The evolution of robust & cost-effective, isolated DC/DC converters
2023. 6. 16
The isolated power converter has a rich history of bringing modern, complex, efficient, and SAFE electronics to fruition.
1. Introduction
The isolated power converter has a rich history of bringing modern, complex, efficient, and SAFE electronics to fruition. It is important to focus on the key characteristics of a power converter that dictate isolation properties. They drive many of the leading factors that go into today’s cutting-edge power supplies by maintaining Moore’s Law on the load side while optimizing the manufacture, cost, and reliability of critical components such as magnetics and driving other advanced packaging techniques on the supply side.
2. A Brief Overview of Isolated DC/DC Converters
The isolated DC/DC converter has enabled countless applications that would not otherwise be possible. Some prominent examples are medical power supplies, high-speed communication buses, offline power solutions, motor drives, and high-voltage use cases.
Additionally, one could argue that the most valuable contribution of isolated DC/DC converters relates to the isolation itself. Being able to SAFELY process high voltages and/or large amounts of power has been a critical contribution of power electronics to society. Many people may not be aware of all of this and therefore do not have an appreciation for these enabling technologies, but I am sure they are happy with the results in their daily lives. As power electronics engineers (or related), we are very much accustomed to being the unsung heroes that “secretly” make all the electronics of the world operate, even if considered “black magic” or unbeknownst to the user.
First, we should define what isolation is and how it applies to DC/DC converters. Electrical (a.k.a. – galvanic) isolation is the physical separation of conductors to prevent the direct flow of current between them [1]. The quickest test for evaluating if any level of isolation is present in a system is to evaluate the ground potentials between two targets. The grounds between isolated circuits should be at independent (a.k.a. floating) potentials. In addition to safety needs, there are several practical uses of floating grounds in DC/DC converters, which will be covered a little later.
Power conversion circuits use many different types, methods, and implementations of isolation, so we shall quickly overview the most salient here. The classification of isolation depends completely on the physical isolation techniques, they are often accomplished in the transformer assembly/construction and by physical spacing. The table below showcases a more comprehensive overview of isolation in DC/DC converters and their implementation.
It is very important to note that the requirements and aspects of isolation needs are dictated by many different industrial/safety standards, which can be highly dependent on the application and/or geographical location of usage. So be sure to capture any safety/certification requirements for your system early in the design process. It is imperative to research the specific requirements of the application at hand as the metrics, spacings (in 2D & 3D), isolation levels, and verification test methodology/setups can vary greatly and often be the difference maker between a smooth development and unexpected cost/time budget overruns. For instance, please see the excerpt below for the voltage spacing requirements of uninsulated conductors from IPC-9592B. While it clearly calls out minimum spacing based on conductor potentials, it also notes that creepage/clearance requirements in a related standard (IEC 60950 in this case) may be more stringent and precedence should be taken. Supporting a medical and/or high-reliability application will also come with many application-specific guidelines/requirements in this regard.
Fig. 1: IPC-9592B Uninsulated Conductor Voltage-Spacing Requirements Excerpt [3]
While isolation is commonly accomplished in transformer construction, it can also be implemented in other ways, particularly for smaller signals (i.e., control feedback, digital communications, etc.). It is very common to isolate communication buses, such as that of CAN-bus in automotive and industrial applications by using a small, isolated DC/DC or even capacitive isolation for digital signals. The small-signal feedback information from an isolated power converter’s output can be fed back to the input via an optoisolator. The optoisolator converts a signal’s electrical energy to optical, then back to electrical, thus passing along the critical control info while preserving the galvanic isolation between input and output.
Modern improvements in transformer construction/materials, 3D power packaging (3DPPR) techniques, and other novel geometries and manufacturing processes have made great strides in this space. Higher levels of reinforced insulation along with improved assembly techniques enable a design to meet isolation requirements, while still shrinking the overall size of the solution and utilizing the benefits of more automated manufacturing processes. This enhances overall quality and reliability, while concurrently taking advantage of economies of scale so that the robustness and power density improvements do not come at the detriment of cost. A prime example is how previously manually-wound toroids are now automatically controlled by implementing a planar structure that embeds windings in printed circuit boards (PCB) and directly incorporates the magnetic core materials into the surrounding geometry.
Additionally, one could argue that the most valuable contribution of isolated DC/DC converters relates to the isolation itself. Being able to SAFELY process high voltages and/or large amounts of power has been a critical contribution of power electronics to society. Many people may not be aware of all of this and therefore do not have an appreciation for these enabling technologies, but I am sure they are happy with the results in their daily lives. As power electronics engineers (or related), we are very much accustomed to being the unsung heroes that “secretly” make all the electronics of the world operate, even if considered “black magic” or unbeknownst to the user.
First, we should define what isolation is and how it applies to DC/DC converters. Electrical (a.k.a. – galvanic) isolation is the physical separation of conductors to prevent the direct flow of current between them [1]. The quickest test for evaluating if any level of isolation is present in a system is to evaluate the ground potentials between two targets. The grounds between isolated circuits should be at independent (a.k.a. floating) potentials. In addition to safety needs, there are several practical uses of floating grounds in DC/DC converters, which will be covered a little later.
Power conversion circuits use many different types, methods, and implementations of isolation, so we shall quickly overview the most salient here. The classification of isolation depends completely on the physical isolation techniques, they are often accomplished in the transformer assembly/construction and by physical spacing. The table below showcases a more comprehensive overview of isolation in DC/DC converters and their implementation.
| Isolation grade class | Description | Example use case |
|---|---|---|
| Functional | The output is isolated, but there is no protection against electric shock | 
Ring Core Transformer with Functional Isolation |
| Basic | The isolation offers shock protection as long as the barrier is intact | 
Bobbin Transformer with Basic Isolation |
| Supplementary | An additional barrier to basic, required by agencies for redundancy | 
Example of a Reinforced Transformer Construction with a Basic and Supplementary Layer of Insulation (shown as the thick black lines in the diagram |
| Reinforced | A single barrier equivalent to two layers of Basic insulation |
Table 1: Common Isolation Grade Overview Table, From “Understanding isolation in DC/DC converters” Blog [2]
It is very important to note that the requirements and aspects of isolation needs are dictated by many different industrial/safety standards, which can be highly dependent on the application and/or geographical location of usage. So be sure to capture any safety/certification requirements for your system early in the design process. It is imperative to research the specific requirements of the application at hand as the metrics, spacings (in 2D & 3D), isolation levels, and verification test methodology/setups can vary greatly and often be the difference maker between a smooth development and unexpected cost/time budget overruns. For instance, please see the excerpt below for the voltage spacing requirements of uninsulated conductors from IPC-9592B. While it clearly calls out minimum spacing based on conductor potentials, it also notes that creepage/clearance requirements in a related standard (IEC 60950 in this case) may be more stringent and precedence should be taken. Supporting a medical and/or high-reliability application will also come with many application-specific guidelines/requirements in this regard.
Fig. 1: IPC-9592B Uninsulated Conductor Voltage-Spacing Requirements Excerpt [3]
While isolation is commonly accomplished in transformer construction, it can also be implemented in other ways, particularly for smaller signals (i.e., control feedback, digital communications, etc.). It is very common to isolate communication buses, such as that of CAN-bus in automotive and industrial applications by using a small, isolated DC/DC or even capacitive isolation for digital signals. The small-signal feedback information from an isolated power converter’s output can be fed back to the input via an optoisolator. The optoisolator converts a signal’s electrical energy to optical, then back to electrical, thus passing along the critical control info while preserving the galvanic isolation between input and output.
Modern improvements in transformer construction/materials, 3D power packaging (3DPPR) techniques, and other novel geometries and manufacturing processes have made great strides in this space. Higher levels of reinforced insulation along with improved assembly techniques enable a design to meet isolation requirements, while still shrinking the overall size of the solution and utilizing the benefits of more automated manufacturing processes. This enhances overall quality and reliability, while concurrently taking advantage of economies of scale so that the robustness and power density improvements do not come at the detriment of cost. A prime example is how previously manually-wound toroids are now automatically controlled by implementing a planar structure that embeds windings in printed circuit boards (PCB) and directly incorporates the magnetic core materials into the surrounding geometry.
3. Impacts of Isolation on Converter Design
The most common figures of merit (FOM) regarding the design and optimization of power solutions are their size, weight, and power (a.k.a. SWaP factors). When combined with a cost metric, this can also be referred to as SWaP-C factors [4]. Given the different methods and levels of isolation, a design may require, these support needs can have big impacts on overall SWaP-C factors, particularly in filter components. Most systems cannot ship without signoff/certification for meeting (sometimes) multiple safety and functional standards so these are not “nice to have” kind of solutions, but are critical to market as well as inject cost and time into a project development schedule that did not account for the resources required to support these contingencies.
For instance, the table in the last section demonstrates the tradeoff in voltage versus spacing needs when packing conductors at different potentials into tight spaces. The class of isolation grade determines how many isolation protection features must be present and their minimum characteristics (i.e., in terms of material, thickness, and/or redundancy) for meeting the isolation spec (typically conveyed in terms of voltage level and withstand time for an exposure to such voltage(s) to still be functionally viable). This translates into a very typical tradeoff analysis between wanting to shrink overall solutions for optimizing SWaP, yet also coming at the detriment of cost when more expensive components (such as triple-insulated wire or TIW) can help to meet requirements in more compact arrangements.
Other technical factors such as thermal mitigation and supporting wide, high-voltage ranges may drive the compactness of a solution. Like any engineering development, reasonable compromises need to be made between meeting the core, functional/safety requirements of the system, cost impacts to development schedules/budgets, warranty/reliability needs, and time-to-market (TTM) targets. Given magnetics manufacturing remains among the last manual component assembly needs on the line, it should be reiterated that relegating as much of that to automation and non-hand-soldered assembly as possible can help optimize many of the most crucial elements of optimizing SWaP-C and improving reliability in a design.
At this point, it seems prudent to provide a quick mapping of isolated solutions to common power topologies and implementations. While a comprehensive topology overview is beyond the scope of this paper, this is meant to give a brief overview of which power conversion topologies support isolation and why.
For instance, the table in the last section demonstrates the tradeoff in voltage versus spacing needs when packing conductors at different potentials into tight spaces. The class of isolation grade determines how many isolation protection features must be present and their minimum characteristics (i.e., in terms of material, thickness, and/or redundancy) for meeting the isolation spec (typically conveyed in terms of voltage level and withstand time for an exposure to such voltage(s) to still be functionally viable). This translates into a very typical tradeoff analysis between wanting to shrink overall solutions for optimizing SWaP, yet also coming at the detriment of cost when more expensive components (such as triple-insulated wire or TIW) can help to meet requirements in more compact arrangements.
Other technical factors such as thermal mitigation and supporting wide, high-voltage ranges may drive the compactness of a solution. Like any engineering development, reasonable compromises need to be made between meeting the core, functional/safety requirements of the system, cost impacts to development schedules/budgets, warranty/reliability needs, and time-to-market (TTM) targets. Given magnetics manufacturing remains among the last manual component assembly needs on the line, it should be reiterated that relegating as much of that to automation and non-hand-soldered assembly as possible can help optimize many of the most crucial elements of optimizing SWaP-C and improving reliability in a design.
At this point, it seems prudent to provide a quick mapping of isolated solutions to common power topologies and implementations. While a comprehensive topology overview is beyond the scope of this paper, this is meant to give a brief overview of which power conversion topologies support isolation and why.
| Topology | Fundamental circuit | Impacts of isolation/regulation |
|---|---|---|
| Unregulated push-pull (Royer) | 
Unregulated push-pull converter circuit |
|
| Inverting buck-boost (Flyback) | 
Basic Flyback converter circuit |
|
| Half-Bridge (push-pull) | 
Basic Half-Bridge converter circuit |
|
| ... | ... |
|
Table 2: Power Converter Topology Isolation/Regulation Comparison Table
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