“Even with very careful layout and proper selection of active and passive components, if there are additional very strict specification requirements (such as long cables, not shielding, etc.), then Class B compliance cannot be achieved without filters of high-power DC/DC converters. However, we can anticipate and arrange suitable filters in advance.
In Part 1, we explained how choosing the right capacitor type, power Inductor, switching frequency, and semiconductor are critical to the efficiency of a DC/DC switching controller, and demonstrated the development of a buck-boost converter to specified specifications example of a task. We also explore how to choose the best capacitors and inductors to create a filter matched to the converter, allowing for very low inductance and a compact layout. In Part 2, we’ll cover board layout and EMC considerations, select input and output filter components, and use thermal imaging to verify functional circuits.
There are a few factors to consider when laying out the board. For example, input and output loops that result in high ΔI/Δt values should be kept compact by placing filter ceramic capacitors close together. The bootstrap circuit should be compact and close to the switching regulator IC. A wideband pi filter is required to decouple the switching regulator’s internal power supply. Use as many vias as possible for a low-inductance, low-impedance connection between the internal power GND plane and the bottom layer of the board. While a large area of copper can achieve better thermal performance and lower RDC, the copper area should not be too large to avoid capacitive and inductive coupling with adjacent circuits.
Filterless EMC measurements (100W output power).
For most applications, the converter should meet Class B (household) limits for both conducted (150kHz to 30MHz) and radiated (30MHz to 1GHz) interference emissions. In addition to insertion loss, it is also important that high current applications require the inductive components to have as low an RDC as possible to keep efficiency and heat generation within acceptable limits. Unfortunately, low RDC also means larger size. Therefore, it is especially important to select state-of-the-art components that balance the factors of RDC, impedance, and size. Both the WE-MPSB series and the compact design WE-XHMI series are suitable for this situation. For capacitive filter components above 10µF, low-cost aluminum electrolytic capacitors can be used. There is no need to worry about high ripple current because the filter inductor effectively suppresses current variation. Therefore, a larger ESR is not a concern, it will result in a lower quality factor of the filter, preventing unwanted resonances. The additional losses caused by the filter are due to the ohmic losses of the inductor.
Select input and output filter components
The most important point in the filter component selection criteria is the ability to achieve broadband interference suppression from 150kHz to 300MHz, thereby suppressing conducted and radiated EMC. Filtering can be reduced if shorter cables or no cables are used for the input or output. Figure 6 shows the effective frequency range of each filter element.
Figure 6: Block diagram of filter elements, each with 3 different frequency ranges.
Figure 7: PCB top view including all filter components, CISPR32 Class B compliant
Measured temperature and efficiency of circuit with filter at 100W output power (Ta = 22°C)
The maximum temperature of the components measured by the thermal imaging camera is lower than 64°C (Figure 8), which means that there is enough safety margin to deal with the higher ambient temperature, and the stress on the components is also less. Efficiency is also at a very high level (buck mode: 96.5%; boost mode: 95.6%), especially considering all filter components are accounted for.
Figure 8: Temperatures measured at the top and bottom.
Figure 9: Radiated interference emissions measured for a circuit with filters on the input and output. Sufficient distance from limit values (horizontal and vertical) can be maintained over the entire measuring range.
Figure 10: Measured conducted emissions with filter at the input. Average and quasi-peak values are below the limits over the entire measurement range.
Figures 9 and 10 show the improvement in circuit measurements with the filter installed. With the filter, the peak value of the conducted interference emission in the low frequency range and the complete measurement curve of the radiated interference emission are below the required limits with sufficient margin.
Even with very careful layout and proper selection of active and passive components, if there are additional very strict specification requirements (such as long cables, not shielding, etc.), then Class B compliance cannot be achieved without filters of high-power DC/DC converters. However, we can anticipate and arrange suitable 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 placed on opposite sides of the board. Design and simulation software such as REDEXPERT and LTSpice help achieve desired design goals quickly and cost-effectively.
Andreas Nadler, FAE, Field Application Engineer Würth Electronics,[email protected]