The 0 voltage that one of advantages of LLC are comparing MOSFET of former edge of the implementation inside wide laden range namely are enlightened (ZVS) , the enlightened loss of MOSFET is theoretic fall for 0. Want to assure the ZVS of MOSFET of LLC former edge, need below full enough 3 postulate:

 

1) fluctuation switch is in charge of 50% occupy sky comparing, the driving voltage of 1800 semmetry is undee;

 

2) perceptual and syntonic antrum has enough perceptual electricity;

 

3) should enough dead band time maintains ZVS.

 

The circuit of resonance of LLC series connection that graph A) is a model. Graph B) is the working weaveform of the MOSFET below perceptual load. Because perceptual load falls, electric current is phasic go up meeting lead voltage, because of the ZVS that this assured to MOSFET moves. Want to make sure MOSFET moves in perceptual area, the syntonic electric current on syntonic inductance must enough big, in order to ensure MOSFET source leaks equivalent the charge of the memory on parasitism capacitance is OK be released completely clean inside dead band time.

 

 

Be in when the MOSFET of former edge when closing position, medium syntonic electric current meets circuit of series connection resonance be in charge of the equivalent of MOSFET to output capacitance to undertake full discharging to switch. Following plan institute show the equivalent circuit when MOSFET closes:

 

 

Pass the analysis that pursues to going up, can reach need satisfies two requirement of ZVS, following:

 

 

Although,formula looks simple, however an actual condition that outputs capacitance Ceq about MOSFET equivalent, the equivalent parasitism capacitance that is MOSFET is a source the function of Vds of leakage pole voltage, the article before has had detailed theory and actual introduction to the equivalent parasitism capacitance of MOSFET. , that is to say, the size that equivalent capacitance is worth can change as the change of Vds. Following plan institute are shown, it is with the IPP60R190P6 of Infineon exemple:

 

 

The cent of Vds discharge process of MOSFET of circuit of resonance of LLC series connection is 4 phase, following plan institute are shown, (I) 380V-300V; (II) 300V-200V; (III) 200V-100V; (IV)100V-0V.

 

 

Can see from inside the graph, (I) and (two parts of IV) held Vds discharge time near 2/3, the inductance electric current of syntonic antrum is basic and right now constant. This two parts held the major time that Vds discharges, main reason is close to at dropping when Vds at 0 when, the addition of the meeting index of parasitism capacitance Coss between MOFET source leakage. Because this wants to release the charge of this one part completely, need longer LLC syntonic cycle and release time.

 

The MOSFET(with accordingly right choice is enough) of small equivalent parasitism capacitance, crucial to the implementation of ZVS, be close to when Vds especially at 0 when, equivalent output electric capacity wants enough small, still can reduce dead band time further so and improve the work efficiency of LLC.

 

Next graphs explain how to choose appropriate ZVS plan further.

 

 

Graph (A) : Ideal ZVS is undee;

 

Graph (B) : Vds has not dropped 0, vgs has appeared. Below this kind of circumstance, resonance of LLC series connection can produce hard switch. Answered plan needs to reduce the excitation electric current of transformer, perhaps increase dead band time appropriately (if IC make choice of, dead band time was secured commonly) ;

 

Graph (C) : Realized ZVS, but the abidance that the electric current of syntonic antrum can’t maintain the diode inside MOSFET body guides.

 

Graph (D) dead band time is too long, can reduce the work efficiency of whole LLC.

 

Anyhow, it is crucial that the equivalent of MOSFET outputs the capacitance implementation to MOSFET ZVS of LLC former edge. If MOSFET already make choice of, syntonic antrum need is calculated carefully, debug and set, choose right dead band time, will cover all laden application range. The design objective that the hard switch that in applying actually, runs to stable state can have correction achieving stability thereby moving through the design. Switch on the mobile phone however the hard switch in the process (soft open high frequency in low frequency process) , switch on the mobile phone especially a cycle of a few switch in the process, to some designs and plan, hard switch cannot avoid.