the ones drawn in the picture below.

That's correct, too. But unless you are winding a transformer by hand just don't worry about this. Winding machines used for serial production almost always turn in the same direction during the entire winding process. In this case, the start (or end) point matters.

As you suggest, for windings on the same core section, wound in the same direction, the first turn of each winding has the same "polarity". In this context, "direction" is best expressed in terms of how each turn of the wire is passed through the hole in the core as being wound (i.e. down or up).

One easy way to think about this is consider windings on a single wound auto-transformer. Each section is clearly in phase as connected, with the "no dot" end of one part windings, joined to the "dot" start of the next. This remains true even if the windings are separated.

• Words like "direction" and "polarity" are too ambiguous IMO. The important part is only whether the windings curl CW or CCW around the core crosssection. But I have no better word to offer to describe this. "Winding sense"? Nov 19, 2023 at 9:56
• Agree I could do with better word, just bot sure what they should be! Obviously edit uf you can improve!. That was why I used examples autotransformer, as that felt clearer. Nov 19, 2023 at 15:09