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| Help with Pulse Transformer Design |
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| KurtK:
Hi All. This is my first post so please be gentle. I'm designing a special purpose pulse transformer, and since I really can't find a suitable guides for that on the internet I have to experiment to find find the most efficient design. And I therefore need to know the efficiency of each design so that I can compare them. Here is my first attempt, it's an simple air-core design, how do I calculate the efficiency of this transformer? My idea is to multiply current with voltage for the whole pulse duration to obtain instantaneous power, and from that calculate average power. The divide primary average power with secondary average power to get a measure of efficiency, would that be correct way to do it? Cheers KK |
| MagicSmoker:
--- Quote from: KurtK on August 22, 2019, 10:09:25 am ---... I'm designing a special purpose pulse transformer.... Here is my first attempt, it's an simple air-core design... --- End quote --- A good pulse transformer has very high magnetizing inductance with the lowest possible distributed capacitance, which tend to be conflicting requirements because the former implies lots of turns while the latter implies minimizing the turns. The usual approach, then, is to use a high permeability core material that is much larger than necessary from a flux swing standpoint to get both a high magnetizing inductance with few turns. Neither of which can be accomplished with an air core. |
| KurtK:
Hi MagicSmoker. Yes I have read exactly that in some technical articles, and I do have some ferrite cores that I would like to try, but unfortunately I don't have any datasheet for them, so I will have to experiment. And if I have to experiment I need a way to measure/calculate efficiency otherwise I will be experimenting in the dark. Can you tell me if my idea of calculating efficiency is correct? Or is there another way? |
| MagicSmoker:
Efficiency is not usually a top design consideration of a pulse transformer as it generally handles very little power, so I am wondering if there might be a "lost in translation" issue here? That said, no, your method won't give you an accurate measurement of efficiency because the energy stored in the magnetizing inductance of the primary is returned back to the supply, minus some losses in the winding resistance and from driving the core (if present), while the actual load current drawn by the secondary is reflected back to the primary and incurs losses in the resistance of both windings. As a result, a pulse transformer without any kind of ferromagnetic core will either have a very high magnetizing current, which incurs a high I^2R loss, or the windings will have an excessive number of turns to minimize magnetizing current, and still incur a high I^2R loss... As us Yanks say, "you can't win for losing." Separating out all of these loss contributions is rather tedious and usually only worth the effort for a transformer that handles significant power which, again, doesn't typically apply to a pulse transformer. |
| KurtK:
--- Quote from: MagicSmoker on August 22, 2019, 02:54:14 pm ---Efficiency is not usually a top design consideration of a pulse transformer as it generally handles very little power, so I am wondering if there might be a "lost in translation" issue here? --- End quote --- Perhaps "lost in terminology". I would have preferred to just to call it a (step-down) "transformer" but that usually implies 50HZ sine AC. So regardless of the term used I just want to know how to measure/calculate efficiency of the "transformer" at hand. Obviously I want it to be as efficient as possible. --- Quote from: MagicSmoker on August 22, 2019, 02:54:14 pm ---That said, no, your method won't give you an accurate measurement of efficiency because the energy stored in the magnetizing inductance of the primary is returned back to the supply, minus some losses in the winding resistance and from driving the core (if present), while the actual load current drawn by the secondary is reflected back to the primary and incurs losses in the resistance of both windings. As a result, a pulse transformer without any kind of ferromagnetic core will either have a very high magnetizing current, which incurs a high I^2R loss, or the windings will have an excessive number of turns to minimize magnetizing current, and still incur a high I^2R loss... --- End quote --- I don't quite understand that. In a (50HZ since AC) transformer the efficiency is calculated as primary power / secondary power * 100, and power is calculated as Vrms * Irms * Power factor. So why couldn't I do something similar? I mean in a (50HZ since AC) transformer the energy stored in the magnetizing inductance of the primary is also returned back to the supply etc.? So if I use the entire pulse duration as basis for the calculation, it must include in theory everything including losses you described? --- Quote from: MagicSmoker on August 22, 2019, 02:54:14 pm ---As us Yanks say, "you can't win for losing." --- End quote --- You should come live with us then, here everybody is a winner - even the loosers. ;-) --- Quote from: MagicSmoker on August 22, 2019, 02:54:14 pm ---Separating out all of these loss contributions is rather tedious and usually only worth the effort for a transformer that handles significant power which, again, doesn't typically apply to a pulse transformer. --- End quote --- The application is not a typical pulse transformer, so could we perhaps call it a "power pulse transformer"? In any case, how do I by experimenting with cores and windings and maybe some assistance from general math, obtain a efficient say 90% @ 34KHz "power pulse transformer"? |
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