What to know before choosing a low EMI power supply that complies with EN55022

introduction

As the market demands for improved performance of information technology and communication equipment, today's system designers face the challenge of having to design  EMI  compliant products. Before being sold, all information technology equipment (ITE), which is usually specified as having a modulated clock signal above 9kHz, must meet relevant government standards such as FCC Part 15 Subpart B in the United States and EN55022 in the European Union, which specify the maximum allowable radiated emissions for industrial and commercial environments (Class A) and domestic environments (Class B). Given such stringent  EMI  standards, engineering manpower constraints and the need for quick time to market, the popularity of power modules certified to  the EN55022  standard has increased. However, it is important to know the electrical operating conditions under which the power module was certified to avoid surprises later in the design process. An understanding of EMI interference sources and field strength factors in switch-mode regulators will help design engineers select the best components to mitigate electromagnetic emissions, especially in cutting-edge equipment that requires higher current levels.

Figure 1: FCC emission limits (USA) and EN55022 Class B emission limits (EU)

Thinking about electromagnetic theory will help readers better understand the impact of EMI on high-power buck converters operating at higher voltages and higher output powers. Let us recall Gauss' Law, which states that the electric field strength in a closed area is proportional to the total charge inside it.

For example, the amount of charge is proportional to the amount of current passing through a PCB trace. (1A equals 1 coulomb of charge per second.) The higher current required to increase the output power will therefore produce a stronger magnetic field, and it is natural that the electric field is always changing in a switching mode regulator because the AC current path operates at different points within a cycle. Maxwell's equations then tell us that this changing electric field will produce a proportionally stronger magnetic field, causing a self-sustaining EMI wave to radiate from the conductors (all current paths within the buck converter). This discussion is not exhaustive; for example, the effects of magnetic components surrounding the current path and rapid polarity changes on the power inductor have not been addressed, but the effect of higher output power on radiated  EMI  is clearly visible. Sources of EMI Radiation

Due to their special properties, switching power supplies generate electromagnetic waves that are radiated into the surrounding atmosphere. Pulsed voltages and currents will appear due to the switching action and directly affect the intensity of the radiated electromagnetic waves (see sidebar). In addition, parasitic devices inside the converter will also generate electromagnetic radiation. Figure 2 shows a typical buck converter, which includes parasitic inductors and parasitic capacitors of the power MOSFET.

Figure 2: Buck switching regulator with parasitic inductor and capacitor

In each switching cycle, the energy stored in the parasitic inductor will resonate with the energy stored in the parasitic capacitor. When the energy is released, a large voltage spike will be generated on the switch node (VSW), which can be up to twice the input voltage, as shown in Figure 3. When the current capability of the MOSFET increases, the energy stored in the parasitic capacitor tends to increase. In addition, the switching action also pulsates the input current and the current flowing through the top MOSFET (ITOP) and the bottom MOSFET (IBOT). This pulse current will generate radio waves on the input power cable and PCB board traces (which act as a transmitting antenna), resulting in radiated and conducted emissions.

As input voltage and output current increase, the voltage spike on the switch node when the power inductor changes polarity in each cycle will also increase. Moreover, the higher the output current, the larger the pulse current generated inside the circuit loop. Therefore, radiated emissions are highly dependent on the electrical operating conditions of the device under test. In general, radiated noise will increase with increasing input voltage and output power (especially output current). Because linear regulators as a low-noise alternative are too inefficient and dissipate too much heat at high voltage and high power levels, design engineers have to overcome the challenges caused by adopting the most advanced switching power supply solutions, in which  EMI  suppression becomes quite tricky.

Figure 3: Typical switch node voltage spike and ringing in a 12V input buck switching regulator

EMI Suppression

Alternative approaches for reducing radiated EMI  from switch-mode power converter designs  face additional challenges. One traditional approach is to add an EMI shield around the power solution, which will contain an  EMI  field within the metal enclosure. However, EMI shielding increases design complexity, size, and cost. Placing an RC snubber circuit on the switch node (VSW) can help reduce voltage spikes and subsequent ringing. However, adding a snubber circuit will reduce operating efficiency, thereby increasing power dissipation, resulting in higher ambient and PCB temperatures. A final countermeasure is to adopt good PCB layout schemes, including the use of local low-ESR ceramic decoupling capacitors and short PCB trace spacing for all high-current paths to minimize the parasitic effects shown in Figure 2, but at the cost of increased engineering design time and delayed time to market.

In summary, engineers must have extensive power design experience and make difficult trade-offs to simultaneously meet size, efficiency, heat dissipation, and  EMI  specifications, especially in high input voltage, high output power applications (for the reasons mentioned above). Circuit designers often need to spend a lot of time and effort to evaluate the trade-offs and design a power converter that meets EMI standards and meets all system requirements. Solutions to ensure EMI compliance

To provide assurance of a simple and  EMI  compliant high output power supply design, Linear Technology submitted the LTM4613 8A step-down µModule® regulator demonstration board (DC1743) to an independent certified test laboratory (TUV Rheinland), whose 10-meter EN55022 test chamber is officially recognized by the US National Institute of Standards and Technology (NIST). The LTM4613 demo board was found to meet the EN55022 Class B limits at 96W output power from a 24V input. With only input capacitors, output capacitors and a few other small components, a "worry-free" solution that complies with the EN55022 standard can be easily implemented, especially when using the DC1743 Gerber files that can be downloaded free of charge.

Figure 4: Demonstration of EN55022 compliance of the LTM4613 (DC1743) at 96W output power (performed by an independent test house)

The LTM4613 offers the highest output power and efficiency of all power modules on the market that have been verified to meet the EN55022B specification . With a carefully designed integrated filter, meticulous internal layout, shielded inductor, internal snubber circuit and power transistor driver, the LTM4613 achieves a perfect balance between size, output power, efficiency and EMI radiation. The LTM4613, along with many other members of the EN55022B certified µModule regulator family (with various output power levels), eliminates the need for additional magnetic shielding and external snubbers, reducing overall solution size, cost and some cumbersome design tasks.

Summarize

Designing Information Technology Equipment (ITE) products for  EMI  compliance is a requirement that requires extraordinary skill and time. Although cumbersome, such restrictions are critical to ensuring the proper functioning of adjacent electronic equipment or other dependent components within the system itself. To meet this need, the industry has introduced power modules such as the LTM4613 that are certified to the  EN55022  Class B standard. However, since EMI field strength depends heavily on many factors (such as input voltage, output current, output voltage and PCB layout), when comparing products, be sure that the EN55022 certification is performed under similar electrical conditions (performed by an industry-recognized  EMI  test laboratory at input voltage and output power levels similar to your design). Linear Technology publishes EMI test results and demo board Gerber files for the most common operating conditions, providing customers with the peace of mind of a good power supply design certified by an authorized independent laboratory.