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DIY Injection Transformer for Power Supply Control Loop Response Measurements
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David Hess:

--- Quote from: rx8pilot on October 24, 2017, 03:28:56 pm ---Picotest wants to get a lot of money for a transformer....
--- End quote ---

It is not a lot of money for someone who gets paid for the sort of work where the transformer would be useful.  Greater than $5000 DSOs are not expensive in that respect either.  Think of the cost in comparison to the salary of the engineer using it.

For hobbyists, there are other less expensive but more time consuming methods to use like driving the injection point with a transconductance output or using a 60 Hz AC current transformer in reverse.  I usually get by with just measuring the small signal and large signal load transient response.
rx8pilot:

--- Quote from: David Hess on October 24, 2017, 05:18:54 pm ---
--- Quote from: rx8pilot on October 24, 2017, 03:28:56 pm ---Picotest wants to get a lot of money for a transformer....
--- End quote ---

It is not a lot of money for someone who gets paid for the sort of work where the transformer would be useful.  Greater than $5000 DSOs are not expensive in that respect either.  Think of the cost in comparison to the salary of the engineer using it.

For hobbyists, there are other less expensive but more time consuming methods to use like driving the injection point with a transconductance output or using a 60 Hz AC current transformer in reverse.  I usually get by with just measuring the small signal and large signal load transient response.

--- End quote ---

True enough.
I am in the purgatory of power electronics design. I just don't do it enough to justify the bench full of specialized test gear, so I muddle through the best I can. I looked into renting the Bode100 VNA with all the accessories and it is about $750-$850 per month rental. About 8 rentals would pay for it. I would use it for 3-4 designs per year - so buying it breaks even at around 2 years for the dedicated FRA.

Measuring transient response will definitely reveal most control problems - I do the same over a range of operating conditions. I have been able to get some use out of the FRA built into my Keysight scope although it takes some doing when testing noisy switching supplies. The automated FRA application cannot deal with the noise, but manually I can step through the frequencies and use math filters and averaging to get a measurement of gain/phase in the control loop. Slow - but eventually gets to a reasonable answer of phase margin.


Jay_Diddy_B:
Hi,

I described my injection transformer in another thread.

https://www.eevblog.com/forum/projects/dynamic-load-bode-plot-using-hp-35665a-dsa/

Here are some of the details:

Schematic:



Board Design:



Construction






I normally use an HP 3577A VNA for measuring control loop response. The HP3577A does a great job, but it is big and heavy.

Jay_Diddy_B

rx8pilot:
Other than size - the 3577A is an interesting option since it goes to 5hz.

Any maintenance issues with it?

Your injector looks pretty cool.....off to read that thread now. Thanks for posting that one.
Jay_Diddy_B:

--- Quote from: rx8pilot on October 24, 2017, 09:23:21 pm ---Other than size - the 3577A is an interesting option since it goes to 5hz.

Any maintenance issues with it?

snip ...


--- End quote ---

The only issue that I have had with the 3577A is the CRT. The CRT assembly is the HP1345A display module. I changed mine with one I got from another instrument. If it happened again I would fit one the LCD retrofit kits.

It is a great instrument.

Regards,

Jay_Diddy_B
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