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| Design Idea DI-76 (R) LinkSwitch Application Appliance/Industrial Control Device LNK520P 3.1 W Low Parts Count Power Supply Power Output 3.1 W Input Voltage 100-375 VDC Output Voltage 12 V Topology Flyback Design Highlights * Replaces a linear transformer based power supply at the same or lower cost, but with much better performance * High Efficiency: >75% at >100 mA of load current * Primary side regulated: requires no optocoupler * <300 mW no-load power consumption at 375 VDC * Very low parts count: only 9 components! * 12 V output: ideal for driving motors, relays, or SCRs restart (for hard short circuits) and thermal shutdown effectively produce a very rugged design. LinkSwitch derives all feedback information from the primary side, thus no optocoupler is required. During output diode (D4) conduction, the output voltage is reflected through the transformer turns ratio, providing a primary feedback voltage (VOR), which is rectified by diode D3 and held by capacitor C4. Resistor R3 limits the effects of transformer leakage on the feedback voltage. Resistor R2 feeds the VOR signal as a current to the CONTROL pin of the LinkSwitch, controlling duty cycle (and primary current limit during overload conditions). When there is little or no feedback signal (such as a short circuit) the LinkSwitch goes into auto-restart, limiting output current to approximately 40 mA. Capacitor C3 provides device decoupling. The extra winding on pin 3 of T1 is a shield that reduces EMI. Operation The very low parts count LinkSwitch flyback circuit shown below can be used as a general-purpose power supply or linear adapter replacement. The design provides regulated output voltage during normal loads as well as a soft output-current limit during overload conditions (such as a stalled motor or defective relay). Adding to this, the other LinkSwitch self-protection features of autoLinkSwitch D S C T1 12 V, 260 mA + U1 LNK520 R2 17.8 k C3 1 F 10 V 69T 34 AWG 43T 35 AWG 1 5 + 30T 30 AWG T.I.W. 6 DC IN 100-375 VDC *C2 100 nF 400 V D3 1N4937 R3 150 C4 0.1 F 100 V FILM 4 C5 220 F 25 V 3 55T 38 AWG EE13 Lp = 2.484 mH PI-3883-070604 D4 UF4003 1A 200 V RTN *Optional Component Figure 1. DC-Input, Isolated LinkSwitch 3.12 W Flyback Converter. DI-76 www.powerint.com July 2004 DI-76 Key Design Points * The value of C3 (1 F) allows time for the output voltage to develop at startup. * A larger R3 value will provide flatter voltage regulation if a higher no-load voltage can be tolerated. * A lower R2 value will reduce the output voltage while proportionally increasing the maximum output current. PI-3884-070104 * Output diode D4 needs a sufficient peak inverse voltage (PIV) rating for high line withstand, requiring an ultrafast 200 V type (UF4003). For lower output voltages (PIV <100 V), Schottky diodes work well and enable improved efficiency. * The LinkSwitch PIXls spreadsheet tool included in PI ExpertTM Design Software can be used to redesign the transformer for different output voltages. 20 18 16 14 TRANSFORMER PARAMETERS Core Material Bobbin TDK PC40 EE13, ALG = 199 nH/T2 EE13 Horizontal 8-pin Shield 1: 55 T, #38 AWG, 1 Layer Primary-1: 69 T, #34 AWG, 2 Layers Primary-2: 43 T, #35 AWG, 1 Layer Secondary: 30 T, #30 T.I.W., 2 Layers Shield 1: (3-Float), Primary-1 (1-4), Primary-2 (4-3), Secondary (6-5) 2484 H 10% Voltage (V) 12 10 8 6 4 2 0 0 100 200 300 400 MIN MAX 105 VDC 150 VDC 315 VDC 370 VDC Winding Details Load (mA) Figure 2. Output Regulation vs. Load Current Curves. PI-3885-070104 Winding Order (pin numbers) Primary Inductance 80 Efficiency (%) Primary Resonant 500 kHz (minimum) Frequency Leakage Inductance 70 H (maximum) 70 60 50 40 105 VDC 150 VDC 315 VDC 370 VDC Table 1. Transformer Design Parameters. 0 50 100 150 200 250 300 Load (mA) Figure 3. Efficiency vs. Output Current Curves. For the latest updates, visit www.powerint.com Power Integrations may make changes to its products at any time. Power Integrations has no liability arising from your use of any information, device or circuit described herein nor does it convey any license under its patent rights or the rights of others. POWER INTEGRATIONS MAKES NO WARRANTIES HEREIN AND SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATIONS, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (including circuits external to the products and transformer construction) 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. The PI logo, TOPSwitch, TinySwitch, LinkSwitch and EcoSmart are registered trademarks of Power Integrations. PI Expert and DPA-Switch are trademarks of Power Integrations. Copyright 2004, Power Integrations Power Integrations MAIN PHONE NUMBER +1 408-414-9200 A 7/04 5245 Hellyer Avenue APPLICATIONS FAX +1 408-414-9760 San Jose, California 95138 For a complete listing of worldwide sales offices, please visit www.powerint.com APPLICATIONS HOTLINE +1 408-414-9660 www.powerint.com |
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