AN2644 datasheet(42/64 Pages) STMICROELECTRONICS

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The LLC resonant half-bridge converter

AN2644

42/64

Unless the converter is running at very high frequency, which would make turn-off losses

dominant, RDS(on) is usually the major source of power loss in power MOSFET. In the total

loss budget, however, the power wasted in power MOSFETs is usually a minor contributor,

especially when the converter is powered from the output of a PFC preregulator (400 V

typ.). It is not uncommon to see power MOSFETs running cool with minimum heat sinking.

The resonant capacitor Cr dissipates because of its own ESR. For this reason, especially

when very high efficiency is required, Cr should be a low-loss one, suited for AC/pulse

applications. Polypropylene film capacitors are the preferred choice.

Concerning the transformer, high frequency copper losses need to be particularly

addressed. In fact, eddy currents and proximity losses are considerable, especially in the

side-by-side winding arrangement because of the high transverse flux. Litz-type or

multistrand wire is a must for both primary and secondary windings.

Switching losses are essentially located in the power MOSFET Q1 and Q2. As previously

stated, the value of the switched current at heavy load (IRmin) is a trade-off between the

need for ensuring ZVS for both Q1 and Q2 and their turn-off losses. The higher the

switched current is, the higher the margin for ZVS will be, but at the expense of larger

switching loss due to voltage-current overlap. It is intuitive that the optimum design, in terms

of total dynamic losses minimization, is the one that uses for IRmin the minimum value

necessary to achieve ZVS. It still provides zero capacitive turn-on losses with minimum

switching losses at turnoff. Since component tolerance must be accounted for, and losing

ZVS must be avoided not only for efficiency reasons but also to prevent troubles, adequate

margin needs to be considered and the typical operation will be suboptimal.

Secondary rectifiers are usually the components where the majority of power losses occur.

Assuming the rectifiers are identical, their cumulative conduction losses are given by:

Equation 16

where Vth is the rectifier's threshold voltage and Rd its dynamic resistance. With bridge

rectification, losses are almost double because there are always two diodes in the

conduction path (we say "almost" because of the lower blocking voltage rating, so that Vth

and Rd are expected to be slightly lower for the same current rating). Hence this

arrangement is preferred when the output voltage is high. Firstly, the efficiency loss due to

the rectifiers (which is

∝ 2 VF/Vout) becomes less significant. Secondly, the higher the

output voltage is, the more a lower blocking voltage requirement becomes beneficial.

As compared to the ZVS Asymmetrical Half-bridge and the Forward converter, conduction

losses (for the same technology and blocking voltage) would be slightly greater because of

the worse current form factor that would increase the term, but this is compensated by the

absence of recovery and its associated losses. In the end, considering also that in the LLC

resonant half-bridge there is no secondary choke with its associated losses, it is expected

that the total secondary losses will be lower.

2.8

Small-signal behavior

It is essential to know the small-signal behavior of the LLC resonant converter to be able to

design the feedback loop. In line with the approach followed to describe its steady-state

thIout

–

dIout

rms

–

thIout

–

dIout

rms

–

()

⎩⎭

⎪⎪

⎨⎬

⎪⎪

⎧⎫

full - wave rectification

bridge rectification

AN2644 Datasheet(PDF) 42 Page - STMicroelectronics

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