To add;
The top snubber cap (or transistor) is just as likely to be a problem. This cap's value is a bit more critical. For snubbers I would use the original value. The couplers don't matter.
JI am not responding to you here Phil, but the whole bunch.
Everyone wants to get into all this esoteric shit here and really an induction top doesn't run at microwave speeds. Simple RF. I mean television SMPS RF.
The bottom keeps shorting, well lessee here, what can cause one transistor in a totem type pole circuit to short ?
** An intermittent fault ( ie internal short) in the load.That induction heating coil is a non- trivial device running at high voltage & frequency.
The briefest short or arc between adjacent turns would take out one or both IGBTs instantly.
BTW:
Stop being such a PITA jerk.
.... Phil
On Wed, 28 Nov 2012 19:22:28 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:
current capabilities are up to snuff. The value does not mean shit as long as it doesn't go too low.
Wouldn't increasing the capacitance from 0.68uF to 1uF result in a 50% increase in cooking energy?
Also, wouldn't each capacitor take longer to charge, and if the capacitor wasn't fully charged when the IGBT switched off, wouldn't this result in an interruption of the current in the coils, with a potentially damaging back-EMF? Or am I completely misunderstanding how this appliance works?
- Franc Zabkar
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On Thu, 29 Nov 2012 09:31:06 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:
for capacitive reactance ? Apply it and just guess the frequency is over an octave above the sonic range.
tuned system, they are being used as coupling caps.
rail are effectively at the same AC potential, so it's not 0.68, it's aready
1.36 uF. That is almost a piece of wire at 20 Khz. Almost, but we are dealing with a quite higher frequency here.Here is an induction cooker design where the capacitors and coil are said to form a "resonant tank":
The second IGBT's body diode allows the coil current to decay gracefully when the first IGBT switches off. I forgot about that.
- Franc Zabkar
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On Fri, 30 Nov 2012 13:54:20 +1100, Franc Zabkar put finger to keyboard and composed:
The above design references the following document:
Induction Heating System Topology Review:
- Franc Zabkar
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current capabilities are up to snuff. The value does not mean shit as long as it doesn't go too low.
It's a serie LC resonnance driven by a half bridge switcher, if I'm correct.
I've been following this with curiosity and amusement.
I suspect this product is a marginal design that fails more-often than it should, but not often enough to be considered outright "defective". (The NuWave product advertised on late-night TV seems to be plagued with similar problems.) There is probably no way to "fix" it, short (hmmm) of a complete bottom-up redesign. It's a shame, because a counter-top induction "burner" is a good idea. (I almost ordered the NuWave until I learned how unreliable it -- and its seller -- are.)
The Infinity "SWAMP" switching amplifier from the late '70s is an example of such a marginal design. It blew its output transistors far too often. The designer later found the problem, and admitted that adding two cheap diodes per channel would have prevented it.
On Fri, 30 Nov 2012 09:31:06 +0100, cLx put finger to keyboard and composed:
current capabilities are up to snuff. The value does not mean shit as long as it doesn't go too low.
Yes, I see that now. I should have researched the topic instead of relying on the OP's rough circuit diagram. The absence of the flyback diodes from the drawing left me wondering how the coil current would decay after the IGBTs switched off, so I assumed that the current had to be zero when this happened.
- Franc Zabkar
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On Thu, 29 Nov 2012 09:31:06 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:
for capacitive reactance ? Apply it and just guess the frequency is over an octave above the sonic range.
tuned system, they are being used as coupling caps.
XC = 1/wC = 1 /(2 x pi x 20000 x 1.36 E-6) ~ 6 ohms
My research would suggest that a typical inductance value for the coil would be of the order of 50uH.
XL = wL = 2 x pi x 20000 x 50 E-6 ~ 6 ohms
So XL = XC, ie resonance.
rail are effectively at the same AC potential, so it's not 0.68, it's aready
1.36 uF. That is almost a piece of wire at 20 Khz. Almost, but we are dealing with a quite higher frequency here.If those 0.68uF capacitors were coupling capacitors, then it wouldn't matter how large they were. In fact the larger, the better. So let's assume for the sake of analysis that they were infinitely large. This means that the voltage at their junction would be constant (Vsupply /
2), irrespective of the induction coil current.So when the upper IGBT is on, the voltage across the coil would be Vsupply - Vsupply/2 = Vsupply/2. Similarly, when the bottom IGBT is on, the coil voltage would be -Vsupply/2. This results in a linearly increasing coil current, first in one direction and then in the reverse direction, ie a symmetrical triangular current.
Vsupply/2 = L . dI/dt
I would think that a triangular current would be undesirable.
- Franc Zabkar
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On Thu, 29 Nov 2012 10:21:43 +0100, cLx put finger to keyboard and composed:
I would measure the voltage at the junction of the two 0.68uF capacitors, with the coil disconnected, and confirm that it sits at half the DC supply voltage. This might identify any imbalance in the capacitor characteristics, ie leakage, reduced capacitance.
- Franc Zabkar
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I didn't mean it like it sounded I guess.
Anyway I understand what you say about the Xc, but I am assuming that the f requency is much higher than 20 Khz. I'd bet it's somewhere around 75 Khz w hich takes the tuned circuit out of the picture. The Xc of the caps would b e much lower and the Xl of the coil much higher. If the whole shebang was 1
2 ohms total, one burner would pull almost 27 amps providing the supply is 320 DC. The numbers fall into line at around 75 Khz or so.At that point, there is nothing tuned about it. I didn't mean to offend, I just know that so many have a few misconceptions about these things, actual ly not even these things pty se, SMPSes that use a similar configuration. I f you don't have those misconceptions great.
I try to think from the engineer's standpoint. Why would I waste energy on Xc ? There is a reason to do it for example in the horizontal sweep ciscuit of a TV but usually it's all or nothing. Block the DC and pass the AC.
JOn Fri, 30 Nov 2012 16:20:16 -0800 (PST), snipped-for-privacy@gmail.com put finger to keyboard and composed:
frequency is much higher than 20 Khz. I'd bet it's somewhere around 75 Khz which takes the tuned circuit out of the picture.
That Fairchild design note I alluded to in an earlier post discusses several topologies (including the OP's), all of them based on resonance principles. The resonant frequency was chosen as 24kHz while the operating frequency "of the system is set at 28kHz, which is higher than the resonance frequency, in order to avoid noise generated within the audio frequency band".
Wikipedia also suggests that 24kHz is a common design target:
The above article tabulates the skin depth of various materials at
24kHz.However, Wikipedia also states that "Panasonic Corporation in 2009 developed a consumer induction cooker that uses a higher-frequency magnetic field, and a different oscillator circuit design, to allow use with non-ferrous metals."
As for some real examples, here are two service manuals for induction cookers:
Both manuals refer to a diagnostic mode whereby the microprocessor is accessible via an RS232 interface. There are several sample outputs which indicate that the operating frequency is 20080 Hz.
- Franc Zabkar
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On Sat, 01 Dec 2012 16:27:24 +1100, Franc Zabkar put finger to keyboard and composed:
frequency is much higher than 20 Khz. I'd bet it's somewhere around 75 Khz which takes the tuned circuit out of the picture.
I found this interesting application note:
"Induction cooking functions based on the principle of the series L-C resonant circuit, where the inductance L is the cooking element itself. By changing the switching frequency of the high voltage half-bridge driver, the alternating current flowing through the cooking element changes its value. The intensity of the magnetic field and therefore the heating energy can be controlled this way."
It describes the OP's circuit as a resonant circuit consisting of an L-C resonant tank. It states that "the IGBTs are driven by high frequency complementary square waves with 50% duty cycle", and that the frequency of "the PWM signal applied to the driver input pin ... varies in a range between 19 kHz and 60 kHz". The two capacitors are
680nF 600V types, and the IGBTs are STGY40NC60VD.The design has 9 power levels, with a frequency of 60kHz for lowest power, and 25kHz for highest power. At 60kHz the plate current appears to be roughly triangular, while at 25kHz it appears sinusoidal.
- Franc Zabkar
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On Sat, 01 Dec 2012 17:12:28 +1100, Franc Zabkar put finger to keyboard and composed:
The datasheet for the STGY40NC60VD states that the IGBT is good for "high frequency operation up to 50kHz". Hmmm ...
- Franc Zabkar
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Very interesting Frank. You do good research. However there is no real cont radiction. The inductance must be higher than we assumed because if the Xl is only 6 ohms, it would pull a hell of alot more current. It had to be one or the other.
Even more interesting is how they're making it work with non-ferrous vessel s. Technically it should work with anything that conducts electricity, coil s in a transformer are not ferrous and you can induce current in them right ? It actually does work on non-ferrous I guess, just extremely inefficient ly. They aimed to increase the efficiency. It says reduced efficiency and I saw a chart, but it wasn't boiled down to just HOW inefficient a given uni t will be on specific cooking materials - as if the layman would usually ev en know. It would still be trial and error.
Now if they can make it work on glass cookware, they get my vote for the No bel prize.
Very interesting, thanks for bringing that in.
contradiction. The inductance must be higher than we assumed because if the Xl is only 6 ohms, it would pull a hell of alot more current. It had to be one or the other.
vessels. Technically it should work with anything that conducts electricity, coils in a transformer are not ferrous and you can induce current in them right ? It actually does work on non-ferrous I guess, just extremely inefficiently. They aimed to increase the efficiency. It says reduced efficiency and I saw a chart, but it wasn't boiled down to just HOW inefficient a given unit will be on specific cooking materials - as if the layman would usually even know. It would still be trial and error.
Nobel prize.
Making it work with glass would impress must anybody. However i wonder if they can be made to work on slightly salty water (saline{?}).
Just a thought.
?-)
Actually if they can really penetrate the non-ferrous they can heat the foo d directly. I gues an upside down microwave or something.
Hell, it might be all the rage range now because your steak heats the pan, not the other way around.
By now you know most of that "Visions" type cookware got recalled right ? T he stuff exploded on a few folks and they went back to metal pots and pans.
Funny what it takes to induce common sense sometimes eh ? My Mother had the m but got rid of them over that shit. Really, they would probably be alrigh t for boiling and casseroles etc, but I think trying to fry crispy chicken in a glass pan might not be the smartest idea.
But then lead could be put in the glass, or barium. Maybe it would glow or something.
Colored glass I guess in a way could be considered "doped". A study of the various types' reaction to bombardment like this would be intereasting.
frequency is much higher than 20 Khz. I'd bet it's somewhere around 75 Khz which takes the tuned circuit out of the picture.
When it was brand new, i've measured the frequency at ~35KHz, modulated by
100Hz (double rectification of the mains' frequency).See:
I've been using Corning glassware and Corningware for years, and have never had a piece break while cooking. Dropping them is a different matter.
y 100Hz (double rectification of the mains' frequency). "
So they don't bother filtering, like a microwave. Hmmm.
er had a piece break while cooking. Dropping them is a different matter. "
Visions were different. Some sort of superglass. The commercial showed them melting a conventional metal pan in their glass pan. I would imagine it wa s not stainless, IIRC the melting point is quite high compared to aluminum or steel, or cast iron.
But these things didn't just break, they exploded. They are no longer on th e market, at least with the old formula.
It is pretty cool to be able to see what you're cooking like that. There wa s always Pyrex as well. One of the things visions touted was an extremely h igh thermal mass. In most cases if you wanted to cook say spaghetti, once i t came to a boil again with the pasta in the water, you supposedly could sh ut the heat off and it would cook fully.
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