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2020.09.09 AC/DC

Points to Note Relating to PCB Layout

Design of a Secondary-Side Synchronous Rectifying Circuit to Improve the Efficiency of an AC/DC Converter

This article addresses points to be noted relating to the trace and component layout of the mounting board (PCB) in this design.

Points to Note Relating to PCB Layout

Taking the following circuit diagram (low-side type) as an example, points to be noted relating to the PCB layout are summarized. There are some requirements that are specific to this circuit, but nearly all the issues involved are based on the fundamentals of layout in a switching power supply circuit. Points ① to ⑦ requiring attention are indicated in the circuit diagram.

Point①: When the VCC line is affected by switching noise, malfunctioning is possible. Hence it is recommended that the capacitor CVCC be connected by an independent line between the VCC pin and the SR_GND pin, as close to the pins as possible.

Point②: The line connected to the SH_IN pin is a high-impedance line. In order to avoid crosstalk, it should be made as short as possible, and should be arranged so as not to be parallel to a switching line.

Point③: Because the MAX_TON pin can affect the forced off time if it is itself affected by switching, RTON, R3, and C1 should be connected as close as possible to the MAX_TON pin, and an independent line should be used to connect these to the SR_GND pin.

Point④: Because in synchronous rectification control the voltage VDS2 generated by the secondary-side MOSFET M2 must be precisely monitored, independent lines must always be used to connect the IC drain pin to the M2 drain and the SR_GND pin to the M2 source.

Point⑤: It is recommended that the shunt regulator ground (SH_GND) be connected to the secondary-side output ground and that the feedback resistors RFB1 and RFB2 be connected to the secondary-side output VOUT using independent lines.

Point⑥: The drain pin is on a switching line with an amplitude of roughly 0 to 100 V, and so the line should be as short and as narrow as possible.

Point⑦: When inserting a snubber circuit between the drain and source of the MOSFET M2, an independent line that is as short and thick as possible should be used to connect the transformer output and the M2 source.

An example of a PCB layout is shown below, where the top is on the left and the bottom is on the right. Of the above points, ① through ⑥ are shown. It should be referred to as a model of PCB layout.

The next article will present a final summary.

Key Points:

・In secondary-side synchronous rectification as well, nearly all the important points pertaining to PCB layout are based on the fundamentals of layout in switching power supply circuits.

Design Example for PWM Flyback Converter

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