PFC Power Supplies

PFC Power Supplies

Why are active PFC power supplies desirable for military applications?

In today’s military environment many platforms (ships, planes, tanks etc.) are adding more and more electronics in order to increase the capabilities and functionality of these platforms. This is placing higher and higher demands on existing prime AC sources of input power. Most electronic systems take in the prime AC input power, rectify this voltage and use a large filter capacitor to create the DC voltage needed to run the system electronics. This creates large current pulses on the input mains. These current pulses can cause high levels of total harmonic distortion (THD) which interferes with other components of the system and reduces the amount of power that is available from the primary source.

Ships, planes and other platforms have a need for low-THD military power supplies. One solution to this issue is to use active PFC circuits in the custom military power supplies for the system components. An active power factor corrected power supply makes the electronic system appear as a resistive load to the primary input source. A resistive type load reduces the chances of interference between other systems and enables the primary AC power source to supply power closer to its rated capabilities.

Typically there are two measures of input characteristics that determine the impact that a load has on the primary AC input source, Power Factor (PF) and the amount of input current harmonic distortion. A resistive load would have a PF = 1.0 and the military has a number of specifications that define acceptable levels of harmonic distortion in a PFC military power supply. For example MIL-STD-1399 defines these requirements for shipboard applications. The following is a summary of the major requirements of a power factor corrected power supply that are defined in this specification.

MIL-STD 1399, Section 300A, Electric Power, Alternating Current

  • Type I power
    • The ship service power distribution system supplied by the ship’s generators is 440 V, 60 Hz, three-phase, ungrounded.
    • Through transformers, 115 V, 60 Hz, three-phase, ungrounded is distributed throughout a ship.
  • Power interruptions of 100 msec.
  • Power factor; 0.8 lagging and 0.95 leading
  • Voltage Spike; line-to-line and line to ground
    • 2500 V for 440 V systems
    • 1000 V for 115 V systems
  • The operation of user equipment shall not cause single harmonic line currents to be generated that are greater than 3% of the unit’s full rated load fundamental current between the second and thirty-second harmonic.
  • Current unbalance; <5% (3% for submarines)

PFC Military Power Supplies from ACT Using active PFC, ACT has produced a number of custom, low-THD military power supplies that meet the requirements of MIL-STD-1399. ACT has designs that can operate over the 440 V and 115 V three phase and single phase inputs.