7 considerations for PCB power supply design

Have you ever wondered how power is delivered within a complex PCB? Yes, it is a challenging job for PCB designers to design a power supply that provides the required power to each PCB component (IC, capacitor, etc.) because the power requirements of each of these components vary. Only a perfect power supply design can help overcome this challenge.

As circuit design density and complexity increase, the complexity of power supply design is also amplified. PCB designers are provided with many possibilities for PCB power supply design and layout. Although PCB power supply designs are diverse, designers must follow certain rules and deal with common issues associated with them.

Some common issues to deal with in power supply design are, electromagnetic interference, trace design to handle high currents, reducing current loops, component selection, and following datasheet layout recommendations.

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In this article we will cover the following topics:

PCB power supply design

PCB Power Supply Design Considerations Choosing the Right Regulator for the PCB Power Supply Thermal Management of the Power Supply Ground and Power Planes Provide Better PCB Power Supply Decoupling Capacitors and Bypass Capacitors EMI Filtering Frequency Response of the Power Delivery System Power Integrity (PI) )

The purpose of power supply design is not just to convert power from AC to DC. The purpose of a power supply is to provide power to circuit components at the correct voltage and current. Devices with voltages as low as 1.8V and 1.2V will be common in the future. Low voltage brings low tolerance to power supply noise.

The power supply also requires current limiting to limit the maximum current. Therefore, the important parameters of a power supply are voltage, maximum current, voltage ripple and heat loss at maximum current.

Typical power flow block diagram in electronic circuits

Typical power flow for powering electronic circuits is shown in the figure above. Electronic circuits require voltages in the range of 1.8V to 12V. 1.2V, 1.8V, 3.3V, 5V and 12V are the most commonly used voltages.

In the first stage, the input AC voltage of 230VAC/110VAC is converted to an isolated DC voltage in the 6-12V range. The second stage uses a buck switching regulator to convert 6-12V to 5V or 3.3V. Additionally, use an LDO (Low Dropout Regulator) to convert 3.3V to 1.8V or 1.2V.

Before the advent of switching power supplies (SMPS – Switching Power Supplies), iron core transformers were used to convert high voltage of 230VAC/110VAC to 12VAC. This is further rectified by a diode bridge rectifier to a DC voltage with a maximum value of approximately 12 x 1.4 = 16.8 VDC. Linear regulators are used to step down the voltage to a desired level. The disadvantages of this circuit are poor power efficiency (less than 80%), high heat loss, large PCB footprint, and poor power supply ripple. The use of switching power supplies increases the efficiency of converting voltage to lower levels, reduces the PCB footprint of the power supply (it is very small and lightweight), and reduces ripple.

In linear regulators, a lot of power used to be lost due to higher voltage drops. For example, consider the linear regulator LM7805. The LM7805 (5V) typically has a voltage drop of approximately 7.5V and requires at least approximately 2.5V difference between the input and output voltages. Therefore, for a 1A regulator, the power loss in the regulator at 7.5V input is 2.5V x 1A = 2.5W. Using the low dropout voltage regulator LM1117-5.0, the dropout voltage is 6.2V, requiring the input voltage at the input terminal to be Vout +1.2V. For critical applications, a combination of switching regulators and LDOs are used to increase efficiency. For example, starting with the first stage, if 7.5V is available, it will step down to 3.3V using a buck converter and then to 1.8V using the linear regulator LM1117-1.8.

PCB power supply design considerations

When designing a power supply, the importance of a well laid out PCB cannot be overstated. Additionally, designers must understand the importance of power supply operation in order for the job to be successful.

For power supply design, the designer needs to execute the PCB layout well and plan an effective power distribution network. Additionally, designers need to ensure that noisy digital circuit power supplies are separated from critical analog circuit power supplies and circuitry. Some important things to consider are discussed below:

Voltage stabilizing circuit board

1. Choose the right voltage regulator for your PCB power supply

Typically, designers have two options when selecting a power supply regulator, linear regulators and switch-mode regulators. Linear regulators provide low noise output, but it has higher heat dissipation, which requires a cooling system. Switch-mode regulators are efficient over a wide current range, but switching noise can cause response spikes.

A linear mode requires the input voltage to be higher than the required output voltage because there will be minimal voltage drop. Linear regulators will have considerable power losses and heat dissipation, which will reduce the efficiency of the linear regulator. If you are considering a linear regulator PCB design for your device then you must consider a regulator with low voltage drop and thermal analysis must be done before fabrication. Beyond that, linear mode regulators are simple, cheap, and provide exceptionally noise-free voltage output.

This switching regulator converts one voltage to another by temporarily storing energy in an inductor and then releasing that energy at different voltages at different switching times. In this type of power supply, fast switching MOSFETs are used. The output of these high-efficiency regulators can be adjusted by changing the duty cycle of pulse-width modulation (PWM). Efficiency depends on the heat dissipation of the circuit, which in this case is very low.

The PWM switching of a switching regulator can cause noise or ripple in the output. Switching currents can cause noise crosstalk in other signals. Therefore, switching power supplies need to be isolated from critical signals.

Switch mode regulators use MOSFET technology, so obviously these regulators emit EMI (Electromagnetic Interference) noise. We cannot completely eliminate EMI from any circuit, but we can minimize it through EMI-reducing measures such as filtering, reducing current loops, ground planes, and shielding. Electromagnetic compatibility (EMC) measures should be considered before adding a switch-mode regulator to your design.

When choosing a voltage regulator, linear regulated power supplies and switching regulated power supplies are the two obvious choices. Linear controlled power supplies are cheaper, but are less efficient and dissipate more heat. At the same time, switching regulated power supplies are more expensive, require more passive components to be connected, and are less likely to generate heat.

2. Power thermal management

The performance of a power supply is directly dependent on heat dissipation. Most electronic components will heat up whenever current flows through them. The amount of heat dissipated depends on the power level, characteristics and impedance of the component. As mentioned before, choosing the right voltage regulator can reduce heat dissipation in the circuit. Switching regulators are very efficient because they dissipate less heat.

Electronic circuits operate more efficiently at lower temperatures. To ensure equipment operates at ambient temperatures, designers should consider appropriate cooling methods.

If the designer chooses a linear regulator, it is recommended to use a heat sink or other cooling method if the system permits. If the device dissipates a lot of heat, fans can be built into the design to ensure forced cooling.

Heat dissipation may be uneven throughout the PCB. Components with high power ratings may dissipate large amounts of heat, creating hot spots around them. Thermal vents can be used near these components to quickly move heat away from the area.

A combination of thermal technologies and cooling methods can create efficient power supply designs. Designers can use conductive cooling methods (such as heat sinks, heat pipes, thermal vias) or convection cooling methods (such as cooling fans, thermoelectric coolers, etc.).

3. Ground and power planes for better PCB power

Ground plane and power plane connections overlap

Ground and power planes are low-impedance paths for power transfer. Power supplies require separate ground planes to distribute power, reduce electromagnetic interference, minimize crosstalk and reduce voltage drops. Power planes are dedicated to delivering power to the required areas of the PCB.

PCB designers need to handle each part of the ground network individually. In a multilayer PCB, one or more layers can be dedicated to ground and power planes. Also, they can reduce interference and crosstalk by placing a ground plane between the two active signal layers, effectively surrounding the signal traces with ground.

4. Decoupling capacitors and bypass capacitors

Power flow in common power supply designs

When power is distributed to components across the board, different active components can cause ground to bounce and ring in the power rails. This can cause a voltage drop near the component’s power pins. In these cases, designers use decoupling and bypass capacitors near the component’s power pins to provide short spikes that meet the device’s current requirements.

The concept behind decoupling is to reduce the impedance between power and ground. This decoupling capacitor acts as an auxiliary power supply, providing the current required by the IC. and acts as a local charge source to support switching events.

Bypass capacitors bypass noise and reduce fluctuations on the power bus. They are placed closer to the device or IC and connected between power and ground to compensate for changes in power and ground plane potential when many ICs switch simultaneously.

Bypass capacitors are used to suppress inter-system or intra-system noise within the system grid. All decoupling capacitors must be connected close to the IC’s power pins with the other end connected directly to a low-impedance ground plane. Short traces to the decoupling capacitors and ground vias are required to minimize the added series inductance of this connection.

There are several aspects to consider when selecting a local bypass capacitor. These factors include selecting the correct capacitor value, dielectric material, geometry, and location of the capacitor relative to the IC. A typical value for the decoupling capacitor is a 0.1μF ceramic capacitor.

5. EMI filtering

Any power cord entering or exiting the power supply enclosure may generate EMI radiation. PCB designers want power supplies to keep their EMI levels below their defined spectrum limits. Therefore, use EMI filters at the power input point to reduce conducted noise.

Read more: 7 Tips and PCB Design Guidelines for EMI and EMC

The architecture of the EMI filter allows it to block high frequency noise. It is critical that designers carefully place filter circuit components to prevent components from transferring energy into the traces connecting them

6. Frequency response of power transmission system

When the power supply is suddenly loaded, such as from no load to full load, the voltage output will tend to drop briefly and return to normal voltage. In some cases, the output will oscillate for a period of time before the voltage settles to normal. If the oscillation exceeds the design limits, the output capacitor and compensation capacitor will need to be adjusted. For example, for the LM7805, it is recommended to place a 0.1μF capacitor next to the output pin. Likewise, sudden unloading of the regulator can cause overshoot and oscillation.

To get better response from your circuit design, make sure the components you select are within the design constraints. Regardless of whether the circuit is AC or DC, they all respond differently. AC and DC circuits should be considered separately.

7. Power Integrity (PI)

Designers should ensure power integrity of the power supply design. Power integrity is simply the quality of power delivered to a circuit. It is a measure of how effectively power is delivered from the source to the loads within the system, ensuring that all circuits and devices are providing the appropriate power to achieve the required circuit performance.

A less noisy power supply ensures greater power integrity. Power integrity design is nothing more than managing power supply noise. There are simulation tools that can help estimate the power quality in a circuit. Such tools can help estimate voltage drops, recommend the placement of decoupling capacitors, and can also identify high-current hot spots in a circuit.

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In conclusion

A good power supply is key to the accurate operation of electronic equipment. As we have seen, PCB designers have several options when considering power supply designs. Among these considerations, the selection of voltage regulators, capacitors, and EMI filtering are important. Likewise, thermal effects and load response should be considered when designing power systems.

Also, follow the recommendations mentioned in the power IC datasheet. Trace thickness and component placement play a vital role in power supply design

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