Why do 1:1 DC/DC isolation transformers have different inductance values?

Question

I am new to power supply design. I need to design an isolated, unregulated DC/DC power supply, 28VDC, 45W. The purpose of this circuit is just to provide isolation with 1:1 voltage and power.

As I was going through 1:1 transformers, I came across "https://www.we-online.com/catalog/en/WE-FLEX#/articles/WE-FLEX-EFD20".

Even though they are all 1:1 DC/DC isolation transformers, why do they have different inductance values? yhere's 3.4uH at the lower end and 196uH on the higher end.

Does it depend on the primary voltage?

If, yes how? How do I select a 1:1 transformer for a 1:1 DC/DC isolated power supply design?

Answer 1

If you are designing an unregulated isolating DC-DC converter, the simplest topology, and probably the easiest to design is a push-pull DC-DC converter.

In this topology, the transformer is operated with traditional transformer action, rather than as an inductor with two coils. That is, energy is transferred simultaneously from the primary and to the secondary. This is in contrast to topologies such as flyback, in which energy is transferred from primary to magnetic field during one phase of the cycle, and from the magnetic field to the secondary during the next phase of the cycle.

In the flyback and related topologies, the transformer often has an air gap, so that it is wound with a higher inductance. Sometimes the air gap is distributed throughout the core in the form of binding material.

In traditional transformer action, higher inductance can reduce copper losses due to excitation current. However, the designed inductance value is less critical to power capacity than in fly-back and related topologies. Traditional transformer action transformers generally do not purposefully add an air-gap.

So, before deciding what inductance your transformer needs, you first need to decide whether the converter is going to use the transformer in traditional transformer action mode, or in the mode of stored-energy inductor. Your transformer specs will depend upon that choice.

www.ti.com/lit/slyt790 provides a walk-through for designing a push-pull isolating DC-DC converter.

Answer 2

Depending on your converter topology (probably flyback), the combination of inductance, input voltage, and output power will determine your switching frequency.

Using a lower inductance will generally result in a higher switching frequency. Assuming discontinuous mode operation, generally...

$$f \ge \frac{W}{\frac{1}{2}I_{sat}^2*L} $$

For example 749196540 is 3.4uH and has saturation current of 4.18A.

$$\frac{45W}{\frac{1}{2}*4.18A^2*3.4uH} = 1.51MHz $$

We can calculate the switching frequency for all the parts in this series. The results are shown below.

Typically, one would expect that the frequency is at least 20% higher to account for efficiency losses.

What we see is that for this transformer series the available combinations of inductance and saturation current result in very high switching frequencies (and therefore high switching losses).

Wurth actually has a really good tool called Red Expert that can help you pick a transformer.

Another thing to consider is the on-time of your power switch. Generally the power switch can't be on for more than...

$$T \le \frac{I_{sat}*L}{V_{in}} $$

So for example...

$$\frac{4.18A*3.4uH}{120V} = 120ns $$

Which is actually pretty short. It may be too short if your switching transistor or controller chip can't switch that fast.