New HF Driver Idea

[Liam David]

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.


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

[Werner Engel]

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

my ferrit core with air gap of about 6mm (thanks to Philipp for helping me winding the wires)

[Liam David]

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


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.

[Werner Engel]

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

[Chris Bradley]

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.

[Bob Reite]

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

[Liam David]

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.

[Bob Reite]

You need to have a few microseconds delay between the turn off of one side and the turn on of the other.

[Liam David]

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.

[Rich Feldman]

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.