PCB Routing guidelines - Part 1 (Power Supply)

Power supply design is one of the critical factor in circuit design. Designer may design a flawless design but if they don't give sufficient guidelines to the layout engineer or if the layout engineer doesn't follow the appropriate guidelines the whole effort to put in during the design phase may be wasted. Whether you are working with AC or DC, or whether you are working with linear supply or switching regulator, following the recommended guidelines is a must. The thermal performance, functional qualification, certification (EMI/EMC) depends on the proper layout of power supply also. Finally, a good stable power supply is a result of good layout design practices. We always have to 

remember that switching power supplies are the main sources of EMI in the circuits as they contain the switching elements. Based on the switching that is happening in the switching mode power supply the noise is either radiated or coupled to other circuits. 

The collection of layout guidelines that need to be followed by the layout engineer are listed below.

1. Have a separate plane for power and signals. 

2. Isolate the power and signal planes with continuous ground plane.

Note: For the above two points we are talking about the multi-layer boards.

3. The ground path loop should be as small as possible. 

4. Have a complete copper pour underneath the switcher/linear regulator and connect this to thermal pad. This helps to sink the heat to the PCB.

5. Placement is very critical in regulators. Placement must be in such a way that all the components should not be distributed randomly.

Note: For the above point reason is that the return current should not change direction.

6. Never try to connect the grounds in a daisy chaned way. The noise on one circuit may propogate to other easily. Try to use a star type routing for ground. 

7. Isolate the signal and power grounds and have a common node for connecting all together. 

8. Place the capacitors at the input and output of regulator as close as possible. 

Reason: The additional trace acts as inductor affecting the transient response of the switcher.

9. The grounding of the capacitor should be as close as possible to the supply regulator. Avoid longer ground traces. Having a lesser ground trace lengths help to eliminate the unnecessary ground loops.

10. The most important point to remember is that layout engineer should always know the amount of current that flows on the power traces. Correspondingly, the width of the must be layout. 

11. When there are via to connect different points of the circuit, have sufficient stitching via to support desired current.

12. The power trace connecting the various points should not be free flown. The length should be maintained as short as possible.

13. The board will have signal traces and power traces as well. Power traces should be maintained thicker to allow sufficient current to flow. Irrespective of lowest current limit, the power trace should be > 12mils always and further based on current flow, the width will change.

14. Switching regulators have a inductor at their output, take care that you don't route the low power traces near the inductor. 

15. The placement of digital circuit must be such a way that they should be placed away from the inductor.

Reason: The reason for the above two points is that inductors are the source of EMI on the boards.

16. The feedback path in the regulator is very critical to determine the output of regulator. Try to keep the resistive dividers in the feedback path as close as possible to the feedback pin. If not possible, route the feedback trace thicker. 

17. The feedback trace must be away from all the noisy paths. If required provide ground isolation to the feedback paths from other circuits/traces.

18. The components at the output of regulator (inductor, capacitor, schottky) should be placed as much closer as possible. 

19. Have a perfect ground plane around and underneath the regulator. This helps any EMI from the power circuit to ground directly.

What are the effects of bad layout? The effects of layout are more seen as the load current is more.

1. Desired regulation may not happen.It meant the output may vary from the desired output.

2. Too much noise at the output of the regulator.

3. Increases design iterations and hence increases project cost.

4. Has to have a good mechanical design to compensate for bad layout practices.

5. Ground bounce leading to improper ground reference.

6. Simultaneous switching noise affecting signal integrity.