Isolated DC/DC Converters

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.

1. About DC/DC Converters

DC/DC converters are power supplies that can change a direct current (DC) voltage into another DC voltage; in other words, they can act like an isolating transformer or a step-up or step-down transformer, but with direct current instead of alternating current (AC) supply.

As transformers only work with AC, all DC/DC converters are internally DC-to-AC-to-DC modules:


Fig. 1: Basic layout of a DC/DC converter

2. Isolated DC/DC Converters

Although DC/DC converters exist without input-to-output isolation, most DC/DC converters use an internal transformer, and the output is electrically (galvanically) isolated from the input. The separated output can be used to provide either a floating power source or to generate different voltage rails and/or dual polarity rails (Figure 2). Since the output is isolated from the input, the choice of reference voltage for the input or output side can be arbitrary (Figure 3); for example, a DC/DC can be used to change the voltage polarity (e.g. -5V out from +5V in), add a voltage (e.g. +12V from a +5V supply) or generate a dual output from a single supply (e.g. ±5V from a 12V battery). This feature makes DC/DC converters very versatile. Having outputs that are floating with respect to the input is also quite useful; the isolation breaks ground loops and thus eliminates noise in electrical systems, the output polarity can be freely chosen, and of course the isolation barrier is an important safety element, as a safeguard against electric shock or short circuit hazards.



Fig. 2: Example DC/DC supply configurations

3. Classes of Isolation

There are three main classes of isolation:

  • 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?

4. Operational/Functional Isolation

The input and output windings are wound directly over one another on a ring core, relying on the thickness of the wire lacquer for isolation (Figure 3).

This method has the advantage of a very compact sized transformer which, despite the small size, can withstand up to 4kVDC/1s isolation voltage testing (e.g. RFMM, RKE series).

Fig. 3: Ring core transformer example
Another type of transformer construction is to wind the input and output windings on an insulating bobbin core (Figure 4).

This method still relies on lacquer insulation around the wires, but the plastic bobbin isolates the conductive ferrite core from the windings. This method has the advantage of a very compact sized transformer that can deliver more power and offers isolation withstand voltage of up to 6kVDC/1s (e.g. REC5/H6 series).


Fig. 4: Bobbin transformer example

5. 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 can be used in larger sized transformers where there is enough room to add layers of tape or film in between the windings (e.g. RPA60 series).

Fig. 5: Bobbin transformer with basic isolation
For very compact low power DC/DC converters, other ways must be found to provide basic isolation without making the transformer too large. Figure 6 shows a transformer which uses a plastic separation bridge to physically separate the two windings.

In addition, the ferrite ring core is completely enclosed, so that it is also independently isolated from the windings (e.g. RxxPxx, RxxP2xx and RV series).

Fig. 6: Bridged transformer
There is also another way of making a basic insulated transformer, namely the potted core. In this method of construction, the core and one winding are placed inside a plastic pot which is filled with epoxy. A lid is fitted and then the second winding is wound around the whole construction through the hole in the middle. This type of construction is used in the RP series.



Fig. 7: Potted core transformer construction

6. Reinforced Isolation

With reinforced isolation, the input and output windings are separated by at least two separate independent physical barriers (Figure 8), and the transformer has increased creepage and clearance dimension requirements compared to basic isolation. Examples of RECOM converters with reinforced isolation are 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)

7. Clearance & Creepage

Clearance is the shortest distance between two points measured point to point (arcing distance). Creepage is the shortest distance between two points measured by following the surface (tracking distance).



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.


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 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 the construction:

Functional designs have internal clearances equal only to the thickness of the transformer wire lacquer, e.g. 0.016mm. The bridged transformer construction has creepage and clearance equal to the thickness of the separation bridge (2mm), whereas the pot core construction has a clearance equal to double the wall thickness of the plastic pot (0.5mm + 0.5mm), but a creepage of a minimum of 3mm.

Reinforced transformers using triple insulated wires (TIW) or fully insulated wires (FIW) can meet the requirements for reinforced insulation within the transformer, using only the wires themselves, but still need to meet clearance requirements between the transformer and adjacent components. The standard creepage and clearance separations also apply to all the other components; for example, the opto-coupler and any EMC capacitors bridging the isolation gap ...

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