AN1031 Datasheet, PDF (1/15 Pages) Fairchild Semiconductor – Considerations in Designing

April 30, 1999
AN1031
Considerations in Designing the Printed Circuit Boards of Embedded
Switching Power Supplies
Marty Brown
1. Introduction
The importance of a good printed circuit board (PCB) layout in switching power supplies cannot be
overstated. Developing the schematic and debugging the breadboard is a good start, but the final,
critical challenge is to layout the PCB. Fortunately, understanding the phenomenon behind the
operation of the typical switching power supply makes the effort much easier.
2. Discussion
The primary rule for the designer is to be involved with every aspect of the design of the switching
power supply, including the PCB. He or she is the only person who best understands the functional
requirements of the power supply within the final product. In doing this, the power supply designer
should never allow a PCB designer to use the auto-routing routines within the PCB layout program.
The autorouter routine only strives to connect nodes that utilize the same signal as stated in the
netlist. It disregards the length of the traces needed to accomplish this. The autorouter also
considers all grounds the same signal and connects them together without consideration of the
actual types of signals running through certain traces. For the power supply designer and the PCB
designer to execute a good PCB layout, knowing the signals that flow between components is very
important.
Appreciating the subtle “black magic” aspects to the PCB layout is essential to the success of the
product. These layout factors can affect the performance of the switching power supply and can
also affect the product’s ability to be released into the market. The aspects of the product’s opera-
tion that affect the printed circuit board design are: radiated electromagnetic interference (radiated
EMI), conducted EMI, power supply stability, efficiency and operational longevity. The two forms
of EMI are tested by regulatory approval bodies such as UL, IEC, and numerous other regulatory
bodies throughout the world. The product must pass these stringent EMI tests before it can be
sold into its respective market. The remaining factors affect the product's basic operation and
customer satisfaction.
3. Current Loops
Switching power supplies have large current pulses with very sharp edges flowing within the power
supply circuit. These large current pulses have the greatest effect on the creation of EMI, and
should be the primary focus of the PCB designer. These currents flow in definable “loops” and the
circuits carrying these currents should be laid-out first. The low-level control circuitry is then
subsequently coupled into specific spots in the layout. These loops are diagrammed in figure 1 for
the three major basic topologies of switching power supplies. All of the other topologies are varia-
tions of these three.
Rev A, July 1999
1