Ask the Expert: Jeff VielAugust 15, 2012
Jeff Viel our Director of EMI/ Engineering for NTS Boxborough, MA is a US Marine Veteran, bringing 20 years of industry-specific knowledge and experience. Jeff has a BSEE from North Eastern University and 14 years with NTS.
A question that is often asked of Jeff is, “Will a COTS power supply pass MIL-STD-461 Testing?
This is a difficult question to answer, but in most cases I would say no, not without a few modifications. The main reason for this is due to test requirement variances between defense and commercial standards.
For example, MIL-STD-461 uses Test Method CE102 to evaluate the conducted emissions levels on a product’s input power lines similar to methods described in CISPR and FCC. Both utilize 50 uH LISNS of similar type construction, and relatively similar resolution bandwidths. However one notable difference is MIL-STD-461 requires peak detection, versus quasi-peak, and average detection to capture the worst case emissions envelop.
A second notable difference is that the MIL-STD-461 CE102 conducted emissions measurements start at 10 kHz, where most commercial specifications do not perform this measurement below 150kHz. Based on these commercial certification requirements almost all COTS power supplies switch below 150kHz, many even switch as low as 70-75kHz, so that the 2nd harmonic of the switching frequency is also below the 150kHz limit. This requires less lowpass RF filtering to meet the commercial specs, thus reducing the cost to build and reducing the chances of failing costly EMI testing. However, as a result of these design implementations, measurements between 10 kHz and 150 kHz are generally problematic.
A common solution is to tune the power line filter to a lower cutoff frequency in an effort to provide adequate switching noise suppression. How this is done depends on what procurement group it is being sold to. The U.S. Navy has strict limitations on the amount of line-to-ground (common-mode) capacitance a power line filter can have. Line-to-ground capacitance for each line shall not exceed 0.1 microfarads (μF) for 60 Hertz (Hz) equipment or 0.02 μF for 400 Hz equipment. For submarine DC-powered equipment and aircraft DC-powered equipment, the filter capacitance from each line-to-ground at the user interface shall not exceed 0.075 μF/kW of connected load. For DC loads less than 0.5 kW, the filter capacitance shall not exceed 0.03 μF. In these cases, the limitation on common mode capacitance will require the introduction of bulky series inductive chokes to achieve the same performance.
The added weight, cost, and footprint restrictions are all common challenges which must be resolved prior to becoming certified to MIL-STD-461. If pre-compliance test data is available which shows the frequencies, and amplitudes which need to be attenuated, a series of band-reject (Notch) filters can be implemented. This will substantially reduce the size and weight of the compliance filter as it is only attenuating the discrete frequencies of interest.
If you are interested in more information related to this question or have any others for Jeff you can contact him at Jeff.Viel@nts.com