Isolated DC/DC Converters: Key Concepts, Applications, and Safety Standards
Feb 5, 2019
Our whitepaper shows the concepts and components to create the desired isolation in DC/DC converters, which are used in critical applications, where a higher level of isolation and therefore a higher level of safety is indispensable.
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About DC/DC Converters
DC/DC converters are power supplies that convert one direct current (DC) voltage into another. In essence, they function similarly to isolating, step-up, or step-down transformers—but for DC instead of alternating current (AC). Since transformers operate only with AC, all DC/DC converters internally perform a DC-to-AC-to-DC conversion.
Fig. 1: Basic layout of a DC/DC converter
Isolated DC/DC Converters
Although some DC/DC converters operate without input-to-output isolation, most use an internal transformer, providing electrical (galvanic) isolation between input and output. The isolated output can serve as a floating power source or be used to generate different voltage rails and/or dual polarity rails (Figure 2). Since the output is isolated from the input, the reference voltage for either side can be chosen freely (Figure 3).
For example, a DC/DC converter can reverse polarity (e.g. -5V out from +5V in), add voltage (e.g. +12V from a +5V supply), or produce dual outputs from a single supply (e.g. ±5V from a 12V battery). This versatility makes DC/DC converters highly adaptable. Floating outputs are especially beneficial: isolation breaks ground loops, reducing electrical noise, allows flexible output polarity, and acts as a critical safety barrier against electric shock or short circuits.
Fig. 2: Example DC/DC supply configurations
For example, a DC/DC converter can reverse polarity (e.g. -5V out from +5V in), add voltage (e.g. +12V from a +5V supply), or produce dual outputs from a single supply (e.g. ±5V from a 12V battery). This versatility makes DC/DC converters highly adaptable. Floating outputs are especially beneficial: isolation breaks ground loops, reducing electrical noise, allows flexible output polarity, and acts as a critical safety barrier against electric shock or short circuits.
Fig. 2: Example DC/DC supply configurations
Classes of Isolation in DC/DC Converters
There are three main classes of isolation:
So how do these definitions translate into practical transformer construction?
- Operational or Functional (the output is isolated, but there is no fault protection)
- Basic (the transformer offers single fault protection)
- Reinforced (two independent means of insulation that offer double fault protection)
So how do these definitions translate into practical transformer construction?
Operational/Functional Isolation
The input and output windings are wound directly on top of each other around a ring core, with isolation provided by the thickness of the wire lacquer (Figure 3). This design allows for a very compact transformer that, despite its small size, can withstand isolation voltage testing up to 4kVDC for one second (e.g. RFMM, RKE series).
Another type of transformer construction is to wind the input and output windings on an insulating bobbin core (Figure 4). While this approach still depends on lacquer insulation around the wires, the plastic bobbin provides additional isolation by separating the conductive ferrite core from the windings. This design enables a compact transformer capable of delivering more power and withstanding isolation voltages up to 6kVDC for one second (e.g. REC5/H6 series).
Another type of transformer construction is to wind the input and output windings on an insulating bobbin core (Figure 4). While this approach still depends on lacquer insulation around the wires, the plastic bobbin provides additional isolation by separating the conductive ferrite core from the windings. This design enables a compact transformer capable of delivering more power and withstanding isolation voltages up to 6kVDC for one second (e.g. REC5/H6 series).
Fig. 3: Ring core transformer example
Fig. 4: Bobbin transformer example
Basic Isolation
In a bobbin-type transformer, the input and output windings are not wound directly over one another but are separated by an independent barrier, such as an insulating film (Figure 5). Minimum creepage and clearance distances also apply (see Table 1). This method is suitable for larger transformers, where there is enough space to add layers of tape or film between the windings (e.g. RPA60 series).
For very compact, low-power DC/DC converters, alternative methods are needed to provide basic isolation without increasing transformer size. Figure 6 shows a transformer that uses a plastic separation bridge to separate the two windings physically. Additionally, the ferrite ring core is completely enclosed, providing independent isolation from the windings (e.g. RxxPxx, RxxP2xx and RV series).
For very compact, low-power DC/DC converters, alternative methods are needed to provide basic isolation without increasing transformer size. Figure 6 shows a transformer that uses a plastic separation bridge to separate the two windings physically. Additionally, the ferrite ring core is completely enclosed, providing independent isolation from the windings (e.g. RxxPxx, RxxP2xx and RV series).
Fig. 5: Bobbin transformer with basic isolation
Fig. 6: Bridged transformer
Another method for constructing a basic insulated transformer is the potted core. In this design, the core and one winding are placed inside a plastic pot filled with epoxy. A lid is then fitted, and the second winding is wound around the entire assembly through the hole in the center. This type of construction is used in the RP series.
Fig. 7: Potted core transformer construction
Fig. 7: Potted core transformer construction
Reinforced Isolation
With reinforced isolation, the input and output windings are separated by at least two independent physical barriers (Figure 8), and the transformer must meet increased creepage and clearance requirements compared to basic isolation. Examples of RECOM converters with reinforced isolation include the RxxPxx/R, REC6-RW/R and REM medical grade series.
Fig. 8: Example of a reinforced transformer construction with increased creepage separation and two layers of insulation (shown as heavy black lines in the illustration)
Fig. 8: Example of a reinforced transformer construction with increased creepage separation and two layers of insulation (shown as heavy black lines in the illustration)
Clearance & Creepage
Clearance is the shortest distance between two points, measured directly (arcing distance). Creepage is the shortest distance between two points, measured along the surface (tracking distance).
Fig. 9: Clearance and Creepage Definition
Note: Creepage and clearance are based on the sum of the input and output voltages (e.g. 24V±10% in, 5V out = 31.4VDC working voltage), and not on the primary power supply voltage—unless the converter is specified for a particular working voltage (e.g. 250VAC).
The internal clearances and creepages within the transformer also depend on its construction:
Functional designs have internal clearances equal only to the thickness of the transformer wire lacquer, e.g. 0.016mm. The bridged transformer construction provides creepage and clearance equal to the thickness of the separation bridge (2mm), whereas the pot core construction has a clearance equal to twice the wall thickness of the plastic pot (0.5mm + 0.5mm), and a creepage of at least 3mm.
Reinforced transformers using triple-insulated wires (TIW) or fully insulated wires (FIW) can meet the requirements for reinforced insulation using only the wire itself, but still need to satisfy clearance requirements between the transformer and nearby components. Standard creepage and clearance distances also apply to other components, such as the opto-coupler and any EMC capacitors bridging the isolation gap. The values specified in many standards for creepage and clearance are based on ...
Fig. 9: Clearance and Creepage Definition
| Isolation Class | Input Voltage | 15VDC / 12VAC | 36VDC / 30VAC | 75VDC / 60VAC | 150VDC / 125VAC | 300VDC / 250VAC |
|---|---|---|---|---|---|---|
| Operational / Functional* | Clearance | 0.4mm | 0.5mm | 0.7mm | 1.0mm | 1.6mm |
| Creepage | 0.8mm | 1.0mm | 1.3mm | 2.0mm | 3.0mm | |
| Clearance | 0.8mm | 1.0mm | 1.2mm | 1.6mm | 2.5mm | |
| Creepage | 1.7mm | 2.0mm | 2.3mm | 3.0mm | 4.0mm | |
| Clearance | 1.6mm | 2.0mm | 2.4mm | 3.2mm | 5.0mm | |
| Creepage | 3.4mm | 4.0mm | 4.6mm | 6.0mm | 8.0mm |
Table 1: Typical values for clearance and creepage relative to the input voltage
* For functional isolation, the clearance and creepage are measured outside of the transformer.
* For functional isolation, the clearance and creepage are measured outside of the transformer.
Note: Creepage and clearance are based on the sum of the input and output voltages (e.g. 24V±10% in, 5V out = 31.4VDC working voltage), and not on the primary power supply voltage—unless the converter is specified for a particular working voltage (e.g. 250VAC).
The internal clearances and creepages within the transformer also depend on its construction:
Functional designs have internal clearances equal only to the thickness of the transformer wire lacquer, e.g. 0.016mm. The bridged transformer construction provides creepage and clearance equal to the thickness of the separation bridge (2mm), whereas the pot core construction has a clearance equal to twice the wall thickness of the plastic pot (0.5mm + 0.5mm), and a creepage of at least 3mm.
Reinforced transformers using triple-insulated wires (TIW) or fully insulated wires (FIW) can meet the requirements for reinforced insulation using only the wire itself, but still need to satisfy clearance requirements between the transformer and nearby components. Standard creepage and clearance distances also apply to other components, such as the opto-coupler and any EMC capacitors bridging the isolation gap. The values specified in many standards for creepage and clearance are based on ...
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Applications
