It's very hard to answer this question. You should give more detail about what system you want to find the transfer function, how many variables, parameters.... Basically, the transfer function in Power Electronics is calculated based on the small-signal modelling.
I am not sure if I understood the problem, but if it is correct that your output is divided by the input, then you have a nonlinear system and a transfer function cannot be found simply. You need to linearize the model, either by using an identification method for linear systems (if you do not know the nonlinear model) or by using a Taylor decomposition if you know the nonlinear function.
On top of the suggestions given by Dr. Nguyen and Dr. Gambier, it depends on whether you have the time domain data or the frequency-response data. There are techniques available to estimate the Transfer Function from the sampled signals. For more info, please visit the following link: