MIL-SPEC COTS EMC input filter for DC-DC converters

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MIL-SPEC COTS EMC input filter for DC-DC converters

April 26 , 2023 0

The switching action of a DC-DC converter may cause undesirable common-mode and differential mode noise, creating unacceptable interference at many points in the spectrum. Front-end (or powerline) filters are intended for use before DC-DC converters to mitigate electromagnetic interference (EMI). These customizable or off-the-shelf purchased front-end filters can be implemented with vendor switching power supplies (SMPS) or DC-DC converters designed to comply with Electromagnetic Compatibility (EMC) regulatory standards, such as FCC, ETSI, CISPR, MIL-SPEC, etc.

These off-the-shelf front-end filters are custom-designed based on the electromagnetic characteristics of the power conversion equipment. However, other electrical (e.g., voltage spikes, ripples), mechanical (e.g., vibration, shock), and environmental (e.g., high altitude) design constraints must also be considered to meet the needs of military equipment. This paper discusses the design considerations of front-end filters and the testing requirements of DC power modules for military equipment.

What is a front-end filter?

The design of this input filter is critical to meet electromagnetic Compatibility (EMC) standards and objectives. Front-end or input filters are used for a variety of purposes:

■ Suppress noise and surges that may enter the first stage of the power supply.

■ Reduce the fundamental frequency (i.e. switching frequency) and its harmonic emission noise

Switching power supplies are increasingly common in electronic devices, with rich spectral content that may be conducted through physical contact to other parts of the circuit and interfere with nearby sensitive circuits. As switching speeds increase, noise becomes an increasing problem, especially when fast-switching transistors cause an interruption in the flow of current (resulting in voltage spikes and high-frequency noise).

These current interrupts can be found at the input end of the step-down converter, the output end of the boost converter, and the input and output end of the flyback and step-down converters.

Noise sources for various voltage regulators

The input end of the DC/DC step-down converter has the fast opening and closing of the fast switching device, resulting in sharp rising and falling edge (high di/dt) intermittent current on the capacitor. This will cause the fundamental frequency and several harmonics (usually lower order harmonics) not to meet the standard. A boost converter operating in continuous conducting mode (CCM) will experience EMI at its output due to the need for a fast reverse recovery diode which, while greatly reducing power loss, is more aggressive with current change (di/dt), increasing EMI. In discontinuous conduction mode (DCM), the main current ripple is larger. The ripple will create a changing signal that travels through the common contact conductors to the rest of the system.

EMI: Emission of radiation and conduction

Typically, the emission of conduction correlates with frequencies below 30MHz, while the emission of radiation usually falls in the frequency range above 30MHz (usually 50 to 300MHz). However, the emission of conduction and radiation still overlaps. In switching power supplies, voltage surges (high dV/dt) are usually the source of radiated noise. As mentioned earlier, conducted EMI is usually derived from interrupted current (high di/dt) and can be decomposed into common-mode (CM) and differential mode (DM) noise.

Differential mode and common mode noise

DM current is usually dominated by di/dt and will flow between the power line and the loop; DM noise dominates lower frequencies. It is often difficult to change the behavior of di/dt without fundamentally altering the circuit. di/dt can be reduced by using passive low-pass EMI filters (e.g., R-C dampers, L-C, Pi segments, T segments, etc.) to suppress oscillations caused by intermittent currents.

CM current is usually a function of dV/dt and will flow between each power cord and the ground. When CM current is coupled to a long conductor or cable, the cable can act as an antenna, making CM noise more prominent at high frequencies. Depending on the length of the cable and the distance between conductors and the reference ground plane, the loop area of the accidental loop may be very large. Effective layout designs can greatly inhibit CM, such as moving conductors closer to the reference ground plane, carefully deploying safety capacitors, shielding connected cable bundles, or placing CM inductor coils in the CM current path. The CM inductor coil also provides a high-impedance series path that allows CM current to flow out of the converter through the Y capacitor to form a shunt path for the EMI ground.

Both DM and CM will contribute to EMI, and it is often necessary to quantify DM and CM noise components to comply with industry EMC standards before designing EMI filters. Input EMI is usually quantified using a line impedance stabilization network (LISN) at the input of the device under test (DUT) as well as a spectrum analyzer.

Design considerations for front-end filters

In general, passive EMI filtering is the most common method for noise suppression; However, this may prove difficult when the filters terminate with different load impedances and different noise sources for the SMPS. These filters are usually resistors, capacitors, and inductors of various permutations. The basic components and the first few harmonics have the largest sizes and will make the greatest contribution to the overall noise, while the amplitudes of higher order harmonics gradually decrease as the frequency increases. The ability of the filter to suppress these noise components also increases with frequency, so mitigating noise at the fundamental and lower order harmonics is a prominent design challenge.

In general, large passive filters can mitigate low frequency emission; However, due to its parasitic nature (such as the equivalent series resistance of the capacitor and the inductor and the shunt capacitance of the inductor), high-frequency emission may require additional design considerations. Other EMI filtering techniques often involve active components: one of these techniques uses spread spectrum or jitter to modulate the switching frequency of the power supply to reduce the peaks of the fundamental and lower order harmonics found in the frequency domain. Ultimately, the technology adopted depends on the unique noise characteristics of SMPS as well as design costs, size, and regulatory constraints.

In addition to complying with EMC regulatory standards, EMI filters can include the ability to suppress high current transients reflected from the load to the SMPS input power supply. The predictive transient characteristics of each SMPS will be different, so customizations are often required to adequately suppress surges. This is certainly an additional design consideration for MIL-SPEC power electronics. Military equipment will have a range of electrical, mechanical, and environmental design considerations that will cause manufacturers to carefully design power electronics from the ground up -- material screening and electrical, mechanical, and harsh environmental performance requirements must be met.

Common military standards for power supplies

MIL-STD-461 sets conduction and radiation emission limits for electrical devices, guiding the correct measurement of EMI. If the SMPS exceeds these limits -- and it often does -- it will need an EMI filter to bring it "back to spec." However, choosing any off-the-shelf EMI filter does not necessarily cause the power supply to suddenly conform to the standard requirements; The device can be so noisy that attaching any EMI filters to the input will still cause the part to fail. The various requirements for MIL-STD-461 and their descriptions can be found in Table 1. MIL-STD-461 compliant electronic devices typically list details of meeting specific CE, CS, and RE requirements.

EMI is not the only consideration for the correctness, reliability and safety of electronic equipment. The power supply must also be able to operate under a variety of voltage conditions, including reverse polarity, voltage spikes, and voltage surges. The MIL-STD-1275E standard provides test conditions to be applied to 28V electrical power system inputs, as well as expected performance parameters for the device. These systems are expected to be used in military ground vehicles, civilian off-road vehicles, and military and civilian heavy equipment.

Other military standards -- such as MIL-STD 704F for aircraft power characteristics and DO-160G for airborne equipment -- will specify environmental conditions and test procedures to adequately simulate various voltage conditions. The MIL-STD-810 standard includes test conditions and requirements for devices that will experience mechanical shock, vibration, and high altitude. It may also be a necessary consideration to ensure the long life and reliability of supplies in harsh environments. Power supplies operating in military vehicles and air systems will likely need to meet all aspects of MIL-STD-1275e /MIL-STD 704F, MIL-STD-461, and MIL-STD-810 to be considered fit for use. P-Duke offers a range of MCF catch-front filters that can be matched with selected DC/DC converters to meet all of these specifications.

P-DUKE MCF series

The MCF family provides EMI filtering and transient protection to meet the conducted emission, conducted-sensing, and radiative emission requirements in MIL-STD-461G, surge/spike requirements in numerous military standards, and height/shock/vibration requirements in MIL-STD-810 (Table 2).

The series features active input overvoltage protection that limits overvoltages of up to 50ms to less than 40V, absorbs a peak voltage of +/-250V, and has an internal protection circuit (Figure 1).

As shown in Figure 2, the series includes other active protection features such as remote on/off control, overload protection, output short-circuit protection, reverse polarity protection, boot current limitation, etc.

You can see a significant improvement in EMI performance in Figure 3. The EMI performance of the 200W HAE200 DC/DC converter was tested before and after applying the MCF filter. This greatly reduces the cost and design effort of custom EMI filters and peripheral circuits. The MCF front-end filters are rated up to 250W and can be used in a wide range of military equipment.

MCF filters can be easily integrated into military systems

EMI filters cannot just be randomly selected to make the power supply conform to the standard, and the process of designing these filters can be very complex with high non-repetitive engineering costs (NRE). This is especially true in military equipment, where airborne subsystems in military vehicles and aircraft are powered by 24V batteries or 28V generators and have stringent requirements for many aspects that COTS equipment and custom solutions may not be feasible. The P-DUKE offers 15W to 250W DC-DC converters paired with associated MCF front-end filters to meet military requirements for EMC and surge suppression.