Defense systems with 28-V power requirements must meet a number of noise and power related standards, including MIL-STD-461 (Requirements For the Control of Electromagnetic Interference Characteristics of Subsystems and Equipment), MIL-STD-704 (Aircraft Electric Power Characteristics), and MIL-STD-1275 (Characteristics of 28 V DC Electrical Systems in Military Vehicles). Each of the standards encompasses multiple revisions, any one of which may be enforced by the application. Additionally, each standard features subsections that apply as dictated by the end installation. To satisfy the standard, designers need to introduce EMI filters. Let's take a closer look at how to design an effective filter to ensure compliance.
Of the various power-related standards, MIL-STD-461 is of particular interest. The standard addresses a number of characteristics including conducted emissions, conducted susceptibility, radiated emissions, and radiated susceptibility. Emission refers to the noise a device generates as it impacts the source to which it is connected. Susceptibility is the vulnerability of a system to incoming noise.
The latest substantive revision, MIL-STD-461E, includes a number of substandards for various components such as power leads, antenna terminals, and antenna ports, over defined frequency bands (see table 1). [Ed. note: a subsequent revision, 461F, does exist but it is just a validation and does not change specifications.]
Table 1: Substandards for MIL-STD-461E define performance requirements at a component and system level.
As table 2 shows, not all sections of MIL-STD-461E apply universally; hence, most power conversion suppliers focus on achieving compliance to the subset that affects all installations, and in particular to the conducted, rather than radiated, sections. These standards are CE102, CS101, CS114, and CS116. Frequently, manufacturers will also reference CE101 because the frequency band of interest (30 Hz to 10 kHz) is well below the switching frequencies of most DC-DC converters. Conducted emission and susceptibility requirements, not radiated requirements, are quoted because radiated sections are significantly dependent upon the physical layout, external output circuitry, and the enclosure in which the power supply resides. With a valid filter design and good PCB layout, most components can easily meet conducted requirements.
Table 2: The applicability of MIL-STD-461E substandards varies by platform.
There is not much difference between revision E and the earlier revision D; in fact, of sections CE101, CE102, CS101, CS114, and CS116, only CS101 and CS114 differ as follows:
• CS101 - No change up to 5 kHz; above 5 kHz:
MIL-STD-461D: Required level drops 20 dB/decade to 50 kHz MIL-STD-461E: Required level drops 20 dB/decade to 150 kHz
• CS114 - No change up to 30 MHz; above 30 MHz:
MIL-STD-461D: Required level drops 10 dB/decade to 400 MHz MIL-STD-461E: Required level drops 10 dB/decade to 200 MHz
Basics of EMI Now that we have introduced the standard, how do we gain compliance? What follows is a general guide for EMI filter design. We will focus on CE102 for our discussion.
Let's start with an overview of EMI. We can separate EMI measurements into two parts: conducted and radiated. Conducted measurements are measurements of either voltages or currents flowing in the leads of the device under test, as dictated by the standard. Common-mode conducted noise current is the unidirectional (in phase) component in both the positive and negative inputs to the module. This current circulates from the converter via the power input leads to the DC source and returns to the converter via the output lead connections. This represents a potentially large loop cross-sectional area that, if not effectively controlled, can generate magnetic fields. Common-mode noise is a function of the rate of change of voltage (dv/dt) across the main switch in the converter and the effective input to output capacitance of the converter. Differential-mode conducted noise current is the component of current, at the input power terminal, that is opposite in direction, or phase, with respect to each other.
For our purposes we will concentrate on MIL-STD-461, CE102 that is a voltage measurement into 50 O.
E-field radiated emissions are caused by currents conducted by a suitable antenna, such as the power leads of the device under test. If we can greatly reduce the conducted emissions then we will reduce the radiated emissions, as well. The enclosure of the device under test, lead geometry, and other devices running within the device under test will affect the emissions. Radiated emissions due to B-fields are best addressed by shielding with a suitable material and proper layout.