How to Choose Modular EMI Filters for AC-DC Converters

Modular AC line filters are commonly found in end equipment, either mounted on a chassis or integrated on a connector, especially in applications such as communications, healthcare and industrial electronics. The purpose of a filter is to attenuate radiation generated by a combination of power supplies, built-in electronics, high-speed data lines, and more.

Modular AC line filters are commonly found in end equipment, either mounted on a chassis or integrated on a connector, especially in applications such as communications, healthcare and industrial electronics. The purpose of a filter is to attenuate radiation generated by a combination of power supplies, built-in electronics, high-speed data lines, and more.

The built-in power supply usually meets the statutory emission standards by itself, usually EN55011/EN55032, why do you need an additional filter? Compatible components do not necessarily guarantee a compatible system. If there are multiple AC-DC converters, their radiation may increase. Additionally, an AC-DC will be tested under specific conditions of AC line impedance, load, component orientation and location relative to the ground plane, including set lengths and cable routing and passive loading. When installed in equipment, the converter will not see these same conditions and the radiation may be higher. Connecting to the terminal equipment outlet may also receive radiation from other system components, increasing conducted interference.

EMI compliance may require additional filters

A typical solution is a modular filter installed in or near the appliance power outlet connector. To get the best performance and cost, choosing from the many filters available is not an easy task, so to help you, let’s refer to a typical filter diagram and see what each component does. (figure 1).

  How to Choose Modular EMI Filters for AC-DC Converters

Figure 1: Typical modular EMI filter circuit

CX attenuates differential mode noise, i.e. noise from line to neutral. It is an “X” rated capacitor capable of withstanding certain AC line transients, depending on the “overvoltage” rating of the environment. Types X1, X2 or X3 are available according to EN60384-14 with peak working voltage ratings of 4kV, 2.5kV and 1.2kV respectively. If the capacitor is short-circuited due to stress, there is a risk of fire, so the components must pass through a safety mechanism. The capacitance value of CX can be very high, limited only by practical considerations: when the AC line is disconnected by a downstream circuit or R1, it must discharge within a specified time to avoid leaving potentially dangerous voltages on the connector pins . According to the communication safety standard EN62368-1, if CX is greater than 300nF, after two seconds, the limit is less than 60V, for values ​​less than 300nF, higher voltages are allowed, or in environments only accessible by trained personnel. In medical facilities, limited to 60V for 1 second according to EN60601-1, but no requirement if CX is less than 300nF.

R1 must be rated for high continuous line AC voltage and must also withstand transient voltages of not more than 10% deviation in resistance value if installed before a fuse according to EN62368-1. For large values ​​of CX, R1 must be a relatively low resistance to meet the discharge time specification, resulting in significant continuous power dissipation. This can be problematic when trying to meet the no-load or standby loss limits imposed by the US Department of Energy and European ERP directives.

L is a “current compensated” two-winding Inductor that attenuates common-mode noise, typically generated by high switching frequency voltages, driving noise currents to ground through internal supply capacitors and back through line and neutral. As shown, the magnetic field created by the normal operating current cancels out and the “common mode” currents on the line and neutral together “see” a high impedance. These two capacitors divert the current so that it circulates locally rather than through the AC power source. L1 can be a high value when the operating current field cancels out and prevents core saturation, but sometimes the winding coupling is intentionally reduced to allow some leakage inductance, which contributes to differential mode attenuation, possibly reducing the value of CX.

The two capacitors CY must also go through a safety mechanism, because if one of them fails short circuit and the device ground is disconnected, the device case can be live. Even in the absence of a capacitor failure, if the ground connection is accidentally disconnected, the enclosure will have sufficient “leakage current” to strike, so the capacitor value is limited to provide large “contact” and “enclosure” leakage currents according to applicable standards. In some industrial areas with hard-wired grounding, the limit may be milliamps, and in “heart-floating” medical environments, the limit is less than 10 microamps. Capacitors are also specified according to the AC power class; Y1, Y2, Y3 and Y4, with peak test voltages up to 8kV.

The fuses in Figure 1 are often included in panel-mounted modular filters, such as the popular IEC320-C14 type (Figure 2). Some standards require that only the line connections be fused, while others in medical and communications require both the line and neutral to be fused. On a single blow, if the input is accidentally reversed, the fuse is at neutral, and when neutral and ground are shared, they are bypassed, making protection dependent on upstream fuses or circuit breakers that may be connected to Shared with other devices and therefore has a high current trip rating. Current entering filtering equipment may represent a fire hazard. Line and neutral are blown, reverse power is now covered. However, if the fuse in the neutral connection opens due to a fault such as line-to-neutral overcurrent, the device is clearly “dead” but has a live connection inside. To fix this, the neutral fuse can be made a step higher than the line value so that the line fuse will open normally first.

Modular filters are available in a variety of mechanical forms; board mount types, usually with a six-sided shield attached directly to a grounded board, are very effective for shorting fuses and jack connectors. IEC inlet connectors with built-in filters are a common choice, available in screw-on or snap-in mounting, with one or two fuses depending on the application. The rated current of the C14 type is 10A, and the rated current of the C20 type is 20A and above.

Versions of each type are available without a “Y” capacitor and can be used in medical applications with large leakage currents, typically 5µA. This reduces common-mode noise attenuation, for example, cascaded filters may be required to allow for common-mode noise attenuation.

Load power sets the rated current of the filter, allowing low input voltage and load power factor. For example, for a power factor of 0.8, small 90VAC, and a load of 300W, the current draw is 300/(0.8x90VAC) = 4.16A, which indicates a 5A rated filter is required.

Modular Filters have published graphs of attenuation with frequency, which can be used to select a type by measuring performance without using a filter and then subtracting it from the target to get the desired filter attenuation. Filter performance data is obtained under specific test conditions, typically with 50 ohm source and load impedances, so the attenuation actually depends on the termination circuit. AC source impedance can be normalized using a Line Impedance Stabilization Network (LISN), but the load can be very different from 50 ohms, vary with frequency, and even show negative incremental impedance. Also, the danger of resonance with other series filters can lead to unexpected results, even amplifying rather than attenuating EMI at certain frequencies.

As an experiment, the EMI performance of XPPower’s PBR500PS12B AC-DC converter was plotted under 230VAC and 180W load, and the results are shown in Figure 2. As required by the standard, the converter meets the EN55032 curve class B emission limits with good margins for quasi-peak detection. An additional filter has been added, the model is XPPowerFCSS06SFR, and the attenuation characteristics are shown in Figure 3. The solid line is the common mode and the dotted line is the differential mode attenuation.

Figure 4 presents a graph of the combined effect on radiation, showing that at approximately 1MHz, the total attenuation (unit: dB) is the original plus filter value. But above a few MHz, the attenuation is smaller than expected. This is due to the fact that the filter does not “see” the 50 ohm termination at high frequencies, reducing its attenuation effect and confirming the need for actual measurements to confirm compliance with emission limits.

How to Choose Modular EMI Filters for AC-DC Converters

Figure 2: AC-DC power supply, internal filter only

How to Choose Modular EMI Filters for AC-DC Converters

Figure 3: XPFCSS06SFR Modular Filter

How to Choose Modular EMI Filters for AC-DC Converters

Figure 4: AC-DC power supply with external filter added

The type and performance of a modular EMI filter is a complex choice that is critical to avoiding potential redesign costs in EMC testing as product launch approaches. Overkilling with large filters is expensive and can even produce unexpected results. Manufacturer XPPower can help, complementing its AC-DC converter offering with a wide range of filters, with a choice of current ratings and attenuation characteristics in IEC and board-mount formats. Versions are available for all applications, including low leakage types of medical, but more are planned for high current and three phase applications. Multistage filters are also available to increase attenuation and to customize versions as needed.

XPPower provides comprehensive application support for its products and can assist with pre-compliance testing of customer products through the use of in-house EMC testing facilities in various locations.

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