The power supply is the most important part of a system, because nothing works without a power source. The power supply is the “heart” of any system. A power supply is a charge for the public AC network or a DC battery. Choosing a power supply is not easy, but an optimal choice can be made rationally following the 18 criteria explained below.
1. Maximum power required in continuous use at maximum temperature
2. Constant voltage source or constant current source
3. Field, real working environment and extreme limits
4. Reliability requirements (MTBF, estimated lifetime, warranty)
5. Natural or forced cooling of the source
6. Maximum current absorbed by the source at start-up
7. Range for input voltage
8. Electrical characteristics imposed by the application
9. Various inlet and outlet protections
10. Dynamic characteristics
11. Characteristics related to indications, interface and communication
12. Power supply system in which the source is included
13. Size and constructive form
14. Certifications and approvals required
15. Special requirements for source components
16. Standard product or special product
17. Preventive maintenance requirements
1. Maximum power required in continuous use at maximum temperature. A source is chosen for the power supply of one or more known consumers (load) to which is added a power reserve of at least 30%. Inductive loads such as electric motors, transformers and relay coils may require a current greater than 2… 5 times the rated current for the first input voltage for several input voltage wave cycles. If one source supplies another source that has capacitors at the input (discharged at first) a high current pulse is required for charging (eg a 12Vdc source is supplied from the 24Vdc source). Cold resistive loads also require a higher current at start-up, until the resistance increases with heating (eg filament bulb, heating element).
Mandatory, the source must provide power peaks when starting electric motors that have their own inertia when starting at rest, but also a mechanical load (eg a water pump in a boiler). High-performance switching power supplies or power converters specify that they can provide, for a limited time, a current higher than the steady state current when starting inductive or capacitive loads.
2. Constant voltage source or constant current source. The application imposes a minimum of initial requirements for a power supply: if constant voltage or constant current is required (eg LEDs are supplied with constant current), if the output voltage or current is fixed or adjustable, if required multiple outputs, if it is required to connect the voltage sources in series to obtain a higher voltage or to connect the current sources in parallel to obtain a higher current.
3. Field, real working environment and extreme limits. Field of application (industrial, indoor or outdoor lighting schemes, LED display panels, commercial, medical, laboratory devices, surveillance and alarm systems), source for independent use or incorporated in another product or cabinet (source with / without housing), outdoor mounting with exposure to sun, moisture, rain, dust… or indoors in office environments, household, warehouses, degree of protection IP xy to the real working environment, respectively to moisture, dust, bodies foreign matter that can enter the source (metal powders, chips, pieces of wire…), abuse by extreme demands of temperature and humidity, frequent starts and stops, mechanical vibrations and shocks, disturbing electromagnetic fields, corrosive vapors, explosive environment.
4. Reliability requirements (MTBF, estimated lifetime, warranty). Ultra-safe operation requirements (especially in the fields of medical, military and surveillance systems). Cheap sources have not tested reliability under specified limit conditions.
Burn-in tested sources must be chosen (Note 1), specifying that the temperature sensitive parts (electrolytic capacitors) have high reliability (long life) and at the temperature of max. 105 ° C. The warranty should be 2 or 3 years.
5. Natural or forced cooling of the source. The source may have forced cooling with at least one fan running continuously or intermittently controlled by a temperature sensor or convection cooling (by free, natural circulation, air) or conduction cooling (by mounting on a good heat conductive metal support). It is mandatory to check in the technical specifications of the source, the decrease of the output power depending on the source temperature (power – temperature diagram), because the power given by the source decreases with increasing temperature. Quality sources give this diagram in specifications. An additional cooling device shall be provided if the rated power of the source is required at maximum working temperature. For example. at 50 ° C, the delivered power can be reduced from 100% to 70%. Depending on the maximum power required and the place of installation (plastic or metal box or in the cabinet with or without vents, on the rail, on the table, on the wall, in street lighting…) sources with outdoor cooling are chosen or forced with a fan.
6. Maximum current absorbed by the source at start-up.