New HF Driver Idea

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Liam David
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New HF Driver Idea

Post by Liam David » Sat Oct 25, 2014 3:48 pm

I watched a video a couple days ago about H-bridges and how they can switch the direction of a motor's rotation. I got the idea that I could build a DC to AC converter, but possibly drive it up to high frequencies using a timer or oscillator. Here is my first circuit schematic.
Image

The resistors in the 555 timer part would be determined by the formula above the schematic. What would be a good target frequency? 10kHz, 100kHz?
Would this work, or would I end up blowing everything out?

-Liam David

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Werner Engel
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Re: New HF Driver Idea

Post by Werner Engel » Sat Oct 25, 2014 4:07 pm

The frequency you should use depends on a lot of parameters.
There is the inductivity, the material of the core, the load, the geometric of the core (air gap), ...

I found out it's quite difficult to build a HV transformer.
Try to vary the frequency and measure the voltage.
Working on it - see the picture.

BR,
Werner
HF trafo test.jpg
my ferrit core with air gap of about 6mm (thanks to Philipp for helping me winding the wires)

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Liam David
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Re: New HF Driver Idea

Post by Liam David » Sat Oct 25, 2014 4:16 pm

The transformer I'm trying to build this for is one I bought from Andrew Robinson.
Image

The core is 2 separate pieces and about 1 inch square. You might be able to see a small bump on the left side of the core where it separates. The gap between the coils and the core is as thick as the plastic case, so about 5mm.
I guess I'd better find some core saturation and induction equations.

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Werner Engel
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Re: New HF Driver Idea

Post by Werner Engel » Sat Oct 25, 2014 4:23 pm

And what is the core made of?
I used the following: http://uk.rs-online.com/web/p/transform ... s/0180834/

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Chris Bradley
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Re: New HF Driver Idea

Post by Chris Bradley » Sat Oct 25, 2014 5:11 pm

You are re-inventing what does not need re-inventing if you do not use FET drivers. Your 'NPN' transistor arrangement looks poor for a variety of reasons that you can find in most good texts on the subjects. Save your efforts and buy high current (for maximum efficiency) non-inverting (to avoid feedback) gate drivers.

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Bob Reite
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Re: New HF Driver Idea

Post by Bob Reite » Sat Oct 25, 2014 7:30 pm

You have to be careful that the two sides of the bridge do not turn on at the same time. That's how MOSFETs get blown out. I would duplicate Steve Ward's "High Votage Capacitor Charging PSU" circuit shown at http://www.stevehv.4hv.org/ccps1.htm
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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Liam David
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Re: New HF Driver Idea

Post by Liam David » Sat Oct 25, 2014 8:51 pm

That's why I (tried) to add a NOT gate for one of the bridge sides, to ensure that I don't get a short. Only one side is on at a time. It's the part of the circuit with the transistor labeled NPN. I'll look into Steve's design. Thanks for the lead.

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Bob Reite
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Re: New HF Driver Idea

Post by Bob Reite » Sun Oct 26, 2014 12:01 am

You need to have a few microseconds delay between the turn off of one side and the turn on of the other.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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Liam David
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Re: New HF Driver Idea

Post by Liam David » Sun Oct 26, 2014 12:13 am

I have seen that there are a couple of microseconds of shoot-through every time the circuit switches, and with 120v, this might be an issue. I'll see how I can build in a delay between the switches.

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Rich Feldman
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Re: New HF Driver Idea

Post by Rich Feldman » Sun Oct 26, 2014 1:57 am

Hi Liam.
The core in your transformer picture has rounded corners, a style I'm used to seeing in tape-wound laminated steel cores.
To follow Werner Engel's question, are you sure it's ferrite? The magnetic properties and the efficient frequency range are very different for steel.

Here is one of many reviews of core material categories.
http://www.ti.com/lit/ml/slup124/slup124.pdf

Either way: since you can take the core apart, I suggest that you put it together
without the high voltage windings, while you use it in experiments for learning.
You can make your own experimental windings on folded cardboard bobbins.
The electrical behavior of a primary winding
with an unloaded (open circuit) secondary
is practically the same as that with _no_ secondary.
All models are wrong; some models are useful. -- George Box

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