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Iso-Buck Converters for Smaller, Simpler Isolated Power Supplies

May 31, 2017

Reno Rossett  By: Reno Rossetti
 PhD, EE, Maxim Integrated 

Even though input voltages below 60V are considered to be safe to touch, there’s still a need for isolation in this operating range. It’s important to protect the power-supply electronic load, which is typically a delicate and expensive microcontroller. So, while isolated DC-DC voltage regulators are more complex than non-isolated variations, isolation remains useful in low-voltage power conversion systems.

Isolation guards against ground loops, which can produce parasitic currents that could disrupt the output voltage regulation and also introduce galvanic corrosion of the conducting traces, degrading equipment reliability. You’ll likely see isolated power supplies used when there’s a need to protect sensitive loads and preserve the long-term reliability of equipment.

A great example of low-voltage isolated systems are the I/O modules that are essential for process control in an automated factory. See Figure 1 for a block diagram of a digital I/O module and factory system. Here, a central hub takes the AC line power and converts it to 24V DC, delivering it to the I/O module along with the corresponding digital input (DI) and digital output (DO) data. Considering how harsh a factory environment is, it’s no wonder why sensitive electronics need protection from electric and magnetic interferences and over-voltages. 

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Figure 1. Digital I/O module and factory system block diagram

The programmable logic controller (PLC) in each module is powered by an isolated step-down voltage regulator. A rugged voltage-level translator interface at the digital input module (DIM) powers the sensor, receiving its information and passing it along to the PLC via a digital isolator or optocoupler. On the digital output module (DOM), a similar power, signal, and isolation chain leads to the on-board driver, interfacing to the external actuator. Modern systems call for a power-efficient and compact implementation of the isolated step-down converter in the input and output modules. 

Iso-Buck Converters Lower Costs

Classic architectures call for an optocoupler to provide the isolated feedback. The optocoupler and several passive components are, however, generally costly and space-consuming. By comparison, an iso-buck converter uses a new and highly integrated architecture that greatly reduces the bill of materials (BOM). See Figure 2 for a block diagram of the iso-buck converter architecture, which eliminates the need for an optocoupler while still remaining isolated. Since various external components aren’t required, the iso-buck architecture is more cost-efficient compared to other traditional isolated converters, reducing the number of BOM components by up to 50% and yielding board-space savings of up to 30%".

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Figure 2. Iso-buck converter diagram

For a deeper dive into iso-buck converters, read my Design Solution, Iso-Buck Converter Enables Smaller, More Efficient Isolated Power Supplies.