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Induction Cooktop Circuit Design

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fethiyeli

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Hi, i need to design induction cooktop circuit having 4kW output power. I have 1kW and lower power levels SMPS background but i dont have any experience in resonant converter topologies. As i have read from app notes, it is very similar to resonant converters except transformer. In formulas, i see transformer turns ratio and output voltage but in this design, i dont know what the output voltage is. I would like to ask whether i can apply the resonant converter formulas to induction cooktop circuit design ? I am struggling applying formulas especially turns ratio and output voltage calculations.
 

Your "core" is the pan and your secondary is the pan
and the secondary "burden resistor" is the pan as
well. As this is highly end-user-variable I'd think that
attention should be paid to resonance tuning for
some coupling behavior (as bounded by wallplug,
performance and regulatory realities).

You might look at industrial / metalworking
induction heaters for inverter designs, there are
many, RF LDMOS or GaN FETs are likely for the
power devices, I don't recall much thought given
to real time tuning in these applications but you
are looking also for fine heat control where dudes
melting beer cans for fun, just go full out.

You could always look for service schematics
for high end appliances. Although these days it's
probably "module level replacement by qualified
personnel".
 
Your "core" is the pan and your secondary is the pan
and the secondary "burden resistor" is the pan as
well. As this is highly end-user-variable I'd think that
attention should be paid to resonance tuning for
some coupling behavior (as bounded by wallplug,
performance and regulatory realities).

You might look at industrial / metalworking
induction heaters for inverter designs, there are
many, RF LDMOS or GaN FETs are likely for the
power devices, I don't recall much thought given
to real time tuning in these applications but you
are looking also for fine heat control where dudes
melting beer cans for fun, just go full out.

You could always look for service schematics
for high end appliances. Although these days it's
probably "module level replacement by qualified
personnel".

Hi, thank you for your comment. Could you also advice for resonant controller ic ? I have found resonant controller ics such as ncp1399, fan7631 and ucc256303. I also would like to ask whether isolation is needed. We will use plastic enclosure.
 

I'm not sure a "resonant controller" is needed or wanted.
That naming usually applies to one / some of the resonant
transformer DC-DC converter topologies. But this in turn
relates to the complex switch drive needed for snubber,
source / shunt switches in such topologies.

In your case what you want is resonant frequency match
to the funky all-of-a-piece core / secondary / load mess.
That's not really like the converter application where you
have a single known fixed inductor-bundle (multiwinding
xfmr) - you have the problem of a varying LM{LRC} thing
which you may conceive as a transformer but good luck
getting secondary-side feedback.

Wireless power transmission outfits like ChargEdge have
been putting out a lot of stuff about near field power
transfer and how to make frequency-tuning for power
efficiency and scale, work.

Your cooktop would presumably have a tempered high
temp glass face that would be the human safety barrier
at the "business end". There could be "sneak paths"
that pertain to control knob, case, etc. and you'd
be wanting a power conversion products type to tell
you what can and can't be fielded, when it comes
to that. But for now I expect this is a bare bones
"get the water to boil, then worry about the logo"
effort that ought to focus on whether you get the
top line job, done. Resist scope-creep until someone
puts a value to it.
 
Do i need isolation ? As i have seen in induction cooker, only the control circuit was isolated, not the power stage. I missunderstood the project. This project is going to use to heat a metal pipe where water will circulate. Is it ok to insulate only between the coil and metal pipe ?
 

That ought to do, especially if the pipe is also earthed.
Probably wants some attention to durability of the
insulation against various fault scenarios like water
intrusion, gross overtemperature (like induction-heating
a dry pipe) and so on. FMEA fun, miss one and smell the
freshly baked torts.

Now you sound like an on-demand water heater, which
maybe you can find construction tear-downs for that
can show you what's managed to make it to market with
adequate (or at least compliant) safety qualities.
 
There will be temperature feedback from the metal pipe for over-temperature. For the safety, metal pipe i think should be earthed. We have observed in a product having only plastic insulation between the coil and metal pipe. Maybe we can put HF transformer at resonant converter power stage for the safety.
 

Hi,

I am stuck with designing impedance matching. I also need to use transformer for both galvanic isolation and impedance matching. There is not many resource for induction heating as i did resarch. I have been also doing simulation in LTSpice but i got pretty different results. How can i do calculations for impedance matching for 4kW output ?
 

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