Design Idea DI-119 ® LinkSwitch-LP Cordless Phone Linear Adapter Replacement with 10 kV Surge Withstand Application Device Power Output Input Voltage Output Voltage Topology Cordless Phone/Adapter LNK562P 1.6 W 85-265 VAC 7.7 V, 210 mA Flyback Design Highlights • Simple, low-cost, low parts count CV/CC solution • Low no-load input power: <180 mW at 265 VAC • Dramatically improved regulation over line frequency linear transformer • Meets CEC / ENERGY STAR requirements for active mode efficiency (63% vs. 53% requirement) • Small, low-cost EE16 core size allows compact design • >15 dBµV margin to EN55022B conducted EMI limits • No Y-capacitor gives low (<10 µA) line frequency leakage current • Meets 10 kV common mode and 2 kV differential mode surge (EN 1000-4-5 Class 4) Operation Supplies for cordless phones or answering machines often require a 10 kV common mode surge withstand capability to prevent damage to the telephone network during local lightning strikes. The design shown in Figure 1 meets this requirement while still being simple and low cost. L RF1 10 Ω 2.5 W D1 1N4937 The LNK562 device (U1) provides primary side sensed output voltage and current regulation, eliminating the need for an optocoupler. Using the PI Transformer Designer software, shield windings were included in the transformer design. This allowed the circuit to meet EN55022 B conducted EMI limits without the use of a Y-rated safety capacitor bridging the primary to secondary isolation barrier. The AC input is rectified and filtered by D1, D2 and C1, C6. The input capacitance is split to form a π filter with L1 and L3, with R5 damping the self resonance. Varistor RV1 provides surge protection for differential surges while RF1 provides filtering and fusing. The internal MOSFET of U1 drives the transformer primary, but the normal primary clamp network is not required due to the low current limit of U1. Skipping switching cycles based on the voltage sensed from the bias winding provides output regulation. Should the output of the supply be overloaded, then U1 lowers the switching frequency to limit the output current until ~2 V, when the unit enters auto-restart. L1 1 mH 1 T1 J1 C4 D4 100 µF 1N4933 25 V J4 NC R5 4.7 kΩ J5 2 7.7 V, 210 mA J3-1 R8 4.7 kΩ J3-2 RTN 3 85-265 RV1 VAC 275 VAC N C6 3.3 µF 400 V D2 1N4005 C1 3.3 µF 400 V C3 10 µF 50 V R7 4.7 kΩ L3 1 mH 4 D3 1N4005 EE16 R6 100 kΩ J2 LinkSwitch-LP U1 LNK562 D R1 22.1 kΩ 1% FB BP S C2 100 nF 50 V R2 3.01 kΩ 1% PI-4491-092806 Figure 1. LNK562 Linear Adapter Replacement Schematic. DI-119 September 2006 DI-89 DI-119 Elimination of the optocoupler and Y1 capacitor allowed the necessary printed circuit board (PCB) clearance and creepage to be obtained to withstand a 10 kV surge. The use of triple insulated wire for the secondary winding that terminates onto the PCB as flying leads (J4 & J5) allows the necessary creepage from primary to secondary. Figure 2 shows the PCB layout. Key Design Points • • TRANSFORMER PARAMETERS Core Material Bobbin EE16, gap for ALG of 113 nH/T2 4+4 pin horizontal Winding Details Bias/Shield: 29T, 2 × 37 AWG Primary: 176T, 37 AWG Shield: 15T, 2 × 32 AWG Secondary: 17T, 30 AWG TIW Winding Order (pin numbers) Bias/Shield (3-4), tape, primary (2-1), tape, shield (NC-1), tape, 7.7 V (FL-FL), tape Inductance Figure 2. PCB Layout – Extended Clearance and Creepage Provided by Slot (A). Spark Gap Provided to Route High Currents Back to Input and Around Electronics (B). 12 PI-4501-090606 • • Verify maximum drain voltage is <650 V at high line, maximum overload. For EMI repeatability, the transformer must be manufactured consistently. This is especially important in designs with no Y-capacitor. Using a fast diode for D1 improves EMI. Make sure the PCB layout provides 10 mm clearance and 15 mm creepage distance (use a slot in the PCB to increase creepage distance). Provide a path for surge discharge currents to go around sensitive electronic components (see spark gap (B) in Figure 2). To prevent arcing, keep the PCB surfaces clean. Remove flux and any other contaminants. 85 VAC 265 VAC 10 Output Voltage (V) • 8 6 4 2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Output Current (A) Figure 3. Typical Output Characteristics. Primary: 3.5 mH ±10% Leakage: 105 µH (max) Primary Resonant 250 kHz (min) Frequency Table 1. Transformer Design Parameters. TIW = Triple Insulated Wire, NC = No Connect, FL = Flying Lead For the latest updates, visit www.powerint.com Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and circuits external to the products) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign patent applications assigned to Power Integrations. A complete list of Power Integrations' patents may be found at www.powerint.com. Power Integrations grants its customers a license under certain patent rights as set forth at http://www.powerint.com/ip.htm. Power Integrations 5245 Hellyer Avenue San Jose, CA 95138 Phone: 1-408-414-9200 Apps: 1-408-414-9660 Apps Fax: 1-408-414-9760 The PI logo, TOPSwitch, TinySwitch, LinkSwitch, DPA-Switch, PeakSwitch, EcoSmart, Clampless, E-Shield, Filterfuse, PI Expert and PI FACTS are trademarks of Power Integrations, Inc. Other trademarks are property of their respective companies. ©Copyright 2006, Power Integrations, Inc. For a complete listing of worldwide sales offices, please visit www.powerint.com Rev. A 09/06