Optimizing EMC and Efficiency in High Power DC/DC Converters Part 1

Choosing the right capacitor type, power inductor, switching frequency, and semiconductors is critical to the efficiency of a DC/DC switching power supply controller. Making the right choice is not easy, but even if the right choice is made, the controller must be highly efficient and meet EMC requirements to be on the market.

For DC/DC converters with higher input and output power, filters must be used at both the input and output to reduce interference emissions. However, with large input and output currents, it is difficult to strike a balance between parameters such as efficiency, size, filter attenuation and cost, and actual power level. Figure 1 is an example of a 100-watt buck-boost DC/DC design that shows what factors should be considered in terms of layout and component selection.

Figure 1: 100W Buck-Boost Converter Demonstration Board

Task

Develop a buck-boost converter with the following specifications:

Output power 100W when output voltage is 18V, input voltage 14-24V DC, maximum input current 7A, maximum output current 5.55A

The efficiency is greater than 95% when the output power is 100W

Meets CISPR32 Class B emission standards (conducted and radiated)

Low output ripple voltage (less than 20mVpp)

Unable to block

The input and output cables are longer (both are 1 meter long)

As compact as possible

Keep costs as low as possible

The above requirements are quite stringent, and a low parasitic inductance and compact layout must be created, coupled with filters that match the converter. In terms of EMC, the main antennas are the input and output cables, which have a frequency range extending all the way to 1GHz . Depending on the operating mode, both the input and output of the converter have high-frequency current loops (as shown in Figure 2), so both must be filtered. The filter prevents the high-speed switching MOSFET from radiating high-frequency interference through the cable. The application in this example has a wide input voltage range of up to 60V DC, an adjustable switching frequency, and the ability to drive four external MOSFETs, which provides a high degree of design freedom.

Figure 2: Schematic diagram of a switching power supply, where the high-frequency loop is in red and the key switching node is in green, depending on the DC/DC operating mode.

The design uses a six-layer double-sided printed circuit board with a switching frequency of 400kHz. The current ripple on the inductor should be about 30% of the rated current . The 60V MOSFET uses a low on-resistance (RDS(on)) and low thermal resistance (Rth) type. Figure 3 shows a simplified circuit layout.

Figure 3: Simplified schematic diagram of power circuit design

Selecting an Inductor

The REDEXPERT online design platform can help you select the inductor quickly and accurately. In this example, all operating parameters, including input voltage Vin, switching frequency fsw, output current Iout, output voltage Vout, and ripple current IRipple, must be entered for the buck mode of operation first, and then again for the boost mode of operation. The buck mode results in a higher inductance and a smaller maximum peak current (7.52µH, 5.83A). The boost mode results in a smaller inductance but a larger maximum peak current (4.09µH, 7.04A).

The design platform selected the WE-XHMI series 6.8µH, 15A rated current shielded inductor coil. It has a very low RDC and is extremely compact, measuring only 15 mm × 15 mm × 10 mm (length × width × height). The innovative core material enables mild saturation characteristics that are not affected by temperature.

Selecting capacitors

Due to the high pulse current through the blocking capacitor and the low ripple required, a combination of aluminum polymer capacitors and ceramic capacitors is the best choice. By determining the maximum allowable input and output voltage ripple, the required capacitance can be calculated as follows:

(D = Duty Cycle, set to 0.78 in REDEXPERT) 6 × 4.7µF / 50V / X7R = 28.2µF selected (WCAP-CSGP 885012209048)

By using REDEXPERT, the DC bias of the capacitor (MLCC) can be easily determined, resulting in a more realistic capacitance value. A 20% reduction in capacitance is expected at 24V input voltage. This results in an effective capacitance of only 23µF, but still sufficient. A 68µF/35V WCAP-PSLC aluminum polymer capacitor is connected in series with a 0.22Ω SMD resistor and in parallel with the ceramic capacitor. Its purpose is to maintain the stability of the negative input impedance of the voltage converter combined with the input filter. Since this capacitor is also affected by high pulse currents, an aluminum electrolytic capacitor is not very suitable because it heats up quickly due to its higher ESR.

The output capacitor can be selected in the same manner.

Selected 6 × 4.7µF / 50V / X7R = 28.2µF - 15% DC bias = 24µF (WCAP-CSGP 885012209048)

In addition, aluminum polymer capacitors (WCAP-PSLC 220µF/25V) can provide fast enough transient response capability.

Part 2 of this article will cover practical considerations such as board layout, EMC and the important task of selecting input and output filter components, as well as thermal verification of the functional circuit.

author:

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