SEPIC LED driver.....injection point for Loop gain and phase measurement?

[treez]

Hello,

I have done a coupled inductor SEPIC LED driver, which regulates its own input current. We wish to do a stability analysis using an AP300 frequency analyser, so that we can get the gain and phase Bode plots for the converter.

My question is, where should we inject the perturbation signal?, (i.e. the test signal which is to sweep over the frequencies of interest in order to get the Bode Plot?)

Schematic.
https://i46.tinypic.com/jttquc.jpg

Vin is a 6V battery.

The load is any LED lamp which could be 5V up to 40V.

The average input current is set by applying a control voltage into the non-inverting input of the U7 opamp. (677mA per volt applied).

Now, in order to get the Bode plots of gain and phase of the sepic, we need to inject a signal into it, as described in the following web page)

**broken link removed**

Here is the schematic with the loop injection resistor added (R32)…

SEPIC schematic with loop injection resistor (R32)
https://i48.tinypic.com/2lo6hdl.jpg

…The criteria for the loop injection point is that one side of the injection place must be high impedance and the other side must be low impedance. (The output impedance of the opamp buffer, U17, provides the low impedance side).

So do you think that R32 is a good place for the voltage injection, for the purposes of getting the loop gain and phase?

 

[mtwieg]

It doesn't necessarily need to be placed next to a very low impedance. So long as the combination of the transformer inductance and shunt resistance being added does not significantly affect the loop response, it can be anywhere. A more general rule of thumb is that the impedance seen on one side of the injection should be much higher than the injection shunt, and the impedance on the other side must be much lower than the shunt.

That location seems fine. Putting it at the input of buffer U17, or pin 7 of U1, should also work.

 

[treez]

more general rule of thumb is that the impedance seen on one side of the injection should be much higher than the injection shunt, and the impedance on the other side must be much lower than the shunt.

Ok thanks mtwieg, but i am worried by your above remark, because the injection resistor is 22R, and the output impedance of the opamp will be about 100R, so my set up does not conform to your requirements?

Also, putting the injection resistor in line with pin 7 doesnt really conform to your requirement, because the impedance seen looking back there is 220R, which is much more than the 22R injection resistor, -according to your requirement, this 220R is therefore too high value?

Also, where i have put the resistor, do you think the 1K (R16) resistor presents a high enough resistance?

I'm really not sure about putting the injection resistor at the input of buffer U17, because the output of U6 is not that low output impedance.