NCP1246 ONSEMI [ON Semiconductor], NCP1246 Datasheet - Page 20

• Manufacturer Part Number: NCP1246
• Description: Fixed Frequency Current Mode Controller for Flyback Converters
• Manufacturer: ONSEMI [ON Semiconductor]

X2 Cap Discharge Feature
of the NCP1246. This feature save approx. 16 mW – 25 mW
input power depending on the EMI filter X2 capacitors
volume and it saves the external components count as well.
The discharge feature is ensured via the startïup current
source with a dedicated control circuitry for this function.
detector inside the X2 capacitor discharge control circuitry.
See the Figure 50 for the block diagram for this structure and
Figures 51, 52, 53 and 54 for the timing diagrams. The basic
idea of ac line unplug detector lies in comparison of the
direct sample of the high voltage obtained via the high
voltage sensing structure with the delayed sample of the high
voltage. The delayed signal is created by the sample & hold
structure.
is without hysteresis inside. The resolution between the
slopes of the ac signal and dc signal is defined by the
sampling time T
These parameters ensure the noise immunity as well. The
additional offset is added to the picture of the sampled HV
signal and its analog sum is stored in the C
capacitor. If the voltage level of the HV sensing structure
output crosses this level the comparator CMP output signal
resets the detection timer and no dc signal is detected. The
additional offset N
the HV pin. If the comparator output produces pulses it
means that the slope of input signal is higher than set
resolution level and the slope is positive. If the comparator
output produces the low level it means that the slope of input
signal is lower than set resolution level or the slope is
negative. There is used the detection timer which is reset by
any edge of the comparator output. It means if no edge
comes before the timer elapses there is present only dc signal
or signal with the small ac ripple at the HV pin. This type of
the ac detector detects only the positive slope, which fulfils
the requirements for the ac line presence detection.
the direct sample of the high voltage obtained via the high
The X2 capacitor discharging feature is offered by usage
There is used a dedicated structure called ac line unplug
The comparator used for the comparison of these signals
In case of the dc signal presence on the high voltage input,
SAMPLE
OS
can be measured as the V
and additional internal offset N
HV(hyst)
1
storage
http://onsemi.com
OS
NCP1246
on
.
20
voltage sensing structure and the delayed sample of the high
voltage are equivalent and the comparator produces the low
level signal during the presence of this signal. No edges are
present at the output of the comparator, that’s why the
detection timer is not reset and dc detect signal appears.
by the ration between the maximum hysteresis observed at
HV pin V
minimum detectable slope and the amplitude and frequency
of the sinusoidal input voltage:
frequency 35 Hz, if the maximum hysteresis is 5 V and
sampling time is 1 ms.
controller operation mode to ensure this safety feature. The
detection timer is reused for the time limiting of the
discharge phase, to protect the device against overheating.
The discharging process is cyclic and continues until the ac
line is detected again or the voltage across the X2 capacitor
is lower than V
quite big X2 capacitors used in the input line filter to the safe
level. It is important to note that it is not allowed to
connect HV pin to any dc voltage due this feature. e.g.
directly to bulk capacitor.
is kept above the V
mode of device operation to supply the control circuitry.
During the discharge sequence is not allowed to startïup the
device.
V
The minimum detectable slope by this ac detector is given
Than it can be derived the relationship between the
The minimum detectable AC RMS voltage is 16 V at
The X2 capacitor discharge feature is available in any
During the HV sensing or X2 cap discharging the V
max
+
+ 22.7 V
HV(hyst),max
2 @ p @ f @ T
V
HV(hyst),max
HV(min
CC(off)
S
min
sample
and the sampling time:
). This feature ensures to discharge
+
voltage by the SelfïSupply in any
V
+
HV(hyst),max
T
2 @ p @ 35 @ 1 @ 10
sample
5
ï3
(eq. 1)
(eq. 2)
CC
net