To achieve ultra-deep dimming ratio in LED driver application, the resonant process of LLC resonant converter considering the critical parasitic components has been analyzed in detail with time-domain solution in this paper. The effects of three parasitic components, including the equivalent parasitic capacitor Cp in parallel with the transformer, the equivalent output capacitor Ca of the bridge switches and the parasitic capacitor CL of resonant inductor, on the resonant current during the dead-time and switching frequency under light-load condition have been taken account. Brief equivalent circuits during the resonant process of LLC resonant converter and corresponding mathematical functions based on time-domain analysis are presented. After that, accurate frequency curve of the converter and required dead-time in variation with output current have been calculated out.Theoretical analysis show that the LLC resonant converter can achieve ultra-low dimming ratio without entering the burst mode with appropriate external capacitor in parallel with the output of the bridge and zero voltage switching (ZVS) of switches by self-adjusting the dead-time. Finally, an experimental prototype has been built up to verify the theoretical analysis and 0.5% LED dimming ratio has been achieved. The theoretical analysis results meet the experimental results very well, which demonstrates that the time-domain solution presented in this paper can achieved very high accuracy.
INDEX TERMSLLC resonant converter, Light-emitting diode (LED) driver, wide dimming range, parasitic components, time-domain analysis. NOMENCLATURE VFMvariable frequency modulation Vo -output voltage Io -output current Lr -resonant inductor Cr -resonant capacitor N -turns ratio of transformer Q1upper arm switch of bridge Q2upper arm switch of bridge Cossoutput capacitor of switch Q1 or Q2 Cd -junction capacitor of a secondary side rectifier Ctf -parasitic capacitor of the transformer on the primary side CL -parasitic capacitor of the resonant inductor Lr Cp -equivalent parasitic capacitor in parallel with the primary winding of the transformer Ca -equivalent output capacitor of bridge switches Ce -external capacitor in parallel between the midpoint of bridge and primary ground vgH and vgL -driving signals of primary switches; ir -resonant current im -magnetizing current of transformer vcr -voltage across resonant capacitor Cr
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