AN2644 datasheet(6/64 Pages) STMICROELECTRONICS

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Classification of resonant converters

AN2644

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switching inverters driven by this kind of switch network are considered part of the group

called "class D resonant inverters".

As to resonant tanks, with two reactive elements (one L and one C) there are a total of eight

different possible configurations, but only four of them are practically usable with a voltage

source input. Two of them generate the well-known series resonant converter and parallel

resonant converter considered in [2] and thoroughly treated in literature.

With three reactive elements the number of different tank circuit configurations is thirty-six,

but only fifteen can be used in practice with a voltage source input. One of these, commonly

called LCC because it uses one inductor and two capacitors, with the load connected in

parallel to one C, generates the LCC resonant inverter commonly used in electronic lamp

ballast for gas-discharge lamps. Its dual configuration, using two inductors and one

capacitor, with the load connected in parallel to one L, generates the LLC inverter.

As previously stated, for any resonant inverter there is one associated DC-DC resonant

converter, obtained by rectification and filtering of the inverter output. Predictably, the above-

mentioned class of inverters will originate the "class D resonant converters". Considering

off-line applications, in most cases the rectifier block will be coupled to the resonant inverter

through a transformer to guarantee the isolation required by safety regulations. To maximize

the usage of the energy handled by the inverter, the rectifier block can be configured as

either a full-wave rectifier, which needs a center tap arrangement of transformer's secondary

winding, or a bridge rectifier, in which case tapping is not needed. The first option is

preferable with a low voltage / high current output; the second option with a high voltage /

low current output. As to the low-pass filter, depending on the configuration of the tank

circuit, it will be made by capacitors only or by an L-C type smoothing filter. The so-called

"series-parallel" converter described in [2], typically used in high-voltage power supply, is

derived from the previously mentioned LCC resonant inverter. Its dual configuration, the LLC

inverter, generates the homonymous converter, addressed in [3], [4] and [5], that will be the

subject of the following discussion. In particular we will consider the half-bridge

implementation, illustrated in Figure 3, but the extension to the full-bridge version is quite

straightforward.

Figure 3.

LLC resonant half-bridge schematic

Vin

Vout

Center-tapped output with full-wave

rectification

(low voltage and high current)

Single-ended output with bridge

rectification

(high voltage and low current)

a:1:1

a:1

LLC tank circuit

Preferably integrated into a single

magnetic structure

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