NCP1360, NCP1365
www.onsemi.com
20
When the primary power MOSFET is turned on, the
primary current is illustrated by the green curve of
Figure 46. When the power MOSFET is turned off the
primary side current drops to zero and the current into the
secondary winding immediately rises to its peak value equal
to the primary peak current divided by the primary to
secondary turns ratio. This is an ideal situation in which the
leakage inductance action is neglected.
The output current delivered to the load is equal to the average
value of the secondary winding current, thus we can write:
I
out
+t i
sec
(
t
)
u+
I
p,pk
2N
ps
t
demag
t
sw
(eq. 3)
Where:
• t
sw
is the switching period
• t
demag
is the demagnetizing time of the transformer
• N
ps
is the secondary to primary turns ratio, where N
p
and N
s
are respectively the transformer primary and
secondary turns:
N
ps
+
N
s
N
p
(eq. 4)
• I
p,pk
is the magnetizing peak current sensed across the
sense resistor on CS pin:
I
p,pk
+
V
CS
R
sense
(eq. 5)
Internal constant current regulation block is building the
constant current feedback information as follow:
V
FB_CC
+ V
ref_CC
t
sw
t
demag
(eq. 6)
As the controller monitors the primary peak current via the
sense resistor and due to the internal current setpoint divider
(K
comp
) between the CS pin and the internal feedback
information, the output current could be written as follow:
I
out
+
V
ref_CC
8N
ps
R
sense
(eq. 7)
The output current value is set by choosing the sense
resistor value:
R
sense
+
V
ref_CC
8N
ps
I
out
(eq. 8)
Primary Side Regulation: Constant Voltage Operation
In primary side constant voltage regulation, the output
voltage is sensed via the auxiliary winding. During the
on−time period, the energy is stored in the transformer gap.
During the off−time this energy stored in the transformer is
delivered to the secondary and auxiliary windings.
As illustrated by Figure 47, when the transformer energy
is delivered to the secondary, the auxiliary voltage sums the
output voltage scaled by the auxiliary and secondary turns
ratios and the secondary forward diode voltage. This
secondary forward diode voltage could be split in two
elements: the first part is the forward voltage of the diode
(V
f
), and the second is related to the dynamic resistance of the
diode multiplied by secondary current (R
d
w I
s
(t)). Where this
second term will be dependant of the load and line conditions.
Figure 47. Typical Idealized Waveforms of a Flyback Transformer in DCM
0V
pa
IN
N
ps
V
N
*
pa
out
N
ps
V
N
V
AUX
(t)
()
sec
pa
out f
N
ps
VVI
N
+
time
t
demag
t
sw
time
time
,
,
ppk
spk
I
ps
I
N
=
,p
I
pk
t
on
I
p
(t)
I
s
(t), I
OUT
I
OUT
= <I
s(t)
>
