DN05033/D Design Note – DN05033/D A 12V, Off-line Power Supply With Primary Side Feedback Device NCP1251B NDD04N60 Application Input Voltage Output Power Topology I/O Isolation White Goods, Small Instruments, EMeters, Industrial Equipment 90 – 267 Vac 20 Watts DCM Flyback 2.5 kV primary to secondary Other Specification Output Voltage Ripple Nominal Current Max Current Min Current Output 1 Output 2 Output 3 Output 4 12 Vdc +/- 5% < 2% 1.8 Amps 2 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) Nominal Efficiency Inrush Limiting / Fuse Operating Temp. Range Cooling Method / Supply Orientation Signal Level Control No >80% Yes 0 to 50C Convection NA No Others Input EMI filter output voltage. The load regulation over the usable load range and the nominal output voltage can be further optimized by adjusting the values of R5 and R11, respectively. Circuit Description This Design Note describes a very simple, low cost, primary side controlled, off-line flyback power supply using ON Semiconductor’s NCP1251B current mode controller (TSOP6 package) and an NDD04N60 D-Pak Mosfet. The design dispenses with the conventional TL431/optocoupler feedback scheme and uses primary side voltage sensing on the NCP1251’s Vcc rail which is derived from an aux winding on the transformer. This scheme provides tight line regulation and a load regulation of +/- 5% which is more than adequate for most applications. The acceptable load regulation is achieved by tight coupling (bifilar wound) between the main 12V secondary and the 12V aux winding in the transformer. The voltage sensing circuit is comprised of Z2 and Q2. The zener voltage of Z2 plus the VBE drop of Q2 sets the nominal June 2012, Rev. 0 Over-current limiting is provided by sensing the peak current in the MOSFET Q1 via R8. Once the 800 mV threshold level on U1’s pin 4 is exceeded, the circuit will go a “hiccup” mode until the over-current condition is removed. An optional output OVP clamp is implemented via Z1 to limit the maximum no load output voltage. Key Features Primary side voltage sensing – no optocoupler Input EMI filter for conducted EMI compliance Schottky output rectifier for high efficiency Current mode control with adjustable output current limit setpoint www.onsemi.com 1 DN05033/D Schematic1 L1A F1 C1 C2 0.1uF "X2" 0.1uF L1B "X2" R1 4.7 ohms 3W 10 D1-D4 MRA4007 X4 820uH 2A 250Vac AC Input T1 C4 R14 47uF 400V R2 R3 360K 360K 360K R9 1K C5 1nF 2 1N4937 MBRS360 R6 10 5 R7 R8 1 ohm 1/2W 10K R4 3 D5 Q1 D6 C6B D8 U1 5 1 4 J2 C9A C9B R13 3K 14V 1W + Z1 C10 0.1uF 50V 12 Vout _ C12 1K 6 2 D9 6 R10 NCP1251B 10nF EE20/10/6 9 NDD03N60Z C3 820uH C8 4.7nF 1 1kV 43K 1W MMSD4148 J1 22uF 25V R5 D7 24 MMSD 4148 MMSD 4148 C6A 4.7uF 50V R11 10 Q2 1nF "Y1" 3 8 C7 220pF Z2 MMBT 2222A MMSZ5241B (11V) R12 680 NOTES: 1. Crossed lines on schematic are NOT connected. 2. Z1/R13 are optional OV clamp/pre-load for zero load protection. 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. R5 should be adjusted for optimum load regulation depending aux winding leakage inductance (typically 15 to 68 ohms). 7. Z2 sets nominal Vout. 8. R11 is for Vcc/Voutput trimming. 9. R8 sets max output current. 10. See drawing for T1 details. 20 Watt NCP1251 Power Supply with Primary Side Sense (Rev 7A) 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] June 2012, Rev. 0 www.onsemi.com 2 DN05033/D MAGNETICS DESIGN DATA SHEET Project / Customer: 24 watt, 12 vout NCP1251 Flyback with primary side sensing Part Description: 24 watt flyback transformer, 12Vout (Wurth part # 750313284) 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. 12V Secondary/Vcc (5 - 6, 3 - 8) 6 turns trifilar of #26 triple insulated wire over one. layer. The outer winding on schematic is Vcc (3 - 8). The 12V secondary (5 - 6) uses two wires. Self leads to pins per drawing. Primary B (2 - 9) Same as Primary A. Insulate with tape and selfleads to pins. Hipot: 2.5 kV from primaries & Vcc to secondary for 1 minute. Lead Breakout / Pinout Schematic 10 3 (Top View of Bobbin) 1 5 10 9 8 7 6 9 12V sec Pri A Pri B 2 6 Trifilar Wound 0.15" pin separation (8 places) 0.60" pin rows 8 1 2 3 4 5 June 2012, Rev. 0 www.onsemi.com 3 DN05033/D 2 Output Load Regulation Efficiency Plot References: NCP1251 Data Sheet: http://www.onsemi.com/pub_link/Collateral/NCP1251-D.PDF ON Semiconductor NCP1251 Design Notes: DN05012, DN05017, DN05028, DN05029 Flyback Transformer Design Aid: http://www.onsemi.com/PowerSolutions/supportDoc.do?type=tools&rpn=NCP1075 June 2012, Rev. 0 www.onsemi.com 4