NCP1200
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:
V
: the clamping ripple, could be around 20 V
Another option lies in implementing a snubber network
ripple
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
L
leak
by:
where C
represents the
lump
V
+ Ip @
Ǹ
max
C
lump
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.
R
clamp =
2 @ V
@ (V
* (V
) Vf sec) @ N)
out
clamp
clamp
L
@ Ip2 @ Fsw
leak
V
clamp
Available Documents
C
+
clamp
V
@ Fsw @ R
ripple
clamp
“Implementing the NCP1200 in Low–cost AC/DC
Converters”, AND8023/D
with:
“Conducted EMI Filter Design for the NCP1200’’,
AND8032/D
V
: the desired clamping level, must be selected to be
clamp
between 40 to 80 volts above the reflected output voltage
when the supply is heavily loaded.
“Ramp Compensation for the NCP1200’’, AND8029/D
V
+ Vf: the regulated output voltage level + the secondary
TRANSient and AC models available to download at:
http://onsemi.com/pub/NCP1200
out
diode voltage drop
L
: the primary leakage inductance
NCP1200 design spreadsheet available to download at:
http://onsemi.com/pub/NCP1200
leak
N: the Ns:Np conversion ratio
F
: the switching frequency
SW
ORDERING INFORMATION
Device
Type
Marking
1200P40
200D4
Package
PDIP8
SO–8
Shipping
NCP1200P40
F
F
F
F
= 40 kHz
= 40 kHz
= 60 kHz
= 60 kHz
= 100 kHz
= 100 kHz
50 Units / Rail
2500 Units /Reel
50 Units / Rail
2500 Units /Reel
50 Units / Rail
2500 Units / Reel
SW
SW
SW
SW
NCP1200D40R2
NCP1200P60
1200P60
200D6
PDIP8
SO–8
NCP1200D60R2
NCP1200P100
NCP1200D100R2
F
SW
F
SW
1200P100
200D1
PDIP8
SO–8
http://onsemi.com
13
ETC [ ETC ]
