Vishay – Generating isolated supplies for industrial applications using a synchronous buck regulator in an isolated buck topology

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Industrial power applications typically require a high input voltage. Standard voltage rails are 24V, 36V and 48V. The DC- DC buck switching regulators and controllers used to power the circuits in industrial applications are required to provide solid, reliable performance and to withstand the effects of noise, power surges and outages.

It is common for the power-supply requirements in industrial systems to be complex. Galvanic isolation is often needed to meet safety standards, as well as to break ground-loop interference in noise-sensitive applications. For example, in factory-automation systems such as PLCs and I/O modules, an increasing number of I/O channels leads to a requirement for higher sensing accuracy. As a result, isolation between different voltages is preferred, providing for digital/analogue signal isolation, or channel-to-channel isolation to block interference from a common ground, as shown in Fig. 1.

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Fig. 1: Typical architecture of the power supply of industrial equipment carrying digital and analogue signals

The traditional way to provide such an isolated supply would be to use a flyback converter operating from the main supply to generate the various voltages needed by the bias supply. Flyback designs typically use asymmetric transformer turn ratios for the primary and secondary windings, together with an optocoupler and reference voltage, or an auxiliary winding, for feedback regulation.

Flyback converters need an elaborate compensation design for stability. This entails a tedious design process and produces a bulky circuit with a high component count and cost. There is, however, a simpler way to provide an isolated power supply at the voltage levels required in industrial equipment, without the use of a flyback topology.

Isolated buck converter topology
An isolated buck converter uses a synchronous buck converter with coupled inductor windings to create isolated outputs, as shown in Fig. 2. Isolated converters in this topology use a smaller transformer for a given power requirement when compared to a flyback converter, as the transformer’s primary and secondary turn ratios are better matched. There is also no need for an optocoupler or auxiliary winding, as the secondary output closely tracks the primary regulated output voltage, resulting in a smaller circuit and lower cost.

This topology has several advantages, including:
• Isolated positive and negative supplies are easy to generate
• The primary-side supply is available to power loads that do not require isolation from the input
• The design is simpler than that of a traditional flyback converter
• The circuit contains fewer components and is smaller than that of a flyback converter

An isolated buck converter circuit can be made with a part from the SiC46x series of fully integrated synchronous buck regulators from Vishay. These devices, which are featured on page 5, offer high power-conversion efficiency and high power density with low electrical parasitics, a characteristic of their excellent silicon and of Vishay’s advanced packaging design techniques.

The SiC46x parts can operate from an input-voltage range of 4.5V to 60V, which makes them suitable for industrial power supplies running from a distribution bus at up to 48V. Users also benefit from features such as multiple power-saving modes for operation at very low output current, adjustable operating frequency, fast transient response, cycle-by-cycle current limiting, and a comprehensive set of protection features. The SiC46x regulators use an all-ceramic capacitor solution for both input and output capacitance.

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Fig. 2: An isolated synchronous buck converter generating two outputs

Conclusion
PLCs and I/O module power-supply designs have become complex, since they are required to provide multiple isolated voltages, floating bias voltages, and negative output voltages. Required to supply various isolated rails for gate drivers, op amps and communication interfaces such as RS-485 and RS-232, designers can adopt the isolated buck converter topology in place of the traditional flyback converter and benefit from a lower component count, smaller PCB, a compact, low-profile system and a simpler design.

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