NCP1015 ON Semiconductor, NCP1015 Datasheet - Page 14

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

current flowing through R
the V
is fired. The NPN base can also receive a signal from a
temperature sensor. Typical bipolar can be MMBT2222 and
MMBT2907 for the discrete latch. The NST3946 features
two bipolar NPN + PNP in the same package and could also
be used.
Power Dissipation and Heatsinking
dissipation DSS current-source (when active) and the
dissipation of MOSFET. Thus P
When the PDIP7 package is surrounded by copper, it
becomes possible to drop its thermal resistance
junction-to-ambient, R
dissipate more power. The maximum power the device can
thus evacuate is:
Design Procedure
not differ from that of a standard circuit using a controller
and a MOSFET. However, one needs to be aware of certain
characteristics specific of monolithic devices:
R
The power dissipation of NCP1015 consists of the
The design of a SMPS around a monolithic device does
hold
CC
ensures that the SCR stays on when fired. The bias
ramp up (8.5 V) and down (7.5 V) when the SCR
Figure 24. A Possible PCB Arrangement to Reduce the Thermal Resistance Junction-to-Ambient
Figure 23. Two Bipolar Transistors Ensures a Total Latch-off of the SMPS in Presence of an OVP
0.1 mF
OVP
qJA
hold
DC
down to 75°C/W and thus
should be small enough to let
10 k
tot
= P
10 k
DSS
+ P
Rhold
BAT54
12 k
Clamping Elements
MOSFET
http://onsemi.com
NCP1015
.
14
which gives around 1 W for an ambient of 50°C. The losses
inherent to the MOSFET R
following formula:
where I
input), D is the converter operating duty-cycle and R
the MOSFET resistance for T
valid for Discontinuous Conduction Mode (DCM)
operation where the turn-on losses are null (the primary
current is zero when you re-start the MOSFET). Figure 24
gives a possible layout to help dropping the thermal
resistance. When measured on a 35 mm (1 oz.) copper
thickness PCB, we obtained a thermal resistance of 75°C/W:
+
CV
CC
1. In any case, the lateral MOSFET body-diode shall
never be forward biased, either during start-up
(because of a large leakage inductance) or in
normal operation as shown by Figure 25.
p
1
2
3
4
is the worse case peak current (at the lowest line
P
P
mos
max
+
+ 1
8
7
5
3
T
J(max)
@ I
To Secondary Diode
DS(on)
p
J
2
= 100°C. This formula is only
@ D @ R
Transformer
* T
R
qJA
can be evaluated using the
AMB(max)
DS(on)
(eq. 12)
(eq. 13)
DS(on)