L6599

Application information

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Application information

The L6599 is an advanced double-ended controller specific for resonant half-bridge

topology. In these converters the switches (MOSFETs) of the half-bridge leg are alternately

switched on and OFF (180° out-of-phase) for exactly the same time. This is commonly

referred to as operation at "50% duty cycle", although the real duty cycle, that is the ratio of

the ON-time of either switch to the switching period, is actually less than 50%. The reason is

MOSFET and the turn-ON of the other one, where both MOSFETs are OFF. This dead- time

is essential in order for the converter to work correctly: it will ensure soft-switching and

enable high-frequency operation with high efficiency and low EMI emissions.

To perform converter's output voltage regulation the device is able to operate in different

modes (Figure 20), depending on the load conditions:

1.

Variable frequency at heavy and medium/light load. A relaxation oscillator (see

"Oscillator" section for more details) generates a symmetrical triangular waveform,

which MOSFETs' switching is locked to. The frequency of this waveform is related to a

current that will be modulated by the feedback circuitry. As a result, the tank circuit

driven by the half-bridge will be stimulated at a frequency dictated by the feedback loop

to keep the output voltage regulated, thus exploiting its frequency-dependent transfer

characteristics.

2.

Burst-mode control with no or very light load. When the load falls below a value, the

converter will enter a controlled intermittent operation, where a series of a few

switching cycles at a nearly fixed frequency are spaced out by long idle periods where

both MOSFETs are in OFF-state. A further load decrease will be translated into longer

idle periods and then in a reduction of the average switching frequency. When the

converter is completely unloaded, the average switching frequency can go down even

to few hundred Hz, thus minimizing magnetizing current losses as well as all frequency-

related losses and making it easier to comply with energy saving recommendations.

Figure 20. Multi-mode operation