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Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
I am swapping out the ferrites in my boost converter to up the power a bit. My question is how leakage can be factored into the minimum turns formula to get the minimum number of turns on a 153mm^2 Ae core handling 170v for 30uS. I havnt found any sources that even go into air gaps let alone the calculations, so the biggest thing is to find some data on the subject.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
You can use simple formlas such as; Change in Flux = Integral (volt.seconds) = (170V/N)x30E-6 = 5.1/N mV.s = 5.1/N mWb Flux Density = Flux/Area = 5.1 mWb/(N.153E-6) = 33.333/N Tesla So, if you want your peak flux density to change by say 0.1 T (1,000 gauss) 0.1 = 33.333/N .... N = 333.33 turns To decide on the airgap you must choose what inductance you want which is determined by what change in current you want per cycle based on operating frequency, duty cycle and power output. Will this be a continuous-mode or discontinuous-mode boost converter?
lots of design info. at Fair-Rite, Magnetics inc., epcos (ferrite manufacturers websites) etc.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
To a first approximation, the air gap doesn't change the volt-second capacity of the core, so the minimum number of turns is the same as for the ungapped case.
Explanation: The air gap increases the amp-turns for saturation, but it also reduces the inductance in the same proportion.
An air gap increases the energy storage capacity of the core, as this is 0.5*L*I^2. If we double the current rating and halve the inductance, the energy goes up by a factor of 2.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
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Posts: 600
I still don't quite get it. Sulaiman, I get your calculations, but this applies to any ferrite gapped or ungapped doesn't it? And steve, I understand that, but I really don't care about energy storage as long as I don't saturate the core I have some ferrite u cores that i will put some turns on(~minimum + a few). Then I will increase the gap until it can withstand the 17A average, 23A peak I will be putting through it. My question is how I can calculate how far. And also, because the vt handling doesn't change, the minimum turns is still given by s-man's formula or the other ferrite b=vt/aeb one correct?
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837
Here is an article for a discontinuous flyback which runs through calculations for pretty much the same thing you need (except you don't use the flyback phase) :
In addition to Sulaiman's formula it has a formula for L given the air gap and turns, and the L you want is V*T/I ... so that gives you the air gap.
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Air gap calculations
I have some ferrite u cores that i will put some turns on(~minimum + a few). Then I will increase the gap until it can withstand the 17A average, 23A peak I will be putting through it. My question is how I can calculate how far. And also, because the vt handling doesn't change, the minimum turns is still given by s-man's formula or the other ferrite b=vt/aeb one correct?
