Ion Nitriding Project... Electronicaly inept I need help....

Hello,

First a small introduction about myself, I'm a french 40 years old, recently laid of, I used to work in the banking industry in computer science (not one of those how got those fat bonuses, so I'm theoretically one of the good guys....)

I'm reconverting in engineering (a longtime passion of mine) and one of the projects I'm working on requires a nitriding process, since I already have some prior experience in vacuum technology and since I want to keep this process on the environmentally and staff friendly side (ie no ammonia gas or cyanide salts...) I'm going for the go for the ion nitriding hot wall process.

Basically this process consist of heating up a workpiece in a near vacuum (2 to 10 Torr, you can gets this low with only a primary pump like a double stage vane pump, no turbo molecular or diffusion pump needed) in a trace hydrogen and nitrogen atmosphere and establishing a potential between the workpiece (the cathode) and the enclosure (the anode) of 500 to 1000 V DC, this ionizes the nitrogen and propels those ions towards the workpiece where nitrogen diffusion occurs (it's actually a bit more complex, depends on the alloys etc etc...) the big advantage of this process is that it is done at a "low" temperature (anything from 350°C to 650°C depending on the alloy and that this temperature is bellow the phase change temperature of the alloy thus no phase change thus very little distortion, thus little post machining required (which is all for the best since the nitride layer can go up to 78 HRC)

Sorry for the long explanation, I just feel it's only fair to let people understand the big picture.

So her's what I need help with:

I need to build a square wave generator (low = 0V high = 500V to 1000V) that ennables me to vary the frequency (from 1KHz to about 25KHz) and the duty cycle (from 2.5% to 85%) thet current requirement is relatively low, you need about 3 A/m² of workpiece, in my case the workpiece will be .5m² max so 1.5 A should be enought.

I have very limited knowledge in electronics, did a little work in university on binary circuits and such but nothing quite so specific.

I suppose that the solution would break down into 4 parts:

1 - The pulse generator :
555 timer based ?
or maybe better with a digital circuit ? Like for instance a clock running at max frequency feeding a counters that outputs high while count (mod max freq) is below Time Up, and then low while (mod max freq) is above Time Up ?

2 - a driver circuit to tie the pulse generator into a power circuit

3 - a power circuit that takes the pulse signal gets feed high voltage and outputs the resulting wave (IGBT i guess i think a APT 11GF120KR is able to handle 1200V and could probably deal with 25kHz

4- a variable voltage supply to feed the power circuit.


Now I can probably remember enough of my classes to build the pulse generator (I already simulated a 555 circuit for the less than 50% duty cycle and one for the more than 50% duty cycle) but the rest is driving me nuts....

So is this the proper way to start ?
Do any of you have any suggestions ?

Almost forgot to mention.... It would be apreciated if one could stand on the ground and touche the enclosure ie anode set to 0V = GND and Cathode set to - high voltage.

Regards,

Why not simply use a a flyback transformer?

It can be connected directly to a MOSFET driven by the 555.

The only problem is you'll need to wind your own transformer.

Hero999 said:

The only problem is you'll need to wind your own transformer.

Click to expand...

I thought we have variable transformers? Just me piping, uncle Hero.

Hero999 said:

Why not simply use a a flyback transformer?

It can be connected directly to a MOSFET driven by the 555.

The only problem is you'll need to wind your own transformer.

Click to expand...

Ok If I understand the difference between MOSFET and IGBT (in laymans terms that is) :
- MOSFET = low voltage, high current, high switching speed
- IGBT = high voltage, high current, but "low" "switch off" time.

So the solution you are proposing is to build a low voltage flyback driver (say 50V DC @ 60A) and to get the high voltage (600 to 1000 V DC à 3A max) from a transformer.

The thing is I really need for the wave form to have near "vertical" transition between high a 0 voltage, the reason is :
- going from high to low, you need a clean cut off to snub off any arc formation and
- going from low to high you need to quickly apply the volts to get the plasma to form (this part is less critical though)

If I run a square wave through a transformer, especially if I'm running on a setting with a rather high duty cycle (say 75%) and low frequency (say 1kHz), wont the output of the (less than ideal transformer despite my impressive transformer winding abilities (actually other than a few 3 phase motors and some tesla coils when I was a kid it's been a while since I did something like this....)) have spikes and distort the wave form ?

My understanding is that the output potential difference arises from a variation of the magnetic flux, so with such an "uneventful wave form stay up 75% of it's time...) wont the output only exhibit a voltage potential during high to low transition and vice versa ?


Is it feasible to build a transformer that accepts such a wide operating range ? (again duty cycle needs to be able to vary from 2.5% to 85% and frequency from 1KHz to maximum 25KHz)

Actually the frequency might sometimes go as low as 500Hz but rarely needs to go beyond 15KHz, the 25KHz setting is for nitriding parts with very small thickness and small holes that require operating at higher pressures (in the 10Torr) region for better plasma penetration, so if the wide frequency range is too much of a problem it could probably be reduced.

Also the range probably doesn't need to be completely adjustable, having a series (say 20 or so) of preset settings for each parameter should be fine.

And finally, the setting are adjusted for each load prior to running the device but once it's running adjustment are fine tuning, you'll never run into a scenario where the device need the full frequency range for a single process.

PS Funy, most post are people trying to go from DC to a square wave to a nice sin wave to build inverters.... and here I am trying to go from my nice clean 3 phase mains to a blocky square wave... can't possibly be so hard to do can it ?