Author Topic: Need a little help with my design (first DRSSTC)  (Read 580 times)

Offline nabzim

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Need a little help with my design (first DRSSTC)
« on: December 25, 2018, 01:04:51 AM »
Hi
So I'm currently designing my first DRSSTC. I'm aiming for similar specifications to loneoceans' DRSSTC-2 (http://www.loneoceans.com/labs/drsstc2/). I have been referencing his page as much as I can, however it's quite difficult without a schematic. Looking at other schematics everywhere I can find, still, there is nothing that gives all the answers to things I don't understand.

1. First thing that's bugging me, is that most sources recommend using TVS diodes across the Collector and Emitter of each IGBT, but from his pictures, it looks like Gao chose not to include them in his bridge design. Is it just not important for for half-bridge modules? I know there are already diodes inside, so...
2. Second thing, Does the voltage rating of the Snubber capacitor need to be so high? Gao is using 2000V 1uF, but that is somewhat hard to find at a good price... but 1200V is easy to find and more affordable... Though the price differences are not that big, I'm just trying to save a few bucks where ever I can.
*(3.) additional question at bottom of my post.

That's pretty much it for now... just want to be certain about the items in my Digikey shopping-cart before I checkout  ;D

In the shopping cart:
Fiber TX/RX and cable, epcos "T38 material" ferrite cores (for fb & ocd), heatsink, 1.5KE33CA for gates

I already own:
UD-2.5, (1x)CM200DU-24H, (10x)942C20P15K-F (9 for MMC, 1 for pri. strike protection),
Magnetics 0J43813TC (J material) for GDT, ...(can't think of everything right now)...

Planning to purchase in near future (or, add to shopping cart):
cheap 24v SMPS, ..... , other things, ....

I plan to finalize the major component choices/purchases and have a clear-cut design plan, before I attempt to build anything.
The build will go like this:
Build/install all circuitry in base/enclosure --> Build/install Secondary & toroid --> Build/install Primary
(I want to have the secondary installed first so that I can measure it's Fres at the correct height)

Gao designed his secondary with 34awg on a 3.5" form, 13" length, giving a Fres of 122 kHz. I have found through JavaTC simulation, that if I wind 32awg on a 4.5" form, with the same length of 13", gives me nearly the same Fres, about 122.7 kHz. So as far as I can tell, this will be a much better choice, for peace of mind, not requiring the dreadfully-thin 34awg.
*(3.) Gao's secondary frequency drops to about 106 kHz under spark load, and tuned his primary to account for such; with my proposed secondary, can I expect the same % F drop?

Thank you very, very much.

Offline sjsimmo

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Re: Need a little help with my design (first DRSSTC)
« Reply #1 on: December 27, 2018, 02:57:47 AM »
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.
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Offline nabzim

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Re: Need a little help with my design (first DRSSTC)
« Reply #2 on: December 27, 2018, 10:29:41 AM »
Thanks sjsimmo,
I guess, as long as they can't do more harm than good, I may as well add the tvs just to be on the safe side... although I am planning my bus layout to be just as compact as Gao's.

The most important thing for snubbers is their peak current rating.
So, uh... what about their peak current rating?  ???

Offline nabzim

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Re: Need a little help with my design (first DRSSTC)
« Reply #3 on: December 27, 2018, 08:17:45 PM »
Sorry, my last comment was kind of condescending...  ::)

 I just right-now read something in Mads' DRSSTC design-guide that clears this up for me. He said this in regards to the snubber:

"The overall power dissipation is however very low due to the very low duty cycles used in a DRSSTC and it is not necessary to take extra precautions in regard to RMS current if IGBT snubber capacitors mounted directly on the terminals are used."

Offline Mads Barnkob

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Re: Need a little help with my design (first DRSSTC)
« Reply #4 on: December 29, 2018, 02:04:27 PM »
Hi
So I'm currently designing my first DRSSTC. I'm aiming for similar specifications to loneoceans' DRSSTC-2 (http://www.loneoceans.com/labs/drsstc2/). I have been referencing his page as much as I can, however it's quite difficult without a schematic. Looking at other schematics everywhere I can find, still, there is nothing that gives all the answers to things I don't understand.

1. First thing that's bugging me, is that most sources recommend using TVS diodes across the Collector and Emitter of each IGBT, but from his pictures, it looks like Gao chose not to include them in his bridge design. Is it just not important for for half-bridge modules? I know there are already diodes inside, so...
2. Second thing, Does the voltage rating of the Snubber capacitor need to be so high? Gao is using 2000V 1uF, but that is somewhat hard to find at a good price... but 1200V is easy to find and more affordable... Though the price differences are not that big, I'm just trying to save a few bucks where ever I can.
*(3.) additional question at bottom of my post.

That's pretty much it for now... just want to be certain about the items in my Digikey shopping-cart before I checkout  ;D

In the shopping cart:
Fiber TX/RX and cable, epcos "T38 material" ferrite cores (for fb & ocd), heatsink, 1.5KE33CA for gates

I already own:
UD-2.5, (1x)CM200DU-24H, (10x)942C20P15K-F (9 for MMC, 1 for pri. strike protection),
Magnetics 0J43813TC (J material) for GDT, ...(can't think of everything right now)...

Planning to purchase in near future (or, add to shopping cart):
cheap 24v SMPS, ..... , other things, ....

I plan to finalize the major component choices/purchases and have a clear-cut design plan, before I attempt to build anything.
The build will go like this:
Build/install all circuitry in base/enclosure --> Build/install Secondary & toroid --> Build/install Primary
(I want to have the secondary installed first so that I can measure it's Fres at the correct height)

Gao designed his secondary with 34awg on a 3.5" form, 13" length, giving a Fres of 122 kHz. I have found through JavaTC simulation, that if I wind 32awg on a 4.5" form, with the same length of 13", gives me nearly the same Fres, about 122.7 kHz. So as far as I can tell, this will be a much better choice, for peace of mind, not requiring the dreadfully-thin 34awg.
*(3.) Gao's secondary frequency drops to about 106 kHz under spark load, and tuned his primary to account for such; with my proposed secondary, can I expect the same % F drop?

Thank you very, very much.

TVS diodes is only nescesary if you have a high inductance primary circuit design, like wires going everywhere etc. If you construct it from copper busbar and make it as compact and low inductance as possible, TVS diodes is no longer fast enough to catch the transients such a brigde will produce.

Snubber capacitors needs a voltage rating to withstand the short and energy weak switching transients. So 1200V will properly work okay, you can always just put two in series...

Zeners for the gates is a good precaution against high IGBT C-E currents from gate over-voltage.

Sounds like a good build plan and yes you can properly expect the same drop in loaded resonant frequency or at least somewhere around 10% lower tuned primary. You can simulate a loaded secondary by adding a branched piece of copper wire to its output terminal and measure the resonant frequency with a signal generator and oscilloscope.
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Re: Need a little help with my design (first DRSSTC)
« Reply #4 on: December 29, 2018, 02:04:27 PM »

 


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