Power Supply Pcb Design Guidelines

[Suanne Forte]

Linear power supply circuits are simple in nature, with few components that are straightforward to mount on a PCB. The challenge is that these circuits are inefficient, which results in the need to manage significant power losses into radiated and conducted thermal energy. This problem can be challenging when temperature-sensitive components are mounted on the PCB or enclosed within an environmentally sealed casing for protection, limiting cooling options.

Unless space is constrained, allow for a board design to include a solid power supply ground plane to provide electromagnetic shielding. If devoting a whole layer is not possible, consider as a minimum a ground polygon that covers the entire area under the PCB power supply components.

The ground plane for the PCB power supply design should be separated from the common ground for the rest of the circuit to minimize noise coupling effects. In addition, the connection between these two grounds should be limited to one point on the board to prevent ground loops.

Keep traces for PCB power supply circuits as short and broad as possible to reduce resistive losses and electromagnetic noise emissions. Where space allows, the recommendation is to use polygon pours. This is particularly relevant for linear power supplies where thermal conductivity can be critical.

It would be best to include solid fill internal layers that use vias for connection for power and ground planes in the board design for maximum effect. Using vias to switch PCB power supply traces from one layer to another should be avoided as the via will act as a point of increased impedance. Multiple vias linking polygons offer a better solution.

Another option for increasing conductivity is to add a solder layer to the outer board layers through changes to the solder mask. However, you can obtain better performance by adding PCB bus bars or external wires between the points on the board where PCB power supply design components are mounted.

Driven by the requirement for traces to be as short as possible, PCB power supply components should be located as close together as possible in the optimum orientation to achieve short trace lengths. This can include mounting parts on both sides of the board to achieve this.

Traces carrying sensitive signals should ideally be routed away from power supplies on an unconnected board layer separated from the 12V power supply PCB layout design traces by a ground plane. Signal traces should never run parallel to power traces carrying power to prevent noise-coupling from the layout power supply to the signal. If proximity is unavoidable, signal traces should cross PCB power supply traces at 90 degrees to minimize noise coupling effects.

All layout power supply circuits generate heat, so the board design will need to include thermal management. Therefore, the first layout consideration should be component placement to separate heat-generating components from heat-sensitive components if possible while maintaining short trace lengths.

The next consideration is using the copper of the board to provide thermal conductivity to distribute heat more evenly away from hotspots and to areas that allow heat dissipation.

A potential issue for switch-mode PCB power supplies is that the feedback control circuit often contains temperature-sensitive components that need colocation with the heat-generating switching components. If unchecked, hot spots can cause layout power supply instability and exacerbate thermal issues.

Mark Harris is an engineer's engineer, with over 16 years of diverse experience within the electronics industry, varying from aerospace and defense contracts to small product startups, hobbies and everything in between. Before moving to the United Kingdom, Mark was employed by one of the largest research organizations in Canada; every day brought a different project or challenge involving electronics, mechanics, and software. He also publishes the most extensive open source database library of components for Altium Designer called the Celestial Database Library. Mark has an affinity for open-source hardware and software and the innovative problem-solving required for the day-to-day challenges such projects offer. Electronics are passion; watching a product go from an idea to reality and start interacting with the world is a never-ending source of enjoyment.

Have you ever thought about how power is being transferred within complex PCBs? Yes, it is a challenging job for PCB designers to design a power supply that delivers the required power to each PCB component (ICs, transmitters, capacitors, etc.) since the power requirement for each of these components varies. Only a perfect power supply design can help overcome this challenge.

With the increase in circuit design density and complexity, the complexity of power supply design has also magnified. PCB designers are offered several possibilities for PCB power supply design and layouts. Despite the varieties in PCB power supply designs, designers must follow certain rules and deal with common problems concerning them.

The purpose of the power supply design is not just to convert the power from AC to DC. The function of the power supply is to deliver power to the circuit components at the correct voltage and current. In the future, it will be common to have voltages as low as 1.8V and 1.2V devices. Low voltages bring low tolerance to power supply noises.

Power supplies also require current limits to limit the maximum current. Thus, the important parameters for a power supply are voltage, max current, voltage ripple, and heat loss at maximum current.

The typical power flow of an electronic circuit for a power supply is shown in the above figure. Electronic circuits need voltages from 1.8V to 12V range. 1.2V, 1.8V, 3.3V, 5V, and 12V are the most common voltages used.

In the first stage, input AC voltage at 230VAC/110VAC is converted to an isolated DC voltage in the range of 6-12V. A buck-switching regulator is employed in the second stage which converts the 6-12V to 5V or 3.3V. Further, the 3.3V is converted to 1.8V or 1.2V using LDOs (Low Dropout regulators).

The disadvantages of such circuits were poor power efficiency (less than 80%), high heat loss, a larger PCB footprint, and poor power ripple. The use of switching power supplies has improved the efficiency of converting voltages to lower levels, reduced the PCB footprint of power supplies (very small and light in size), and reduced ripple.

A larger amount of power was previously lost in linear regulators due to higher dropout voltages. For example, consider the linear voltage regulator LM7805. LM7805 (5V) typically has a dropout voltage of approximately 7.5V, requiring a minimum of approximately 2.5V difference between input and output voltage. Therefore, for a 1A regulator, the power loss at 7.5V input will be 2.5V x 1A = 2.5W.

With the low dropout regulator LM1117-5.0, the dropout voltage is 6.2V requiring an input voltage of Vout +1.2V at the input. A combination of switching regulators and LDO is used to increase efficiency for critical applications. For example, from stage one, if 7.5 volts is available, this will be dropped to 3.3V with a buck converter and then dropped to 1.8V using a linear regulator LM1117-1.8.

The importance of a well-laid-out PCB can not be overstated when it comes to designing power supplies. Also, the designer must understand the importance of power supply operation to make the effort a success.

For power supply design, the designer needs to execute a good PCB layout and plan an efficient power distribution network. The PCB designer must ensure that noisy digital circuit power supplies are separated from critical analog circuit power supplies and circuits. Some of the important things to be considered are discussed below:

Generally, designers have two options in choosing power supply regulators: linear regulators and switched-mode regulators. The linear regulators provide low noise output but have higher heat dissipation, which requires cooling systems. The switched-mode regulators are highly efficient over a broad current range, but the switching noise causes spikes in response.

A linear-mode regulator requires an input voltage higher than the required output voltage because there will be a minimum voltage dropout. Linear regulators have considerable power loss and heat dissipation, which makes them less efficient.

If you are considering a linear regulator for your PCB design, then you must consider a regulator with low voltage dropout, and the thermal analysis must be done before going to fabrication. Besides that, linear-mode regulators are simple, cheap, and provide exceptionally noiseless voltage outputs.

The switching regulators convert one voltage to another by temporarily storing energy in inductors and then releasing that energy at a different voltage at different switching times. In such power supplies, fast switching MOSFETs are used. The output from these highly efficient regulators could be adjusted by altering the duty cycle of the Pulse Width Modulation (PWM). The efficiency depends on the heat dissipation of the circuit, which is low in this case.

The PWM switching of switching regulators causes noise or ripple in the output. Switching currents can cause noise crosstalk in other signals. Thus switching power supplies need to be isolated from critical signals.

Switched-mode regulators use MOSFET technology, so it is obvious that they emit EMI (Electromagnetic Interference) noise. We cannot eliminate EMI from any circuit completely, but we can minimize it by EMI-reducing measures like filtering, reducing current loops, ground planes, and shielding. Electromagnetic Compatibility (EMC) measures should be considered before incorporating switched-mode regulators in your design.

If the designer chooses a linear regulator, a heat sink or other cooling methods are recommended if the system allows it. Fans can be incorporated into the design to ensure forced cooling if a device has high heat dissipation.

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