10kW 240Vac-line-powered ZVS induction heater?

[davekni]

Still in the planning/simulation stage, but time to seek input/suggestions.  This is an induction heating driver powered directly from rectified 240Vac line.  No bulk capacitance, so induction power will track line voltage.  Output will be ferrite-transformer coupled to work coil, for both isolation and to reduce voltage.

    Up to 10kW (for use on a 40A 240V circuit).
    ~100kHz frequency (perhaps 80kHz minimum, ~200kHz maximum)
    Resonant capacitors on primary side.  (Transformer handles resonant current.)
    IXYH24N170CV1 24A 1700V TO247 IGBTs, 4 total, 2 parallel pairs.
    SCT2H12NY 4A 1700V SiC FET for gate drive to IGBTs
    GD10MPS17H 10A 1700V SiC diodes across IGBTs (which have no internal diodes)
        (ZVS circuit ideally has no reverse IGBT voltage, but slow turn-off requires diodes for momentary conduction.)

My thought for the LARGE output transformer:
    4:1 ratio (with the idea to match voltage from 60V direct-drive ZVS systems).  Would love to learn more about inductance and frequency of typical work coils for <= 10kW power.
    Two sets of large U93/76/30 U-cores arranged to look like an even larger E-core set.
    4-turn primary made of 90mm x 0.2mm copper foil wound as four layers around E-core center.  Leads (copper pipe or bus-bar) exit one end of the E-core set.
    1-turn secondary, 90mm x 0.5mm copper, over (around) primary.
    Windings spaced radially to allow for axial forced-air cooling.
    Each work coil can be soldered to its own 1-turn primary if desired.  Core is separable, so new secondaries can be slid into place.
    90mm x 0.5mm secondary winding can be extended as parallel-plate leads in order to locate work coil farther from transformer without adding significant additional parasitic inductance.

For circuitry, here's my modifications to the simple ZVS oscillator in order to accommodate larger forward drop of HV devices while keeping Vge low voltage close to 0V:


Any feedback and suggestions would be appreciated.  Likely to be some time before this project gets to the top of my list.

Also thinking of a variation w/o ferrite transformer for driving my SSTC.

[Twospoons]

I would have thought fat litz > foil for the transformer.  Looking at the transformer in the Panasonic microwave oven inverter on my desk they've used multifilar (not true litz wire) wire on both primary and secondary.  The primary looks to be at least 50 strands of 0.2mm enameled wire. This is only a 1kW inverter.

If you have a gap in your core you might find some odd current distributions turning up in the foil.  Easy to run a sim with FEMM as a sanity check.

I'm interested to see what you come up with as I'm toying with the idea of a more modest 2kW induction heater, also offline.  But I really need to finish a lot of other projects first 

[Mads Barnkob]

Since you want to run this as a cheap induction hob, you could maybe find some more inspiration in how its made?

Induction hobs in Europe are built to 2300W, as 230VAC at 10A is the most common fused group in houses.

My analysis of how it achieves that power rating straight from mains without bulk capacitance is here about quasi-resonant inverter https://kaizerpowerelectronics.dk/general-electronics/hacking-ikea-2kw-induction-hob/

[davekni]

My analysis of how it achieves that power rating straight from mains without bulk capacitance is here about quasi-resonant inverter https://kaizerpowerelectronics.dk/general-electronics/hacking-ikea-2kw-induction-hob/

Yes, that is roughly the circuit I used for my 23kHz eddy-current levitation box.  It was tricky getting it to work with high-Q loads.  If I were to build that box again, would likely try a two-transistor ZVS oscillator.  Induction cooktops hare relatively low-Q loads, with sensors that turn the unit off when no pan is present (ie. when Q is high).

I would have thought fat litz > foil for the transformer.  Looking at the transformer in the Panasonic microwave oven inverter on my desk they've used multifilar (not true litz wire) wire on both primary and secondary.  The primary looks to be at least 50 strands of 0.2mm enameled wire. This is only a 1kW inverter.

Yes, litz (multifilar) would be my normal thought.  However, I think it is difficult to get equal voltage on each strand when the turn-count is extremely low (ie. 1 or 2 in this case).  I gather it can be more efficient in such cases to use copper sheet and force the field to (mostly) avoid penetrating the windings.  Saw a tear-down video somewhere of a large TIG welder.  It's transformer was constructed similarly to my thought here, wound with copper sheet.

If you have a gap in your core you might find some odd current distributions turning up in the foil.  Easy to run a sim with FEMM as a sanity check.

No gap planned.  I don't have experience with any finite-element software (except for viewing results others generated).  Is there a free or cheap application that is reasonably easy to learn?

BTW, unrelated, but I just had to repost this edit, as well as my initial reply this evening.

[Twospoons]

  I don't have experience with any finite-element software (except for viewing results others generated).  Is there a free or cheap application that is reasonably easy to learn?

https://www.femm.info/wiki/HomePage
Free 2D and 2.5D finite element simulator for magnetics, electrostatics, current  flow and heat flow.  Surprisingly useful, as many problems can be reduced to a 2d slice and still yield good results.
I think the hardest thing to get to grips with is setting up the boundaries around the problem - I usually use periodic boundaries to simulate the rest of the universe. There's a pretty good manual, included with the software, that describes all this.

[davekni]

https://www.femm.info/wiki/HomePage
Free 2D and 2.5D finite element simulator for magnetics, electrostatics, current  flow and heat flow.  Surprisingly useful, as many problems can be reduced to a 2d slice and still yield good results.
I think the hardest thing to get to grips with is setting up the boundaries around the problem - I usually use periodic boundaries to simulate the rest of the universe. There's a pretty good manual, included with the software, that describes all this.

Thank you!  This looks interesting and useful.  Do you happen to know if it can handle radial symmetry?  In other words, could I simulate a vertical plane slice through a Tesla coil from center out to "infinity", handling the equations correctly for the wedge-shaped slice?  Or is that what 2.5D means?

[Twospoons]

Yes thats what 2.5D means. Also know as "axisymmetric ".  Good for any "solenoidal" type of problem.

[davekni]

https://www.femm.info/wiki/HomePage
Free 2D and 2.5D finite element simulator for magnetics, electrostatics, current  flow and heat flow.  Surprisingly useful, as many problems can be reduced to a 2d slice and still yield good results.
I think the hardest thing to get to grips with is setting up the boundaries around the problem - I usually use periodic boundaries to simulate the rest of the universe. There's a pretty good manual, included with the software, that describes all this.

Finally got to learning FEMM.  Very glad I did.  Thank you again for the pointer!  GUI is not typical Windows style, so takes some learning.  Worth the trouble.  (LTSpice also has an atypical GUI and is well worth learning.  Unfortunately, not the same atypical GUI as FEMM.)  So far I've simulated both versions of this transformer and some QCW coil geometry ideas to optimize coupling.

I would have thought fat litz > foil for the transformer.

For a 1-turn secondary, I can get to roughly 1/5th power dissipation with 100,000 strands of 44AWG wire per FEMM.  However, a real 1-turn litz coil will not meet one of FEMMs assumptions, that each strand is uniformly distributed across the bundle.  With one fat turn, it is not practical to get each strand to be equally on the inside and outside of the turn.  I'm not sure how much uniformity is possible with a 1-turn coil.  In the extreme of no uniformity (strands layered from inside out), power dissipation will be higher than for copper foil, around 2x for 25% fill factor.  (That is for foil thickness of 2x skin-depth or more.)

[Twospoons]

I'm glad you found FEMM to be useful. Numerical simulation (done with the appropriate caveats in mind) is great for getting answers where analytical solutions are prohibitively difficult. Analysing magnetic systems is definitely one of those situations.  I've been fortunate enough to have access in the past to Ansys Maxwell, a full 3D electromagnetic FEA software package. Very capable, very slow on complex problems (days to run some of my simulations), and of course very expensive (NZ$30k per CPU core!).  I was using it to simulate PCB coils for wireless charging, so the geometry was very complex, hence the long simulation times.