NCP4326
http://onsemi.com
21
The new patented soft−start is based on the flux
transformer reconstruction concept with leading edge mode
control during a startup sequence only.
A startup sequence can be arisen with the 3 following
cases:
1. The power supply unit is just plug on the main
supply, in this case there is a general startup.
2. The power supply unit is running but one or the
both outputs are disabled, thus by enabling the
output a new startup happened.
3. The power supply unit is running but the
secondary controller is in standby mode (STBY
pin grounded), when the standby mode is left, a
startup sequence happen if at least one of the
outputs is enable.
The idea of this soft−start is to reconstruct the flux image
inside the flyback transformer, and to compare this image
with a slow ramp up voltage on enable pin, to generate a
smooth increasing duty cycle in leading edge mode. The
leading edge mode control guarantees that the peak current
ramps up smoothly. Because the secondary duty cycle
finishes at the OFF−time end and starts just before.
At the end of the off time period and due to the primary
controller running in critical conduction mode; the flux in
the transformer is null, so the peak will start from zero to
reach the nominal value.
Figure 44 illustrates the driver synchronization in
soft−start sequence.
0
t
0
t
0
t
Verror
VCt
DRV
0
t
0
t
ID2
Transfo Flux
Vsync
OFF TimeON Time
EN voltage
Figure 44. Startup Sequence Illustrating the Leading Edge Mode Control
Due to the internal current source and the external
capacitor connected on enable pin (EN1 and EN2 pin); a
voltage ramp is generated that it fixes the soft−start time; by
playing with the capacitor value the soft−start time can be
adjusted to fit the application startup time.
