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[SOLVED] resonant frequency tunning

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Newbie level 4
Jan 1, 2015
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I am working in full bridge series resonant frequency. and the results shows that the current was not flowing probably. beside the L and C frequency was measured to be the same with switching frequency. so is nay one can help me about how can I tune the resonant tank to work at it is resonant frequency???



What are you talking about?

Is it an RF amplifier or a power supply?

A schematic or block diagram could help.


series resonant frequency
resonant tank

I like to collect oscillator circuits, and I have seen just a few where the oscillations come from a bare LC tank and transistors. They are tricky to adjust, in order to make them start up and maintain oscillations reliably. (This appears true in simulations and in real circuits I've tried.)

The L:C ratio needs to cooperate with the amount of Amperes going through the circuit. For example, when less current is available, then use a smaller capacitor and/or larger inductor.

Here is an oscillator where an LC tank is suspended between a full H-bridge (as you describe). It may not be the same as you are using, nevertheless it illustrates a concept of operation.

Notice the LC tank can have greater current going back and forth in it, than the current levels through the mosfets.

The LC tank may also be capable of generating higher voltage than the supply voltage. Notice the scope traces indicate the mosfets are shut off at the peak of the sinewaves.

Usually the relevant inductance of a series resonant converter is the transformer leakage inductance plus some wiring inductance. It's almost stable, but maybe you measured it incorrectly? A series resonant converter has low Q and shouldn't be too sensitive to resonance frequency deviations of 5 or 10 percent. Typically it's only tuned once during design and then reproduced as designed.

A simple way to check the correct tuning in operation is to monitor the phase difference between bridge output and voltage and current under load, which should zero.

Besides telling some circuit details, can you show basic waveforms to clarify that you actually have a tuning problem?

in LLC converter your LC resonant frequency should be whatever it is..but then your switching frequency should be between the upper and lower resonant frequencies...(W0 and Wp)

You can go above wo but get higher switching loss...but go below wp and your circuit go bang

AN-4151 by fairchildsemi have the graphs you then see that for a given vin and vout, you need a certain q factor, and a certain ratio of magnetising inductance and leakage inductance.

If you have wide vout range then you need low LP/LR...and this is a bit disappointing as it means higher circulating current.

Its also good to have a low Q so that you can get a wide load range....but low Q can mean a bigger Cr which means poorer output short circuit performance, and more inrush when there is a transient in the load.

Good luck, because the subject is shrouded in industrial secrecy...only this forum tells the secrets it seems to me.

Nice you mention AN-4151, but the OP is aking about series resonance converter, not LLC.
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Thanks, but are we sure?, poster says "series resonant frequency" and not "series resonant converter"......I think we need more info from the OP.
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