How to Select Modular EMI Filters for AC-DC Converters

An ac-dc converter is a power supply device that converts alternating current into direct current. Simply put, it is a switching power supply that uses the PWM principle. The MOS tube works to produce a changing waveform, which is then converted into voltage through a transformer and rectified for output. The conversion of AC to DC is called rectification, and the conversion of DC to AC is called inversion. An EMI filter, also known as an "electromagnetic interference filter," is an electronic circuit device used to suppress electromagnetic interference, especially noise in power lines or control signal lines. Modular electromagnetic compatibility filters are typically added at the AC inlet to the terminal device. There are many current and attenuation ratings, including versions with or without fuses and switches. Low leakage parts can also be used in medical applications. This article describes the function of the filter and how to match it to the intended application for best results.

For AC mains powered equipment, modular AC line filters are often used, mounted on the connector or as a board mounted component, particularly suitable for use in professional environments such as industrial, medical and communications. The equipment usually includes an embedded AC-DC converter or power supply, which can also be board mounted, sometimes rack mounted or PCB mounted. In each case, the power supply will always meet the statutory requirements for emissions as a separate component, usually the EN55011/EN55032 standards for conducted and radiated interference.

One might ask why an additional filtering module might be necessary, but experienced equipment designers have long known that simply using compliant components does not guarantee that the EMC compliance of the final product will "pass". The reasons are varied: for example, compliance testing of the equipment AC-DC converter is performed under very specific conditions of assumed AC line impedance, output load, cable length and routing, and the position of the components relative to ground. When the final product is tested on an internally mounted AC-DC converter, all of these conditions change, resulting in a different and generally worse conducted EMI signature. Radiated EMI from other components can also be picked up via the power cables, adding to the conducted level. In filtering circuits, there are many special filtering elements (such as ferrite beads). They can improve the filtering characteristics of a circuit, and the correct design and use of filters is an important part of anti-interference technology.

1. Modular filters can make the system compliant with electromagnetic interference

An external modular filter could be the solution, but, with hundreds to choose from, which one is best? Let’s first look at the internal circuitry of a typical commercial filter and consider how each component contributes (Figure 1).

Figure 1: Schematic diagram of a typical modular EMI filter

Capacitor CX attenuates differential mode noise, signals and spikes from line to neutral produced by rapid changes in current within the converter and is rated X1, X2 or X3 to withstand voltage transients on the AC line. L is a common mode or current compensated choke with two phase windings as shown. Common mode noise, produced by rapid changes in voltage inside the converter, from line and neutral to ground, sees the choke as a high impedance and each CY capacitor diverts the noise current to ground. Normal operating current through the two windings on the choke causes the magnetic fields in the core to cancel, so high inductance values ​​can be used without worrying about magnetic saturation. Typically, L is wound with less than perfect coupling between the windings, so some leakage inductance is generated, which appears as a separate series inductor, which increases differential mode attenuation.

While CX can be any capacitance value within practical limits, the two CY values ​​are limited by earth leakage current requirements and are available in Y1, Y2, Y3 and Y4 types with reduced rated working voltages and transient voltages. Leakage currents through Y capacitors are a potential problem because they cross a safety barrier - the line and neutral grounds. If the protective ground connection to the metal parts of the equipment fails, the housing "floats" to the line voltage through the Y capacitor and can cause electric shock. Therefore, the value of the Y capacitor is limited to allow no more than specified current to flow, depending on the standard used for the application environment. Limits range from tens of milliamps in industrial applications to less than 10 microamps in cardiac floating medical applications.

R1 is a high value resistor, typically 1 megohm to discharge CX if the AC supply is suddenly disconnected and the load cannot be relied upon to drain the charge, leaving potentially dangerous voltages on the AC connector pins. Standards such as IEC62368-1 specify that R1 should discharge the capacitor to less than 60V after 2 seconds for CX>300nF, with higher voltages permitted for CX<300nF. The permitted voltage limits are also higher for equipment that is accessible only to trained personnel. Other standards vary, for example IEC60601-1 for medical equipment requires less than 60V discharge after 1 second, but not if CX is less than 100nF. Standards such as IEC62368-1 also place requirements on resistors, such as the ability to withstand transient voltages with a resistance deviation of no more than 10% if the resistor is installed before the fuse. Therefore, R1 will be a high specification part. In some applications, the power dissipation of R1 under normal conditions is a limit to comply with standby or no-load loss limits set by agencies such as the US Department of Energy (DoE) and the European ErP Directive.

In commercial applications, a single fuse in the line is normal. If the fuse element meets the standards, the specifications of the downstream components (such as R1) are simplified. Some applications, such as medical equipment and Class II communications, require that both the line and neutral be fused to cover the possibility of accidental reversal of connections, which in the single fuse case would leave the live line unfused and rely on upstream fuses or circuit breakers in the power opening to short from live to protective earth. These upstream devices may be rated for high currents to protect the wiring of multiple loads and are not guaranteed to open quickly in the event of a device failure, which could result in a fire. Double fusing does have disadvantages, however, where a line-to-neutral overcurrent may only open the neutral fuse, leaving the equipment apparently dead but with live connections still inside.

2. Select filter

The filter block diagram is a natural starting point; IEC input receptacles with screw or snap-on mounting are available, with switch options and no fuses, one fuse or two fuses, depending on the application requirements. IEC inlet types are rated at 10A for C14, 16A for C20, and chassis mount parts are rated at 20A and higher. Chassis mount filters, typically with 6-sided shielding and direct fixing to conductive grounded metalwork, provide very effective EMI attenuation.

For all types of products, medical versions can omit the Y capacitor to reduce the leakage current to a maximum of 5μA. This necessarily means that the common mode attenuation is reduced and may need to be compensated elsewhere, such as by cascading filters. The rated current can be easily calculated from the load power requirement given the minimum input voltage and load power factor. For example, a load of 200W on the filter at 90VAC and a power factor of 0.9 will produce a current of 200W/(0.9x90VAC)=2.47A, in which case a 3A rated filter can be selected.

The best way to select the required attenuation from a filter is to measure the performance without the filter installed and then calculate the additional value required from the external filter. The attenuation curves in the filter datasheet give an indication of performance, but remember that the datasheet performance is under specified test conditions, usually 50 ohm source and load impedance. Although AC-DC power supplies can be standardized through the use of line impedance stabilizer networks (LISN), the application load can vary greatly. In AC-DC power supplies, filter modules cascaded with internal filters can also lead to unexpected results, creating potential resonances and even causing EMI amplification at critical frequencies. For example, the EMI graph taken from a typical AC-DC converter from XPPower, part PBR500PS12B, running at 230VAC and 180W is shown in Figure 3. The graph shows compliance with the EN55032 curve B quasi-peak detection emission limit line. The filter was then plugged into the XPPower FCSS06SFR model AC line and its attenuation characteristics are shown in Figure 4. The dashed line is the differential mode and the solid line is the common mode attenuation. The overall result of the results is shown in Figure 5.

Figure 3: AC-DC power supply, internal filter only

Figure 4: XPFCSS06SFR modular filter

Figure 5: AC-DC power supply with external filter added

It can be seen that up to about 1MHz the filter attenuation reduces emissions by the expected amount, but at 10MHz and above the improvement is not consistent, meaning that the modular filter is not “seeing” the 50 ohms as a termination at these frequencies and is giving less attenuation than expected. This confirms the need to make actual measurements to confirm compliance.