Onsemi NCP1249 | Demoboard NCP1249AD65GEVB

Description

The primary side uses a flyback topology, providing theadvantage of a cost effective power stage design. The designoperates in both CCM (continuous conduction mode) andDCM (discontinuous conduction mode), allowing it toaccept a wide universal input voltage range. The CCMoperation provides desired full load performance with agood efficiency and low ripple of the primary current. TheDCM operation permits an increased efficiency under lightload conditions by decreasing the switching losses.The device switches at 65 kHz which represents a goodtrade−off between switching losses and flyback transformersize. The adapter consists of several important sections. Thefirst is an input EMI filter to reduce the conducted EMI to theac line at the input of the adapter. The EMI filter is formedby common−mode inductor L4 and capacitors C22, C12,C13 and C11 with differential mode inductor L2. Thevaristor R33 is used protect the adapter against the lineovervoltage peaks. When the power supply is disconnectedfrom the AC mains, X capacitors C22, C11 and Y capacitorsC12 and C13 are discharged through HV pin via thefollowing path: rectifying diodes D1, D4, surge protectionresistive network R1, R2. This feature replaces commonlyused discharging resistors and saves approximately 25 mWof input power consumption at 230 V(AC) line.A following block is the rectifier with a bulk capacitor. Themain power stage of the flyback converter utilizes the lowRDSon MOSFET SPP11N60C3 along with a custom designedtransformer TR1 KA5038−BL, from the Coilcraft company.The detailed design procedure of flyback adapter can befound in the application notes AND8461/D and AN9115/Dat ON Semiconductor website: http://www.onsemi.com.The secondary side rectification is done by a low forwardvoltage drop Schottky diode NTST30100SG fromON Semiconductor. A simple RC snubber (R12 and C5)across the secondary rectifier damps any high frequencyringing caused the unclamped leakage inductance of thetransformer secondary side. The secondary−side controllerNCP4355B provides the output voltage regulation. Outputvoltage is set by voltage divider R26, R32, R37 and R41.Regulation output is coupled via the optocoupler (OK1) to theprimary side controller NCP1249. The secondary controlleralso detects a very light load via D19, R42, C27, R43 and R44via OFFDET pin. When light load condition is detected,primary controller is switched into the OFF mode operationvia optocoupler OK2 through the ONOFF pin. The NCP4355controls the primary controller with an “Active ON” signal,meaning that it only drives optocoupler current during the ONmode to minimize consumption during the OFF modeoperation. The built−in LED driver of the NCP4355indicates primary side operation (when SMPS is not in OFFmode). The LED driver switches with a 1 kHz frequencyand 12% duty cycle in order to optimize LED efficiency.

Features

• Constant voltage regulation
• Very low input power at light and no load
• High−voltage start−up
• Peak power excursion
• Overpower protection
• Universal mains operation

More information

• Datasheet
• IC product information