Anytime your PCB prototype comes in, it is worth running through some basic power-on tests to ensure the design meets its basic operating requirements. The exact set of tests and checks varies across designs, but there is a common set of processes used to power on a brand new device. The goal is to prevent damage to components in the initial power-on process, as well as isolate problems that might be observed during the first power up.
Common Power-On Process
When you first receive a prototype, there could be problems lurking within the assembly, even if the board visually appears correct or as you expect. Fabrication and assembly defects could be present, incorrect or backwards components could be placed, or the design could have errors that cause the prototype to be mis-functional. A basic power-on process can help prevent damage to a device as it is brought online and ideally isolate a few problems along the way.
To properly power on a device for the first time, slowly turn up the output power on a regulated bench power supply in constant voltage mode. The slow ramp of the output voltage will bring up components one-by-one, including slow ramp of the regulators that power each rail in the design.
A basic benchtop power supply is essential for testing prototypes.
A fast turn-on can damage the components on the device, so manually turning up the power can slowly bring up power rails without overshoot and inrush at the input. The reason for the damage is that fast turn-on can create overshoot in the main rail and the outputs from any regulators in the device. Overshoot from power regulators can create a secondary inrush current into any loads connected to the regulator, and the result could be damage to components.
If the prototype PCB does power on without components exploding, you can go through a basic inspection checklist.
Basic Inspection Checklist
After the initial power-up phase is completed, you can walk through a basic checklist that examines the high level functions of the device to ensure it is booting into a stable state.
- Verify LEDs are correctly on, off, or blinking
- Measure input power and current as the device is brought online
- Check all exposed testpoints for expected voltages
- Check all connectors and terminals for expected output voltages
- For AC connectors, hook up a scope probe to measure for the presence of signals
- For embedded devices, verify the device can enter programming mode
- For embedded devices, verify the device can receive commands from a computer
Assuming all of these points pass muster, you can start experimenting with the expected functions of the device. Check to see whether it performs the main functions intended in the design. If everything works, then no debugging should be needed and you can move on to other areas like reliability testing. Many times, however, another round of testing is needed that involves deeper measurement of the device functions.
Advanced Inspection Checklist
More advanced tests may also be needed, either as part of debugging or as part of reliability testing. Some of these checks can help identify some basic EMI problems and SI/PI problems, or more complex functionality problems that are not apparent from a basic power-on test.
- Check for power overshoot with a scope by switching the power supply on and off
- Check that power rails have working safety/protection mechanisms
- Verify at what loads any regulators enter short circuit mode
- Monitor I/Os (either at connectors, pins, or testpoints) to sense ground bounce/ringing
- Check for excessive temperature on the highest power components (can be done by touch)
- Check mechanical mounting with a scope, including solder joints and through-hole pin mounting
- For a power system: bring the device to low-load mode to verify when its internal safety mechanisms begin acting.
- Manually toggle enable pins on specific ICs if they are accessible
- Verify flashing and debug features for the host processor through the programming interface
- Monitor power sequencing on groups of components with a oscilloscope
- Toggle any switches while monitoring their I/Os and any code execution in the embedded firmware
There are even more advanced aspects of prototype inspection, such as mechanical and ESD testing to verify reliability in idealized operating conditions. Once prototype functioning is verified from the above checklist, the design team can move on to more advanced aspects of testing.
Before You Start: Compile Your Requirements
Right after finalizing the design but before you send out your order, make sure you compile a complete set of expected features, functions, and voltage readings across the device. If possible, make sure your requirements list includes a set of potential fixes if you happen to observe something different from what you expect. If you compile these expectations, and you designed your board for testing and debugging, it will be much easier to walk through your checklist and determine any problems.
When it’s time to test your PCB prototype, cross-probing between your PCB layout and schematics can help you track down defects and verify your design’s functionality. Make sure you build your systems using the best set of PCB design features in OrCAD from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.