Although the PCB design process can be fascinating and challenging, it is important to take all necessary precautions to ensure correct circuit operation, especially when dealing with high-power PCBs. As the size of electronic equipment continues to decrease, appropriate consideration must be given to design aspects such as power supply and thermal management. This article describes some guidelines designers can follow to design a PCB suitable for supporting high-power applications.
Width and thickness of the trace
In principle, the longer the orbit, the greater its drag and heat to emit. Since the goal is to minimize power losses to ensure high reliability and durability of the circuit, it is recommended to keep the circuitry that conducts large currents as short as possible. To correctly calculate the width of a track and know the maximum current it can pass through, designers can rely on the formula contained in the IPC-2221 standard, or use an online calculator.
As for the wire thickness, the typical value of the inner layer of a standard PCB is 17.5 m (1/2oz/ft2), and the typical value of the outer layer and adjacent layer is 35 m (1oz/ft2). High-power PCBs usually use thicker copper to reduce the line width at the same current. This reduces the space taken up by wiring on the PCB. Thicker copper ranges in thickness from 35 to 105 μm (1 to 3oz/ft2) and is usually used for currents greater than 10A. Thicker copper inevitably comes at an additional cost, but because of its higher viscosity, it saves space on the card. The required track width is much smaller.
Board layouts should be considered from the early stages of PCB development. An important rule that applies to any high-power PCB is to determine the path that the power follows. The location and amount of power flowing through the circuit is an important factor in assessing the amount of heat the PCB needs to emit. The main factors affecting the layout of printed circuit boards include:
The power level flowing through the circuit;
Evaluate the ambient temperature at which the board works;
Affect the air flow of the plank;
Materials used in the manufacture of PCB;
The density of the components that make up a circuit board.
Even with modern machinery, this need is less pressing, but in the case of directional changes, it is recommended to avoid right angles and instead use 45° angles or curves
It is critical to first determine the location on the PCB of high-power components (such as voltage converters or power transistors) that are responsible for generating large amounts of heat. High power components should not be installed near the edge of the circuit board as this can lead to heat buildup and significant temperature increases. Highly integrated digital components (such as microcontrollers, processors, and FPGAs) should be placed in the center of the PCB to allow heat to spread evenly across the board, thus reducing temperature. In no case should the power components be concentrated in the same area to avoid the formation of hot spots. In contrast, a linear arrangement is preferred.
Placement should begin with power equipment whose wiring should be kept as short as possible and wide enough to eliminate noise generation and unnecessary grounding loops.
In general, the following rules apply:
Identify and reduce current loops, especially large current paths.
Minimize resistive pressure drops and other parasitic phenomena between components.
Keep the high power circuit away from the sensitive circuit.
Take good grounding measures.
In some cases, components can also be placed on several different boards, as long as the size of the device allows.
Proper thermal management must be carried out to maintain each component within a safe temperature range. The junction temperature shall not exceed the limit indicated in the manufacturer’s data sheet (usually between +125°C and +175°C for silicon-based devices). The heat generated by each component is transferred to the outside through the packaging and connecting pins. In recent years, electronics component manufacturers have built an increasing number of thermally compatible packages. Even with the development of these packages, cooling became more and more complex as the size of integrated circuits continued to shrink.
The two main techniques used to improve PCB thermal management include creating larger layers and inserting thermal conductors. The first technique allows you to increase the heat dissipation area on the PCB. Typically, these surfaces are connected to the upper or lower layers of the circuit board to maximize heat exchange with the surrounding environment. However, the inner layer can also be used to extract some of the power consumed by the device on the PCB. Instead, heat through holes are used to transfer heat from one layer of the same plate to another. Their function is to direct heat from the hottest point on the plate to the other layers.
Many components used in electronic circuits, such as regulators, amplifiers, and converters, are extremely sensitive to fluctuations in the surrounding environment. If they detect significant thermal changes, they can alter the signal they produce, creating errors and reducing the reliability of the device. Therefore, it is important that these sensitive components be thermally insulated so that they are not affected by the heat generated on the circuit board.
Resistance welding membrane
Another technique used to allow the wires to carry more current is to remove the solder mask from the PCB. This exposes the underlying copper material, and additional solder can then be added to increase the thickness of the copper and reduce the overall resistance of the PCB’s current-carrying elements. Although it can be seen as more of a solution than a design rule, this technique allows PCB wiring to withstand greater power without increasing the wiring width.
Active components can produce dangerous phenomena, such as ground bouncing and ringing, when distributing and sharing power lines between multiple circuit board components. This may cause a voltage drop near the component’s power pin. To overcome this problem, decoupling capacitors are used: one terminal of the capacitor must be placed as close as possible to the pin of the component receiving the power supply, while the other terminal must be directly connected to the low impedance connection layer. The purpose is to reduce the impedance between the power guide and the ground. The decoupling capacitor acts as a secondary power source, providing the required current to the component during each transient (voltage ripple or noise). There are several considerations when choosing a decoupling capacitor. These factors include the selection of the correct value of the capacitor, the dielectric material, the geometry and the position of the capacitor relative to the electronic components. Typical values of decoupling capacitance are 0.1μF for ceramics.
The design of high-power PCB requires the use of materials with special characteristics, first of all thermal conductivity (TC). Conventional materials, such as low-cost FR-4, have a Tc of about 0.20W/m/K. For high-power applications where heat gain is minimized, it is best to use specific materials such as RogerSRT laminates. The material has a Tc value of up to 1.44W/m/K, which minimizes temperature rise at high power levels.
In addition to using to low loss material processing power and heat, the PCB must also use a very similar coefficient of thermal expansion (CTE) of conductive and thermal conductive materials to manufacture PCB, in this way, because of the high power or high temperature lead to any expansion or contraction of material at the same rate, so as to maximize reduce the mechanical stress on the material.
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