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N Channel Mosfets

Bradley

Mar 17, 2015
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I'm only using the small heatsink on the bottom for testing.
 

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Bradley

Mar 17, 2015
26
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Mar 17, 2015
Messages
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So more inductance and a bigger hea
Yes well that's why that version worked then :)

My theory was that if I can drive a flyback with a Mosfet Than I should be able to drive a tesla coil as well :) same concept except for the ferrite core.. lol. so what I'm getting from this, is that my Mosfets are getting cooked cause there isn't enough inductance on the primary and the air gap is preventing the energy transfer between coils...? I've tried wrapping the primary directly to the secondary and came up with poor results. Any suggestions?
 

Arouse1973

Adam
Dec 18, 2013
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Yes you hit the nail on the head. Another thing you have to remember about switching high power Mosfets is the transient gate current and that if you want to switch the MOSFET on quickly, which is what you need to do from a dissipated power point of view. You may need to supply quite a bit of current. For example to switch the FET on fully within 1us and a total gate charge of 42 nC would require over 40 mA of gate current as a transient. If the device driving this gate can't supply enough current then this is just going to slow things down even more and your power dissipation goes through the roof.
 

Bradley

Mar 17, 2015
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So how many turns do you have on the secondary?
Adam

I wanna say roughly 1200 turns on 2" PVC. I don't remember right off the top of my head, but I'll be home in a few hours :) I'll be able to find out then.. Another question.. With a circuit such as this, does it require resonate caps? Or would that only be for the SGTC?
 

Arouse1973

Adam
Dec 18, 2013
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I wanna say roughly 1200 turns on 2" PVC. I don't remember right off the top of my head, but I'll be home in a few hours :) I'll be able to find out then.. Another question.. With a circuit such as this, does it require resonate caps? Or would that only be for the SGTC?

I don't know much about Tesla coils. They look like just a basic transformer with effectively an air core. So yes I guessing they would need to be tuned, and that's both primary and secondary. You don't have any core material to help with energy transfer, so you need to resonate the circuit to give a higher voltage across the transmitter coil and the receiver coil time to pickup the energy from the transmitter coil.

I don't know if the secondary coil has enough capacitance between the windings and then you just match the primary to this and this will determine your resonant frequency. But just looking at the ratio of windings your going to need a much higher voltage on the primary. Let me see if I can work something out for you.

Adam
 

Bradley

Mar 17, 2015
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I don't know much about Tesla coils. They look like just a basic transformer with effectively an air core. So yes I guessing they would need to be tuned, and that's both primary and secondary. You don't have any core material to help with energy transfer, so you need to resonate the circuit to give a higher voltage across the transmitter coil and the receiver coil time to pickup the energy from the transmitter coil.

I don't know if the secondary coil has enough capacitance between the windings and then you just match the primary to this and this will determine your resonant frequency. But just looking at the ratio of windings your going to need a much higher voltage on the primary. Let me see if I can work something out for you.

Adam


The problem I was having with my Mosfets getting cooked was exactly what we talked about. There is no longer an Airgap Between the Primary and the Secondary. Which allowed me to pump up to 50v Through the primary pulling between 5A to 10A without any Fet failure :) Primary got warm but it's working like a charm!! Definitely an improvement on the output of the secondary! It's still very far from its potential due to improper tuning but that's the next step, getting the Primary and secondary tuned properly... With the amount I'm putting into the primary i should be getting some nice breakout, but I'm not :(

Thank you for all your help!! Look forward too seeing what you come up with :D
 

Bradley

Mar 17, 2015
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IMAG0366.jpg Also i through this coil together today for kicks and giggles, its only roughly 300 turns on 4" PVC but the EMF field was quite large!! I was quite impressed..
 

Arouse1973

Adam
Dec 18, 2013
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Ok I am still working on it, going to take me a while to put something together. But you need to look at getting the transmitter to resonate to get the voltage up. A series tuned circuit will be the best for this.
Adam
 

Arouse1973

Adam
Dec 18, 2013
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I think we are looking at a complete re-think on the electronics. With such a small primary coil compared to the secondary we are going to need to increase the voltage and current that's applied across the primary coil. As you know you would usually use a spark gap and charge a capacitor up to 14 KV or more. And the cap then discharges huge amounts of current into the primary. Let me have a look at a driver for this.
Adam
 

Arouse1973

Adam
Dec 18, 2013
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Ok I have started on a few bits. Using an example for a secondary coil construction to show the required voltage needed. You can see it's not a trivial thing. At the bottom of the calculations it shows the voltage required (19.8 KV) to produce over 300 KV with a VSWR of 20. I need to next break down the VSWR formula, it's a bit complex I have to work out mutual inductance for this. Give me more time.

I have been looking into a driver for this and I think we can use a bridge circuit, but the voltage to the bridge is going to be several hundred volts.

Adam

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