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NCP1207DR2
PWM Current-Mode Controller for Quasi-Resonant Operation
AND8127/D
Implementing NCP1207 in
QR 24 W AC-DC Converter
with Synchronous Rectifier
Prepared by: Petr Lidak
ON Semiconductor
IntroductionThe NCP1207 is a controller dedicated for driving the
current−mode free running quasi−resonant Flyback offline
converter.
This converter is designed for consumer products like
notebooks, offline battery chargers, consumer electronics
(DVD players, set−top boxes, TVs), etc.
The growing interest for EMI pollution reduction,
efficiency improvement, and maximum safety has been
taken into account while designing the NCP1207.
By implementing the NCP1207 one can build a power
supply that can meet all those requirements. This can be
achieved with help of the following NCP1207 main
features: Current−Mode Control: Cycle−by−cycle primary
current observation is helping to prevent any significant
primary overcurrent which would cause transformer’s
core saturation and consequent serious power supply
failure. Critical Mode Quasi−resonant Operation: Prevents the
converter operation in Continuous Conduction Mode in
any input and output condition. It is provided by the
zero crossing detection of the auxiliary winding’s
voltage. By addition of the reasonable delay the switch turn−on
instant can be shifted to the minimum (valley) of drain
voltage. This improves EMI noise and efficiency. Dynamic Self−Supply: Ensures IC proper operation in
applications where the output voltage varies during
operation like battery chargers. The DSS also supplies
the IC when the overvoltage event is being latched and
converter operation is stopped. Overvoltage Protection: By sampling the plateau
voltage on the auxiliary winding, the NCP1207 enters
into latched fault condition whenever the overvoltage is
detected. The controller stays fully latched until the
VCC decreases below 4.0 V, e.g. when the user unplugs
the power supply from the mains outlet and re−plugs it. Over−Load Protection: by continuously monitoring the
feedback loop activity, NCP1207 enters hiccup
operation as soon as the power supply is overloaded. As
soon as overload condition disappears, the NCP
resumes operation.
The 24 W AC−DC Adaptor Board SpecificationThe adaptor has following maximum and performance
ratings.
The schematic diagram of the adaptor can be seen in
Figure 1.
Transformer DesignThe bulk capacitor voltage than can be calculated:
(eq. 1)
Vbulk−min� VAC−min2� � 180�2� � 255 VDC
(eq. 2)
Vbulk−max� VAC−max2� � 240�2� � 339 VDC
The requested output power is 24 W.
Assuming 87% efficiency the input power is equal to:
(eq. 3)PIN� POUT � 24
0.87� 27.6W
The average value of input current at minimum input
voltage is:
(eq. 4)IIN−AVG� PIN
Vbulk−min� 27.6
255� 108 mA
