What is injection transformer?

[fonograph]

I recently watched Daves Bode 100 video where he raves about some wideband injection transformer,it seems to have very wide and very flat frequency response.What is injection transformer and what is it used for? How is injection transformer different to other transformers?

[JS]

He explained all that in the video where he used it. It's used to inject a signal into a feedback loop to measure the response and stability. The difference depends ob what you compare it to, there are too many different transformers to make a general comparison but the wide band of operation and the lab type of enclosureand connections are some of the properties they have.

JS

[fonograph]

1. Why would you want to inject signal into feedback loop?
2. Why do you need transformer to inject signal into feedback loop?

[JS]

1. Why would you want to inject signal into feedback loop?
2. Why do you need transformer to inject signal into feedback loop?

1 to test the circuit, to measure the response to a known and controlled perturbation.
2 to make it in the less intrusive way possible to affect its response as little as possible, so what you measure is way it actually is. Also you need a way to make it floating so any signal present at the node passes by and a transformer is a good easy to accomplish that.

JS

[fonograph]

What you mean by less intrusive? Isnt purposefully injecting signal into feedback loop in order to destabilize it a intrinsicaly intrusive thing to do?

Is it for galvanic isolation? I am noob but,what does the transformer have to do with the floating,what is node?

[JS]

No, you want to inject some signals without making it less stable, so you can see how it reacts and how stable it is, if you modify the characteristics of the circuit you can't know how the circuit behaves once you removed the transformer.

Floating means you apply a voltage at the terminals of the transformer, but anything you put at one end is seen at the other, as it's not necessary referenced to ground or some arbitrary point but rather to whatever is in the other end. Galvanic issolation isn't strictly necessary here, you could use an active floating signal to inject but that is harder to behave as it should without modofying the properties of the circuit.

Node is a pretty basic concept in electronics, check kirchoff laws as you really need them to make any sense of electronica. Basically, a node is a point where you connect stuff, the entire conductor connecting all it's components is a node, like a trace in a PCB.

With an injection transformer you would be pinching a node, cutting into it and introducing the signal. When the signal is not present the node behaves as usual, when you add a signal it's pressent referenced to that node if seen from the other end.

JS

[rbola35618]

Here is a video that kind of explain it, or I show how to use an injection transformer to measure the gain and phase of a power supply.

[Smokey]

Is there any need for the injection transformer if you had a floating output on the function generator?

[xavier60]

I successfully used my  FY6600 function generator to measure the stability phase margin of a bench PSU project.
The function generator has no mains earth connection. I connected its output ground to the low impedance side of the node, the PSU's output.

[David Hess]

Is there any need for the injection transformer if you had a floating output on the function generator?

Or there are other ways to create a floating output to inject.  I think Cleverscope has an application note about doing the same thing with a floating isolated function generator output.

[MathWizard]

AN-1889 How to Measure the Loop Transfer Function of POWER SUPPLIES

I need to try testing my homemade PSU, I have a couple of tiny tiny 120V to 12V transformers that should be good enough for this

[electron_plumber]

I successfully used my  FY6600 function generator to measure the stability phase margin of a bench PSU project.
The function generator has no mains earth connection. I connected its output ground to the low impedance side of the node, the PSU's output.

This might work in theory, but here is some more context as not to confuse the noobs:

Professional EE labs aren't going to want to measure loop response manually with a cheap floating function gen. You're going to want a nice high dynamic range instrument that can resolve lower frequency loop dynamics accurately. Those instruments will of course be grounded, requiring an injection transformer. Injection transformers also (mostly) all come with an inline fuse to protect your bench equipment in the event that you hook them up wrong. So, it's a no-brainer to spend $400 on a transformer to protect your fancy tools.

Edit: I just bought two transformers for my lab and did a little review of the market. For anyone interested, here's a table of the main market options I found available in US:

PN	Usable Bandwidth	Saturation current	Winding Capacitance	Connector type	Enclosure material	Price (USD)	Comments
FIT1045A	10Hz - 45 MHz	14.75 mA	85 pF	BNC	Metal	$399 + free shipping (prime)
BWIT-100	1Hz - 10 MHz	10 mA	110 pF	BNC/Banana	Metal	$590 + shipping	.
J2101A	10Hz - 45 MHz	10 mA	150 pF	BNC/Banana	Plastic	$590 + shipping	.
Universal Injector	0.1Hz - 30MHz	(?)	(?)	BNC	Metal	$650 + shipping	.
TBJT01	10Hz - 45MHz	12 mA	90 pF	BNC/Banana	Plastic	$389 + shipping	<!>Warning

<!>Warning: The TBJT01 has internal 5Ω termination! So, be mindful of that if you plan to put it in parallel with in-circuit termination. Kind of odd in my opinion.

[Calvin]

Hi,

here are a couple of sources you might find helpful.

- AN1889 "How to Measure the Loop Transfer Function of Power Supplies", or Application Report SNVA364A – October 2008–Revised April 2013 - TexasInstruments
- An3245 "Injection Transformers for Closed-Loop D"C-DC Converter Network Analysis" -March 31. 2005 - Maxim
- "Use op-amp injection for Bode analysis", Martin Galinski, Micrel Semiconductor, San Jose, CA, EDN  September 16, 2004 p90-91
- "Make your own insertion transformer for measuring loop gain-phase of power converters", Roland van Roy Feb 13, 2018
- "Injection Transformers", Electronic Projects for Fun, https://electronicprojectsforfun.wordpress.com/injection-transformers
- "Measuring Commercial and Homebrew Injection Transformers", https://electronicprojectsforfun.wordpress.com/injection-transformers (Subfolder)
- "A Loop Stability Measurement Solution for the Poor", https://electronicprojectsforfun.wordpress.com/a-loop-stability-measurement-solution-for-the-poor/
- "Measuring a PicoTest J2111A Current Injector", https://electronicprojectsforfun.wordpress.com/measuring-a-picotest-j2111a-current-injector/
- WWVBlod-thread, https://www.eevblog.com/forum/testgear/picotest-injector-teardowns/new/#new
- ITP-10 Injection transformer - https://coretechgroup.com/wp-content/uploads/2017/01/Datasheet_ITP-10.pdf.

I especially liked the infos regarding DIY at "Projects for Fun", where a reengineered Bode B-WIT100 and other Injection transformers are described., built and characterized.

regards
Calvin

[mawyatt]

Some additional info the OP might find useful.

https://www.eevblog.com/forum/testgear/diy-transformer-for-use-with-bode-plots/

https://www.eevblog.com/forum/testgear/injection-transformers-bode-plots-application/

Best