l6599atn STMicroelectronics, l6599atn Datasheet - Page 12

• Manufacturer Part Number: l6599atn
• Description: Improved High-voltage Resonant Controller
• Manufacturer: STMicroelectronics

Application information
6
12/31
Application information
The L6599AT is an advanced double-ended controller specifically designed for resonant
half-bridge topology (see
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
actually less than 50%. The reason is that there is an internally fixed dead-time T
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
1.
2.
Figure 3.
Variable frequency at heavy and medium/light load. A relaxation oscillator (see
Section 6.1: Oscillator
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.
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.
Multi-mode operation of the L6599AT
(Figure
3), depending on the load conditions:
Figure
for more details) generates a symmetrical triangular waveform,
Doc ID 15534 Rev 1
4.). In these converters the switches (MOSFETs) of the
D
L6599AT
inserted