Hello,

I am trying to make open loop simulasion for a LLC circuit. I am confused at some point. I am designing my half bridge LLC converter for a specific output voltage of 12V at 250W. Then I change the load(input voltage is the same) and redo my calculation to find new switching frequency. Output voltage is not 12V but far away from it.

So, I think I do not know how to calculate new switching frequency at different laods with the same input voltage. I have read all application notes available and none helps. I need someone else's answer with previous experience of designing LLC circuit.

Thanks,
Alper

 

post your attempt

 

Attached below is my line of thought about designing a LLC where I am confused at.

 

An open loop circuit, by definition, has no feedback and thus will never be able to work with more than one set of conditions. So it will never be possible to have a simulation work for more than one condition.

 

Of course. Open loop works for one set of parameters.

Could you tell me how I set new switching frequency when the load changes from 250W to 60W while every other conditions stays the same.

 

SWitchmode power supplies do notadjust frequency to the load or input, but rather the duty cycle. and usually the filter circuit is not resonant.
SO FOR ANY DISCUSSION TO MAKE ANY SENSE AT ALL it will help to show what the design is and what the intention is. If this is to be a DC power supply then resonance in the filter must be avoided. If this is not intended to be a DC power supply then an explanation of what it IS must be provided.

 

I assume that it is an LLC-resonant converter.
It is necessary to avoid the resonant frequency, and the output voltage is controlled by varying the frequency not the mark-space ratio. A higher frequency moves further from the resonance and reduces the output.
It's not efficient at low loads.
Power Integrations make some ICs, and they have good application notes about how to design one. OnSemi's NCP1396 is another control IC which requires external MOSFETs.
Each has a frequency-setting resistor, and the feedback opto transistor connects in parallel with it, to increase the frequency as the output voltage increases.
I've made them successfully, but I didn't worry too much about excessive amounts of calculation. Just work out the resonant frquency, set the minimum frequency 10% above and the maximum frequency 10x the resonant and see if it works. It probably will, but it won't be able to regulate very well if the load is disconnected.
The main problem is finding the bobbins because they need to be two-section (which means the design is great for isolation between primary and secondary). Miles Platts will happily send you one sample, but you can't order less than a thousand.

 

I am working on open loop simulation of LLC resonant circuit and trying to understand correlation between load and switching frequency while all other parameters are the same. It is easy to understand frequency increases while load decreases and vice versa.

But the question is:

How do we calculate new switching frequency?