Author Topic: Schmitt trigger schematic question  (Read 175 times)

Offline Michelle_

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Schmitt trigger schematic question
« on: May 14, 2024, 09:45:38 PM »
So in the attached schematic I think both of the right facing triangle symbols are part of the same Schmitt inverter, the numbers are the pins; pin 2 is connected to pin 3. Is that right?

I'm assuming both of the triangles represent a channel of the Schmitt inverter, why is one channel going into another one then out? What's going on here exactly?

This is part of Steve Ward's SSTC5 schematic (https://www.stevehv.4hv.org/SSTC5/miniSSTCfnlsch.JPG) and the only part that I am not really following. I think the purpose of this is to convert the feedback from the antenna into a square wave.


Offline Benjamin Lockhart

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Re: Schmitt trigger schematic question
« Reply #1 on: May 14, 2024, 11:00:00 PM »
Yes that's correct. 2 of the inverters are being used so the signal is inverted then uninverted again. You can actually omit the inverters though because the UCC gate drive ICs have hysteresis on the inputs already. Also I prefer to use current transformer secondary base feedback for SSTCs.

Here is the schematic of my small half bridge SSTC which is similar but very simplified and works great.
The schematic shows a 12V regulator for the gate drive supply but you could use 15V as well it would probably be better. The single UCC27425 works fine as long as you're only driving a half bridge with it. For a full bridge that requires twice the drive current I would use the 2 separate UCC chips.
.

You can see the little coil here. It does about 6" sparks but the secondary is about 400kHz which is a bit high, putting the primary impedance higher than it would be with a lower frequency.

Offline Michelle_

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Re: Schmitt trigger schematic question
« Reply #2 on: May 14, 2024, 11:12:22 PM »
Yes that's correct. 2 of the inverters are being used so the signal is inverted then uninverted again. You can actually omit the inverters though because the UCC gate drive ICs have hysteresis on the inputs already. Also I prefer to use current transformer secondary base feedback for SSTCs.

Here is the schematic of my small half bridge SSTC which is similar but very simplified and works great.
The schematic shows a 12V regulator for the gate drive supply but you could use 15V as well it would probably be better. The single UCC27425 works fine as long as you're only driving a half bridge with it. For a full bridge that requires twice the drive current I would use the 2 separate UCC chips.


You can see the little coil here. It does about 6" sparks but the secondary is about 400kHz which is a bit high, putting the primary impedance higher than it would be with a lower frequency.
/>

Thanks that's very helpful! I wonder why he included the inverters then, just good practice?

You aren't the first person that's suggested secondary feedback over the antenna. Would you use the same kind of ferrite core as the gate driver? What kind of wire do you use to make 50 turns, magnet wire? And does the secondary just pass through the middle or does it need a loop? (sorry for all the question lol).

Thanks again for the answer by the way. I see what you mean about the single UCC as well.

Offline Benjamin Lockhart

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Re: Schmitt trigger schematic question
« Reply #3 on: May 15, 2024, 04:58:02 AM »
The inverters don't hurt but it works fine without them, and then you don't need the 5V rail to power just one inverter IC.

Yes the same kind of ferrite as the gate drive transformer is good for feedback current transformers. I used an N87 core for both, material 77 and 75 are also good.
You can use quite a small core for a CT and the secondary base wire passes through only once, no loops. Turns are counted by the number of times the wire passes through the core.

If you have some CAT5 cable you can use an individual wire from it, or just use any hookup wire. I would avoid magnet wire though, some people had problems with it arcing to the core.

The UCC27425 is an inverting and non inverting gate driver in one, but it's only rated for 4A instead of 9A like the UCC27322/UCC37321.

The questions are all good, when you're new to electronics things that are easy for many to take for granted are not always obvious.

Offline Michelle_

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Re: Schmitt trigger schematic question
« Reply #4 on: May 15, 2024, 03:32:57 PM »
The inverters don't hurt but it works fine without them, and then you don't need the 5V rail to power just one inverter IC.

Yes the same kind of ferrite as the gate drive transformer is good for feedback current transformers. I used an N87 core for both, material 77 and 75 are also good.
You can use quite a small core for a CT and the secondary base wire passes through only once, no loops. Turns are counted by the number of times the wire passes through the core.

If you have some CAT5 cable you can use an individual wire from it, or just use any hookup wire. I would avoid magnet wire though, some people had problems with it arcing to the core.

The UCC27425 is an inverting and non inverting gate driver in one, but it's only rated for 4A instead of 9A like the UCC27322/UCC37321.

The questions are all good, when you're new to electronics things that are easy for many to take for granted are not always obvious.

Thank you this is very helpful. You're right that many things aren't always obvious when you're new, even if the overall concept is understandable!

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Re: Schmitt trigger schematic question
« Reply #4 on: May 15, 2024, 03:32:57 PM »

 


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