Designing extreme circuits can be a complicated and stressful ordeal. Back in college, I designed a direct spark ignition circuit at my co-op, which stepped 120 VAC up to several thousand volts. I foolishly prototyped this design on a protoboard without adequate insulation and when I plugged it in… well, there were a few fireworks. High power PCBs may not be prone to explosions, but designing them comes with its own unique challenges. These kinds of boards are often found in industrial applications where they drive high current components like motors or LED arrays. The main thing you need to look out for is the heat generated by all that current. The first thing you’ll want to do is make sure your copper is the right weight and your traces can conduct the current without significant power loss. Next, you’ll need to devise a plan for conducting or radiating away all the heat generated in the power path. Lastly, there are a couple of tips and tricks with fuses and sensors that can help save you if everything goes south.
When I built my doomed spark ignition circuit, I overlooked the layout of the copper on the protoboard, resulting in the incoming power arcing all over the board. Pay attention to your copper and don’t make the same mistake I did.
Firstly, you’ll need to specify copper thickness on your board. For high power circuits, you’ll want at least 2 oz thickness which will conduct more heat and current than a thinner layer. Usually you can trust your contract manufacturer (CM) with handling this, but some processes can leave the copper thinner in some areas and take you below the 2 oz threshold. Be sure to confirm that your CM has the capabilities to hit the thickness you need.
The second element you’ll want to pay careful attention to is your traces. For the high power path, you’ll want to keep traces short and thick. Longer traces are more resistive and will generate higher heat, which can lead to significant power loss. Your trace thicknesses should be based on IPC-2221. Since that standard is technically for longer traces, some designers like to push the envelope and go below the recommended width. The inherent risk there is not usually worth the reward, since a very thin trace can actually act as a fuse and burn up if it conducts too much current.
Now that we’ve reviewed the basics, it’s time to dive into the deep end and discuss general thermal considerations. Thermal planes and thermal (stitch) vias are great ways to dissipate and conduct heat in your PCB. Exposed pad components can help you when it comes to specific elements, and eliminating thermal reliefs in planes can ensure all your current and heat are directed as planned.
Copper planes are an important part of heat dissipation in high power PCB design. Dedicated heat sinks are best, but in boards that lack the space, copper planes are the next best thing. It’s likely you have power and ground planes on your board and possibly even a signal plane. These planes will help dissipate heat, but are not always connected to high power components or placed for maximum thermal radiation. For high power PCBs, you may also want to include a “thermal” plane that efficiently removes heat from your board. It’s best to place a thermal plane on the outer layers of your board where it can dissipate heat into the environment. Heat conducting planes should be connected to hotter inner layers with stitch vias that act as heat pipes. Larger stitch vias will conduct more heat but can also wick away solder during reflow, leading to manufacturing defects. This is particularly a concern when they’re placed inside pads. In that case, use multiple smaller vias in lieu of one large one. That way they still conduct heat well but involve less manufacturing risk.
You’ll most likely have conducting vias inside thermal pads for exposed pad (EP) components. Many high power components, like switching transistors, have exposed pads that can be soldered directly to a copper plane or thermal pad. This allows them to cool off more quickly and operate at a higher level. If you’re designing a high power board, you might want to look for EP components that will conduct heat into your dissipation scheme rather than becoming radiating hotspots. Your board may have other unique components that need to be soldered directly to a copper plane. In these cases, the ring often conducts heat to the plane too well for solder reflow, so a thermal relief is added to reduce conduction. This is obviously a problem when it comes to conducting heat and current during operation. If your high power board is a prototype or low volume, consider removing the thermal relief and hand soldering those components in with a high power iron. That way, you can ensure the solder wets correctly and still get good heat and current-carrying capability during operation. If you can’t afford to lose the thermal relief, make sure the method you choose for it can still conduct an adequate amount of heat and current.
Failsafes and Monitors
We all know that even the best-laid plans of experienced engineers sometimes go awry. That’s why you should think about including some failsafes into your design to help you mitigate worst-case scenarios. Two of the more simple additions are fuses and an onboard thermal sensor.
Short circuits in a normal circuit can be devastating but are not always dangerous. If the power source can’t source much current, the board may fry but may not catch fire or explode. High power PCBs are often sourcing tens of amps at relatively high voltages of 12 V or 24 V. In this case, a short circuit could source a significant amount of current and possibly catch something on fire. A good way to avoid this is to fuse your board’s inputs and outputs. That way, if a short does occur, the fuse will blow before too much current can be pulled into or out of your circuit.
It can also be prudent to add an onboard temperature sensor to your PCB. That way, your software can monitor the actual temperature of a critical component or hotspot and adjust accordingly. This may not be a great option for simple or low-level circuits, but for more complex control circuits, this could be an invaluable way to check the pulse of your board.
Failure in a normal PCB may mean intermittent operation or reduced function. In a high-voltage or high-power board, sparks just might fly. That’s why it’s imperative that you design your board with caution. Make sure your copper and traces are thick enough to conduct large amounts of heat and current effectively. You’ll also want to go over your thermal design and diligently examine your copper planes and how they connect to hotspots with vias. Components also make a difference, especially when they have exposed pads or require thermal relief for solder reflow. Last but not least, it's a good idea to include failsafe fuses to mitigate short circuits, and you may want to include temperature sensors to monitor important components or hotspots. A good manufacturer will help you keep your copper within tolerance and may even have some recommendations for thermal designs that are economical to make. Here at Tempo Automation, we like to set our customers up for success by offering our expertise when applicable.
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