Choke formula - Marty Brown

[Elvee]

Elvee: you are right,. But remember that energy storage is gobernated via the main switcher by the PWM controller, and a feedback loop, so energy entered into the "mutually coupled inductor" (NOT TRANSFORMER)

What is that distinctive difference between mutually coupled inductors and transformers?

equals energy removed from secondary, so it will not explode by accumulated energy not delivered to secondaries (And a tertiary if there is a coil winding for auto maintain the primary side as in PC monitors or notebook SMPS´s).

A coil is unable to keep the energy accumulated unless it is shorted (and perfect).

Again, normally the most of the SMPS´s I see, repaired and designed, enters a sort of low frequency start and stop when no load (Hiccup mode or pulse skiping or so) and maintaining mean output value although with higher low-frequency ripple. I never saw a uncontrolled SMPS generating "megavolts" at it output under no load conditions.

Obviously no. As soon as the regulation loop is open, the snubber becomes unable to absorb the excess energy, and the switching element fails instantly due to the overvoltage.
Typical failure mode of these supplies, never a risk of megavolts.

 

[Osvaldo de Banfield]

What is that distinctive difference between mutually coupled inductors and transformers?

There are two main differences:

1) Transformer ideally doesn´t store energy, practically a low amount is stored in leakage inductance. Flyback, and boost converter DO store energy and returns to the circuits some time later.

2) Voltage ratio in transformers is governed be turns ratio. Mutually coupled don´t. In fact, a boost can step up 24 to 100V in a single winding coil. And a flyback inductor, a 1:1 winding can give 1:1 voltage ratio as well as 10:1 or 1:10. The difference is the volts*second stored in the AIR GAP, not in the core itself.

Best regards.

 

[Elvee]

1) Transformer ideally doesn´t store energy, practically a low amount is stored in leakage inductance. Flyback, and boost converter DO store energy and returns to the circuits some time later.

There are many types of transformers: 50/60Hz transformers are designed to store a minimal amount of energy, but many electronic transformers for resonant supplies, Royer converters, etc need to store energy.
Flyback transformers also do. And they are actual transformers: the turns ratio does matter, and the coupling is very strong, stronger in fact than in ordinary transformers, because their leakage inductance generates losses and overvoltages.
BTW, ordinary 50/60Hz transformers do not store significant amounts of energy in their leakage inductance: under no load condition, some energy is stored in the magnetizing inductance, and leakage inductance only comes into play at higher currents.

2) Voltage ratio in transformers is governed be turns ratio. Mutually coupled don´t. In fact, a boost can step up 24 to 100V in a single winding coil. And a flyback inductor, a 1:1 winding can give 1:1 voltage ratio as well as 10:1 or 1:10. The difference is the volts*second stored in the AIR GAP, not in the core itself.

Mutually coupled windings having a k approaching 1 are called transformers; in most european languages anyway.
For a flyback transformer, both the turns ratio and the boost ratio matter.

The boost ratio can (in theory) be chosen freely, by adjusting the regulated or clamped voltage. In practice though, boost ratios in excess of 10 put an excessive stress on the switch element and windings, which is why it is complemented by the transformer's ratio for high voltages.

Anyway, I am not particularly for or against any topology: they all have advantages and disadvantages, and any application has probably one, or two at most ideally suited ones.

One thing has to be remembered: there are no free lunches in engineering, and if you need a large choke to make a converter function properly under certain conditions at a certain frequency, you won't be able to escape it: whether it is a smoothing coil in a forward converter, or part of a gapped flyback transformer, you will need it.
In the case of the flyback, it will need to be twice the size, because of the asymetric/unidirectional operation, but you will get away with only one switch element.

The control aspect is simply another element to take into account: in the case of the flyback, the ultimate voltage will be limited by the breakdown voltage of the weakest element.
That is something that may need to be taken into account for sensitive circuits, but basically, you don't plan for something going wrong, so that's secondary. But it may require a crowbar if some after-disaster survivability is necessary.

 

[Osvaldo de Banfield]

Elvee: perhaps you are right, but doesn´t agree in all items with I had read and learned from the books and papers I had read. I know some about SMPS´s, but not sufficient to establish any concept as 100%, so I´ll not discuss anything because I may be wrong.

Regards.