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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi Marko
Marko wrote ...
I do not think this circuit you drawn would work since current fed converter *needs* shoot through to work, because link choke current must never be interrupted.
In your schematic transistors would short out the tank cap if they go on simultaneously.
I don't think so.. I've made a crappy waveforms in MS paint just to illustrate:
wrote ...
If you are only into driving ferrite transformers, 50..100kHz, it may all be a bit of overkill. Do you really need perfect ZVS and ZCS so badly that you want to go through all of that complexity, to drive a flyback?
I want to build a multipurpose HV inverter, possibly in the 1-3kW maximum real power output range, multiply by 4 and you have the maximum reactive power - 4 to 12KVA in this case, not really achievable with standard FETs or IGBTs.
(Why multiply by 4: You have a power supply with constant impedance. This power supply puts out most power at 1/2 output voltage. Agree? At this time, 1/2 of the reactive power circulates through the impedance/reactance and the other 1/2 is the real power. So at this time- reactive power = 2x real power. When you short circuit the output, the reactive power rises 2 times again (you go from 1/2 output voltage to 0V). And 2x2=4, so in the case of short circuit, 4x more reactive power circulates than the maximum real power the PSU can put out.)
wrote ...
Did you use feedback or a fixed oscillator? IH guys had success with simple direct feedback from the tank current.
I did use fixed oscillator but i tried the tank circuit feedback too, and my current limiting lightbulb was really bright (this means a problem - something would explode if I connect it up without the bulb limiting).
wrote ...
Matching inductor should never resonate with tank capacitor, only with DC blocking cap and tat is only if you take current feedback from inverter output. I'd expect inverter to blow up instantly in that condition...
Problem - the inductance of the primary winding varies greatly with load, it is small with shorted output and very big with open output- this is the worst condition.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I don't think so.. I've made a crappy waveforms in MS paint just to illustrate:
Ahh, I really hoped someone (richie, steve, etc.) would come to fill up, as your understanding of these things is apparently better than mine...
Don't know for fullbridge, but looking at half-bridge inverter of this kind, if only the upper side had choke, the bottom transistor would ''voltage feed'' the tank circuit. (short out the tank cap each time it turns on), there's nothing stopping it from doing so.
There must have been some reason after all they used split choke on their CFL inverter.
The choke's core will see just the same flux as you drawn in first picture, only it will come from two different windings.
(Why multiply by 4: You have a power supply with constant impedance. This power supply puts out most power at 1/2 output voltage. Agree? At this time, 1/2 of the reactive power circulates through the impedance/reactance and the other 1/2 is the real power. So at this time- reactive power = 2x real power. When you short circuit the output, the reactive power rises 2 times again (you go from 1/2 output voltage to 0V). And 2x2=4, so in the case of short circuit, 4x more reactive power circulates than the maximum real power the PSU can put out.)
Yeah, (having a hard time reading this ), the *reactive power* will increase 4 times from match to short. I confused it with apparent power on the inverter which should increase for sqrt2.
I'm not sure how much more stress would this pose to the inverter? It would switch only sqrt2 more current.
I did use fixed oscillator but i tried the tank circuit feedback too, and my current limiting lightbulb was really bright (this means a problem - something would explode if I connect it up without the bulb limiting).
I still really don't know what would prevent LCLR from saturating the core at all.with no load present. You definitely need much larger inductance than an IH has and big airgap in the core. I can hardly tell more.
Problem - the inductance of the primary winding varies greatly with load, it is small with shorted output and very big with open output- this is the worst condition.
As far as I know, that's the problem of *all* resonant systems we mentioned here.
The transformer always needs to be ballasted inductively or capacitively to remove this problem, and still, your magnetizing inductance will vary over large range because of arc loading.
Your inverter would need to track frequency over high range and take care not to ever saturate it, which looks like a serious problem to me.
It's much easier with stable loads like filter caps or fluorescent tubes where magnetizing inductance of the transformer is going to stay relatively fixed.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I want to build a multipurpose HV inverter, possibly in the 1-3kW maximum real power output range, multiply by 4 and you have the maximum reactive power - 4 to 12KVA in this case, not really achievable with standard FETs or IGBTs.
12KVA isnt all that hard, and i used standard $5 IGBTs in my SLR inverters to do about "10kVA" with no sweat. Because of the ZCS, the switching losses were just about zero, which in many cases greatly outweighs the extra conduction losses caused by the higher sinusoidal currents. My huge ferrite core got warmer than my IGBTs in this case.
I think you guys might be going a bit over the top with trying to fix this "problem". There likely are some tricks that havent been exploited much in the hobbyist community, but i doubt you guys are missing out on anything really important.
Instead of immediately throwing out the SLR, LCLR, or current fed inverter topologies, run some actual power loss calculations and see which one works best for what you intend your power supply do to!
(Why multiply by 4: You have a power supply with constant impedance. This power supply puts out most power at 1/2 output voltage. Agree?
I already disagree. Constant impedance? I guess if it was a battery... but most of these inverter-drive schemes certainly arent constant impedance. A "constant current" source (like the SLR) had better NOT be constant impedance, otherwise it would suck as a constant current source.
The problems are 1) you need to switch at zero voltage not zero current so CT is not an option,
Maybe a rogowski coil, or a CT with a differentiator, which should give you a representation of the voltage (at least for the LC pair) could work.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve Ward wrote ...
(Why multiply by 4: You have a power supply with constant impedance. This power supply puts out most power at 1/2 output voltage. Agree?
I already disagree. Constant impedance? I guess if it was a battery... but most of these inverter-drive schemes certainly arent constant impedance. A "constant current" source (like the SLR) had better NOT be constant impedance, otherwise it would suck as a constant current source.
Hi Steve, I was talking about hard-switched fixed frequency inverters where the output impedance is constant.
Your post got me interested again in the SLR inverter, but the problems I had previously were with open-circuit performance - the ZCS was lost and the transistors were heating a lot. IS there any way to better the open circuit performance?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
The SLR converter behaves as a constant current source, so it is inherently short-circuit protected. An open-circuit load is meaningless for a current source because it implies infinite output voltage.
All resonant-mode power converter topologies have a range of load conditions over which ZCS or ZVS can be maintained due to the circulating energy. You cannot expect them to maintain this perfect operating condition with Q's ranging from 0 to infinity! Most only operate with optimal efficiency over a small load range.
As Steve W said, define the requirements of your power supply thoroughly, then choose from one of the industry's existing solutions to your problem.
Hard switched PWM bridge and ZVS phase-shifted bridge both approximate constant voltage sources and can be made s/c proof with cycle-by-cycle current limiting or by using peak current-mode control. Conversely, the series-load-resoanant converter (SLR) and parallel-load-resonant converter (LCLR or LLC) can both approximate a constant current source.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Your post got me interested again in the SLR inverter, but the problems I had previously were with open-circuit performance - the ZCS was lost and the transistors were heating a lot. IS there any way to better the open circuit performance?
Im almost willing to bet money that 1) your transformer had too much leakage inductance of its own, and 2) you didnt have an external, discrete inductor. From what i found, all the leakage inductance on the primary circuit should be external to the transformer itself, that is a separate inductor, not coupled in any way to the transformer. With this setup i achieved the cleanest short-circuit and loaded operation, and the open circuit condition was very safe with only a few watts of power consumed by the IGBTs, and the current was about 1/10th of the peak current during short circuit or capacitor charging.
Indeed, if i had a gap in my ferrite transformer, i experienced very similar problems to what you mention. You have to be very careful to avoid having an open circuit resonance! Normally, the magnetizing inductance of the transformer should completely destroy the possibility of resonance with open circuit output (that means Lmag should be >> Lstray).
I even had a cracked ferrite once that i glued back together, thinking it would still be OK. Well, it was just "OK". Changing it out to a new core made a very large improvement on the open circuit behavior compared with the repaired ferrite that had just a hairline crack through it.
So the point im getting at is, dont give up on the SLR yet! It can seem unstable until you figure out the small problems (somewhat detailed above). It can be made to work wonderfully for making arcs for fun, or for actually charging capacitors. I use mine to charge my 12kV 72uF pulse cap, and it gets the job done *fast*. I also have fun drawing arcs off the output.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
One thing is spinning in my mind.
Would it be possible to build a HV supply/CCPS without any ferrite transformers at all, only utilizing a series impedance matching network?
Just like the DRSSTC primary tank circuit, but much higher characteristic impedance to match the inverter properly.
A bit more complex current control would be required to keep it from blowing up without load, but I don't see anything that would make it impossible? Why haven't I seen something like that anywhere?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
The parallel load resonant converter basically does this Marko, but a transformer is usually used to provide electrical isolation between the input and output sides. Remember that a high-speed rectifier is required to convert the high-frequency AC back to DC to charge the output capacitor so the negative terminal of the capacitor can't share the same node as the inverter's negative bus without an isolation barrier.
High voltage step-up ratio's are best done with a transformer though because high Q resonant networks imply large circulating currents and hence increased I^2R losses.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve Ward wrote ...
Im almost willing to bet money that 1) your transformer had too much leakage inductance of its own, and 2) you didnt have an external, discrete inductor.
yes. Really, to minimize Lleak you want to wind the pri. under the sec., and I don't like this as you can not adjust primary turns/wire thickness after you have wound the secondary over it.
I had a look again at the current-fed halfbridge that Marko posted (with a dual-winding DC choke with each winding from each supply rail to the transistors), and so far it seems like my choice. I just have one question: am I right that the peak voltage across the transistors will be pi/2 times the supply voltage so I can use 600V devices for 325VDC supply?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I just have one question: am I right that the peak voltage across the transistors will be pi/2 times the supply voltage so I can use 600V devices for 325VDC supply?
I thought so, since royer doubles switch voltage I'd expect it too to be pi/2.
I forgot although how is this derived, inductor current is fullwave-rectified like with mean magnitude of 2/pi and waveform factor of pi/2sqrt2.
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The probem of short-circuitable inverters
), the *reactive power* will increase 4 times from match to short.
Really, to minimize Lleak you want to wind the pri. under the sec., and I don't like this as you can not adjust primary turns/wire thickness after you have wound the secondary over it.