PCB layout considerations for non-isolated switching power supplies

Henry J. Zhang, Applications Engineering Manager, Power Products, Linear Technology Corp. -July 20, 2012

Separation of Input Current Paths Among Supplies
Figure 9 shows an application with several onboard switching supplies sharing the same input voltage rail. When these supplies are not synchronized to each other, it is necessary to separate the input current traces to avoid common impedance noise coupling between different power supplies. It is less critical to have local input decoupling capacitor for each power supply.

Figure 9. Separate the Input Current Paths Among Supplies

PolyPhase®, Single Output Converter
For a PolyPhase, single output converter, try to have symmetric layout for each phase. This helps to balance thermal stresses.

Layout Design Example - 1.2V/40A Dual Phase Buck Converter
Figure 10 provides a design example of a 4.5V to 14VIN to 1.2V/40A max dual phase synchronous buck converter using PolyPhase current mode step-down controller, the LTC3855. Before the start of PCB layout, one good practice is to highlight the schematic traces for the high current traces, the noisy high dv/dt traces and the sensitive small signal traces with different colors, so the PCB designer understands the differences between these traces.

Figure 10. Dual Phase 1.2V/40A Max LTC3855 Buck Converter

Figure 11 shows the power stage layout example of the power component layer of this 1.5V/40A supply. In this figure, the QT is the top side control MOSFET and QB is the bottom side synchronous FET. An optional QB footprint is added for even more output current. A solid power ground plane layer is placed just underneath the power component layer.

Figure 11. Example of Power Stage Layout of Dual-Phase, Single-VOUT Buck Converter

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