Application information

L6599AT

12/31

Doc ID 15534 Rev 3

6

Application information

The L6599AT is an advanced double-ended controller specifically designed 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 interval. 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

between the turn-off of either MOSFET and the turn-on of the other, where both MOSFETs

are off. This dead- time is essential for the converter to work correctly; it ensures soft-

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

To perform output voltage regulation of the converter, the device is capable of operating 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,

to which the MOSFETs' switching is locked. 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, thus exploiting its frequency-dependent transfer

characteristics.

2.

Burst-mode control with no or very light load. When the load falls below a certain 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 an OFF-state. A further load decrease is translated into longer

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

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

even a few hundred hertz, thus minimizing any 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 L6599AT