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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I have a question for all you power supply guru's
This is the basic schematic of the inverter:
It is basically the "mazzili" inverter adapted for half-bridge operation, so the same choke design applies to it.
I'm having problems designing the dc choke (L1). The supply voltage is 325V (rectified mains), frequency of operation will be between ~20-100kHz (as I'll play with the tank circuit) and the maximum input power it'll ever see is planned to 3kW (10A average on the DC buss).
What are the criterias for choosing the dc choke core size, number of turns on each winding and core gap? What is the impact of its inductance on the operation of the inverter? Do I have to avoid its saturation in all cases? How does the current through the choke look, is it constant (in each halfwave) or not?
A lot of questios i know, but I'm lost in the choke design and would like to learn more about the inverter operation
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Choke core size: Biggest in your junk box Turns: As many turns of 10A rated wire as will fit. Say 40 bifilar. The two windings should be made as one bifilar winding for best coupling. Airgap: 1 thickness of cereal packet or business card.
This rough estimate should get you going, and you can feel the core temperature rise and look at the current waveforms to help you optimize it.
The inductance just needs to be large enough that the reactance of the choke is "infinite" at the operating frequency. In other words, it's so large that you can ignore it and assume the circuit is fed by a perfect source of constant current. And as this implies, yes, the choke current should be constant and the choke should not be allowed to saturate.
For true current-fed operation you also need to drive it with a constant current power supply.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks a lot Steve! I have one question though, you said fit as much turns as I can- but the more turns I fit, the lower will be the saturation current right?
Calculating saturation current with 0.5mm gap and 80 (40+40) turns @0.3T ( ) gives roughly 1.5 amps. With more turns it gets worse. Am I missing something? It would take something like 5mm gap to get the saturation current I need.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Oh, hmm, I see the problem. More turns avoids saturation in a transformer, but in a DC choke it makes it worse. And this component is both a transformer and a choke.
So you need enough turns to withstand a lot of volt-seconds (325V at 20kHz - or is it 160V?) while keeping the AC ripple low enough that the core won't overheat or saturate with the combined DC and AC peak flux, and yet not so many turns that you need a huge airgap to prevent saturation from the DC flux, as this would decrease the inductance, defeating the first objective. That's a tradeoff, so maybe 20+20 turns and 3 thicknesses of cornflake packet
Edit: I figured out a more scientific method: Choose the inductance to get, say, 20% ripple current at 20kHz, then iterate number of turns and airgap size until you can also get a saturation current of 10A plus an extra 20%.
(Note the current only flows in one or other winding at a time, so you should have used 40 turns in your calculation, afaik)
This reminds me of when I was experimenting with a Mazzilli oscillator with no centre tap on the resonant circuit. I decided to use two separate magnetic components for the two functions. A bifilar wound choke on an ungapped ferrite core to do the coupling (the DC flux cancelled so no gap was needed) and a stack of iron powder toroids to provide the DC link inductance.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Calculating with 1.5mm gap and 2x30 turns gives roughly 12A, much better. Then there's the problem if the inductance won't be small enough to interact with proper opeartion of the inverter.
wrote ... Edit: I figured out a more scientific method: Choose the inductance to get, say, 20% ripple current at 20kHz, then iterate number of turns and airgap size until you can also get a saturation current of 10A plus an extra 20%.
I'm afraid i'm void of formulas How do I choose inductance to get x % ripple at y Hz? What do you mean by iterating turns and gap size? I also need to know how to calculate inductance of choke based on core size (+core material), turns and airgap, I'm sure these are some basic formulas, but I have never used them nor we had them in school. Maybe there's a site with some magnetics design formulas so I don't need to ask for all of them?
Edit> I think there's another problem, with 10A choke current the average dc buss current will be only 5A (?), so I need to calculate the choke for 20A?!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Could powdered iron (material 26) cores maybe give better performance than ferrite at those frequencies? It beats ferrite by max B but is lossy. But, if Jan has it available in high quantities (dead PC power supplies?) a large stack of them might work well.
Also, it might give better mutual coupling between windings than ferrite due to distributed airgap.
More turns avoids saturation in a transformer, but in a DC choke it makes it worse. And this component is both a transformer and a choke.
Hm, conner, can you check if this sentence makes sense? (it doesn't to me);
The pictured component is a transformer (as it contains two windings), but other that it performs exactly like choke in a royer oscillator; fluxes of L1 and L2 add together and the core sees exactly the same flux waveform it would see in Mazzili flyback driver or whatever.
The DC flux is what dominates and thus it will always be proportional to number of turns, thus core will always saturate at lower current with more turns.
So you need enough turns to withstand a lot of volt-seconds (325V at 20kHz - or is it 160V?) while keeping the AC ripple low enough that the core won't overheat or saturate with the combined DC and AC peak flux, and yet not so many turns that you need a huge airgap to prevent saturation from the DC flux, as this would decrease the inductance, defeating the first objective. That's a tradeoff, so maybe 20+20 turns and 3 thicknesses of cornflake packet
Well, yeah, the basic trade off is really between saturation flux limiting the number of turns we can use and core losses occurring from AC flux in it. AC flux is especially bad with powdered iron toroids but they can stand more DC flux. I guess that's the basic trade off between ferrite and powdered iron.
Some of those low-loss powdered iron (red toroids?) might be superior to both but probably unneeded for such low frequency.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Dr. Kilovolt wrote ...
What happens if the inductor saturates? For my first "zvs" driver I used ungapped ferrite core and it worked OK.
Yes, many reported ungapped ferrite working well in royer oscillators - after thinking about it more, I don't really see why would it be so bad as it would first appear. Closed ferrite inductor will at first have very high inductance, but it will drop almost immediately as it saturates, and as permeability drops down enough it will just settle down to some lower inductance; permeability drop has negative feedback effect on flux itself, so inductance won't really suddenly rocket to 0 in any case.
It will be sort of like the inductor ''gapped itself'' to some point, and can store energy; AC flux will be larger in the inductor but ferrite can take it relatively well.
It's similar to saturable reactors, they have no air gaps either but can still act as series ballast inductors and store lots of energy once saturated with DC flux.
I guess permeability drop would be bigger problem for iron powder toroids, since they are lossy and would not tolerate AC flux well.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko wrote ...
It's similar to saturable reactors, they have no air gaps either but can still act as series ballast inductors and store lots of energy once saturated with DC flux.
I always thought they "chop" the ac current they are being fed with, like a triac.
Anyway, can someone else comment on the inductor saturating? Would it really be such big problem?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
I have not encountered the switching topology you have drawn before, so don't really have much to comment on here. Do you have any references for this design showing simulated or actual switching waveforms? Usually papers covering switching topologies will also cover the basic equations governing component choices.
Re the choice of mangetics: In general gapped Mn/Zn ferrite is good for DC chokes when the operating frequency is high and current ripple is 20% or more. Hold-up under DC bias is initially good. Permeability (and hence inductance) remain reasonably constant until the ferrite starts to saturate then they drop off quite abruptly! In contrast Iron powder materials are best suited for DC chokes operating at lower switching frequencies with typically 20% current ripple or less at full load. The permeability of iron-powder starts to drop almost as soon as any DC bias is applied (resulting in a "swinging choke" type effect,) but Hold-up under very heavy DC bias is actually better with iron-powder.
Iron powder is often used for buck chokes because it is cheap, compact, and the output current-ripple of a buck is usually designed to be very small to minimise voltage ripple across the output cap's ESR. In this application Iron powder is also a lot more tolerant of current overloads without going into hard saturation.
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CF parallel resonant inverter: choke design
How do I choose inductance to get x % ripple at y Hz? What do you mean by iterating turns and gap size? I also need to know how to calculate inductance of choke based on core size (+core material), turns and airgap, I'm sure these are some basic formulas, but I have never used them nor we had them in school. Maybe there's a site with some magnetics design formulas so I don't need to ask for all of them? 
