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NCP1253 Datasheet(PDF) 12 Page - ON Semiconductor

Part No. NCP1253
Description  Current-Mode PWM Current-Mode PWM Power Supplies
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

NCP1253 Datasheet(HTML) 12 Page - ON Semiconductor

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solves this potential issue by adding an extra capacitor on the
auxiliary winding. However, this component is separated
from the VCC pin via a simple diode. You therefore have the
ability to grow this capacitor as you need to ensure the
self−supply of the controller without jeopardizing the
start−up time and standby power.
Triggering the SCR
The latched−state of the NCP1253 is maintained via an
internal thyristor (SCR). When the voltage on the Vcc pin
exceeds the internal latch voltage, the SCR is fired and
immediately stops the output pulses. When this happens, all
pulses are stopped and VCC is discharged to a fix level of 7 V
typically: the circuit is latched and the converter no longer
delivers pulses. To maintain the latched−state, a permanent
current must be injected in the part. If too low of a current,
the part de−latches and the converter resumes operation.
This current is characterized to 32
mA as a minimum but we
recommend including a design margin and select a value
around 60
mA. The test is to latch the part and reduce the
input voltage until it de−latches. If you de−latch at Vin =
70 Vrms for a minimum voltage of 85 Vrms, you are fine. If
it precociously recovers, you will have to increase the
start−up current, unfortunately to the detriment of standby
The most sensitive configuration is actually that of the
half−wave connection proposed in Figure 25. As the current
disappears 5 ms for a 10 ms period (50 Hz input source), the
latch can potentially open at low line. If you really reduce the
start−up current for a low standby power design, you must
ensure enough current in the SCR in case of a faulty event.
An alternate connection to the above is shown below
(Figure 26):
1 Meg
1 Meg
Figure 26. The Full−wave Connection Ensures Latch
Current Continuity as well as a X2−Discharge Path.
In this case, the current is no longer made of 5 ms “holes”
and the part can be maintained at a low input voltage.
Experiments show that these 2 M
W resistor help to maintain
the latch down to less than 50 Vrms, giving an excellent
design margin. Standby power with this approach was also
improved compared to Figure 25 solution. Please note that
these resistors also ensure the discharge of the X2−capacitor
up to a 0.47
mF type.
The de−latch of the SCR occurs when the injected current
in the VCC pin falls below the minimum stated in the
data−sheet (32
mA at room temp).
Frequency Foldback
The reduction of no−load standby power associated with
the need for improving the efficiency, requires a change in
the traditional fixed−frequency type of operation. This
controller implements a switching frequency foldback when
the feedback voltage passes below a certain level, Vfold, set
around 1.5 V. At this point, the oscillator enters frequency
foldback and reduces its switching frequency. The peak
current setpoint is following the feedback pin until its level
reaches 1.05 V. Below this value, the peak current freezes to
Vfold/4.2 (250 mV or 31% of the maximum 0.8−V setpoint)
and the only way to further reduce the transmitted power is
to diminish the operating frequency down to 26 kHz. This
value is reached at a feedback voltage level of 350 mV
typically. Below this point, if the output power continues to
decrease, the part enters skip cycle for the best noise−free
performance in no−load conditions. depicts the adopted
scheme for the part.

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