Electronics > Projects, Designs, and Technical Stuff
DIY Injection Transformer for Power Supply Control Loop Response Measurements
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diyaudio:
Its clear an Injection Transformer is required to perform signal isolation when performing loop response measurements,
however they cost big bucks, a Picotest J2101A costs $525.00  :--


https://www.picotest.com/products_J2101A.html





I want to start building a DIY Injection Transformer for my MSOX3104T, does anyone know what transformer material I can use to support a 1:1 10Hz to +/-40MHz signals.


   
jahonen:
I have just used an ordinary mains transformer (230/5 volts or something like that, voltages are not that critical), I think it works sufficiently well up to at least few tens of kHz, but that is often enough for switch mode regulators. When both reference and output are measured, the transformer passband flatness is not so critical. I actually used a 192 kHz sound card and ARTA software for the measurements, good enough for most of my needs. This ordinary mains transformer is also used in this TI appnote: http://www.ti.com/lit/an/snva364a/snva364a.pdf

Regards,
Janne
2N3055:
a bifilar winding on a toroidal ferrite core... Maybe a bit of experiment as to how many turns...
MatteoX:
It is extremely difficult to acheive 10Hz - 45MHz transformer bandwidth (that is 6 1/2 decades or 22 octaves!).
Picotest mentiones use of specially annealed cores. If you look at Picotest J2101A Data Sheet, you will notice
resonance at 11MHz

https://www.picotest.com/downloads/INJECTORS/J2100A-J2101A_Spec_Sheet_FinalV2_Email.pdf

Many years ago I tried to make an isolation transformer for the same purpose. My goal was 10Hz to 1Mhz bandwidth
but I couldn't reach it so I give up after several experiments.

There were several discussions on the Internet that critiqued TI app note listed above and its use of mains
transformer.

I would be really curious if someone knows how to acheive such bandwidths.

Maxim App note 325  https://www.maximintegrated.com/en/app-notes/index.mvp/id/3245 has
some interesting details
2N3055:
50-100Hz to few Mhz shouldn't be problem on common ferrite cores...
45 MHz not so easy, but I see no reason to that high on testing control loops on PSU...

22 nsec rise time in PSU control loop not so common...  Most PSU have bandwidth in 10s of kHz
testing to 100-200 kHz to make sure all is stable is good practice...
Testing to few MHz for fast PSU I get... 45 MHz i have no use for..
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