Did you know that one of the first personal computers was so badly plagued by EMI problems that it was eventually discontinued? In the late 1970s, the Radio Shack TRS-80 helped pave the way for personal computing, but it generated so much EMI that it would disrupt other electronics. Gamers soon learned to use this to their advantage, however as the buzzing it created in AM radios would match the intensity of the computer’s activity. This created a pseudo sound effect for games, which was welcome since the computer lacked its own sound system.
Most of the time, however, electromagnetic interference (EMI) is something to be avoided in electronics. Not only should a printed circuit board not emit EMI that can disrupt other electronics, but it should also be protected from incoming EMI from external sources. EMI can be generated in a number of different ways on a PCB, with one of the biggest problems coming from the onboard power supply circuitry. Let’s take a closer look at this problem and some design ideas for reducing EMI in switching power supplies.
The Importance of Reducing EMI in Switching Power Supplies
The power supply circuitry that is designed into a printed circuit board is typically either a linear or a switch-mode power supply (SMPS) style, with the SMPS being the more popular choice. This is due to their increased level of efficiency, their ability to produce more current with less heat, and their smaller size of interconnected circuitry and components. There is a trade-off, however, and that is in the amount of noise, or EMI, that the high-frequency switching of the SMPS can create.
Switch-mode power supplies use a switching regulator to convert the power, and the output voltage is regulated using a process called pulse width modulation. Depending on the power supply requirements, an SMPS can be configured as a variety of types including buck and boost converters, which will either step down or step up the voltage. The pulse width modulation will generate some high-frequency noise, however, which must be filtered with circuitry or layout refinements.
The EMI that is radiated from an SMPS occurs when the voltage switches between the on and off states. The faster the rise time of the switching voltage, the higher the level of EMI that will be generated. This can be controlled with the use of filtering components as long as the efficiency of the SMPS isn’t compromised. The other control method is to incorporate tighter PCB layout methodologies into your design.
PCB Design Tips to Remove the Potential for EMI in Your Power Supplies
A good PCB layout of your power supply circuitry starts with the placement of the components. The parts in a power supply need to be close enough for short and direct connections, while still incorporating enough space between them to pass the design for manufacturing (DFM) rules. You will want to make sure that the components that are carrying the high currents, such as the inductor and the IC, have the shortest connection lengths to each other. You will also want to keep them on the same side of the board as well for added ease in trace routing.
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A good PCB layout of your power supply circuitry starts with the placement of the components. |
When it comes to routing, here are some guidelines that can help with the reduction of EMI in your power supply circuitry:
- Keep the trace routing as short and as direct as possible. The shorter the routing, the less potential there is of generating EMI.
- Keep the traces for routing as wide as possible too. The wider the trace, the lower the inductance, which will also help to minimize EMI. At the same time, take care that the trace to pad connections do not violate any DFM requirements with their extra width.
- Route high-current lines at 45 degrees instead of at right angles. Or, if possible, round the corners instead.
- Minimize the use of vias as they can also add inductance. With the power supply components on the same side of the board though, there shouldn’t be any reason to use vias except to connect to the ground plane and/or for thermal dissipation.
When it comes to your power and ground planes, take care in how they are managed. This starts at the beginning of the design with the configuration of the layers in a multi-board stackup. The goal is to create the most optimum shielding for the power supply. You will also want to isolate the ground plane of the power supply from the digital ground plane for the rest of the board. This will further reduce the chance of EMI. Also, be careful about routing sensitive digital lines through the power supply area. These lines can have an effect on the power supply circuitry or vice versa.
These guidelines represent a good start in reducing EMI in your power supply circuitry, but there is still more that can be done as well. Here is where getting the help of some people with experience in designing circuit boards for EMI reduction can be a huge benefit to you.
How Your PCB Contract Manufacturer Can Help with Your Design
Your local PCB CM can be a great source of information when it comes to specific PCB design problems, such as reducing EMI in the power supply circuitry. They have been building similar circuit boards for their different customers for a long time, and understand the challenges of EMI from both the design and manufacturability perspectives. Additionally, they will know how to incorporate the design for test (DFT) standards into your board that will work with the EMI design techniques that you are using. They will also work together with you to make sure that your design complies with all the appropriate EMI standards for its product family.
At VSE, we have been working with PCB design innovators for a long time, and we have a very good understanding of what it takes to successfully reduce EMI in power supply circuitry. As an example, we helped one customer to manage their EMI by re-engineering their design in order to create a Faraday cage. This was done by isolating specific areas of GND and tying those areas to stitching vias, and then completing the project with a custom metal cage for additional shielding. This ended up helping that customer quite a bit, and we make it a practice to offer the same level of expertise to all of our customers.
