Electronics > Projects, Designs, and Technical Stuff
DIY Injection Transformer for Power Supply Control Loop Response Measurements
Kleinstein:
If you don't use the transformer very heavily, you could get away in splitting the measurement and use 2 separate transformers, one for the low frequency (e.g. 1 Hz -1 kHz) and one for the higher frequencies (e.g. 100 Hz - 10 MHz). This very much eases on the design. Some overlap can also help to check for limitations.
For the low frequency range one could even use some kind of balances current injection or other electronic circuit instead of a transformer - so one could measure down to essentially DC.
2N3055:
AcHmed99,
I think you didn't quite understand how it's done..
You don't inject current into PSU output.. You take PSU, you break feedback loop and insert injection transformer in series with feedback loop.. So it gets amplified by error amplifier, not suppressed......
diyaudio:
--- Quote from: Floyo on October 10, 2016, 11:03:51 am ---I dabbled a bit with this after being unsatisfied with the mains transformer technique, or using a common mode choke as Jay_Diddy_B did in one of his threads. I then found some inductors I have had lying around for ages, I think they came out of some telecom gear. They have a stupid amount of primary turns, and a stupid high permeability. the primary inductance is around18 Henry(!). I plotted the response with my analog discovery, see attachments.
The secondary is terminated into 50Ohms, and the primary is being fed in "low impedance" output mode of the AD signal gen. Resonance happens at around 5.5Mhz, and the low cutoff is around 5 Hz, though you could get a bit more out of it by virtue of the loop measurements being relative, and by changing the drive impedance. If there is interest I could give some more details, but I'm at work right now, so that will have to wait a bit.
--- End quote ---
This looks very interesting, I too saw some possible isolation transformers in modem equipment I may possibly use one, can you upload some more details on your transformer?
As a side note: I don't think I need that high frequency response for the transformer is required...up to 1MHz signal will do to test compensation loops.
Jay_Diddy_B:
Hi,
In this thread:
https://www.eevblog.com/forum/projects/dynamic-load-bode-plot-using-hp-35665a-dsa/msg309192/#msg309192
I showed how I used a common mode choke for injecting the signals.
In this message I showed how to estimate the useful bandwidth of the transformer based on magnetizing inductance and leakage inductance.
https://www.eevblog.com/forum/projects/dynamic-load-bode-plot-using-hp-35665a-dsa/msg310991/#msg310991
I prefer to use my HP3577A VNA analyzer for doing this measurement. It has tracking filters, which can be set as narrow as 10Hz, these reject all the frequencies except the one that you are injecting. The narrow bandwidth helps lower the noise floor.
The injection transformer does not need to be flat. 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.
The injection transformer is connected between the output and the top of the divider resistor. The control loop will try and hold the top of the divider constant so all the injected signal should appear on the output. At low frequencies the voltage on the divider should be very small. I normally inject a signal that is 0.5 to 1% of the output voltage. so for a 5V supply I will use 25mV rms. If the loop gain is 40dB only 25/100 =0.25mV is at the top of the divider. As the injection frequency is increased, the loop gain is reduced and the ratio of the signals changes. You do not want to make the injection amplitude too big. If you do you may not be measuring the small signal behaviour.
Since this I have developed a new injection transformer that uses a common choke with 6.5mH of inductance.
I believe that the Picotest injector, must use special high frequency techniques, like a Pearson current transformer.
I would love to see a teardown of Picotest transformer or any of the other picotest units.
There is another way of doing this, using op-amps. This would work very well at low frequencies, but I have forgotten the details.
Regards,
Jay_Diddy_B
T3sl4co1l:
--- Quote from: diyaudio on October 10, 2016, 05:09:19 pm ---As a side note: I don't think I need that high frequency response for the transformer is required...up to 1MHz signal will do to test compensation loops.
--- End quote ---
Beautiful! We have all the numbers we need to work with!
Suppose the transformer is 1:1 at 50 ohms. It should be made of 50 ohm transmission line, in maximum electrical length 1/8 wave at 1MHz. If velocity factor is 0.67, then 25 meters is the allowable length.
Which is a pretty damned good bit of wire!
It should be something like #34 AWG or finer magnet wire (cough*), twisted together. This won't be quite as low as 50 ohms, but it'll be close enough.
Good cores:
Hi-mu toroids, especially nanocrystalline (obtain from Mouser or Elna Magnetics?)
Silicon steel, with fine laminations (under 10 thou), GOSS or better
Pot cores (hi-mu ferrite, e.g. 3E27 or better)
Making a core size choice isn't obvious. You do have a known quantity of wire to wrap around the thing. You can probably get on the order of 500 turns around a 5uH/t^2 core. Which is 1.25H, which gives a -3dB cutoff at 6.4Hz. Not too bad. Actually half that, because you'll have a 50 ohm signal generator, acting in parallel with the 50 ohm secondary termination resistor. So 3.2Hz.
You should be able to obtain higher inductivity (>20uH/t^2) in the fancier steels (NiFe, permalloy) and amorphous and nanocrystalline materials. This would push the cutoff below 1Hz.
The winding should be bifilar, wound in layers (with tape between each), to prevent too much crosstalk from adjacent layers. That will help avoid weird dips and peaks in the response.
*Checking... you may actually want much heavier wire, since 25m of #34 is 21 ohms. And that's just for one winding! #28 is about the smallest you can get away with (< 3dB insertion loss), and #25 would be ideal...
Needless to say, larger wire will invite much larger cores and windings. A UR style core (the kind they make flyback transformers with) might be attractive, because of the wide cylindrical winding area available.
Needless to say, any excuse you can make to reduce the winding length will save on both resistance and HF response. :)
Tim
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version