AN2644 datasheet(12/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

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Figure 7.

Equivalent schematic of a transformer including parasitic capacitance

2.2

The switching mechanism

Still another way of classifying resonant converters would include the LLC resonant half-

bridge in the family of "resonant-transition" converters. This nomenclature refers to the fact

that in this class of converters power switches are driven in such a way that a resonant tank

circuit is stimulated to create a zero-voltage condition for them to turn-on.

To understand how this can be achieved in the LLC half-bridge, it is instructive to consider

the circuits illustrated in Figure 8 where the switches Q1 and Q2 that generate the square

wave input voltage to the resonant tank are power MOSFETs. Their body diodes DQ1, DQ2

are pointed out because they play an important role. In circuit a), the drain-to-source

parasitic capacitances Coss1, Coss2 are pointed out as well. In fact, as far as voltage

changes of the node HB are concerned, the parasitic capacitances Cgd and Cds are

effectively in parallel, then Cgd+Cds=Coss has to be considered.

Additionally, other contributors to the parasitic capacitance of the node HB (e.g. that formed

between the case of the power MOSFETs and the heat sink, the intrawinding capacitance of

the resonant inductor, etc.) are lumped together in the capacitor CStray. Note that Coss1, like

Coss2, is connected between the node HB and a node having a fixed voltage (Vin for Coss1,

ground for Coss2). Then, as far as voltage changes of the node HB are concerned, Coss1 is

effectively connected in parallel to Coss2 and CStray. It is convenient to lump all of them

together in a single capacitor CHB from the node HB to ground, as shown in the circuit b):

Equation 4

which we will refer to in the following discussion. Note also that Coss1 and Coss2 are non-

linear capacitors, i.e. their value is a function of the drain-to-source voltage. It is intended

that their time-related equivalent value will be considered (see Appendix A).

OSS1

OSS2

Stray

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