NCP1200
www.onsemi.com
14
If the leakage inductance is kept low, the MTD1N60E can
withstand accidental avalanche energy, e.g. during a
high−voltage spike superimposed over the mains, without
the help of a clamping network. If this leakage path
permanently forces a drain−source voltage above the
MOSFET BVdss (600 V), a clamping network is mandatory
and must be built around Rclamp and Clamp. Dclamp shall
react extremely fast and can be a MUR160 type. To calculate
the component values, the following formulas will help you:
R
clamp
=
2 @ V
clamp
@ (V
clamp
* (V
out
) Vf sec) @ N)
L
leak
@ Ip
2
@ Fsw
C
clamp
+
V
clamp
V
ripple
@ Fsw @ R
clamp
with:
V
clamp
: the desired clamping level, must be selected to be
between 40 V to 80 V above the reflected output voltage
when the supply is heavily loaded.
V
out
+ Vf: the regulated output voltage level + the secondary
diode voltage drop
L
leak
: the primary leakage inductance
N: the Ns:Np conversion ratio
F
SW
: the switching frequency
V
ripple
: the clamping ripple, could be around 20 V
Another option lies in implementing a snubber network
which will damp the leakage oscillations but also provide
more capacitance at the MOSFET’s turn−off. The peak
voltage at which the leakage forces the drain is calculated
by:
V
max
+ Ip @
L
leak
C
lump
Ǹ
where C
lump
represents the total parasitic capacitance seen
at the MOSFET opening. Typical values for Rsnubber and
Csnubber in this 4W application could respectively be 1.5
kW and 47 pF. Further tweaking is nevertheless necessary to
tune the dissipated power versus standby power.
Available Documents
“Implementing the NCP1200 in Low−cost AC−DC
Converters”, AND8023/D.
“Conducted EMI Filter Design for the NCP1200’’,
AND8032/D.
“Ramp Compensation for the NCP1200’’, AND8029/D.
TRANSient and AC models available to download at:
http://onsemi.com/pub/NCP1200
NCP1200 design spreadsheet available to download at:
http://onsemi.com/pub/NCP1200
ORDERING INFORMATION
Device Type Marking Package Shipping
†
NCP1200P40G
F
SW
= 40 kHz
1200P40 PDIP−8
(Pb−Free)
50 Units / Rail
NCP1200D40R2G 200D4 SOIC−8
(Pb−Free)
2500 / Tape & Reel
NCP1200P60G
F
SW
= 60 kHz
1200P60 PDIP−8
(Pb−Free)
50 Units / Rail
NCP1200D60R2G 200D6 SOIC−8
(Pb−Free)
2500 / Tape & Reel
NCP1200P100G
F
SW
= 100 kHz
1200P100 PDIP−8
(Pb−Free)
50 Units / Rail
NCP1200D100R2G 200D1 SOIC−8
(Pb−Free)
2500 / Tape & Reel
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specifications Brochure, BRD8011/D.
