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Transformer gap values

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bowman1710

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Hi guys,

Quick question, how do you determine whether the gap in a flyback transformer is too large or too little? Are they any rules of thumb so to say?
 

schmitt trigger

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Gap width and its cross sectional should not be guessed. One has to really determine the proper gap based on performance requirements and operating conditions.

There was a printed EPCOS app note, which may be available online, with the required formulas.
 

brushhead

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As a guide the increase in airgap will reduce inductance but then raises the saturation current because if the increased reluctance of the magnetic circuit.

Phobos1's document link looks quite good BTW. There are also some TI documents that were originally Unitrode's which were the docs everyone used to refer to in days of old (yes i'm an old fart)
 

Warpspeed

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Hi guys,

Quick question, how do you determine whether the gap in a flyback transformer is too large or too little? Are they any rules of thumb so to say?
The gap is determined by the required power level and mode of operation.

The supply can be designed for complete energy transfer every cycle, in which case you adjust the gap to operate that way at maximum power.
If the gap is made wider than it needs to be, then peak currents will be much higher and the "on" times much shorter.

This increases losses in both the switching device and the transformer. So you set the gap to just reach the required peak current at maximum available duty cycle to reach the required output power level you need.

You may decide to run the whole thing in incomplete energy transfer mode, which will be slightly more efficient still.
Its really all a bit too complicated to go into here, but incomplete energy transfer mode complicates the feedback design, and you can easily run into instability problems that can be difficult to fix without a deeper understanding of the whole system.

Basically just set the gap so the output power just reaches your goal at full available duty cycle, with the minimum expected input voltage.
That is the simplest answer.
 

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