# Induction heater - question about resonance frequency for LC filter design

#### Marcus2012

Joined Feb 22, 2015
425
Hey everyone

I'm designing my induction heater and I'm a bit confused about something and was hoping someone could clear it up for me please.

Basically its the relationship between LC resonance frequency and supply frequency. I am current unclear on the relationship between these two. Is the ideal aim to match the LC filter resonance frequency to the supply frequency as closely as possible? and then use a L-match filter to match impedance? I'm currently looking at the 10-50KHz range in order to heat shallow/medium thickness materials.

Sorry if this seems a silly question but the maths on this has got me a bit confused so thanks in advance to anyone who can help

Cheers guys

#### #12

Joined Nov 30, 2010
18,224
The obvious answer is: "The power supply frequency has nothing to do with this." Therefore your question is probably incomplete.

#### Marcus2012

Joined Feb 22, 2015
425
Hey #12, thanks

But I'm just a little confused as I read this in an article (link below) but as I said my understanding is still limited so maybe I read it out of context, I am just really unsure now.

REVIEW:
• A capacitor and inductor directly connected together form something called a tank circuit, which oscillates (or resonates) at one particular frequency. At that frequency, energy is alternately shuffled between the capacitor and the inductor in the form of alternating voltage and current 90 degrees out of phase with each other.
• When the power supply frequency for an AC circuit exactly matches that circuit's natural oscillation frequency as set by the L and C components, a condition of resonance will have been reached.
http://www3.eng.cam.ac.uk/DesignOffice/mdp/electric_web/AC/AC_6.html

#### #12

Joined Nov 30, 2010
18,224
I'm sticking with the first answer. 50 Hz is so far from 10,000 Hz that the effect will be microscopic.

Joined Jul 18, 2013
28,013

#### #12

Joined Nov 30, 2010
18,224
"The resonant tank is coupled to the power source with a coupling transformer. The transformer is connected to the inverter."

"The inverter chops the DC power source at a particular frequency. This is the resonant frequency of the tank."

I only see 2 frequencies here, the power supply and the resonant tank. Color me puzzled.

#### Marcus2012

Joined Feb 22, 2015
425
so my supply frequency from my inverter (from full bridge to tank) has to match my tank circuit resonance frequency? or be close. right?

EDIT fyi no transformer in my version

#### #12

Joined Nov 30, 2010
18,224
Now you say you're using an inverter. When you place a capacitor across the output of the inverter, you discard its frequency...mostly. Are you planning on running a switch mode inverter with no output capacitor?

#### Marcus2012

Joined Feb 22, 2015
425
Apologises if I caused confusion there, I could of articulated that better.

I am using a full bridge MOSFET inverter. I have designed this and now I am just trying to understand the relationship between my inverter output and the tank circuit resonance. The inverter in question has no output capacitor, I have seen on diagrams but was unsure to its purpose at the time.

#### #12

Joined Nov 30, 2010
18,224
Are you talking about an inverter to lower the power line voltage or an inverter to create the heating frequency?

Joined Jul 18, 2013
28,013
The Mazilli/Royer flyback transformer principle is very simple as shown in the link for the one I made.
Self oscillating.
Max.

Last edited:

#### #12

Joined Nov 30, 2010
18,224
In the Royer schematic there are only 2 frequencies: DC and the heating frequency.

Are you saying that NOT matching the load frequency resonance to the driver frequency is an option?

Joined Jul 18, 2013
28,013
In the Royer schematic there are only 2 frequencies: DC and the heating frequency.
Yes, the resonance of the tank!
Max.

Joined Jul 18, 2013
28,013
.
• When the power supply frequency for an AC circuit exactly matches that circuit's natural oscillation frequency as set by the L and C components, a condition of resonance will have been reached.
Or, Resonant frequency occurs at the point the inductive reactance equals the capacitive reactance.
Max.

#### Marcus2012

Joined Feb 22, 2015
425
Are you talking about an inverter to lower the power line voltage or an inverter to create the heating frequency?
An inverter to create the heating frequency that's what I have designed

#### #12

Joined Nov 30, 2010
18,224
I thought resonance in the load tank was imperative. Why else would anybody attach 27 capacitors in parallel with the load coil?

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#### Marcus2012

Joined Feb 22, 2015
425
This was what I am aiming for minus the gate driver circuitry with MOSFETs operating in pairs diagonally with a L-match network, so if I use that as an example the tank resonance is 211920kHz (this would be the desired frequency for the specific work piece presumably?). My H-bridge, will that have to generate a modified sine wave of the same frequency from the DC source?

Source http://www.richieburnett.co.uk/indheat.html

#### #12

Joined Nov 30, 2010
18,224
That page you referenced says how to deal with the frequency and resonance several times.

"The worst thing that can happen is that the tank circuit becomes detuned such that its natural resonant frequency is just above the operating frequency of the inverter."

"In order to reduce the power throughput the inverter is normally detuned on the high side of the tank circuits natural resonant frequency."

"(This can be contrasted with the situation that would occur if the inverter were detuned on the low side of the work coil's resonant frequency. ZVS is lost, and the free-wheel diodes see forced reverse-recovery whilst carrying significant load current.)"

#### #12

Joined Nov 30, 2010
18,224
Adding a core to an air core inductor usually increases its inductance and that lowers the tank frequency. This is good because it doesn't trigger the penalties in post #19. When the curie point is reached, the inductor goes back (close) to the inductance it had as an air core inductor. Apparently your goal is to adjust the driver frequency to the resonance of the tank circuit, or just a bit higher.