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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Finn, why don't you just use a capacitive load as it would be in order for the supply? It may be noisy, but is the *right* way for the supply to work. You may use water to diischarge and muffle the cap into..?
And you can easily visualize output power by speed it charges the cap. AFAIK, those supplies are actually rated in J/s for power output for capacitor charging. I think it has to do with that it behaves much more like a current than a voltage source.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Marko wrote ...
Finn, why don't you just use a capacitive load as it would be in order for the supply? It may be noisy, but is the *right* way for the supply to work. You may use water to diischarge and muffle the cap into..?
And you can easily visualize output power by speed it charges the cap. AFAIK, those supplies are actually rated in J/s for power output for capacitor charging. I think it has to do with that it behaves much more like a current than a voltage source.
Marco,
I have not built a suitable cap yet, and: (blushing with shame) The honest answer is that I think slowly. I would like to sit and watch the scope at prolonged periods, pondering the waveforms, the ringings and such. If I don`t do this, I will learn nothing, the stuf I build will blow up and I will not have a clue to base the formulation of a question over.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
In reply to Steve Ward's comment about unsafe open-circuit voltages:
The SLR is designed to drive a load that appears as a short circuit at high frequencies. Its behaviour with an open load is undefined, and as Steve pointed out, undefined usually means smoke.
My solution would be to make sure that the unit never tries to drive an open circuit. Since this is a CCPS, I would integrate the output rectifier, voltage sensor and a small capacitor (maybe as part of a Terry Filter) into the apparatus. Then turning on without a capacitor bank connected will charge the small capacitor very quickly, and the voltage feedback will shut the converter down.
The energy in this capacitor will be wasted in the Terry filter resistor every time the Tesla coil fires, so it should be made as small as possible, provided the open load behaviour stays safe. It really is bad form if your converter catches fire every time you forget to plug it into the Tesla coil.
Marko pointed out a while ago that the behaviour of a SLR converter with a Jacob's Ladder load is also undefined. It sounds like Steve Ward did some work to define it, though, so if you want to make a Jacob's Ladder driver, ask him. (Or just insulate heck out of the transformer and use those "extra resonances" to help get the arc going...)
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Steve,
Thanks for your look into the (hopefully) not too distant future. A Terry filter will be built into the supply for sure, and the idea of a small internal capacitor is good. We sure don`t want to fry the supply every time. =:-O
Now, about running into an open circuit. I have not covered this in detail, like I have not taken scopeshots of it, but from memory, what we saw was a triangle current wave, like in a SSTC, with a ripple overlayed. Ripple ,I think, the same 5-6MHz that seems to be inherent in the supply, due to unaccounted circuit parameters. I had previously looked at an open circuit as equivalent to a full capacitor, but this is wrong, of course, because the cap, although full at the design top voltage, still is a low impedance load to overshoots.
OTOH, we didn`t just set the jacobs ladder to infinite distance, so whatever abnormalities, they were caught and transferred into plasma.
I`l be good and not hook the supply to a jacob ladder again, unless I want to explore the fact/myth (that the combination JL/CCPS is a catastrophical one) in detail.
Making videoes of the scope shots, showing the effect of parameter changes, voltage or frequency, is new i think. Any of you guys get usefull information out of that? Should I talk and point out what to look for, would that be better? Daniel and I are talking about writing the cookbook of CCPS, Based on this supply, and this thread, and we would like to know if the dynamic scopeshots are any good.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The open circuit condition can in fact be very safe and tame, but it can also be catastrophic. I tweak my operating frequency (among other parameters) until im satisfied with the primary current (during O.C.) and open circuit voltage. But, my later controllers allowed a form of PWM control where you can drop the operating frequency and there is a definite voltage peak at a little less than the highest possible frequency (so it should be avoided if possible).
As Conner says, insulate the heck out of it and have fun . Core saturation should really limit the output voltage to some degree but depending on how conservative you were in the design, that could be rather high voltage! Ive seen upward of 30kV on some of my earlier designs that had too much leakage inductance in the transformer.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
All,
Richie said we should make the gate drive as slow as possible, and since the gate resistors were already 10ohms, the current into the gates is as low as 2A. Why use a hotrodded DRSSTC worthy gatedriver for that? Below are gate waveforms for first, the hotrodded driver:
Driving the gates directly from the HP optocouplers shows, that they sink curent slightly worse. They get quite warm too:
If I remove the 5V negative bias, which I am not sure I need, I get this situation, with cool running opto`s:
In this the third iteration of the CCPS, I would like to remove all the overkill that sneaked into it, during investigations. The gate driver is one such thing. The low inductance busses were another. We never got an elegant power supply for the gates either, instead we took the term "Breadboarding" way too literal:
I am going to try out Terry`s source follower supply from his DRSSTC.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Aww, why did you remove low-inductance bus? It looked so cool.
Gate drive needs something to be done although... maybe just directly use a larger GDT?
PS. As far as I figured out, terry is powering his drivers from HV bus directly via dropper resistors? Am I looking at right schematic? That exact approach is not going to work in CW unless you are prepared to dissipate huge amounts of power in resistors.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Marko wrote ...
PS. As far as I figured out, terry is powering his drivers from HV bus directly via dropper resistors? Am I looking at right schematic? That exact approach is not going to work in CW unless you are prepared to dissipate huge amounts of power in resistors.
Marko
Oh no, it is really elegant. The solid state equivalent to a servo: The source follower. Here is a schematic:
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Oh no, it is really elegant. The solid state equivalent to a servo: The source follower. Here is a schematic:
Sorry but I see nothing of elegance.
It is not going to work for CW without a charge pump or some kind of power supply.. DRSSTC is pulsed and really doesn't require much gate drive power, unlike your huge IGBT's in CW.
Looking at rest of terry's schematic I found nothing like that.
As far as I understood it all you are going to dissipate hundreds of watts to get enough drive speed.
Apart from that, opto-drivers are slow. It may not be a problem though.
ANd it doesn't drive the gate negative. I guess I'm misunderstanding at some part.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Marko wrote ...
Sorry but I see nothing of elegance.
Marko wrote ...
It is not going to work for CW without a charge pump or some kind of power supply..
It _is_ the power supply. during initial startup, where the supply voltage is ramped up, the 3 caps, are charged to full potential. C1 reaches the zener voltage and forms a bias supply for the mosfet. The mosfet conducts as long as it`s source is more than 5V below this, so the caps c2 and c3 end up with Vzener - 5V on them.
Marko wrote ...
DRSSTC is pulsed and really doesn't require much gate drive power, unlike your huge IGBT's in CW.
The total cate charge is 2uC. at 50kHz that translates into 100mA gate current/avg.
Marko wrote ...
As far as I understood it all you are going to dissipate hundreds of watts to get enough drive speed.
So for dissipation, I get in R1, which I will make 100K at 560V ,3.5mArms => 1.225W in R2 which I will make 200ohm, I get 0.1^2 * 200 => 2W
I will put a diode on top of the resistors to keep the caps from discharging while the IGBT is conducting.
Marko wrote ...
Apart from that, opto-drivers are slow. It may not be a problem though.
Slow in what way? The scope shots showed that they can deliver the current, and thay are part of the supply I use at this moment, so speed is no problem.
Marko wrote ...
ANd it doesn't drive the gate negative. I guess I'm misunderstanding at some part.
Can you give a good reason why I should need a negative supply?
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CCPS (Capacitor Charging Power Supply)
. Core saturation should really limit the output voltage to some degree but depending on how conservative you were in the design, that could be rather high voltage! Ive seen upward of 30kV on some of my earlier designs that had too much leakage inductance in the transformer.
