“Selecting the correct type of capacitor, power Inductor, switching frequency and semiconductor is critical to the efficiency of a DC/DC switching power supply controller. Making the right choice is not an easy task, but even when the right choice is made, the controller must be efficient and EMC-compliant before it can be marketed.
“
Selecting the correct capacitor type, power inductor, switching frequency, and semiconductor is critical to the efficiency of a DC/DC switching power supply controller. Making the right choice is not an easy task, but even when the right choice is made, the controller must be efficient and EMC-compliant before it can be marketed.
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 balance the parameters of efficiency, size, attenuation and cost of the filter, and practical power stage. Figure 1 is an example of a 100-watt buck-boost DC/DC design that shows what should be considered in layout and component selection.
Figure 1: 100W Buck-Boost Converter Demonstration Board
Task
Develop a buck-boost converter with the following specifications:
• When the output voltage is 18V, the output power is 100W, the input voltage is 14-24V DC, the maximum input current is 7A, and the maximum output current is 5.55A
• Efficiency greater than 95% at 100W output power
• Complies with CISPR32 Class B emissions (conducted and radiated)
• Low output ripple voltage (less than 20mVpp)
• Can’t block
• Longer cables for input and output (both 1m 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 with a filter matched to the converter. In terms of EMC, the main active antennas are the input and output cables, and their frequency range extends all the way to 1GHz. Depending on the mode of operation, both the input and output of the converter have high-frequency current loops (as shown in Figure 2), so both must be filtered. Filters prevent high-frequency interference from high-speed switching MOSFETs radiating through the cable. The application in this example has a wide input voltage range up to 60V DC, adjustable switching frequency, and the ability to drive four external MOSFETs for a high degree of design freedom.
Figure 2: Schematic diagram of a switching power supply, where the red box is the high-frequency loop, and the green box is the critical switching node, depending on the DC/DC mode of operation.
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 MOSFETs are available in low on-resistance (RDS(on)) and low thermal resistance (Rth) versions. Figure 3 shows a simplified circuit layout.
Figure 3: Simplified power circuit design schematic
Choosing an Inductor
The REDEXPERT online design platform can help you select inductors 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 first for the buck operating mode, and then once for the boost operating mode. The result in buck mode is higher inductance and lower maximum peak current (7.52µH, 5.83A). Boost mode results in a smaller inductance but a larger maximum peak current (4.09µH, 7.04A).
The design platform selected the 6.8µH, 15A rated current shielded inductor coil of the WE-XHMI series. It has a very low RDC and an extremely compact size of only 15mm x 15mm x 10mm (L x W x H). Innovative core material enables mild, temperature-independent saturation characteristics.
choose capacitor
Due to the high pulse current through the DC blocking capacitor and the required low ripple, 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 to obtain a more realistic capacitance value. Expect a 20% reduction in capacitance at 24V input voltage. That’s only 23µF of effective capacitance, but it’s still enough. Connect a 68µF/35V WCAP-PSLC aluminum polymer capacitor in series with a 0.22Ω SMD resistor and then in parallel with the ceramic capacitor. Its purpose is to maintain the stability of the negative input impedance when the voltage converter is combined with the input filter. Since this capacitor is also subject to high pulse currents, an aluminum electrolytic is less suitable as it heats up rapidly due to the higher ESR.
The output capacitor can also be selected in the same way.
6 × 4.7µF / 50V / X7R = 28.2µF – 15% DC bias = 24µF selected (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 the important task of board layout, EMC and selection of input and output filter components, and practical considerations such as thermal verification of a functional circuit.
author:
Andreas Nadler, Field Application Engineer Würth Electronica FAE, [email protected]
The Links: PD104SL5 LM057QC1T01