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µModule regulator family with various output power levels eliminate the need for magnetic shields and external snub- bers reducing overall solution size, cost, and design effort. Designing an information technol- ogy equipment (ITE) product for EMI compliance is a nec- essary requirement, which requires great skill and time.

PCB layout including local low ESR ceramic decoupling cap a citors with short PCB trace distances for all high- current paths, to minimise the parasitics, shown in Figure 2, at the cost of additional engineering design time and a longer time to market. Overall, a lot of power design expe- rience and tough tradeoffs are required of an engineer to meet size, efficiency, heat and EMI specifications together, especially in high-input voltage, high- output power applications for the reasons explained.

Often a huge amount of time and energy is required for a circuit designer to evaluate the tradeoffs and design an EMI compliant power converter, which meets all the needs of the system.

EMI compliance To guarantee a simple EMI compliant high-output power supply design, Linear Tech nology developed the LTM4613 8A step-down µModule regulator demo board (DC1743) and presented it to an independent certi- fied test laboratory, TUV Rheinland whose 10 meter EN55022 test chamber is accredited by the U.S. National Institute of Standards and Technology (NIST).

Under a 24V input delivering 96W of output power, the demo board was found to be compliant with EN55022 Class B limits. With only input capaci- tance, output capacitance, and a few other small components required, a worry-free EN55022B compliant solu- tion is easy to achieve particularly with Gerber files freely available online. The LTM4613 delivers the highest output power and efficiency among all proven EN55022B compliant power module products on the market. With a carefully designed integrated filter, meticulous internal layout, shielded inductor, internal snubber circuitry and power transistor driver, the power module has already achieved a perfect balance between size, output power, efficiency and EMI emissions. This device along with many other members of the EN55022B certified

Electronics MARCH 2012

While cumbersome, such limits are critical to guaranteeing proper func- tionality of neighbouring electronic equipment or other sub components within the system itself. To address this need, EN55022 Class B certified power modules such as the LTM4613 have been introduced. However, since EMI field strength is highly dependent on multiple factors such as the input voltage, output current, output voltage and PCB layout, when comparing products be certain the EN55022 certification was performed under similar electrical conditions at an input voltage and output power-level similar to your design within an industry approved EMI test facility.

Figure 3 (Top left): Typical Switch Node Voltage Spike & Ringing in a 12V Input Buck Switching Regulator

EMI Radiation & Electromagnetic Theory

“Often a huge amount of time and energy is

required for a circuit designer to evaluate the tradeoffs and design an EMI compliant power converter”

A reflection on electromagnetic theory will give a better understanding of the EMI implications of high-power step-down converters operating at ever-higher voltages and higher output power. Recall Gauss' Law, which states that the strength of an electric field in a given area over an enclosed volume is proportional to the total charge inside it.

That charge is proportional to the amount of current passing through a PCB trace, for example. (One ampere equals 1 coulomb of charge per second.) Therefore higher currents required for increased output power create stronger electric fields, which are of course constantly changing in a switch mode regulator as alternating current paths are active at different times within a cycle.

Publication of EMI test result Linear Technology has made public the EMI test results for the most popular operating conditions and demo board Gerber files giving customers a known good power supply design certified by an authorised independent laboratory for your peace of mind. Linear Technology Enter 208

Figure 4:

Demonstrated EN55022 Compliance LTM4613 (DC1743) at 96W as conducted by Independent Tests

Maxwell's equations then tell us this constantly varying electric field creates a proportionally stronger magnetic field resulting in a self sustaining EMI wave radiating from the conductor, all the current paths within step-down regulator. The discussion presented here is by no means complete, for example the implica- tions of the magnetic component surrounding the current paths and the rapid voltage polarity changes on the power inductor have yet to be discussed however, the impact of higher output power on radiated EMI is particularly clear.


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