I want to increase the voltage on my electric fence charger
Don't do it.  What you're wanting to do is dangerous from a safety point of view, but also it would make you much more liable in the case of an accident.  Instead, buy an electric fence that does want you want.  Here's some articles that demonstrate what I'm talking about:

6-Year Old Girl Killed on Electric Fence

2000 Darwin Award Winner Kills Self with DIY electric fence

Electric Fence Kills 77 Year Old Woman

Electric Fence Laws

Now, you might be saying to yourself, "I wouldn't be as stupid as those people".  But do you want to take that risk?  At least if you buy a commercially made one then they have to go through all the safety testing and they assume some or most of the liability in case of a fatality.  Just my opinion.