Datasheet
NCP1351
http://onsemi.com
19
Latch Input
The NCP1351 features a patented circuitry which
prevents the FB input to be of low impedance before the V
CC
reaches the VCC
ON
level. As such, the circuit can work in
a primary regulation scheme. Capitalizing on this typical
option, Figure 24 shows how to insert a zener diode in series
with the optocoupler emitter pin. In that way, the current
biases the zener diode and offers a nice reference voltage,
appearing at the loop closure (e.g. when the output reaches
the target). Yes, you can use this reference voltage to supply
a NTC and form a cheap OTP protection.
Figure 24. The Latch Input Offers Everything Needed
to Implement an OTP Circuit. Another Zener Can
Help combining an OVP Circuit if Necessary
C2
100n
V
CC
R1
2.5k
C1
100nF
LatchFB
C3
100nF
5V
OVP
D2
R
pulldown
Figure 25. You can either directly observe the V
CC
level or add a small RC filter to reduce the leakage inductance
contribution. The best is to directly sense the output voltage and reacts if it runs away, as offered on the right
side.
C4
100n
R4
2.2k
C5
1n
V
CC
Latch
C3
100nF
L
aux
R
OVP
D2
1N4937
R
pulldown
CV
CC
20F
V
CC
Latch
C3
100nF
C1
100nF
Aux
Sec
U1A
D4
OUT
CV
CC
22F
U1B
+
+
Design Example, a 19 V / 3 A
A Universal Mains Power Supply Designing a
Switch-Mode Power Supply using the NCP1351 does not
differ from a fixed frequency design. What changes,
however, is the regulation method via frequency variations.
In other words, all the calculations must be carried at the
lowest line input where the frequency will hit the maximum
value set by the C
t
capacitor. Let us follow the steps:
V
in
min = 100 Vdc (bulk valley in low-line conditions)
V
in
max = 375 Vdc
V
out
= 19 V
I
out
= 3 A
Operating mode is CCM
= 0.8
F
sw
= 65 kHz
1. Turn Ratio. This is the first parameter to consider.
The MOSFET BV
dss
actually dictates the amount
of reflected voltage you need. If we consider a
600 V MOSFET and a 15% derating factor, we
must limit the maximum drain voltage to:
V
ds_max
+ 600 0.85 + 510V
(eq. 17)
Knowing a maximum bulk voltage of 375 V, the clamp
voltage must be set to:
V
clamp
+ 510 * 375 + 135V
(eq. 18)
Based on the above level, we decide to adopt a headroom
between the reflected voltage and the clamp level of 50 V. If
this headroom is too small, a high dissipation will occur on
the RDC clamp network and efficiency will suffer. A
leakage inductance of around 1% of the magnetizing value
should give good results with this choice (k
c
= 1.6). The turn
ratio between primary and secondary is simply:
ǒ
V
out
) V
f
Ǔ
N
+
V
clamp
k
c
(eq. 19)
Solving for N gives:
N +
N
s
N
p
+
k
C
ǒ
V
out
) V
f
Ǔ
V
clamp
+
1.6
(
19 ) 0.8
)
135
(eq. 20)
+ 0.234