What You Can Learn
Component and circuitry choices in PCB design for power electronics.
Thermal management ideas for PCB design.
PCB layout tips for power integrity.
A power supply on a circuit board
Power can be a fickle thing. When it works we don’t think much about it, but when there is a problem it can really make life interesting. When my brother was a teenager, he discovered that his old car had developed a problem in the charging system when he noticed that the lights varied their intensity with the speed of the engine. He decided to test this by stepping hard on the gas and promptly blew out every light bulb in the car.
My brother would agree that wasn’t his best decision in life, but it sure illuminated how power can turn on you if not managed correctly. This is why we put a lot of effort into the design of printed circuit boards with onboard power supplies. Not only do we want to protect users from shock, but the resulting circuit noise can be detrimental to other circuitry on the board. Let’s take a look at some of the best practices in PCB design for power electronics.
Components and Circuitry Choices in PCB Design for Power Electronics
There are different power electronics that you can design for your circuit board depending on the need. In the case of converting AC power to DC, you will most likely use a regulated power supply that is either a linear or a switch-mode power supply. For applications that consume low power, these supplies can be designed into the PCB as an integrated circuit component (IC) making them ideal for smaller devices that plug straight into a wall socket.
To convert DC to DC power, such as stepping a voltage up or down, there are different power conversion circuits that you can design onto your circuit board. For example, a boost conversion circuit will step up the output voltage while a buck converter will step it down. There are also other variations of DC to DC conversion circuits, such as the buck-boost converter where the voltage can be stepped up or down depending on how it is controlled.
No matter what kind of power supply that you are designing into your schematic, you will be using a mixture of both active and passive components. You will need to use parts that are designed for higher voltages which have dielectric material and coatings that are specified to withstand higher resistance and temperatures. You must also make sure that your design doesn’t exceed the manufacturer’s specified operating values for the parts that you are using to ensure that the part will not fail prematurely. To make sure that the power supply design you create in your schematic will work correctly on the final PCB, you should simulate the design before going to PCB layout.
Thermal Management of Power Electronics
Power supply electronics can run hot depending on the amount of power that they are converting, and that heat must be managed on your circuit board. For those supplies that are very high powered, you may want to look into alternate board materials such as ceramics or some of the PTFE (Polytetrafluoroethylene or “Teflon”) laminates. Be warned though that some of these materials have non-standard fabrication processes that will drive up your manufacturing costs.
Another method of managing the heat in your design is in how the PCB layer stackup is configured with power and ground planes. Although ground planes are important for the management of power integrity, they also help to manage the heat from the surface layers. The heat from component thermal pads and metal conductors will get conducted down through the vias and into the planes where it can dissipate. It is important in your layout to use traces with a higher copper weight to carry high current, which will help with heat dissipation as well.
Component placement also plays an important role in thermal management. Although the parts of an individual power supply should be placed close together, the different power supply circuits on the board should be spread out whenever possible. This will avoid creating hot spots and ensure a more uniform temperature across the board. You should also consider passive cooling methods with your placement such as heat sinks, or active cooling devices such as fans, for very high-temperature applications.
A power converter layout on a printed circuit board
PCB Layout Tips for Power Integrity
When it comes to actually laying out the power supply, here are a few important categories to focus on:
Components: To keep your power routing as short as possible, keep the component placement as tight as you can. Start with the main components of the supply first, such as the converter IC. From there, place the remainder of the power parts starting with the input capacitor and the inductor followed by the output capacitor. You need to keep these parts as close to each other as possible to reduce EMI, and you will want the parts on the same side of the board to avoid the impedance caused by using vias.
Traces: Your trace routing should also be as tight as possible. To keep the inductance low, keep your traces short and make them wide. Try to route corners at 45-degree angles, or even with rounded corners. Also, keep other signal traces not associated with the power supply out of this area to avoid noise contamination to them from the power components.
Ground: The best grounding for a power supply is to use a solid plane instead of traces. Not only will this help with power integrity, but it will also help with thermal management as we’ve previously discussed. You should also isolate the power supply ground plane from the common ground plane for the rest of the board. They can be tied together at one point, but you want to avoid return signal paths coming through an area of noisy ground returns if both grounds are using the same plane.
To help yourself with designing power electronics on your printed circuit board, you need design tools that are equal to the challenge. You will need simulation and analysis tools to verify that your power supply design will satisfy the needs of the board. You also want to use your design rules and constraint capabilities to their fullest in order to identify and group power components and nets. This will allow you to assign the correct spacing and trace widths in the layout.
Don’t forget to use the 3D capabilities of your tools to ensure that the taller power components on your layout do not conflict with other mechanical objects or the system enclosures. And lastly, make good use of the different routing features of your tools to create the shortest connection paths between your power components.
That may sound like a lot to demand of any design tools, but the good news is that there is a PCB design system available to you today that can handle all of the requirements needed for designing power electronics. OrCAD PCB Designer, from Cadence, has all the tools and functionality that we have been referencing here. With OrCAD, you have access to all of the schematic capture, SPICE simulation, and PCB layout features that you need.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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