Why design a printed circuit board?
A printed circuit board (PCB) was designed to safely convert power from a 48V 30A source, constructed by the Mate Rov team, to multiple 12V 20A outputs. Additionally, the PCB must supply power to an Arduino Uno Mega, responsible for sending a PWM signal to control each motor’s electronic speed controller. As a part of a broader electrical engineering system, the PCB had to meet various design requirements.
How was it made?
The following power distribution module PCB was designed using Altium Designer, which is an industry standard PCB design software. Various libraries from Snap EDA and Mouser were used to generate ECAD models for each component. Digikey was used to find available components, as it featured user-friendly settings that allowed for rapid navigation between each of the components. Lastly, JLCPCB was used to manufacture the PCB, as the WashU Racing, WashU Robotics, and WashU Rocketry team have historically had excellent experiences with JLCPCB’s services.
What were the final results?
Schematic
The schematic for the power PCB is shown in the figure above. The two leftmost boxes, featuring a total of 3 35-pin Deutsch connectors are responsible for every wire entering the board. These wires carry signals of varying uses. There are 14 AWG wires that are responsible for carrying current from the buck converters to the motors safely, each connected to the same Deutsch connector. There are a variety of lower gauge wires that carry signals that are entered into connectors in the box on the lower right hand side. These two Deutsch connectors are responsible for expanding the output of the Arduino. Although many connectors that claimed to meet the 20 amp specifications required by our design were considered, we eventually chose the 1-776231-4, which are described as PCB Mount Header, Vertical, Wire-to-Board, 35 Position, 4 mm [.157 in] Centerline, Fully Shrouded, Gold, Through Hole – Solder, Sealable, AMPSEAL on TEConnectivity. The compatible female connector chosen is the 776164-1, described as, Wire-to-Device, 35 Position, .157 in [4 mm] Centerline, Sealable, Black, Power & Signal, AMPSEAL. This set of connectors claim to carry 17 amps safely. Although this is slightly lower than the 20 amp max that the buck converters provide, we can run the motors safely at lower power.
The following figure depicts the circuitry behind each current sensing circuit, which monitors the current behind each of the sensors. The current sensor chosen was the ACS758LCB-050B-PFF-T, which is a bidirectional current sensor found on Digikey that can monitor up to 50 amps of current safely. An automotive fuse holder has been electrically connected to the current source to limit the amount of current flowing through the PCB. In cases of failure, the fuse will pop underwater, essentially disabling the motors from having any power at all. Additionally, to prevent voltage spikes, a BZX85C15 R0G Zener diode has been added in parallel with the output of the power source. The other three pins are power, signal, and ground for operating the sensor.
Layout
The second step in PCB design is creating the layout. To start, each layer of the board can be discussed based on its primary function. The external layers, for instance, were designated to carry the highest amounts of power. [1] To determine the appropriate trace width for these layers, the Digikey PCB Trace Width Calculator was used. This tool helps ensure that the copper width is high enough to safely carry a current of up to 20 amps. The calculator uses specific formulas, calculating the area of the trace (A) as a function of the anticipated temperature rise of the board (T rise), the thickness of the copper layer (t), and the desired current (I). In addition, constants (K), (B), and (C), are predefined constants by the IPC-2221 standard for internal and external layers. These formulas may be written as follows.
The current was set at 17 amps, the maximum draw for the motors, while the PCB used had a copper thickness of 0.0356 mm, as specified by Altium. We used a standard temperature rise of 10 degrees Celsius, although for future Mate Rov designers this value must be adjusted to the internal temperature of the rover. This should be possible with the internal temperature sensor of the PCB. Constants K, C, and B were chosen for an external layer of the board. The formula resulted in a trace width of 578 mil, or about 0.6 inches. Consequently, all the power polygons were made 0.6 inches wide and placed on the external layer to optimize for space.
More considerations in polygon placement were made with respect to common standards for printed circuit board design. Internal layers were grounded, following standards to minimize electromagnetic interference and capacitive coupling between the board’s layers. Additionally, high-power circuitry was physically separated from the microcontroller circuitry to reduce electromagnetic interference between the higher-power traces that drive the motors and the low-power PWM signals that control them. On the second revision of the PCB, we drew in the power polygons 50 mil away from the board’s edge, as recommended by Scott Chaney, to prevent potential shorts at the edges.
Thermal relief protection was applied to each component’s vias to ensure strong and reliable connections between components and connecting traces. This often involved increasing the via width from 10 mil to 40 or 50 mil. Designators for each component were placed close to their respective components to improve readability once the PCB was manufactured. Overall, the design process took considerable time and effort, but it’s rewarding to see that it worked in the end.
References
[1] Digikey, “PCB Trace Width Calculator, ” https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-pcb-trace-width (accessed May. 5th, 2024).