Views: 0 Author: Site Editor Publish Time: 2024-06-21 Origin: Site
High-power PCB is a printed circuit board made of heavy copper. Compared with other circuit boards, high-power PCB is able to handle higher current rates, it can resist high temperatures for a long time, and provides strong connection points.
High Power PCB Features
High power PCB designs are manufactured for specific devices that require higher amounts of current and are often subject to varying temperatures.
In order for them to perform effectively, high power PCB designs contain the following features: Copper layers in high power PCB designs are thicker and heavier than copper layers in other PCBs, capable of conducting higher currents.
This ability to conduct higher currents is combined with the ability to dissipate heat, which helps ensure that short circuits do not occur during the operation of a device made from the circuit board. For these reasons, high-power PCBs are able to resist and adapt to the fluctuating temperatures of device use.
1. Types of High-Power PCB Design
There are many high-power PCBs available on the market. Here are 3 common classification standards:
1. Double-sided high-power PCB
These are high-power printed circuit boards that allow components to be installed on both sides. This is an entry-level product made using high-power PCBs.
Using vias, alternating wiring between the top and bottom layers makes them more efficient and reliable compared to single-sided high-power printed circuits.
2. Rigid-Flex High-Power PCB Design
High-power printed circuits are composed of rigid and flexible circuit substrates.
Typically, rigid-flex high-power boards are composed of multiple layers of flexible substrates, which are then connected to one or more rigid boards.
Rigid-Flex PCB Design
This attachment is done internally or externally, and the intended application of the high-power rigid-flex board is critical in determining how the connection is done.
In addition, flexible components are designed to be flexible at all times. This flexibility is useful in corners and areas where extra space is needed. Rigid substrates are helpful in areas where additional support is needed.
With these features, it is ensured that these high-power rigid-flex boards can bend during manufacturing and installation. Rigid-flex technology enables high-power PCBs to fit into smaller applications, which leads to enhanced performance and convenience.
3. Multi-layer high-power PCB design
Multi-layer high-power circuit boards have at least three conductive layers. Cross-circuit board plated through holes are the most commonly used electrical connection strategy in these boards.
Depending on the purpose of manufacturing the circuit board, the conductive layers can be as many as twelve layers. However, some companies are now manufacturing PCBs with up to 100 layers, providing room for manufacturing some of the most complex high-power PCB applications.
Multilayer PCB
2. Advantages of High-Power PCB Design
High Power PCB Design
1. Increase tolerance to thermal strain
The thick copper of high-power PCB enables it to withstand the thermal stress it is subjected to. Therefore, devices made of high-power PCBs are able to resist thermal fluctuations, making them reliable, so they are generally used in the manufacture of military applications.
2. Increase current carrying capacity
Heavy copper also enables high-power PCBs to conduct large currents without much stress. High currents on PCBs with lighter copper are prone to failure and breakdown.
Devices such as power transformers are exposed to very high currents, and without high-power PCBs, they are likely to fail or cause some circuit disasters.
3. Increase mechanical strength of connector parts and PTH holes
The heavy copper used to make high-power PCBs gives them mechanical strength, which is very important for supporting components mounted on the board. The connector parts are strengthened in high-power PCBs. This extends to the through holes, which are also made of copper.
4. Reduce product size
High-power PCB design also helps to reduce product size. This is achieved by combining multiple copper weights onto the same layer of the circuit, explaining its preference in military applications, as most products must be portable.
5. Heat transfer to external heat sink
Using heavy copper plated through-holes, high current transfer through the circuit board can be achieved. This helps transfer heat to an external heat sink, making high-power PCBs the most effective circuit board for applications that require high current to operate efficiently.
Heat sinks in high-power PCB designs can also be plated directly on the board, which explains why high-power PCB designs are often used in industry.
3. Example High Power PCB Design Schematic
Here is an example of a high-power PCB design based on the Atmega328 microcontroller. The board controls two DC motors with an integrated H-bridge driver. Since the H-bridge can also drive almost any inductive/resistive load, it can also drive a high-current LED panel, specifying that each output drives a 15A load, for a total of 30A.
High Power PCB Design
The schematic above uses two VNH5019A integrated H-bridge drivers, each of which can drive 30A continuously. The Atmega328 will control the logic of the driver, and a single 12VDC supply will power the board.
The dropout from the switching regulator will provide the 5VDC supply to the ATmega. The VNH has pulled high all the logic, except for the Ina/b lines which provide the direction of rotation for the motor. If more control is needed, you can control the ENa/b pins from the ATmega.
The VNH is able to handle most of the flyback protection, only a 1000uF electrolytic capacitor is required. A 74651195R 85A screw terminal is used here as the main 12VDC power input, and two 1792229 30A snap terminal blocks are used as the motor outputs.
Each VNH driver has a 30A input fuse and a 15A fuse. The fuse for the driver is before the driver, and because the driver can provide 30A, the fuse should blow before the driver is overloaded.
This ensures that both sides are cut off when the fuse blows, because the driver has no power at all. Another option is to fuse the two outputs of the H-bridge driver, but this may cause one side to still be hot after the short circuit.
5. High-power PCB design tips
1. Consider safety
As with any circuit, the main concern for high-current circuits is to ensure that they operate safely. There are some unique potential problems with boards that drive such high-power loads, and the main thing to pay attention to is heat. No matter how you design and arrange the board, it will generate more heat than a standard board.
This must always be considered when manufacturing the enclosure, and external vents/fans should be used. What is done for all designs that drive more than a few A is to install a dedicated temperature sensor on the PCB. This is a great firmware-based fault protection. With the ability to monitor temperature, you should always be able to react to any overheating conditions. To reduce the heat generated by the board itself, it is best to choose components with low resistance.
The next potential safety hazard is about short circuits. Since this board is designed to drive high-power devices, it will be able to provide considerable current when shorted. It is critical to consider this possibility during the design phase. The simplest way to handle a short circuit is to install a fuse on all outputs leaving the board, as well as an input fuse. Fuses should always be rated for less than the current that the wires used can handle. They should also be rated for less than/equal to the amount of current the board trace/pouring is designed for. It is also a good idea to use a driver with built-in short-circuit prevention.
2. PCB Power Design
The establishment of the power path is the most important rule for high-power PCB circuits, which is critical to determining the location and amount of power that should flow through the circuit, as well as the location of the IC and the amount of heat dissipation required by the board.
There are many factors that influence the layout of a given design:
The first consideration should be the amount of power flowing through the circuit
Equally important is the ambient temperature of the device and board design
The expected amount of airflow around the device and even the board should also be considered
Another consideration is the board material that will be used
A final and equally important factor is the IC density of the board that is intended to be used
3. PCB Design Layout
The board layout 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 are important factors in evaluating the amount of heat that the PCB needs to dissipate. The main factors that affect the layout of the printed circuit board include:
The power level flowing through the circuit;
The ambient temperature in which the board operates;
The amount of airflow that affects the board;
The materials used to manufacture the PCB;
The density of components that populate the board.
But it is usually preferred to divide such a board into low-power and high-power sections. This ensures that all high-power traces are as close to the power supply and output as possible. The board will be 2 layers with 2 ounces of copper.
Something I learned when doing high current PCBs is to do a rough initial layout with 8 mil traces on everything to ensure that the components are placed in an optimized manner. This helped a lot with this example because it showed exactly where the high current paths were and how to best position the H-bridge drivers.
Rough board layout with 8 mil traces
The image above shows the initial layout of all the components, along with the 8 mil traces that will be used to specify the path for all the final traces. Power will enter from the bottom terminals, go to the input fuse, branch out to the H-bridge drivers, and low current power will go up through the center of the board to the 5V regulator.
For the H-bridge drivers, power will enter them through large electrolytic capacitors on the bottom layer, connecting the top layer and pads through many stitching vias.
PCB high power design
4. Component Selection
High current designs and power systems often get most of their reliability from components. As obvious as it sounds, make sure you factor in component safety margins during the selection process. Generally, it’s best to start by looking at two specifications:
Rated current, especially for MOSFET and inductor components
Thermal resistance
You can use the estimated or designed operating current (if available) to determine the power dissipation, or use the first specification above to get a worst-case value. Both will help with thermal management, which requires using thermal resistance values to estimate temperatures. For some components, you can determine if a heat sink is needed to ensure reliability.
Other components that are important to high current boards, such as connectors, may have very high ratings and are useful in power systems. Two examples of machine screw terminal connectors that can handle very high currents are shown below.
Connector
5. Proper Copper Weight
The copper resistance used in the traces will generate some DC power losses, which will be dissipated in the form of heat. For designs with very high current, this becomes very important, especially when the component density is very high.
The only way to prevent DC losses in high current PCBs is to use copper with a larger cross-sectional area. This means that either heavier copper is needed or wider traces are needed to keep Joule heating and power losses low enough.
Use a PCB trace width vs. current table to determine the copper weight and/or trace width required to prevent excessive temperature rise.
6. Grounding
PCB high power systems can require the same kind of safety failure measures. A certain degree of safety and EMI can be achieved with a proper grounding strategy. Generally, grounding should not be separated, but power systems involving high current and/or high voltage are an exception. Grounding needs to be separated between the input AC, unregulated DC, and regulated DC sections.
A good starting point is the grounding strategy you can find in an AC system or isolated power supply. Typically, for high current power systems, you will use a 3-wire DC arrangement (PWR, COM, GND), where the GND connection is actually a ground connection. Your board may use an isolation strategy where the output side is disconnected from GND while the input side is grounded to ensure safety in the event of a fault.
7. Component Placement
It is critical to first determine the location of high-power components on the PCB, such as voltage converters or power transistors, which are responsible for generating a lot of heat.
High-power components should not be mounted near the edge of the board, as this can cause heat accumulation and significant temperature increases. Highly integrated digital components, such as microcontrollers, processors, and FPGAs, should be located in the center of the PCB to achieve uniform heat spreading across the board, thereby reducing temperatures. In any case, power components must never be concentrated in the same area to avoid forming hot spots; instead, a linear arrangement is preferred. The figure below shows a thermal analysis of an electronic circuit, with the areas with the highest heat concentration highlighted in red.
Thermal Analysis of PCB High Power Design
The layout should start with the power devices, whose traces should be as short as possible and wide enough to eliminate noise generation and unnecessary ground loops. In general, the following rules apply:
PCB 元器件放置
Identify and reduce current loops, especially high current paths.
Minimize resistive voltage drops and other parasitics between components.
Keep high power circuits away from sensitive circuits.
Take good grounding measures.
In addition to the above layout considerations, it is also necessary to avoid mixing different power components on the board. To achieve thermal balance of the board, make sure these heat components are evenly distributed throughout the board.
This will also effectively protect the board from warping. Therefore, you can ensure that the heat on the board is reduced and sensitive circuits are protected. Signals will also be equally protected during operation.
8. IC and component installation
Whenever there is power flow in a circuit, it is obvious that all components will generate heat. When passive components and ICs generate heat, the heat is likely to dissipate. This heat is dissipated into the cooler ambient air around the device.
IC component mounting
This dissipation is achieved through the lead frame of the device or through the package. Therefore, most IC packages are designed without much space for external heat sinks.
In addition, this requires a way to extract heat from the device. Exposed pads are such a method. For best thermal performance, use a bare die inside the package.
This die should have an EP directly connected to it. These ICs can then be properly mounted on the board. This way, the heat transfer from the package to the board will be optimized.
9. Heat sink
The purpose of using heat is to prevent heat from wicking into the surrounding copper pour when soldering. For a lot of high power PCB designs, it is generally hand soldered internally using a high power iron. Even on 2Oz copper, it can make quick work of solid pads. I tend to use heat sinks on all non-power nets and use solid connections on the power nets.
Fill plane showing heat relief
The image above shows where the heat sink is placed. The main input power, fuses and outputs do not use heat, all other nets do. This technique has worked very well in multiple designs, hundreds of boards have been produced, and there have rarely been problems with soldered components coming loose or any other problems related to cold solder joints.
10. Trace Thickness and Width
When designing any circuit board, you need to be aware of the minimum trace width. This becomes critical when dealing with high-power PCBs.
In principle, the longer the track, the greater its resistance and the greater the heat dissipation. Since the goal is to minimize power losses, it is recommended to keep traces that conduct high currents as short as possible in order to ensure high reliability and durability of the circuit. To correctly calculate the width of the track, knowing the maximum current that can pass through it, designers can rely on the formulas included in the IPC-2221 standard, or use an online calculator.
As for trace thickness, typical values for standard PCBs are around 17.5 µm (1/2 oz/ft 2 ) for inner layers and around 35 µm (1 oz/ft 2 ) for outer layers and ground planes. High-power PCBs often use thicker copper to reduce the trace width for the same current. This reduces the space occupied by the trace on the PCB.
Thicker copper thicknesses range from 35 to 105 µm (1 to 3 oz/ft 2) and are typically used for currents greater than 10 A. Thicker copper inevitably incurs additional cost, but helps save space on the card because the viscosity is higher and the required track width is much smaller.
Trace thickness and width
11. Solder Mask
Another technique that allows traces to carry large amounts of current is to remove the solder mask from the PCB. This exposes the copper material underneath, which can then be supplemented with additional solder to increase the copper thickness and reduce the overall resistance of the PCB current-carrying components. While it may be considered a workaround rather than a design rule, this technique allows PCB traces to handle more power without increasing the trace width.
12. Decoupling Capacitors
When a power rail is distributed and shared between multiple board components, active components can develop dangerous phenomena such as ground bounce and ringing. This causes a voltage drop close to the component power pins.
To overcome this problem, decoupling capacitors are used: One terminal of the capacitor must be as close as possible to the pin of the component receiving the power, while the other terminal must be connected directly to a low-impedance ground plane. The goal is to reduce the impedance between the power rail and ground. The decoupling capacitor acts as an auxiliary power source, providing the required current to the component during each transient (voltage ripple or noise).
There are several aspects to consider when selecting a decoupling capacitor. These factors include choosing the right capacitor value, dielectric material, geometry, and placement of the capacitor relative to the electronic component. A typical value for a decoupling capacitor is a 0.1μF ceramic capacitor.
13. Double the layers
A technique that is used in many high power circuits that is not used often is to double the copper pours and stitch them together with vias. This double layer allows twice the amount of copper to be in the same area. For this board, the copper on the main power input was doubled from the terminal to the input fuse. The image below shows this.
When you use this technique, the chances of creating a current loop increase because there is a section where no return current can flow. I do not believe in using two layers from the input fuse to F3/F4 on the net because this is where a lot of the return current flows.
Double layer close-up of main power input
The minimum width of this pour is 460 mils, but because it is on the top and bottom layers, the actual width is twice that, resulting in a much smaller voltage drop across the net. The smaller the voltage drop, the less heat is generated.
14. Copper pour
Regardless of what type of board you are designing, you will generally try to use a copper pour for all power nets. When dealing with dedicated high-current designs, all nets that carry high power should be a single pour. Copper pours can significantly increase the width of copper that can be mounted on the board.
Layout using copper pours on all high current nets
The image above shows a high current portion of the board where copper pours are used on all high current nets. By pouring instead of traces, the amount of copper is able to be increased significantly. A trick used to help the design go a little faster is to use a 20mil grid and use it to ensure that all pours are symmetrical at 45 degree angles.
6. High Power PCB Design Steps
1. Prepare the Substrate
Before the manufacturing process begins, the laminate must be thoroughly cleaned. This pre-cleaning is essential as the copper coils used in high power PCB design often have anti-rust properties, and these are usually done by suppliers to provide anti-oxidation protection.
2. Generation of Circuit Patterns
When designing high power PCBs, two main techniques will be used to achieve this goal. These techniques include:
Screen Printing – This is the most preferred method because of its ability to produce the desired circuit pattern. This can be attributed to its ability to be accurately deposited on the surface of the laminate.
Photo Imaging – This is the oldest technique used in designing high power PCBs. However, it is still a common method for delineating circuit traces on the laminate.
This technique helps ensure that the dry photoresist film consisting of the intended circuit is placed on the laminate. The resulting material is exposed to UV light. As a result, the pattern on the photomask is transferred to the laminate. The film is chemically removed from the laminate. This gives the laminate the intended circuit pattern.
3. Etching the Circuit Pattern
When designing high-power PCBs, this is usually done by immersing the laminate in an etching tank. Alternatively, they can be sprayed with an appropriate etchant solution. To achieve the desired result, both sides are etched simultaneously.
4. Drilling Process
After etching, the next step is drilling. In this step, holes, pads, and vias are drilled. To drill precise holes, you must ensure that the drilling tool is high-speed, and laser drilling methods are used when creating ultra-small holes.
5. Through-hole Plating
When designing high-power PCBs, this is a step that must be handled very carefully and precisely. After drilling the desired holes, copper is deposited in them.
Unlike other circuit boards, this is done in large quantities and made thicker. They are then chemically plated. The result is the formation of electrical interconnections across layers.
6. Application of Coverlay or Cover Coating
Protecting both sides of the board is essential in high-power designs. This can be achieved by applying a coverlay.
The importance of this lies in providing protection from harsh environments. This is essential for high-power PCBs as they are subject to temperature fluctuations. Such coverlays also provide protection from harsh chemicals and solvents.
Polyimide film supported with an adhesive is the most commonly used coverlay material, and screen printing can press the coverlay onto the surface.
Curing is achieved using UV irradiation. Controlled heat and pressure are applied during the lamination process of the coverlay. There is a significant difference between coverlay material and coverlay. Coverlay is a laminated film, while coverlay refers to a material that can be applied directly to the surface of the substrate.
There are many factors that determine the type of cover. They include the methods used in the manufacturing process, the materials used, and the application area. Both coatings are essential to enhance the electrical integrity of the entire assembly.
7. Electrical Testing and Verification
The circuit board undergoes a series of electrical tests that carefully check factors such as performance. You also need to use the design specifications as a threshold to evaluate the quality.
7. High-power PCB processing
The following are the basic steps:
Print inner layer
Align layer
Drilling
Copper plating
Outer layer imaging
Copper plating and tinning
Final etching
Apply solder mask
Apply surface finish
Apply silk screen
Anvil
1. Heavy copper circuit structure
In high-power PCB design, thick copper circuits are used. This usually requires special etching technology.
One-stop high-power PCB design
The technology used for weaving here is also quite different from that used for other PCBs, using high-speed plating and differential etching.
When plating thick copper circuits, you can continue to increase the thickness of the board. You can also mix thick copper with standard features on a single board. This is also called power link. This will translate into many advantages, including a reduced number of layers. Power will also be distributed efficiently.
This will also allow high-current circuits and control circuits to be incorporated on the board. In addition, a simple board structure is also provided.
2. Current carrying capacity and temperature rise
Estimate the maximum current that the trace can easily carry. This can be determined by finding a method that can estimate the heat rise. This is related to the current you apply.
The ideal situation is to reach a stable operating temperature, in which case the heating rate is equal to the cooling rate. When you = your circuit can withstand temperatures up to 100°C, you are ready to go.
3. Board strength and survivability
You can choose from a variety of dielectric materials. Among them is FR4, which has an operating temperature of up to 130°C. Another dielectric material is high-temperature polyimide, which can operate at temperatures up to 250°C.
Higher temperatures require the use of special materials so that they can survive extreme conditions. There are several methods that can be used to test and determine the thermal integrity of a finished product. One of these methods is to use a thermal cycle test. It helps to check the resistance of the circuit while performing an air-to-air thermal cycle. This cycle is checked from 25°C to 260°C.
Increased resistance can lead to a breakdown in electrical integrity through cracks in the copper circuit. For this test, make sure to use a chain of 32 plated through holes. This is because they are considered the weakest point in the circuit, especially when they are subjected to thermal stress.
Thick copper circuits often reduce or eliminate the failures inherent in these boards. This is because the copper circuits become impermeable during the mechanical stress phases due to thermal cycling.
4. Thermal Management
Heat is usually generated during the operation of electronic devices and must be dissipated from the source and radiated to the external environment. If this is not done, the component may overheat, resulting in failure.
Heavy copper helps reduce heat. It conducts heat away from the component, which greatly reduces the failure rate. Use a heat sink to achieve proper heat dissipation from the heat source. The heat sink will likewise dissipate the heat away from the source where the heat is generated. This is done by conducting and dissipating the heat to the environment.
Connections are made with copper vias to bare copper areas on one side of the board. Classic heat sinks can be bonded to the base surface of the copper. This is achieved with thermally conductive adhesives. In other cases, they are riveted or bolted.
These heat sinks are usually made of copper or aluminum, and built-in heat sinks are created when manufacturing high-power PCBs. This does not require additional assembly. Copper Circuit Technology allows the addition of thick copper heat sinks to any part of the board surface.
