Application information

L6699

12/38

Doc ID 022835 Rev 2

6

Application information

The L6699 is an advanced double-ended controller specific to resonant half bridge topology.

In these converters the 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 symmetrical

operation at “50% duty cycle”, although the real duty cycle, i.e. the ratio of the ON-time of

either switch to the switching period, is actually less than 50%. The reason is that there is a

one, where both MOSFETs are off. This deadtime is essential in order for the converter to

work correctly: it enables soft-switching and, then, high-frequency operation with high

efficiency and low EMI emissions.

best fits the transition times of the half bridge midpoint (adaptive deadtime). This allows the

user to optimize the design of the resonant tank so that soft-switching can be achieved with

a lower level of reactive energy (i.e. magnetizing current), therefore optimizing efficiency

under a broader load range, from full to light load.

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

modes (

Figure 1), 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 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 the 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

specifications.

Figure 4.

Multimode operation of the L6699