More methods to salvage leakage energy in a flyback?

Folks,

Have to do a flyback-PFC but this time with a really good efficiency, >90%. The output voltage is high enough that sync rectification isn't needed. AFAICT the only other way to boost efficiency is to not flare off the leakage inductance spike via zener or snubber but to recycle it instead. Unfortunately a recuperating winding only works if the duty cycle stays under 50% at all times. Or if it's a double winding 66%, similar in a tapped design, et cetera. Long story short that doesn't work in a PFC because the duty cycle is much higher around the zero crossings of the AC going into the unit.

A non-dissipative snubber such as in Weinberg converters can be iffy because the results are highly dependent on component tolerances in the FET and in the flyback transformer.

Are there any other tricks to sponge off the leakage spike and dump it back into the pot? Maybe a fast and low capacitance "siphoning buck" that only takes off energy when it sees 2x input voltage or something? Must limit kind of hard though.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
Loading thread data ...

In my opinion, leakage is best dealt with first, in and of itself. What winding options do you have on this job? Custom OK? Current stock only?

You want LL/(stray C) ~= Vsw / Isw. Too much interleaving is just as bad as too little.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
 Click to see the full signature
Reply to
Tim Williams

Vladimir Vassilevsky posted this a while back:

formatting link

--
----Android NewsGroup Reader---- 
http://usenet.sinaapp.com/
 Click to see the full signature
Reply to
bitrex

It's a flyback, needs an airgap. The transformer will be custom so we can decide what we want. We can't interleave much because safety isolation almost requires a two-chamber bobbin or at least a thick separation layer.

It is the usual rock hard spot situation.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

That's a good name :-)

A friend of mine always gets it wrong when we talk about SEPIC converters. He calls the septic converters.

Unfortunately we need isolation so I can't have a cap of tens of nanofarads across the barrier. If I heave the leak energy across the barrier it would have to be via a small secondary transformer. Best would be to dump it into the source.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Den mandag den 2. februar 2015 kl. 22.42.39 UTC+1 skrev Joerg:

I know nothing of PFCs but this is one of the first hits on google

formatting link

-Lasse

Reply to
Lasse Langwadt Christensen

Thanks, interesting. With a PFC this could be tough because the clapm level either needs to be controlled and persistently high or the energy in the cap C2 would have to "waft around" with the incoming AC phase. Which might be doable but requires an elaborate switcher to employ the stored energy for something useful.

I am currently simulating an active clamp method, similar to this but not on the high side:

formatting link

It's a pain. Even with an Intel i7.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Folks,

Have to do a flyback-PFC but this time with a really good efficiency,

needed. AFAICT the only other way to boost efficiency is to not flare off the leakage inductance spike via zener or snubber but to recycle it instead. Unfortunately a recuperating winding only works if the duty cycle stays under 50% at all times. Or if it's a double winding 66%, similar in a tapped design, et cetera. Long story short that doesn't work in a PFC because the duty cycle is much higher around the zero crossings of the AC going into the unit.

A non-dissipative snubber such as in Weinberg converters can be iffy because the results are highly dependent on component tolerances in the FET and in the flyback transformer.

Are there any other tricks to sponge off the leakage spike and dump it back into the pot? Maybe a fast and low capacitance "siphoning buck" that only takes off energy when it sees 2x input voltage or something? Must limit kind of hard though.

--
Regards, Joerg 

http://www.analogconsultants.com/ 
 Click to see the full signature
Reply to
Harry D

Bullcrap. HV transformers are easy, including flyback types.

You just have to understand that a high winding count means that the latter winds need to be isolated from earlier turns. Not from the other winding. That gets done by the bobbin. Each layer of turns in the secondary gets taped, and subsequent turns get wound on top of that. The margin at the bobbin faces should be such that there is not an upper layer turn near the face and other layer turns.

I had secondary designs where the bobbin hub got 75 turns of say #43, and there was about a half mm space between the winding start and the finish, and the bobbin faces. That way, the tape which goes on top of that first layer tapes to the bobbin hub .5mm at each face and the next layer is inboard too. So there is always a half mm space at the edges of each layer where the tape provides layer to layer creepage, and the turns number keeps the differential low. Volts per turn and all that.

So I could lay down 1500 turns on my secondary, and place a 15 turn, single layer say #26 bifilar (or single) on top of that.

But you can bet none of those higher layer conductors ever go anywhere near the lead-in line or lower layers. The lead in gets a piece of Teflon tubing over it, as does the output lead.

So I could pump 4kV from just a couple CW multiplier stages, instead of counting on more stages to get there, which is less efficient.

Reply to
DecadentLinuxUserNumeroUno

The search term would be non dissipative snubber Which yields this

formatting link

The old unitrode app notes (which is now ti) are gold. Mark

Reply to
makolber

I've worked on one project where they were aiming for high efficiency, and used energy-conserving snubbers.

IIRC the snubbers charged a cap that was then discharged with an auxiliary switcher (I think a buck) back into some sensible part of the circuit.

The guy doing the circuit design had a name for it, but I don't remember what it was.

--
www.wescottdesign.com
Reply to
Tim Wescott

  • 2 switch forward (energy returned to the rails)
  • Quasi resonant flyback (drain capacitance controlled)
  • Quasi resonant flyback (detect bottom of negative ringing for start of next cycle)
  • Active clamp Cheers

Klaus

Reply to
Klaus Kragelund

ad

Never find any recuperating winding in Flyback topologies. AFAIK Energy is stored in the airgap of the Xformer during Ton on primary side and restitue d on secondary during Toff primary. Where is the recuperating windings here ? Are you speaking on Forward topo converters ?

Reply to
habib.bouaziz

That's off course a 2 switch flyback......

Reply to
Klaus Kragelund

Medical?

Or to put it another way, any bobbin wide enough to handle that isolation by tape and padding alone would be kind of hopeless? Even with "triple insulated" wire?

You can wind alternate sections of a really heavily divided bobbin. Like starting with one of those parts used for CCFL inverters. Or... this reminds me...

...I once pulled a one of these from, I believe it was a CRT monitor's power supply:

formatting link
roughly 100W, flyback. ETD30-ish size by the looks of it. No clue why they needed the isolation, but they've got probably more than you even need on there!

This is the primary side, and you can see it goes into a few alternating sections. Bottom left is the aux winding. The secondary side is... kind of cryptic. It's a CRT, so they needed a lot of voltages...

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
 Click to see the full signature
Reply to
Tim Williams

They truly are and I cherish the old databook from them. But the non-dissipative snubber doesn't work well for a PFC-flyback, tried it. I guess I need an active recycler, somthing like a super-simple hysteretic buck from a 2xVCC rail or something down to VCC. One that is controlled by input voltage instead of output side.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Not this time but safety-critical as well.

That's frowned upon by agency folks but in a pinch can be donw (only the tape method though). Tape increases leakage also.

Cool!

In our case the plastic volume would eat a bit much space.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Well, most of it. A not insignificant amount of energy is stored in the leakage inductance that results from having an air gap. This energy has nowhere to go and thus will drive the drain or collector of the switching device into the sky unless you cap it. Usually it's snubbered or zenered, either method essentially turning this energy into unwanted heat.

No, flybacks. It looks like this:

formatting link

But be careful, that only works if the duty cycle is hard-limited. In the case of a 1:1 winding it must always remain below 50%. So it's no good for PFC.

[...]
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Copied, your news reader still doesn't work right or maybe you wrote underneath your own sig line:

Joerg, you need help. What is your input AC voltage range and output Volts and Amps? Sounds like you require I/O isolation. A Flyback at >50W or >0.93PF requires a professional.

Answer:

I have designed them before, it's not a problem to even get the PF much higher than that. It's just that this time I need a little more efficiency and that requires not letting the energy in the inevitable leakage inductance go to waste. At least not all of it. A piece of cake in a non-PFC flyback, but more difficult in a PFC version. I can always build a cheapo buck to recoup that but was wondering if there's an easier way that tolerates duty cycles past 90%.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

That's what I am looking at also but I wanted to avoid another buck in there. But maybe it just has to be that way or we have to move away from a flyback architecture so there is no leakage inductance.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.