Application information

L6599AF

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DocID028175 Rev 3

6

Application information

The L6599AF is an advanced double-ended controller specific for resonant half bridge

topology (see Figure 4). 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%.

either MOSFET and the turn-on of the other one, where both MOSFETs are off. This

deadtime is essential in order for the converter to work correctly: it ensures soft-switching

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

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

modes (Figure 3), depending on the load conditions:

1.

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

Section 6.1: Oscillator for more details) generates a symmetrical triangular waveform,

which the MOSFET switching is locked to. The frequency of this waveform is related to

a current that is modulated by the feedback circuitry. As a result, the tank circuit driven

by the half bridge is stimulated at a frequency dictated by the feedback loop to keep the

output voltage regulated, therefore 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 enters 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 is 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 hertz, therefore minimizing magnetizing current losses as well as all

frequency-related losses and making it easier to comply with energy saving

recommendations.

Figure 3. Multi-mode operation of the L6599AF