# Transformer maximum voltage rating specifications

I don't know how to interpret the voltage values found in datasheets for transformers. For example, in the datasheet below:

where it says 15v, 50mA near the winding schematic. Is this a maximum or a test condition or nominal value? Is there generally a maximum output voltage for a transformer other than the dielectric breakdown voltage which is in this case specified at 1500V? Do i have to respect something like not go above 50mA*15V watts? If i want to use this transformer at an output of 75-100V for each of the 2 secondaries will i run into problems? How can i make sure the output of a flyback converter using this transformer is not too high, using the information provided in the datasheet?

• I forgot to mention; at the primary is it allowed to have below 15 V. According to the laws of physics i hardly see how that would be a problem, so i don't understand the meaning of 15-40V. Dec 9, 2017 at 11:56
• Makes very little sense in a flyback transformer. I normally just list the tuns in the drawing. Dec 9, 2017 at 11:58

For a flyback design, your principal design considerations will be primary inductance and the turns ratios from primary to each secondary. You need to design according to the specs for your flyback controller IC. That will give you the information you need to specify transformer. Flyback as well as other switching topologies, allows varying input voltage such as 15-40, but with stable output voltage voltage.

To answer your question more directly - output windings conductors size is chosen according to maximum current rating - drawing more current than the transformer is designed for will increase internal temp of transformer. Output Voltage is more related to turns ratio, but is not just a simple linear ratio for flyback design or other switching topolgies.

With respect to output voltage of 75 to 100V, assuming same turns ratio, your input voltage range would be similar of 75 to 100 or so. This would work fine if input and output voltages were increased - but again, you need to choose based on primary inductance required, and that will vary by overall design. You also need to be careful not to exceed power dissipation in transformer or you will have serious thermal issues.

Not sure why you would consider flyback with turns ratio of 1:1 however.

It seems like this transformer is not intended to be used with voltages as high as 100V, and I guess this transformer was designed especially for specific TI's LM5017 Flybuck IC. That is the reason, why transformer datasheet lacks max voltage spec.

Nevertheless, I guess it may work with such voltages. The limiting factor here is not dielectric strength between primary and secondary coils (that is much higher due to inter-winding extra insulation), but between adjacent turns, i.e. coil wire's insulation, which is weaker but is still enough for inter-turn voltages.

I'm not sure if it is enough for full voltage of the winding (about 100 V), so construction of winding and leads becomes of value (how many layers winding consists of, how much leads are spatially separated).

So, in summary, it's better to ask the manufacturer.