1:1 and flyback transformer at low frequency and high temperature
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You MUST give more information.
The question cannot be answered sensibly at present.   


Do you want flyback transformer design information or circuit information?
What research have you done so far?
What voltage in? 
What power level?   
What application? 
How high a temperature? 


Telling us what you are trying to achieve will VERY VERY greatly assist us in getting you a good answer. 



Temperature:

Temperature MUST be specified.


If using ferrite the material curie point is a major factor in maximum temperature.
If using iron core the insulation is liable to be a major factor.
If using powdered iron core - usually the material of choice at significant power levels - the core lifetime will be affected by elevated temperatures.