Electronics > Beginners
LLC vs LCC Converters for High Output Voltage
T3sl4co1l:
--- Quote from: MagicSmoker on May 15, 2019, 02:33:42 pm ---Finally, the higher the high turns ratio (in either direction) the higher the leakage inductance, usually, (and the higher the losses from proximity effect, which, fortunately, does not apply to the flyback).
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Hm? Explain?
Proximity effect occurs in any multilayer section. It's simply the image current of one layer induced upon the adjacent layer, that happens to already be carrying the same current in the same direction.
It probably isn't important at these currents and frequencies -- the wire used will be quite small.
It's like how the smallest wire I have in stock is 37 AWG, so when I want to wind something for high voltage, well, it's automatically capable of ~30mA continuously, even if I only needed a few mA... ::)
So, which, actually... thanks, I never thought about this before but it's quite true:
In any flyback transformer, with multiple layers of windings, regardless of interleave, proximity effect occurs.
This is simply because the primary and secondary currents are not coincident, so they can't cancel out their proximity. (Whereas you can do this in a forward converter with interleaved single layer sections.)
So flybacks with many layers are somewhat prohibitive, and one or few layers are preferred, which means wider bobbins should also be preferred (i.e., to maximize turns/layer). Cool!
(Interleave is still critical for leakage, however!)
--- Quote ---While the clamp diodes in the two-switch variant do return all the energy stored in the leakage inductance back to the supply, while they are doing so the output diodes can't deliver energy to the load... So, a lower turns ratio secondary feeding a half-wave doubler/tripler/quadrupler could be highly advantageous here for that reason, as well.
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In any case, the 2-switch discussion is spot on. Also, the secondary peak voltage rating is the same (total) whether the secondary is whole, or broken into sections with a diode each.
Incidentally, HV diodes are all just stacks of lower voltage diodes -- I don't think they can make high speed (<= 250ns or so?) diodes over 1.5kV or so. (In fact, an industrial "hockey puck" rated for 6kV is "high speed" if it has a recovery of 2us!) So you see V_F climb -- multiple junctions in series.
The trick is, they're matched diodes, and packaged together, so recovery and capacitance are equal. This is a lot harder to pull off with discrete diodes*, so there is value in the high voltage types.
*What happens is, one recovers first, and gets all the voltage drop and immediately goes into avalanche, burning a huge amount of power. Then the next one recovers, the total voltage drop rises and not as much power is burned, and so on until the total avalanche rating exceeds the applied (reverse) voltage, and conduction stops. And it's a runaway condition, because recovery gets slower at higher temperature.
At least it's not as brutal as with a forward converter, where the reverse current is switched in -- a flyback in DCM has soft recovery, with dI/dt set by the transformer inductance.
Actually, an even more subtle point is that recovery itself is a sort of dynamic avalanche condition: effectively, the off-state (blocking) voltage of the diode increases over time, eventually reaching its nominal rating. This probably happens simultaneously for all diodes in the stack, but the result is still that one diode dissipates much more recovery loss than all the others.
But anyway -- if 20kV diodes are unobtanium, at least the secondary can be split into sections, and as many 3 to 10kV diodes can be used, which are hopefully more obtanium!
--- Quote ---And if you put all of these conflicting issues together you start to see why I keep recommending a flyback with a voltage multiplier,
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Also -- note that a multiplier needs a different control circuit than a bog-standard (current mode) flyback. The multiplier draws current during switch-on, so either a current-limiting switch is needed, or something that acts in the same way. It may be adequate, for example, to use a normal peak-current-mode controller, but increase the transformer's leakage, so that the switch turns on for a moment, charges the multiplier through the leakage inductance, then turns off. (The increased leakage would suck for a one-switch converter, but would be very much in favor of the two-switch or full-bridge version.) The operational result will be, lower maximum power output at low output voltages, because the switch turns off much sooner than it would otherwise. If you don't need a wide output voltage range, and don't need a lot of startup current, that should be quite feasible.
A current limiting switch (i.e., a linear device) is probably completely out because of heat and size.
Proof that it can be done, though: https://www.seventransistorlabs.com/Images/Deadbug_Sch.png This is hardly a watt, and the switching transistor can get quite toasty under some conditions, so it's hard to recommend at any kind of scale!
--- Quote ---Yes, a properly designed (and protected) LCC or other resonant converter would achieve a higher efficiency and likely take up less space, but forget this being the deep end of the pool; at that point you are swimming with the sharks out in open blue water. If you have sufficient dedication and budget (in both time and money) to dick around with this, then by all means attempt a resonant converter design - it will be better for the job - but if the real world has placed practical restrictions on you then, no, none of the exotic topologies are really appropriate. Note that I am not saying you can't or won't be able to get a more exotic topology working - who am I to judge what you are capable of? - I'm just saying that it will be a much more difficult task, relatively speaking.
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And speaking of budget -- do consider hiring a consultant to help guide you. Or, heck, just contract out the whole thing, if it just needs to be done -- give or take how many lessons you want to learn from the project as well.
Which, on that note, I'm pretty busy right now actually, but if your timeline extends to late summer I'd be glad to help. Otherwise, there may be high voltage / switching professionals here who are available; doesn't hurt to ask. :-+
--- Quote ---And you won't be running nearly as high as a switching frequency as you appear to think is possible - after all, a mere 10pF of stray capacitance has an impedance of 160k at 100kHz, so a 10kV square wave applied across this will result in 63mA of current. Whether that current is flowing across the junction of a diode that is supposed to be off, or across a transformer secondary, it amounts to a pure loss...
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Well..... that's the whole point of the resonant converter, right?
It does tend to be loss, in a traditional flyback converter that needs wide bandwidth and can only discard energy that gets trapped in reactances.
It does seem unlikely to construct a transformer with bandwidth of multiple MHz (that would make a 20-100kHz flyback feasible), so that is a point strongly in favor of resonant.
The tradeoff is then, a flyback that's easy to design but has terrible efficiency, or a resonant that's hard to design but easily gets good efficiency. :-\
Tim
MagicSmoker:
--- Quote from: T3sl4co1l on May 15, 2019, 03:12:07 pm ---
--- Quote from: MagicSmoker on May 15, 2019, 02:33:42 pm ---Finally, the higher the high turns ratio (in either direction) the higher the leakage inductance, usually, (and the higher the losses from proximity effect, which, fortunately, does not apply to the flyback).
--- End quote ---
Hm? Explain?
Proximity effect occurs in any multilayer section. It's simply the image current of one layer induced upon the adjacent layer, that happens to already be carrying the same current in the same direction.
--- End quote ---
Yes, an important point I failed to make - that proximity effect applies whenever a winding has more than one layer because the fluxes from each layer add together, so it was misleading of me to say that proximity losses aren't a problem at all in the flyback, it's just that you can't do anything about them except minimize the layer count and that is true of any magnetic component that experiences a significant flux swing. The one thing the flyback has going for it compared to forward mode converters is that the current waveform is a ramp and proximity losses are also proportional to the rate of change in the current (ie - flux).
--- Quote from: T3sl4co1l on May 15, 2019, 03:12:07 pm ---Incidentally, HV diodes are all just stacks of lower voltage diodes -- I don't think they can make high speed (<= 250ns or so?) diodes over 1.5kV or so. (In fact, an industrial "hockey puck" rated for 6kV is "high speed" if it has a recovery of 2us!) So you see V_F climb -- multiple junctions in series.
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Yep - the Vf spec for the 2CL2FM I mentioned earlier is something like 15-18V, but the junction capacitance is probably so low it is unmeasurable.
--- Quote from: T3sl4co1l on May 15, 2019, 03:12:07 pm ---The trick is, they're matched diodes, and packaged together, so recovery and capacitance are equal. This is a lot harder to pull off with discrete diodes*, so there is value in the high voltage types.
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It's not as much of a problem as you might think AS LONG AS you always use a static balancing resistor and dynamic balancing RC network across each diode in series. Incidentally, the design of the RC network for a series string of diodes is much the same as for a dV/dt snubber for a thyristor.
--- Quote from: T3sl4co1l on May 15, 2019, 03:12:07 pm ---
--- Quote ---And if you put all of these conflicting issues together you start to see why I keep recommending a flyback with a voltage multiplier,
--- End quote ---
Also -- note that a multiplier needs a different control circuit than a bog-standard (current mode) flyback. The multiplier draws current during switch-on, so either a current-limiting switch is needed, or something that acts in the same way. It may be adequate, for example, to use a normal peak-current-mode controller, but increase the transformer's leakage, so that the switch turns on for a moment, charges the multiplier through the leakage inductance, then turns off. (The increased leakage would suck for a one-switch converter, but would be very much in favor of the two-switch or full-bridge version.) The operational result will be, lower maximum power output at low output voltages, because the switch turns off much sooner than it would otherwise. If you don't need a wide output voltage range, and don't need a lot of startup current, that should be quite feasible.
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Yes, another excellent point - I know this trick has been done when you need to supply precisely stepped voltages to the anodes (dynodes) of a photomultiplier tube, but that hardly qualifies as a load; probably not nearly as practical if you need to draw more than a few tens of mA. Standard peak current mode controllers handle it just fine, btw (again, for relatively low output powers...)
--- Quote from: T3sl4co1l on May 15, 2019, 03:12:07 pm ---And speaking of budget -- do consider hiring a consultant to help guide you. Or, heck, just contract out the whole thing, if it just needs to be done -- give or take how many lessons you want to learn from the project as well.
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That's the rub - the problems with getting a resonant converter to work may very well be so difficult for the first time designer that not much might be learned in the process. As some old wag once quipped - multiplication doesn't make any sense until you learn addition.
--- Quote from: T3sl4co1l on May 15, 2019, 03:12:07 pm ---...
The tradeoff is then, a flyback that's easy to design but has terrible efficiency, or a resonant that's hard to design but easily gets good efficiency. :-\
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Bingo. A flyback run at as low a switching frequency as you can get away space/volume permitting, will be much, much easier to get working.
MagicSmoker:
--- Quote from: MagicSmoker on May 15, 2019, 04:49:48 pm ---
--- Quote from: T3sl4co1l on May 15, 2019, 03:12:07 pm ---
--- Quote ---And if you put all of these conflicting issues together you start to see why I keep recommending a flyback with a voltage multiplier,
--- End quote ---
Also -- note that a multiplier needs a different control circuit than a bog-standard (current mode) flyback. The multiplier draws current during switch-on, so either a current-limiting switch is needed, or something that acts in the same way. It may be adequate, for example, to use a normal peak-current-mode controller, but increase the transformer's leakage, so that the switch turns on for a moment, charges the multiplier through the leakage inductance, then turns off. (The increased leakage would suck for a one-switch converter, but would be very much in favor of the two-switch or full-bridge version.)
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The above comment got me to thinking so I pulled out an LTSpice circuit I drew up awhile ago for a two-switch flyback and modified the transformer parameters so it would produce 5kV/0.1A which could be fed to a doubler (Villard and C-W were tried). I didn't bother finding a proper HV diode model so I just used the default diode (infinite blocking voltage! / infinitely fast!) with a few puff of simulated junction capacitance.* Unsurprisingly, the leakage inductance parameter needed to be set to something realistically high** for this to even work, and trying to pull more than a few mA caused the voltage to collapse to the peak output of the transformer. And note that this is without any parallel capacitance across the transformer secondary, which would only make things worse.
So, I rescind my recommendation to use a flyback with a lower voltage secondary feeding a voltage multiplier - it might be fine for biasing the anode(s) on a CRT or photomultiplier, but not if any kind of real current must be drawn.
I'll attach the .asc file for the flyback with a conventional rectifier output if anyone wants to play with it. The transformer core is a Magnetics, Inc. EC 70 shape in P material with a 0.5mm gap and a 1:30 turns ratio and if not optimal for the application, is reasonably close. Other caveats: I did not optimize the feedback compensation; the transformer secondary still doesn't have a parallel capacitance specified; the primary side RC damper to suppress ringing is not accurately specified since the secondary diodes and transformer parallel capacitance aren't specified.
* - LTSpice seems to have a hard time if ideal diodes are used in power circuits; putting a small capacitance across them is the trick that makes it happy.
** - e.g., 0.965 gives a leakage of about 7%, which is very good, but not impossible to achieve, for a HV transformer with a 30:1 turns ratio.
T3sl4co1l:
Oh neat, haven't seen a GDT drive like that before. Makes sense. Think I'd put a diode across Q4 too, but it's not required.
Tim
state_of_flux:
Thanks guys.
--- Quote --- So, I rescind my recommendation to use a flyback with a lower voltage secondary feeding a voltage multiplier - it might be fine for biasing the anode(s) on a CRT or photomultiplier, but not if any kind of real current must be drawn.
--- End quote ---
So, from re-reading this thread, that only really leaves me with the buck current-fed full bridge, or the LCC running at fixed frequency, fixed duty, with a buck pre-regulator to achieve control? This is a project that I plan on running into next year so I do have time to attempt a more complex solution if that is the case and it doesn't seem as though I have many more options available.
MagicSmoker, I have one point in regards to a previous point you made:
--- Quote --- It's not so much a matter of lower efficiency if you open-circuit a series resonant converter or short-circuit a parallel resonant one, it's more a case of instant switch destruction, though it is typically easier to protect the former from abuse than the latter.
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Isn't it the case though, with the LCC converter, that it is naturally protected in overloading and short-circuit conditions due to the parallel capacitor? In my application, short-circuits can and do occur - so again this could point towards the use of the LCC.
One issue I do have with the LCC/LLC resonant converters is their controllability - it is to my understanding that both these multi-resonant converters have limited usefulness due gain variations and chaotically moving poles and zeroes in their dynamic power transfer function. I've seen Texas Instruments use what's called Hybrid Hysteretic Control to overcome this, https://www.mouser.co.uk/new/Texas-Instruments/ti-ucc256303-controller/. It says that the system is always stable with proper frequency compensation, but again in my case where this is most likely not okay - would it make these devices inapplicable and thus meaning a terrible control characteristic regardless of choosing LLC and LCC? Or could this be alleviated through the use of a pre-regulating buck converter again?
The load of my converter is typically pulsed on/off with a pre-determined duty cycle, and the regulation is quite stringent - so I believe this to be an important point I neglected to mention.
Also, T3sl4co1l, I have dropped you a quick message in regards to consulting, in case I am still banging my head against the wall in late Summer. :palm:
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