Housed in a tiny TSOP6 package, the NCP1256 lends itself very well to designing moderate to high output power converters. This application note demonstrates the part capabilities in a 60W AC−DC adapter, typical of what is needed for the high−volume net/notebook market.
The adopted schematic appears in Figure. You can see the NCP1256 surrounded by components implementing brownout sensing, over voltage and over temperature protection. We start the description by the left side of the board. The mains is applied on the rectifying diode bridge through an EMI filter made of a 10−mH common−mode choke. Its leakage inductance is used together with C11 to form a differential mode filter. Resistors (R15, R27, R17 and R20) perform the dual function of brown−out sensing and also ensure the discharge of the X2−capacitor when the power cord is unplugged. These resistors must be carefully selected to fulfill the IEC−950 safety standards as they can obviously hamper the no−load standby power. Additional filtering and protection devices can be necessary (VDR, spark−gaps) to improve the filter as you start qualifying the final prototype for safety and surge robustness. On the other hand, it is also possible to differently design the EMI filter to adopt an X2 capacitor of 0.1 mF. In this case, discharge resistors are no longer needed and brown−out sensing resistors can be slightly increased to the benefit of standby power.
The controller drives a 5 A / 650 V power MOSFET with a small PNP transistor which helps reduce the turn−off event for an improved efficiency.
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