In a variety of systems including automotive, industrial automation, telecommunications, computers, white goods and consumer electronics, there is an increasing need to reduce high busbar voltages to lower voltages to power ICs and other loads. The challenge for designers is how to achieve this step-down conversion with the highest efficiency, minimal thermal load, low cost, and the smallest possible solution size.

Traditional asynchronous buck converters offer a potential low-cost solution, but their conversion efficiency is low and cannot meet the needs of many electronic systems. Designers can utilize synchronous DC/DC converters and synchronous DC/DC controllers to develop compact, high-efficiency solutions.

This article briefly introduces the performance requirements of electronic systems for efficient DC/DC conversion and reviews the differences between asynchronous and synchronous DC/DC converters. Then, several synchronous DC/DC converter designs from Diodes, Inc, STMicroelectronics, and ON Semiconductor are presented, along with evaluation boards and design guides. These scenarios help jump-start the development of efficient solutions.

Why do you need a synchronous DC/DC converter?

The ever-increasing efficiency requirements and increasing complexity of all types of electronic systems have driven power system architectures and power conversion topologies to evolve accordingly. Distributed power architectures (DPAs) are being used in an increasing number of electronic systems as more and more independent voltage domains can support an increasing number of functions.

Instead of using multiple isolated power supplies to drive different loads, the DPA consists of only one isolated AC/DC power supply for relatively high distribution voltages, and multiple smaller buck converters. Among them, a buck converter is used to reduce the distribution voltage to a lower level according to the requirements of each load (Figure 1). The advantages of using multiple buck converters are small size, high efficiency, and excellent performance.

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Figure 1: Distributed power architecture showing the main isolated AC/DC power supply (front end), and multiple non-isolated DC/DC converters supplying low voltage loads. (Image credit: Digi-Key Electronics)

When choosing an asynchronous or synchronous buck converter, there is a trade-off between cost and efficiency. If the lowest cost solution is required that can accept both lower efficiency and higher thermal loads, an asynchronous buck scheme may be the first choice. On the other hand, if efficiency is a priority and you want an operating scheme that generates less heat, a more expensive synchronous buck converter is usually the better choice.

Comparison of Synchronous and Asynchronous Buck Converters

A typical asynchronous buck converter application is shown in Figure 2. ON Semiconductor's LM2595 is a monolithic integrated circuit that includes the main power switch and control circuitry. The device uses internal compensation to minimize external component count and simplify power supply design. Its typical conversion efficiency is 81%, and the heat loss accounts for 19% of the power, while the typical conversion efficiency of the synchronous buck scheme is about 90%, and the heat loss accounts for only 10% of the power. This means that the heat loss of an asynchronous buck converter is almost twice that of a synchronous buck converter. Therefore, using a synchronous buck converter can reduce heat generation, greatly simplifying the thermal management challenge.

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Figure 2: Typical asynchronous buck converter application with output rectifier (D1), output filter (L1 and Cout) and feedback network (Cff, R1 and R2). (Image credit: ON Semiconductor)

In a synchronous buck converter, such as the ST1PS01 from STMicroelectronics, the output rectifier is replaced with synchronous MOSFET rectification (Figure 3). Compared to the output rectifier in an asynchronous buck converter, the lower "on" resistance of the synchronous MOSFET reduces losses and thus significantly improves conversion efficiency. The synchronous MOSFET is an internal part of the IC and does not require an external rectifier diode.

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Figure 3: Synchronous buck application circuit showing external output rectifier diodes removed. Output filtering and feedback components are still necessary. (Image credit: STMicroelectronics)

Higher efficiency and lower thermal load can be achieved with a synchronous buck converter, but this comes at a cost. Asynchronous buck converter controllers are much simpler and less bulky because they only contain a power switching MOSFET and a rectifier diode, and do not have to consider the possibility of cross-conduction or "shoot-through" or use synchronous FETs for control. much smaller. A synchronous buck topology requires more complex drivers and anti-cross-conduction circuitry to control both switches (Figure 4). To ensure that both MOSFETs do not turn on at the same time and create a direct short circuit requires a more complex circuit, which in turn requires a larger and more expensive IC.

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Figure 4: Synchronous buck converter IC block diagram showing two integrated MOSFETs (pins labeled "SW" next to them) and added driver/anti-cross-conduction circuitry. (Image credit: STMicroelectronics)

While PWM-controlled synchronous buck converters are more efficient at moderate or full load conditions, asynchronous buck converters typically have higher conversion efficiency at light loads. However, this situation is gradually diminishing, as the latest synchronous buck converter implementations include multiple operating modes that allow designers to optimize low-load efficiency.

Synchronous Buck for 5V and 12V Distribution

For designers using 5-volt and 12-volt power distribution in consumer products and white goods, Diodes, Inc. introduces the AP62600 device. This is a 6 amp (A) synchronous buck converter with a wide input range of 4.5 to 18 volts. The device integrates a 36 milliohm (mΩ) high-side power MOSFET and a 14-mΩ low-side power MOSFET for high-efficiency step-down DC/DC conversion.

Since the AP62600 uses constant on-time (COT) control, very few external components are required. The device also features fast transient response, easy loop stabilization, and low output voltage ripple. The AP62600 is optimized for electromagnetic interference (EMI) suppression. The device uses a proprietary gate driver scheme that prevents switching node transients without sacrificing MOSFET turn-on and turn-off times, thereby reducing high-frequency radiated EMI noise caused by MOSFET switching. The device is available in a V-QFN2030-12 (Type A) package.

Equipped with a "good power" indicator to alert the user to possible fault conditions. Programmable soft-start mode controls inrush current at power-up, enabling designers to implement power sequencing when using multiple AP62600s to power large integrated devices such as field programmable gate arrays (FPGAs), application-specific ICs (ASICs) ), digital signal processors (DSPs) and microprocessor units (MPUs).

The AP62600 provides three operating modes for designers to choose from to meet the specific needs of individual applications (Figure 5). High efficiency for all loads is achieved through pulse frequency modulation (PFM) operation. Other modes available include Pulse Width Modulation (PWM) for optimum ripple performance, and Ultrasonic Mode (USM) to avoid audible noise at light loads.

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Figure 5: The AP62600 provides designers with a choice of three operating modes to meet the needs of individual applications: PFM, USM, and PWM. (Image credit: Diodes, Inc.)

To help designers get started with the AP62600, Diodes, Inc. also offers the AP62600SJ-EVM evaluation board (Figure 6). AP62600SJ-EVM has a simple layout and can access corresponding signals through test points.

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Figure 6: The AP62600SJ-EVM evaluation board provides a simple and convenient evaluation environment for the AP62600. (Image credit: Digi-Key Electronics)

Synchronous Buck for 24 Volt Bus

STMicroelectronics' L6983CQTR has an input range of 3.5 to 38 volts and output currents up to 3 A. Designers can use the L6983 for a wide range of applications, including 24-volt industrial power systems, 24-volt battery-operated equipment, distributed smart nodes, sensors, and always-on and low-noise applications.

Based on a peak current mode architecture with internal compensation, the L6983 is housed in a 3 mm x 3 mm QFN16 package, thus minimizing design complexity and size. The L6983 is available in Low Consumption Mode (LCM) and Low Noise Mode (LNM) versions. The LCM maximizes light-load efficiency by controlling output voltage ripple, making the device suitable for battery-powered applications. LNM mode keeps switching frequency constant and minimizes output voltage ripple for light load operation for low noise applications. The L6983 allows selection of switching frequency from 200 kilohertz (kHz) to 2.3 megahertz (MHz) with optional spread spectrum for improved EMC.

STMicroelectronics offers the STEVAL-ISA209V1 evaluation board to help designers explore the capabilities of the L6983 synchronous monolithic buck regulator and jump-start their designs.

Synchronous Buck Controllers for Computing and Telecom Designs.

ON Semiconductor's NCP1034DR2G is a high-voltage PWM controller for high-performance synchronous step-down DC/DC applications with input voltages up to 100 volts. The device is designed for 48-volt non-isolated power conversion in embedded telecom, networking, and computing applications. The NCP1034 drives a pair of external N-channel MOSFETs, as shown in Figure 7.

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Figure 7: Typical application circuit for the NCP1036 synchronous buck controller IC, showing the high-side and low-side MOSFETs (Q1 and Q2, respectively). (Image credit: ON Semiconductor)

The NCP1036 has a programmable switching frequency range of 25kHz to 500kHz and a sync pin for externally controlling the switching frequency. By providing these two methods of frequency control, the designer can select the optimum value for each specific application and synchronize multiple NCP1034 controllers. The device also includes user-programmable undervoltage lockout and hiccup current limit protection. For low voltage designs, an internal trimmed 1.25V reference can be used for more precise output voltage regulation.

Four undervoltage lockout circuits are included to protect equipment and systems. Three circuits are dedicated to protecting specific functions; two protect the external high-side and low-side drivers; and one protects the IC from premature start-up before VCC falls below a set threshold. Designers can program a fourth undervoltage lockout circuit with an external resistor divider: the controller remains off as long as VCC is below a user-programmed threshold.

To help designers get started with the NCP1034, ON Semiconductor also offers the NCP1034BCK5VGEVB evaluation board (Figure 8). The evaluation board is designed with several options to meet various system needs. There is a linear regulator to power the IC; for this, the designer can decide whether to use a Zener diode or a high voltage transistor by choosing the appropriate resistor. Designers can also select Type II (voltage mode) compensation or Type III (current mode) compensation, ceramic or electrolytic output capacitors, and various input capacitance values. Equipped with two headers: one for easy connection of an external sync pulse source, allowing the board to connect directly to another NCP1034 demo board; the other header is connected to the SS/SD pins, which can be used to shut down the controller by connecting to ground .

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Figure 8: The NCP1034BCK5VGEVB evaluation board offers several options to help designers jump-start new designs. (Image credit: Digi-Key Electronics)

Epilogue

In a variety of systems such as automotive, industrial automation, telecommunications, computers, white goods, and consumer electronics, there is an increasing need to reduce high busbar voltages to lower voltages to power ICs and other loads.


Reviewing Editor Huang Haoyu

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