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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I see. I still think the CT approach is pretty bullet-proof, but they have failed before in ATX supplies. If you want to sample the supply rails which are DC, the only method I can think of is using a current sense resistor. You'll want isolation and level-shifting, an optoisolator should work for that. Just set the resistance so the voltage hits the forward voltage at max current. Be aware that the forward voltage doesn't remain constant and varies for each device. Using a CT is much more precise, in fact you might want to employ both methods for ultimate durability.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys...
My overcurrent circuit was nothing more than a CT, one diode and small signal SCR which tripped after voltage on the burden got over one point and shut down the SG3525 and power needs to be cycled in order to restart. I can draw it for you if you like but I warn you that it's actually a poor solution, quite unstable and small signal SCR's like that are not very available.
I would recommend to use a comparator, like LM393, to sense the burden voltage and maybe another comparator as a latch. (I believe latching overcurrent protection is much better than some kind of current-mode control).
Further upgrade could be dI/dt sensing protection which would allow larger overcurrent pulses in short time if you need that.
steve wrote ...
JMartis' circuit is the one I was thinking of. AFAIK, you can't make a current-fed inverter as a halfbridge, it has to be a fullbridge.
I was actually referring to a resonant inverter... but regarding schematic Jan linked, would it solve the massive-reactive-power problem? I understand there would be no more duty-cycle regulation possible and it must be done with a buck converter, and some shot through would actually be needed in order for the circuit t work well.
But we basically transferred the buck inductor from secondary side (which is impossible with a flyback) to primary side as shown on schematic, so I think driving capacitive and troublesome low-impedance loads should then be much less problematic.
It might all just work but I don't see any advantage against a resonant CF inverter, apart from lack of feedback.
Now, steve:
I referred to this note several times but I apparently didn't get much attention.
It's the circuit at page 5.
Each transistor must have a series choke (if only one had, the another would still slam dunk the tank cap into itself with each cycle).
The choke seems to be on the same core, wound in same direction so shotthrough currents *don't* cancel out. The core basically feels the same waveform as in a royer.
The bridge then drives a parallel LC... but I don't know what is separate C3 and Cr about. Cr looks like it's decoupling the bridge O_o
That's all I know about this topology - and although it looked promising and interesting I don't have enough balls to even try building it.
Uzzors, the problem of your over current detection is that if one fet fails, the other one dumps the filter caps to it, ultimately destructing it. I'm going to make an OCD that rather samples current on one of the power rails. This way if one FET fails and the other one starts turning on, the OCD will (hopefully) switch it off before it explodes.
Jan, a fet doesn't fail if your CT overcurrent protection works well, so you don't have to worry about that :p
Sensing rail current will be a quite tricky thing to do. A shunt will need to dissipate very large amounts of power to give measurable voltage (just think of 10 amps for 1 diode drop).
That is usually done with hall current transducers, if you can get one...
Some people also seemed to use success with diode-shunted CT's on pulsating current. It might work since it never sees >50% duty cycle.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
edit- then theres always the way of the CTCT (center-tapped CT)
Windings need to be phased out in order not to put DC on the transformer - and then shoot through currents will cancel out. Center tap would not work because the core would see pure DC at 50% duty cycle.
As I already said you'l need to use reverse diode in order to make the a CT work the way you want.
Or you might use differential transformer to detect shot through only (so 2 CT's) but I think all that's too much hassle.
I'm not sure if you'll be able to shut everything off quickly enough to save the mosfet's life once few hundred amps have started to dump into it from bus cap...
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well I think I now have an idea why my original circuit actually *could* work but again I don't have enough experience with magnetics/LC circuits. The primary voltage cannot rise indefinitely because the core will saturate sooner or later, this implies that the current too cannot rise indefinitely (actually it should stay at a sensible value). Am I right?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
The primary voltage cannot rise indefinitely because the core will saturate sooner or later, this implies that the current too cannot rise indefinitely (actually it should stay at a sensible value). Am I right?
After the core saturates all that happens is that it's inductance will drop and current will spike. It doesn't matter how high the voltage gets when your inverter will simply see a short on output since energy can't be transferred further to load.
Only way to keep series RLC stable is to keep the R large, that is to have load present all the time, and it must be large enough to keep the core from saturating.
If you just want to drive flybacks I don't see much merit in this approach.
From all I learned until now, I think my ultimate off-line flyback driver would be PFC buck-boost converter powering a current-fed ZVS oscillator.
Royer is so good because it shifts all reactive power to the tank circuit, so the switches only really see the real current. This is very good for poor pf and unstable loads like arcs.
If I was just going to charge caps, power a SGTC for example, I'd go for a SLR converter.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
After the core saturates all that happens is that it's inductance will drop and current will spike. It doesn't matter how high the voltage gets when your inverter will simply see a short on output since energy can't be transferred further to load.
Actually, you can still transfer energy to the secondary when the core is saturated. If you wind the primary and secondary on the same axis, then this helps greatly as they are still decently coupled without a core present, so any B, beyond Bsat will be outside of the core, but can still link the primary and secondary coils. I wouldnt recommend operating a transformer like this of course.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
What about this? Does it look as off as my other ideas? I think this is no longer ZCS but the reactive current should still circulate in the LC circuit ...?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve Conner wrote ...
That's a LCLR circuit, and the same theories discussed in the OMG Induction Heater thread apply.
Ok, I've read the thread. I just have some basic questions: 1) How high voltage will see the tank circuit? Can it be higher than drive voltage? 2) How do I calculate the drive inductor? (I think its called matching inductor) 3) Where is better to put the CT, across the capacitor or inductor? 4) And finally, is there any reason why it would not work for a flyback?
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ZCS resonant driver idea for (HV) ferrite power transformers
I think this is no longer ZCS but the reactive current should still circulate in the LC circuit ...?
