Delivering the required power to each component on a PCB can be a complex challenge. Designers must manage converting AC to DC while also delivering the correct voltage and current to each component. A well-designed PCB results when the designer takes power supply seriously—paying close attention to the effects that power delivery can have on surrounding components, such as through heat management or signal interference.
Maintaining Power Quality and Integrity
The International Electrotechnical Commission defines power quality as:
A set of parameters defining the properties of the power supply as delivered to the user in normal operating conditions in terms of continuity of supply and characteristics of voltage (magnitude, frequency, waveform).
No matter how well designed your power supply is, it can only control the quality of voltage. It cannot control the current that various loads might draw.
Power integrity is achieved when the quality of power delivered to a circuit remains constant and predictable. For a circuit to achieve its desired performance, it needs power to be effectively transferred from the power supply to every component, circuit, and device as expected. Power integrity is measured by how effectively power is transferred from the power source to components in the system. A design needs to ensure that all components are supplied with the appropriate power level to achieve target performance of the entire circuit.
Power quality can be negatively affected by noise, so one important design goal is to minimize that noise. You should start, of course, by being familiar with the power supply requirements for all your ICs and components. Higher quality power supplies can produce less noise, as well.
Keeping It Inside
If your board has three or more layers, keeping the power and ground planes on one of the internal layers can make a lot of sense. Not only can this help add structure to the board, but it gives easy access to both power and ground from other layers while helping to keep your PCB design clean.
Since it is better to supply power to certain components in parallel rather than serial, designing your power plane on an internal layer can really help keep the layout clean on the outer layers. This can keep you from backing yourself into a corner with power delivery and forcing you to daisy-chain your ICs together. You will easily be able to create wide, solid, common power rails without interfering with the rest of your PCB design.
Separating your ground and power planes can also help reduce electromagnetic interference while distributing power. Not only will this reduce strange signal patterns, but it can help prevent unexpected voltage drops, too.
Heat dissipation directly impacts the performance of a power supply. Almost every component will emit some amount of heat when current is applied, and the amount of heat depends on the level of power applied, the particular characteristics of the component, and impedance. Higher temperatures impact the performance of a circuit, which means that power supply design also includes cooling design.
If your design uses a linear regulator to maintain steady voltage and manage power quality, you may need to integrate a cooling method such as heat sinks or fans. However, the right regulator can reduce heat dissipation in a circuit. Switched regulators, in particular, can be helpful for this since they dissipate less heat.
Using Power Supply Problems to Diagnose Circuits
Remember that the power supply can only control the quality of the voltage it provides. It has no control over the current being pulled by various components or loads. Luckily, this means that sometimes power supply issues can help you locate faults in your PCB design. For instance, voltage sags or interruptions can indicate a fault or short circuit somewhere in your design.
Power spikes and unexpected voltage drops could indicate that certain components are drawing unpredictable amounts of power under certain loads. If you are seeing this behavior in your circuit, consider using decoupling and bypass capacitors to mitigate voltage drops and maintain power quality.
A decoupling capacitor reduces impedance between power and ground. It acts like a secondary power source for components by smoothing over voltage drops. Use a bypass capacitor to help reduce flutter, or power fluctuations, which are usually the result of too many ICs pulling voltage simultaneously. Bypass capacitors are usually placed closer to ICs or components.
Good Power Supply Design is a Core Practice
No matter how solid the rest of your PCB design is, without good power supply design, your circuit will not reach the level of reliability you would probably like. You will need to pay attention to not only the physical layout of the power supply on the PCB, but the way voltage fluctuates, spikes, and drops across segments of your circuit. You will also need to watch for heat issues and electromagnetic interference, all of which are core issues that good power supply design is intended to mitigate.
This column originally appeared in the April 2021 issue of Design007 Magazine.