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Registered Member #1571
Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44
Hi there,
I’d like to say at the start I am not an electrical engineer but wonder if I might pique the interest of one or more on this forum.
You see I have a friend who has been playing with high voltage… as have I. I, however, failed to achieve the high voltage I aimed for ~50kV peak – I achieved instead 25kV peak and it then blew the bridge when I went a little higher.
My rig was mains input (AUS) into an isolation transformer, into a variac, then bridge rectified and then fed into a full bridge that was connected to a primary of 16 turns/secondary of just over 3000 turns (an air coil – 28uH primary/2.185H secondary).
Why did it blow up… excessive current… The primary having an inductance of just 28uH meant the circuit was seeing pretty much a short circuit. (that’s what I think is the case but it would be nice for someone a lot more informed than me to confirm my idea here).
Now I contacted a friend (in another country) who did a better job than me on this and his rig was different to mine. He informed me that I needed to use a current source converter and a primary that is centre tapped – he stated on the centre tapped primary:
“I used only 2 power mosfets with a center tapped primary, this also contributes to limit surge currents, compared with a full bridge.â€
…he also used a coil that had a core. In particular he states:
“I used a 80 * 56 * 320 mm I-form core, and if I had more sheets, I had piled more sheets to get 80 mm (square core). With such a non closed, 320 mm long straight core, you get a huge benefit of better coupling between primary and secondary, much higher secondary voltage for less input voltage and current, and no drawbacks as I had first with a closed core.â€
My questions are here and I am wondering if those that might take the time to answer, answer each specifically or particular ones as it would greatly help my understanding of the problem space…
1. What is a "Current source inverter" as apposed to a "Voltage source inverter"?
2. I used a voltage source converter it seems… I got excessive current as the inductance of my primary was woefully small. Will adding a large inductor – ballast – in series with the circuit convert this to a voltage source converter?
3. Will this change successfully limit the current and run a primary with such a small inductance?
4. Given that I need to turn <100V into ~50kV is current source converter a much better idea?
5. How is having a large gapped core an asset in this circuit as apposed to a non gapped core why the gap in this circumstance?
6. How does a centre tapped primary assist (as my friend mentions) in limiting the surge current given his topology(Circuit attached).
Hoping someone might help me understand these points....
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
I'll respond to some of those points here.
wrote ...
My rig was mains input (AUS) into an isolation transformer, into a variac, then bridge rectified and then fed into a full bridge that was connected to a primary of 16 turns/secondary of just over 3000 turns (an air coil – 28uH primary/2.185H secondary).
Without a core you'll have more leakage inductance, meaning a higher energy spike at the transistor drain on turnoff. I see in your schematic you have what looks like a snubber there to try to handle it. This section may merit further scrutiny, particularly with air core transformer.
wrote ...
Why did it blow up… excessive current… The primary having an inductance of just 28uH meant the circuit was seeing pretty much a short circuit. (that’s what I think is the case but it would be nice for someone a lot more informed than me to confirm my idea here).
An inductor is not a short at high frequency, if you apply an input voltage Vin, then the current ramps up at Vin/L amps per second. When switching on and off at high frequency the current might not be very big.
wrote ...
1. What is a "Current source inverter" as apposed to a "Voltage source inverter"?
2. I used a voltage source converter it seems… I got excessive current as the inductance of my primary was woefully small. Will adding a large inductor – ballast – in series with the circuit convert this to a voltage source converter?
3. Will this change successfully limit the current and run a primary with such a small inductance?
4. Given that I need to turn <100V into ~50kV is current source converter a much better idea?
5. How is having a large gapped core an asset in this circuit as apposed to a non gapped core why the gap in this circumstance?
6. How does a centre tapped primary assist (as my friend mentions) in limiting the surge current given his topology(Circuit attached).
1. It means you have something approximating a constant current input, rather than contant voltage. If you smooth out an input voltage with a shunt capacitor you have a constant voltage input. If you smooth out the input current with a series inductor, you have something like a constant current input. I'm not too sure what the reasoning is behind using a current source inverter, I think it may make the feedback easier if you are regulating the transistor duty cycle to get a constant output voltage.
2. Higher primary inductance will decrease the magnetizing current, though not the current that flows to the load, but how high of an inductance you need depends on your transistor sizing and switching frequency. Adding a series inductor turns it into a current source inverter.
3. You can most easily increase primary inductance by using a transformer core.
4. I dunno, sorry.
5. The gapped core helps ensure that the core does not saturate, by limiting the magnetic flux. If you have too many amp-turns on the input and this creates too high of a flux (generally something like 0.3-0.5 Teslas) then the core will saturate and will stop and like an inductance and start acting like a short.
6. Center-tapped with a push-pull means you need fewer transistors than a full bridge, but you need a bigger transformer. If I remember correctly the voltage stress on the transistors is also doubled.
Registered Member #1571
Joined: Wed Jul 02 2008, 03:26AM
Location: Bendigo Victoria Australia
Posts: 44
Hi there,
First of all thanks for your answer Mattski... its helped.
I am still grappling with the idea of current and the fact that my full bridge blew in the face of the settings in the original post.
So from what I understand the current, if you use the formula H = Wb * t / A or A = Wb * t / H and Wb = V * s / t (t = turns) then a coil with 16 turns and a frequency of 15000Hz then you would get:
A = V * s * t/H or
= 40 * 1/15000 *16/ 0.000028
= 1523 Amps
I reckon I might have calculated this incorrectly but it would indicated that the coils inductance was no where near big enough to keep the current from blowing up my bridge.
Now I would like to ask a question of the forum here... What about putting a choke/ballast (a coil with much greater inductance) in series with the bridge??
Would the bridge now be limited to the choke's current?
In the example in which I had the system running I had each leg of the bridge switching via a drive transformer. The only point where there would be a drop in current would be when there was a cross over and one side of the bridge switched off while, with a little bit of dead time, the other leg switched on.
The fact that the only time there was a significant change in the current was this momentary switching during dead time... My last question...
Would the fact that there was bugger all change in the current required from the bridge render the choke irrelevant ??
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
You need to figure out the inductance of your transformer. 16 turns without a core of any sort will tend not to be a high inductance.
Very approximately (using the formula for a long solenoid), your inductance will be L=mu*N^2*A/l: and some of the formulas on wikipedia might be better.
Then if you apply a constant voltage when the bridge is on, the magnetic current will ramp up at rate I=Vin*time/L. There will also be the "reflected load current" which depends on your load, and is equal to the output current on the secondary times the ratio of secondary to primary turns.
All of this is complicated by the air core, you will have lots of flux that is not coupling the two windings which creates spikes on the transistors and effectively changes the turns ratio. And it substantially reduces your inductance by a factor of 10's to 1000's depending on material, which is exacerbating the current issue.
Putting a coil in series with the bridge could help, though you need to think about where the current will be going if there is some dead time between the two halves of the bridge switching, because that could also cause voltage spikes on the transistors. I think some people do it though.
What current was changing during the dead time?
I think the best thing you can do is read up a bit on transformer design and get yourself a ferrite core.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
A few of us are working on similar projects, Si, and we're starting with lots of ferrite.
I'm currently working with four of the largest 'c' cores I could get, sandwidged together, others have used 8.
The reason for this is that the more ferrite you have in your core (the larger the csa) the more volts per turn you can have before the core goes into saturation.
This approach can result in cooling problems with the core though, but most of us are placing our transformers under oil anyway for insulation, which also facilitates oil cooling.
I'd reccomend you also follow this approach, although it is quite feasible to use an air core transformer to achieve these voltages (just read up on some of the tesla coil circuits here)
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Questions regarding the generation of 50kV AC....
