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DRSSTC Calculator - Design Strategy & Budgeting

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meawoppl

Tue Dec 22 2015, 07:28AM

Registered Member #57545 Joined: Wed Oct 21 2015, 01:28AM
Location:
Posts: 8

I am spending some time this weekend working on my web based DRSSTC design tool. I already have some basic parts in place, like calculation of the primary/secondary LC's, and tools to estimate the topload capacitance, coil parasitic capacitance etc, save/load online, compare to known coils etc.

More generally I want to understand the process by which people spec out a DRSSTC design. It seems to me that the most common sequence seems to be something like:

Approximate secondary coil size based on budget
Pick a topload that gets you into a rough frequency range
Pick a primary that matches the above, balancing between L and C, with approximately equal Power handling.
Design bridge consistent with Primary frequency and power requirement

There are a number of angles to come at it from, and I want to make a tool that becomes very usable to this community for design or teaching others etc.

Is this a fair approach to the design process of a DRSSTC?

Any thoughts appreciated.

loneoceans

Tue Dec 22 2015, 07:40AM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

meawoppl wrote ...

I am spending some time this weekend working on my web based DRSSTC design tool. I already have some basic parts in place, like calculation of the primary/secondary LC's, and tools to estimate the topload capacitance, coil parasitic capacitance etc, save/load online, compare to known coils etc.

More generally I want to understand the process by which people spec out a DRSSTC design. It seems to me that the most common sequence seems to be something like:

Approximate secondary coil size based on budget
Pick a topload that gets you into a rough frequency range
Pick a primary that matches the above, balancing between L and C, with approximately equal Power handling.
Design bridge consistent with Primary frequency and power requirement

There are several really great tools available such as Teslamap and Javatc just to name a few, which do a lot of really cool calculations.

I'm sure many people have different opinions on how to design a coil but I always design the coil with the over-arching performance and physical size goal first. As with all engineering problems, developing a solution becomes clearer once you have more constraints, so I add on constraints and then develop a final product based on what I have on hand or what I can purchase. So it could go either way, starting with for example, a pre-wound secondary coil, or starting with a set of transistors, then going on from there. After building a bunch of coils, you start to get an intuition on what works well and what does, and can easily flesh out a coil. So part engineering, and part art :)

What I think can be a useful reference would be a good database of existing coils. I think that's what the *add your coil here* thread in the sticky tries to capture, but it's a really long list and might be a good idea to make them in some sort of pivot-able database sorting in terms of type, size, spark length and so on with their associated specifications. Or even just a more organized list.

Mads Barnkob

Tue Dec 22 2015, 08:43AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

I already made a MMC calculator

and I am nowhere near finished with a DRSSTC design guide

For some time I also wanted to simplify the JavaTC calculator, but there just isn't a reason to, it is such a great calculator that uses advanced calculations to get all the properties and interactions between components correct.

I have tried mapping different DRSSTCs in a database, but there is simply too many parameters to make any real consistent data on selecting only a few design highlights like physical size and power input since there are so many different designs and some bad and some good.

I cooked it down to a list of proven switching speeds for IGBTs at whatever peak current they ran with in the respective coils:

How many DRSSTCs did you build yourself? What is clear to me is that my tools and guides are made from my point of view and it is certainly not all that design in that particular order. I build my MMC to suit my application, others build their application to suit their MMC.

In the end it all comes down to how much money do you want to spend, that is a key design parameter :)

meawoppl

Thu Dec 24 2015, 05:26AM

Registered Member #57545 Joined: Wed Oct 21 2015, 01:28AM
Location:
Posts: 8

Yeah, the database of known coils would be really helpful. There are some fairly complicated parts to the design process that require a bunch of inside knowledge to grock.

What mosfet/igbt are appropriately scaled for a bridge of a given power and frequency?
How do TPD and coil size scale?
How are frequency and coil size related?

There are lots of less than obvious design curves running around that a database would make more clear.

I was thinking to get the tool 90% working, then start inviting people to add their coils + pictures etc.

Do either of you have a coherent mental list of the real design variables?

Primary Dimensions, wire shape, L
Topload dimensions, C
Secondary Freq (nominal)
Primary Geom + L
Primary C
H-bridge
Snubber bank size
Bus voltage

Any other big ones I am missing?

nzoomed

Thu Dec 24 2015, 09:30AM

Registered Member #54503 Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288

Alot of those calculators are very useful.

My biggest issue for me is i have no idea how much power my coil will use, only that i know it can draw alot of current if i increase the BPS enough from the MIDI interrupter.

I am using two CM300 IGBT's and they are running at 680VDC on the primary bus.
I believe my IGBT's can handle up to 1500A from reading what others say here, but if i power my coil over 1000A, i will need a larger MMC to take the current and i have no idea how to know how large your coil has to be in order to stop too many arcs to the strike ring if its too powerful for its size.

Infact, i dont know how it is possible to get 2m long arcs on a coil that is under a metre in height without them all striking the ring?

meawoppl

Tue Dec 29 2015, 01:21AM

Registered Member #57545 Joined: Wed Oct 21 2015, 01:28AM
Location:
Posts: 8

How many DRSSTCs did you build yourself?

Two now. One fairly small (~30cm secondary coil), the other in conjunction with a larger group, and is the biggest I have seen 16" x 96", running a CM600 full bridge on 680. We tapped out our 68kW generator before encountering any equipment failures.

I think what I find hard is that there are a number of less than obvious design/performance constraints, and I suspect some fairly clear patterns when plotted out along side a hundred or so others.

Sweet design guides Mads. I will poke through those a bit for sure!

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