In the context of spacecraft

Spacecraft

A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

, the power processing unit (PPU) is a module containing the electrical subsystem responsible for providing electrical power to other parts of the spacecraft. The PPU needs to be able to cope with varying demands

Dynamic demand (electric power)

Dynamic Demand is the name of a semi-passive technology for adjusting load demands on an electrical power grid. The concept is that by monitoring the frequency of the power grid, as well as their own...

 for power output and provide that power in the most efficient manner possible.

There are two main constrains placed on PPU:

• Power generation
  Electricity generation
  Electricity generation is the process of generating electric energy from other forms of energy.The fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday...
  
   from for instance a solar array or radioisotope thermoelectric generator
  Radioisotope thermoelectric generator
  A radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...
  
   where power generation can vary based on external conditions.
• Power utilization which varyies depending of the current internal activities performed by the spacecraft such as burst radio transmission or external demand factors like outside temperature and the need to provide heat to maintain constant internal temperature.

Major considerations in building PPUs are weight

Weight

In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...

, size

Measurement

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

 and efficiency

Efficiency

Efficiency in general describes the extent to which time or effort is well used for the intended task or purpose. It is often used with the specific purpose of relaying the capability of a specific application of effort to produce a specific outcome effectively with a minimum amount or quantity of...

.
Most PPUs process and supply direct current

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 because that is what is generated by a solar array.

The PPU is also responsible for voltage

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

conversion and supplying the required voltage to other subsystems of the spacecraft.