The term power supply covers a multitude of products from a sub 1W DC/DC converter for local chip regulation, through to multi kW systems for telephone exchanges and mass data storage systems. With this in mind, Electronics Sourcing challenged Relec Electronics sales director, John Stone, to provide readers with a ten-step guide to the key points in defining power requirements and choosing the best supply for the application Inputs and Outputs
On the face of it, the basic requirements for all power systems are defined by their inputs and outputs. Key issues here are:
DC or AC waveforms
Input voltage range
Output current (+ pulse requirements)
Regulation (how accurate does the output voltage need
Once defined, the best product can be matched to each application.
The space available for the power supply is often one of the last things defined by design engineers. However, it is one of the most important. Speak to the supplier at the earliest possible stage about the approximate power requirements. They can help allocate sufficient space and cooling to meet the systems needs.
Knowing the full range of temperatures the equipment is going to operate in is vitally important. The power supply is likely to be one of the biggest heat generators in the equipment. At the upper end, you need to consider how to get the heat out of the system. Is there air blowing across the power supply to keep it cool or is the unit suitable for mounting on a cold wall? It is known that the mean time between failures for power supplies halves with every 10C increase in operating temperature. Thus, try to ensure the unit runs as cool as possible. At the opposite end of the scale, it is equally important to ensure the unit will start correctly when operating at sub-zero temperatures.
Other environmental conditions
Shock and vibration (both in operation and transit) are important considerations for a power supply. Units made on double sided PCBs will withstand rugged environments far easier than those mounted on single sided boards. Also, seriously consider humidity. Some power supplies can be supplied with a conformal coating (or potting) which protects against moisture ingress.
Along with variations in case size, consider carefully the method of getting current into and out of the power supply. Options include PCB mounting,screw terminals and cage clamps for bare wires or plug and socket arrangements. If choosing the latter, ensure the connectors mating halves are freely available, preferably from the power supply distributor.
Isolation and safety considerations
It goes without saying that mains input power supplies have to meet generic safety requirements and EN60950 is a good start point. Regarding DC/DC converters, there is a wide choice of isolated and non-isolated products on the market. Check carefully what isolation voltage is required between various parts of the system. Will the power supply work continuously at these levels?
One of the most contentious power supply issues is EMC approvals. If using a component power supply in the system, you will still have to approve your product according to relevant specifications. Generally speaking, data published on the power supply will be tested against a simple resistive load and may not represent the performance of the final equipment.
By the nature of their construction, power supplies tend to use transistors and capacitors which get hot. If equipment is designed to operate for many years, check the quality of components used by the supplier. As mentioned earlier, the hotter a PSU runs, the shorter its life. Also, ask the supplier if they can supply field returns data as well as calculated MTBF figures.
Price & delivery
As well as finding a product which meets your technical requirements, you need to ensure it is freely available at a price which suits the product. Also, try and ensure the power supply is likely to be available for the lifetime of your product. Obsolescence can have a nasty impact in terms of getting products re-approved.
Ensure your supplier discusses the issues above and is interested in supplying the best product for your application and not just the product which best meets their targets.