STMicroelectronics 19V - 90W Adapter with PFC for Laptop computers using the L6563H and L6699 EVL6699-90WADP EVL6699-90WADP Fiche De Données
Codes de produits
EVL6699-90WADP
L6699
Application information
Doc ID 022835 Rev 2
19/38
Figure 11.
Comparison of initial cycles after startup: traditional controller (left), with
L6699 (right)
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.
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
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
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
, this
transition is nearly seamless and just a small perturbation of the tank current can be
observed.
observed.
Using capacitor values significantly different from those provided in
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.
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
circuit from pin 4 (
circuit from pin 4 (
RF
min
) to ground (see
Initially, the capacitor C
SS
is totally discharged, so that the series resistor R
SS
is effectively
parallel with RF
min
and the resulting initial frequency is determined by R
SS
and RF
min
only,
as the optocoupler's phototransistor is cut off (as long as the output voltage is not too far
away from the regulated value):
away from the regulated value):
Equation 4
The C
SS
capacitor is progressively charged until its voltage reaches the reference voltage (2
V) and, consequently, the current through R
SS
goes to zero. This conventionally takes 5
times the constants R
SS
·C
SS
, however, the soft-start phase really ends when the output
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