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| Design Idea DI-80 (R) LinkSwitch-TN 0.6 W Non-Isolated Utility Meter Power Supply Application Utility Meter Device LNK302 Power Output 0.6 W Input Voltage 85-265 VAC, 47-63 Hz Output Voltage 12 V, 50 mA Topology Buck Design Highlights * * * * * * * * * Universal AC input voltage range Meets EN62053-21:2003 (<1 W real and <5 VA apparent input power at 265 VAC) Long holdup time (140/580 ms at 115/230 VAC) High efficiency (>60 % at 265 VAC) Very low no-load consumption (0.15 W at 265 VAC) EMI compliant (EN55022B, CISPR-22B) Extended line surge withstand with integrated 700 V MOSFET Small size and light weight Scalable output power using larger LinkSwitch-TN family members In meter applications, capacitors C1 and C2 are sized for the required holdup time to write the meter reading to non-volatile memory after a power outage. LinkSwitch-TN uses on/off control to regulate the output voltage. When the current into the FEEDBACK (FB) pin exceeds 49 A, MOSFET switching is disabled for the next switching cycle. The 49 A threshold is specified at an FB pin voltage of 1.65 V (7% over the entire operating temperature range), allowing it to be used as a reference. The power processing stage is formed by the LinkSwitch-TN, freewheeling diode D3, output choke L1, and the output capacitor C5. To a first order, the forward voltage drops of D3 and D4 are equal. Therefore, the voltage across C4 tracks the output voltage. The voltage across C4 is sensed and regulated via the resistor divider R4 and R3 connected to the LNK302 FB pin. The values of R3 and R4 are selected such that, at the desired output voltage, the voltage at the FB pin is 1.65 V. Resistor R5 establishes a small 3.6 mA pre-load and is only necessary for applications where regulation to zero load is required. The value of L1 is selected according to the LinkSwitch-TN Design Guide (AN-37) or the PIXIs design spreadsheet (part of PI ExpertTM power supply design software). Both are available at www.powerint.com. Operation Using the buck topology, the circuit shown in Figure 1 generates a non-isolated 12 V, 50 mA output. Typical uses include utility meter and industrial and motor control applications. The AC input is rectified and filtered by D1, D2, C1, C2, RF1, R1, and R2. Two rectifiers improve both line surge withstand (2 kV) and conducted EMI. Resistor RF1 should be a fusible flameproof type, whereas R1 and R2 can be flameproof only. RF1 8.2 2W D1 1N4007 D2 1N4007 R1 220 0.5 W C3 100 nF FB D BP S R3 1.82 k 1% R4 11.0 k 1% C4 1 F 16 V L1 1.5 mH 130 mA D4 1N4005GP 12 V, 50 mA 85-265 VAC C1 4.7 F 400 V R2 220 0.5 W LinkSwitch-TN C2 LNK302 4.7 F D3 400 V UF4005 C5 47 F 16 V R5 3.3 k RTN PI-3918-081104 Figure 1. LinkSwitch-TN 0.6 W, 12 V, Universal Input Power Supply Using a Non-Isolated Buck Topology. DI-80 www.powerint.com September 2004 DI-80 Key Design Points * * * * The output is not safety isolated from the input. The circuit shown in Figure 1 has a total output voltage tolerance of 10% (-20 C to +85 C ambient temperature). To prevent noise coupling and to decrease EMI, place the input filter components physically away from the source node of the LinkSwitch-TN and L1 inductor. The DC input filter capacitors C1 and C2 can be placed as a barrier between the AC input and LNK302 and L1. Their size depends on the minimum operating input voltage and required holdup time. Select ESR of output filter capacitor C5 based on the output voltage noise requirement. * Diode D3 should be an ultra-fast type. A reverse recovery time trr 75 ns should be used at an ambient temperature of +70 C or below. If the ambient temperature is higher, then a diode with trr 35 ns should be used. Feedback diode D4 can be a low-cost slow diode such as 1N400X series, however it should be specified as a glass passivated type to guarantee a specified reverse recovery time. Pre-load R5 is only required for output regulation down to zero load. Use LNK304 for up to 110 mA output current (<2 W true input power and <6 VA apparent input power at 265 VAC). Select output inductor L1 value and current rating accordingly. See LinkSwitch-TN Design Guide for in-depth information. * * * * * 13.0 PI-3916-081004 80 70 3.0 2.5 2.0 1.5 1.0 0.5 0.0 PI-3917-081204 3.0 2.5 2.0 1.5 1.0 Output Voltage (V) 12.5 60 50 12.0 40 30 11.5 85 VAC Load Regulation 265 VAC Load Regulation 85 VAC Efficiency 265 VAC Efficiency 20 10 0 50 True Input Power (W) Apparent Input Power (VA) 0.5 0.0 265 11.0 0 10 20 30 40 85 115 145 175 205 235 Output Current (mA) Figure 2. Output Regulation (left Y-axis) and Efficiency (right Y-axis) Over Load Current at 85 VAC and 265 VAC Input. Input Voltage (VAC) Figure 3. Real Input Power (left Y-axis) and Apparent Input Power (right Y-axis) AC Input Voltage at 50 mA Load Current (0.6 W Load). 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, DPA-Switch and EcoSmart are registered trademarks of Power Integrations. PI Expert and PI FACTS are trademarks of Power Integrations. Copyright 2004, Power Integrations Power Integrations MAIN PHONE NUMBER +1 408-414-9200 B 9/04 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 Apparent Input Power (VA) Real Input Power (W) Efficiency (%) |
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