Switch Mode Power Supplies

When using components such as switch mode power supplies (SMPS), the PCB layout is critical.

Faults in the PCB layout cause a number of problems including switching jitter, poor output voltage regulation and possible failure with the PCB design.

Issues like this can be avoided, saving money and time on scrapped circuit boards and PCB modifications.

When putting together a PCB layout the best approach is to review datasheets for devices such as SMPS. A respectable manufacturer such as Texas Instruments will offer guidelines and reference designs to follow for your PCB layout.

Usually a layout guide will show a PCB design with all of the parts, copper tracks and vias arranged for an optimum performance. It will show lots of space with no other components interfering. This does not happen in the real world, most of the time there is not enough room to keep parts and tracks away from critical areas.

The best way is to follow the guide and bend the rules only when it’s absolutely necessary to do so.

Below is a part of the schematic that show the SMPS and the related components. The second diagram is the recommended PCB layout from the manufacturer.

The decoupling caps (input and output) must be close to the regulator. The most important aspect of SMPS is to reduce high current loops. The recommended layout shows a GND plane under the component, flooding over other components connecting to GND. There is also a skinny blue line representing a track on the bottom side of the board, this is the feedback signal, the track is on the bottom side to reduce interference and must not be in the high current path.

The PCB layout must follow this guide for it to function correctly.

Here is the schematic and PCB layout to highlight the key areas, orange for input, purple for output and burgundy is feedback.

The PCB layout shows the input signal running from a plane (using vias) through the decoupling caps through the inductor and into the module (REG1 highlighted in yellow) via more caps. The output signal runs through decoupling caps and away through the inductor. The GND signals connect to the GND plane through vias at the module and at the decoupling caps.

The burgundy feedback signal is routed from pin 4 of the module, through the resistor and capacitor and connecting to the output signal via C16.

Here is the recommended footprint and PCB design as a comparison.

The recommended footprint shows more GND copper on the component side, but is not that practical on the PCB layout due to a lack of space and the input signal is connected to other pins on the module.

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