Datasheet

NCP1028
http://onsemi.com
21
This difference might not be seen as a problem, but some
design specifications impose stringent conditions on the
maximum output current capability, regardless the line
input. Hence the need for an OPP input…
Since we want to limit the power to 12.8 W at high line,
let us calculate the needed peak current:
From equation 6: I
peak
+
2P
out
F
SW
L
p
h
Ǹ
= 693 mA to
deliver 12.8 W at high line.
Compared to our 735 mA, we need to decrease the
setpoint by 6% roughly when V
in
equals 350 Vdc.
The NCP1028 hosts a special circuitry looking at the
couple voltage/current present on pin 7. Figure 36 shows
how to arrange components around the controller to obtain
Over Power Protection.
Current
Setpoint
Over Power
Protection
OPP
Bulk
ROPPU
ROPPL
GND
Figure 36. A resistive network reduces the
power capability in high-line conditions.
First, you need to know the required injected current and
the voltage across pin 7 to start activating OPP.
Experiments consist in wiring Figure 36 circuit and
running the power supply in conditions where it must shut
down (e.g. highest input voltage and maximum output
current per specification). For this, R
OPPL
can be put to
10 kW and R
OPPU
made of a series string of 4 1.0 MW
resistors plus a 10-turn 1.0 MW potentiometer set at its
maximum value. An amp-meter is inserted in series with
pin 7 and a volt-meter monitors its voltage with respect to
ground. Once the power supply is powered, slowly rotate
the potentiometer and observe both voltage and current
going up at pin 7. At a certain time, as voltage and current
increase, the controller will shut down the power supply.
The current at this time is the one we are looking for.
Suppose these experiments lead to 80 mA with a pin 7
activation voltage of 2.45 V. Final resistor equations are:
VbulkH = 375 Vdc ; the maximum voltage at which OPP
must shut down the controller
V
bulkL
= 200 Vdc ; the minimum voltage below which
OPP is not activated
I
OPP
= 80 mA ; the current in pin 7
V
f
= 2.45 V ; the voltage of pin 7 at the above
condition
R
OPPL
+
V
bulkH
-V
bulkL
I
OPP
(V
bulkL
-V
f
)
V
f
+ 27kW
(eq. 7)
R
OPPH
+ R
OPPL
V
bulkL
-V
f
V
f
+ 2.2MW
(eq. 8)
If the OPP feature is not needed for some designs, it is
possible to ground it via a copper wire to the adjacent
ground pin. This can help to develop a larger copper area
in an application where the thermal resistance is an
important parameter.
Ramp Compensation
When operating in Continuous Conduction Mode
(CCM), current-mode power supplies can exhibit
so-called sub-harmonic oscillations. To cure this problem,
the designer must inject ramp compensation. The ramp can
either be added to the current sense information or directly
subtracted from the feedback signal. Figure 37 details the
internal arrangement of the ramp compensation circuitry.
Gate Reset
Ramp
RR
Vp
V
DD
IRR
Control
Figure 37. The Internal Feedback Chain and the Ramp Compensation Network