PCB Power Supply Design Essentials

Key Takeaways

• No power supply system is immune from signal integrity or power integrity problems, but implementing some simple PCB power supply design best practices can help prevent a redesign.

• There are two principal options for regulating DC output from power supplies: a linear regulator or a switching regulator.

• Depending on the type of regulator used in your power supply, there are some circuits you should consider adding to your board, and some layout choices can help suppress conducted and radiated EMI.

PCB power supply design

Whether your next device runs on battery power, a solar element, or plugs into the wall, you’ll need to design circuitry to regulate the power leading to your device. No power supply or the device connected to it is immune from signal integrity or power integrity problems, but implementing some simple design procedures can help prevent a redesign. These best practices include proper component arrangement, decoupling/bypassing, and stackup design.
 

Choosing the Right PCB Power Supply Design Type

The first step in the power supply is to choose the type of power supply you wish to use for your device. Unregulated power supplies are a down-and-dirty option for converting AC power from a wall outlet to a DC voltage. The output from these supplies will contain a ripple waveform as the output is not smoothed with a regulator. Modern applications use a regulated power supply, where this ripple is minimized.

There are two principal options for regulating DC output from power supplies: a linear regulator or a switching regulator, sometimes called a switched-mode power supply. These components pass the DC output from a full wave rectifier to a regulation circuit, which smooths the superimposed ripple waveform on the desired DC output. These regulators can also directly regulate a DC power source like a battery. Linear regulators have very low noise but tend to be bulky due to heatsinks or other active cooling measures required for thermal management. The significant heat dissipation in these power supplies is responsible for their low efficiency.

In contrast, a switched-mode power supply provides much higher efficiency over a broad current range, allowing these power supplies to take on a smaller form factor. However, these power supplies use a pulse width modulation (PWM) circuit to smooth and regulate the output voltage, which requires an active switching component, usually a metal-oxide-semiconductor field-effect transistor (MOSFET). This means the system radiates strong EMI, and the output will contain spikes due to switching noise. This switching noise can appear as a ringing signal on the output (i.e., conducted EMI), which needs to be filtered from the output.

Best Practices for PCB Power Supply Design

For low-power applications, linear regulators and switching regulators are available as integrated circuits. These ICs are ideal for mobile devices or other devices that might plug into a wall outlet but require low power consumption. Regardless of the power consumption in your device, there are some basic PCB design considerations to consider to ensure power integrity and signal integrity.

3D layout with power connection at the edge

Depending on the type of regulator used in your power supply, there are some circuits you should consider adding to your board, and some simple layout choices can help suppress conducted and radiated EMI. In extreme cases, such as with a high current power supply or generally with a switching regulator, you may need to include shielding in your board to ensure signal integrity in nearby circuits.PCB power supply design is about more than just power conversion. Ensuring power integrity in the output from your power supply will help solve some signal integrity issues. Thermal management is also important in power supplies as components dissipate heat during conversion. Consider the following points during PCB power supply design.

Choosing a Regulator in PCB Power Supply Design

The output from linear and switching regulators includes some noise, although the source and effects of noise on your downstream circuits will vary. A linear regulator has less noise but is also less efficient and dissipates more heat. In contrast, a switching regulator replaces ripple noise on the input for switching noise on the output. However, it is easy to control the output voltage from a switching regulator (i.e., as a buck-boost converter) by adjusting the duty cycle of the PWM signal, which provides switching action. The switching regulator will dissipate much less heat thanks to its high efficiency.

Read more about signal integrity in your PCB power supply design.

Power integrity problems can affect signal integrity in any of these traces

Thermal Management Options for Large Regulated Power Supplies

With high current power supplies, you’ll most likely build a regulator circuit you need from discrete components, as the system size will be too large to fit in a standard integrated circuit package. In this case, you must consider thermal management options for any ICs in your power supply’s PCB. If you convert power from a wall outlet to DC, a straightforward solution is to mount a fan on the enclosure and power it using the input AC signal. With DC-to-DC power supplies, you’ll need a PWM signal to run a fan to cool your components.

Your layer stack also plays a role in thermal management. Designing your PCB power supply  on a multilayer board can aid thermal management as the internal copper plane layers help evenly distribute heat throughout the board. Using thermal vias and pads below components that dissipate significant heat can help quickly transport heat away from these components. The goal is to prevent hot spots on your board by allowing heat to dissipate quickly from critical components.

If you’d like to learn more about thermal management in PCB power supply design, read more about thermal analysis for PCBs.

Bypassing and Decoupling for Power Integrity

Once power is sent downstream to your components, various active components can cause ground bounce and ringing in a power bus when ICs switch. This can lead to bit rate errors when many ICs switch simultaneously, as it affects the power received by components and the potential difference between the ON and OFF states in digital signals.ICs that run at lower supply voltage are more prone to these problems as they have a smaller voltage difference between the ON and OFF states.

These problems can be solved by designing a decoupling network and choosing bypass capacitors between the ground and power pins on an IC. The goal in placing bypass capacitors is to compensate for changes in the ground potential when many ICs switch simultaneously. Similarly, a decoupling network is designed for transient oscillations (i.e., ringing) in the power bus when ICs on the bus switch. One tool for designing your power delivery network and decoupling network is to use circuit analysis tools to design the equivalent RLC network that forms these circuits. With the right component choices, you can critically damp transient oscillations in your power delivery network and compensate for ground bounce.

If you’d like to learn more about suppressing transients in a power delivery network, read about using SPICE simulations for time domain analysis in RLC networks.

Transient response in an overdamped RLC network

Conducted EMI Suppression

Noise output from a regulator or an unregulated power supply can impact downstream components and conducted EMI. Severe noise on the power bus can affect the output level from downstream components. Large ripple voltages and switching noise in a switching regulator can create these problems, especially when the power supply provides a high current.

In this case, conducted EMI should be filtered from the power supply output. Since one generally desires a DC output, filtration can be used to remove these higher-frequency components from the power supply output. This is where simulations for filters become essential, as this helps you choose the components you need to build your filter.

Here is some more information on filter design and analysis.

Shielding With Switching Regulators

Switching regulators emit EMI that can affect signal integrity in downstream circuits, especially in analog components. Low-level switching regulators may not produce many problems unless placed close to sensitive components. However, power supplies with high output current can cause involuntary switching in nearby digital circuits or noise spikes in analog circuits, which appear as a transient response in the nearby circuit.

Band stop filtering at the circuit’s natural frequency can effectively remove these current spikes, but this is not practical when working with a large number of components on a board. Instead, it is easier to take advantage of shielding provided by ground planes in your layer stack and arrange sensitive components farther away from the switching regulator. You may need to place some shielding on sensitive components if they are near the switching regulator as this will block radiated EMI.

If you’d like to learn about some strategies for suppressing EMI, read more about EMI suppression techniques in PCB design.

Cadence Brings Layout and Analysis Together for PCB Power Supply Design

With the layout rules required to ensure signal and power integrity in your power supply and overall board, you’ll need the right design, analysis, and adaptable layout tools for any application. Your PCB power supply design and analysis tools should take data directly from your schematic and help you determine the best layout choices for your system.

Your trace and component layout is critical in PCB power supply design

Cadence’s full suite of PCB design and analysis tools is adaptable to any application, including high-speed design. The SI/PI Analysis Point Tools provide designers with power integrity analysis features directly applicable to PCB power supply design. You’ll have access to a complete electronics design and analysis solution when you work with Cadence’s industry-standard suite of design tools, including our new OrCAD X PCB design tool.