My recent post on ‘Quora’ on flyback magnetics and gaps:
Air gaps in flyback ‘transformers’
Why do flyback transformers need a gap in their cores? First let’s think why we need a core in a flyback converter in the first place: mainly to constrain the magnetic field inside the core to stop it coupling with other components causing losses and reduced coupling to the intended secondary coil. (forward converters are different). Let’s look at an example to show how it can work without a core, ignoring leakage field. Take a 10V supply, 20W output power P (ignore efficiency), 100kHz operating frequency (10µs cycle time T, 5µs on-
From the ‘solenoid’ equation in air (µr =1), L= µr µ0 n^2 Area x 1000/length, this can be formed from an air coil 20mm long, 20mm diameter and 18 turns. Quite reasonable size for a 20W transformer with no core! The leakage field is the big problem though so a ferrite core is used to constrain it. Ferrite has much higher permeability µr (magnetic conductance) than air, maybe 2000x, so from the solenoid equation, inductance goes up in proportion to 12.5mH assuming the core area is the same as the coil area. We need to get back to 6.25µH so we leave the core in and reduce the turns. If we left the inductance at 12.5mH, the current would not have time to ramp to 8A in the on-
Given an existing inductor, introducing a gap and adjusting turns to get the inductance you want can reduce flux density and prevent saturation especially if there is a DC current component but this is not the usual starting point.
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