NCP1015 ON Semiconductor, NCP1015 Datasheet - Page 17

• Manufacturer Part Number: NCP1015
• Description: Self-supplied Monolithic Switcher For Low Standby-power Offline Smps W/out Ovp
• Manufacturer: ON Semiconductor

whereas a 10 ms ripple naturally affects the final voltage
available on the transformer end. Once the Bulk capacitor
has been selected, one should check that the resulting ripple
(min V
As an example, to benefit from the largest operating range,
a 7 W board was built with a 47 mF bulk capacitor which
ensured discontinuous operation even in the ripple
minimum waves.
according to Equation 15. This option is only valid for low
power applications, e.g. below 5 W, otherwise chances exist
to destroy the MOSFET. After evaluating the leakage
inductance, you can compute C with Equation 15. Typical
values are between 100 pF and up to 470 pF. Large
capacitors increase capacitive losses.
network. You calculate R
following formulas:
R
value N x (V
a MUR160 represents a good choice. One major drawback
of the RCD network lies in its dependency upon the peak
CVcc
clamp
Please note that these calculations assume a flat DC rail
Figure 26A: The simple capacitor limits the voltage
Figure 26B: The most standard circuitry called the RCD
V
clamp
HV
+
bulk
2 @ V
is usually selected 50-80 V above the reflected
?) is still compatible with the above calculations.
out
C
clamp
clamp
+ V
NCP1015
1
2
3
4
A
f
@ (V
). The diode needs to be a fast one and
+
L
V
6
5
clamp
8
7
leak
ripple
clamp
@ I
* (V
@ f
V
p
Figure 26. Different Options to Clamp the Leakage Spike
2
clamp
C
sw
@ f
out
and C
@ R
sw
) V
clamp
HV
f
clamp
sec) @ N)
CVcc
using the
(eq. 27)
(eq. 28)
http://onsemi.com
Rclamp
NCP1015
B
17
NCP1015
1
2
3
4
MOSFET Protection
excursion to a safe value, e.g. below the MOSFET BVdss
which is 700 V. Figures 26A, B, and C present possible
implementations:
current. Worse case occurs when I
and V
all three but it offers the best protection degree. If you need
a very precise clamping level, you must implement a zener
diode or a TVS. There are little technology differences
behind a standard zener diode and a TVS. However, the die
area is far bigger for a transient suppressor than that of zener.
A 5 W zener diode like the 1N5388B will accept 180 W peak
power if it lasts less than 8.3 ms. If the peak current in the
worse case (e.g. when the PWM circuit maximum current
limit works) multiplied by the nominal zener voltage
exceeds these 180 W, then the diode will be destroyed when
the supply experiences overloads. A transient suppressor
like the P6KE200 still dissipates 5 W of continuous power
but is able to accept surges up to 600 W @ 1 ms. Select the
zener or TVS clamping level between 40 to 80 volts above
the reflected output voltage when the supply is heavily
loaded.
As in any Flyback design, it is important to limit the drain
Figure 26C: This option is probably the most expensive of
Cclamp
8
7
6
5
D
out
is close to reach the steady-state value.
HV
CVc
c
p
and V
NCP1015
2
3
4
1
in
C
Dz
are maximum
8
7
6
5
D