Optimizing EMC and Efficiency in High Power DC/DC Converters Part 2

Optimizing EMC and Efficiency in High Power DC/DC Converters Part 2

In Part 1, we explained how choosing the right capacitor type, power inductor, switching frequency, and semiconductors is critical to the efficiency of a DC/DC switching controller, and presented an example of the task of developing a buck-boost converter to specified specifications. We also explored how to choose the best capacitors and inductors to create a filter that matches the converter, achieving very low inductance and a compact layout. In Part 2, we will cover board layout and EMC considerations, select input and output filter components, and use thermal imaging to verify a functional circuit.

Layout Guide

There are several things to consider when laying out the board. For example, the input and output loops, which result in high ΔI/Δt values, should be kept compact by placing filtering ceramic capacitors close together. The bootstrap circuit should be compact and close to the switching regulator IC. A wideband π-type filter is required to decouple the internal power supply of the switching regulator. Use as many vias as possible to achieve low-inductance, low-impedance connections between the internal power GND layer and the bottom layer of the board. Although a large copper area can achieve better heat dissipation and lower RDC, the copper area should not be too large to avoid capacitive and inductive coupling with adjacent circuits.

EMC measurement without filter (100W output power).

For most applications, the converter should comply with Class B (domestic) limits for interference emissions in both the conducted (150kHz to 30MHz) and radiated (30MHz to 1GHz) ranges . Besides insertion loss, it is important for high current applications to have inductive components with the lowest possible RDC to keep efficiency and heat generation within acceptable limits. Unfortunately, low RDC also means larger size. Therefore, it is important to choose state-of-the-art components that balance RDC, impedance and size. The WE-MPSB series and the compact WE-XHMI series are suitable for this. For capacitive filter components above 10µF , low-cost aluminum electrolytic capacitors can be used. Since the filter inductor effectively suppresses current changes, there is no need to worry about high ripple currents. Therefore, a larger ESR is not a problem, which leads to a lower filter quality factor, thus preventing unwanted resonances. The additional losses caused by the filter are due to the ohmic losses of the inductor.

Selecting Input and Output Filter Components

The most important filter component selection criteria is the ability to achieve broadband interference suppression from 150kHz to 300MHz to suppress conducted and radiated EMC. If shorter or no cables are used at the input or output, the degree of filtering can be reduced. Figure 6 shows the effective frequency range of various filter components.

Figure 6: Block diagram of filter components with three different frequency ranges.

Figure 7: Top view of the PCB including all filter components, compliant with CISPR32 Class B

Temperature and efficiency of the circuit with filter at 100W output power (Ta = 22°C)

The maximum component temperature measured with a thermal imager was below 64°C (Figure 8), which means there is enough safety margin to cope with higher ambient temperatures, while also placing less stress on the components. The efficiency is also at a very high level (buck mode: 96.5%; boost mode: 95.6%), especially considering that all filter components are accounted for.

Figure 8: Temperatures measured at the top and bottom.

Figure 9: Measured radiated interference emissions of a circuit with filters at input and output. A sufficient distance to the limit values ​​(horizontally and vertically) can be maintained over the entire measuring range.

Figure 10: Measured conducted emissions with filter at the input. Both the average and quasi-peak values ​​are below the limits over the entire measurement range.

Figures 9 and 10 show the improvement in the circuit measurement results after installing the filter. With the filter, the peak value of the conducted interference radiation in the low-frequency range and the complete measurement curve of the radiated interference emission are both below the required limit with sufficient margin.

Summarize

Even with very careful layout and selection of the right active and passive components, a high-power DC/DC converter that complies with the Class B standard cannot be realized without filters if there are additional very stringent specifications (e.g. long cables, no shielding, etc.) . However, we can anticipate and arrange the right filters in advance. The result is a flexible, efficient, Class B-compliant 100W buck-boost converter. To make the printed circuit board more compact, the two filters can be rotated 90° or arranged on the opposite side of the board. Design and simulation software such as REDEXPERT and LTSpice can help to achieve the desired design goals quickly and cost-effectively.

author:

Andreas Nadler, FAE, Würth Elektronik, appnotes@we-online.de, Tel: +49 7942 945 - 0