# Resonant Converter Simulation Problem

1. ## Resonant Converter Simulation Problem

Hi,

I need to simulate following circuit via LTSpice but when i set LC tank resonance frequency to 25kHz, i am not able to get sinusoidal current in LC tank network. If i set resonance frequency to about 732kHz, current gets sinusoidal. Whats the wrong with circuit ? Any idea ?

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2. ## Re: Resonant Converter Simulation Problem

How do you determine resonant frequency? Resonant converter can be hard switching series resonant LC converter or better soft switching LLC converter. Resonance frequency is mainly determined by the leakage inductance and series capacitance.

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3. ## Re: Resonant Converter Simulation Problem

Originally Posted by FvM
How do you determine resonant frequency? Resonant converter can be hard switching series resonant LC converter or better soft switching LLC converter. Resonance frequency is mainly determined by the leakage inductance and series capacitance.
Actually, this circuit is for 4kW induction heating. May i derive this circuit from LLC circuit ? I am really confused with induction heating. In normal LLC configurations, i see series L and C before transformer but in induction heating i see parallel L and C at output. Also sometimes C or L are used for impedance matching. In LLC, output is known but in IH, i dont know what the output voltage is.

Is attached circuit series LLC right ?

How can i apply parallel LLC to induction heating ?

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4. ## Re: Resonant Converter Simulation Problem

I think you are mixing several topics.

- An induction heater may use a parallel capacitor to compensate the low coil inductance (air coil or coil with large air gap)
- To convert the output switcher square wave to sine, you need a series inductor, implemented either as separate inductor or a by the transformer leakage inductance, and a series capacitor to put it in resonance. The function can be achieved by C1 || C4 in your schematic, C6 isn't needed.
- Finally, you want a certain amount of inductive load to allow zero voltage switching. Respectively, a parallel LC circuit would be tuned to a resonance above the switching frequency.

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5. ## Re: Resonant Converter Simulation Problem

Originally Posted by FvM
I think you are mixing several topics.

- An induction heater may use a parallel capacitor to compensate the low coil inductance (air coil or coil with large air gap)
- To convert the output switcher square wave to sine, you need a series inductor, implemented either as separate inductor or a by the transformer leakage inductance, and a series capacitor to put it in resonance. The function can be achieved by C1 || C4 in your schematic, C6 isn't needed.
- Finally, you want a certain amount of inductive load to allow zero voltage switching. Respectively, a parallel LC circuit would be tuned to a resonance above the switching frequency.
Yes, i absolutely mixed several topics.

As i know, in order not to saturate transformer in half bridge, a capacitor in series with L3 is needed but when i removed C6, circuit was working. Transformer is also required for the isolation in this project.

why does the parallel resonance frequency must be above the switching one ?

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6. ## Re: Resonant Converter Simulation Problem

why does the parallel resonance frequency must be above the switching one?
Make the total load impedance inductive to achieve zero voltage switching and reduce switching losses. General consideration about LLC operation modes apply.

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7. ## Re: Resonant Converter Simulation Problem

Originally Posted by FvM
Make the total load impedance inductive to achieve zero voltage switching and reduce switching losses. General consideration about LLC operation modes apply.
Here is the final circuit having leakage inductance 15u. L4 and C5 has resonance frequency about 5MHz. I know their value seem incorrect but i got the sine current in primary and secondary in this case. When i set down the this resonance frequency little bit, sine current gets distortion and/or triangular wave.

In LLC design, we have fixed output voltage but in induction heating, we dont have specific fixed output voltage which is required for LLC design.

Is it OK to design only with simulation ? I asked it because i dont know how to calculate the currents or voltage for selecting the components in induction heating ?

I am really struggling at how to apply LLC calculations for induction heating.

8. ## Re: Resonant Converter Simulation Problem

I am now totaly confused. When i make the load inductive which means parallel LC tank resonance is above the sw frequency and the series LC network before transformer, sine currents get distorted in LC tank network. I have been simulating in LTSpice but nothing worked. I also used FHA model in app note of TI about LLC but it uses sine input voltage and gives nothing about ZVS. When i use transformer before LC parallel tank, LC parallel tanks become 211nF and 7uH (primary referred). Secondary referred values are 3.3uF and 437nH. I can also step down to 230VAC to 48VDC for H-Bridge. I dont know which way is better.

As i know, parallel LC tank must be used in resonance but when i use them in resonance, they can make the total circuit capacitive.

Another question is whether i should use the FHA model in LTSpice for induction heating ?

I really need help, this is my first project about resonance topology.

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