L6699

Application information

Doc ID 022835 Rev 2

19/38

Figure 11.

Comparison of initial cycles after startup: traditional controller (left), with

L6699 (right)

It goes without saying that when either MOSFET is turned on for the very first time, this

occurs with a non-zero drain-to-source voltage. Therefore, strictly speaking, hard-switching

is still there. However, this type of one-shot hard-switching, where the body diode of the

other MOSFET is not reverse recovered, is of little concern. In fact, the related capacitive

power loss is thermally insignificant and, with a proper gate-drive circuit, spurious turn-on of

the other MOSFET through Cgd injection is easily prevented.

The timing diagrams of

Figure 11 compare the startup behavior of a resonant converter

driven by a traditional resonant controller with that of a converter driven by the L6699.

During the initial phase, the ramps of the oscillator are synchronized to the zero-crossings of

the tank current, so that a trapezoidal waveform appears across CF. As a result, the duty

cycle of the half bridge is initially considerably less than 50% and the tank current changes

its sign every half-cycle. The device goes to normal operation after approximately 50 µs

from the first switching cycle. If the timing capacitor CF is selected according to

Table 6, this

transition is nearly seamless and just a small perturbation of the tank current can be

observed.

Using capacitor values significantly different from those provided in

Table 6 might cause

large perturbations during the transition. This might bring the half bridge close to losing soft-

switching with a consequent activation of the capacitive-mode detection function.

With the L6699 the soft-start function is easily realized with the addition of an R-C series

as the optocoupler's phototransistor is cut off (as long as the output voltage is not too far

away from the regulated value):

Equation 4

voltage has got close to the regulated value and the feedback loop has taken over, so that

()

SS

min

start

R

//

RF

·

CF

·

3

1

=

f