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Back to basics: Meeting EMI for AC-DC systems

EMI is seen as one of the key challenges of power conversion design and system integration, particularly for AC-DC systems. Our latest technical tip by David Fletcher, Principal Engineer from Vicor’s Westcor Division, looks at the causes of EMI and its impact on a system, before going on to make some practical suggestions on how to minimize its effect.

Causes of noise

We have to start with what engineers affectionately call noise. This is generated whenever rapid voltage (dv/dt) and/or current (di/dt) transitions take place. AC-DC power converters typically utilize a number of power switching topologies, including fully resonant, quasi-resonant and PWM (Pulse Width Modulation), the most typical control in the front-end section of the converter.

PWM-controlled converters use a rectangular control signal with a continuously varying pulse width in response to the operating conditions of the converter. The result is typically a “white noise” energy distribution spectrum. If this was not filtered and shielded it would interfere with consumer electronic equipment using the same AC mains.

“Noise” currents exiting the power converter through the AC power lines and/or via output power cables are known as “conducted emissions.” The noise manifests itself in two forms: differential and common-mode. The definition of differential-mode is noise that is only on the power lines and is not present on the earth ground lead, and can be measured with respect to the power lines. The definition of common-mode is noise that can only be measured from earth ground to one of the power lines.

AC-DC power converters employ EMI filters within the power converter enclosure. These filters contain noise suppression topologies containing inductive and capacitive components. These components are called “X” and “Y” elements. The “X” components are placed across the power lines and filter the differential-mode noise; the “Y” components are placed between the power lines and earth ground and filter common-mode noise.

How to minimize EMI filtering

AC-DC power converters are designed to meet various regulatory and safety standards, including various EMI standards. The individual details and the standards met are generally covered thoroughly in the product literature. However, during a system integration of various components, including an AC-DC power converter, a system design engineer may find it necessary to add additional EMI filtering to enable the integrated system to comply with all relevant agency standards. If this approach becomes necessary, it is recommended to minimize the additional filter components required by empirically gathering test information. It is recommended that a few areas be given special attention:

1. Test the system without any extra filters. This will give the system engineer a good baseline.
2. Identify the problem frequencies on the EMI plots.
3. Try to relate these frequencies to areas of the integrated hardware within the system.
4. Re-check all points of earth grounding and re check cable routing — all these should be in accordance with good engineering practice. These include but are not limited to separating signal cables from power cables, using twisted pair techniques (3 to 5 twists per inch if possible) on cable carrying signals and /or high current and their ground returns. If using this technique is deemed impractical then strip lining the cable as close to one and other is an alternative to minimize noise. Consider shielding cables with fast voltage and/or current transitions. In integrated systems that have several grounding points a careful study of the system integration block diagram is advisable to detect the presence of grounding loops. If these are present they must be eliminated.

What to consider when adding external filtering

If EMI problems still persist, then adding an external filter may be necessary. There are areas to consider when considering an off-the-shelf filter:

1. The filter must be able to handle the full rated worst case current of the system.
2. The filter must provide enough attenuation at the noise frequencies of concern to provide enough margin for system-to-system manufacturing variations.
3. The amount of extra leakage current (touch current) caused by the additional “Y” capacitance inherent in the filter should be calculated. If the leakage current is not stated on the filter data sheet the following formula can be used to calculate the leakage current: 2 x π x (AC-Volts-max) x (AC-frequency-max) x (Y capacitors) = I leakage 20% to 25% should be added to this value before you add this to the overall leakage current. If the total leakage current exceeds the allowable regulatory level you may want to consider a filter that has an internal series inductive component in the ground lead. These filters tend to have no “Y” capacitors and may be referred to as medical filters.
4. The filter should have an internal resistor between the power lines to ensure that upon disconnection of the AC line input the internal “X” capacitors discharge properly – typically within a second, as recommended by VDE 0806 and IEC380.
5. The effect of adding an external filter means additional inductive elements are now in series with the power converter’s AC input. Care should be taken to prevent a phenomenon called “ring-up.” This occurs when the forward current through a series inductor is high enough to saturate the filter inductor, so that when the resultant magnetic field collapses a very high peak may occur, with a voltage amplitude up to twice the original impulse. These peaks may be caused by lightning strikes, external power company grid switching, or heavy inductive transitions due to motor loading in a building. Commercial converters have input protection schemes and comply with regulatory standards. However with the addition of an external filter through “ring-up” it is possible to exceed the protection components and cause damage to the power converter. External protection components, such as MOV (Metal Oxide Varistors) may be needed across the external filter to add robustness to the overall system.
6. The filter should be placed as close as possible to the AC input of the system enclosure. This will maximize the EMI filtering. All ground connections to the filter should be as short as possible to minimize series inductance and impedance.