Electronics > Projects, Designs, and Technical Stuff
DIY Injection Transformer for Power Supply Control Loop Response Measurements
diyaudio:
--- Quote from: rx8pilot on October 12, 2016, 02:43:58 am ---I have been using a Jensen VB-1BB isolation transformer used for analog video. Not perfect, but not bad assuming that 10hz is not a necessity. It does ok down to rather low frequencies though. I paid $20 on eBay. BNC 75 Ohms in/out
http://www.jensen-transformers.com/product/vb-1bb/
I tried to use it with the Keysight power application on my MSOX6000 but was quickly stopped by it's inability to deal with the noise in SMPS. The newer software on the 3k's and 4k's seems to be a lot better at filtering and allowing a LUT for the injection voltage to get a better SNR on a range of frequencies. I was able to get measurements, but manually stepping through the frequencies, injection levels, averaging, and math filtering. A bit slow, but I think I was able to get believable measurements. After getting some pricing on dedicated FRA's, I am determined to figure out how to get what I need on the scope. I will design about 4-5 SMPS's per year so I need this capability for a grand total of maybe a week per year as I work out the supply design.
Having a real VNA would be nice, but certainly not a must.
--- End quote ---
-- Johnnie Hancock
Yes, we recently updated the firmware in the 3000T and 4000A X-Series. We implemented a completely different measurement algorithm that improves the dynamic range of frequency response measurements significantly. We also implemented a much better user-interface, as well as overlaying the gain and phase traces on the same plot. We plan to update the firmware in the 6000 X- Series next year. I don't know exactly when, but when it comes out, it will be a free upgrade.?
rbola35618:
Hi EEVBLOGGER,
I posted this video awhile ago here on the blog. I wound my injection transformer using a P42/29 core. I basically replicated a Venable injection transformer that we have at work. I get good results with the transformer. I now use the Cleverscope to do my FRA measurement and do not need an injection transformer. The Cleverscope has an isolated function generator output and all I have to do is connect across the injection resistor. Since there is no injection transformer used, I get a bandwidth all the way to DC. The Cleverscope work well for me.
diyaudio:
I found a unused complete Panasonic EMI Line filter core (from the junk box 10 years old), doesn't look bad at all.
http://www.datasheetspdf.com/PDF/ELF18D290x/530267/3
rx8pilot:
--- Quote from: rbola35618 on October 13, 2016, 03:51:54 pm ---Hi EEVBLOGGER,
I posted this video awhile ago here on the blog. I wound my injection transformer using a P42/29 core. I basically replicated a Venable injection transformer that we have at work. I get good results with the transformer. I now use the Cleverscope to do my FRA measurement and do not need an injection transformer. The Cleverscope has an isolated function generator output and all I have to do is connect across the injection resistor. Since there is no injection transformer used, I get a bandwidth all the way to DC. The Cleverscope work well for me.
--- End quote ---
Robert -
Are you allowed to describe how the Cleverscope signal generator is isolated? Guessing it is not a transformer and some sort of isolation amplifier? The wide band electronic isolators used for this purpose are double or triple the cost of the the transformers, but seems unlikely they are terribly difficult to design and manufacture (could be sadly mistaken of course).
On the more traditional route, how critical is the transformer performance relative to the goal of FRA on an SMPS control loop? I do not have much knowledge of designing magnetics, but I can intuitively understand there is a challenge making a very wide bandwidth transformer like commercial models. If there is no need to test active PFC's, the low frequency is seemingly unimportant. I am not sure why you would need to go to 45Mhz on the high end either. In a practical sense, what bandwidth would you say is reasonable for the budget minded?
BTW - thank you for your video lessons on YT and the offline pointers you have shared.
Bud:
--- Quote from: Jay_Diddy_B on October 10, 2016, 05:32:25 pm ---. The transfer function of the transformer is not part of the measurement. You are measuring on either side of the transformer with respect to ground.
--- End quote ---
What is the point of having an "isolation" transformer if the measuring inputs not isolated?
Or theay are?
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