Transformer Problems?

[MsStateDawg]

I am designing a half-bridge converter that takes 12 VDC and converts it to 170 VDC. I have successfully got my two mosfets switching using an MC34025 PWM controller and a IR2181 high-low side driver. The 12 VAC wave shape coming off the mosfets isn't the greatest loking waveform but I would say that it works. I have also built a tranformer form scratch. The problem is that the 12VAC that I am feeding to the transformer(it is a high frequency transformer of course 100kHz) isn't being boosted by the transformer. I feed in 12VAC, 100kHz signal and I get out about the same when I measure on the secondary side of the transformer with an o-scope. Also if I feed the output of the secondary to a rectifier and measure on the recitfier I get double the voltage (About 27 VAC clean sine wave). The transformer has 2 turns on the primary and 37 on the secondary. I used the Power Supply Cookbook by Marty Brown to calculate the the turns ratio and I also used the recommendations given by Magnetics Inc. where I bought my cores. Does anyone have any idea why the transformer isn't operating properly and why I am not getting anything near the voltage that I need?

 

[Russlk]

What is the part number of the core? Is it an E core, C core, POT core, or toroid? If it is an E or C core, how are you holding it together?

 

[MsStateDawg]

The core is a Magnetics Inc. Ferrite E core. I am holding the core together with magnetic tape.

 

[Sebi]

Maybe the magnetic tape work as air-gap.

 

[Nigel Goodwin]

One thought, how thick is the wire you are using?, also what sort of power are you trying to obtain?.

 

[jem]

Another thought is maybe you do not have enough magnetizing inductance. Unless the core you are using has a very high permeability, two turns on the primary side seems to be too few. Turns ratio allows you to calculate the primary to secondary voltage ratio, but by itself is not sufficient to allow you to design a transformer. For example, in most cases you cannot expect to have a working 120V to 12V, 60Hz transformer properly work with 10 turns on the primary and one turn on the secondary (even though the turns ratio is correct). Most probably you will short out the 120 AC line if you did this.

Bottom line in all this is try to increase the primary turns, and the secondary turns in the same ratio. Not by too much though, or otherwise you will probably have to tune out the resulting larger leakage inductance.

 

[MsStateDawg2]

I am working on the project with MsStateDawg. I am using 12 guage wire on the primary and 16 on the secondary. 300 Watts continuous power and 600 Watts peak will be the power requirements for this prototype. Does anyone have any suggestions on how many turns I should increase the number of primary turns?

 

[Russlk]

We need to know the permability of the core. How can you hope to design a transformer, not knowing this? The actual part number of the core will allow those knowledgable to determine the applicable parameters.

 

[jem]

Also, at these power levels, core saturation becomes a real problem. Sebi mentioned something about an air gap in a earlier post. You really need to know the characteristics of your core.

Selecting a proper core is a compromise between cost, size and efficiency (losses).

 

[MsStateDawg2]

Transfromer Problems?

The core is a ferrite cor from Magnetics Inc. Part Number 0P44022EC E-Shaped core.

 

[MsStateDawg2]

Would it matter if the secondary is wound on the bobbin first and then the primary? I know that it is possible to split either winding and sandwich the other winding in between, but does the order of which they are wound matter?

 

[Russlk]

It is common practice to put the primary first, but I think that is just a convenience for the factory, they can wind many primaries and put on different secondaries for different transformers. The first winding uses the least wire, so perhaps the high current winding should go first.

 

[grim]

have you actually measured the voltage on the primary, to know it is 12v?

 

[Nigel Goodwin]

have you actually measured the voltage on the primary, to know it is 12v?

You seem to have a fasination with transformer questions?, but perhaps you might try looking how old the thread is, the last post was almost four years ago!.

 

[Hero999]

If I remember rightly transformers are his job and by the looks of it, his life too.

 

[grim]

ahhh, yes. was looking at something else and at the bottom it said 'other related posts' and this one was one of them. guess it should say obsolete related posts....

wouldn't call it a fascinating, but i am in therapy for it