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What you should know before selecting a low EMI EN55022-compliant power supply


Richard Ying Design Engineer & Willie Chan Senior Product Marketing Engineer at Linear Technology Corporation discuss key specifications to consider when designing power modules into an application, to ensure the design and end device remain fully compliant with government standards


A


s a result of market demand for increased performance in infor- mation technology and commu- nications equipment, today's system designers face a big challenge of designing EMI-compliant products. Prior to sale, all information tech -


nology equipment (ITE) commonly defined as having a regulated clock signal greater than 9kHz must meet government standards such as FCC Part 15 Subpart B in the United States and EN55022 in the European Union, which define maximum allowable radiated emission for industrial and commercial environments (Class A) and home environments (Class B). Such strict EMI standards, engineer- ing manpower constraints and fast time to market demands have increased the popularity of EN55022- certified power modules. However, it is important to be aware of the electri- cal operating conditions under which the power module was certified to avoid surprises later in the design. An understanding of EMI sources and field strength factors in switch mode regulators will help the design engineer select the best components to mitigate electromagnetic emissions particularly in leading-edge equipment, which require ever-higher current levels.


EMI radiation sources


By their very nature, switching power supplies generate electromagnetic waves that radiate into the surrounding atmosphere. The pulsating voltages and currents associated with the switching action generate and directly influence the strength of radiated


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MOSFET increases the energy stored in the parasitic capacitor tends to increase as well. Switching action also pulses the input current and the current flowing through both top MOSFET (ITOP


) and bottom MOSFET (IBOT ).


This pulsating current generates electrical waves on the input supply cable and on the PCB board traces, which act as a transmitting antenna generating both radiated and conducted emissions.


electromagnetic waves (see side bar). Furthermore, parasitic devices within the converter also contribute to electromagnetic radiation. Figure 2 presents a typical step-down converter including parasitic inductors and parasitic capacitors of the power MOSFETs used.


Figure 1: FCC Radiated Limits (USA) & EN55022 Class B Radiated Limit (European Union)


During 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 is created at the switch node (VSW


),


which can be as large as twice of the input voltage, as shown in Figure 3. As the current capability of the


Figure 2:


Buck Switching Regulator with Parasitic Inductor & Capacitors


As input voltage and output current increase, so will the magnitude of the voltage spike at the switching node as the power inductor changes polarity during each cycle. Moreover, the higher the output current, the larger the pulsat- ing current generated inside the circuit loop. So radiated emission is highly dependent on the electrical operating conditions of the device under test. In general, radiated noise increases with higher input voltage and higher output power, particularly output current. Design engineers are forced to overcome these challenges. As the low noise alternative, linear regulators are too inefficient and dissipate too much heat at the high voltages and high- power levels enabled by state of the art switching power solutions where EMI mitigation becomes challenging. Alternative methods to reduce the radiated EMI from a switch mode power converter design face other challenges. One conventional method is to add an EMI shield surrounding the power solution, which will contain the EMI field within the metal enclo- sure. However, an EMI shield adds design complexity, size and cost. An RC snubber circuit at the switch- ing node (VSW


) could help to reduce


the voltage spike and subsequent ring- ing. However, adding an RC snubber circuit will reduce operating efficiency thereby increasing power dissipation, which results in higher ambient and PCB temperatures.


The last strategy involves a good MARCH 2012 Electronics


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