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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
High Voltage Planar Ferrite transformers. ( Intial Experiments )
Purpose : To verify some ideas i have on SMPS transformers for aircraft and spacecraft power conversion ( specifically needed since fuel cells and solar cells can't be seriesed past some practical point ). I plan to develop high power, small, high effciency power supplies. Heating, reliability and mass are the primary concerns, though keeping the turns ratio down is also important.
Primary: The device below will have a copper foil primary of less than a single skin depth to avoid proximity effects at high frequency, insulated with Kapton film. There is also a thin copper foil grounded grading plane around the center post of the ferrite. Secondary: A secondary of very thin 0.007" enamel wire. The secondary is meant to have many volts per turn (due to ferrite core area), which are in an insulated groove, as JPsmith and I demonstrated years ago. The secondary turns will be epoxy potted. The wire i have is "double enamled" 0.0056" copper diameter, maikng it 35 AWG. Im aiming for about 40kV, dont know yet if i can from 312 turns.
Ferrite and Bobbin: The Bobbin will be 3D printed in the future from bright blue PLA as it has a 15kV / mil dielectric strength, so at 5kv per mil, my bobbin should insulate to 90-120kV. The whole transformer will be submerged in Canola oil for insulation and cooling. I wanted the surface area of the ferrite to increase faster than volume when scaled as a function of power, for thermal dissipation, so that drove me to the TSC companies planar cores. Bmax is 2000 Guass, as ferrites typically saturate at 3000 Guass. The core will swing 4000 Guass total.
FEMM analysis: to follow!
Principle to be proved: I may have to series the secondaries, and parallel the primaries to get the voltage on the secondary.These primaries can use multiple TO-247 mosfets instead of the mega CM600 packages, which can have switching issues at the gate, which is physically large with loop area.
Circuit:
(Incomplete) this is a half bridge alternately grounding the two halves of the primaries. Some of these componet values are bogus, just to keep the editor happy.
heat : Grooved Bobbins For Segmented Windings (JPsmith's work)
I've got some math to post, but it looks like I'll get 44 Volts per turn, Which might mean 13.7 kv. If i can get to 11kV ill be happy, then i can series 10kv sections to get up to 70kv or so. All the insulation and field-grading has been dimensioned into the design so far. FEMM analysis and a prototype tested to destruction will be used to verifiy this.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Yes but I can't help wonder how you are going to wind it? Or are you going to make a coil on a separate jig and transfer it to the central oval groove afterwards? Is the groove round the outside rim for the primary, perhaps I'm picturing it all wrong.
Is this for very high voltage (like in the realm of flybacks) when you quoted those insulation ratings and thin wire, or just normal SMPSU / inverter applications?
(Edit)
Duh! just read your post again seen you said 40kv must be tired tonight. Tell us more, a flying hv supply or is it part of a bigger plan.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Electra wrote ...
Yes but I can't help wonder how you are going to wind it? Or are you going to make a coil on a separate jig and transfer it to the central oval groove afterwards? Is the groove round the outside rim for the primary, perhaps I'm picturing it all wrong.
Is this for very high voltage (like in the realm of flybacks) when you quoted those insulation ratings and thin wire, or just normal SMPSU / inverter applications?
(Edit)
Duh! just read your post again seen you said 40kv must be tired tonight. Tell us more, a flying hv supply or is it part of a bigger plan.
ive been editing the post as youve been reeding it, dont feel bad!
it will be wound while wet with epoxy, keeping the bubbles out by visual inspection. This is the best i can do till i get a vacuum pump.
the goal is to continue my DeSeversky ionocraft research, MHD high temp thrusters, and laser discharge experiments.
I got way more than 312 turns into this baby! i underestimated the packing effciency, and overestimated the diameter when i estimated 312 turns. Im thinking 400 turns. I dyed the water thin epoxy bright red to verify that bubbles werent being left behind. I had no vacuum pump, but this wet winding was the best i could do at the moment.
Registered Member #3324
Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276
Oooh, this is a cool project. At a convention i went to there was a company that specialised in planar transformers, they had one about the size of a cigerette pack that could handle around 40kW IIRC
Can't wait to see how this turns out bud :) -Thomas
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
I was reading somewhere a few years ago that planar transformers were going to give us amazingly high power densities at both high voltage and low voltage. Nice to see them being developed!
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
yeah i really started trying to understand the ferrite and its ways, instead of ust going bigger and bigger. and Mag-inc want like 700US$ for their biggest cores, which couldnt do much better than these small planar, but for there better winding volumes. so now its just proving high volts per turn is posssible.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Which also improves efficiency, because you have a higher voltage per turn, you can have less turns, and therefore thicker wire, allowing a greater current and less copper/skin loss I'll have to look at using one in my switched mode inverter
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
For 10KW at high voltage, the winding core is going to need to be totally encased with HV lead-out wire, or its going to eat itself in a bright flash!. The intrenal stresses will have to be considered too. You may need sectional windings to limit the stresses.
You might want to use two sets to double the voltage, and actually use larger E cores so you can stack 4 or 8 bobbins on top of each other in one assembly, just a thought.
Epcos cores are pretty cheap anyway, you could probably get a whole flat of them for $20.
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High Voltage Planar Ferrite transformers. ( Intial Experiments )
