L6599A
Application information
7
Application information
The L6599A is an advanced double-ended controller specific for resonant half bridge
topology (see Figure 21). 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 that there is an internally fixed deadtime TD inserted between the turn-off of
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 20), depending on the load conditions:
1. Variable frequency at heavy and medium/light load. A relaxation oscillator (see
Section 7.1: 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 20. Multi-mode operation of the L6599A
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Doc ID 15308 Rev 7
15/35
STMICROELECTRONICS [ ST ]
