EMI Requirements for COTS Power Supplies

June 23, 2014
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This is the second in a series describing the requirements that must be met when developing COTS power supplies. At Vicor, we manufacture a range of COTS power components as well as complete COTS power supplies that are available from our Westcor division. A key requirement is for these products to meet Mil-STD 461, which defines the EMI performance that must be achieved.

MIL-STD-461 covers four key aspects of Electromagnetic Interference

  • Conducted Emissions (CE) – what the power supply feeds back to the power grid
  • Conducted Susceptibility (CS) – the effect that various noise and distortion elements in the power grid have on the power supply
  • Radiated Emissions (RE) – the radiated noise generated by the power supply
  • Radiated Susceptibility (RS) – is the supply performance disturbed by external voltage and magnetic fields

Typically only CE and CS requirements apply to power converters, while RE and RS apply to the system as a whole.

The most problematic parts of this standard for COTS power supplies are CE101 and CE102, which are generally met by a combination of low-noise design and external filtering.

CE 101 is the test procedure that is used to verify that electromagnetic emissions from the Equipment Under Test (EUT) do not exceed the specified requirements for power input leads, including returns, over the 30 Hz to 10 KHz range. It is required for power systems used in submarines, Army aircraft (including flight line) and Navy aircraft with anti-submarine warfare equipment.

Fortunately due to the requirements for Power Factor driven by the European market, most active PFC power supplies do a pretty good job of getting rid of the harmonics of line frequency that make up the bulk of line emissions in this frequency range. Non-PFC supplies face much more of a challenge due to their non-linear AC input current, and are forced to use heavy, bulky passive filters for line harmonics.

CE102 verifies that electromagnetic emissions from the EUT do not exceed the specified requirements for power input leads including returns over the 10 KHz to 10 MHz range, and is required for a wide range of systems including: submarines, Army aircraft (including flight line), Navy aircraft, Air Force aircraft, space systems, ground Army, ground Navy, ground Air Force and surface ships.

Since many PFC supplies switch in the 100-200kHz range, this is more of a challenge, and generally an external filter that notches the switching frequency is required, since few COTS front ends need to meet this requirement when selling to commercial customers

Conducted Susceptibility tests whether the EUT will continue to function properly if various parts of the system are injected with electrical or magnetic disturbances. Conducted Susceptibility is much less of an issue with Westcor PFC supplies as the disturbances are damped by existing front end filtering that removes the switching noise.

In the next post in the series, Jeff describes the challenges imposed by Mil-STD 810, which defines the requirements for mechanical ruggedness.

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