Have you ever thought about how power is transferred in a complex PCB? Yes, it is a tough job for a PCB designer to design a power supply to provide the required power to each PCB component (IC, transmitter, capacitor, etc.) because each component has different power requirements. Only a sound power supply design can help overcome this challenge.
As circuit design density and complexity increase, so does the complexity of power supply design. Several possibilities for PCB power supply design and layout are provided to the PCB designer. Despite the diversity of PCB power supply designs, designers must follow certain rules and deal with common problems associated with them.
Some common issues to deal with in power supply design include EMI, trace design to handle high currents, reducing current loops, selecting components, and following datasheet layout recommendations.
In this article, we will cover the following topics: PCB Power Supply Design, Design Considerations for PCB Power Supplies, Choosing the Right Regulator for PCB Power Supplies, Thermal Management of Power Supplies, Ground and Power Planes for Better PCB Power Supplies, Go to Coupling and bypass capacitors, EMI filtering, frequency response of power transmission systems, power integrity (PI), PCB power supply design.
The purpose of power supply design is not only to convert power from AC to DC. The function of a power supply is to deliver power to circuit components at the correct voltage and current. In the future, voltages will typically be as low as 1.8V and 1.2V devices. Low voltage reduces 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.
A typical power flow of an electronic circuit for a power supply is shown. Electronic circuits require voltages ranging from 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 range of 6-12V. The second stage uses a step-down switching regulator to convert the 6-12V to 5V or 3.3V. Also, use an LDO (Low Dropout Regulator) to convert the 3.3V to 1.8V or 1.2V.
Before the advent of Switch Mode Power Supplies (SMPS – Switching Mode Power Supplies), iron core transformers were used to convert 230VAC/110VAC high voltage to 12VAC. This is further rectified by a diode bridge rectifier to a maximum DC voltage of approximately 12 x 1.4 = 16.8 VDC. A linear regulator is used to step down the voltage to the required level. The disadvantages of such circuits are low power efficiency (less than 80%), high heat loss, large PCB footprint, and poor power ripple. The use of switching power supplies increases the efficiency of converting voltages to lower levels, reduces the PCB footprint of the power supply (very small and lightweight in size), and reduces ripple.
In linear regulators, a lot of power used to be lost due to the high dropout voltage. For example, consider the LM7805 linear regulator. The LM7805 (5V) typically has a dropout voltage of about 7.5V, requiring a minimum difference of about 2.5V between the input and output voltages. So for a 1A regulator, with a 7.5V input, the regulator dissipates 2.5V x 1A = 2.5W. When using the low-dropout voltage regulator LM1117-5.0, the dropout voltage is 6.2V, and the input voltage required to be input is Vout + 1.2V. For critical applications, a combination of switching regulators and LDOs can be used to increase efficiency. For example, from the first stage, if 7.5 volts are available, this voltage will be stepped down to 3.3V by a buck converter and then down to 1.8V using a linear regulator LM1117-1.8.
Design considerations for PCB power supplies, When designing a power supply, the designer must understand the importance of power supply operation in order to make the job successful.
For power supply design, designers need to perform good PCB layout and plan effective power distribution network. In addition, designers need to ensure that noisy digital circuit supplies are separated from critical analog circuit supplies and circuitry. Some important things to consider are discussed below:
1. Choose the right voltage regulator for PCB power supply
Typically, designers have two choices when selecting a power regulator, namely linear regulators and switch-mode regulators. Linear regulators provide low-noise outputs but have high heat dissipation, requiring cooling of the system. Switch-mode regulators are very efficient over a wide current range, but switching noise can cause spiky responses.
A linear mode requires the input voltage to be higher than the desired output voltage because there will be a minimum dropout voltage. A linear regulator will have considerable power loss and heat dissipation, which will make the linear regulator less efficient. If a linear regulator is to be considered for PCB design, one with a low dropout voltage must be considered and a thermal analysis must be done before proceeding to fabrication. Besides that, linear mode regulators are simple, cheap, and provide a noise-free voltage output.
The switching regulator converts one voltage into another by temporarily storing energy in an inductor and then releasing this energy to different voltages at different switching times. In this power supply, fast switching MOSFETs are used. The output of these high-efficiency regulators can be adjusted by changing the duty cycle of the pulse width modulation (PWM). Efficiency depends on the heat dissipation of the circuit, which in this case is 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.
Switching regulators use MOSFET technology, so obviously, these regulators emit EMI (Electromagnetic Interference) noise. We cannot completely eliminate EMI in any circuit, but it can be minimized through EMI reduction measures such as filtering, reducing current loops, ground planes, and shielding. Electromagnetic compatibility (EMC) measures should be considered before including a switch-mode regulator in your design.
Linear and switching regulated power supplies are two obvious choices when choosing a voltage regulator. Linearly controlled power supplies are less expensive, but are inefficient and dissipate more heat. At the same time, switching regulated power supplies are more expensive and need to connect more passive components, which are not prone to generate a lot of heat.
2. Thermal management of power supply
The performance of a power supply is directly dependent on heat dissipation. Most electronic components heat up whenever electricity passes through them. The heat dissipated depends on the power level, characteristics and impedance of the components. As mentioned earlier, choosing the right regulator can reduce the heat dissipation of the circuit. Switching regulators dissipate less heat, so they are very efficient.
Electronic circuits work more efficiently at lower temperatures. To ensure that the device operates at ambient temperature, the designer should consider proper cooling methods.
If the designer chooses a linear regulator, a heat sink or other cooling method is recommended, if the system allows it. Fans can be integrated into the design to ensure forced cooling where the device heat dissipation is high.
Heat dissipation may not be uniform across the PCB. Components with high power ratings can dissipate a lot of heat, creating hot spots around them. Thermal vias can be used near these components to quickly transfer heat away from the area.
A combination of thermal techniques and cooling methods can create an efficient power supply design. Designers can use conduction cooling methods (such as heat sinks, heat pipes, thermal vias), or convective cooling methods (such as cooling fans, thermoelectric coolers, etc.).
3. Ground plane and power plane to improve PCB power supply
Ground and power planes are low-impedance paths for power transfer. Power supplies require a separate ground plane to distribute power, reduce EMI, minimize crosstalk, and reduce voltage drop. Power planes are dedicated to delivering power to the desired areas of the PCB.
PCB designers need to deal with each part of the ground network separately. 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 two active signal layers, effectively connecting signal traces to ground.
4. Decoupling capacitors and bypass capacitors
When power is distributed to components across the board, disparate active components will cause ground bounce and ringing in the power rails. This can cause a voltage drop near the power pin of the component. In this case, designers use decoupling and bypass capacitors near the component's power supply pins to create brief spikes in the device's current demand.
The concept behind decoupling is to reduce the impedance between the power supply and ground. A decoupling capacitor acts as a secondary 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 ripple in the power bus. They are placed close to the device or IC and are linked between power and ground to compensate for changes in power and ground plane potential when many ICs are switching simultaneously.
Bypass capacitors are used to suppress inter-system or intra-system noise within the grid. All decoupling capacitors must be connected close to the power supply pins of the IC, with the other end directly connected to a low-impedance ground plane. Short traces to the decoupling capacitors and ground vias are required to minimize the series inductance of this connection.
There are several aspects to consider when selecting a local bypass capacitor. These factors include selection of the correct capacitor value, dielectric material, geometry and placement of the capacitor relative to the IC. Typical values for decoupling capacitors are 0.1µF ceramic capacitors.
5. EMI filtering
EMI radiation can come from any power wires entering or exiting the power supply enclosure. PCB designers expect power supplies to keep their EMI below their defined spectral limits. Therefore, an EMI filter is used at the power input point to reduce conducted noise.
The architecture of the EMI filter allows it to block high frequency noise. It is critical that the designer carefully lay out the filter circuit components to prevent the components from transferring energy into the traces connecting them.
6. Frequency response of power supply 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 while before the voltage stabilizes to normal levels. If the oscillation exceeds the design limit, it is necessary to adjust the output capacitor and compensation capacitor. For example, for the LM7805, it is recommended to place a 0.1µF capacitor next to the output pin. Also, sudden unloading of the regulator can cause overshoot and oscillations.
To get better response from circuit design, make sure that the selected components are within the design constraints. Whether the circuit is AC or DC, they respond differently. AC and DC circuits should be considered separately.
7. Power Integrity (PI)
The designer should ensure the power integrity of the power supply design. Power integrity is simply the quality of power delivered to a circuit. This is a measure of the efficiency with which power is transferred from the system to the loads within the system, ensuring that all circuits and devices are supplied with the proper power to achieve the desired circuit performance.
A less noisy power supply ensures higher power integrity, and the core of power integrity design is managing power supply noise. There are several simulation tools to help estimate the power quality of a circuit. Such tools help estimate voltage drops, suggest decoupling capacitors, and identify hot spots of high current in the circuit.
A good power supply is critical to the accurate operation of electronic equipment. As we have seen, PCB designers have several options when considering power supply designs. Considering these factors, it is very important to choose a voltage regulator, capacitor and EMI filtering. Similarly, thermal effects and load response should also be considered when designing a power system. In the meantime, follow the recommendations mentioned in the power IC datasheet. Trace thickness and component placement play a critical role in power supply design.
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