Hello Everyone,

I need to design 4kW induction heating circuit that heats up the metal pipe. This metal pipe has circulating water inside. I did some calculations but i am still stuck at both LTSpice simulations and calculations. Here is my basic circuit in LTSpice.



As i know, i can use basic LLC calculations. So, i used the TI's Designing an LLC Resonant Half-Bridge Power Converter app note.

VDCmin = 283V, VDCmax = 358V
Nominal Switching Frequency = 100kHz
Min Switching Frequency = 55kHz
Max Switching Frequency = 150kHz

All the values except C5, L4, L5, R1 are calculated values. This circuit in LTSpice has 55kHz switching frequency at VDCmin and full load. With these values for C5 and L4, primary and secondary currents are almost pure sinusoidal and they are matched with calculated values but when i change parallel resonance frequency of C5 and L4 to lower value about 200kHz, these currents gets higher in value and become distorted.

Do i really need to push the parallel resonance frequency of C5 and L4 to about something 5MHz ? As i know, their parallel resonance frequency needs to be higher than max switching frequency.

How can i calculate the C5 and L4 ?

 

Try on line calculator,

https://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml

 

Try on line calculator,

https://www.daycounter.com/Calculators/LC-Resonance-Calculator.phtml

I mean these C5 and L4 values seem unrealistic. Max primary current in transformer is about 38A and max secondary is about 124A. By using secondary max current, secondary peak voltage and parallel LC resonance frequency, values of C5 and L4 are calculated. For example, i put the resonance frequency to 200kHz and get the value C5 = 1.4uF and L4 = 452nH. When i put these values to simulation currents get higher and distorted.

My question is that when the max switching frequency is about 150kHz, do i really need to push the parallel resonance frequency to about 5MHz in order to get pure resonant sinusoidal current ?

 

Tuned circuits should always be at resonant frequency for maximum Q and power output, if you're running at 200Khz, then set your cap and inductor to resonance.

 

Tuned circuits should always be at resonant frequency for maximum Q and power output, if you're running at 200Khz, then set your cap and inductor to resonance.

When i decrease the LC parallel resonance frequency in the secondary to switching frequency or series resonance frequency, primary and secondary sinusoidal resonant currents get very distorted.