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FAN6755 Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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FAN6755 Datasheet(HTML) 8 Page - Fairchild Semiconductor |
8 / 9 page AN-6755 APPLICATION NOTE © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.2 • 1/14/11 8 Printed Circuit Board (PCB) Layout High-frequency switching current / voltage makes PCB layout a very important design consideration. Good PCB layout minimizes excessive EMI and helps the power supply survive during surge / ESD tests. Guidelines: To improve 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 shown in Figure 18 is C1 – Transformer – MOSFET – RS. The area enclosed by this current loop should be as small as possible. Keep the traces (especially 4→1) short, and wide. High-voltage traces related to the drain of the MOSFET and RCD snubber should be kept 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 for the control circuits should be connected together, then to the current-loop ground 2 at a single point close to the ground connection of capacitor C3. 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 FAN6755 for good decoupling. For high-level surge, this auxiliary ground must be connected to the bulk capacitor directly. This method can improve the surge capability of the system. Two suggestions with different pro and cons for ground connections are offered: GND3→2→4→1: This may avoid common impedance interference for sense signals. GND3→2→1→4: This can be better for EMI testing where the earth ground is not available on the power supply. Regarding the EMI 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. Control circuits should not be placed on the discharge path. Point discharge for common choke can decrease high-frequency impedance and increase EMI 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 EMI; however, the creepage between these two pointed ends should be large enough to satisfy the requirements of applicable standards. Figure 18.Layout Considerations |
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