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Different ways of doing Current sense transformer reset?

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cupoftea

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Hi,
We are doing a 750W Buck, 42Vin to 24Vout.
It has Current transformer sense (as attached). We note the attached two methods of doing the current sense transformer reset as attached. The simple one, with just the diode and Zener, has lower peak magnetising current in the CST secondary, and so appears better. So why do people ever use the other method for CST reset? (the one with the PNP)

PDF schems and LTspice sims attached ...also the CST secondary magnetising current waveforms are shown.
 

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  • Buck with higher magnetising current CST.pdf
    171.2 KB · Views: 225
  • Buck with lower magnetising current CST.pdf
    175.5 KB · Views: 200
  • High magnetising current CST.zip
    3.1 KB · Views: 129
  • Low magnetising current CST.zip
    3 KB · Views: 107
  • Higher magnetising current.jpg
    Higher magnetising current.jpg
    89.8 KB · Views: 151
  • Lower magnetising current.jpg
    Lower magnetising current.jpg
    85.9 KB · Views: 153

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FvM said:
This Unitrode/TI application nodes is narrowing the range where reset circuits may be actually required https://www.ti.com/lit/an/slua174/slua174.pdf
Click to expand...
Yes, that's the document where I originally saw active-reset circuits.
cupoftea said:
Thanks FvM, thats a great article on this subject...its interesting though, that at the bottom of the LHS column of page 1, it says that the leakage inductance is "generally not a problem"....which , by the sims shown above, we know is not correct.
Click to expand...
No, I don't believe practical amounts of leakage will impact CT operation significantly. I can't run your sim because it's missing device models. But k=0.9 is not realistic, should be at least 0.98, unless it's a very bizarre or homemade part.

From a black box perspective, modelling the leakage on the primary/secondary side is just a matter of preference. Both will give the exact same behavior (from a black box standpoint), assuming those models have been derived properly.

So consider: if the leakage is on the primary side, what effect do you expect?
 
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I haven't measured the coupling on a CT for power electronics for a while - but the symmetry of fields suggests it will be higher than 0.95 - I will measure some typ examples next year ....
Click to expand...

Thanks, ive noticed on this particular converter, it has 2 current sense transformers, one for the 750W Buck Converter (42 to 24Vdc), and one for the 850W Half Bridge (390V to 42V).
Both CST's are wound on split bobbins, (a plastic "ridge" exists in the middle of the "spool") with the primary in the Left half, and the single secondary coil in the right half. As such, what would you think is the coupling factor for this?...i would think pretty low?
 

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