DocID025567 Rev 1

3/27

AN4406

Description

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1

Description

To meet the ever-increasing demand for higher power density in consumer applications like

notebooks, high-power adapters (over 150 W), desktop PCs, FPDTV, gaming SMPS,

lighting, power supplies used for telecommunication equipment, mainframe computers and

high-power systems in general (over 500 W), component counts, power loss, heat-sinks

and reactive component sizes must be reduced.

The LLC resonant half bridge [1] represents a new alternative to the typical hard-switched

half (full) bridge topology, whereby the load enables commutation of the bridge switches

with near-zero voltage or current switch conditions, resulting in low switching losses and

thus eliminating power loss due to overlapping switch current and voltage at each transition.

With this technique, the switching losses associated with the main power switching remain

low even when the system operates at high frequencies, allowing for reduced component

reactive sizes and simplified thermal management.

According to the resonance principle, each reactive component in the circuit contributes to

the overall working frequency. As the frequency of the load range in the LLC topology is

influenced by the magnetic transformer, two main working frequency values can be

distinguished.

When the system operates under a light load, the effects of the intrinsic parasitic

capacitances of the power MOSFET can impact both operation and switching power loss,

resulting in decreased efficiency.

Resonant conversion has attracted concerted academic and industry research efforts over

the last few decades because of the associated waveform, efficiency and power density

improvements. However, the use of this technique in off-line powered equipment has long

been confined to niche applications, such as high-voltage power supplies and audio

systems.

Recent applications like flat panel TVs and the introduction of new voluntary and mandatory

regulations concerning efficient energy use are pushing power designers to find increasingly

efficient AC-DC conversion systems, promoting renewed interest in resonant conversion.

1.1

Resonant converters

Resonant converters form an extremely vast family of devices that are not easily gathered

under one comprehensive definition. Generally speaking, they are switching converters with

a tank circuit which influences the input-to-output power flow.

Most resonant converters are based on "resonant inverters", which are systems that convert

DC into sinusoidal voltages (or AC voltages with low harmonic content) and provide AC

power to a load [2]. To do so, a switch network typically produces a square-wave voltage

applied to a resonant tank tuned to the fundamental component of the square wave. In this

way, the tank responds primarily to this component and negligibly to the higher order

harmonics, so that its voltage and/or current, as well as those of the load, are essentially

sinusoidal or piecewise sinusoidal.

Figure 1 shows a resonant DC-DC converter providing DC power to a load by rectifying and

filtering the AC output of a resonant inverter.