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SG6846 Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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SG6846 Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 8 page AN-6846 APPLICATION NOTE © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.3.2 • 9/26/08 7 Printed Circuit Board (PCB) Layout High-frequency switching current/voltage makes PCB layout a very important design issue. Good PCB layout minimizes excessive EMI and helps the power supply survive during surge/ESD tests. Guidelines: To get better EMI performance and reduce line frequency ripples, the output of the bridge rectifier should be connected to capacitor C1 first, then to the switching circuits. The high-frequency current loop is in C1 – Transformer – MOSFET – RS – C1. The area enclosed by this current loop should be as small as possible. Keep the traces (especially 4→1) short, direct, and wide. High-voltage traces related to the drain of MOSFET and RCD snubber should be kept far way from control circuits to prevent unnecessary interference. If a heatsink is used for the MOSFET, connect this heatsink to ground. As indicated by 3, the ground of control circuits should be connected first, then to other circuitry. As indicated by 2, the area enclosed by transformer auxiliary winding, D1, C2, D2, and C3 should also be kept small. Place C3 close to the SG6846 for good decoupling. Two suggestions with different pro and cons for ground connections are offered: GND3 → 2 → 4 → 1: This could avoid common impedance interference for sense signal. GND3→2→1→4: This could be better for ESD testing where the earth ground is not available on the power supply. Regarding the ESD discharge path, the charges go from secondary through the transformer stray capacitance to GND2 first. The charges then go from GND2 to GND1 and back to the mains. Note that control circuits should not be placed on the discharge path. Point discharge for common choke can decrease high-frequency impedance and help increase ESD immunity. Should a Y-cap between primary and secondary be required, connect this Y-cap to the positive terminal of C1. If this Y-cap is connected to the primary GND, it should be connected to the negative terminal of C1 (GND1) directly. Point discharge of this Y-cap also helps for ESD. However, the creepage between these two pointed ends should be large enough to satisfy the requirements of applicable standards. Figure 16.Layout Considerations |
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