As fascinating and challenging as the design process of a PCB can be, it is important to take all necessary precautions to ensure proper circuit operation, especially when dealing with a high-power PCB. Since the size of electronic devices is constantly and gradually decreasing, design aspects such as power source and thermal management must be taken into consideration. This article will provide some guidelines that a designer can follow to design a suitable PCB to support high power applications. Impact width and thickness In principle, the longer the path, the greater its resistance and the amount of heat to be dissipated. Since the goal is to reduce energy losses, in order to ensure high circuit reliability and durability, the recommendation is to keep the effects that lead to high currents as short as possible. To correctly calculate the width of the track, knowing the maximum current that can pass through it, designers can rely on the formulas given in the IPC-2221 standard, or use an online calculator. For trace thickness, the typical value for a standard PCB is approximately 17.5 μm (1/2 oz / ft2) for the inner layers, and 35 μm (1 oz / ft 2) for the outer layers and for ground planes. Typically, high-powered PCBs use thicker copper to reduce the path width for the same current. This reduces the space taken up by traces on the PCB. The thickness of the thickest copper ranges from 35 to 105 μm (1 to 3 oz / ft2), and is usually used for streams greater than 10A. The thicker copper definitely comes at an additional cost, but it can help save space on the cards since, with higher viscosity, the required track width is much smaller. Layout of PCB Board design should be considered from the very early stages of PCB development. An important rule that applies to any high-powered PCB is determining which path the energy will follow. The location and amount of energy flowing through the circuit is an important factor in evaluating the amount of heat the PCB needs to dissipate. The main factors affecting the design of a printed circuit board include: the level of power flowing through the circuit; Ambient temperature at which the board will operate; The amount of air flow affecting the plate; Materials used to manufacture PCBs; The density of the components that fill the plate. Although this need is less urgent with modern machinery, it is recommended in directional changes to avoid right angles, but by using 45 ° angles, or curved lines, as shown in Figure 1. Figure 1: Angle orientation on positioning of PCB components Of primary importance is firstly the positioning of high-power components on the PCB, such as voltage transformers or power transistors, that are responsible for generating a large amount of heat. High-power components should not be installed near the edges of the panel, as this causes heat build-up and a significant temperature rise. Highly integrated digital components, such as microcontrollers, processors and FPGAs, must be located in the center of the PCB, allowing for the uniform spread of heat across the board and thus a drop in temperature. In any case, the power components should never be concentrated in the same area to avoid the formation of hot spots; Alternatively, a linear arrangement is preferred. Figure 2 shows the thermal analysis of an electronic circuit, with the regions with the highest heat concentration highlighted in red. Figure 2: Thermal analysis of the electronic circuit Figure 2: Thermal analysis of the electronic circuit The placement should begin from power devices, whose effects should be kept as short as possible and wide enough to eliminate the generation of unnecessary noise and ground loops. In general, the following rules apply: Determination and reduction of current loops, in particular high current paths. Reducing resistor voltage drops and other parasitic phenomena between components. Place high-power circuits away from sensitive circuits. Take good grounding measures. In some cases, it may also be preferable to place components on several different plates, as long as the form factor of the device allows this. Thermal Management Adequate thermal management is essential to keep each component within safe temperature limits. The junction temperature should never exceed the limit indicated on the manufacturer’s data sheet (generally between +125 ° C and + 175 ° C for silicone-based devices). The heat generated by each component is transferred to the outside through the beam and connecting pins. In recent years, electronic component manufacturers have increasingly built heat-compatible packages. Even as this package progresses, the heat dissipation becomes increasingly complex as the size of the integrated circuits continues to shrink. The two main techniques used to improve thermal management of PCBs are to create large floor levels and insert thermal vents. The first technology allows you to increase the space available on the PCB for heat dissipation. Often times, these planes are attached to the upper or lower layer of the plate to maximize heat exchange with the surrounding environment; However, the inner layers can also be used to extract a portion of the energy dissipated by the devices on the PCB. Instead, heat pipes are used to transfer heat from one layer to another on the same plate. Its function is to direct heat from the hottest areas on the board to the other layers. Many components used in electronic circuits, such as regulators, amplifiers, and transformers, are very sensitive to fluctuations in the surrounding environment. If they detect large thermal differences, they can change the signal they emit, generate errors, and reduce the reliability of the device. Therefore it is important to isolate these thermally sensitive components, so that they are not affected by the heat generated on the plate. Welding mask Another technique used to allow tracking to carry larger amounts of current is to remove the welding mask from the PCB. This exposes the core copper material which can then be supplemented with additional casters to increase the copper thickness and reduce the overall resistance in the current carrier components of the PCB. Although it can be viewed as more of a solution than a design rule, this technology allows PCB traces to carry more power without the need to increase the trace width. Separation capacitor When an electric rod is distributed and shared between multiple panel components, the active components can generate dangerous phenomena, such as ground recoil and resonance. This can cause a voltage drop near the component power pins. To overcome this problem, separation capacitors are used: one end of the capacitor must be placed as close as possible to the terminal of the component receiving the power supply, while the other end must be connected directly to a low resistance ground level. The goal is to reduce the impedance between the power supply rail and the ground. Separating capacitors act as a secondary power source, supplying components with the current they need during each transient period (voltage or noise ripple). There are several aspects to consider when selecting a separation capacitor. These factors include choosing the correct capacitor value, dielectric material, geometry, and the location of the capacitor in relation to the electronic component. Typical value for separation capacitors is 0.1μF ceramic. Materials The design of high-power PCBs requires the use of materials with certain properties, first and foremost, thermal conductivity (TC). Conventional materials, such as the low-cost FR-4, have a TC of about 0.20 W / m / K. For high-power applications, where heat gain is required to be minimized, it is best to use specific materials, such as Rogers RT sheets. With a TC value of 1.44 W / m / K, this material deals with high energy levels with minimal warming. In addition to using materials that can handle energy and heat with low losses, PCBs must be manufactured using conductive and thermal materials with a very similar thermal expansion coefficient (CTE), so that expansion or contraction of materials due to high energy or temperatures occur at the same rate. , Which reduces the mechanical stress on the material. .
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