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Old 26th August 2012, 08:05 AM   #1
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Join Date: Jun 2011
Default LLC converter

Hello,

I am doing an LLC resonant converter for guitar amplifier supply.

I am not using an integrated trafo as its too expensive.

I am using a simple trafo with one layer pri and sec, then an external "leakage" inductor.

I am assuming, since the "internal" leakage is low, that my design equations are as per the page 3 of AN4151:

AN4151
http://www.fairchildsemi.com/an/AN/AN-4151.pdf



I am finding that i always need to use an "m" value of around 4.5 or less.

This is because if any higher, then the "f0" frequency is too close to the peak gain frequency, and there is then the danger that i may end up in the capacitive region after load transients.

Do you find this too?

Anyway, if operated at the resonant frequency [w = 1/(sqrt(Lr.Cr))] then a LLC resonant converter should not need to change switching frequency as the load changes.........this is fantastic as it means that the transient response will be very quick......do you agree?

.......However, highly transient loads cause a problem for LLC resonant converters.....do you agree?......this is because the sudden draw of current can overcharge the resonant capacitor and cause it severe overvoltage stress.

An LLC control chip should have protection against this, but all the LLC controllers that i've seen do not have quick accurate shut off of the switching FETs when primary overcurrents occur..

In all cases , i find that the reference voltage into the overcurrent comparator is always poorly toleranced, being always at least +/40%.

Also, many LLC controllers have a 1 or 2us delay between the tripping of the overcurrent comparator and the switch off of the FETs.

This is just not acceptable, and means that LLC resonant converters just can't be used for Class D amplifier supplies....do you agree?
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