Modular AC line filter, EMI AC line filter

What is an EMI AC line filter? What does it do? For equipment powered by AC power, a modular AC line filter is often used, either as part of the connector or as part of the chassis, especially in In professional environments such as industrial, medical and ITE. This equipment usually includes an embedded AC/DC converter or power supply, which can also be chassis-mounted or sometimes rack-mounted or PCB-mounted. In each case, the electricity supply will always meet the statutory requirements for emissions as a separate part, typically EN55011/EN55032 for conducted and radiated disturbances. But additional filtering may still be needed.

Experienced equipment designers know that simply using compatible components does not guarantee that the final product meets EMC requirements. There are many reasons. For example, compliance testing of a device's AC-DC converter is performed under very specific conditions, including assumed AC line impedance, output loading, cable length and routing, and the location of the component relative to the ground. When a final product is tested, this AC-DC converter is installed inside, and all of these conditions are different, resulting in a different and often worse conducted EMI signal. Radiated electromagnetic interference from other components can also be detected on power cables, increasing conduction levels.

Modular filters enable system electromagnetic interference compliance

An external module filter may be the solution, but with hundreds of options, which one is best? Let's first look at the internal circuitry of a typical commercial filter and consider the role of each component.

Capacitor CX attenuates differential mode noise, signal, and spikes that appear from line to neutral due to rapidly changing currents in the converter. Capacitors are rated for X1, X2, or X3 because of their ability to withstand voltage transients on AC lines. The inductor L is a common mode or current compensated choke with two windings phase controlled as shown. Common mode noise is caused by rapid changes in voltage within the converter from line and neutral to ground. The choke is high impedance and each CY capacitor directs noise current to ground. Normal operating current passes through the two windings on the choke, causing the magnetic fields in the core to cancel, so high inductance values ​​can be used without fear of magnetic saturation. Often, the coupling between the L windings is less than perfect, creating some leakage inductance that appears as a single series inductance, increasing differential mode attenuation.

While CX can be any capacitance value within the practical range, these two CY values ​​are limited by ground leakage current requirements. There are types Y1, Y2, Y3 and Y4, and the rated operating voltage and transient voltage are both reduced. Leakage currents through Y capacitors are a potential problem because they bridge the safety barrier line and neutral to ground. If the protective grounding of the equipment's metal enclosure fails, the enclosure can float to line voltage via the Y-capacitor and potentially cause an electric shock. Therefore, the value of these Y capacitors is limited to not allow more than the specified current through the bushing, this amount is set by the standards of the specific application environment. Limits can range from tens of milliamps for industrial systems down to less than 10µA for cardiac mobile medical applications.

Resistor R1 is a high value resistor, typically 1M ohm, used to discharge if the AC power is suddenly disconnected and the load cannot be relied upon to pull the charge away, leaving a potentially dangerous voltage on the AC connector pins CX. Standards such as IEC 62368-1 state that R1 should discharge the capacitor to less than 60V after two seconds for CX > 300nF (nanofarad), while higher voltages are allowed for CX < 300nF. Likewise, the allowable voltage limit is higher for equipment that is accessible only to trained personnel.

However, other criteria are different. For example, IEC60601-1 medical equipment requires that the discharge be less than 60V after one second, but there is no requirement if CX is less than 100nF. Standards such as IEC 62368-1 also require resistors to withstand transient voltages and that the resistance does not vary by more than 10% if installed in front of the fuse. Therefore, resistor R1 will be a high specification part. In some applications, R1's power consumption under normal conditions may limit its opportunity to comply with standby or no-load loss limits imposed by agencies such as the U.S. Department of Energy (DoE) and the European ErP Directive.

The fuse shown in Figure 1 can be included in modular filters, especially panel mount types like the popular IEC320-C14 type (Figure 2).

In commercial applications, a single fuse in the line is normal. If the fuse element complies with the standard, it will simplify the specification of downstream components, as stated in R1. Some applications, such as medical equipment and Class II IT, require lines and neutrals to be fused simultaneously to cover the possibility of accidental connection reversal. In the case of a single fuse, if a reversal of connection occurs, it will render the live line unstable and dependent on the upstream fuse or circuit breaker at the power opening, resulting in a protective short to ground. However, these upstream devices may be rated for high amperage values ​​to protect lines with multiple loads and are not guaranteed to open quickly in the event of equipment failure, which may result in a fire hazard. However, dual fuses also have the disadvantage that an overcurrent connected to the neutral may only open the neutral's fuse, leaving the device apparently dead but still having a live connection internally.

Select filter

The mechanical format of the filter is a natural starting point for the selection process. Mechanical variants are available as IEC imports with screw or snap-on mounting, with a choice of switches and zero, one or two fuses depending on application requirements. IEC import types are rated C14 at 10A and C20 at 16A, with chassis mount parts available for 20A and above. Chassis mounted filters, typically with 6 side shielding and fixed directly to conductive ground metal, provide very effective attenuation of electromagnetic interference.

For all types, medical versions are available, which omit the Y capacitor to reduce leakage current to typically 5µA maximum. This omission necessarily means that the common-mode attenuation is reduced, which may need to be compensated for elsewhere, such as by cascading filters. The rated current requirement of the filter can be easily calculated from the load power requirement, given the minimum input voltage and load power factor. For example, if the filter load is 200w and the power factor is 0.9, at 90vac, it will produce a current of 200w / (0.9 x 90 VAC) = 2.47A. In this case, a class 3a filter can be chosen.

Selecting the required attenuation of a filter is best accomplished by measuring system performance without the filter installed and then calculating how much additional attenuation is required from the external filter to meet the specifications. The attenuation curve in the filter datasheet will give an indication of the filter's performance, but remember that datasheet performance is under specified test conditions, typically 50 ohms source and load impedances. While AC power can be standardized using a line impedance stabilization network (LISN), the application load can vary significantly from the datasheet test conditions.

In AC and DC power supplies, filter modules cascaded with internal filters can also produce unexpected results. Possible resonances can even lead to amplification of electromagnetic interference at critical frequencies. As an example, an EMI diagram taken from a typical AC-DC converter of an XP power supply, part PBR500PS12B, operating at 230 VAC and 180 W is shown in Figure 3. The pseudo-peak detection in the figure complies with the en55032 curve B emission limit line. Then a filter, XP Power FCSS06SFR, is inserted into the AC line, and the resulting attenuation characteristics are shown in Figure 4. The dotted line is the differential mode and the solid line is the common mode attenuation. The overall combination results are shown in Figure 5.

It can be seen that around 1mhz the attenuation of the filter reduces the expected emission, but at 10mhz and above the improvement is not as expected, meaning the modular filter does not "see" 50 ohms at these frequencies of termination. It gives lower attenuation than expected. This result confirms the need for actual measurements to determine compliance.

Ask the Experts

Proper EMC compliance at the initial stages is crucial to avoid costly failures in final product testing. However, the solution is not simply to use an oversized modular filter at the AC inlet, which may add unnecessary costs or even be counterproductive, with unexpected attenuation results. Instead, designers should perform tests and take measurements to determine the actual filter requirements for their application.

Power supply manufacturers like XP Power can help, offering a range of AC and DC power products with modular filters in versions tailored for ITE, industrial and low leakage medical applications, among others. Some companies even offer customers comprehensive application design support and free use of their in-house EMC pre-compliance testing facilities. The above is the relevant analysis of EMI AC line filters. I hope it can help you.