NCP1201
http://onsemi.com
14
APPLICATION INFORMATION
Power Dissipation
The NCP1201 can be directly supplied from the DC rail
through the internal DSS circuitry. The average current
flowing through the DSS is therefore the direct image of the
NCP1201 current consumption. The total power dissipation
can be evaluated using:
(V
HVDC
− 11 V) × I
CC2
.Ifthe
device operates on a 250 VAC rail, the maximum rectified
voltage can go up to 350 VDC. At T
A
=25C, I
CC2
=2.1 mA
for the 60 kHz version over a 1.0 nF capacitive load. As a
result, the NCP1201 will dissipate 350 V x 2.1 mA =
735 mW (T
A
=25_C). The SOIC--8 package offers a
junction--to--ambient thermal resistance R
θ
J--A
of 178C/W.
Adding some copper area around the device pins will help
to improve this number, 12mm x 12mm copper can drop
R
θ
J--A
down to 100C/W with 35 m copper thickness (1 oz.)
or 6.5mm x 6.5mm with 70 m copper thickness (2 oz.). With
this later number, we can compute the maximum power
dissipation the package accepts at an ambient of 50C:
P
max
=
T
jmax
-- T
Amax
R
θJ--A
= 750 mW
(T
Jmax
= 125_C),
which is acceptable with our previous thermal budget. For
the DIP8 package, adding a min--pad area of 80mm
2
of 35 m
copper (1 oz.), R
θ
J--A
drops from 100C/W to about 75C/W.
In the above calculations, I
CC2
is based on a 1.0 nF output
capacitor. As seen before, I
CC2
will depend on your
MOSFET’s Q
g
which I
CC2
I
CC1
+ F
sw
x Q
g
.Final
calculation should thus account for the total gate--charge Q
g
your MOSFET will exhibit.
If the power estimation is beyond the limit, supply to the
V
CC
with a series diode as suggested in Figure 28 can be
used. As a result, it will drop the average input voltage and
lower the dissipation to
350 V × 2
π
× 1.6 mA = 356.5 mW.
Alternatively, an auxiliary winding can be used to disable
the DSS and hence reduce the power consumption down to
V
CC
x I
CC2
. By using the auxiliary winding supply method,
the rectified auxiliary voltage should permanently stays
above the V
CCOFF
threshold voltage, keeping DSS off and
is safely kept well below the 16 V maximum rating for
whole operating conditions.
Non--Latching Shutdown
In some cases, it might be desirable to shut off the device
temporarily and authorize its restart once the control signal
has disappeared. This option can easily be accomplished
through a single NPN bipolar transistor wired between FB
and ground. By pulling FB pin voltage below the V
SKIP
level, the output pulses are disabled as long as FB pin
voltage is pulled below the skip mode threshold voltage. As
soon as FB pin is released, the the device resumes its normal
operation again. Figure 33 depicts an application example.
Figure 33. A Method to Shut Down the Device Without a Definitive Latchoff State
ON/OFF
Q1
8
7
6
5
1
2
3
4
Fault Protection
In applications where the output current is purposely not
controlled (e.g. wall adapters delivering raw DC level), it is
often required to permanently latchoff the power supply in
presence of a fault. This fault can be either a short--circuit on
the output or a broken optocoupler. In this later case, it is
important to quickly react in order to avoid a lethal output
voltage runaway. The NCP1201 includes a circuitry tailored
to tackle both events. A short--circuit forces the output
voltage to be at a low level, preventing a bias current to
circulate in the optocoupler LED. As a result, the FB pin
level is pulled up to 4.2 V, as internally imposed by the IC.
The peak current set--point goes to the maximum and the
supply delivers a rather high power with all the associated
effects. However, this can also happen in case of feedback
loss, e.g. a broken optocoupler. To account for those
situations, NCP1201 included a dedicated overload
protection circuitry. Once the protection activated, the
circuitry permanently stops the pulses while the V
CC
moves
between 10--12 V to maintain this latchoff state. The system
resets when the user purposely cycles the V
CC
down below
3.0 V, e.g. when the power plug is removed from the mains.
In NCP1201, the controller stops all output pulses as soon
as the error flag is asserted, irrespective to the V
CC
level.
However, to avoid false triggers during the startup sequence,
NCP1201 purposely omits the very first V
CC
descent from
12 to 10 V. The error circuitry is actually armed just after this
sequence, e.g. V
CC
crossing 10 V. Figure 34 details the
timing sequence. The V
CC
capacitor should be calculated
carefully to offer a sufficient time out during the first startup
V
CC
descent.
