Thank you for your response.In this course
SMPS course_little folders
https://drive.google.com/file/d/1WvElhDNfHI0EBei4rouSNEdWWTO1oCtJ/view?usp=sharing
...there is a folder called switchingpowermagazine.com.....in the top three PDFs it tells how to do it.
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SMPS Feedback loop measurements
The Ridley engineering group make a gain phase analyser ( AP300) which is good for measuring the gain and phase margin of an SMPS…
http://www.ridleyengineering.com/analyzer.html
The following 4 videos go through use of the AP300 frequency analyser, which you can use to measure gain and phase margin of the SMPS feedback loop.
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Attached also is a Buck in LTspice with the injection source going in......you can check the phase of the signals either side of the 50R injection resistor, with respect to gnd, to get the phase margin.....the amplitude difference tells the gain margin....sorry its a bit messy....you dont see it until after 20ms.
...You usually calc this with complex numbers..only if your loop bandwidth is really high do you need to measure compensator......and if you did, you do it in isolation..no need to do it in PSU.2) I would also like to measure the compensator transfer function so I though this time to hook the FRA at FB and Va. again is this a good way to measure the compensator gain?
No, you measure the open loop gain & phase margins by the injection technique....to work out Con to output..you would subtract out the error amplifier from that . The vids deffo tell this.1) In real life I think to inject a signal at point 4 (I have access to this point) and hook the Frequency Response Analyzer (FRA) at Vb and Vout. Is this a good way to measure the control to output transfer function?
the question still remains - is it possible to measure the control to output by just changing the position of the probes of the FRA.
How come? correct me if I'm wrong but you measure a sine wave the at the error amplifier output and measure also the sine wave after the modulator + plant then divide the amplitudes and measure the phase difference - you have the control-output bode....that pg 7 is basically saying that the error amp isnt affecting that measurement, but it is...so i dont agree with that.....you cant JUST measure power_stage_and_modulator Transfer function if the error amp is still there.
It's possible. A full featured FRA like AP300 has a source output and two probe inputs, so it's just the question how you connect it.the question still remains - is it possible to measure the control to output by just changing the position of the probes of the FRA.
The problem with a filter is that it introduces new ... and maybe unknown ... phaseshift.Only possible way is with an extremely narrow band tracking filter with a bandwidth of perhaps 1Hz.
Its very easy to check, just connect both channels to the same source and do a sweep. Phase and gain should sit at 0 degrees and 0 db. I am using 100Khz quartz crystals in my two filters, and its not difficult to tweak a matched pair for both frequency and Q to close enough limits to be still be very useful.The problem with a filter is that it introduces new ... and maybe unknown ... phaseshift.
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