Air Gap: why and how much?

[HighVoltageRulezz]

Hi-

I laready researched this topic but it did not help much..

Could somebody explain in Laymans Terms how to specify the airgap?

My setup is planned the following way:

CCPS Topology according to https://www.stevehv.4hv.org/ccps1.htm

Big Ferrite Core. My driver has can be controlled in frequency and duty cyle I plan for a frequency of perhaps 30-40 kHz. Output is planned to be around 15kV and currents of perhaps 200mA.

So how big must my air gap be? What if I make it bigger / smaller?

Thanks
Roman

[klugesmith]

If the big ferrite parts are for a transformer, not a flyback converter,
then I don't see any benefit from a deliberate air gap in the core.
Other readers may know better.  Perhaps to lower the inductance in a resonant design?

We can talk about air gaps in flyback cores, after you explain the difference between transformer operation and flyback converter operation.

[johnf]

As far as i know you use the air-gap to get the required inductance that I worked out for the drive frequency.
The air gap also helps to stop saturation effects taking out the drive silicon. With no air gap when you saturate, the core disappears and your transformer is equivalent to having the same inductance as if you have removed the core altogether.
Normally you look up the magnetic path length and core area and design around these and the ferrite properties. You then calculate the required turns and I usually go 10 to 25 % over so I can shim the core gap to get the required inductance. Suitable shim material = kapton tape which is approx 60 - 68 microns per layer.
For final production transformers in the E core type I have the central core of one half ground back to the required gap. Doing this allows the windings to shield the magnetic fringing around the gap that may affect other magnetic devices nearby or induce currents in circuit traces

[profdc9]

The air gap increases the reluctance of the magnetic circuit that contains the primary and secondary windings. 

https://en.wikipedia.org/wiki/Magnetic_reluctance

Some increase in reluctance can be beneficial as the amount of flux in the circuit will be reduced given a certain of ampere-turns of your winding.  The gap which is useful for switching power supplies like flyback transformers that transfer the energy stored in the gap.  It also increases the amount of current tolerated before the transformer saturates.

For other kinds of switching supplies that use direct coupling of the primary and secondary windings, an excessive large gap can create too much leakage inductance that reduces the current and can create large voltage spikes on the switches (MOSFETs or IGBTs). 

You can use a piece of tape to introduce a small gap in a U/I core transformer.  I have example formulas on calculating the gap size in this document:

https://drive.google.com/file/d/0BwFicJLV0O4jYlQ0ejJ6NE9XVE0/view