High voltage > Transformer (Ferrite Core)
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:
--- Quote ---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/
--- End quote ---
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).
--- Quote ---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.
--- End quote ---
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.
--- Quote ---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.
--- End quote ---
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:
--- Quote from: davekni on July 07, 2021, 05:47:12 AM ---
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?
--- End quote ---
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.
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