Transfomer primary inductance

[bowman1710]

I'm designing a DC=DC transformer for a push-pull design, how do I know what the optimum primary inductance for this will be? Many thanks in advance.

 

[schmitt trigger]

You don't design transformers based on inductance, you design them based on flux density.
The resultant primary inductance is a only consequence of the number of turns and cross sectional area, which are required to support the maximum applied volts-time.

Are you familiar with transformer design equations?

 

[bowman1710]

Yes I am, I've contacted a planar transformer company that makes custom magnetics. They have calculated/ most of the requirements but have asked me the primary inductance when not giving me the core type etc. I just wondered what to spec as my desired primary inductance. I heard that a magnetising current of <5% of the load current is a rule of thumb to calculate the required primary inductance.

 

[schmitt trigger]

5% magnetizing current sounds about right. Ask them also, to give you the core type and size they are proposing.

With that information, and the maximum calculated flux density, you can estimate your core losses, which IMHO is a most important parameter.

 

[bowman1710]

Well they estimated at 100uH primary a 3W core loss at 200W. So I'm hoping with the value i give them it may come off better, I have done the calculations for transformer before but never had to spec an inductance first, usually i just obtained it from the calcs i have done from the core materials etc. I wondered if there was a rule of thumb for inductance and hope they would just optimise the core size/material to bring the losses down.

 

[schmitt trigger]

.......... but never had to spec an inductance first, .

Me neither.
Usually the resulting inductance, which BTW is inductance assuming initial permeability, is used to quality-check during final tests to ensure that the correct number of turns have been wound, that there are no unwanted airgaps between cores, and suchlike.

But if you are contracting the design, you have the right to ask these questions to them.

 

[BradtheRad]

Primary inductance is a key parameter in a transformer. There is a wide range of suitable values. This simulation should illustrate (although it is not exactly the same as your project).

The other factors mentioned in this thread are also important, of course.

The four circuits below are identical except for primary inductance. It changes in steps of 10x.



The middle two circuits produce output which could be called satisfactory. They are not horribly inefficient. You could obtain satisfactory performance from a wide span of primary inductance values.

The other two circuits have lopsided waveforms, and are inefficient.