Use for the Tesla Switch

[StevanC]

(snip)
Devising a driving method for Floating MosFets
isn't too difficult. But, it requires some understanding
of the principles.


Download This Document The discussion on Gate drive transformers
begins on page 30. The whole document is outstanding.

Study this web page. It has a TON of good info.

Downoad this one too. It is good technical information.

This one is good also.

That should pretty well cover it.

Then we can get down to putting the pieces
together...

Do note:
they consider 50Hz the slowest rate
a 2.0...4.0 mH xfrm is the one used in "so called" 300W PC power supplies
and that's the big one (3.3 mH)
looks like a bit of an overshoot? But is doable.
(still have to go trough the rest of the documents)

Best regards,
Stevan C.

[nvisser]

My sentiments exactly.
The circuit Dave (DoubleD) posted does all that and is probably as simple as you could get it.
It took him and some other engineers about 3 years to get it where it is now.
Transformer drive circuits could maybe simplify it.
Like on some of the pdf,s that Seamonkey posted today

http://tonic-lab.com/pdf/reading_room/transformer%20isolated%20gate%20drive.pdf

[StevanC]

Can you show us this particular circuit Stevan?

If it isn't living up to your expectations we need
to analyze why. Somehow, we must find the
answer!

What are the sizes of your capacitors?

Well,
I used for both capacitors each one:
10000uF 35V
8x 10000uF 50V
3x 10000uF (now permanently fixed)

And I did take "current" measurement across a 15A amp-meter by o-scoping it's terminals.

Once I had a circuit with 10000/50V banks (8 pieces each bank) that managed to kick 30A for a split usec (3000A/usec dI/dt) but as soon as i replaced the capacitors with 25V ones and altered the layout (all same components) it was gone (???)

The "good" one was all in 22AWG? (not thicker) and had even screw-terminals (??)

The present one is both in more heavy gauges and more asymmetric.

I have to try run a o-scope across the circuit wire sections and look what i see...

thanks for the considerations,
I will try to take photos, the circuit is basically the CP You posted with the only difference in using two BJT for the "output" and using Schottky paired diodes for the "top-cap"

Stevan C.
P.S.
It's a pain to get the camera from the kids (teenagers), so I guess the phone-shots will have to do...

[StevanC]

A SPICE MODEL is long walk home yet

Stevan
Could you redraw the Palindrome's circuit into an understandable format and maybe give some specs on the trafo's you are talking about. I cant even see them in the diagram.

Vissie,
I would gladly provide a more obvious and practical circuit, but there are few basic considerations in regards to SPICE in general:

1. SPICE is practical to an extent, but one has to know to do without to be able to take advantage by doing with SPICE: there is really not much value in lanenal's design in terms of a blueprint - it is a model - an abstraction far from real world issues and drawbacks: like using a child's sketch to build a race bicycle.

2. lanenal's proposed "revelation worth" simple MOSFET switch has been known for long time and been a first consideration has it not been the problem of driving the supply of floating voltage to get the gates 10 (or preferably 18V) volts a top of the source.

3. Then he uses P-channel MOSFETS: less than optimal by twice the cost and half the performance. Not to mention driver issues possible

4. Then he uses transformer to drive a FET: shoot-trough, runaway, overshoot, failure conditions, a myriad of plaques and problems not even tried to solve, let alone design in the circuit: he could as well give six "voltage sources"; define them as pulses and "apply" them floating, each to a MOSFET, and call it "simplest". Yes model wise, but how does one implement that?

5. To drive a MOSFET's gate off a floating secondary one takes a high-side driver (such as IRS21851 already is, or LTC4440) and feed it's Vboost-Vs capacitor with a rectified (one diode sufficient) current. Therefrom You let the driver "clamp" the Vboost to 20V and let the driver take care of feeding the gate with "best diet" for it.

6. Once You do include those famous drivers to the circuit, You find the "neat simplicity" is all gone for now You see You ned not less than 30V (and more) of surplus voltage (from bottom "line") to feed the Vboost of the "upper" two MOSFETS or a AC generating circuit with two floating rectified 20V outputs.

7. To generate a AC one needs what's called oscillator - and this takes time to build, and power to run - so farewell "simple TS design" and the nightmare is all back again...


8. While we doing this all "so" to get 12V across a 10Ohm load (~1A), we need to feed the MOSFETS with 18V at least (10mOhm batteries considered) and that costs _power_, to get approx same results with BJT (namely MJL21194) we need just BD140 and optos (total six of each) - and just reuse the "Quer" of the PDF I posted.
And MJL will go up to 16A (dropping from 12 to about probably 8V across the switch)
that's at least 128W of usable power, at the cost of 64W of dissipation.

9. I did begin to consider a MOSFET Brandt-TS but I feel not ready to "ride the bull" yet...

I just don't know...
Stevan C.

[nvisser]

I have read boat load tonight.

I'll read that stuff to.

Thanks Man
Matt

I like it.

[SeaMonkey]

My commutator is 15" diameter, 12 cycles per revolution holding flattened copper 3/4" pipes for the contacts. It's this big to have enough room to keep the commutator rpms down to 4 or 5 a second. If I only had 4 rotating contacts the rpm would unreasonably high.

I'm making a spot welder pretty much with these currents, I know. 12v at 150amps is nothing to scoff at. If I have too much trouble with sticky brushes I'll turn down the amperage a bit with a fat resistor. If I had the money I would buy more batteries and some better parts...but bottomless accounts are hard to come by these days. Unfortunately I have only what I have.

And besides...Tesla only had brass and wood

Where there is a will there will be found a way!

Do you have a rough diagram of how you are wanting
to apply power to your spot welder?

What sort of a battery bank do you have?

Rather than a commutator switch mechanism have
you considered the possibility of slow speed cam
operated really heavy duty copper contacts?
Only the contacts and the wiring would need to
be copper, the remainder could be wood, or
any other convenient material.

[SeaMonkey]

I would love to see a schematic from you. John B, Matt Jones, Vissie, John Koorn, StevenC, Citfa, Ren, and myself, along with a whole list of others have contributed towards the progression of this goal. SeaMonkey, Let's see some of your work, instead of "We", Let's see "YOUR" work. "We" share, why not you?

Jeff

Primacy.

We'll get there in due time.

All of your contributions are important. Hopefully,
all concerned have a good understanding of the
needed techniques and the enabling principles.

Some are presently getting 'up to speed' and
will soon have confidence.

"We" is good!

[chainmailleman]

Unless your planning on using Mercury Brush's or 3 inch cubes of carbon your gonna have few problems with commutator also at kilowatts. Trust me they can make alot of heat. You have to consider not only friction but current as well.

If you haven't built one yet I would build one to smaller scale first so you can realize what kind of issues you gonna have.

Matt

My commutator is 15" diameter, 12 cycles per revolution holding flattened copper 3/4" pipes for the contacts. It's this big to have enough room to keep the commutator rpms down to 4 or 5 a second. If I only had 4 rotating contacts the rpm would unreasonably high.

I'm making a spot welder pretty much with these currents, I know. 12v at 150amps is nothing to scoff at. If I have too much trouble with sticky brushes I'll turn down the amperage a bit with a fat resistor. If I had the money I would buy more batteries and some better parts...but bottomless accounts are hard to come by these days. Unfortunately I have only what I have.

And besides...Tesla only had brass and wood

[Bit's-n-Bytes]

That should pretty well cover it.

Then we (SeaMonkey)can get down to putting the pieces
together...

I would love to see a schematic from you. John B, Matt Jones, Vissie, John Koorn, StevenC, Citfa, Ren, and myself, along with a whole list of others have contributed towards the progression of this goal. SeaMonkey, Let's see some of your work, instead of "We", Let's see "YOU'RE" work. "We" share, why not you?

Jeff

[Matthew Jones]

I have read boat load tonight.

I'll read that stuff to.

Thanks Man
Matt

[SeaMonkey]

Ya these mosfets drive off of a low voltage. If I am reading the sheet right.

http://ixdev.ixys.com/DataSheet/0acf6981-a5f0-43ec-b89a-58e0bba5550f.pdf

The Vgs(th) is 2.0 volt min and 4.0 max.

What I found while testing was that no matter the switch the base or gate will only take what it needs (For lack of a better description.)
When 2 series batteries are connected you general have no significant voltage between the 2 connected poles. But if you put your meter on the base or the gate, you'll see a difference. Depending on the device the voltage shown at that point is usually somewhere between the min and the max turn on.
I am not sure if that valid but its something I have looked at.

So is a Mosfet Driver a component or something you build or buy or what. Could you point me to one that might run the above mosfets?
I'd like to see how they are laid out.


Matt

The MosFets you referenced are good heavy duty
devices. They would work just fine.

The Threshold Voltage (Vgs(th)) is the amount of
Gate to Source voltage that will just bring the
MosFet into conduction. The Drain to Source
resistance at that point will be quite high as it
is the beginning of the linear conduction curve
for the device.

To get into the region of minimum Rds(on) you still
need the rated 10 Volts Gate to Source. Up to
about 15 Volts is acceptable.


Devising a driving method for Floating MosFets
isn't too difficult. But, it requires some understanding
of the principles.


Download This Document The discussion on Gate drive transformers
begins on page 30. The whole document is outstanding.

Study this web page. It has a TON of good info.

Downoad this one too. It is good technical information.

This one is good also.

That should pretty well cover it.

Then we can get down to putting the pieces
together...

[Matthew Jones]

Ya these mosfets drive off of a low voltage. If I am reading the sheet right.

http://ixdev.ixys.com/DataSheet/0acf6981-a5f0-43ec-b89a-58e0bba5550f.pdf

The Vgs(th) is 2.0 volt min and 4.0 max.

What I found while testing was that no matter the switch the base or gate will only take what it needs (For lack of a better description.)
When 2 series batteries are connected you general have no significant voltage between the 2 connected poles. But if you put your meter on the base or the gate, you'll see a difference. Depending on the device the voltage shown at that point is usually somewhere between the min and the max turn on.
I am not sure if that valid but its something I have looked at.

So is a Mosfet Driver a component or something you build or buy or what. Could you point me to one that might run the above mosfets?
I'd like to see how they are laid out.


Matt

[SeaMonkey]

@SeaMonkey, I don't think it matter what the output is That would be dependent on the load you need. I am growing more curious by the day about driving the transformer as I have started to see some neat results from a test model I have running. So we can go with that for a basic model.

I think this how I did it.
Mosfet Tesla Switch.
...

I used 2, 12 volt 1 amp halogens. Each one running a different direction. This schematic could be used to drive the transformer as RB did, or you could rectify the load with 4 more switch's as I outlined in post #3091.

You don't need any resistance on the gates, nor do you need resistance when using a BJT. In both cases they just lead to heat and low potential produced in the transistor. This is something I tested just recently. Feel free to test it for yourself. I don't know if this is correct I just noticed more heat when using resistance on the base or gate, so I left it out.

The biggest problem I had and the only reason I didn't pursue it was when I started using an inductive load the serial switch's burnt up. I added a fast diode from the ground back to the hot (Picture the arrow for diode pointing toward the hot side of the serial connection). And that work fine but then Positive side switch's burnt and when I added protection at that point I had to slow switch's down to get the load to respond and lots of heat showed up the in the Positive side switch's.

It was all done in a bread board.

I spent a little time looking into better ways to drive them but its all above my head at the moment. So she just got set off to the side as I went back to mosfet driven SSR's. Which I am looking into building myself. So this circuit may pan out to applicable if driving them gets a little easier or robust or something...
No doubt we'll have to find a better mosfet, that doesn't burn up so easy.

Matt

OK, so you used the Brandt configuration which
was designed to work into a split primary transformer.

I like the transformed AC output idea also.

Yes, there is no need for series resistors in the Gate
drive circuit for low frequency operation. They're
only useful for very high frequency switching
where the Gate current can become
very substantial. The series resistance will dissipate
some of the 'heat' that might otherwise build up
in the Gate Driver chip to protect it. Performance
is sacrificed in favor of reliability.

The Brandt configuration has way too many diodes
for my taste. I fully understand why he used them
at the time he built his circuit with Germanium
devices; but they're really unnecessary and add
to the power losses.

When driving an inductive load there will always be
the possibility of intense flyback pulses which may
hazard the MosFets. Once they go into 'avalanche'
it takes some time for them to recover; if the
flyback pulses are coming at them fast and furious
they'll stay in avalanche and will cook themselves
to death.

The flyback energy must either be absorbed by a
transient absorber protective device or diverted
into energy re-capture capacitors via diodes for
some useful purpose. Or both.

MosFets that are 'burning up' may not be getting
sufficient Gate Drive. The gate must be charged
to at least 10 Volts to achieve minimum Rds(on).
There are Logic Level MosFets which will work well
with 5 Volts of gate drive, but even those work best
with 10 Volts to 15 Volts.

For most applications the MosFet Driver Chip is
the only way to assure that the Gate is driven
adequately to assure minimal losses and heating.

When a MosFet is driven fully it will switch heavy
current with very little heat development. Many
times it is not even necessary to use a heat sink.

Low Voltage MosFets are becoming available that
work with 2 Volts of gate drive for use in single
cell switching boost converters but they're not
able to handle the really heavy current.

[SeaMonkey]

I would not be too dismissive to that design?
Once I'm done with PV switchers I'm coming to Brandt types, and it will be MOSFET or bust

...

3. The CP (charge pump) layout seems to be lacking the "kick" my booster has:
Since it does not insulate the capacitors the instant it commences the "dump" Que, they seem to kick both sides: a part of the kick is "back fired" and thus lost as far as I can tell? Is this really the case or am I missing something?

And i did have a excellent kick (+30A peak) with the 7BJT version (=more losses) but now with the 2 parallel BJT version I have it not?

Circuit symmetry? Can symmetry pay more toll than wire gauge?

Best regards,
Stevan C.

Can you show us this particular circuit Stevan?

If it isn't living up to your expectations we need
to analyze why. Somehow, we must find the
answer!

What are the sizes of your capacitors?

[Matthew Jones]

No, for the reasons because I can't make a transistor from scratch and when working with kilowatts, solid state valves are very limited and very expensive.

Unless I work with large triodes, I am stuck with mechanical switching. Commutators are faster than relays and have less "chatter".

Unless your planning on using Mercury Brush's or 3 inch cubes of carbon your gonna have few problems with commutator also at kilowatts. Trust me they can make alot of heat. You have to consider not only friction but current as well.

If you haven't built one yet I would build one to smaller scale first so you can realize what kind of issues you gonna have.

Matt