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| Design Idea DI-37 DPA-SwitchTM Application Telecom Device DPA424R 16.5 W DC-DC Converter Power Output 16.5 W Input Voltage 36-75 VDC Output Voltage 3.3 V Topology Forward Sync. Rec. Design Highlights * * * * * * * Low cost 400 kHz synchronous rectification design Low component count Efficiency - 87% at 48 VDC No current sense resistor or current transformer required Output overload, open loop and thermal protection Integrated UV meets ETSI standard voltages and linearly reduces the maximum duty cycle with input voltage to prevent core saturation during load transients. Resistor R3 programs the DPA-Switch current limit to 60% of nominal to limit fault and overload power. Drain voltage clamping is provided by Zener diode VR1. Transformer core reset is controlled by the gate capacitance of Q1. Resistor R15 charges the gate of Q2, the forward synchronous rectifier MOSFET. The catch synchronous rectifier MOSFET (Q1) is directly driven by the transformer (T1) reset voltage and operates only when Q2 is off. Diode D2 provides a conduction path for the output inductor (L2) current when the transformer reset is complete. Operation DPA-Switch greatly simplifies the design compared to a discrete implementation. Resistor R1 programs the under/over C7 1 nF 1.5 kV + VIN 36-75 VDC L1 1 H 2.5 A R14 10 L2 C12 C11 C10 100 F 100 F 100 F 6.3 V 6.3 V 6.3 V C13 1 F 0805 3.3 V, 5 A R1 619 k 1% R15 10 D2 B540C Q1 Si4800 DY D1 BAV 19WS RTN T1 Q2 Si4800 DY C4 4.7 F 20 V U2 R7 10 k C1, C2 1 F 100 V R10 3.24 k 1% C16 100 nF DPA-Switch D L CONTROL U1 DPA424R C U2 PC357N1T D3 BAV19WS R6 150 C15 10 F 10 V U3 LM431AIM3 R12 5.1 S X F R9 220 VR1 SMBJ 150 VIN R3 11.1 k 1% C5 220 nF R4 1.0 C6 68 F 10 V C14 1 F R11 10.0 k 1% PI-3650-072004 Figure 1. DPA424R-16.5 W, 3.3 V, 5 A DC-DC Converter. DI-37 www.powerint.com July 2004 DI-37 Key Design Points * Transformer core reset is critical in this design. MOSFET gate loading will affect the transformer reset waveform. Capacitor CQ1gs will load transformer reset. Choose Q1 MOSFET such that CQ1gs provides sufficient reset at low line and safe maximum drain voltage at high line. Choose synchronous rectifier MOSFETs which have both low RDS(ON) and also low Qg (combination of gate charge Qgs and Miller capacitance Qgd). * * * Reduce transformer leakage inductance by filling each winding layer across the entire width of the bobbin. Choose a low-drop Schottky diode (such as Vishay SL44 - Vf = 0.42 V), to increase high line efficiency. Choose a larger DPA-Switch to increase efficiency at low and medium input voltages. * TRANSFORMER PARAMETERS Core Material Bobbin Winding Details Winding Order and Pin Numbers Primary Inductance Epcos P/N: P 14 x 8 N87, ungapped 8-pin P 14 x 8 surface mount bobbin Primary 10T + 10T, 1 x 26 AWG 3.3 V, 4 x 26 AWG Primary-1 (4-NC), 3.3 V (5-6), Primary-2 (NC-1) 600 H 25% at 400 kHz INDUCTOR PARAMETERS Core Material Bobbin Winding Details Winding Order and Pin Numbers Inductance Epcos P/N: P 14 x 8 N87, gap for inductor 8-pin P 14 x 8 surface mount bobbin Main 10T, 2 x 24 AWG Bias 30T, 1 x 32 AWG Main (7,8-5,6) Bias (1-2) Pin (7,8-5,6): 16 H 10% at 400 kHz Primary Resonant 3.8 MHz (minimum) Frequency Leakage Inductance 1 H (maximum) Table 2. Inductor Design Parameters. Table 1. Transformer Design Parameters. 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 B 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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