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Uspring

Thu Nov 15 2012, 11:39AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

Goodchild wrote:

Another interesting thing that I have noted is that high frequency needs less voltage as well. If I remember right Steve Ward showed this with his QCW when he took voltage measurements at 380KHz and then 500KHz and was able to get the same sparks but with less voltage.

Thank you for the info. When I compared my measurements with Wards, it also looked like higher frequencies required less voltage. My coil runs at 220kHz and needed 70kV for mere 30".

I'm curios if this also relates to less spark capacitance with an increase in frequency or if it's strictly based on overall volume and density of the plasma.

That is an interesting question. I'd also like to know, whether higher frequency affects the spark capacitance. For the unbranched sparks you're looking for, capacitance is probably less than for branched ones.
The model suggests lower voltage requirements for higher frequencies because of the heavier load the capacitances create. That makes the spark hotter and more conductive. Lower voltage is not an advantage, though, since the power requirements rise with frequency for a given spark length. It is not so difficult to achieve high voltage. The problem is to keep it up, once the arc begins to load it. That is where you need power.
I believe, that this is the main difference between QCWs and SGTCs. A SGTC needs a lot of voltage since the arcs don't have much time to grow.

Ash Small

Thu Nov 15 2012, 11:58AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'll throw something in here.

Higher frequency = more current per unit of time.

power = voltage x current.

You therefore get the same power for less voltage. (same heating, etc)

Higher frequency results in a hotter plasma, requiring less voltage to maintain.

Steve Ward

Thu Nov 15 2012, 07:41PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I still think you need to work on you cooling system.

As it stands, you have built a thermal capacitor with that big block of copper. On top of that, copper represents thermal resistance, ideally youd be putting water in direct contact with the Si with no copper in the way at all.

"Poor" thermal management is what caused my first dual-bridge QCW system to explode at ~12kVA input power. Even with a significantly better water cooling system than your proposed one, it still had its breaking point. The problem? too much copper between my Si and water!

It may not matter on a single shot basis, but you will be severely crippling the power capability of those transistors with the design you have now.

Patrick

Thu Nov 15 2012, 08:54PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

the cooling system is a bottle neck for many well and poorly designed systems, as Steve Ward points out, copper and aluminum bolcks merely store and delay heat rise...

ideally the math and physics do support Steve wards point on zero copper and max water in contact with silicon, but practically youll need some well justified (through math and physics) metal solution, like: (1) large cross section for initial conduction, for the (2) spreading and interfacing from the silicon to the water and (3) but a minimum path length to prevent thermal resistance. So thats a machined face of copper to your silicon, then fins (high SA) to the fluid... yet it should be highly physically (dimensionally) optimised.

If he plans to use a single fluid path for all his different potentials, he may need to switch from DI water to canola food oil or diala/univolt oil. If he does plan to use fluid, mica or silicone isolators would be unwise.

thats what i think.

Remember, when we oh-so-smart engineers and scientists F-up thermal matters, nature steps in and rights the situation with noise, flames and plasma...

Since Goodchild has planned a fluid cooled primary and copper block transistor head, he may have to switch to oils... even DI water can be made to explosively decompose.

EDIT: i dont want to put words in Steve Wards mouth, but i think Ward is implying a single hole through the one-inch thick copper block will be insuffcient for fast fliud heat transfer. (single hole, i gather from high res on flickr)

Goodchild

Thu Nov 15 2012, 10:36PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

I agree that water directly in contact with the package is the best way to go, however that represents a rather complex heatsink to manufacture at least the way I'm thinking about it. Currently there is a 3/8 water channel though the center of the block and we have a rather large pump to shove water through it at a high pressure. I would also like to add that we got the copper free so it's also a case of stuff we had on hand.

This being said it's not our goal to run the transistors on the absolute edge of their SSOA, that is why we have 8 bridges. So that we can run them at a modestly high level without heating issues. The water blocks are merely a way to stay away from forced air approach and make the bridges more compact. Currently I'm not even convinced that with our available power (about 15Kw) that we will even come close to the overall thermal rating of the system.

The other great thing about our bridges is that it is very easy to change out the block in the future to something more like what Steve Ward proposed with water flow directly over the transistors.

Patrick

Thu Nov 15 2012, 11:06PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Goodchild wrote ...

The other great thing about our bridges is that it is very easy to change out the block in the future to something more like what Steve Ward proposed with water flow directly over the transistors.

I figured youhad put some thought into this, but remember, direct fluid contact may not be enough. if you cuase boiling you could increase the C'/W resistance rating. Then youd be stuck with thermal run away, and load shedding or hogging on different bridges.

Goodchild

Fri Nov 16 2012, 01:12AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Patrick wrote ...

Even if we had boiling which is unlikely given the amount of water flow and the fact that we will not be running at the edge of failure; it would be impossible to have a thermal runaway. As referenced in the first post the IGBTs we are using have a positive tempco under all operating conditions unlike 60N60s. On top of this we have a matching network for all the bridges further making it very unlikely of any substantial mismatch.

Patrick

Fri Nov 16 2012, 03:24AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

a positive tempco is a good thing to look for, but it wont just cuase load shedding to the other bridges, it will accerlerate the heating, even at the same or lower power. As for derating, thats your best bet for controlling this situation. over spec the components and under power the system.

You can also back-calculate the thermal resistance from temp rise, and predict your power red line.

...

Fri Nov 16 2012, 04:15AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

A single 3/8" line is a little on the small side, but it is amazing what having a decent pump (a few gpm at 50psi is what I usually see) can do to make up for lack of area on the waterblock. I started out using elaborate waterblocks designed for cpu watercooling, and when I saw the cooling channels in my big laser (

) I thought it had no chance of working, but the 500w of heat over the roughly 5in^2 of copper/water area is enough and keeps it at about 5 degrees above the water temperature.

In any case it looks like an amazing project, can't wait to see the sparks!

Patrick

Fri Nov 16 2012, 04:44AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

... wrote ...
.....but the 500w of heat over the roughly 5in^2 of copper/water area is enough and keeps it at about 5 degrees above the water temperature.....

this is good to know. i guess at a few GPM?

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