AN2644 datasheet(41/64 Pages) STMICROELECTRONICS

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AN2644

The LLC resonant half-bridge converter

41/64

This phenomenon is well-known in push-pull and half-bridge topologies and is sometimes

referred to as "flux doubling" because the transformer's magnetic flux excursion, which

normally swings by 2Bpk from -Bpk to +Bpk under steady state operation, in the first cycle

goes from 0 to 2Bpk. Fortunately, if the integrated magnetics approach is used, there will be

no risk of saturation. Flux doubling will concern only the resonant inductor Ls, which is

mostly associated to the transformer's leakage inductance that, by definition, cannot

saturate because the relevant flux is developed in air.

Although not inherently hazardous, (capacitive mode and ZVS loss will typically occur in the

first two-three cycles, then the associated stress level is practically negligible) this

phenomenon is not nice to see. To eliminate or, at least, minimize it, the split capacitor

configuration of Figure 4 can be used in some cases. In fact, the initial voltage across each

capacitor will be close to Vin/2 (equal to Vin/2 if the two capacitors had exactly the same

value), then there will be only a minimum transient.

However, this is ineffective with many control ICs with high-side MOSFET driving capability

using capacitive bootstrap. To guarantee an adequate precharge of the bootstrap capacitor

to correctly drive the high-side MOSFET Q1 since the first cycle, the low-side MOSFET Q2

is turned on for some time before starting to operate (as shown in Figure 26), which

discharges completely the lower Cr/2 capacitor. This bootstrap precharge mechanism,

shown in Figure 27, makes ineffective also any pull-up that could precharge Cr.

In this case the initial current peak cannot be completely eliminated, just reduced by either

using a higher starting frequency or by forcing the duty cycle to start from a value

considerably smaller than 50% and letting it widen progressively.

Figure 27.

High-side driving with bootstrap approach and bootstrap capacitor

charge path

2.7

Analysis of power losses

Conduction power losses on the primary side are located in the power MOSFETs, in the

resonant capacitor Cr and the transformer (for convenience the secondary winding losses

can be incorporated). On the secondary side, losses will be essentially located in the

secondary rectifiers, although those in the output capacitors cannot be neglected, at least

as far as capacitor selection is concerned.

Vin

Vcc

DBOOT

CBOOT

Cr / 2

a:1:1

Cr / 2

Vcc

Level

Shifter

Control

Logic

IBOOTCHARGE

Vin

Vcc

DBOOT

CBOOT

Cr / 2

a:1:1

Cr / 2

Vcc

Level

Shifter

Control

Logic

IBOOTCHARGE

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