Several Elements to Understand When Choosing a Low EMI Power Supply

Due to the market demand for improving the performance of information technology and communication equipment, today's system designers are facing the great challenge of designing EMI compatible products. Before sale, all information technology equipment (ITE) usually specified as having a regulated clock signal higher than 9KHz must meet relevant government standards, such as fccpart15subpartb of the United States and en55022 of the European Union, which specify the maximum allowable radiation emission in industrial and commercial environment (classA) and home environment (ClassB). In view of such strict EMI standards, engineering human resources restrictions and rapid product launch requirements, the popularity of power modules certified by en55022 standard has been improved. However, it is important to know the electrical operating conditions of the power module at the time of certification, so as to avoid surprise in the subsequent design process. An understanding of EMI interference sources and field strength factors in switching mode regulators will help design engineers select the best components to reduce electromagnetic radiation, especially in cutting-edge equipment with high required current level.

Figure 1: FCC radiation limit (USA) and en55022 ClassB radiation limit (EU)

Figure 2: step down switching regulator with parasitic inductors and capacitors

Due to its special properties, switching power supply will produce electromagnetic waves radiated into the surrounding atmosphere. Pulse voltage and current will appear due to switching action and directly affect the intensity of radiated electromagnetic wave. In addition, parasitic devices inside the converter will also produce electromagnetic radiation. Figure 2 shows a typical step-down converter, which includes parasitic inductors and parasitic capacitors of power MOSFET.

EMI radiation source

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 switching node (VSW), which can be up to twice the input voltage, as shown in Figure 3. When the current capacity of MOSFET increases, the energy stored in parasitic capacitor often increases. In addition, the switching action also makes the input current and the current flowing through the top MOSFET (iTOP) and the bottom MOSFET (IBOT) pulsation. This pulse current will generate radio waves on the input power cable and PCB printed line (which acts as a transmitting antenna), resulting in radiation emission and conduction emission.

Figure 3: typical switching node voltage spikes and ringing in a 12V input step-down switching regulator

When the input voltage and output current increase, the voltage spike on the switching node will also increase when the polarity of the power inductor changes in each cycle. Moreover, the higher the output current, the greater the pulse current generated in the circuit loop. Therefore, the radiation emission largely depends on the electrical operating conditions of the tested device. Generally speaking, the radiated noise will increase with the increase of input voltage and output power (especially output current). Due to the low efficiency of linear regulator as a low-noise alternative and excessive heat dissipation at high voltage and high power level, design engineers have to overcome the problems caused by adopting the most advanced switching power supply solutions, in which EMI suppression becomes very difficult.

Figure 4: demonstration of en55022 standard compliance of ltm4613 (dc1743) under 96w output power (implemented by an independent test organization)

EMI suppression

Alternative methods for reducing radiated EMI from switching mode power converter design face other challenges. A traditional approach is to add EMI shielding around the power solution, which will contain an EMI field in the metal housing. However, EMI shielding increases design complexity, size, and cost. Placing an RC damper circuit on the switch node (VSW) can help reduce voltage spikes and subsequent ringing. However, adding a shock absorber circuit will reduce the working efficiency, increase the power dissipation, and increase the ambient temperature and PCB temperature. The last countermeasure is to adopt an excellent PCB layout scheme, including the use of local low ESR ceramic decoupling capacitors, and adopt a short PCB routing interval for all high current paths to minimize the parasitic effect shown in Figure 2, but at the cost of increasing the engineering design time and delaying the product launch process. In general, in order to meet the requirements of size, efficiency, heat dissipation and EMI specifications at the same time, engineers must have rich power design experience and make difficult trade-offs, especially in high input voltage and high output power applications (for the reasons described above). In order to evaluate the compromise strategy and design a power converter that meets EMI standards and meets all system requirements, circuit designers often need to spend a lot of time and energy. Solutions to ensure compliance with EMI standards

In order to guarantee the design of a simple and EMI compliant high output power supply, linglilte submitted ltm46138a step-down model to an independent certified test laboratory (tuvrheinland) μ Module regulator demonstration board (dc1743). The 10m en55022 test chamber of the laboratory has been officially recognized by the U.S. National Institute of standards and Technology (NIST). When a 24V input provides 96w output power, it is found that the ltm4613 demonstration board complies with the limit of en55022classb. With only input capacitance, output capacitance and a few other small components, a "worry free" solution conforming to en55022 standard can be easily realized, especially when dc1743 photo (Gerber) files can be downloaded free of charge.

Ltm4613 can provide the highest output power and efficiency among all power module products verified to comply with en55022b specification on the market. With a carefully designed integrated filter, detailed internal layout, shielded inductor, internal damper circuit and power transistor driver, the ltm4613 achieves a perfect balance between size, output power, efficiency and EMI radiation. Ltm4613 and en55022b certified μ Many other members of the module regulator family (which have different output power levels) together eliminate the need for additional magnetic shielding and external shock absorbers, thereby reducing the overall size, cost and some troublesome design tasks of the solution.

summary

It is an inevitable requirement to design information technology equipment (ITE) products according to EMI compliance, which requires extraordinary skills and time. Although troublesome, such restrictions are crucial to ensure the normal functional operation of adjacent electronic equipment or other auxiliary components within the system itself. In order to meet this demand, the industry has launched power modules such as ltm4613 certified by en55022classb standard. However, since the EMI field strength largely depends on many factors (such as input voltage, output current, output voltage and PCB layout), when comparing products, it should be determined that en55022 certification is carried out under similar electrical conditions (implemented by the EMI testing laboratory recognized by the industry at the input voltage and output power level similar to your design). Linglilte published EMI test results and photo drawing documents of demonstration board for the most common working conditions, thus providing a good power supply design certified by an authorized independent laboratory, which gives customers a "reassurance".