DN05053/D Design Note – DN05053/D NCP1129, 12 Watt, Off-line Buck Regulator Device Application Input Voltage Output Power Topology I/O Isolation NCP1129 MBRS3200 Smart Meters Electric Meters, White Goods 85 to 265 Vac 12W at 12Vout Off-Line 65 kHz Buck Non-isolated Output Specification Output Voltage Output Ripple Typical Current Max Current Min Current 12 to 28 Vdc depending on selected Z1 zener value Less than 1% 250 mA to 1 amp 1 amp maximum (12 Vout) zero PFC (Yes/No) Efficiency Inrush Limiting / Fuse Operating Temp. Range Cooling Method / Supply Orientation Signal Level Control No, Pout < 25 watts Dependent on Freewheel diode selection Fused input 0 to +50°C (dependent on U1 heatsinking) Convection None Circuit Description This design note describes a simple, low power (15 W or less), constant voltage buck power supply intended for powering electronics for white goods, electrical meters, and industrial equipment where isolation from the AC mains is not required. The efficiency limitations of the offline buck converter are also discussed with emphasis on switching losses caused by the freewheel diode recovery characteristics. The output voltage can be set from 12 to approximately 28 volts (or higher) by using the appropriate zener diode for Z1 and selection of trim resistor R4. The new ON Semiconductor NCP1129 copackaged controller/Mosfet in a DIP 8 package is utilized as the buck switching element. Output reguation is accomplished by utilizing a simple zener/optocoupler sensing scheme. The optocoupler is necessary because the control logic is at a switching node common to the freewheel diode (D3) cathode. This was done along with half-wave input line rectification to allow a common connection from the input line neutral to the negative output terminal. The main problem associated with off-line buck converters is the efficiency associated with the combination of low duty cycle (Vout less than 50V) and the power Mosfet and freewheel diode turn-on switching loss. ON Semiconductor application note AND8318 discusses one solution for both of these issues by using a tapped buck inductor. This solution, however, typically requires a non-standard inductor which may not be an “off-the-shelf” component. October 2013, Rev. 0 At initial Mosfet turn-on, the freewheel diode recovery characteristics causes a significant leading edge spike of current to flow through both devices that can easily exceed 3 to 5 times the normal peak load current of the devices. This is particularly acute at high input line (230 Vac) if a fast recovery type of diode is used where the reverse recovery characteristics are slower with increased diode PRV voltage rating. Conventional high voltage Schottky diodes (typically several in series) and silicon carbide (SiC) diodes will greatly reduce the turn-on switching loss but there will still be a leading edge current spike associated with the devices’ junction capacitance. The table and waveforms shown below compare the representative efficiencies and switching characteristics when using the three diode types. The SiC diode definitely exhibited the best performance, but the present day cost of this part may be prohibitive for the intended application, and a pair of (or 3) series standard Schottky rectifiers are probably the best compromise. Tests have shown that the use of a typical ultra-fast recovery rectifier for D3 may prohibit continuous output currents much greater than about 500 mA due to excessive heating of the Mosfet and/or the freewheel diode itself. Obviously good pc board layout with liberal copper clad for heatsinking will certainly help the thermal issues. Protection features in the NCP1129 include peak overcurrent limiting which can be set by proper selection of current sense resistor R7, and the Vcc pin will latch the chip off if the Vcc exceeds 27 volts, allowing for overvoltage detection. Over-temperature protection at 150C (internal) is also included. www.onsemi.com 1 DN05053/D Schematic Line F1 1A AC input 85 - 265Vac L1, 820 uH 0.5 A D1 MRA4007 C2 C1 100nF "x" R1 1M 0.5W 100nF "x" D2 MMSD4148A 1 NCP1129-65 5 U1 C3 330uH, 1.5A 1.5 ohms 6 4 1 3 2 8 47 uF, 400Vdc L2 R7A R5 C7 R7B 2.4K C8 D3B 0.1uF, 50V D4 R6 + C5 4.7uF 50V 1nF C4 U2 3 1 2 1000uF 0.1uF 16V 50V Z2 _ 1N5352B (15V, 5W) MURA160 100 ohms R2 4 Output 12V, 1A C9 MBRS 3200 x2 100pF C6 Neutral + D3A 33 Z1 MMSZ5241B R4 Vadj. R3 680 Notes: 1. Vout set by Z1 (Vout = Vz + 1V approx). 2. L1 is Wurth 7447728215; L2 is Wurth 7447709331. 3. Thick lines indicate recommended ground plane area. 4. U1 should use clad heatsinking around pins 5 and 6. 5. D3A/B should have large pad areas for heatsinking. 6. Z2 is optional output OVP zener. 7. Crossed schematic lines are not connected. 8. U2 is NEC PS2561L-1 optocoupler or similar (CTR > 0.5) 9. R4 will adjust Vout upwards only. 10. Red ground plane area should be minimized to lower capacitance (switched node). 11. Clad pour recommended around U1 pins 5 and 6 for heatsinking. Off-Line Buck Converter Using NCP1129 With Optocoupler Voltage Sensing (Rev 1A) 1 © 2013 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Frank Cathell, e-mail: [email protected] October 2013, Rev. 0 www.onsemi.com 2 DN05053/D Mosfet Source Voltage (yellow) and Current (blue) 12V, 1A output, 120 Vac input with ultra-fast recovery freewheel diode. Expanded drain current waveform showing current spike magnitude. October 2013, Rev. 0 www.onsemi.com 3 DN05053/D Waveforms with 1.5A, 600V SiC freewheel diode. Waveforms with pair of 3A, 200V MBRS3200 Schottky freewheel diodes in series. October 2013, Rev. 0 www.onsemi.com 4 DN05053/D Freewheel Diode Efficiency Comparisons Freewheel Device Efficiency (12V/1A output; 120Vac input) 3A/600V Ultra-fast 76% (unable to run at 230Vac without eventual thermal shutdown) 1.5A/600V SiC diode 83% (operation at full load at 230Vac without thermal issues) 3A/200V Schottky x 2 (MBRS3200 in series) 81.5% (operation at full load at 230Vac with acceptable thermals) Summary The NCP1129 co-packaged controller/Mosfet in a DIP 8 package can be used to design a simple, yet effective off-line buck converter for applications that don’t require input to output isolation. Care must be taken, however, in the selection of the freewheel diode due to its reverse recovery characteristics and the overall impact on conversion efficiency. Utilizing the buck controller at the high-side switching node in conjunction with optocoupler feedback provides the most effective, low noise technique for sensing and regulating the output voltage. References: ON Semiconductor Application Notes: AND8318, AND8328 ON Semiconductor Design Notes: DN05014, DN06011, DN06052 ON Semiconductor NCP1129/1126 Data Sheet October 2013, Rev. 0 www.onsemi.com 5