For AC power-powered equipment, the usual practice is to use modular AC line filters, which are integrated into connectors or installed as chassis mounting parts, especially in professional environments such as industry, healthcare, and ITE. The equipment usually includes an embedded AC-DC converter or power supply, and may also be installed on a chassis, or sometimes on a rack or PCB. In each case, the power supply as an independent component will always meet the statutory requirements for emission, usually in accordance with EN55011/EN55032 for conducted and radiated interference. But additional filtering may still be necessary.
Experienced equipment designers have long known that simply using compliant components does not guarantee the EMC compliance of the final product "passes". The reasons are many and varied. For example, the conformance testing of equipment AC-DC converters is carried out under very specific conditions, namely assumed AC line impedance, output load, cable length and wiring, and the position of the components relative to the ground. When using this internally mounted AC-DC converter to test the final product, all of these conditions will change, resulting in different and often worse conducted EMI characteristics. Radiated EMI from other components can also be picked up on the power line, thereby increasing the level of conduction.
Modular filters can achieve system EMI compliance
An external modular filter may be the solution, but there are hundreds to choose from, which one is the best? We first look at the internal circuit of a typical commercial filter and consider the role of each component (Figure 1).
Figure 1 This typical modular EMI filter uses CX capacitors to attenuate differential mode noise, and uses an inductor-capacitor combination to reduce common mode noise.
The capacitor CX attenuates differential mode noise, signals, and spikes that appear from the line to the neutral point due to the rapid changes in the current in the converter. Capacitors will be rated X1, X2 or X3 because they can withstand voltage transients on the AC line. Inductor L is a common mode or current compensation choke with two phase windings as shown in the figure. Common-mode noise is generated by the rapid voltage changes in the converter, from the line and neutral to the ground, the choke is regarded as high impedance, and each CY capacitor transfers the noise current to the ground. The normal operating current through the two windings on the choke will cause the magnetic field in the core to cancel, so high inductance values can be used without worrying about magnetic saturation. Usually, the coupling between the windings of L is not perfect, so some leakage inductance will be generated, which will appear as a separate series inductance, which will increase the differential mode attenuation.
Although CX can be any capacitance value within practical limits, the two CY values are limited by ground leakage current requirements. They are available in Y1, Y2, Y3 and Y4 types with reduced rated operating voltage and transient voltage. Leakage current through Y capacitors is a potential problem because they bridge the safety barrier-line and neutral to ground. If the protective ground connection to the equipment metal product fails, the case will "float" to the line voltage through the Y capacitor, and may cause electric shock. Therefore, these Y-capacitor values are limited to allow no more than a specified current to flow through the enclosure, and this value is set by the standard for the specific application environment. Limits can range from tens of milliamps in industrial systems to less than 10 µA in cardiac floating healthcare applications.
Resistor R1 is a high-resistance resistor, usually 1M ohm. If the AC power supply is suddenly disconnected and the load cannot be used to discharge the charge, it is used to discharge CX, leaving a potential hazard on the AC connector pins Voltage. IEC 62368-1 and other standards for the safety of ITE and media equipment stipulate that when CX> 300nF (nafara), R1 should discharge the capacitor to less than 60V after two seconds, and when CX <300nF, a higher value is allowed. Voltage. Similarly, for equipment that can only be used by trained personnel, the allowable voltage limit is higher.
But other standards are different. For example, IEC 60601-1 for medical equipment requires discharge to less than 60V after one second, but if CX is less than 100nF, there is no requirement. Standards such as IEC 62368-1 also require resistors. If the resistor is installed before the fuse, the resistor must be able to withstand the transient voltage with a resistance deviation of no more than 10%. Therefore, the resistor R1 will be a high specification part. In some applications, the power consumed by R1 under normal conditions will limit its chances of meeting the standby or no-load loss limits set by agencies such as the U.S. Department of Energy (DoE) and the European ErP Directive.
The fuses shown in Figure 1 can be included in modular filters, especially panel mount types, such as the popular IEC320-C14 type (Figure 2).
Figure 2 Fuse panel mounted EMI filter (eg IEC320-C14) is a popular modular filter option.
In commercial applications, a single fuse in the circuit is normal. If the fuse element meets the standard, the specifications of downstream components (such as the aforementioned R1) can be simplified. Certain applications, such as medical equipment and Class II IT, require both the circuit and the neutral wire to be fused to prevent the possibility of accidental connection reversal. In the case of a single fuse, the reverse connection will make the live wire not blown, and rely on the upstream fuse or the circuit breaker in the power disconnection when a short circuit from the live wire to the protective ground occurs. However, the rated current of these upstream devices may be very high to protect the wiring of multiple loads, and it cannot be guaranteed to be quickly disconnected when the device fails, which may cause a fire hazard. However, the double fuse does have a disadvantage, that is, the overcurrent connected to the neutral line may break the neutral fuse, making the device appear to be dead, but there is still a live connection inside.
The mechanical format of the filter is the natural starting point of the selection process. Depending on the application requirements, the mechanical variant can be used as an IEC inlet, with screw or snap-on installation, with a choice of switch and no fuse, one or two fuses. The IEC inlet type has a C14 rating of 10A, C20 has a rating of 16A, and the chassis-mounted components have a rated current of 20A or higher. Chassis-mounted filters usually have 6-sided shielding and are directly fixed to conductive grounded metal products, which can provide very effective EMI attenuation.
For all types, a medical version is available, which omits the Y capacitor to reduce the leakage current to the usual maximum of 5 µA. This omission necessarily means that common mode attenuation is reduced, which may need to be compensated elsewhere, such as through cascaded filters.
Given the minimum input voltage and load power factor, the rated current demand of the filter can be easily calculated according to the load power requirements. For example, at 90 VAC, the load on a filter with a power factor of 0.9 is 200 W, which will consume 200 W/ (0.9 x 90 VAC) = 2.47A. In this case, a 3A class filter can be selected.
Choosing the required attenuation of the filter is best done by measuring the performance of the system without the filter installed, and then calculating how much is needed from the external filter to meet the specifications. The attenuation curve in the filter data sheet will give an indication of the filter performance, but remember that the data sheet performance is under the specified test conditions, usually 50 ohm source and load impedance. Although it is possible to use the Line Impedance Stabilizer Network (LISN) to standardize AC power, the applied load may be very different from the test conditions of the data sheet.
The filter module cascaded with the internal filter in the AC-DC power supply can also cause unexpected results. Potential resonance can even lead to EMI amplification at critical frequencies. For example, the EMI diagram is taken from XP Power's typical AC-DC converter, part PBR500PS12B, which runs at 230 VAC and 180 W, as shown in Figure 3. This graph fits well with EN 55032 curve B emission limit line quasi-peak detection. Then insert a filter in the AC line XP Power FCSS06SFR, and the resulting attenuation characteristics are shown in Figure 4. The dashed line is the differential mode attenuation, and the solid line is the common mode attenuation. The overall combination result is shown in Figure 5.
Figure 3 EMI curve of AC-DC power supply with internal filter shows good compliance with emission limits.
Figure 4 EMI diagram of XP FCSS06SFR modular filter shows differential mode and common mode attenuation.
Figure 5 The total attenuation of the AC-DC power supply of Figure 3 above 10 MHz with the external filter of Figure 4 added is smaller than expected, which indicates the need for confirmation measurements.
It can be seen that until about 1 MHz, the filter attenuation reduces the expected emission, but at 10 MHz and above, the improvement does not meet expectations, which shows that the modular filter does not "see" the 50 ohm termination at these frequencies. Its attenuation is lower than expected. This result confirms the need for actual measurement to confirm compliance.
Obtaining correct EMC compliance at the earliest stage is essential to avoid costly failures in the final product test. However, the solution is not simply to use a large modular filter at the AC inlet, which will increase unnecessary costs and even backfire due to unexpected attenuation results. Instead, designers should perform tests and make measurements to determine the actual filter requirements for their application. Power supply manufacturers such as XP Power can help by providing a series of AC-DC power products with modular filters, the versions of which are designed for ITE, industrial, and low-leakage medical applications. Some even provide customers with comprehensive design application support and free use of their internal EMC pre-compliance testing facilities.
Gary Bocock is the technical director of XP Power.
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