NCP1280_05 ONSEMI [ON Semiconductor], NCP1280_05 Datasheet - Page 13

• Manufacturer Part Number: NCP1280_05
• Description: Active Clamp Voltage Mode PWM Controller for Off-Line Applications
• Manufacturer: ONSEMI [ON Semiconductor]

Feedforward Ramp Generator
compensate for changes in line voltage. A FF Ramp
proportional to V
line voltage changes, the FF Ramp slope changes
accordingly. The duty cycle will be adjusted immediately
instead of waiting for the line voltage change to propagate
around the system and be reflected back on V
feedforward current (I
charging an internal 10 pF capacitor (C
current proportional to I
(capacitor is discharged) once the Oscillator Ramp reaches
2.0 V. Please refer to Figure 2 for a functional drawing of the
Feedforward Ramp generator.
operating frequency and the required duty cycle. If the
operating frequency and maximum duty cycle are known,
I
where V
Max DC Comparator and t
FF
The NCP1280 incorporates line feedforward (FF) to
A resistor between V
I
FF
is calculated using the equation below:
is usually a few hundred microamps, depending on the
DC(inv)
I FF +
V
V
AUX(on)
AUX(off)
is the voltage on the inverting input of the
OUT2
OUT1
in
V
AUX
0 V
2 V
0 V
0 V
0 V
0 V
C FF
is generated and compared to V
FF
6.7 kW
in
). The FF Ramp is generated by
Soft−Start Voltage
on(max)
FF
V DC(inv)
and the FF pin (R
UV/OV Voltage
Figure 31. Soft−Start Timing Diagram (Using Auxiliary Winding)
. The FF Ramp is finished
t on(max)
is the maximum ON time.
125 kW
FF
) with a constant
EA
FF
.
) sets the
EA
http://onsemi.com
. If the
NCP1280
13
Figure 18 shows the relationship between I
For example, if a system is designed to operate at 200 kHz,
with a 60% maximum duty cycle at 100 V, the DC
can be grounded and I
feedforward resistor is calculated using the equation below:
controlled predominantly by the value of R
resistance seen into the FF pin is only 12 kW. If a tight
maximum duty cycle control overtemperature is required,
R
R FF +
FF
For a minimum line voltage of 100 V, the required
From the above calculations it can be observed that I
t on(max) + DC MAX
should have a low thermal coefficient.
I FF +
I FF
V in
T + 1
* 12.0 kW + 100 V
+
C FF
10 pF
f
+
SOFT−START
6.7 kW
200 kHz
6.7 kW
FF
V DC(inv)
1
0.888 V
is calculated as follows:
55.2 mA
T + 0.6
+ 5.0 ms
t on(max)
3.0 ms
* 12.0 kW [ 1.82 MW
125 kW
125 kW
5.0 ms + 3.0 ms
FF
+ 55.2 mA
and DC
FF
, as the
MAX
MAX
FF
pin
is
.