Which is better – isolation or no isolation in power converters?


Exploring the Distinctions Between Isolated and Non-Isolated Power Supplies

In brief, an insulated power converter establishes a segregation between the input and output, both electrically and physically. This separation impedes direct current flow between the two, a feat typically accomplished through the application of a transformer. Conversely, a non-insulated power converter operates with a solitary circuit, permitting current to traverse between input and output. For those unfamiliar with power supplies, this raises additional inquiries: What advantages do isolated and non-isolated power supplies offer? And how do I determine which suits my specific application?


Foundations of Isolation

Galvanic isolation, often simplistically termed as isolation, entails the physical and electrical partition between different sections of a circuit. An outcome of this isolation is that each circuit section possesses its individual return or ground reference. In a non-isolated converter, as depicted on the left side of Figure 1, the input and output share a common ground, allowing current to course freely between them. In contrast, an isolated converter, illustrated on the right side of Figure 1, has the input and output returning to their distinct grounds, eliminating any direct current path from one to the other.

Non-isolated buck converter and Isolated Flyback converter

Fig.1 – Non-isolated buck converter and Isolated Flyback converter


Despite the prohibition of current flow between input and output in isolated converters, the transfer of power and information remains essential. Numerous methods facilitate this transfer, with power conveyed through electromagnetic fields via transformers or coupled inductors, while signals traverse isolation via signal transformers or optically through opto-isolators.

It’s crucial to recognize that isolation is not absolute. At sufficiently high voltages, insulation breakdown occurs, allowing current to flow. Datasheets typically specify the isolation voltage, indicating the maximum voltage applicable across the isolation for a brief period without instigating current flow. This isolation rating differs from the working voltage, denoting the maximum continuous voltage applicable across the isolation without inciting isolation breakdown.


Advantages of Isolation

Various scenarios necessitate or benefit from an isolated power supply. These include conformity to safety standards, disruption of ground loops, and level shifting.

Ensuring Safety Compliance

Safety regulations frequently mandate the use of an isolated power converter. In cases where converters are energized by high and potentially perilous voltages, such as AC-DC converters powered by AC mains, isolation shields the output from hazardous input voltages.

In considerations of safety, the insulation grade assumes significance. Safety standards stipulate the required insulation level for a given application. Insulation grades encompass functional, basic, supplementary, and reinforced insulation.

  • Functional insulation: The simplest form providing isolation but lacking protection against electric shock.
  • Basic insulation: A single protective layer against shock.
  • Supplementary insulation: Basic insulation with an additional redundancy barrier.
  • Reinforced insulation: A singular barrier equivalent to two layers of basic insulation.

Disruption of Ground Loops

Isolated supplies, given their absence of a shared ground between input and output, prove advantageous in breaking up ground loops. Circuits susceptible to noise benefit from this, as their ground is disengaged and isolated from potentially disruptive circuits.

Floating Outputs and Level Shifting

Another merit of isolated converters lies in their floating outputs. While maintaining a fixed voltage between output terminals, isolated outputs lack a defined or fixed voltage concerning voltage nodes in the circuits they isolate from. These outputs, termed as floating, may have one terminal connected to another circuit node to anchor it to a specific voltage. This characteristic facilitates the shifting or inversion of the output concerning another circuit point.

For instance, Figure 2 illustrates that linking the +Vout terminal to the input ground terminal compels the output ground below the input ground by an amount equivalent to Vout. Before this connection, the voltage between Vin and Vout remained undefined, whereas now, a common potential connects both sides.

Inverting connection

Fig.2 – Inverting connection


Connecting the output ground terminal to the +Vin terminal, as shown in Figure 3, results in the +Vout terminal equating to (Vin+Vout) concerning the input ground. In both scenarios, the isolation between input and output dissipates as the two sides establish a direct connection.

Additive configuration

Fig.3 – Additive configuration


Multiple isolated converters with floating outputs may be serially connected to amplify the output voltage or construct +/- rails, as depicted in Figure 4.

+/-Rails created using two isolated single outputs

Fig.4 – +/-Rails created using two isolated single outputs


Caution must be exercised to ensure authentic output floating. If the output ground terminals of two isolated converters connect to the chassis, they cease to be floating relative to each other. If these outputs are linked in series, a short circuit may occur, as both terminals connect to the chassis. In AC-DC converters, the output ground terminal occasionally ties to the earth, forfeiting its floating status, despite being isolated.


Advantages of Non-Isolation

While isolation offers numerous benefits, there are compelling reasons to opt for a non-isolated converter, including cost considerations, size constraints, and enhanced performance.

Economic Considerations

Isolated converters generally incur higher costs than non-isolated counterparts. A primary cost factor lies in the use of custom-built transformers, in contrast to the off-the-shelf inductors common in non-isolated converters. If a heightened level of insulation is mandated, the cost disparity escalates. Furthermore, components like opto-couplers may be integrated into an isolated design, adding to costs absent in a non-isolated configuration.


Non-isolated converters typically boast smaller dimensions than their isolated counterparts. The additional components mentioned earlier, contributing to increased costs, also occupy more space. In addition, the substitution of transformers with inductors in non-isolated converters, coupled with higher operating frequencies, further diminishes the size of magnetic components and capacitors.

Operational Efficiency

The efficiency and regulation of non-isolated converters generally surpass those of isolated models. Transformers and opto-couplers, substantial contributors to performance disparities, are absent in non-isolated designs. The lack of an isolation barrier permits direct sensing and precise control of the output, resulting in superior regulation and transient performance. Their reduced size also allows for closer placement to the load, mitigating transmission line effects.


In Conclusion

The decision between isolated and non-isolated converters hinges on diverse factors. Certain applications demand isolation for safety reasons, while others reap benefits from a floating output, disrupting ground loops, or referencing voltage shifts. Nevertheless, in scenarios where isolation is unnecessary, a non-isolated converter may yield cost reductions, space savings, and improved efficiency. Discerning the costs and benefits of isolation proves paramount in selecting the optimal converter for a streamlined design.