I used to trail run with a friend who is an urban planner. As part of her devious plan to get me to run further before I was too tired and bored, she’d tell me about the various considerations that go into the zoning and construction in a town. There were also a lot of shenanigans with local politics that provided a juicy distraction.
Although my friend would disagree, layouts for a high voltage PCB have some similarities to complex urban planning. In addition to all the other considerations that go into a PCB design, a high voltage PCB also needs a layout that can control and optimize field strength across the board to provide the best performance and lifetime protection in your final product.
Isolate high voltage areas
The same way that a town creates zoning areas and restricts what land can be used for, you want to group your high voltage circuitry together to minimize the impact it will have on the rest of your board. By partitioning high voltage areas from low voltage areas, you can decrease the risk of arcing on your board.
One way to physically isolate high voltage areas is to add an insert around them. When you lay out the board, be sure to include a slot that will be routered out where you’d place the insert. Talk to your manufacturer to be sure this is feasible, and to find out what size tolerance they have for the slot.
Since you are placing the slot near the highest voltage region of your board, overvoltage conditions are likely. Proto Express recommends that you design the slot to survive repeated arcing. The minimum slot width should provide adequate protection at the highest voltage expected on the board. By adding a little margin to the slot size, you can ensure the PCB will stay undamaged even if the edges of the slot carbonize during corona or arcing. This is important because the resistance of the PCB material drops along the edges that suffer arc damage.
During manufacturing, the slot will be routered like any other feature or via on the board. Then you can place an inert insulator material into the slot to form a vertical barrier. For lower voltages, you can use PCB material, but at higher voltages, you’ll want something like polyester or Teflon. The inserts can be held in place with clips, glue, or by designing the shape of the slot and insert to lock in place.
Partitioning a high voltage PCB is important to provide a gradual decrease of voltage across the board.
Decrease voltage across the board gradually
After you’ve isolated the highest voltage regions on the board, you should still lay out the rest of the board in “zones” that let you step the voltage down gradually. By placing circuitry that operates at lower potential around the main conductors, you can redistribute the electric field. The lower field strength over an area lessens the potential for corona and arcing.
Voltage floating rings, or field grating rings, can also be used to manage the field distribution of high voltage designs. They can serve as a termination or be coupled to resistors and/or capacitors, depending on the AC/DC characteristics of the high voltage source that your design will protect. These are pretty advanced design components. If you’re considering using them, it is worth diving deep into their literature.
Isolate noise sources
If there’s a single universal rule of PCB design it’s to either use short traces or isolate your noise sources. This is still the case when designing high voltage PCBs. Just like you don’t want to live next to a loud manufacturing plant, you don’t want signal or power noise to reign-free on your board. If noise couples through parasitic capacitance on the board or in the insulation, it can easily propagate to very sensitive areas of the board.
You should minimize interconnects on your PCB. Using fewer interconnects reduces the opportunities for transient generation in your design. It will also minimize the propagation of high voltage across the board. Like noise, any transients or unexpectedly high voltage areas can damage sensitive components or reduce performance.
PCB interconnects make it easier for the electric field to propagate across the board, which is highly undesirable.
Use design checks
Make sure your PCB design tool has a thorough design rules checker. That checker is the urban planner of your board, making sure that spacing and positioning throughout the design and assembly of your board are up to spec. All cables, connectors, and components can affect the performance of any board, but a high voltage product poses a particular risk to itself and users if the design goes awry.
Looking for great design layout software for getting started? Look no further than Altium. Design rules checking can be automated and performed in real time, saving you a tremendous amount of work in the small tweaks that are needed to get the board just right, especially for high voltage!
Have a question about high voltage design? Contact an expert at Altium.
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