DN05028/D Design Note – DN05028/D A 20 Watt, High Efficiency, Off-line Power Supply Using Synchronous Output Rectification Device Application NCP1251B NCP4303 NCP431 White Goods, Small Instruments, EMeters, Industrial Equipment Input Voltage 90 – 267 Vac Output Power Topology I/O Isolation 20 to 25 Watts DCM Flyback With Synchronous Rectifier 3 kV Other Specification Output Voltage Ripple Nominal Current Max Current Min Current Output 1 Output 2 Output 3 Output 4 12 Vdc < 2% 1 to 1.8 Amps 1.8 Amps zero N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A PFC (Yes/No) Typical Efficiency Inrush Limiting / Fuse Operating Temp. Range Cooling Method / Supply Orientation Signal Level Control No 84% at full load Yes 0 to 50C Convection NA No Others Input EMI filter Circuit Description This Design Note describes a very simple, low cost, yet high efficiency off-line flyback power supply using ON Semiconductor’s NCP1251B current mode controller (TSOP6 package) in conjunction with the NCP4303 synchronous MOSFET rectifier controller. This Design Note is an extension of DN05012/D for engineers who require the highest possible efficiency for a 12 volt, 15 to 25 watt application. Other output voltages from up to 28 Vdc are easily implemented by modifying the values (or ratings) of the secondary circuit components and T1’s secondary turns. The primary power circuitry and transformer design is identical to the circuit of DN05012/D. The secondary circuitry contains the NCP4303 February 2012, Rev. 0 controller driving synchronous MOSFET rectifier Q3. Details of the various features and parameters of this controller can be found in the devices’ data sheet available at the ON Semiconductor web site. The plot shown below compares the efficiency curves of the original design using Schottky rectifiers with this synchronous rectifier version. Key Features Input EMI filter for conducted EMI compliance Simple, single switch DCM flyback topology Very high efficiency with synchronous output rectification Current mode control with adjustable output current Low cost components www.onsemi.com 1 DN05028/D Schematic1 L1A C1 0.1uF "X2" R1 4.7 ohms 3W C2 NDD03N60Z C3 47uF 400V 820uH R3 R4 470K 470K 5 4.7nF 1 1kV 1nF D5 6 2 10K 6 U1 5 Vcc R5 10 ohms 4 10K 4.7 Vcc C15 3 MMSD 4148 R21 20K 8 C7 220pF 1nF 15K C12 C6 10uF 50V _ 0.1uF 50V U4 8 1 5 NCP4303 2 3 7 4 6 0.1uF 50V D7 1000uF 16V x 2 R20 R19 100 C13 R10 1 R18 NVD6820NL R8A/B 2 ohms 1/2W 12Vout C10 Q2 R7 1K 10K 68 470pF 1N4937 10 + R16 C14 R17 R6 NCP1251B 2 R9 1K C5 Q1 D6 D8 MRA 4007 3 10nF R2 43K 1W 9 0.1uF L1B "X2" C8 J2 C9A C9B 10 EE20/10/6 2A 250Vac AC Input T1 C4 D1-D4 MRA4007 X4 820uH R22 F1 MMSD4148 J1 1nF "Y1" R12 39K R11 1K R15 R14 C11 U3 10K U2 4 1 NCP431 NOTES: 0.1 10K R13 10K 3 2 1. Crossed lines on schematic are NOT connected. 2. U2 is NEC PS2561L-1 or equivalent optocoupler (CTR > 50%). 3. R1 is for inrush limiting - use carbon comp or wire wound. 4. L1A/L1B are Wurth 7447728215 components (820 uH, 500mA). 5. Output caps (C9A/B) are radial lead, low impedance types (UCC LXV series or similar). 6. Heavy schematic lines are recommended ground plane areas. 7. R5 is for Vcc trimming (< 28Vmax), typically 10 ohms. 8. R8A/B sets max output current (primary peak current sense). 9. See drawing for T1 details. 20 Watt NCP1251 Power Supply with Output Synchronous Rectifier (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] February 2012, Rev. 0 www.onsemi.com 2 DN05028/D MAGNETICS DESIGN DATA SHEET Project / Customer: ON Semiconductor - 24 watt, 12 vout NCP1251 Flyback Part Description: 24 watt flyback transformer, 12vout (Wurth Electronics part # 750312495) Schematic ID: T1 Core Type: EE20/10/6 ferrite core; 3C90 material or similar Core Gap: Gap for 190 +/- 200uH across Primary A (pins 1 - 10) Inductance: 750 uH total (+/- 5%) measured from pin 1 to pin 9 with pins 2 and 10 connected Bobbin Type: 10 pin horizontal mount for EE20/10/6 Windings (in order): Winding # / type Turns / Material / Gauge / Insulation Data Primary A (1 - 10) 30T of #28HN over 1 layer (25 TPL). Insulate for 1 kV to next winding. Self leads to pins. Vcc (3 - 8) 7 turns of #28 HN over 1 layer, spiral wound over primary A. Self leads to pins. Insulate to 1 kV to next winding with tape. 12V Secondary (5 - 6) 6 turns bifilar of #24 triple insulated wire over one. layer (two strands). Self leads to pins. (Note: #26 is also acceptable here if the fit is too tight for one layer) Primary B (2 - 9) Same as Primary A. Insulate with tape and selfleads to pins. Hipot: 3 kV from primaries & Vcc to secondary for 1 minute. Lead Breakout / Pinout Schematic 10 (Top View of Bobbin) Pri A 1 9 Pri B 2 10 9 8 7 6 5 12V sec (bifilar) 0.15" pin separation (8 places) 0.60" pin rows 6 8 1 2 3 4 5 Vcc 3 February 2012, Rev. 0 www.onsemi.com 3 DN05028/D Efficiency Plot Comparison (Schottky vs Synchronous Rectifiers) References ON Semiconductor Design Note DN05012/D ON Semiconductor Design Note DN05017/D ON Semiconductor NCP4303 Synchronous Rectifier Controller Data Sheet ON Semiconductor NCP1251 Current Mode Controller Data Sheet ON Semiconductor NCP431 Programmable Zener Data Sheet February 2012, Rev. 0 www.onsemi.com 4