Selecting and Placing High Power Connectors in Your PCB
My early PCBs were focused on function, and I will admit that I didn’t always pay attention to technical specifications. It’s embarrassing when you build a high current PCB and the wonderful smell of burning plastic starts filling the air. I was confused as to why this occurred, until I started reading data sheets for PCB connectors.
You might be tempted to select high power connectors for PCBs based purely on appearance or form factor, but there are other important points to consider when choosing connectors for power electronics.
Choosing High Power Connectors for PCBs
With high power systems, you’ll still need to account for the number of pins and type of connection required for your board. However, there are other aspects that are certainly more important in order to ensure the reliability of the connector itself and the connection it forms on your board. The last thing anyone needs in a power system is for the board to heat up and for plastic components to catch fire.
With high power connectors for PCBs, the current capacity and operating temperature are the most obvious points to consider. However, there are other important aspects of connectors to account for. The temperature rise in the connector is related to the contact resistance of your connector, so it is important to know how the contact resistance when selecting connectors for power connections.
Some low quality connectors can withstand several amps or more of current without problems, but their contact resistance might be relatively high, reaching milli-Ohm levels. This is complicated by the fact that some connectors can have wildly varying contact resistance, and manufacturers will only list a maximum contact resistance instead of a mean and tolerance. This means that IR drop across a connector can be difficult to predict during design.
With AC power electronics, you’ll also need to account for the frequency limitations. Some connectors have inductive impedance, meaning power dissipation in the connector will increase at higher frequency. The quoted value for maximum frequency is meant to ensure that your board dissipates a safe level of power when operating at the current capacity. This then helps keep your operating temperature at a safe level.
Finally, we would be negligent if we did not mention duty cycle. This is not related to the on/off ratio of a digital signal. Rather, this refers to the number of times a connector will be disconnected from its cable. With high power systems, this tends to be less of a consideration as these cables are seldom disconnected.
Designing Connections to Your Board
If you look through some options for high power connectors for PCBs, you’ll notice that many of these options are through-hole components rather than SMT components. There is a good reason for this. Through-hole connectors take up less mounting space than SMT connectors with comparable current capacity and operating temperature. SMT connectors require a larger solder pad with thicker copper, while through-hole connectors can be soldered to a plated mounting hole.
SMT connectors require large solder pads, as seen in this high power LED for industrial lighting.
Other connectors will be through-hole mounted components with a threaded post, meaning that they will need to be bolted onto the board to provide a tight connection. These connectors are very rugged and are typically used in boards that must accommodate very high current levels that might otherwise damage solder joints. Some examples include applications in aerospace and military systems.
Accommodating through-hole connectors means you will need plated mounting holes in your board, and it is a good idea to extend these mounting holes all the way through your stackup. These mounting holes should include annular rings on the surface layer. With bolted through-hole connectors, make sure to extend the annular ring out far enough to match the size of the bolt and washer. When you work with higher currents, you will need to use higher copper weight when plating these mounting holes.
What Happens when Power Reaches Your Board?
Even if you have chosen a connector with the right current carrying capacity, designed your mounts for it properly, assembled the board correctly, and used the right wire gauge leading to your connector, the board will likely fail if it is not designed to withstand high current and high heat dissipation. The IPC-2221A standard provides a general formula for calculating the current carrying capacity that will produce a 20 °C temperature rise in a copper trace with a given weight.
Some manufacturers that specialize in PCBs for power systems offer extremely heavy copper that exceeds 100 oz./square foot. If you have a circuit that needs to withstand hundreds of amps and tens of kW worth of heat dissipation, then you will need to go this extreme route to ensure that your board will not burn up. Once you hit kiloamp levels of current, you’ll be working with extremely thick traces (one the order of inches), and working on a PCB quickly becomes impractical.
These situations are uncommon and can be found in systems running at extremely high DC voltage and current. With AC power systems, the output current and voltage from transformers are inversely related, so you can easily step down the current to the point where it is safe enough for your particular board. However, the power dissipated in these boards will still be the product of voltage and current. High voltage between neighboring traces can cause the dielectric to break down if the traces are too close, leading to a strong discharge and creating an extreme fire hazard.
ESD sparks can quickly burn up your board
Selecting and placing the right connectors for high power systems is easy when you use the right PCB layout and design software with a full suite of design tools. The design features in Allegro PCB Designer and Cadence’s analysis tools help you lay out and verify power integrity in your power electronics systems. You can find and place the high power connectors in your PCBs quickly and easily.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.