Datasheet

NCL30000
http://onsemi.com
13
Figure 25. On Time Control
COMP
Ct
+
−
PWM
+
DRV
I
charge
t
on
V
Control
− Ct
(offset)
t
on(max)
V
Ct
V
Ct(off)
V
DD
DRV
V
Control
Ct
(offset)
V
EAH
V
Control
Off Time Sequence
In a fixed on-time CRM flyback converter, energy stored
in the primary of the flyback transformer varies directly with
input line voltage on a cycle-by-cycle basis. When the
switching cycle is terminated, the energy stored in the
transformer is transferred to the secondary. The auxiliary
winding used to provide bias to the NCL30000 is also used
to detect when the current in the secondary winding has
dropped to zero. This is illustrated in Figure 26.
Figure 26. Ideal CrM Waveforms with ZCD Winding
DRV
V
CL(NEG)
V
ZCD(TRIG)
V
ZCD(ARM)
V
CL(POS)
V
ZCD(WIND),on
V
ZCD(WIND),off
V
ZCD(WIND)
V
out
I
secondary
I
primary
MOSFET Conduction
Output Rectifier Conduction
t
on
t
diode
t
off
t
SW
0 V
0 V
0 V
0 V
0 A
0 A