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

#### Dodgydave

Joined Jun 22, 2012
11,234
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?

View attachment 81822
Source http://www.richieburnett.co.uk/indheat.html
The resonant frequency with those values is 211Khz, so your oscillator for the H-bridge will have to be in that frequency range, here is a site with 3 versions http://danyk.cz/induk_en.html

#### alfacliff

Joined Dec 13, 2013
2,458
using the heating coil in an oscilator would correct the problem of detuning when putting the object to be heated in the coil field. using an invertor would require setting the frequency of the invertor drive to the output stage to keep the invertor at the same frequency the output tank resonates.

#### Marcus2012

Joined Feb 22, 2015
425
Thanks everyone, I think I finally understand this . With my square wave output from the inverter I have to match impedance between the inverter and the tank circuit so with the full bridge I attached the matching impedance is split across the two bridge legs, has anyone know how of a calculator to calculate the inductance of the matching inductors? I have been searching and I've found a few calculators but they seem like different networks.

#### Dodgydave

Joined Jun 22, 2012
11,234
Thanks everyone, I think I finally understand this . With my square wave output from the inverter I have to match impedance between the inverter and the tank circuit so with the full bridge I attached the matching impedance is split across the two bridge legs, has anyone know how of a calculator to calculate the inductance of the matching inductors? I have been searching and I've found a few calculators but they seem like different networks.

use the formula and substitute your values http://www.1728.org/resfreq.htm

#### Marcus2012

Joined Feb 22, 2015
425
use the formula and substitute your values http://www.1728.org/resfreq.htm
Hi
Thanks but I thought this was for the tank circuit? I need one for the L-match network, can I work that out from this or would I just end up with the same values as my tank circuit?

I've tried to understand this one but I'm not sure what values to use as I don't think I need a DC blocking capacitor with a full bridge. (presumably load capacitance and inductance are my tank circuit values?)

http://www.daycounter.com/Calculators/L-Matching-Network-Calculator.phtml