Datasheet

NCP1351
http://onsemi.com
12
Figure 10. The Current Injected into the Feedback Loop Adjusts the Switching Frequency
I
Ct
= 10 A
Controlled by the
FB Current
V
Ct
Maximum Frequency
Minimum Frequency
P
out
Decreases
P
out
Increases
Figure 11. In Light Load Conditions, the
Oscillator Further Delays the Restart Time
Figure 12. C
t
Voltage Swing at a Moderate
Loading
C
t
Voltage
C
t
Voltage
In light load conditions, the frequency can go down to a
few hundred Hz without any problem. The internal circuitry
naturally blocks the oscillator and softly shifts the restart
time as shown on Figure 11 scope shot.
Delays The Restart Time
In lack of feedback current, for instance during a startup
sequence or a short circuit, the oscillator frequency is pushed
to the limit set by the timing capacitor. In this case, the lower
threshold imposed to the timing capacitor is blocked to
500 mV (parameter V
fault
). This is the maximum power the
converter can deliver. To the opposite, as you inject current
via the optocoupler in the feedback pin, the off time expands
and the power delivery reduces. The maximum threshold
level in standby conditions is set to 6 V.
Over Power Protection
As any universal-mains operated converters, the output
power slightly increases at high line compared to what the
power supply can deliver at low line. This discrepancy
relates to the propagation delay from the point where the
peak is detected to the MOSFET gate effective pulldown. It
naturally includes the controller reaction time, but also the
driver capability to pull the gate down. If the MOSFET Q
g
is too large, then this parameter will greatly affect your
overpower parameter. Sometimes, the small PNP can help
and we recommend it if you use a large Q
g
MOSFET:
Figure 13. A Low-Cost PNP Improves the Drive
Capability at Turn-off
D1
1N4148
Q1
2N2907
DRV
GND