Electronics > Projects, Designs, and Technical Stuff

Simple induction heater

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sandalcandal:

--- Quote from: T3sl4co1l on October 17, 2021, 10:51:12 am ---Like to power up under load only, like a cooktop?

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Similar but more to prevent self-induced damage under a no load condition. [Although I guess that's what cooktops do?]

--- Quote from: T3sl4co1l on October 17, 2021, 10:51:12 am ---For the ZVS, just sensing supply current is enough; easy enough to do with an MCU, voltage and current sense, and a supply controller of some sort (switch, or linear or buck regulator), or even discrete logic if you're into that sort of thing.

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I was thinking along the lines of something much more analog, minimalist in the sense of using the least sophisticated components as a design challenge rather than minimalist in terms for making the design process as simple as possible. I was imagining using a feedback coil to generate negative feedback into the drive circuitry somehow to attenuate the drive power. Although I didn't think too much about how that would actually be achieved. When no load is applied I guess the voltage in the feedback coil would be increased. Not too sure how that could be used to attenuate power, particularly since we'd prefer to do so by increasing the drive frequency above resonance. Perhaps some analog conditioning to provide a (non constant) phase lead then mixing it into the drive signal? I can't remember the technical name for this sort of control, any RF people?  This shifts the switching point "forward" making it switch sooner each cycle, thereby increasing the switching frequency, this feeds back each cycle up to a point where the feedback amplitude can't drive the switching frequency any higher.

A more "complicated" means of achieving no-load drive attenuation in terms of the maths and modelling but "simpler" and minimalist in that it can be implemented with "primitive" analog components.

Edit: or was it phase lag that's needed for this? I think it was phase lead.

--- Quote from: T3sl4co1l on October 17, 2021, 06:12:36 am ---By popular request, a thread dedicated to this item:

Which if you wanted something stand-alone, just use a, like, 555, with pin 3 tied to pins 6+2 with a variable resistance, and 6+2 to GND with a cap, leave 7 open, 1 GND, 4+8 VCC.  Uh might want to adjust the supply voltage, 15V is pushing it for a 555, and nearly for the gate driver, so maybe reduce coil turns and run at 10-12V for instance.

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or an IR2153, it's a "555", half bridge gate driver, and a ~15V zener clamp for supply, the D version even have the bootstrap diode build in

T3sl4co1l:

--- Quote from: sandalcandal on October 17, 2021, 12:42:28 pm ---Similar but more to prevent self-induced damage under a no load condition. [Although I guess that's what cooktops do?]

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Ah, sure. One could implement a current or voltage limiting scheme.  Which is what ZVS does: the output characteristic is constant voltage (because again, it's a power mixer, so reflects whatever the input characteristic is), so least power comes at maximum Q.  For the CV-driven series resonant, it has to be either input voltage reduced, or driven frequency raised, to maintain output; basically resonance acts to invert the characteristic (so you get maximum current draw at maximum Q instead).

Frequency control has its own issues, as the dominant pole of the control loop is given by the difference between driven and resonant frequencies -- again, the inverter is a power mixer, and you get the superposition of driven and resonant modes on the tank, which interfere to give apparent amplitude modulation (plus whatever it does to phase, when measured by zero crossings, say).  But that's just a matter of going slow enough (with control compensation) for a known worst case coil Q.

--- Quote ---I was thinking along the lines of something much more analog, minimalist in the sense of using the least sophisticated components as a design challenge rather than minimalist in terms for making the design process as simple as possible. I was imagining using a feedback coil to generate negative feedback into the drive circuitry somehow to attenuate the drive power. Although I didn't think too much about how that would actually be achieved. When no load is applied I guess the voltage in the feedback coil would be increased. Not too sure how that could be used to attenuate power, particularly since we'd prefer to do so by increasing the drive frequency above resonance. Perhaps some analog conditioning to provide a (non constant) phase lead then mixing it into the drive signal? I can't remember the technical name for this sort of control, any RF people?  This shifts the switching point "forward" making it switch sooner each cycle, thereby increasing the switching frequency, this feeds back each cycle up to a point where the feedback amplitude can't drive the switching frequency any higher.

A more "complicated" means of achieving no-load drive attenuation in terms of the maths and modelling but "simpler" and minimalist in that it can be implemented with "primitive" analog components.

Edit: or was it phase lag that's needed for this? I think it was phase lead.

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Speaking of IR2153, there's an application circuit for it, I think, that uses output voltage to feed back to the timing components, say for an LLC resonant converter.  I think such an arrangement lacks one key feature, that it can't throttle down to zero -- most LLC controllers go to a burst/hysteretic mode at light load, while there's no way to arrange this here.  So it would work for supplies with known minimum loads.  Which is the case here, basically.

A downside is, as a "555", its output duty cycle may suck.  It doesn't have the T flip-flop of, like, TL494 and friends.  Does mean the input is AC (phase) sensitive, so you might in fact use AC feedback -- a recent thread here discussing an ultrasonicator used one of these this way.

Current is best sensed with a transformer, but a shunt could be used as well.  Or a capacitor divider, if you put the cap on the low side.  Voltage, easy enough by resistor divider.

Tim

--- Quote from: T3sl4co1l on October 17, 2021, 11:00:01 pm ---
--- Quote from: sandalcandal on October 17, 2021, 12:42:28 pm ---Similar but more to prevent self-induced damage under a no load condition. [Although I guess that's what cooktops do?]

--- End quote ---

Ah, sure. One could implement a current or voltage limiting scheme.  Which is what ZVS does: the output characteristic is constant voltage (because again, it's a power mixer, so reflects whatever the input characteristic is), so least power comes at maximum Q.  For the CV-driven series resonant, it has to be either input voltage reduced, or driven frequency raised, to maintain output; basically resonance acts to invert the characteristic (so you get maximum current draw at maximum Q instead).

Frequency control has its own issues, as the dominant pole of the control loop is given by the difference between driven and resonant frequencies -- again, the inverter is a power mixer, and you get the superposition of driven and resonant modes on the tank, which interfere to give apparent amplitude modulation (plus whatever it does to phase, when measured by zero crossings, say).  But that's just a matter of going slow enough (with control compensation) for a known worst case coil Q.

--- Quote ---I was thinking along the lines of something much more analog, minimalist in the sense of using the least sophisticated components as a design challenge rather than minimalist in terms for making the design process as simple as possible. I was imagining using a feedback coil to generate negative feedback into the drive circuitry somehow to attenuate the drive power. Although I didn't think too much about how that would actually be achieved. When no load is applied I guess the voltage in the feedback coil would be increased. Not too sure how that could be used to attenuate power, particularly since we'd prefer to do so by increasing the drive frequency above resonance. Perhaps some analog conditioning to provide a (non constant) phase lead then mixing it into the drive signal? I can't remember the technical name for this sort of control, any RF people?  This shifts the switching point "forward" making it switch sooner each cycle, thereby increasing the switching frequency, this feeds back each cycle up to a point where the feedback amplitude can't drive the switching frequency any higher.

A more "complicated" means of achieving no-load drive attenuation in terms of the maths and modelling but "simpler" and minimalist in that it can be implemented with "primitive" analog components.

Edit: or was it phase lag that's needed for this? I think it was phase lead.

--- End quote ---

Speaking of IR2153, there's an application circuit for it, I think, that uses output voltage to feed back to the timing components, say for an LLC resonant converter.  I think such an arrangement lacks one key feature, that it can't throttle down to zero -- most LLC controllers go to a burst/hysteretic mode at light load, while there's no way to arrange this here.  So it would work for supplies with known minimum loads.  Which is the case here, basically.

A downside is, as a "555", its output duty cycle may suck.  It doesn't have the T flip-flop of, like, TL494 and friends.  Does mean the input is AC (phase) sensitive, so you might in fact use AC feedback -- a recent thread here discussing an ultrasonicator used one of these this way.

Current is best sensed with a transformer, but a shunt could be used as well.  Or a capacitor divider, if you put the cap on the low side.  Voltage, easy enough by resistor divider.

Tim

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the data sheet shows a fet to short ct to ground as shutdown

there's another one variable frequency and 50% duty cycle, https://www.infineon.com/dgdl/dt98-4.pdf?fileId=5546d46254e133b401554de8f0885e14

SiliconWizard:

--- Quote from: sandalcandal on October 17, 2021, 09:32:14 am ---I wonder what a minimalist load detection circuit could be like. Maybe, a feedback/detection coil and circuit that compares the driven coil to the feedback coil?

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You could also use the same coil and a different driver circuit in parallel, just for this purpose.