High Power Induction Heat Generator

Hi all,

I'm on designing a generator for Induction Heating. Please see my spice sim.

Power in controlled by {phase_shift} parameter.

Any wise advice of guys with experience on this topic is welcomed ; otherwise please be gentle and learn.

Habib.

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-1104 -32 0 FLAG -976 128 0 FLAG -1072 288 0 FLAG 704 816 Vbus FLAG -336 -432 Vbus FLAG -336 288 Vbus FLAG -976 560 0 FLAG -1104 688 0 FLAG -976 864 0 FLAG -1104 1024 0 FLAG -1296 64 0 FLAG -1328 800 0 FLAG -288 784 shunt_B FLAG -336 944 0 FLAG -304 32 shunt_A FLAG -336 192 0 FLAG 656 1008 0 FLAG 512 1008 0 FLAG 576 1008 0 FLAG -1472 176 0 FLAG -1456 992 0 FLAG -704 944 0 FLAG 96 992 0 SYMBOL Digital\\and -960 -336 R0 SYMATTR InstName A1 SYMATTR SpiceLine Vhigh=5 SYMBOL res -1120 -256 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 3k SYMBOL cap -1120 -128 R0 SYMATTR InstName C1 SYMATTR Value 200pF SYMBOL schottky -1136 -176 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D1 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL Digital\\and -960 -16 R0 SYMATTR InstName A2 SYMATTR SpiceLine Vhigh=5 SYMBOL res -1088 64 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 3k SYMBOL cap -1088 192 R0 SYMATTR InstName C2 SYMATTR Value 200pF SYMBOL schottky -1120 144 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D2 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL voltage -16 800 R0 WINDOW 0 -42 6 Left 2 WINDOW 3 -44 93 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 -142 118 Left 2 SYMATTR InstName V6 SYMATTR Value SINE(0 320 50) SYMATTR SpiceLine Rser=0.01 SYMBOL nmos -384 -416 R0 SYMATTR InstName M1 SYMATTR Value STW11NM80 SYMBOL e -704 0 R0 SYMATTR InstName E1 SYMATTR Value 3 SYMBOL res -480 -32 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 5 SYMBOL res -480 -352 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 5 SYMBOL e -704 -320 R0 SYMATTR InstName E2 SYMATTR Value 3 SYMBOL schottky -608 -432 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D3 SYMATTR Value 1N5818 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky -608 -112 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D4 SYMATTR Value 1N5818 SYMATTR Description Diode SYMATTR Type diode SYMBOL res -480 -432 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R8 SYMATTR Value 5 SYMBOL res -480 -112 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R9 SYMATTR Value 5 SYMBOL e -704 736 R0 SYMATTR InstName E3 SYMATTR Value 3 SYMBOL res -480 704 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 5 SYMBOL res -480 400 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 5 SYMBOL e -704 432 R0 SYMATTR InstName E4 SYMATTR Value 3 SYMBOL schottky -608 320 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D5 SYMATTR Value 1N5818 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky -608 624 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D6 SYMATTR Value 1N5818 SYMATTR Description Diode SYMATTR Type diode SYMBOL res -480 320 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R10 SYMATTR Value 5 SYMBOL res -480 624 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 5 SYMBOL Digital\\and -960 416 R0 SYMATTR InstName A3 SYMATTR SpiceLine Vhigh=5 SYMBOL res -1120 496 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R13 SYMATTR Value 3k SYMBOL cap -1120 624 R0 SYMATTR InstName C4 SYMATTR Value 200pF SYMBOL schottky -1136 576 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D7 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL Digital\\and -960 720 R0 SYMATTR InstName A4 SYMATTR SpiceLine Vhigh=5 SYMBOL res -1120 800 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R14 SYMATTR Value 3k SYMBOL cap -1120 928 R0 SYMATTR InstName C5 SYMATTR Value 200pF SYMBOL schottky -1136 880 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D8 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL Digital\\inv -1296 -48 R0 SYMATTR InstName A7 SYMATTR SpiceLine Vhigh=5 SYMBOL Digital\\inv -1328 688 R0 SYMATTR InstName A8 SYMATTR SpiceLine Vhigh=5 SYMBOL res -352 800 R0 SYMATTR InstName R24 SYMATTR Value 0.001 SYMBOL res -352 48 R0 SYMATTR InstName R25 SYMATTR Value 0.001 SYMBOL nmos -384 336 R0 SYMATTR InstName M7 SYMATTR Value STW11NM80 SYMBOL nmos -384 640 R0 SYMATTR InstName M8 SYMATTR Value STW11NM80 SYMBOL polcap 640 880 R0 SYMATTR InstName Cx SYMATTR Value 2.2m SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=450 Irms=1.43 Rser=0.021 Lser=0 SYMBOL cap 528 944 R180 WINDOW 0 24 56 Left 2 WINDOW 3 24 8 Left 2 SYMATTR InstName C9 SYMATTR Value 1\B5 SYMATTR SpiceLine V=450 Irms=5.65 Rser=0.00828976 Lser=0 SYMBOL nmos -384 -96 R0 SYMATTR InstName M2 SYMATTR Value STW11NM80 SYMBOL polcap 560 880 R0 SYMATTR InstName C10 SYMATTR Value 2.2m SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=450 Irms=1.43 Rser=0.021 Lser=0 SYMBOL ind 32 336 R0 SYMATTR InstName L2 SYMATTR Value 7\B5 SYMBOL voltage -1456 848 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 WINDOW 3 -36 183 Left 2 SYMATTR Value PULSE(0 5 0 100n 100n 4.5u 9u) SYMATTR InstName V3 SYMBOL voltage -1472 32 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 WINDOW 3 -114 194 Left 2 SYMATTR Value PULSE(0 5 {phase_shift} 100n 100n 4.5u 9u) SYMATTR InstName V1 SYMBOL ind 112 48 R0 SYMATTR InstName L4 SYMATTR Value 0.7\B5 SYMATTR SpiceLine Ipk=100 Rser=0.001 SYMBOL cap -16 192 R180 WINDOW 0 24 56 Left 2 WINDOW 3 24 8 Left 2 SYMATTR InstName C6 SYMATTR Value 3\B5 SYMATTR SpiceLine V=600 Irms=0 Rser=0.009864 Lser=0.328211f SYMBOL res 144 288 R180 WINDOW 0 36 76 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName R7 SYMATTR Value 0.026 SYMBOL diode 176 768 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D9 SYMATTR Value RFN30TS6D SYMBOL diode 352 768 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D10 SYMATTR Value RFN30TS6D SYMBOL diode 176 896 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D11 SYMATTR Value RFN30TS6D SYMBOL diode 352 896 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D12 SYMATTR Value RFN30TS6D SYMBOL ind 32 -144 R0 SYMATTR InstName L1 SYMATTR Value 7\B5 TEXT 88 536 Left 2 !.tran 100m startup TEXT -1552 -488 Left 2 ;D E A D T I M E G E N E R A T O R TEXT -824 -488 Left 2 ;I N V E R T E R / DC/AC Converter TEXT -1592 256 Left 2 !.step param phase_shift list 500n 1u TEXT 88 560 Left 2 !.options method = gear TEXT -88 664 Left 2 ;MAINS RECTIFIER LINE Normal 784 1040 784 656 2 LINE Normal 784 1056 784 1040 2 LINE Normal -157 1061 -157 1061 2 LINE Normal -157 1061 -157 1061 2 LINE Normal -157 1061 -157 1061 2 RECTANGLE Normal -883 1061 -1607 -520 RECTANGLE Normal -179 1061 -837 -512 RECTANGLE Normal 787 1061 -160 624

Yikes, 41kW from a quad of 11A MOSFETs? Half that is being dissipated in the MOSFETs, right? :)

That's about a 3x underestimate -- use this instead:

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It takes into account skin effect, proximity effect, even waveguide effects, so the results are accurate from DC to, I think somewhere past the first resonance (which would be ~100MHz for a coil like that). The numerical accuracy is better than you can measure a solenoid (~1%).

But obviously, it'll still be a gross underestimate without a model for the load -- which sucks, because modeling a load is nontrivial (most working in this field use a 3D field solver :-/ ).

Simple stop-gap solution is to add series or parallel resistance until the Q is about right. There are hand-waving ways to estimate the Q of a workpiece (based on geometry and material). If it's within a factor of 2, that's as good a guess as you need; any more accurate and you're setting yourself up for destruction, anyway (the inverter needs to be able to handle the extra VAs of a somewhat mistuned load, otherwise you'll never survive passing Curie temperature).

This isn't a frequency tracked design?

I have concerns...

Yes, that swamps the load change, but it, well, swamps the load, too. You burn a ton more power in the wasted inductance, and tank capacitor and wiring, all just because... the circuit is too dumb to do better? :-/

I have seen several times where this had to be done, in practice. The reason is usually: a. because someone goofed the coupling transformer ratio, b. because the machine has a terrifically narrow window of operation, or c. they were trying to force-fit a situation that simply wasn't suitable for that machine.

In all cases, when it's needed, it's caused by a complete failure of design.

Since this is new design, you have all the options in the world!

Transformers are better for this -- they are commercially available, too, if not cheap. Not that anything is cheap at this power level. :^)

This circuit is known as the LLC, or series, parallel resonant configuration.

The L's act in parallel, in this case, (L1 + L2) || L4. That gets you your basic resonant frequency.

Another way to think of it: a part of C6 resonates with (L1 + L2) as a series resonant tank, onto the midpoint of which a parallel resonant tank (the rest of C6 || L4) is attached.

Yet another way to think of it is, (L1 + L2) and (part of C6) is an L-match network. In RF circuitry, this is used for medium to narrow bandwidth impedance matching. How it works is: the load impedance ZL (the resistance of rest-of-C6 || L4, at resonance) is transformed to an input impedance Zin, when Zser = sqrt((L1 + L2) / C6) is geometrically between Zin and ZL. That is, ZL is as many times larger than Zser, as Zin is smaller.

The practical effect is this: if you need a modest ratio between inverter and load impedances, the ratio goes as sqrt(Q) * Zout/Zin.

For Zin ~= Zout, you use (L1 + L2) ~= L4. For Zin Zout, the ratio goes from there.

Q factor is the ever-important quality factor of the resonant tank. For this transformation to be effective, Q needs to be modest: below about 3, the math breaks down; and for large impedance ratios, Q needs to be at least that ratio.

Failure to meet these constraints will result in insufficient power (L1 + L2 too large, relative to the tank L and Q) or excessive power (too small), and subsequent release of magic smoke. :)

I don't like the LLC network, because it wastes extra reactive power (at high Q and low ratio, (L1 + L2) contains many times the reactive power circulating in L4 -- and you thought L4 was the "work" coil!) and accomplishes nothing that a transformer can offer. Worst yet, many designs inevitably need both (i.e., a transformer for isolation and/or matching, AND the series inductor). I recommend series resonant for most applications.

If you'd like more information, I'm open for consultation.

Cheers, Tim