DN05036/D Design Note – DN05036/D A 12V, Monolithic Off-line Power Supply With Primary Side Regulation (PSR) Device Application Input Voltage Output Power Topology I/O Isolation NCP1075 White Goods, EMeters, Industrial Equipment 90 – 267 Vac 10 Watts Flyback 2.5 kV primary to secondary Output 1 Output Voltage Ripple Nominal Current Max Current 12 Vdc +/- 5% 100 mV 800 mA 1.2 Amps 50 mA for Vout < 13V Min Current Others Input EMI filter Circuit Description This Design Note describes a very simple, primary side controlled, 10 watt, off-line flyback power supply using ON Semiconductor’s NCP1075 current mode controller (SO-223 package). The power supply design is intended for low power applications such as E-meters, white goods, instruments, and industrial equipment where an output voltage regulation of +/-5% over a load range of 5% to 100% is acceptable with a Vout of 12 volts or greater. . The design dispenses with the conventional TL431 and optocoupler sensing/feedback scheme and uses primary side voltage sensing on the NCP1075’s Vcc rail which is derived from an aux winding on the transformer. This scheme provides tight line regulation and a typical load regulation of +/- 5% which is more than adequate for most applications. The acceptable load regulation is achieved by tight coupling (bifilar winding) between the main 12V secondary and the 12V aux winding in the transformer. The voltage sensing circuit is comprised of Z1 and Q1. The zener voltage of Z1 plus the VBE drop of Q1 sets the nominal output voltage. The regulation over the usable load range and the August 2012, Rev. 0 nominal output voltage can be further optimized by adjusting the values of resistors R5 and R6, respectively. Over-current limiting is provided by sensing the peak current in the NCP1075’s internal MOSFET. Once the 450 mA threshold level is exceeded, the circuit will go into a “hiccup” mode until the over-current condition is removed. An optional output OVP clamp is also implemented via Z2 to limit the maximum no load output voltage. The output at no load will typically rise to about 15 volts (for Vout nominal = 12V) where a small 500 mW zener will clamp it to an acceptable level. If desired the clamp level can be lowered to 13 or 14 volts with the appropriate zener diode. Key Features Primary side voltage sensing – no optocoupler for feedback Up to 10 watts output Input EMI filter for conducted EMI compliance Schottky output rectifier for high efficiency Current mode control with 700 volt rated internal MOSFET www.onsemi.com 1 DN05036/D Schematic J1 L1A F1 1.5A C1 AC Input T1 1 mH 0.1uF "X" R1 4.7 ohms 3W R2A C2 0.1uF "X" L1B 3.3M D1-D4 MRA4007 X4 C3 R3 C4 68K 1/2W 1nF 1kV D5 22uF 400V U1 R2B 3.3M 1 mH Primary Ground Plane 4 1N4937 NCP1075 (100 kHz) J2 C9A C9B + 7,8 MBRS 260T3 R4 MMSD 4148A R5 1K D6 22 C5 22uF 25V 4 D7 3 2 1 1 C6 Q1 Secondary Ground Plane 2 22uF 25V R6 47 0.1uF 50V 5,6 3 C10 12Vout Z2 1000uF, MMSZ 5245B 16V x 2 C11 _ 0.1uF 50V C8 1nF "Y1" C7 1nF Z1 R7 MMSZ 5241B 680 NOTES: 1. Crossed schematic lines are not connected. 2. R4 value dependent on aux winding turns and Vout setpoint. 3. R1 is optional inrush limiting resistor. 4. U1 tab (pin 4) and D7 should have heatsinking clad pours. 5. Heavy lines indicate recommended ground plane areas. 6. L1A/L1B are Wurth 7447728102. 7. Q1 is MMBT2222A or similar. 8. Z1 sets Vout (Vout = Vz + 0.6V for Ns:Naux = 1:1). 9. R6 trims Vout (up only). 10. Z2 is optional no load OVP zener (Vz = Vout + 2V) 10 Watt NCP1075 Power Supply with Primary Side Sensing (Rev 1) 1 © 2012 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] August 2012, Rev. 0 www.onsemi.com 2 DN05036/D MAGNETICS DESIGN DATA SHEET Project / Customer: ON Semiconductor - NCP1014/1075 10W PSU Part Description: 10W flyback transformer, Primary side sensing; 100kHz, 12V/1A out Schematic ID: T1 Core Type: EF16 (E16/8/5); 3C90 material or similar Core Gap: Gap for 1.7 to 2 mH inductance across primary (pins 1 - 4) Inductance: 1.85 mH +/-5% Bobbin Type: 8 pin horizontal mount for EF16 Windings (in order): Winding # / type Turns / Material / Gauge / Insulation Data Primary (4 - 1) 100 turns of #35HN over 2 layers evenly. Insulate with a layer of Mylar tape. 12V Secondary/Vcc (8-5, 3-2) 7 turns bifilar of #24 triple insulated wire over one layer if possible. Terminate to pins as shown in schematic below. Pins 3 to 2 is the aux winding (primary side referenced). Varnish assembly Hipot: 2.5 kV from primary to main secondary for 1 minute Lead Breakout / Pinout Schematic (Bottom View - facing pins) 1 Primary 4 8 3 Bifilar wound 2 5 secondary and Vcc aux 4 3 2 1 5 6 7 8 Wurth Electronics par # 750313390 Rev 00 August 2012, Rev. 0 www.onsemi.com 3 DN05036/D 1 Output Load Regulation Efficiency Plot August 2012, Rev. 0 www.onsemi.com 4 DN05036/D References: NCP1075 Data Sheet: http://www.onsemi.com/pub_link/Collateral/NCP1072-D.PDF ON Semiconductor Applicable Design Notes: DN05018, DN05033 See also Design Notes for NCP1014 controller. Flyback Transformer Design Aid: http://www.onsemi.com/PowerSolutions/supportDoc.do?type=tools&rpn=NCP1075 August 2012, Rev. 0 www.onsemi.com 5