Fig 6 – Simple Gate Driver
COOL THING #6 – Low drive requirement allows super simple 2-transistor driver!
Wow, a long list of really great stuff. But alas, there is one pretty significant drawback when considering the use of this circuit for homebrew induction heating of metal: The work coil is “hot”.
Hot? You ask. Hot from awesome melting metal?
Yes, but also electrically hot. Meaning that the massive (1kV) tank ringing is right on the wire of that work coil. Touch it with the steel rod you’re trying to heat and you get killed – boo! Whacking the work coil with the workpiece is one of the commonplace hazards of actual workingman’s induction heating. You don’t want to do it, but it happens all the time.
So while this is a great piece of circuitry, we can’t say that it’s suitable for one of those cool levitation melting vids you see on YouTube. Yet.
It’s probably possible to magnetically couple (through a ferrite toroid or other transformer core) this circuit to another work coil made from copper tubing, but we haven’t tried yet. In that case, the dangerous voltages can all stay inside the protective case of the heater, and the work coil can happily run at a low voltage with an even HIGHER current. Cool!
The trick there will be the impedance matching. This circuit wants to run with a 50uH-ish inductance, and you don’t want a ton of turns on the primary side of the output transformer so a ferrite core may be tricky. Air core? Who knows. We have a bunch of experiments lined up on our side, and we’d be happy to hear your suggestions on how to take this cheap and cheerful circuit from the kitchen to the forge.
Attached are our schematics in LTSpice format: Openschemes-Burton_Powerstage. Just unzip everything into one folder, open the schematic (asc file) with LTSpice, and it should be ready to run. We’ve included libraries (LIB files, they’re just text files with spice models) for the comparator, all the diodes, and the IGBT’s. They will all unzip right where they need to be – right next to the asc file – so the simulator will automatically load them at sim time.
So go forth and tinker! Please post here if you discover great things, cool mods, or build your own induction heater based on this design. Once again, be careful and only take on projects you are qualified for – a dead ‘schemer is no good to anyone! 
By Dan January 13, 2011 - 9:44 am
Great post, very informative. Enough that it made me want to start a project! I’m having trouble downloading the LTspice files, the link appears to be broken. Keep up the good work!
By openschemes January 16, 2011 - 8:00 pm
Good call brother! The link was shortcutted and shouldn’t have been. It’s repaired now..
Glad to hear you’re interested in starting a project. We’ve been so swamped with other stuff that ours has been sadly neglected, but we’ll be back soon with TWO new induction heating projects. First, our own take on this design. And second, hacking this actual device for manual operation – it’s easy! And dangerous!
By haxx0r August 10, 2012 - 12:21 am
Since the Capacitor on the comparator allows delay for zero voltage switching, how does it stand up to varying loads? (frequency change?)
By openschemes August 11, 2012 - 7:58 pm
The cap on the comparator is just for filtering the noise, really. Any delay it gives may be looked at as tuning, rather than system level design. The cap you care about is the one across the heater coil. This one is not really for delay, but rather forms an LC tank which sets the ringout (reset) time. This time will change with load, power level, etc so the frequency is often changing. It’s not intended to be fixed frequency in this system, it works quite nicely being variable frequency. One way to think about it is that when the power level is low, the heater is giving small pulses so they build up and ring out quickly. Therefore the frequency is high. When the power level is high the pulses are huge, and so is the ringout time, so the frequency is quite low – less than 10kHz and you can certainly hear it.
By haxx0r August 10, 2012 - 12:39 am
http://www.seekic.com/uploadfile/ic-circuit/201165221122900.gif
Here’s another IH cooker, How does the pwm mechanism work, since it looks a lot different than your teardown design.
By openschemes August 11, 2012 - 7:51 pm
Very interesting. Thanks for posting it. At first glance, it’s not clear HOW the PWM mechanism works but we’ll look a little harder in the upcoming days. It appears that comparator I3 does the job of detecting the ringout, which signifies that a new cycle can start. And the gate driver is a fairly standard implementation (NPN/PNP emitter follower driver with VD5 diode for offset) but between those two points – we agree, it’s very different! But quite interesting, we will let you know what we find.
By h4xxor February 27, 2011 - 6:24 am
The High Voltage power supply inside Panasonic “Inverter Series” Microwave ovens uses the IDENTICAL principle involved except that it uses proprietery IC’s.
By openschemes March 1, 2011 - 8:33 pm
That’s great to know – we’ll have to go check out those devices to see how it’s put together. So how did you learn about those ovens?
Thanks for the tip!
By h4xxor April 26, 2011 - 5:10 am
I started taking panasonic “inverter” microwaves to dig out the IGBT’s. I’m surprised that a huge amout of them failed due to “transformer hv winding arcover” but most of the time, the IGBT’s survived.
It used a circuit identical to the above with the HV transformer primary in place of the pan heating coil. I knew it because when I fired the unit without the HV transformer primary connected, Gate driver signal only does “click” “click” “click”, Assuming it was the starting signal from the IC to sense transformer “ring”.
Those inverters work on three modes. the start mode, The idle mode (also known as the magnetron presence test mode), (inverter powered, no magnetron connected) and the cruise mode. (magnetron connected and inverter powered for 3 seconds) Interestingly, these inverters will fail if the magnetron is disconnected (or fails) when the inverter is in “cruise mode”. Either the IGBT will short out or, continuous arcover of the HV transformer. (90% of the time this happens, as the magnetron fails in mid-use).
Yes, these inverters used for powering magnetrons in Panasonic ovens LACK overvoltage, and short circuit protection! Yes, they go to the grave with the magnetrons!
By openschemes May 4, 2011 - 5:59 pm
Very cool stuff! It sounds like you know quite a bit about these inverter microwaves. If you’re interested in doing a writeup on what you know, and what goodies you can scavenge from them – shoot us an email, we’d be happy to post it up here for you.
By h4xxor May 5, 2011 - 2:07 pm
I had a look at your induction cooktop teardown, I built a similar ciruit (with two comparators and that…), but the circuit keeps stopping. and my question is, how did the startup mechanism worked?
By openschemes May 6, 2011 - 5:19 pm
The startup is achieved by disturbing the first comparator (the one that compares the divided-down SW to the divided-down reservoir cap). When this comparator is disturbed, it fires one pulse which gives one switching cycle. If the power is set high enough, then the SW node will ring up and back down to 0, triggering another cycle.
If your device is running for a while but stopping, then a couple things might be the problem.
1) Most likely is that the power is too low and it can’t regenerate a cycle because SW doesn’t fly back down. Either the inductor is too big, or perhaps you are using a smaller input voltage? We run a lot of tests at 12V, which requires much less inductance than 120V. How big is your coil? If you know the inductance, that’s great. Otherwise, you’ll have to calculate it from the diameter and # of turns.
Another thing that could cause very low power is the wrong values on the INT/VRAMP section of the first comparator. For example, if the RC is so small that the pulse width is tiny, then there will not be enough current built up in the inductor to fly down and retrigger.
One more thing would be if you’re trying to run the device too fast (say, 100kHz). This device is good for a maximum of 40-50kHz at the moment. It’s typical frequency is 20kHz and lower. We are trying to develop a modified device that can be used for aluminum, etc – but these IGBT’s are VERY SLOW (like any IGBT) and getting it all running fast is not trivial.
Have you simulated your circuit in LTSpice? It’s free, and you can use our schematic as a starting point. If you’d like to email us your schematic, or a picture of your board, we’d be happy to advise. IH at openschemes dott com.
By h4xxor May 8, 2011 - 10:32 pm
Yes, I knew the startup circuit of the IHcooker is done, I did realized that the circuit stops only if I used unfiltered AC (like the above), probably because the dc rail goes to zero every half ac cycle. But when I put a total of 6800uf filtering, the circuit operates continuosly, (albeit with the worst power factor I’ve ever seen
)
By Mark March 26, 2011 - 7:01 am
Hooray!!!!
I bought a counter-top induction cooker to perform quick heating of steel rings for a project for my company. I tried it and of course, because it didn’t see enough load or something, because the part is not a complete surface like a pan, it didn’t work. So I casually looked on the internet and saw that there were several chip development sites that displayed proposed circuits and I figured that I could use those to decipher mine and make it do what I wanted. Then I put in the chip number from my device and Voila!!(Wa Laa) you guys pop up and not only have the exact circuit that I purchased but have a hack besides.
God is so good to me!! Really!! I am Irish.
I mean, first, two months ago He had you pull apart and hack this device, then He had me purchase the right device and lastly He suggested that I search based on the chip #.
I don’t hear Him or anything, but I can’t discount the many fortunate coincidences that have happened in my life. So enough about that.
What I really wanted to do was thank you for this article and plead for any further work you have done with it.
I have only just read the article and so now plan to try the potentiometer power control. I will let you know what I find out about the protection circuitry as I can. This is a definite R&D side side project for me though so I don’t get much time for it.
However, you people ROCK!!!
Mark
By Mark O'Hara April 28, 2011 - 12:54 pm
Gentlemen,
Please followup with the manual control of this system as I really want to use it for a production machine I have envisioned.
If the circuit design already exists please direct me to it.
Thanks,
Mark
By openschemes April 28, 2011 - 7:34 pm
Hi Mark,
Glad you’re getting some use out of the article – it’s always great to find exactly the hack you need! Thanks, interweb!
The reason that we’ve not put up our manual control stuff is that there was an “event” where a flying wire used for the startup (strike) drooped over and touched the 1000V heat sink. Pow – dead circuitry! And we’ve been MEANING to order another Burton but just never have. However, another is on it’s way so you should expect the full article soon.
For now, we will generate a placeholder article that will be expanded later. Go look at the main page to find it.
By Mark O'Hara May 3, 2011 - 9:53 am
Gentlemen,
I really need help understanding the circuit which I traced that the enable input (K) is concerned with.
I have a pdf of my sketch (not pretty but accurate) which I would send you if you would give me an address or if you already have the enable circuit and can explain it to me I sure would appreciate the help. It looks to me like “K” needs to be grounded to enable the system.
Thanks,
Mark
By kn3 December 18, 2011 - 6:50 am
Are there two bridges for rectifcation of AC-DC? The IN4007, may not be able to handle high current?
By openschemes December 18, 2011 - 3:49 pm
The 1N4007 is only for simulation. You can find the actual component by looking back at page 5 of the first article.
So, it’s actually a 35A bridge.
By PRG December 19, 2011 - 3:17 pm
Hi, this article has been a great help. I am working on a science project investigating the heating of nano-sized iron oxide particles at various concentrations (I have used them in the past to make ferro-fluid). I am hoping to use a induction cooker (same as that in the article) to raise the temperature of my samples a few degrees but of course they are too small for the cooker to detect and operate. While the manual control sounds like a good way to go, I was wondering if there was an approach that would allow the control panel to still be operational? My thought was to override the appropriate input signals to the micro-controller to make it think it was detecting a suitable load. Any suggestions would be appreciated.
By openschemes December 19, 2011 - 5:26 pm
You should consider making a significant batch – say 3-4oz, and then just placing that entire batch in a glass container right on the hotplate. Since the ferrofluid is magnetic, the cooker will detect that it is a suitable load (as long as the batch size is significant) and will go ahead and cook it. Just like iron powder toroids, magnetic flux can jump between particles even if those particles are not connected together physically. And since the concentration (at least in ferrofluid) is quite dense, the cooker should have no problem detecting and cooking the particles.
It would sure be interesting to see what happens when ferrofluid is induction cooked. The field is much faster (kHz) than it can respond (spiking, etc) but it sure seems like something neat would happen. Maybe an oily blast onto the ceiling – anyone care to try?
Please beware that ferrofluid can be cooked to death – it has a curie temperature which is probably below 100C considering it’s tiny size.
As far as tricking the micro into running without a load – hmm, it can probably be done but the suspicion is that the micro checks the resonance every cycle. So some kind of circuit would have to continually retrigger in order to keep it tricked. Not that it’s impossible, just that it might not be simple.
By PRG December 20, 2011 - 2:12 pm
Thank you for the quick response. I plan to wire up the manual control today and try heating a sample. I’m not sure I can mix up such a large batch of fluid with what I have on hand, but as a test I filled a 4″ diameter flask 1/8″ – 1/4″ deep with a fine black magnetite sand (same stuff I believe, just much larger particles) and put that on the cooker. It wasn’t detected (but did heat up slowly when I placed a metal pot to the side to trigger the cooker). Since magnetite is a ferromagnetic iron oxide, I suspect it doesn’t interact with the field to the same degree that the same mass of iron would. So, it looks like the manual control will be necessary to activate the cooker.
Another issue I am concerned with is whether the frequency will be high enough. From papers I have read 20kHz might be a bit too low to see much, though 40kHz might be enough.
By PRG December 21, 2011 - 1:27 pm
I wired up the control circuit you provided in the other article and was able to try it this morning. One issue that came up was that using a momentary switch to +5 wasn’t causing the circuit to oscillate. I checked with a voltmeter and found that it the PAN line was already high. However, switching the button to ground instead of +5 is working great. I also connected the FAN line to +5 and it is working nicely as well.
Your manual control circuit looks like it will be perfect for my experiment. I already tested it with a small sample of the magnetite sand and a sample of the nanoparticles in water – both showed a small amount of heating (a 5-10 degrees) over the period of a minute. Just what I was hoping for.
Thanks for your help.
By openschemes December 22, 2011 - 10:58 am
Nice work debugging the momentary switch problem – we’ll check your technique here and add it to the article if it works here!
Glad the manual control is working for you, and it sounds like its just the right thing for your nanoparticles. Good luck, and let us know how your experiments turn out!
By PRG December 22, 2011 - 4:48 pm
Thanks.
I do have a couple more questions I would like to ask. One is about the frequency – you mention above that this unit might be good to 40kHz-50kHz (which I would like). Would that just be modifying (lowering) the LC of the coil and associated capacitor or is it something else? I noticed that the frequency ranged from about 31kHz when the power was turned down but was about 20kHz at the high-power end just before the circuit began to “buzz”.
The other question is about the ferrite concentrators that are located below the coil. It appears that they might be heating up a little (the center of the cook top is warm to the touch after a few minutes). I’d like to remove this as a heat source. Would removing them be detrimental to the neighboring circuitry (since the magnetic field won’t be as localized)?
By openschemes December 23, 2011 - 6:45 pm
Sure, you can remove the ferrite concentrators.
The frequency is determined by the on pulse time (made by R C in the comparator circuit) as well as the L C tank of the power stage. That’s why the frequency droops as the power is cranked up – same LC ring but longer on time.
To easily increase the frequency, you may try winding a coil with fewer turns. That mega litz wire is probably not needed for shorter, lower power runs. Some if the web projects get away with hollow copper coil or even AWG 10 and get bolts glowing hot! Beware that this coil is driven to 1200V and would be instantly lethal to touch.
Alternatives to a new coil would be lowering the R or C in the timing stage or lowering the tank cap in the power stage. It might not be good to just halves the tank cap, but would probably work ok for light loads – you will be putting twice the ripple current thru the remaining cap.
By Daniel Admassu January 5, 2012 - 12:13 am
I am trying to build the circuit for my own domestic cooker ambitions. Is there anything on the overvoltage and overcurrent protection circuits? (apart from the 3000:1 current transformer and usage of the two remaining comparators on the LM-339 of course). Looking forward for replies.
By openschemes January 5, 2012 - 3:44 pm
Are you asking if the circuits are available? Not really – they were sketched out at one time, but the sketches are not in the file cabinet so who knows where they went to. If you’re asking if any other circuitry is required to build one, then nope – nothing else is required. We have a couple little ones built that only use an LM319 dual comparator and they work just fine.
By Daniel Admassu January 9, 2012 - 1:04 am
Hi;
Thanks for the prompt reply. I did not expect one so soon so I did not check back.
I was wandering about the general scheme of the overvoltage or overcurrent protection used. For example what parts were protected? The work coil? The IGBTs? And did it use something active such that the voltage or current get regulated or did it go into shutdown mode and needed a restart? If you had any bits that you can share on these, I will much appreciate it. If not, it is all right. It has been one of the greatest circuit articles I have ever seen. Thanks.
By openschemes January 9, 2012 - 5:40 pm
No prob, thanks for the kind comments. The work coil is durable, its the igbt that need protection. As well as limiting the input current to avoid tripping breakers
If memory serves, the ovp/ocp comparators also pull down on the gate driver ( to stop the cycling) if they are tripped. This would probably interrupt the oscillation and require a restrike to get going again.
By Patrick Scully March 17, 2012 - 2:18 am
Wonderful analysis of this apparently very hackable, high quality, bargain basement high frequency/high voltage power source!
I was thinking of cutting off the output inductor and replacing it with a parallel bank of 3 large ferrite flybacks wound to yield approximately 20kV. The primary inductance of this transformer bank (273uH) is far greater than the inductance of the original output coil (45uH), so without offsetting adjustment, the resonant frequency of the tank circuit would be a fraction of the frequency of the drive stimulus. This would effectively ruin the power supply.
However, according to my calculations, the addition of 100nF of high voltage capacitance in parallel with the transformer bank should restore the original 30kHz tank resonance, enabling the system to transfer the roughly 1800w @ 30kHz specified for the induction cooker, only now at 20kV!
Is my simple scheme workable? Your comments and criticism would be dollars and sense/cents to me, because each of those transformers will cost as much as an entire induction cooktop.
OTOH, the prospect of building such an authoritative power source — with brand new parts and for only $250 — is very tantalizing indeed! Roughly equivalent power supplies offered by 1st generation US military/industrial suppliers are $5000 and up….
By openschemes March 19, 2012 - 4:29 pm
Interesting proposal. Offhand, there’s no reason that this device couldn’t be used in flyback configuration. In flyback, we pull the switching node low which reverse biases the output. Field builds in the core. When we let go, our switching node jumps up to VIN+VOUT/N, where N is the turns ratio. So you have to endure a pretty large voltage, but much less than resonance. The field in the core forward biases the secondary diode and pumps up the output. Then, when the field runs out the switching node rings out which SHOULD retrigger this kind of circuit perfectly.
Before you buy your transformers, you would probably want to try it out using the LTspice model to see if it works in sim at least. That is for normal flyback case – the resonant flyback you describe is unfamiliar here, but might work just as you describe. The question is whether resonant flyback is even needed if the circuit will automatically retrigger? You could save some $$$ on those very expensive caps if it’s not needed. On the other hand, trying to turn off the power switch with 15A and a fast rising 600V edge could be a spectacular event. Resonant might be the way to go!
By Jim April 2, 2012 - 5:59 pm
The split capacitor half bridge design if far more superior, Since this quasi-resonant design is the electronics’ world downfall. They DON’T do zero voltage switching, like the theory says, only “low voltage switching”. It CANNOT be done, because when the system is UNDER LOAD, vCE does NOT drop to zero saying that it does is like saying perpetual motion exists. LVS causes eventual degradation of the IGBT. (they actually become leaky, and, poof!)
.
I built my induction stove using an sg3525an IC driving 2 IGBT’s and the frequency is changed by varying the RT (timing resistor’s resistance) through a transistor. it is set in the following: Higher current=higher frequency.= feedback equilibrium
pot on = 1000 watts dissipated. pot off = 1000w reactive power. (yes, I use a rugged ixys 200 volt 6o amp mosfets, off a plasma screen driver)
By openschemes April 2, 2012 - 7:57 pm
Sure, but you need a high side drive for half bridge. The elegance of the junky little circuit in the Burton cooktop is that it’s self resonant and can use a fairly slow low side drive. Your comment about low voltage switching is true in some sense, as any device with current through it will have some IR drop, but the turnoff should be zero current when well tuned, and the turnon should be zero voltage. Hence, getting away with a fairly slow driver.
You should post a writeup on your half bridge for users looking to take it to the next level, it sounds cool!
By hack&mod April 5, 2012 - 10:54 am
does anyone know the simplest way to reduce the induction cooker max power rating?
i want to reduce my 220volt/50hz cooker maximum rating from 2100 watt down to around 1000watt.
but i want to keep their original controller board working normally, so i don’t want to replace it with others, such as pwm or other controllers.
does replace high current rating of the old igbt’s with lower current rating of the new igbt’s will be solve it?
thank you
By openschemes April 5, 2012 - 4:27 pm
No, you would need to limit the power by making the on time smaller. Using smaller igbt would only blow them up. If your circuit is the Burton one here, you can replace the pullup resistor on the INT line with a smaller value. This will make each pulse shorter, so the maximum power will be less. Try reducing it to 2/3 or 1/2 of its original value.
By hack&mod April 5, 2012 - 6:05 pm
dear friend,
and how about the way to reduce their original current into around a half? i am looking for a way to reduce its power by reducing their current and keep their pulses unchanged. i mean, lets leave their on/off cycle pulses as its original controller setting, but reduce their main DC current into a half from original current.
thank you again
By openschemes April 6, 2012 - 10:08 am
Use half the voltage or twice the inductance.
By hack&mod April 6, 2012 - 12:56 pm
did you mean adding similar inductance in serial or just replace and change it’s wire length twice longer? is that the same way? i’m sorry, i’m novice here
thank you
By openschemes April 7, 2012 - 6:53 am
Yes, either of these will work, but changing the inductance of the work coil is probably the best idea. You could re-wind it to a tighter spiral, or yes – you could add some extra turns of wire, if you can get the same wire of the same total thickness. Use a spiral inductor calculator to help you determine how much to add: http://deepfriedneon.com/tesla_f_calcspiral.html
By hack&mod April 7, 2012 - 7:47 am
ok, i will try it. thanks a lot
By Daniel Admassu May 11, 2012 - 1:36 am
Hi;
Sorry to trouble you again. I am planning to build a 240v version of the cooker. What components of the power stage do you think need change? Any suggestion on their values?
By sis June 19, 2012 - 10:26 pm
hello, I’d like to ask you, what should I change if I want to make induction heating to melt the copper from the engine of this stove. thanks
By alekssmitt August 20, 2012 - 4:33 am
Very interesting. But what the mission of R7, C8, D1, and check point PA2 ? I guess some sort of protection ?
By openschemes August 20, 2012 - 6:15 pm
It seems to be how the circuit is first kicked into oscillation, we’ve been calling it “striking” the oscillation. Consider if the device were powered up but no pulsing was taking place. It would be happy to sit there and CONTINUE to not pulse, because the comparator U1 would never trip. Now consider if PA1 quickly dropped from high to low (or high-low-high as the case may be). The cap C8 would pull down further than the diode D1 and force the comparator U1 to give a pulse which would start the whole sequence of oscillation.
By DHANANJAY August 22, 2012 - 2:52 am
can i use power mosfet in place of IGBT ? if yes what should be d rating ? if not why ?(topic- induction cooktop)
By openschemes August 22, 2012 - 5:36 pm
You can use mosfet, but it’s unlikely you will find a mosfet with 20A, 1200V rating. That’s why IGBT are used. If you’d like to make a lower-current version then it will be possible to do it with mosfet.
By alekssmitt August 22, 2012 - 7:19 am
The value of R1 R2 R5 on one hand and R3 R4 R6 R11 on the other make the different value of voltage on the input LM139
Thus it’s no need auxilary R7, C8, D1 elements ?!!
By openschemes August 22, 2012 - 3:15 pm
You can get some insight by calculating the voltage divider for SWREF:(R6+R11)/(R6+R11+R3+R4) and TOPREF: R5/(R5+R1+R2) . If the input voltages are the same (not switching), then the SWREF side will always be lower than TOPREF, meaning that the comparator will never activate (output low) unless it’s disturbed.
By alekssmitt August 24, 2012 - 5:59 am
You wrote above: «Anytime SWREF falls below TOPREF, it’s OK to start a cycle.»
But SWREF below TOPREF in the beginning by mean of the values of R1 R2 R5 on one hand and R3 R4 R6 R11 on the other !
If the output of first comparator is low then the output of the second comparator is high = gate driver is high = output stage is open.
I mean that the values of those resistors, mentioned above, make the whole scheme instable itself. No need to add some components to disturb the scheme. And, anyway R7, C8, D1 can’t do disturb the scheme.
We can imagine that the scheme is hanging. Than what we have:
No current flow in circuit L1 C2
output stage is closed = high
the output of the second comparator is low
The C5 is fully charged
the output of the first comparator is also low
And what ? Adding R7, C8, D1 can’t change the situation.
Anyway the dude, who draw this excellent scheme had some plan about those elements.
P.S. May be this is not the whole scheme, may be PA2 point leads somewhere else, par example to the protection part of the scheme, made from the rest part of LM139 ?
By openschemes August 24, 2012 - 6:39 pm
That’s a very good analysis, thanks for posting it! You are right that the state of the comparator is naturally 0V out. If you have LTSPICE, then please run these experiments on the schematic to see our point.
Change the AC input voltage to a slow ramp, by changing
sine(0 168 50)
to
dc=0 pwl 0 0 10m 120
The comparator starts out, and stays low. C5 cannot charge and the circuit hangs. This is similar to what happens if you include the real components generating V18 and V5. These voltages start up so slow, the comparator is always low and the circuit hangs because C5 never charges up.
Now, to try striking the oscillation you can put a new voltage source on PA2. It’s function will be to be normally 5v, but give one small pulse down to 0V. This statement can do it:
PWL(0 0 10u 5 10m 5 10.001m 0 10.100m 0 10.101m 5)
Now you will see that it can strike the oscillation by giving one small blip.
You’re right that this is not the entire schematic, but the items we’ve deleted are not necessary to show the function of the circuit. This is the core circuitry that can show the function. Hope it helps!
By alekssmitt August 25, 2012 - 3:55 am
I resolve the problem! It takes time, but finally I did it.
The purpose of those elements to stay the sceme on work when the TOPREF is LOW somewere about ZERO !!!!!!!!!!!!!!!!!!!
TOPFER=100 Hz
SWREF=20 KHz
SWREF never falls below ZERO due to DAmping Diods ! Than when the TOPREF pass through ZERO, SWREF can’t falls below ZERO to switch the first comparator U1. But here the R7, C8, D1 elements improve the situation. C8 charged in previous half period by D1 lift-up the voltage on the TOPREF side, making possible to switch the first comparator U1. VOILA !
P.S. Apparently the scheme can work without those elements, if tank Q › 20 with full load. Simply the relaxation oscillations taking place when TOPREF around ZERO. And of course PFC would be out of order during this time.
By openschemes August 25, 2012 - 6:33 am
Great thoughts!
By Wesley September 8, 2012 - 10:47 am
Hi! I want to say that this article is amazing, nice written and include all important infos. I’d like to look extra posts like this .
By Eric October 12, 2012 - 8:58 am
Hi,
I opened up a “Sanki” 1,000W mini hotplate & it looks like a similar circuit. Importantly, one side of the output coil (135 uH flat spiral) is at 120 VAC on my multimeter while the other side goes out of range (>750vac) when powered on.
I suppose there are 2 ways of hacking this… (1) Uncoil the spiral into a cylindrical coil and make sure the whole thing has insulation sufficient for well over 1000V. (2) The second is to couple the output coil to a new coil (i.e. custom transformer).
Assuming #1, (rewind flat coil to cylinder and insulate) does the change in coil geometry change the inductance?
Assuming #2. Would I simply create a matching 1/1 spiral coil with the same number of turns and same gauge wire?
Thanks
By Denny October 18, 2012 - 11:24 am
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upper! Thanks =)
By yunush December 30, 2012 - 7:42 am
i want to make induction cooker my self at home
By Chaitanya January 25, 2013 - 6:48 am
Hi,
First of all many thanks for the article and the LTspice simulation files.I am playing around in LTspice simulations and am new to IGBT stuff. In simulations, if i change the input AC voltage below 130VAC i am not getting the startup and unable to see the resonance effect during IGBT off. Please help me out with changes required in the circuit to make it work down to around 90 or 100VAC. What are the changes required to make it work with universal power supply say 85VAC to 265 VAC.
Thanks in advance,
Chaitanya
By Le Tuan Anh January 27, 2013 - 8:42 pm
What would happen if replace of L1 coil by coil bifilar?
By openschemes January 29, 2013 - 6:06 am
What does “coil bifilar” mean? A centertapped coil? That would work fine if it were made of thick litz wire.
By yunush March 9, 2013 - 10:07 am
i am filling luky
By J March 31, 2013 - 4:18 am
you have to check out my Edit on the IH circuit, it’s truly revolutionary. No zero-switching approximating capacitors need. it is eliminated.
By openschemes April 1, 2013 - 3:05 am
Ok sounds great. Where do we see it?
By g poss April 11, 2013 - 9:04 pm
Any ideas on how to run this power board with a much smaller coil, say 100 by 107 mm made of litz wire, 30 strands of 28 gauge wound into a wire
By openschemes April 12, 2013 - 12:57 am
Use teeny tiny pulses so the current never gets high enough to damage your coil! Or, run the coil from 12V or 24V to keep the current low.