First SSTC

[blahaj]

Hello all, I have been researching and I have found Steve Ward's SSTC 5 (Mini SSTC) schematic and I have several questions.
First, why is a 12v rail needed? The driver chips seem to be rated at a minimum of 4.5v, or is 5v not high enough for operation? The 555 timer also should work at 5v as per the datasheet. I have found out that it is merely the IGBTs should be driven at 12v and not 5v.
Second, I cannot find IRFP260s, just IRFP260MPBF. I assume the PBF means lead free, but what is the M suffix? They seem to have nearly identical specifications.
Third, a capacitor at "cap sub point" is needed, correct? I assume thats what the values in the table for differing sparks. Is a resistor at "resistor sub point" needed, even with the potentiometers?
Fourth, is a core material of T35 sufficient for a GDT?
Finally (whew a lot of questions.. I am pretty new to this), by polypropylene, that is a film capacitor, correct? I just would like to confirm what I have found.
Future thanks for the help!

[Egg]

Most of the time the suffix letters shouldn’t matter. It is generally to identify different manufacturers. Just double check the datasheet when you get confused. Yes, polypropylene is a film capacitor. But be aware there are other types of film capacitors too such as polyester. Those with MKP label are polypropylene. t35 core can be used as gdt but if you are going to make a small coil ( low freq) I would recommend getting a higher frequency rated one like n87.

[ZakW]

Hello blahaj,

Good choice, the SSTC 5 Mini was my first coil. Made it about 11 years ago now.

You're correct, 12v is needed to drive the MOSFETS.

Second, I cannot find IRFP260s, just IRFP260MPBF. I assume the PBF means lead free, but what is the M suffix? They seem to have nearly identical specifications.

I googled a bit and all I managed to find is the the 'M' might be specific to the company that manufactured the parts. 'PBF' means lead free. If you compare the parts IRFP260N and IRFP260M they should be almost identical.

IRFP250s,60s,460s... are old parts. It can take some time but you can usually find comparable, usually better parts now days by using Mouser. I pull up the datasheet or look the part up and then just filter down to available inventory that is close to the specs of the original part. Then I grab a few and compare their datasheets.
Pay attention to:
Lower 'Qg - Gate Charge' means it will be easier to turn on
Lower 'Rds On - Drain-Source Resistance' means less energy wasted as heat

Third, a capacitor at "cap sub point" is needed, correct? I assume thats what the values in the table for differing sparks.

Yes, the sub cap is needed. Larger values make bushy arcs while smaller values make thin arcs. I used a rotary switch that had several capacitors soldered to it. It was easy to switch to different values.

Is a resistor at "resistor sub point" needed, even with the potentiometers?

The sub point resistor is not necessarily needed. It sets the minimum on (or off time) time for the interrupter. For more variability you can use larger 500k POTs and leave out the sub point resistor.

Finally (whew a lot of questions.. I am pretty new to this), by polypropylene, that is a film capacitor, correct? I just would like to confirm what I have found.

No worries, great questions! Yes, you're correct that is a film capacitor.

I am not sure if there is a maximum value for C6 (DC blocking capacitor) but I wouldn't go under 1uF. I use a 3uF in my SSTCs. Lower values cause insufficient gate drive. From what I read it is a good idea to increase the DC blocking cap value for MOSFETS or IGBT that have a large Qg - Gate Charge, or combined gate charge. Since you are driving two MOSFETS their Qg is added.

Finally, if you're interested you could replace both the UCC37322/21 with a single UCC27425. The UCC37322/21 deliver 9A peak power while the UCC27425 only delivers 4A peak. I haven't had any issues using it.


Good luck!
-Zak

t35 core can be used as gdt but if you are going to make a small coil ( low freq) I would recommend getting a higher frequency rated one like n87.

Egg I did not see your post until I finished mine. Smaller coil usually means higher frequency, did you mean to recommend n87 for a smaller higher freq coil?

[Egg]

t35 core can be used as gdt but if you are going to make a small coil ( low freq) I would recommend getting a higher frequency rated one like n87.

Egg I did not see your post until I finished mine. Smaller coil usually means higher frequency, did you mean to recommend n87 for a smaller higher freq coil?

Yes. I forgot to put a comma in my message.

[blahaj]

Thank you for the responses!
Sorry im so late with this reply, its been a lot these past few days haha

For using the UCC27425 instead, how will the lower gate drive current impact the coil, if at all?

[ZakW]

The lower gate drive current from the UCC27425 could result in slower turn on times causing switching losses but it depends on the MOSFETs you're using.

The IGBTs I used in my SSTC had a Qg of 123nC each do 256nC combined. That was not an issue for the 27425 at all.

Looking at the datasheet the IRFP260s have a Qg of 230nC each, 460nC total! That is high. You might want to look for a similar MOSFET with lower Qg if you want to use the single UCC27425.

You could simply overcome or reduce the effects of a high Qg part(s) by using the original UCC37322/21 drivers as they deliver 9A peak which would be plenty for a small half bridge.

Overall, I think it depends on what MOSFETs you choose to go with. Don't sweat it too much if your not sure, just go with the original schematic if you can't find other MOSFETs to use.

[blahaj]

Thanks for the help ZakW, I will just stick with the original dual UCC driver.

Im not sure if I should start a new thread for this, but I'm not clear on designing the primary + secondary.
For the secondary, I believe I have some thick PVC pipe laying around, if I know the O.D., should I just choose a length that gets me somewhere in that aspect ratio range of 1:1.5 to 1:3, like Gao Guangyan says in his SSTC 2 guide?
On the primary, should I play with values in JavaTC to get a desired coupling factor, then experiment as Gao writes?
Also, its a bit difficult to find a long enough roll of wire for the secondary, JavaTC gives me a needed secondary length of above 1000ft for either of those example coils, where do people in the USA get these long rolls from? Haha oops, I wasnt looking hard enough, there are tons of rolls on popular online stores... In this case, what AWG should I aim for? People seem to go with 26-34 AWG, is 28-30AWG alright?

[Mads Barnkob]

A SSTC, with its smaller sparks than a DRSSTC, can usually do fine with the lowest recommendations from my DRSSTC design guide: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/secondary-coil/ where the lowest ratios mentioned is 1:4, you can do fine with 1:2 or 1:3.

The wire size really only has to do with what physical size you want it to end up as and you want to get below the skin effect eating all your performance: https://kaizerpowerelectronics.dk/theory/wire-size-table/

I also wrote down a few specific design things for SSTC here: https://kaizerpowerelectronics.dk/tesla-coils/sstc-design-guide/

Playing with JavaTC is usually the best way, I also made a few different calculators that I like better than those built into JavaTC for like helical coils: https://kaizerpowerelectronics.dk/calculators/

[ZakW]

You're welcome, happy to help!

Sorry, I saw your post and forgot to respond.

I have wound a lot of coils of varying sizes when I was trying to figure out what worked best for my SSTC. Out of all the coils the wider and shorter ratio coils perform better. Longer and skinny had much lower output.

Like Mads said about wire size, I used really fine wire (44awg) to shrink my final coil down to only 2in tall with a resonant frequency of about 500kHz (unloaded).



I found using a plastic roll let me adjust primary coupling really quickly without wasting a lot of wire. Pictures can be found in this post here: https://highvoltageforum.net/index.php?topic=1910.msg14532#msg14532