NCP1239
www.onsemi.com
18
OVER POWER PROTECTION
Over Power Protection (OPP) is a known means to limit
the output power runaway 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. NCP1239 senses the input voltage via HV pin.
This line voltage 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 zero OPP
current at low line (125 V dc), leaving the converter power
capability intact in the lowest operating voltage. Figure 36
presents the internal simplified architecture of this OPP
circuitry.
Figure 36. Over Power Protection is Provided via the Bulk Voltage Present on HV Pin
CS
HV sample
& sampling
HV detection
HV
ROPP
Rsense
offset
To CS
comparator
L1
N
EMI
Filter
Vbulk
Iopp
OPP current
generation
Vfb
The HV voltage will be transformed into a current equal
to 67.5 mA when the HV pin is biased to 125 V. However,
there is an internal fixed sink of 67.5 mA. Therefore, the net
current flowing into R
OPP
is 0 at this low-voltage input
(≤ 125 V dc), ensuring an almost non-compensated
converter at low line: at a 115-V rms input (162 V dc), the
current from the OTA block will induce a 87.5-mA current,
turning into a 20-mA offset current flowing into R
OPP
. Now,
assume a 260-V rms input voltage (365 V dc), the controller
will generate an offset current of:
365 @ 0.54 u * 67.5 u + 130 mA
(eq. 8)
Assume we need to reduce the maximum peak current
setpoint by 250 mV to limit the maximum power at the
considered 260-V rms input. In that case, we will need to
generate a 250-mV offset across R
OPP
. With a 130-mA
current, R
OPP
should be equal to:
250 m
130 u
+ 192 kW
(eq. 9)
A small 100−220-pF capacitor closely connected between
the CS and GND pins will form an effective noise filter and
nicely improves the converter immunity. Now, with this
1.92-kW resistance, the low-line 20-mA offset current will
incur a 38-mV drop, which, in relationship to a 800-mV
maximum peak, generates a small 5% reduction. Assuming
a full DCM operation, the power would be reduced by 0.95
2
or 9.75% only. Please note that the OPP current is clamped
for a HV pin voltage greater than 365 V dc. Should you lift
the pin above this voltage, there will be no increase of the
OPP current.
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 cancel