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Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Uzzors,
I made an Excel spreadsheet a year back with some calculations on CCPS design. To help you and others out there with CCPS design, I’ve realised that it is time to publish this calculator.
Here it is:
]ccps_calculator_v1.2.zip[/file]
Richie, Conner and Ward
Could you guys be so kind and see if there is any errors in this calculator, and give me a PM if you find any?
I have some explaining problems, because my English is really bad, so don’t be hard on me:-)
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Wow, thanks Daniel! When does "Ipeak end" occur? It would probably explain the over-current problems I had.
It is, Finn. I thought a few mm of PVC would stand just 8kV but I guess not. Maybe the pipe isn't any good. In TC duty people use generic plastic pipe which withstands many times the voltage and at higher frequencies, I must really be unlucky. I'm going to try insulating the former with some plastic sheets, hopefully the Litchenberg figures haven't eaten into the windings too.
Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Uzzors,
“Ipeak end†occurs when the capacitor is fully charged, and the current will not get bigger than this.
I don’t understand that you have over current problems, the great thing about SLR is that you don’t get resonant rise, because you switch at the half of the resonant frequency.
How big are your resonant capacitor, switching frequency and input voltage in DC?
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Uzzors wrote ...
It is, Finn. I thought a few mm of PVC would stand just 8kV but I guess not. Maybe the pipe isn't any good. In TC duty people use generic plastic pipe which withstands many times the voltage and at higher frequencies, I must really be unlucky. I'm going to try insulating the former with some plastic sheets, hopefully the Litchenberg figures haven't eaten into the windings too.
You have a corona problem, and more plastic will make it worse. Only remedy is either to exclude the air in the gap, or increase the gap. Corona from a sharp point on the core has burned into the pvc and carbonized it. Then the carbon acts as the plate of a capacitor, grounded trough the corona arc, the plastic as dielectric and the secondary winding the other plate. This capacitor starts to pump energy into ground. And grows outwards forming the lichtenberg. More and more energy is lost this way, but not as an arc to ground, but as a capacitively coupled discharge. You will find that the tube is not punctured. I have seen this along a 60 cm secondary. This stopped my experiments with internal primary coils in tesla coils.
most od the field is across the air in a air/plastic sandwich. Add more plastic, and the field strength in the remaining ais gets bigger, promoting even fiercer corona. Hope this helps.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Uzzors wrote ...
Therefor I'm considering other soft-switching techniques. Are there any that are easy to implement into a full-bridge?
CFPR inverter (current fed parallel resonant). Not exactly easy, because you need some cross conduction or at least clamp winding on the series choke, and a feedback. It is not that hard to get working though. If I got a big ass core like yours the CFPR inverter would be my choice of driving it, because it is very nice on the transistors (they switch only real power).
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Piles of great advice, thanks guys!
Daniel, for my tests I was using a 24nF capacitor and 40µH external leakage inductance. The transformer leakage inductance is around 20µH. Bridge voltage is 325V from rectified mains. Frequency has been up and down, but I've tried fres/2 exactly and tuning for the best waveform. The surge impedance should be high enough to limit the current safely, even with no load. Transformer saturation seems likely, but how can that happen? I've used about 10 turns, which on a 8.8cm^2 core shouldn't saturate at 60kHz with 50% duty. When doing some tests with neither an external inductor or capacitor I found the transformer had a self-resonance point in the 120kHz range. Maybe I need more capacitance to swamp out the transformer's?
Thanks for the heads up, Finn. I had no idea corona was such a pain. I guess I'll need two new secondaries. How can I prevent this in the future? What about wrapping the core with plastic/electrical tape and using large diameter secondaries?
Jan, I was actually thinking of trying LCLR which is sort of the same thing. My concern with parallel resonant converters is the primary tank circuit. Wouldn't large currents be flowing through the primary even in idle, potentially saturating the transformer? Really, I'm not sure how to estimate the primary turns needed when dealing with resonant rise. I've just always imagined a huge sine wave of unpredictable amplitude.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Uzzors wrote ...
Jan, I was actually thinking of trying LCLR which is sort of the same thing. My concern with parallel resonant converters is the primary tank circuit. Wouldn't large currents be flowing through the primary even in idle, potentially saturating the transformer? Really, I'm not sure how to estimate the primary turns needed when dealing with resonant rise. I've just always imagined a huge sine wave of unpredictable amplitude.
LCLR is completely different , I tried it and it did not work well for driving HV transofrmers (I don't say it will not work for you :) ).
Well, the "idle" current in CFPR will be quite big, but the core cannot saturate by principle, because if it does the inductance goes down -> resonant frequency goes up -> feedback compensates for this by increasing frequency. However the core does get quite hot, as the CFPR needs a fair amount of reactive power for the oscillation to keep going (but I think ferrites can work very very hot without damaging). There is no voltage rise because it is parallel resonant (well the voltage is sine so it will peak above your DC buss voltage anyway, I think it will be pi/2 or sqrt(2) times the DC buss voltage).
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Uzzors wrote ...
Thanks for the heads up, Finn. I had no idea corona was such a pain. I guess I'll need two new secondaries. How can I prevent this in the future? What about wrapping the core with plastic/electrical tape and using large diameter secondaries?
Electrical tape will do you no good. More distance will, and potting or oil. For potting to work, you need degassing under vacuum, so no good, needs a lot of practice before it works, and oil is not good in the experiment phase either. More distance is good, and you need to keep the field strength below 3kV/mm, so 5mm distance from winding to core at the corners should be fine.
Registered Member #125
Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155
Hi,
As I can see you only got 300W of power, and about 6Amps of magnetising current in the primary.
I think this is just a little low for the core, as I think something that size can handle 5kW or even more. My first CCPS was made with a bridge of 48N50 MOSFETs, and a core from an old welding inverter, I had it up and run 30Amps in the primary at 2kvA without any problems at all. You definitely need more power to get that core up and running…
If you are afraid that the core is saturating, then run the core without your secondary, and only a few turns 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. And out from this you can calculate volts/turn, and design out from this.
We did this with our first prototype, and it seemed to work just fine:-)
I haven’t read the book Soft Ferrites by E. C. Snelling, so I can’t give you any calculations on primary turns etc.
I would redesign the transformer for high coupling if I was you, just put your primary under the secondary, and adjust with your external inductor and capacitor to get the desired frequency and power.
Yes, you definitely need more capacitor for that core:-)
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Big mo-fo ferrite HV transformer driving (another CCPS/SLR inverter)
I thought a few mm of PVC would stand just 8kV but I guess not. Maybe the pipe isn't any good. In TC duty people use generic plastic pipe which withstands many times the voltage and at higher frequencies, I must really be unlucky. I'm going to try insulating the former with some plastic sheets, hopefully the Litchenberg figures haven't eaten into the windings too.
