NCP1028
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26
clamp
is usually selected 50−80 V above the reflected
value N (V
out
+ V
f
). The diode needs to be a fast one and
an MUR160 represents a good choice. One major
drawback of the RCD network lies in its dependency upon
the peak current. Worse case occurs when I
peak
and V
in
are
maximum and V
out
is close to reach the steady−state value.
Figure 44c: This option is probably the most expensive
of 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 5.0 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
5.0 W of continuous power, but is able to accept surges up
to 600 W @ 1.0 ms. Select the Zener or TVS clamping
level between 40 to 80 V above the reflected output voltage
when the supply is heavily loaded.
Power Dissipation and Heatsinking
The NCP1028 hosting a power switch circuit and a
controller, it is mandatory to properly manage the heat
generated by losses. If no precaution is taken, risks exist to
trigger the internal thermal shutdown (TSD). To help
dissipating the heat, the PCB designer must foresee large
copper areas around the PDI7 package. When surrounded
by a surface greater than 1.0 cm@ of 35 mm copper, it
becomes possible to drop the thermal resistance
junction−to−ambient, R
q
JA
down to 75°C/W and thus
dissipate more power. The maximum power the device can
thus evacuate is: P
max
+
T
j
max −T
amb
max
R
qJA
(eq. 22)
which gives around 930 mW for an ambient of 50°C and a
maximum junction of 120°C. The losses inherent to the
switch circuit R
DS(on)
can be theoretically evaluated, but
the final prototype evaluation must include board
measurements to confirm that the junction temperature
stays within safe limits. Figure 45 gives a possible layout
to help dropping the thermal resistance. When measured on
a 70 m m (2 oz.) copper thickness PCB, we obtained a
thermal resistance of 75°C/W.
Figure 45. A possible PCB arrangement to reduce the thermal resistance
junction−to−ambient.
When routing the printed circuit, it is important to keep
high impedance line very short, like the brown−out signal
and the OPP input if used.
Application Diagram
Figure 46 displays the final application schematic. The
output uses a TLV431 whose low bias current represents an
advantage for low standby power switch mode supplies.
The secondary side features an additional LC filter needed
to remove unwanted spikes, although less problematic than
in DCM operation. On the primary side, a resistive network
senses the input bulk voltage and prevents the controller
from turning on for input voltages below 100 Vdc. The
auxiliary winding delivers 20 V nominal and thus offers
comfortable margin when the converter enters standby. As
we do not use any OPP, pin 7 goes to ground and offers
extended possibility to layout more copper area.
