Here's my short question:
If I use a ferrite transformer with a 0.25mm gap in a push-pull forward converter, what side effects could I expect? Is it safe?
the long story:
Ive taken apart 10 dead Chinese SMPS's which used forward trafos with 2 identical primaries (1reset) and want to reuse the magnetics. Problem is, the cores are assembled so sloppy, that glue and varnish got in-between the core halves, creating a gap. I've sacrificed one trafo, by soaking it in paint stripper overnight, to disassemble it. This revealed the unintentional gap. A 30 turn winding had 600uH before disassembly. After cleaning the ferrite so the halves mate perfectly, 30 Turns measure 4.5mH. The paint stripper damages the wire insulation and the bobbin also, so I don't want to do this to the remaining trafos. They are well made, with shields, margins etc and the turns suit me perfectrly. I'd like tu use them as-they-are.
So a push-pull with a .25mm gap will store some energy during one voltage excursion, hoping to release it to the secondary with opposite polarity in the next excursion, right? But the push-pull already generates a secondary voltage of opposite polarity in the next excursion anyways, so the two processes may be synergistic! Will this stored energy oppose the normal (forward) energy transfer in the next cycle, or help it? To whoever can figure this out, I will concede the rights to patent this new topology: the "flyward" or the "push-back"!
PS. I think maybe many commercial forward trafos may have unintentional glue-gaps in them, since they are assembled by automated machinery, or unhappy slave-workers, and the cores are not closed perfectly. And we live with this...
Joseph
If I use a ferrite transformer with a 0.25mm gap in a push-pull forward converter, what side effects could I expect? Is it safe?
the long story:
Ive taken apart 10 dead Chinese SMPS's which used forward trafos with 2 identical primaries (1reset) and want to reuse the magnetics. Problem is, the cores are assembled so sloppy, that glue and varnish got in-between the core halves, creating a gap. I've sacrificed one trafo, by soaking it in paint stripper overnight, to disassemble it. This revealed the unintentional gap. A 30 turn winding had 600uH before disassembly. After cleaning the ferrite so the halves mate perfectly, 30 Turns measure 4.5mH. The paint stripper damages the wire insulation and the bobbin also, so I don't want to do this to the remaining trafos. They are well made, with shields, margins etc and the turns suit me perfectrly. I'd like tu use them as-they-are.
So a push-pull with a .25mm gap will store some energy during one voltage excursion, hoping to release it to the secondary with opposite polarity in the next excursion, right? But the push-pull already generates a secondary voltage of opposite polarity in the next excursion anyways, so the two processes may be synergistic! Will this stored energy oppose the normal (forward) energy transfer in the next cycle, or help it? To whoever can figure this out, I will concede the rights to patent this new topology: the "flyward" or the "push-back"!
PS. I think maybe many commercial forward trafos may have unintentional glue-gaps in them, since they are assembled by automated machinery, or unhappy slave-workers, and the cores are not closed perfectly. And we live with this...
Joseph
Thank You Poobah,
yes, Push-Pull.
but I thought push-pull is really just like forward (full-forward) as opposed to the one-switch fwd, which would be half-forward.
I see your point, though.
The half-fwd would not send a negative half-cycle to the secondary, and would not rectify the neg. on the secondary so the energy stored in the gap would not have a way to escape..
is that what you mean?
yes, Push-Pull.
but I thought push-pull is really just like forward (full-forward) as opposed to the one-switch fwd, which would be half-forward.
I see your point, though.
The half-fwd would not send a negative half-cycle to the secondary, and would not rectify the neg. on the secondary so the energy stored in the gap would not have a way to escape..
is that what you mean?
Yes, the L(p) is ok for me, 600uH I can use.
There is , expectedly a wide veriation (part-to-part) in L(p) due to the inconsistent gapping provided by the glue layer.
The reason I was concerned, is because I've seen that 0.25mm is a legitimate gap size used in many flyback designs, so there will be for sure flyback action going on here.
BTW, I forgot, the gap is in the outer legs too, so that makes 0.5mm
Joseph
There is , expectedly a wide veriation (part-to-part) in L(p) due to the inconsistent gapping provided by the glue layer.
The reason I was concerned, is because I've seen that 0.25mm is a legitimate gap size used in many flyback designs, so there will be for sure flyback action going on here.
BTW, I forgot, the gap is in the outer legs too, so that makes 0.5mm
Joseph
Right Joe,
You have to reset the core in a FORWARD converter. My concern was that if you have all that energy stored in the gap; that you not use any "R" in snubbing/rectifying the reset winding. Efficiency (1/heat)being the reason.
In a true PP it should be fine... I have the the forgotten other ramifications of big gap... hopefully because they were minor! -
What control strategy, voltage mode? current mode? do you plan on?
You have to reset the core in a FORWARD converter. My concern was that if you have all that energy stored in the gap; that you not use any "R" in snubbing/rectifying the reset winding. Efficiency (1/heat)being the reason.
In a true PP it should be fine... I have the the forgotten other ramifications of big gap... hopefully because they were minor! -
What control strategy, voltage mode? current mode? do you plan on?
Thanks again poobah
Darn!, Iwas sort of hoping that flyback energy could be recovered at the secondary.
What if I full-wave rectify the sec, can't that get the stored energy to come out on the secondary?
(I know , it's asking too much... doing something sub-optimal, and expecting a over-optimal result. )
I use the UC3846 contoller.
And I don't have Aux winding, so I was thinking of a simpler way to supply Vcc to controller:
(OK somebody is going to slap me for this)
use triac lamp dimmer circuit --pot replaced by fixed resistor-- to give 35 V rectified and filtered (the 3846 likes 35V, and the triac only starts reliably above 30V out).
Joseph
Darn!, Iwas sort of hoping that flyback energy could be recovered at the secondary.
What if I full-wave rectify the sec, can't that get the stored energy to come out on the secondary?
(I know , it's asking too much... doing something sub-optimal, and expecting a over-optimal result. )
I use the UC3846 contoller.
And I don't have Aux winding, so I was thinking of a simpler way to supply Vcc to controller:
(OK somebody is going to slap me for this)
use triac lamp dimmer circuit --pot replaced by fixed resistor-- to give 35 V rectified and filtered (the 3846 likes 35V, and the triac only starts reliably above 30V out).
Joseph
Relax Joe,
The energy stored in the gap is returned to the input rails, via the diodes of the OTHER two transistors as soon the CONDUCTING pair shut off.
You may want to parallel the FETS wih some "good" diodes depending on your power level. Look at Inernational Rectifier "FETKY".
Also what is your dc input voltage?
The energy stored in the gap is returned to the input rails, via the diodes of the OTHER two transistors as soon the CONDUCTING pair shut off.
You may want to parallel the FETS wih some "good" diodes depending on your power level. Look at Inernational Rectifier "FETKY".
Also what is your dc input voltage?
If your design has center tapped secondary windings feeding a buck stage (LC filter), then the transformer will be shorted during off-time so the stored energy in the core and the gap won't discharge until the next cycle begins. This means that no special clamps or diodes are required in the primary side.
I've actually tried to gap push-pull transformers and the result is just increased current ripple as seen by the switches (and the control circuit in case it was current mode). The current at the beginning of the cycles gets smaller, and at the end gets bigger than without gap. I've actually used that feature quite successfully to improve noise inmunity with peak current mode control.
The drawback is stronger stray magnetic fields. However, you may try to clean the cores with sandpaper placed over a perfectly clear surface or even with a file. I've done this sometimes to convert gapped cores into ungapped when I hadn't the proper core at hand, and the resulting cores doesn't yield as high inductance as properly factory machined ones, but they get quite close to specs.
I've actually tried to gap push-pull transformers and the result is just increased current ripple as seen by the switches (and the control circuit in case it was current mode). The current at the beginning of the cycles gets smaller, and at the end gets bigger than without gap. I've actually used that feature quite successfully to improve noise inmunity with peak current mode control.
The drawback is stronger stray magnetic fields. However, you may try to clean the cores with sandpaper placed over a perfectly clear surface or even with a file. I've done this sometimes to convert gapped cores into ungapped when I hadn't the proper core at hand, and the resulting cores doesn't yield as high inductance as properly factory machined ones, but they get quite close to specs.
Yes push-pull with CT primary (is there any other kind?)
So... i get it.
There is no way that this parasitic flyback can bring any recoverable energy to the secondary.
All it does is circulate energy among the primary components, generating more heat.
I'll just have to dissolve the soft parts of the remaining trafos with paint stripper, and rewind them to get rid of the gap.
thanks
Joseph
So... i get it.
There is no way that this parasitic flyback can bring any recoverable energy to the secondary.
All it does is circulate energy among the primary components, generating more heat.
I'll just have to dissolve the soft parts of the remaining trafos with paint stripper, and rewind them to get rid of the gap.
thanks
Joseph
Joe,
Only a few percent of your energy is being reflected black... of that, about 1% is getting burned up in the diodes. 1% times 3% = 0.03%... not big deal to leave it as is. There is copper loss too but you get the picture. (you really want to rewind those trannies now... don't you?)
Cheers,
Only a few percent of your energy is being reflected black... of that, about 1% is getting burned up in the diodes. 1% times 3% = 0.03%... not big deal to leave it as is. There is copper loss too but you get the picture. (you really want to rewind those trannies now... don't you?)
Cheers,
of SMPS... please explain???
It's not so complex. It's just that the load connected to the secondary side of the transformer may also easily clamp the inductive kickback. Remember that all windings are coupled and the energy stored in the core and the gap may be recovered through any of them.
Furthermore, the classic circuit made of a transformer with a center tapped secondary whose ends are connected to a buck inductor through two diodes causes the transformer to be effectively shorted during the time when the switches of the primary side are off. This means that no magnetisation energy is dumped back to the storage capacitors of the primary side. Think about it.
Furthermore, the classic circuit made of a transformer with a center tapped secondary whose ends are connected to a buck inductor through two diodes causes the transformer to be effectively shorted during the time when the switches of the primary side are off. This means that no magnetisation energy is dumped back to the storage capacitors of the primary side. Think about it.
Thank you Eva
Speaking of modified topologies....
Bi-Forward Converter
if you gurus have already seen this, then ignore this post.
Otherwise... perhaps a new addition to your armamentarium ?
Joseph
Speaking of modified topologies....
Bi-Forward Converter
if you gurus have already seen this, then ignore this post.
Otherwise... perhaps a new addition to your armamentarium ?
Joseph
This paper is quite funny because no way to reset the transformer is provided in the basic schematic, and not a single word about that is said in the entire document... An additional reset winding and a frewheeling diode are obviously required. That topology appears to be more suited for a flyback converter anyway, what makes the difference is just a center tapped primary (showing two independent primaries in the schematic seems quite obfuscated to me).
Also, the converter is drawing pulsed current from mains line at high frequencies, but not a single word is said about that and its consequences. I think that the document should point out that an additional capacitor of up to 1uF is required anyway after the diode bridge in order to provide a low impedance at HF (not required in standard topologies).
PD. it comes from Spain, so I quite understand these deficiencies...
Also, the converter is drawing pulsed current from mains line at high frequencies, but not a single word is said about that and its consequences. I think that the document should point out that an additional capacitor of up to 1uF is required anyway after the diode bridge in order to provide a low impedance at HF (not required in standard topologies).
PD. it comes from Spain, so I quite understand these deficiencies...
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