Question: My source is a desktop computer power supply, 

It's output is 5 v ,45 amps dc.

I want it to be converted to 20khz or more frequency ac current with same or less volt and same current.

Edit:- I am trying to make a rTMS. Which would change it's poles at 20khz. My electromagnets would be 2,10000 turns coils. Wires used used for coiling are 0.2 mm copper wire.

rTMS:- Repetitive Transcranial Magnetic stimulation

0.2 mm wire,45amps,5v
20khz. I get it it won't work. Thank you for replying. 

A guy from a YouTube channel called applied science made a tms, the coil had 15turns and current source was capacitors (1700v). It did work for him

<https://m.youtube.com/watch?v=B_olmdAQx5s>

Thanks

Answer: 
**You are in over your head**

First piece of advise: Do not attempt to make home-made medical equipment from an old computer supply. The are many ways that can go horribly wrong, and some of them can kill you or your patient.

**The inductance problem**

You say you will have two coils of 10k turns and an (inner?) diameter of 0.5cm. 10k turns of 0.2mm wire will have a cross-section of 400mm^2. So your coil is going to be 20mm tall, 40mm wide, with a 5mm hole down the middle. It will have an inductance of a Henry or so. To push 45A through that at 20kHz will require hundreds of thousands of volts.

Even if you did somehow manage, you have about 60m of wire in those coils, so they will have a resistance of about 30-35 ohms. 45A through 33 ohm will dissipate 67kW as heat.

**You cannot realistically do what you want**

If you really do want to build your own TMS system (and again, I suggest you don't) then you need to spend some time working out what fields, frequencies, and waveforms are commonly used, and how they are usually generated. It certainly isn't a 20kHz AC signal. Then come back and ask specific questions about the implementation you've chosen.
This smells like a major [XY problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem), but an [H-bridge](https://en.wikipedia.org/wiki/H_bridge) would solve this *Y* question.

Now, when you've revealed your X, I'll still say that it's an H-bridge that you want, but that the inductance of the coil will give an impedance that is way too high for 20kHz and 45A. You need **much** higher voltage in order to get the current flowing. I'll just come up with a guess and say that you need at least 100 Volt, and this is ignoring the real resistance in the wire. So let's add 100 Volt on top of this. So my guess is 200 V, minimum. Again, just a **guess**. If you want to **really** know your minimum voltage, then measure your coil's inductance and resistance and then we can come to some actual numbers. 5 volt will **never** do this, not even if you cool it down to 0.1K.