Author Topic: GDT Experiments  (Read 1176 times)

Offline Thunderstruck

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GDT Experiments
« on: March 16, 2019, 06:32:36 AM »
While I'm waiting for some parts to arrive I decided to try and master the dark art of GDT winding. Got some advice on a correct core selection, watched a few videos on the topic and decided to have a go at it.
Well, I wound 5 different cores, and I did not get what I wanted.
I used cores with high AL, Cat5 Network Cable Wire and on one occasion Magnet Wire and got very similar results every time.
I am trying to achieve a good waveform at about 65 - 70kHz

Image 1 - Core specification ( AL5400, T35, 27mm OD )
Image 2 - First attempt
Image 3 - Waveform achieved
Image 4 - Richie Burnett's advice on corrective action for that waveform
Image 5 - He said more  turns  - More turns it is then  ;D
Image 6 - Resulting waveform - Still sloping, and as you can see there is no more room for wire turns

Other attempts had pretty much the same results

I am quite sure that I am not doing something right here - or RS Components sent me wrong cores.




Online Mads Barnkob

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Re: GDT Experiments
« Reply #1 on: March 20, 2019, 01:06:24 PM »
You did also choose cores with almost identical properties, its not even a 1000 permeability in difference, so you will only see the differences from number of turns and how tight coupled the wires are to eachother/the core.

What kind of test jig did you use? A driver IC --> GDT --> MOSFET gates?

Did you see this practical design guide / experiment? http://thedatastream.4hv.org/gdt_practical.htm
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline T3sl4co1l

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Re: GDT Experiments
« Reply #2 on: March 20, 2019, 01:18:29 PM »
The one thing missing from Burnett's advice: reduce the driver impedance.

Was that testing with a function generator (50 ohms)?  Looks about right for it.

Tim

Offline Thunderstruck

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Re: GDT Experiments
« Reply #3 on: March 20, 2019, 09:07:04 PM »
You did also choose cores with almost identical properties, its not even a 1000 permeability in difference, so you will only see the differences from number of turns and how tight coupled the wires are to eachother/the core.

What kind of test jig did you use? A driver IC --> GDT --> MOSFET gates?

Did you see this practical design guide / experiment? http://thedatastream.4hv.org/gdt_practical.htm

Mads, in both cases it was the same core. I did say 5 different cores in my original post, what I wanted to say is 5 different ways ( twisted, straight, trifillar, etc. ). I was expecting to see at least a half decent waveform because it is a high permeability core, especially after watching a few videos.
Test signal was supplied from the FG directly, i do not have a driver circuit to test the GDT with.

I suspected that my FG is not really capable of driving the GDT...
I read that guide a few times, only thing missing is screenshots of waveforms after each attempt.

The one thing missing from Burnett's advice: reduce the driver impedance.

Was that testing with a function generator (50 ohms)?  Looks about right for it.

Tim

Tim, yes I tested the transformer with my FG which is a little bit ancient  :)
« Last Edit: March 20, 2019, 09:14:34 PM by Thunderstruck »

Offline Teravolt

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Re: GDT Experiments
« Reply #4 on: March 20, 2019, 11:15:50 PM »
I dont know if this will help but thare a cuple of articals

http://ferroxcube.home.pl/news/gate%20drive%20trafo.pdf

https://www.coilcraft.com/edu/gate_drive_transformer.cfm

http://seansoleyman.com/solid-state-tesla-coil/

your outputs have good rise time but it looks like they are saturating because the waveform is saging after 7us. usaly your signal generator has a 50 ohm inpedance and may not be able to drive it enough. build a gate driver circuit on a proto board using a mosfet diriving ic or the output of a universal driver. a biger core might help and or using more stacked cores if you had some

Offline Uspring

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Re: GDT Experiments
« Reply #5 on: March 21, 2019, 11:57:53 AM »
If you apply a voltage step to an unloaded (and perfect) transformer, the primary will look like an inductance. It will then respond with a linear current rise, which will induce the desired voltage step in the secondary. In a non perfect situation, you will have a primary winding resistance and also a voltage source, which might sag under the loading current. Both will cause a decrease of the linear current slope and then to a drop of secondary voltage. If the core saturates, the primary current will shoot up, causing more voltage loss without increasing the magnetic field, which would keep the output voltage stable. So this will aggravate the resistance effects.

So first, you have to make sure, that the applied primary voltage doesn't drop. Then you have to check, whether the magnetizing current, i.e. the current supplied to the unloaded transformer isn't excessively large. If so, increase the number of turns or look for a higher u core. A too large magnetizing current can also be caused by a saturating core. Also keep in mind, that the above mentioned resistive losses will increase as you load the secondary.




Offline Thunderstruck

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Re: GDT Experiments
« Reply #6 on: March 25, 2019, 12:56:36 PM »
Thank you for all the comments, a lot of useful information there. It is definitely not a case of just winding some wire on a core.

Clinical definition of insanity is tepeating the same thing expecting a differnet outcome. :o

So I decided to have another attempt  ;D

This time I got 3 different cores:

125-3320 Al 13500 T38 
212-0982 Al 5750   N30
212-0976 Al 7000   N30

I managed to wind 11 turns through the T38 core and got exactly the same result as before.
Logical conclusion is that the way I’m driving the GDT’s is inadequate ( somebody already mentioned this )
I will have to build a test rig, fortunately I got a few PCB’s for profdc9’s SSTC, I reckon that will do just fine.

Strangely enough, yesterday I wound a GDT ( not really caring about how it will perform ) just to test my UD board and got a perfect waveform, go figure....




Offline Thunderstruck

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Re: GDT Experiments
« Reply #7 on: March 27, 2019, 12:46:44 PM »
I built Profdc9's Half Bridge SSTC driver and connected the GDT's I built. This time I'm getting half decent results, waveform actually looks ok.
As you are probably aware, this is all quite new to me, so I am quite excited achieving things that many of you achieved years ago and consider them ordinary. But, it's all about fun, right ?

Screenshots below show output of two different GDT's I made.

First image shows waveform from a GDT I previously tried running from a FG only with terrible results
Second image is a waveform from a GDT made with 11 turns of CAT5 cable - all four pairs of wires. Signal from 2 only shown.


That waveform confused me a bit until I realized that there is "deadtime" in the signal. Only recently I watched a video on YouTube on that topic - quite glad I recognized it in the waveform.  ;D






Offline Teravolt

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Re: GDT Experiments
« Reply #8 on: March 27, 2019, 05:21:43 PM »
Yep thats it, nice work, do you have a link for Profdc9 scematic and witch core did you use if I may ask. If you add a little load resistance in paralell with your transformer output you might be able to get rid of those spikes at each transition

Offline Thunderstruck

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Re: GDT Experiments
« Reply #9 on: March 27, 2019, 08:59:31 PM »
Yep thats it, nice work, do you have a link for Profdc9 scematic and witch core did you use if I may ask. If you add a little load resistance in paralell with your transformer output you might be able to get rid of those spikes at each transition

You can find Prof’s schematic, and a lot of other useful stuff here:
https://github.com/profdc9/DRSSTC-PCB-Pack
It is under Half Bridge SSTC

Core i used in the first screenshot is EPCOS, Al 5400 T35 material, 27mm OD ( ball of wire on the photo below  ;D )

Second core is also EPCOS, Al 13500 T38 36mm OD ( connected to the driver on the photo below )

« Last Edit: March 27, 2019, 09:03:58 PM by Thunderstruck »

Online dexter

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Re: GDT Experiments
« Reply #10 on: March 27, 2019, 10:52:46 PM »
That waveform confused me a bit until I realized that there is "deadtime" in the signal. Only recently I watched a video on YouTube on that topic - quite glad I recognized it in the waveform.  ;D

You have about 1.8us of deadtime in both shoots and i can't figure it out how...
The Half Bridge SSTC schematic doesn't have any built in deadtime and the TC4420 have delay times less than 100ns and matched Rise and Fall times of 25ns or are you using different IC's?

Offline Thunderstruck

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Re: GDT Experiments
« Reply #11 on: March 27, 2019, 11:21:29 PM »
That waveform confused me a bit until I realized that there is "deadtime" in the signal. Only recently I watched a video on YouTube on that topic - quite glad I recognized it in the waveform.  ;D

You have about 1.8us of deadtime in both shoots and i can't figure it out how...
The Half Bridge SSTC schematic doesn't have any built in deadtime and the TC4420 have delay times less than 100ns and matched Rise and Fall times of 25ns or are you using different IC's?

I am using UCC37322 instead of TC4420, so that might be the reason.
I also wondered what causes the dead time ( well to be honest I would not know what does it even if there was a dedicated component )
What controls the dead time was going to be my next question.

I already had spare UCC’s, so I did not want to spend more on TC’s since they are interchangeable in this case.


Online dexter

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Re: GDT Experiments
« Reply #12 on: March 28, 2019, 12:37:42 AM »

I am using UCC37322 instead of TC4420, so that might be the reason.
I also wondered what causes the dead time ( well to be honest I would not know what does it even if there was a dedicated component )
What controls the dead time was going to be my next question.


UCC37322 is faster than TC4420...
Given that you got the same deadtime in both cases at different frequencies i'd say the propagation delay from an extra schmitt inverter on one of the UCC's is what produces the deadtime.
What is the part number of U6 (from schematic) that you are using?

Offline Thunderstruck

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Re: GDT Experiments
« Reply #13 on: March 28, 2019, 12:57:32 AM »
Part number from Digikey is 296-3503-5-ND, CD40106BE.
From what I gather, deadtime is not a bad thing, right ?  Gives time to the devices to switch off and on completely without running into each other ( going linear )

Offline profdc9

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Re: GDT Experiments
« Reply #14 on: March 28, 2019, 04:36:20 AM »
I don't think it should be a problem.  The body diode of the one MOSFETs should go into conduction after the other MOSFET is turned off.   It is a longer dead time than I have observed but it should probably be ok.

Note that the SSTC should be driven using the antenna feedback.  The built-in oscillator is mostly to "jump start" the oscillation of the Tesla coil.



Offline Uspring

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Re: GDT Experiments
« Reply #15 on: March 28, 2019, 11:11:10 AM »
It might be a problem, that the middle voltage in your square wave can cause a situation in which the FET is not completely blocking. That will happen for both FETs at the same time.

Offline Teravolt

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Re: GDT Experiments
« Reply #16 on: March 28, 2019, 05:24:20 PM »
your UCC37322 may have built in deadtimet to prevent lachup but I am surprised to see that it makes it through the core. 1.8us is a bit much but I don't think it is an issue. check it with your scope if you have dead time going into the transformer. If you were using bricks 1.8us would be fine. If you have fead back is it a SSTC or a DRSSTC

Offline profdc9

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Re: GDT Experiments
« Reply #17 on: March 29, 2019, 01:22:05 AM »
I looked on the output of my board.  I get a 880 ns dead time.  I am not sure why you are getting twice as much.  Even this is kind of high for the switching time of the CD40106 with 15 volt supply.  I should probably have a board revision and use a CD4027 and use both the Q and not Q outputs which would change at the same time.  This would also ensure 50% duty cycle, but would add an extra part.

I don't think it's a problem to have zero voltage in the gate drive transformer.  The overcurrent shutoff in fact does exactly this, where it forces both gate drivers low to turn off the MOSFETs. 

I have a plot of the gate drive signal when the antenna port is drive by 200 kHz with a signal generator.  The dead time is long which might lower power output however.



Offline profdc9

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Re: GDT Experiments
« Reply #18 on: March 29, 2019, 02:23:28 AM »
I have a solution to greatly reduce the dead time.

Replace D3 and D4 with 1N4148.   They are 1N5819 in the design.  I will update the schematic.

It should work much better, here's a screenshot.  The dead time is now 320 ns.

Dan


Offline Thunderstruck

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Re: GDT Experiments
« Reply #19 on: March 29, 2019, 05:51:11 AM »
Well ! This certainly took a different turn, I wanted to make a test circuit for GDT’s, and it resulted in a circuit change !

Dan, thank you very much for your help with this. More than likely I will use this driver in a TC build in the future so thanks for troubleshooting and making changes.

One thing that I am not clear with is jumper settings, so far I figured out AC/DC input jumper, but others are a bit harder to figure out from the schematic ( for me at least ).

I’d like to do a test with FG connected to the antenna input next.

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Re: GDT Experiments
« Reply #19 on: March 29, 2019, 05:51:11 AM »

 


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