totempole square wave driver

evostars

totempole square wave driver
«  »Last edited
to amplify the square wave with current.

these powerfull squarewave pulses then get fed into a bifilarcoil (instead of the mosfet on the right).

the Bemf of these strong pulses can be directed in one direction by enforcing (confining) the magnetic field of one side of the coil by using a ferrite layer smeared on one side of the coil.

the bemf is the collapsing magnetic field (from the powerfull squarewave pulses).
the collapsing magnetic field transformes into a powerfull dielectric pulse.

the bemf dielectric pulse is directed into the not ferrite side of the coil.

the ferrite layer also has the function of increasing the inductance, and thereby lowering the resonant frequency of the coil.
Posted: June 13th, 2017, 05:46 AM

the totempole also has the benefit of being able to drive a low impedance load (bifilarcoil).

as i understand both pnp and npn are on together and off together. working together to pull up the voltage and pull down the voltage.

a little different than i had in mind. I thought one transistor for up an one transistor for down alternating. but then the input would need to be inverted. this pnp npn combination seems better.

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chuff1

Re: totempole square wave driver
« Reply #1,  »Last edited
The reason you would need the totem pole driver circuit is because some mosfets have a higher bias
voltageon the gate to allow them to fully turn on.  The drive coming from say a 555 timer at 5v is not
enough for some Mosfets so you would need such a circuit to fully turn it on and off.  These Mosfets are designed for logic level drivers which would eliminate the need for a totem pole circuit.
In addition Mosfets have built in Body diodes that protect the Mosfets from BEMF.

evostars

Re: totempole square wave driver
« Reply #2,  »
thanks chuff1
indeed diodes for bemf are very important. BUT
i need that bemf in another place so being burned through the diodes is not good.
the bemf of a bifilar coil is a little differnt.
so in this case. NO protection diodes.

In first case i would leave out the mosfet.
and only use the totempole to give the square wave enough power. I need powerfull squarewaves. at high frequencies.

but again... my electronic design skills aren't that good...

Matt Watts

Re: totempole square wave driver
« Reply #3,  »Last edited
Quote from evostars on June 13th, 2017, 05:48 AM
as i understand both pnp and npn are on together and off together. working together to pull up the voltage and pull down the voltage.

a little different than i had in mind. I thought one transistor for up an one transistor for down alternating. but then the input would need to be inverted. this pnp npn combination seems better.
Not quite Evo.  One transistor is PNP; the other NPN.  So the signal goes high triggering the NPN and goes low trigging the PNP.  But yes, they do in essence work together--when one is off, the other is on.  This is your typical "strong drive" output for most all digital circuits.  A matched set of these two transistors have a characteristic where they don't step on each other--the rise and fall times are matched.


There's also another technique known as the full bridge driver (or H-Bridge) which is commonly used for motors.  The full bridge driver has two half bridge sections, four transistors in all, that allow the output to swing completely positive or negative from rail-to-rail.  These full bridge drivers allow you to connect a non-center-tapped coil and drive it as though it where center-tapped.



Another benefit of the H-Bridge (with a driver chip like above) is that you can use all the same type of MOSFET or IGBT transistors.  The driver chip has level sense capability (AHS & BHS pins) so the transistors are referenced correctly when gated.  How this will behave with a strong back EMF though, I'm not certain.  I'm hoping the back EMF we can collect external to the coil.  If the back EMF instead chooses to shoot back down the input leads, I think the driver will be badly interrupted, causing the transistors to turn on when they are not supposed to.  When that happens and the transistors fight each other, they blow up.

evostars

Re: totempole square wave driver
« Reply #4,  »
that h bridge i know. looks solid. maybe thats what nelson used in that video with the 60w 230Vdc motor

but it needs a driver, I wonder i we can get it to work at high enough frequencies. minimum 20khz for the skin effect i believe, but much higher probably for the bifilarcoil with ferrite

Matt Watts

Re: totempole square wave driver
« Reply #5,  »Last edited
If we have to, we can make two half bridge circuits that are fast enough and pair them together with some logic to make sure one half bridge  is operating exactly opposite of the other half bridge.  I'm pretty sure we can reach clear out to 1MHz with this method.

I'll keep looking for a full bridge driver chip that is fast enough to do the whole thing with only it, four power MOSFETs and a few other components.  I like simple, so I'll look for simple and see what I can come up with.  If I can't find anything, I still know how to do this old school with some fast optical couplers and DC2DC converters.    :-)


evostars

Re: totempole square wave driver
« Reply #6,  »
:thumbsup2:

im personally also fan of IGBT s
instead of mosfets

Matt Watts

Re: totempole square wave driver
« Reply #7,  »
This one actually looks pretty promising:
http://www.ti.com/lit/ds/symlink/lm5046.pdf

Has its own oscillator too.  Specs show it's capable up to 2MHz.
Re: totempole square wave driver
« Reply #8,  »Last edited
Okay Evo.  I went outside and played with the dog for an hour.  Came in and tried to cool off but the house started warming up fast.  Hairball and I took a nap...

When I woke up, I felt as though I know exactly what we need:

A simple half bridge driver, pulsed at exactly 25% or slightly less duty cycle right on the resonant frequency of the side-by-side pancake coil with one side coated in a ferrite epoxy resin.  This is what I think I'll shoot for on the first test cycle of this concept.

I tossed around one other slight concept modification and couldn't make a decision:  If I use a ferrite epoxy resin, I can make a form/mold on my CNC machine with machineable wax.  I can do this with or without a center hole, pour in the resin and set the finished side-by-side coil on top with a couple of offsets so the coil doesn't sink down in.  Once the resin hardens, I can heat the wax to about 250 degrees for 30 minutes or so and resin flat core will pop right out attached to the pancake coil.

My question is:  Should I cut a center hole in the wax mold or not?

I know Nelson just painted on the resin and didn't have much choice but to leave a hole.  With my method of using a mold, I can do any thickness and can have a hole or not.  The goal is two fold:  Lower the resonant frequency of the coils AND create a magnetic back plate for the vortex ring to push off from.  I'm certain I can do that now, at least good enough to prove the concept.



I also noticed in that LM5046 PDF they show how to boost the gate current up from the normal output of the half bridge driver using (guess what), a totem pole transistor driver.  This is super good because we can use really high current IGBT/MOSFETs and a very simple IR2184 half bridge driver that I can easily connect to my signal generator to find the resonant frequency and set the duty cycle.

I see a few obstacles getting cleared out of the way...

Need to find some powered ferrite to mix with some epoxy resin and I'll be all set.


evostars

Re: totempole square wave driver
« Reply #9,  »
hairball. great name!

use the right ferrite, that is able to work with those high frequencies high currents. the lighter grey stuff.
you could also re-use some ferrite, and grind it to a fine powder and re-use it with epoxy resin.

i am wondering if we would need to cover both sides of the coil. with a speaker wire coil. else we again get a disbalance between the two windings. as they are 2 separate coils.

if we use ferrite on just one side, it must be a tesla patent bifilar coil design with the windings next to eachother. to keep it balanced (ferrite influences both windings)

the hole... I think we need the hole open. (but as an experiment you could close it and open it later).
Re: totempole square wave driver
« Reply #10,  »
the hole is needed for the ring vortex to form.
it might be that the hole set of coils forms one big ring vortex around them all
Re: totempole square wave driver
« Reply #11,  »Last edited
or even a double ring vortex

red the magnetic power pulse coil

dark blue the center coil with dielectric spacing

green the output coil

light blue the double ring vortex around all coils.

maybe?
experiments will show it.

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