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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Hey ! i have alot of experience with HF HV but not much with air core stuff...so my question is what about driving an air coil transformer like a normal transformer, in an SMPS mode is such a thing possible? also is there an absolute power density limit like the saturation of my ferrites? or can you up the power density until you melt your copper and silicon? (regardless of physical dimensions mean)
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
As far as I know the limit to power density for an air-cored coil or transformer is as you suspect limited only by the current carrying capacity of the windings.
For an air-cored transformer leakage inductance/flux and/or insulation and/or inter-winding capacitance will be significant considerations, depending on construction. e.g. you can't have low leakage inductance and high voltage insulation.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
With an air core transformer you can basically increase the power until one of the following occurs:
1. The I²R copper losses become so high that you can't dissipate the heat. (Taking into account skin effect, proximity effect etc.)
2. The windings flash over or some other dielectric breaks down from excess voltage.
The problem with air-cored transformers for switch-mode power supplies is that the coupling factor is always substantially less than unity. That means there is lots of leakage inductance, so choose your switching topology carefully!
Converters like the flyback converter exhibit massive losses in the primary-side clamp if the transformer has much leakage inductance. However, other topologies like the SLR converter turn the situation round and use transformer leakage inductance as a desirable component in the operation of the converter.
Also bare in mind that in resonant converters the transformer may see a VA product that is many times higher than the actual real power being transferred. So be careful to make the distinction between power density and VA density. The transformer design is usually dominated by the combined VA product which may be substantially more than the watts of power you want it to convey. Resonant switching converters are great for minimising switching losses in the semiconductors, but the price you pay for this is often in the form of higher peak voltages or currents through the magnetics and switches!
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
I was told that SMPS operating in the range of 1-10MHz (!) use cores made of ceramic materials... This is becasue at such frequencies there are no (ferro)magnetic materials with reasonable losses... The ceramic material (usually toroidal to minimize flux leakage) is used only as form for the windings.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
yeah i think i see now, copper melting is my big concern. anyway i dont need resonant or soft swithcing, hopefully. th major advantage i see is that i dont need to have an equal and opposite B swing to avoid flux-walking toward saturation, so i can feed 50 pulses of 100kHz (+ to 0 )then 50 pulses from (0 to -) for a beat frequency of 2000 hertz at output yet the aircore will still see 100khz not the 2000 Hz. i think. i hope.
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Power density of resonant and non-resonant air core transformers
