Power supplies are a key component in nearly all electrical equipment, providing power for applications down in the milliwatt range up to the megawatt range.
Written by Display Technology
Typically the power supply is connected to the mains through a single-phase or multi-phase connection depending on the requirements, the power supply then converts the AC (Alternative Current) into a DC (Direct Current) at the required voltage level for the connected load.
Power ratings and peak power
It is important to select a power supply that can deliver enough power to your system when operating under the application conditions, taking into consideration ambient temperature and input voltage. Oversizing the power supply will add additional costs, and when purchasing in higher quantities this can add a lot of unnecessary expenses.
The rated power of a power supply is a measure of the maximum continuous power that can be delivered by the power supply within the specified operating ranges.
The peak power or peak current rating of a power supply is the maximum amount of power that can be delivered for a short period of time. For some applications it can be extremely beneficial to select a power supply that has high peak power handling capabilities, as the converter can be much smaller and cheaper than one rated for the same power rating continuously. Examples of applications and devices that benefit from peak current power supplies are fans, pumps, solenoids, motors, thermal printers. These loads require a high start up current but once they are running the current drawn is much lower, sometimes as much as three times less.
Input and output voltages
Depending on your supply voltage you may need an AC-DC converter or DC-DC converter, some power converters are able to operate from both sources using the same terminals. Most AC-DC power supplies can operate from a range of input voltages, however when supplying with 110Vac instead of 240Vac the maximum power the supply can deliver may be reduced. It is also important to consider if you require a single voltage output, or multiple outputs at different voltages.
Depending on your system design and application you may be able to use a non-isolated converter, this removes the need for a transformer or coupled inductors. Generally, this will increase the efficiency of the converter and allow for a smaller and lighter power supply. Isolated converters are favoured for applications connected to the mains, as it provides a barrier between the dangerous input voltage and the output (often a requirement). Additionally, as the ground voltages are isolated it can prevent ground loop noise which is useful in sensitive applications.
Protection, additional features and EMC
Power supplies generally come as standard with several protection circuits built in, the most common are overcurrent, overvoltage and overtemperature protection. These protection circuits prevent the converter or load from being damaged during abnormal operating conditions, for example when a short circuit occurs on the load side. Another protective feature is under or over voltage lockout, this prevents the converter from operating when the supply voltages are out of specification. In some cases, even a low supply voltage can damage high voltage components.
Some advanced features available on higher end power supplies are remote capabilities and digital control. These allow the converter parameters to be adjusted using digital interfaces such as PMBus. Remote sensing ensures the load receives the correct voltage as there is voltage drop across the wires that deliver the power, for example it would control the converter to provide 12.3V at its output to deliver 12V at the load. Modular power supplies can offer redundancy, allowing systems to remain operational even when a fault occurs with a converter. They are also able to load share to ensure that one supply does not get used more heavily, wearing it out faster during parallel operation.
It is also important to consider what EMI (electromagnetic interference) requirements the complete system will require, as this is often what is certified. It is still important to consider what EMC/EMI characteristics the converter will have, as the power supply can generate a lot of noise or even be susceptible to it.
Dimensions, packaging and thermal requirements
Converters are available in a variety of different packages and mounting styles, the most common are surface mounting (SMT), through-hole mounting (THM), DIN rail mounting, chassis mount, power bricks, open frame, and enclosed power supplies. Depending on the packaging, mounting configuration and location within the system, a variety of cooling options are possible.
The simplest method to manage heat dissipation in a power converter is convection cooling, this is often more effective on an open-frame power supply as air can circulate more freely than an enclosed power supply for example. Conduction cooling often involves designing a power supply or system around the other, to ensure there is an area where a base plate can transfer heat out of the power supply.
Forced air cooling involves using a fan to dissipate heat, the fan can be from the system itself or designed into the power converter. A drawback of forced air cooling is the additional vibration and noise it causes which may not be suitable for some applications, however it is often the most effective method of managing heat when used in combination with conduction cooling.
