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Solid State


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From:  Harri Suomalainen [SMTP:haba-at-cc.hut.fi]
Sent:  Friday, March 20, 1998 10:40 AM
To:  tesla-at-pupman-dot-com
Subject:  Re: Solid State

>ring the primary at the same frequency.  The problem is that getting a
high
>voltage transformer to run at the frequencies of a Tesla coil (about
200kHz
>for mine), is a feat to be seen.  Sure you can try and build your own
>transformer, and this would take remarkable engineering skills.

Switch mode power supplys do have transformers running at high
frequency. Making them is actually not too hard but such project
would need some knowledge of SMPS. Such knowledge *can* be
obtained during the project too but it is cheaper to do low-voltage
projects first. (Trust me, I know. My 1st SMPS was a coil driver!!)

>If you're thinking of maybe running a NST at that frequency, you can
>forget it.  You'll most likely saturate the transformer and it will start
>getting hot.  I tried this method and could only get the NST to 450Hz!!

Normal transformers are definately out.

>There is an alternative scheme that I devised, which involves using
>several transformers, and pulse them each in turn.  The following diagram

Short pulses are not too well produced with normal 50/60Hz transformers.

>By doing this, you can keep the frequency to the indvidual transformers
>within the tranformers capabilities.  You can then combine the outputs
>together to get a frequency, in this case, 4 times the frequency of the
>transformer.  i.e.  if you wanted to operate a 200kHz coil, you could get
4
>transformers and run each at 50kHz.  You can see that this scheme can be
>easily expanded to incorporate more transformers.

With SMPS system that is ok. However, if you have SMPS system is makes
no great difference weather you drive it at 50 or 200kHz.

There are many other possibilities too. In one of my early prototypes I
was
not able to hit the 450kHz the coil required. So I desided to make a test
mostly out of quriosity. I supplied it with 150kHz square wave. 150kHz
square wave just happens to have considerable amount of harmonics
(like all square waves). The third harmonic just happens to be 450kHz.
Output was sure produced and coil did draw sinuoidal current at 450kHz.

The solution is not too good nor recommended. If the fundimental
component of the square has amplitude 1.0 the 3rd harmonic will have
amplitude of 0.3 or so. You require way too much voltage to get anything
real fun. Making any high-voltage transformer put out 3x voltage required
is just plain stupid.

I tried the harmonic approach with 5th harmonic too but output of my
system had much too low power for 5th harmonic drive. It still worked.

I also tried driving the coil with a pulse every now and then. All that
was
needed was once again a lower frequency system putting out short
pulse at fixed intervals. I used 1/3 duty cycle at 1/3 of resonant
frequency.
The result of such (150kHz) drive was a pulse for every third half-cycle
of the resonating coil. Much like driving it with 450kHz square wave with
only every 3rd half cycle present.

This pumping idea worked too. However, as duty cycle is very low there
is much more stress to switching elements (mosfets). No-good solution.

Those funny solution were brought out as an idea. They might actually
be worth the disadvantages if the frequency had to be high. If you are
forced for some reason to do a cw driver for 3MHz coil those might
be handy! Other harmonic based systems like class-E frequency
multipliers might be handy there too.

>To get to the point, you really need to get hold of some high frequency,
>high voltage transformers, for this method of ringing the primary to
work.

And then you might as well driver the high-frequency hv transformer at
output frequency as well. Funny pulsed systems are just too complex
and will put power through ferrites very inefficiently. SMPS approach will
give much better efficiency too! Frequency is still a problem: going much
over 1MHz will be a real pain. Efficiency and costs tend to limit SMPS
driver to below 1MHz.


--
Harri Suomalainen     mailto:haba-at-cc.hut.fi

We have phone numbers, why'd we need IP-numbers? - a person in a bus