“Step-down DC/DC converters are widely used in many Electronic systems, such as 5G base stations, factory automation (FA) equipment, and Internet of Things (IoT) equipment, to effectively reduce high voltages. For example, voltages such as 12 VDC or 48 VDC from a battery or power distribution bus often need to be converted to lower voltages to power digital ICs, analog sensors, radio frequency (RF) components and interface devices.
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Step-down DC/DC converters are widely used in many electronic systems, such as 5G base stations, factory automation (FA) equipment, and Internet of Things (IoT) devices, to efficiently reduce high voltages. For example, voltages such as 12 VDC or 48 VDC from a battery or power distribution bus often need to be converted to lower voltages to power digital ICs, analog sensors, radio frequency (RF) components, and interface devices.
While designers can implement discrete buck converters and optimize for a specific design in terms of performance characteristics and board layout, challenges remain, including selecting the appropriate power MOSFETs, designing feedback and control networks, Inductor design, and choosing Asynchronous or synchronous topology. In addition, the design needs to include numerous protection features to provide maximum efficiency and a small solution size. At the same time, designers must reduce design time and cost, so they need to find more suitable power converter alternatives.
Instead of going the discrete route, designers can use integrated power ICs that combine MOSFETs with the necessary feedback and control circuitry optimized for high-efficiency buck converters.
This article reviews the performance trade-offs between asynchronous and synchronous buck DC/DC converters and how they fit into specific application needs. Using ROHM semiconductor‘s integrated asynchronous buck IC and synchronous buck converter IC solutions as examples, the paper discusses implementation considerations, including the selection of output inductors and capacitors, and printed circuit board layout. Evaluation boards are also discussed. to help designers get started.
Why use a buck converter?
Buck converters are a more efficient choice than linear regulators in applications that require only a few amps (A). Linear regulators are around 60% efficient, while asynchronous buck converters can achieve efficiencies above 85%.
A basic asynchronous buck converter includes a MOSFET switch, Schottky diode, capacitor, inductor, and controller/driver circuitry (not shown) to turn the MOSFET on and off (Figure 1). A buck converter takes a DC input voltage (VIN), converts it into a diode-rectified pulsed AC current, which is then filtered by an inductor and capacitor to produce a modulated DC output voltage (VO). The inductor’s voltage opposes or “steps down” the input voltage, hence the name.
Figure 1: Asynchronous buck converter topology without MOSFET controller/driver circuitry. (Image credit: ROHM Semiconductor)
The controller/driver circuit senses the output voltage and periodically turns the MOSFET on and off to maintain the output voltage at the desired level. As the load changes, the controller/driver changes the on-time of the MOSFET, increasing or decreasing the current supplied to the output as needed to maintain (regulate) the output voltage. The percentage of time the MOSFET is on during a complete on/off cycle is called the duty cycle. Therefore, the higher the duty cycle, the higher the load current supported.
Synchronous Buck
For applications where efficiency requirements cannot be met by asynchronous bucks, designers can use synchronous buck converters, where the Schottky diodes are replaced by synchronous MOSFET rectification (Figure 2). The on-resistance of the synchronous MOSFET (S2) is significantly lower than the Schottky resistance, resulting in lower losses and higher efficiency, but at a higher cost.
This presents a challenge, that is, there are two MOSFETs that need to be switched on and off. If both MOSFETs are turned on at the same time, it will create a short circuit and the input voltage will go directly to ground, destroying or destroying the converter. Preventing this would increase the complexity of the control circuit, increasing cost and design time compared to asynchronous designs.
In a synchronous buck, this control circuit incorporates a “dead time” in the middle of the switching transition, that is, both switches are closed for a short period of time to prevent them from turning on at the same time. Fortunately, designers now have a choice of power ICs that integrate the power MOSFETs and control circuitry needed to form a buck converter.
Figure 2: Synchronous buck converter topology where the Schottky diode is replaced by a synchronous rectifier MOSFET (S2). (Image credit: ROHM Semiconductor)
Integrated Buck Converter IC
Examples of highly integrated buck converter ICs include ROHM’s BD9G500EFJ-LA (asynchronous) and BD9F500QUZ (synchronous) devices, available in the HTSOP-J8 and VMMP16LX3030 packages, respectively (Figure 3). The BD9G500EFJ-LA has a withstand voltage of 80 V, suitable for 48 V power bus in 5G base stations, servers and similar applications, and also suitable for systems with 60 V power bus, such as electric bicycles, power tools, FA and IoT devices. The buck converter IC has an output current of up to 5 A and has a conversion efficiency of up to 85% in the output current range of 2 to 5 A. Built-in features include soft-start, overvoltage, overcurrent, thermal shutdown, and undervoltage lockout protection.
Figure 3: The BD9G500EFJ-LA asynchronous buck converter IC is in the HTSOP-J8 package, and the BD9F500QUZ synchronous buck IC is in the VMMP16LX3030 package. (Image credit: ROHM Semiconductor)
Because the BD9F500QUZ synchronous buck power supply IC has a breakdown voltage of 39 V, designers can reduce the mounting area and cost in FA systems such as programmable logic controllers (PLCs) and frequency converters for systems using a 24 V power bus. component count, thereby reducing system cost. The BD9F500QUZ reduces the solution size by about 60% and has a maximum switching frequency of 2.2 MHz, enabling the use of small 1.5 μH inductors. At an output current of 3 A, this synchronous buck operates up to 90% efficient.
A highly efficient and thermally efficient package means it can reach an operating temperature of approximately 60 °C without any heat sink, saving space, increasing reliability and reducing cost. Built-in features include output capacitor discharge function, overvoltage, overcurrent, short circuit, thermal shutdown, and undervoltage lockout protection.
Choosing Inductors and Capacitors
Although the BD9G500EFJ-LA and BD9F500QUZ integrate power MOSFETs, the designer still needs to choose the best output inductor and capacitor, which are interrelated. For example, in order to obtain the minimum combined size of the inductor and output capacitor, and a sufficiently low output voltage ripple, the optimum inductance value is very important. Transient requirements are also important, which vary from system to system. Load transient amplitude, voltage deviation limits, and capacitor impedance all affect transient performance and capacitor selection.
Designers have a variety of capacitor technology choices, each with different cost and performance tradeoffs. Typically, buck converters use multilayer ceramic capacitors (MLCCs) for their output capacitors, but some designs are more beneficial to use aluminum electrolytic capacitors or conductive polymer hybrid electrolytic capacitors.
ROHM simplifies the inductor and capacitor selection process, providing designers with complete application example circuits in the datasheets for these power ICs, including:
・ Input voltage, output voltage, switching frequency and output current
・ Circuit Schematic
・ Proposed Bill of Materials (BOM) including value, part number and manufacturer
・ Operating waveform
Three detailed application circuits for the BD9G500EFJ-LA, all with a switching frequency of 200 kHz, include:
・7 to 48 VDC input, 5.0 VDC output at 5 A
・7 to 36 VDC input, 3.3 VDC and 5 A output
・18 to 60 VDC input, 12 VDC and 5 A output
The seven detailed application circuits for the BD9F500QUZ include:
・12 to 24 VDC input, 3.3 VDC and 5 A output, 1 MHz switching frequency
・12 to 24 VDC input, 3.3 VDC and 5 A output, 600 kHz switching frequency
・ 5 VDC input, 3.3 VDC and 5 A output, 1 MHz switching frequency
・ 5 VDC input, 3.3 VDC and 5 A output, 600 kHz switching frequency
・ 12 VDC input, 1.0 VDC and 5 A output, 1 MHz switching frequency
・ 12 VDC input, 1.0 VDC and 5 A output, 600 kHz switching frequency
・ 12 VDC input, 3.3 VDC and 3 A output, 2.2 MHz switching frequency
In addition, ROHM provides designers with an application note: “Types of Capacitors and Considerations for Smooth Output from Switching Regulators.”
Evaluation boards speed up the design process
To further speed up the design process, ROHM provides the BD9G500EFJ-EVK-001 and BD9F500QUZ-EVK-001 evaluation boards for the BD9G500EFJ-LA and BD9F500QUZ (Figure 4).
Figure 4: The BD9G500EFJ-EVK-001 (left) and BD9F500QUZ-EVK-001 (right) are evaluation boards for the BD9G500EFJ-LA and BD9F500QUZ buck converter ICs, respectively, helping designers quickly ensure that the device meets their requirements. (Image credit: ROHM Semiconductor)
The BD9G500EFJ-EVK-001 outputs 5 VDC and inputs 48 VDC. The BD9G500EFJ-LA has an input voltage range of 7 to 76 VDC, and its output voltage can be configured from 1 VDC to 0.97 x VIN through external resistors. Alternatively, the operating frequency can be programmed between 100 and 650 kHz using an external resistor.
The BD9F500QUZ-EVK-001 evaluation board outputs 1 VDC and inputs 12 VDC. The BD9F500QUZ has an input voltage range of 4.5 to 36 VDC, and its output voltage can be configured from 0.6 to 14 VDC through external resistors. The power IC has three selectable switching frequencies: 600 kHz, 1 MHz and 2.2 MHz.
Board Layout Considerations
When using the BD9G500EFJ-LA and BD9F500QUZ, general PCB layout considerations:
• Freewheeling diodes and input capacitors should be placed on the same printed circuit board as the IC terminals and as close as possible to the IC.
• Whenever possible, thermal vias should be included to improve heat dissipation.
・ Place the inductor and output capacitor as close to the IC as possible.
• Keep circuit traces in the return path away from noise sources such as inductors and diodes.
For more specific layout details, see the device data sheet and ROHM’s application note on “PCB Layout Techniques for Buck Converters”.
Summarize
As shown, asynchronous and synchronous buck converters can provide higher conversion efficiencies than linear regulators in various FA, IoT, and 5G applications. While it is possible to design a custom buck converter for a given design requirement, it is a complex and time-consuming task.
Instead, designers can choose a power IC that integrates power MOSFETs with control and drive circuitry for a compact and economical solution. In addition, designers have access to a variety of tools to speed time to market, including application notes on capacitor selection and printed circuit board layout, detailed application example circuits and evaluation boards.
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