Zipdox:
Sounds cool. Let us know how it goes, if you get something working

Mads:
The MMC is 2 series 2 parallel 0.1uF/2kv 942c capacitors (942C20P1K-F), so 0.1uF effective capacitance. I have considered reducing it by perhaps adding another set in series, for 3 series 2 parallel giving 0.067uF effective, and also making a new secondary with 1000 turns and a larger toroid, but I'm happy enough with performance at the moment anyway, so I haven't got around to doing it.
The resonant frequency ended up been somewhere close to 160Khz. The primary current figure of 350A peak is going purely off the OCD setting, as I haven't actually been able to measure it directly at full power due to interference crashing my USB oscilloscope. The OCD was showing correct behaviour at 100A, so I presume it to also be correct at the full 350A (a somewhat dodgy assumption, I know).

Sorry that I haven't given any updates on my DRSSTC for a long time (more than a year, as I see now...),
but here is one final post, just to show the completed coil and modifications made since my last post.

After the IGBT failure, I decided to complete the coil with a fiber-optic midi interrupter, as I suspect that the failure was caused due to the Arduino used as a make-shift interrupter crashing. I also replaced 30v TVS that was used to limit voltage on the output of the feedback CT with the Zener diode/Schottky diode configuration used in the UD1.3B, in order to reduce the possibility of phase shift due to transformer saturation.

This is the layout of the driver circuitry, in its final housing (including LV PSU and fiber-optic/Indicator PCBs, which were not previously made):



This is The Midi Interrupter with its housing open:



And Finally, The Coil with the primary secondary installed:



In addition to this, I found out why Steve Wards UD1.3 includes a RC timeout circuit for the feedback signal. If it is omitted (as it was in my original design), the circuit will sometimes start up in a state in which the interrupter cannot start an oscillation. The same thing could occur if the coil lost feedback due to no voltage on the bus etc. As a result, I made the following modification to my driver circuit:



As a result of all these modifications, the coil now operates reliably with 350A peak primary current, and produces sparks up to ~60cm. I am not entirely sure why longer sparks cannot be achieved anymore, but I suspect it is due to the faster current ring-up time and lower on-times that have resulted from slightly less phase shift in the feedback CT. The OCD triggers constantly if I increase on-time past ~40uS at 240VAC input, despite careful tuning. This behaviour may be due to the low impedance of the primary coil, and high impedance of secondary coil (~2000 turns, rather than the generally recommended 1000).

Here is a video, showing the coil operating in midi mode:
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Thanks again to everybody on the forum who gave advice throughout the project 

Condensor Corps website shows how to check what type of dielectric the capacitor is using:http://www.condenser.com/projects/condenser/

It is a Kraft/Polyester not polypropylene, so it's probably not suitable.

You could also use the standard D Flip Flop without enable (instead of the D flip flop w/ enable): https://www.cypress.com/documentation/component-datasheets/d-flip-flop
It can be configured to have both PRE and reset (CLR) inputs (see datasheet linked).

Just note that you'll have to add inverters to the PRE and CLR inputs, to make it logically identical to the 74hc74 (which has the PRE and CLR inputs inverted).

Hi VNTC,

I'm not sure as to the exact reason that you're having failures (I'm fairly new to DRSSTCs myself), however I do notice a couple of things. It looks like you're using TO247 package FGH60N60SMD, and FGA60N65SMD igbts (am I correct?). These have a failure current of ~350A, opposed to the minibrick 60N60s which can typically take over 500A (what you seem to be trying to do).

I also agree with you that your gate waveform doesn't look great. Are you running half-bridge or full bridge? If you're running full bridge, it may be worth trying doubling the UCCs (stack two ICs on top of each other), to increase drive current.

Hope this helps (and sorry I can't help more),
-sjsimmo 

Hi FilipŠebík,

I came across this website a while ago: http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.html when I was trying to design an smps lab power supply (I never got round to actually finalizing the design or building it though...).
It contains calculators which may be helpful with working out the suitability of your core, and required wire thickness. 
I'm sorry I can't help with doing the calculations though, I don't understand much about the mathematics myself...

Hope this helps,
- sjsimmo

Hi nabzim,


TVS diodes are recommended, because of voltage transients on the bus which are hard to avoid without incredibly good bus layout design. They are different to the internal diodes, and are generally required as a safety feature. Gao probably got away without using them due to a good bus layout.


The snubber capacitors only need to have as high a rating as your bus voltage. The most important thing for snubbers is their peak current rating. If you are using less than 400v on the bus, than 600v snubbers should be suitable.


I'm not sure about the frequency drop, but I'd assume it to be similar. If you use copper pipe for you primary, then it's easy to tune the primary with a tapping point. just make sure that you've got an extra turn or two worth of primary so that you have some tolerance for tuning.

Yeah, I've seen that page. It's a really good resource 


I've previously tried reducing the gate resistance from 6.8R to 3.3R with no effect (it's still using the 3.3R). This resistance change was before I doubled up the UCCs which had dramatic improvements, resulting in the waveform in my previous post. Previously it was more of a sawtooth wave. I'd think that needing less than 3.3R would be very unusual, considering I'm using a TO-247 IGBT full-bridge.
One thought that occured to me is that maybe I need to run 3 UCCs in parallel, but this also seems a bit ridiculous, given the IGBTs that I'm using.

On a side note, the biggest spark so far is now 68cm to a grounded object with 300A peak current, 240VAC input and ~50uS on-time. (increasing on-time causes the OCD to fire constantly, may need tuning?)

I'm confused now, that is the datasheet I was using...


The calculated current at switch on is 11.6A, and switch off current is 22.7A. These values are both below the minimum current shown on the graph of figure 15. extending the graph gives a switching loss of about 0.25mJ and 0.4mJ at 80 degrees celsius respectively, or am I reading the graph wrong? 


These values can then be multiplied by 0.2, giving the final values that I wrote down.


The reason for using 1000BPS for the calculations, is because I want to midi modulate it eventually, and I figure that 1kHz should be a sensible limmit... (or is this actually too high?) . As for on-time, I might make that a bit longer, at 200A anything above 80uS has little effect on spark length, but I suppose at 350A this time will grow (although by how much, I'm not sure, since I can almost double bus voltage still.)

Thanks for the guidance

I just finished doing the calculations, and got the following values for my coil at 350A peak, 100uS on-time, 1000BPS:
- Fmax1=1290kHz
- Fmax2=984kHz
These values seem about 5 times bigger than what I was expecting. Am I doing something wrong, or are these IGBTs invincible?


Thanks again,
-sjsimmo