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4hv.org :: Forums :: Tesla Coils
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CCPS (Capacitor Charging Power Supply)

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Finn Hammer
Thu Aug 23 2007, 09:41PM Print View
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
All,
I am building a medium/large Tesla coil together with Daniel Uhrenholt, a fellow coiler and toolmaker here in Denmark.
During a period, where progress was slow, due to me having other commitments, Daniel submitted: I am going to have a go at a CCPS for the coil, I`ve seen Marco`s and Ward has done some work on it also.
I thought:"Yeah, right" and forgot about it, but then some promising videos showed up on youtube


And so I thought Hey, this looks good, I`d better pay attention.
So during the next weekend, we got a microsim model set up.

I had some of the big cores lying around, waiting for an opportunity like this, and we started to make some measurements on them.
By applying 40kHz to them, turning up the voltage and watching the triangular current curve, we looked for the point where the curve got an upward slope. This was also the point where they got warm, indicating beginning saturation.
This happened at 50volts/turn.
We also made mock up windings to measure the leakage inductance of the transformer, and it turned out to be 58uH.
Plugging the numbers into microsim, with a series capacitor of 68nF, a resonant frequency of 80kHz revealed some nice numbers:
At the start of the charging cycle, 19A at the end 39A for 15Arms
Drawing this kind of current from a 560V supply gives 8400VA and with 2 cores available, a 16KVA supply looked feasible.
After this we concluded that there was only one thing to do: Built the damn thing.

I had a pcb for a plasma sonic leftover from another life, and we decided that it would be a good platform for the development stage, until we had a working design. I severed it in half,



dumping the H-bridge section, and modified it to start and stop in syc. with the oscillator.



IGBT``s are not cheap, and sifting trough the maze of 20USD+ devices, I stumbled upon this little beauty:

And at 2USD I decided to build 2 bridges with 8 devices in each.
Since I figured not to change one defective IGBT in the case of a bridge failure, I went for a compact design.



A HP214A was gutted and stuffed with quality parts. 3phase inlet, resistive softstart, timer bypassed. dual 3300uF smoothing caps and bleeders. The transformers and bridges. After some shuffling around, this is the current stage:




It does work!




and this is still with the secondary shorted securely.

I know what you are thinking! You are sitting at your little monitors thinking:
Show us your gate waveforms! Show them to us, so that we can tell you that you are a schmuck!


I won``t disappoint you of course.
With a 22ohm gate resistor, there is no overshoot at the zero transition, and the gate charge gets transferred in a little over 300Sn



As soon as voltage is aplied, things get a bit more ragged, with a nasty 20mHz ripple.
I am starting to run into measurement difficulties, such as different output from a Pearson 2100 and a Tek P6021/134. The latter follows the gate signal with greater fidelity than the pearson.




With 15Arms flowing trough the bridges, and a voltage drop across each device of 1.9V, one would assume 3.8 x 15 = 57W dissiption, and the finns get hot in about 30sec, so I guess a big fan has to be installed.

This weekend, Daniel and I are going to work on a voltage probe, so that we can set the desired output voltage.
We also intend to pass the trigger signal from the trigger generator trough this supply, so that we can derive a delay, to kep it disabled during the coils ringdown, and so that it starts charging the cap after the BRISG has quenched.

And perhaps try it out on a jacobs ladder.

We invite you all to comment, and in particular help us add protective circuitry, to keep this beauty running.

Cheers, Finn and Daniel
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Steve Ward
Fri Aug 24 2007, 05:15AM
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Nice work so far guys.

For snubbing the GDT ringing, i found that around 5-10 ohms on the *primary* side of the GDT was all that was needed (no secondary side gate resistance). This produces very clean waveforms.

My CCPS currently produces a very healthy arc and about 20KV peak output voltage from a voltage doubler. It works great for charging up my beastly 5kJ 12kV pulse cap.

It should run a TC great as well, just be sure to stick some sort of output filter on the thing. I had some strange issues when running a spark gap coil. I think a SISG will be a little nicer than a spark gap system anyway.

Also, you might still want to watch for core saturation. The way i did it was just put another few turns of wire on the core and scope it. As you bring up the voltage, you should get a good idea of the odd voltage waveform. If you begin to saturate, you will likely see the voltage drop off to zero. I think the inverter is pretty well protected against saturation effects since the magnetizing inductance is typically shorted out anyway (which is basically what happens for saturation). Only thing is that the core gets very hot!

Keep us posted!
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Marko
Fri Aug 24 2007, 11:59AM
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Finn

Awesome project guys. You always find those beautiful metal cases for these things.

Any pics of the monster transformer for me?

For 16kW although I would be worried about those heatsink size.. those two look really wimpy even for like 99% efficiency, make sure to do something in order not to overheat the IGBT's.

Other thing could be bad power factor.. But it's natural outcome of need for smooth output.
I imagine it could only be practically helped by a humongous boost converter like Steve's.


Steve: I always wondered how much sense does it make to use all the diode-resistor mess in gate drive.
If my transformer is made well enough, I thought I can avoid gate Zeners after all, just by using clamp-to-rail Schottky diodes on driver outputs?

Still if I want dead time (which you guys have plenty of here) I need separate resistors and diodes for each gate. (Meh.)



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Finn Hammer
Fri Aug 24 2007, 05:11PM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
Steve Ward wrote ...

For snubbing the GDT ringing, i found that around 5-10 ohms on the *primary* side of the GDT was all that was needed (no secondary side gate resistance). This produces very clean waveforms.



I was eager to try this out, but it didn`t work with my setup.
I tried with 10, 5 and 3.3 ohms, about the same results. Shown here are the 5 ohms traces.
Here is a scope shot of the unloaded gate voltage





The miller plateau is not distinctly visible.

This shot shows the gate with voltage on the bridge



The yellow trace shows the corresponding current curve. The turn on is delayed relative to the other bridge.

Next pic. shows the current waveform, the yellow one.




To me, it says that the IGBT is not conducting fully. The leading current pulse is smaller in amplitude than the freewheling one.
And the bridge is heating more.


Marko wrote ...

Hi Finn

Any pics of the monster transformer for me?

For 16kW although I would be worried about those heatsink size.. those two look really wimpy even for like 99% efficiency, make sure to do something in order not to overheat the IGBT's.



To the left is a prototype of the homemade fiberglass former. This is covered with mylar and other dielectric from stock.
Primary wire is 14 turns 1.1 x 5 mm flat wire, over electrostatic shield. Sec is 0.75mm wire. We may rewind this for more clearance at the hot end.



The cooling finns, yeah, I wish I had thought about that before I made it. I just grabbed a sample, without making any calculations.
Silly Me.

Cheers, Finn Hammer

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Marko
Fri Aug 24 2007, 05:56PM
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Finn you are brilliant, and transformer with this flat wire is wicked cool.
I'm eager to see you progressing.

I wondered if there is actually a benefit from having the primary under the secondary.

Few days ago I was drawing arcs from similarly constructed transformer, except on a flyback core and hard-switched mosfet halfbridge.

It was curious how with no ballast I couldn't get more then some 200-300W (very roughly) into the arc, even after I ramped the frequency to 100kHz and V/turn quite up.

At first I used primary ballast what I thought as reasonable, but in the end without it and no gaps I couldn't get satisfactionary amount of power through the core, even though setup should be good for like kW of power at least. (mosfets got barely over room temperature)


It appears that leakage inductance is really a big limiting factor, what can be done for it?










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Steve Ward
Fri Aug 24 2007, 06:54PM
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
To me, it says that the IGBT is not conducting fully. The leading current pulse is smaller in amplitude than the freewheling one.
And the bridge is heating more.


Are you saying this was caused by using a resistor on the primary of the GDT? Well, surely this can change the switching speed of the IGBT, but they are definitely turning on completely for nearly all of the half-cycle. My CCPS also had one current peak smaller than the other. I never really figured out exactly why that happened, but my Pspice sim often produced that same result. BTW, i see the same non-uniformity in current peaks in your original scope shot from post #1.

One reason the primary resistance (vs secondary side gate resistors) might be somewhat desirable is that in the event of a IGBT failure and assuming it fails with all pins shorted, you end up killing the gate drive to the other devices. This is good since it prevents the other switch in the half-bridge from switching ON shorting out the DC bus. In this case, most of the gate drive voltage is dropped across the primary resistor, rather than only the secondary gate resistor. Of course, i dont think ive more than 1 failure since implementing the SLR topology, and that one failure was a single IGBT that had managed to arc through the sil-pad isolator to the heatsink (this happened while powering a SGTC and using no filter between supply and TC).

For 16kW although I would be worried about those heatsink size.. those two look really wimpy even for like 99% efficiency, make sure to do something in order not to overheat the IGBT's.


Well, first, its kVA (not kW), and this distinction is very important for the SLR CCPS because it inherently has a lot of reactive power and a lot less real power (much like the "ZVS flyback driver"). But i agree, the 57W figure is pretty optimistic.
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Finn Hammer
Fri Aug 24 2007, 07:18PM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
Marko wrote ...


It appears that leakage inductance is really a big limiting factor, what can be done for it?


About leakage inductance: If you resonate it with a series capacitor, and drive it at 1/2 the resonant frequency, with 50% duty cycle, it becomes your friend.

Read Marco Denicolai`s thesis for a full coverage of the CCPS topology.


Cheers, Finn Hammer
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Steve Conner
Fri Aug 24 2007, 07:24PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The second half-cycle is always smaller than the first because you're looking at the beginning of a ringdown.

Marko: the leakage inductance is part of the SLR converter circuit, it won't work if there's too little or too much. Winding the primary and secondary coils on opposite core limbs is one way to get the required leakage inductance. I think Steve Ward prefers to wind both coils together and use an extra series inductor to make up the required inductance.

Finn: are you sure your winding window is optimally filled? :P
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Finn Hammer
Fri Aug 24 2007, 07:48PM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
Steve Conner wrote ...



Finn: are you sure your winding window is optimally filled? :P


I guess I am, sort of.
At one point I thought about winding both coils edgewise with the flat wire, but I found out there was ample cross section this way.

Now about those gate waveforms.
I don``t get it. I am not passing a lot of current and not swiching very fast, still this ringing.
I don`t recall having these kind of problems with the DRSSTC`s, which use similar components, but where I whack the gate open as fast as possible,and the current reaches 400A.
Marco D. slows down his gates to 1100nS, but even this does not work here. I am starting to suspect the long leads on the pcb btwn. gate drivers.

If this is not the case, perhaps there is too much leakage inductance in the gate transformer? Perhaps I should reduce the amount of turns.

But that requires a calculation of the volt seconds of the transformer and I can`t do that. Yet!.
Anyone have an explanation of how to do that calculation?
After optimizing the amount of turns, I could reduce the leakage inductance to half by making the primary as shield, surrounding the 4 gate coils, according to James theDatastreams survey.

Cheers, Finn Hammer
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Marko
Fri Aug 24 2007, 09:25PM
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

About leakage inductance: If you resonate it with a series capacitor, and drive it at 1/2 the resonant frequency, with 50% duty cycle, it becomes your friend.


Ok, I ''know'' that much about SLR (what comes from steve's site :p)

I simply wanted to get an idea of difference in leakage inductance with primary-under-secondary and separate style transformer.

What really limits current (with ghetto SG3525-style controllers) when you draw arcs or short the transformer?
There must be a lot of secondary leakage inductance doing this.

If you aren't using a (insanely complex) resonant controller like Marco larger inductor is also needed to stabilize the inductance to a point so it can be tuned by a fixed controller like SG3525. (at least from what I 'figured out').

Apparent inductance drops proportionally with load, so I imagine fixed controller to have trouble without larger inductor.

Finn: I see you copied the transformer much from Denicolai's design. I'm lost with his resonant controller, but how do you keep the thing in tune with only leakage inductance of primary?

Marko

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Finn Hammer
Sat Aug 25 2007, 06:52AM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
Marko wrote ...


I simply wanted to get an idea of difference in leakage inductance with primary-under-secondary and separate style transformer.

The difference is great. With the windings so close you get almost no leakage inductance. So you would need an external choke. Why do that when you can get it for free?
Marko wrote ...


What really limits current (with ghetto SG3525-style controllers) when you draw arcs or short the transformer?
There must be a lot of secondary leakage inductance doing this.

The combination of L and C sets the impedance of the primary, and this determines the current. The fact that you deny the circuit to build up resonant rise is the real current limiting factor.
Marko wrote ...


If you aren't using a (insanely complex) resonant controller like Marco larger inductor is also needed to stabilize the inductance to a point so it can be tuned by a fixed controller like SG3525. (at least from what I 'figured out').

Apparent inductance drops proportionally with load, so I imagine fixed controller to have trouble without larger inductor.

Marco Denicolai`s controller does the same thing as your SG3525 and my TL494. I`s just a question of putting out a 50% duty cycle pulse train, at half the transformers resonant frequency. There is nothing more to it.

It is very clever and very simple. That`s the beauty of it.

The leakage inductance is a fraction of the total inductance based on the geometry of the winding layout relative to the core. It doesn`t change unless the total inductance changes.
Marko wrote ...


Finn: I see you copied the transformer much from Denicolai's design. I'm lost with his resonant controller, but how do you keep the thing in tune with only leakage inductance of primary?


Marco, from what you write, I start to get the feeling that you don`t have enough windings on your primary, so that when you increase the voltage across it, you run into saturation. It may look like a power problem but it is a volt/turn problem.

From my experience, it is very easy to get this topology basically right, (apart from details like gate waveforms) and we have made prototypes on smaller cores. Just make sure there are enough windings on the core, or else, you get problems when you "turn up the wick".

Do you have some documentation of your transformer, and the rest of your drive circuitry?

Cheers, Finn Hammer
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Steve Conner
Sat Aug 25 2007, 10:58AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Finn, how are you going to regulate the output voltage of your CCPS? Are you just going to stop the oscillator when you reach the voltage you want?

Great project BTW :D
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Marko
Sat Aug 25 2007, 12:09PM
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Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
The difference is great. With the windings so close you get almost no leakage inductance. So you would need an external choke. Why do that when you can get it for free?


Finn you are awesome. Thanks a lot.

I`s just a question of putting out a 50% duty cycle pulse train, at half the transformers resonant frequency.


My point was on this (could be one of FAQ's); The transformer does not have stable primary inductance. The cap will be resonating with leakage inductance alone only when secondary is shorted, and with full primary inductance if secondary is open or missing. (as if it was an inductor on ferrite core.) The inductance will wary between those two points depending on load, so I imagined it very hard/impossible to tune for ZCS without feedback.

And as far as I see, denicolai's controller keeps ZCS for the full range of charging.

(sorry, I didn't invent Lenz's rule )


Finn, how are you going to regulate the output voltage of your CCPS? Are you just going to stop the oscillator when you reach the voltage you want?

Great project BTW :D


Steve: CCPS appears to be more constant current than a voltage source, so I don't think there can be real voltage regulation. Denicolai used a HV probe to stop the charging at certain point as you said, and I guess fin will do something similar.

Apart from that I can imagine some kind of complexity which would 'skip cycles' and regulate current that way, but sorry, I'm not completely confident how this works anyway..


Marco, from what you write, I start to get the feeling that you don`t have enough windings on your primary, so that when you increase the voltage across it, you run into saturation. It may look like a power problem but it is a volt/turn problem.


It is a small hard switched design with really nothing special. I calculated the core throughly and got quite close to saturation, but stayed just under it. I could clearly observe the core going into saturation just as I lowered the frequency.
From everything else it appeared just as I had a big ballast on it, which I believed to be due to winding style. I can post more into another thread if you are really interested...

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Finn Hammer
Sat Aug 25 2007, 12:19PM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
Steve Conner wrote ...

Finn, how are you going to regulate the output voltage of your CCPS? Are you just going to stop the oscillator when you reach the voltage you want?

Great project BTW :D


Thanks, Steve.

You are right.
The oscillator is running all the time. The gate driver chips have their enable pins controlled by a 74HC74 D-flipflop, so that they are in sync with the oscillator. The FF is controlled by a CA3031 opamp configured as a comparator with adjustable hysteresis.
The voltage probe is going into this comparator to stop driver chip output, when desired voltage level is reached.

Cheers, Finn and Daniel

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Steve Conner
Sat Aug 25 2007, 12:58PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Marko, this power supply is designed such that the secondary is always shorted from an AC point of view. It uses a bridge rectifier with a large capacitor on the output, and no filter choke as in usual SMPSs. So the resonating inductance is assumed to be always the leakage inductance.

With non-zero output voltage, the waveforms are affected in a strange way as the bridge rectifier commutates. The effect is to lower the Q of the oscillation, as energy is extracted to the load, but the ringing frequency doesn't really change, IIRC. However, if the Q falls too low, hard switching might set in.

Incidentally, it does no harm to run slightly below half the resonant frequency, and it might even give more leeway to let the waveforms change as the output voltage ramps up.

FWIW, the current-fed converter is related to the SLR, but you can regulate it just like a normal SMPS, by PWM of a buck converter in the DC link.
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Marko
Sat Aug 25 2007, 02:47PM
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Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I couldn't get dumber than that... The supply works into capacitive, not the resistive load. Steve makes his perfect point in 1 sentence.

Excuse me.. I'm only a teenager.

*hugs Steve and Finn crying*
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Finn Hammer
Sun Aug 26 2007, 08:13PM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
This has been a rewarding weekend.

When Daniel arrived, and I proudly showed him the CCPS, I wanted to show him how long it took to get hot when the variac was turned up full.
I had done this several times, but right this time it went *crack*, and blew a fuse.
Enter rebuild mode. Thanks god those IGBT`s were cheap.

During the tuning procedure we noticed that the transformers were sensitive like a tesla coil.
We took them out and moved the one around next to the one we had sig. gen. and scope on. Found out they influence each other a lot, depending on wether they are in phase or not. These transformers resonate at 80kHz, but could be brought to resonate at from 70 o 100kHz depending on phasing and distance.
This sensitivity would be an argument for using close coupling in the transformer and external choke, which in turn could be adjusted easily.
Rebuilt, the same bridge blew almost instantly.
Decided to use the one good bridge to test how parallel transformers would share current, and they did well.

Today we went searching in the stock and found a couple of FUJI 2MBI300N-120 . With internal current limiting, not suitable for DRSSTC duty but perfect for this job.
We had given up driving the transformers with IGBT`s barely able to handle the current load, and decided to parallell the primarys and use just one heavy bridge.
How to drive those gates?
The UCC37322 chips didn`t handle the task well. We tried to use the one good bridge, only to realise that it will not deliver a 50% duty cycle. So what to do?

I had in my mind the picture from data sheets, of the TTL output from chips, and took a closer look, well right! it is a PN pair with the gates tied together. Woila! the beefy driver!

We lashed one together, and got some gate waveforms that resembled square waves.
Tried to power up the beast, and, well it didn`t get hardly hot at all, so we made this video of a jacops ladder.


We didn`t get any of the fancy voltage probe stuff done, we built 5 bridges and blew 4 but we got there:
We recorded something like 60 primary amps during this shot which is below 2.4kW because we were in a 10A fuse doing it. Sorry, we were too tired to make better measurements, after 2 days of 14 hours each, full time of learning and doing.

So this has been a fun day: Using the tools from the very first SSTC work, the plasmasonic and the beefy driver, we came out to the bleeding edge with a CCPS.

At present gate drive is per beefy driver and gate transformer, we will change that to dedicated drivers before next update.



Picture shows present status as per this writing moment.

Cheers, Finn and Daniel
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Marko
Mon Aug 27 2007, 12:04AM
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Aw, the number of dead IGBT's completely sucks finn.. I'm glad you've got it working now.

Don't you fear that it may just have been overheating what killed them? Really lot of power and small heatsinks. Bricks don't look happy neither! (How did you fix them to heatsinks smaller than their footprint after all? )

Regarding transformers interfering, maybe you could put some shielding between/around them and just leave-them-alone?

I'm just thinking how could a self-resonant controller be built for such a thing, and how beneficial would it be. (Waveform shape is weird so, some kind of zero cross detector followed by logic)

The choke of few tens of uH is not a too big problem, few turns on #26 toroid would do for lots of current. Much harder is, as I think to construct a transformer with lower leakage inductance.



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Finn Hammer
Mon Aug 27 2007, 06:35AM
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Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
Marko wrote ...


Don't you fear that it may just have been overheating what killed them? Really lot of power and small heatsinks. Bricks don't look happy neither! (How did you fix them to heatsinks smaller than their footprint after all? )


Current handling ability drops with temperature, so.....
Oh, we were getting mean, so the bricks got fixed with tye wraps

Marko wrote ...

Regarding transformers interfering, maybe you could put some shielding between/around them and just leave-them-alone?


It`s ok as it is now. I just thought I`d mention it. When we had 2 separate bridges and one controller, putting a Pearson near one transformer and altering it`s freq. was a disaster.
Marko wrote ...

Much harder is, as I think to construct a transformer with lower leakage inductance.


This would be a matter of putting the windings on the same leg, one on top of the other. No problem at all. With the amount of turns per volt, there is no need for more than one layer anyway, so no extra insulation problems. And there would be room for twice the amount of copper.

Cheers, Finn Hammer
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Finn Hammer
Sat Sep 01 2007, 10:03PM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
During the week I built this gate driver:



It is so strange that the controll circuitry is getting more complex than the circuit it is controlling.. another view:



And here is a shot of it driving the gate of a CM600, without gate resistor. It is 50% duty cycle, so it is really a 97.6 kHz pulse...




The gate driver consists of a IRF530 and a IRF 9540 on top.
The gates of these MosFets are driven by an Agilent HCPL-J312 Optocoupler gate driver, which I used only because I could not get the Fairchild FOD 3180. The latter is a fast optocoupler worthy of use in feedback systems, due to a super low 200nS propagation delay.
Well, this does not matter here, I`m oscillator driven.
The opto gets it`s timing impulses from the TL494, buffered by UCC37321 because I had to invert the signals.

There are +24 and -5 volts regulators on board. I wonder how the high side drivers are going to respond to being swung 560 volts, but we`l see. I feel comfort by the fact that there is 50% dead time.

The gates of the 530/9530 pair had to be fed by a 500 ohm resistor, and discharged trough a 1n5819 diode and a 15ohm resistor. the former to avoid the shoot trough current spikes (100nS pulses of 4A) the latter to keep discharge current lower than the gate drivers 2.5A limit.

The IGBT`s have been changed to SEMITRANS SKM 400GB124D because they are easy to drive.

It may seem overkill, to use these bricks, but I have been reading up on this topology, and this paper

gave me a jolt, because it states that with the resonant capacitor size I have available, I`l only be able to transfer around 2kVA.
I`l have to look into that, but In the end, maby we are into a transformer rewind.

I won`t have time to do more work this weekend, but comments are welcome.

Cheers, Finn Hammer

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Daniel Uhrenholt
Sun Sep 02 2007, 12:56AM
Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Hi Finn.

I think it’s time for me to say a few things about our project .

I like your gate driver, and after an afternoon in your workshop this weekend, with blown optocouplers and a headache, I am pretty satisfied with the results. Too bad we didn’t get a chance to test it on our CCPS…

About the transformer, I must agree with you!

I think we should do it the “Ward” way, and place the primary under the secondary winding, and make an extra series inductor to adjust from. If you agree with me I’ll order two of these from RS (Part number) 213-4470, and use the cores we have in parallel.

We need more space in our HP214A cabinet

Cheers Daniel.
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Steve Conner
Sun Sep 02 2007, 10:40AM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yay, this is looking great

About the matching inductor, I think Steve used an air cored one. You don't need very much inductance if you're transferring a lot of power, as you figured out. If you use ferrite, you'll need a large air gap to avoid saturation. You certainly wouldn't want to use that RS part you mentioned without any airgap!
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Marko
Sun Sep 02 2007, 10:59AM
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Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Steve, Daniel, Finn

You need Only few tens of uH at most, and I think material #26 (yellow-white toroids) would be more than fine for you at these currents.
With rough calculation I think even the size from PC power supply would suffice.


About the matching inductor, I think Steve used an air cored one. You don't need very much inductance if you're transferring a lot of power, as you figured out. If you use ferrite, you'll need a large air gap to avoid saturation. You certainly wouldn't want to use that RS part you mentioned without any airgap!


Steve, I don't think airgap actually does any good with inductors like these.

Inductance will reduce proportionally with permeability and we really get nothing except need to wind more copper onto the core. (OK, with rod cores, this may save some ferrite from other side).

Material #26 has saturation B of about afaik 1T while #77 ferrite is 0.45T max according to CWS table. Most use ferrite to maybe 0.3T.

In any wa ferrite core would need to be larger and more expensive for this application.


The great thing about air cored coil is it's absolute stability, but it is a bad thing when we need 15amps continuously through it at 50kHz.


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Daniel Uhrenholt
Sun Sep 02 2007, 11:56AM
Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Hi Steve and Marko

I did some calculations today on how many Amps we need on the primary to get about 15kVA, and it looks like we need a major rebuild of the supply

I think we have to get a Fo about 50kHz and a C about 1uF

So if P= F*E

E=0.5*560 2* 0.000002 = 0.3136J

P= 50000Hz*0.3136J =15680W

And with a L of 5.07uH we get 248Amps on the primary… That’s a lot of heat we need to get rid off!!


Well I will buy the cores anyway, I want to build a smaller but powerful CCPS when we are done with this coil.

Cheers, Daniel
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Steve Conner
Sun Sep 02 2007, 12:15PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Are you sure about that? The peak current in the resonant circuit should only be something like 3 times what it would be in a hard-switched converter. That gives a loaded Q of 3 even at full output voltage.

For 15kW on a 560V DC bus, a hard-switched converter would run about 30A, so you should have something like 90A peak current. Say 100 since you have an IGBT brick handy

Try designing your circuit such that the leakage inductance and capacitor end up with a surge impedance of (560/100) ohms and a resonant frequency of say 100kHz, or whatever frequency you decide gives acceptable losses. (Switching losses should be low because you have zero current at turn-on and turn-off, so you might be able to aim higher in frequency than the IGBT datasheet would suggest. However, you have to bear in mind that if the loaded Q is too low, you will lose your zero-current turn-off at higher output voltages. This is a trade-off.)
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Daniel Uhrenholt
Sun Sep 02 2007, 12:49PM
Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Hi Steve,

I’m almost sure that my calculations are correct… The I peak of 248A is in the resonance circuit, and looking at other peoples work with high power CCPS`s, their I peak resonant current is in the 100-1100A area..

1120 Amps for their 42kW supply

152A for their 32kHz 15kW supply.

Cheers, Daniel
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Marko
Sun Sep 02 2007, 12:55PM
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
For 15kW on a 560V DC bus, a hard-switched converter would run about 30A, so you should have something like 90A peak current. Say 100 since you have an IGBT brick handy


Right now he has impedance of 2,23 ohms which is quite way far. As far as I understood it, he's limited by total charge he can push through the cap in one cycle, and making the cap big enough makes characteristic Z way too low. If I understood anything of this?

Steve: You didn't answer the thing about cores.. I actually wonder if I'm right or wrong


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Steve Conner
Sun Sep 02 2007, 04:09PM
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Re the thing about cores, it's a known fact that the energy storage before saturation is proportional to the size of the airgap. An ungapped ferrite core can store practically no energy. I've not investigated it myself, but I hear that gapped ferrite isn't nearly so good as iron powder toroids.

Daniel: It always seemed to me that the CCPS tank circuit impedance was a tradeoff between conduction losses caused by higher peak currents, and switching losses caused by it falling out of ZCS at certain output voltages due to not having enough peak current. I think Marco Denicolai explored this with PSpice in a lot of detail. Running at a lower frequency than f0/2 might help too.
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Finn Hammer
Sun Sep 02 2007, 05:13PM
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Hou, Denmark
Posts: 726
Thanks for the input,

These numbers are for a single transformer, topping out at 4kV.

At present:
Z= 25.44, 92kHz
68nF, 44uH, Ipeak, start 25A Ipeak, end 50A , 4kW (1uF to 4kV, 8J, in 2mS)

So we we missed the goal by 50%.....

With a Z of 5, 100kHz,
315nF, 8.04uH, Ipeak, start 100A Ipeak, end 200A (1uF to 4kV, 8J, in 0.4mS = 20kW)

Yes, I get the picture.

The transformer probably has to leave the cabinet, and sure is up for a rewind. The winding window will get *optimally filled* by pie shaped secondary windings.

Waw, this sure is fun stuff.

I feel kind of tempted to skip the negative gate drive on the IGBT`s and adopt Terry`s source follower supplies, from his DRSSTC, but that may have to wait untill the next iteration.

Cheers, Finn Hammer

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Daniel Uhrenholt
Sun Sep 02 2007, 07:51PM
Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Finn,

It looks great, I can’t wait to make an 20kW transformer

Cheers, Daniel
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