How do you select an AC adapter to safely power your microcontroller (MC) development board? Most people don’t think much about the AC adapter (power supply) for their development platform, especially if it is included in the development kit. But more often these days, kits come in small boxes with minimal accouterments, perhaps following the Raspberry Pi model. Or the adapters might be a simple USB charging cable. If you do get a kit that has no power supply included, the huge selection of multiple parameters and features can be overwhelming to choose from.
In selecting a power supply, first, you need to select the correct voltage. Make sure that the supply voltage on your power supply or AC adapter matches your board’s requirements. As for current, it’s alright to have a power supply that is rated for current that’s higher than your board might require. More often than not, the maximum current draw of a development board is not mentioned in the datasheet. If you are using a USB port to power a board, don’t forget that compliant USB chargers with USB 2.0 plugs are limited to about 2A, unless you are using one of the newer USB Type-C cables that are rated for up to a level 5 power delivery that can deliver up to 100W (and USB Type-C cables with USB power delivery standard compliance (USB-PD) are a whole other topic.) If you plan to energize devices from the GPIO on your dev board, you may need more power than a USB charger can provide. The solution is to add an externally powered USB hub or if you have the option, to power the board using an AC adapter that plugs into a non-USB connector (often a 2.1mm barrel jack.)
Choose an isolated AC adapter if given a choice, as they mitigate the introduction of noise into your application. Very noise-sensitive applications such as special medical equipment, instrumentation, or test and measurement equipment might use an isolated AC adapter or in the more sensitive applications, a linear power supply because they are noise-free. However, linear power supplies are impractical for common use because they are large, expensive, and mostly available as bench-top equipment in most engineers’ experience. If noise is a concern for you (maybe you have very low supply voltages or signals down to 1.8v or 0.9v on your board) consider selecting an isolated AC adapter first. They are not much more expensive than non-isolated adapters.
Another characteristic to look for is a high-efficiency AC adapter, and not only because it will consume less energy; it will also dissipate less heat. Efficiency, given as a percent, is the ratio of output power to input power under a known load current with nominal line conditions. Efficiency, operating temperature, and reliability are related, as high operating temperatures can damage or lower the life of the power supply. Reliability has a relationship with the amount of heat the adapter puts off, and less heat is better. A quality AC adapter or power supply will have circuit protection. Circuit protection not only protects the power supply but the load (your board), with components that limit potentially damaging conditions such as over-voltage or over current. When the dc output voltage is above the specified maximum value (typically 120% of rated value) the power supply can shut off to avoid damage to the load (your board). Cheap “wall-wart” type AC adapters might have a non-resettable fuse inside, but not much more. If you are “recycling” old AC adapter wall warts from the junk bin to use with a development board, note that cheap adapters might not have any protection at all. If you are
There are some common protections for power supplies against over-current, over-voltage, and over-temperature. Over-current conditions occur when the output load current is greater than what the power supply specification allows, and one result of more current than is rated for normal operation is an over heated power supply. Good power supplies have overcurrent protection (OCP) for each rail of the supply voltage, which works with current limiting to circumvent damaging conditions. Current limiters are another component that limits in-rush current. Thermistors are current limiters; they have a high resistance when cold and a low resistance when hot.concerned, you can cut the cord and wire in a fuse of your own on the output side and see that the adapter doesn’t get too hot.
The ambient operating temperature of your power supply will affect output current to the load. If your circuit board is operating in extreme conditions, you can cool the board with fans and supply heat sinks to the MCUS, but what about the power supply? If operating in extreme conditions, look at the derating curves for the power supply. The curves will denote the reliable operating current against ambient temperature.Some power supplies have internal temperature sensors for over-temperature protection that will turn off the power supply at a specified maximum temperature. A heat- or temperature-activated fuse that resumes its conductive shape upon reaching normal temperatures is common in household appliances, although it may shut off the power supply without warning and lead to some confusion. An appliance that shuts off suddenly absent any smoke smell is likely just shut down momentarily due to internal protection against excessive heat.
If you want to get very detailed or are having issues that you can’t explain with respect to power, another characteristic to look for is the dynamic response of a power supply. Dynamic response is considered when there are sudden and significant changes in power that the load needs. An MCU that wakes up several peripherals within microseconds may cause a sudden and dynamic load current draw. Depending on the application, a power supply may need to be able to ramp up fast enough to meet sudden rising demand within a specified time interval. When reading the data sheet for a power supply, note that peak power is the momentary but absolute maximum voltage that a power supply can sustain without damage.