DocID025567 Rev 1

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AN4406

Description

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With three reactive elements, there are 36 possible tank circuit configurations, of which 15

are usable with a voltage source input and two popular resonant inverter topologies can be

formed:

•

an LCC (one L and two Cs) resonant inverter – the load is connected in parallel with

one of the capacitors; commonly used in electronic, gas-discharge lamp ballasts;

•

an LLC (two Ls and one C) resonant inverter – the load is connected in parallel with

one inductor.

As previously stated, for any resonant inverter there is a corresponding DC-DC resonant

converter obtained by rectification and filtering of the inverter output. The above mentioned

inverters of course belong to the "class D resonant converters" category.

In off-line applications, the rectifier block is usually coupled to the resonant inverter through

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

efficiency of the inverter, the rectifier block can be configured as:

•

a full-wave rectifier (for low voltage / high current output) with a center tap arrangement

of the transformer's secondary winding

•

a bridge rectifier (for high voltage /low current output) without tapping.

The low-pass filter can be configured with capacitors only or with an L-C type smoothing

filter, depending on the configuration of the tank circuit. The so-called "series-parallel"

converter used in typical in high-voltage power supplies is derived from the LCC resonant

inverter described above. Its mirror configuration, the LLC inverter, generates the converter

with the same name.

We will consider the half-bridge implementation illustrated in Figure 3, but it can be easily

extended to the full-bridge version.

Figure 3. LCC resonant half-bridge schematic

In resonant inverters and converters, power flow is controlled via the switch network by:

•

changing the frequency of the square wave voltage, or its duty cycle, or both

•

by special control schemes such as phase-shift control.

We shall control power flow through frequency modulation by adjusting the frequency of the

square wave closer to or further from the tank circuit's resonant frequency, while keeping its

duty cycle fixed.

GIPG211120131509SR