NCP1256
www.onsemi.com
16
Please note that the IC will restart immediately if the BO
comparator sends the green light while V
cc
is above
VCC
ON
. In that case, as V
cc
is already high, there is no need
to go through a fresh start−up sequence and the part can
switch again.
Over Power Protection
Over Power Protection (OPP) is a known means to limit
the output power excursion at high mains. Several elements
such as propagation delays and operating mode explain why
a converter operated at high line delivers more power than
at low line. NCP1256 senses the input voltage via a resistive
network primarily used for brown−out protection. This line
image is transformed into a current information further
applied to the current sense pin (CS). A resistor placed in
series from the sense resistor to the CS pin will create an
offset voltage proportional to the input voltage variation. An
added current sink will ensure a 0 OPP current at low line,
leaving the converter power capability intact in the lowest
operating voltage. Figure 37 presents the internal simplified
architecture of this OPP circuitry.
CS
BO
vdd
OPPGM
IOPPLL
ICSO
Rupper
Rlower
ROPP
Rsense
To CS
comparator
VFB > VfoldF Iopp3 = I1
VFB < VoppF Iopp3 = 0
I2=0
I1=I2
VFB
Iopp3
I1
I2
C1
Vbulk
VfoldF
V(FB)
offset
Figure 37. Over Power Protection is provided via the bulk voltage image present on Brown−Out pin
We assume the brown−out network is tweaked so that a
80−V rms input voltage brings 0.8 V on the BO pin. This is
the voltage at which the adapter will start working. The
voltage will be transformed into a current by the OPPGM
block. Its transconductance is 115 mS, leading to a generated
current of 92 mA at a 0.8−V bias. However, there is an
internal fixed current sink IOPPLL calibrated so that the net
current flowing into R
OPP
is 0 at this low−voltage input. It
ensures an almost non−compensated converter at low line.
Now, assume a 265−V input voltage, the BO level will be
2.65 V and will generate an offset current of 185 mA as stated
in the specs. In our design, as an example, say we need to
reduce the maximum peak current setpoint by 250 mV to
reduce the maximum power at the 265−V input. In that case,
we will need to generate a 250−mV offset across R
OPP
. With
a 185−mA current, R
OPP
should be equal to 230 m / 185 u =
1.35 kW. A small 100−220 pF capacitor closely connected
between the CS and GND pins will form an effective noise
filter and will nicely improve the converter immunity to
noise. Please note that the OPP current is clamped for a BO
pin voltage greater than 2.65 V. Should you lift the pin above
this voltage, there will be no increase of the OPP current and
the current absorbed by the pin will increase as you approach
the OVP level.
The offset voltage can affect the standby power
performance by reducing the peak current setpoint in
light−load conditions. For this reason, it is desirable to
smoothly cancel its action as soon as frequency folback
occurs. A typical curve variation is shown in Figure 38. At
low power, below the frequency folback starting point,
100% of the OPP current is internally absorbed and no offset
is created through the CS pin. When feedback increases
again and reaches the frequency foldback point, as the
frequency goes up, OPP starts to build up and reaches its full
value at V
foldF
+ 0.7 V.
