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
Operation at no load or very light load
Doc ID 022835 Rev 2
21/38
7
Operation at no load or very light load
When the resonant half bridge is lightly loaded or totally unloaded, its switching frequency
reaches its maximum value. To keep the output voltage under control and avoid losing soft-
switching in these conditions, there must be some current flowing through the transformer's
magnetizing inductance. This current can be kept relatively low because of the adaptive
deadtime function; however, it produces power losses that prevent the converter's no load
consumption from achieving very low values anyhow.
reaches its maximum value. To keep the output voltage under control and avoid losing soft-
switching in these conditions, there must be some current flowing through the transformer's
magnetizing inductance. This current can be kept relatively low because of the adaptive
deadtime function; however, it produces power losses that prevent the converter's no load
consumption from achieving very low values anyhow.
To overcome this issue, the L6699 enables the user to make the converter operate
intermittently (burst-mode operation), with a series of a few switching cycles spaced out by
long idle periods where both MOSFETs are in the OFF-state, so that the average switching
frequency can be substantially reduced. As a result, the average value of the residual
magnetizing current and the associated losses is considerably cut down, therefore
facilitating the converter to comply with energy saving specifications.
intermittently (burst-mode operation), with a series of a few switching cycles spaced out by
long idle periods where both MOSFETs are in the OFF-state, so that the average switching
frequency can be substantially reduced. As a result, the average value of the residual
magnetizing current and the associated losses is considerably cut down, therefore
facilitating the converter to comply with energy saving specifications.
The L6699 can be operated in burst-mode by using pin 5 (STBY): if the voltage applied to
this pin falls below 1.26 V, the IC enters an idle state where both gate-drive outputs are low,
the oscillator is stopped, the soft-start capacitor C
this pin falls below 1.26 V, the IC enters an idle state where both gate-drive outputs are low,
the oscillator is stopped, the soft-start capacitor C
SS
keeps its charge and only the 2V
reference at the
RF
min
pin stays alive to minimize IC consumption and V
CC
capacitor
discharge. The IC resumes normal operation as the voltage on the pin exceeds 1.26 V by 30
mV.
mV.
In the L6699, the half bridge stops and restarts during burst-mode are more accurately
controlled with respect to its predecessors: the last cycle before stopping and the first cycle
after restarting are such that the DC voltage across the resonant capacitor Cr always stays
close to its steady-state value Vin/2. In this way, the anomalous current peaks due to V·s
unbalance in the transformer are minimized.
controlled with respect to its predecessors: the last cycle before stopping and the first cycle
after restarting are such that the DC voltage across the resonant capacitor Cr always stays
close to its steady-state value Vin/2. In this way, the anomalous current peaks due to V·s
unbalance in the transformer are minimized.
To implement burst-mode operation, the voltage applied to the STBY pin needs to be related
to the feedback loop.
to the feedback loop.
input voltage range (e.g. when there is a PFC front-end).
Essentially, RF
max
defines the switching frequency f
max
above which the L6699 enters burst-
mode operation. Once f
max
is
fixed, RF
max
is found from the relationship:
Figure 14.
Narrow input voltage range
Figure 15.
Wide input voltage range
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