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SEPIC LED driver..how measure Loop?

Discussion in 'LEDs and Optoelectronics' started by eem2am, Mar 2, 2013.

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  1. eem2am

    eem2am

    422
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    Aug 3, 2009
    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.
    http://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)

    http://www.ridleyengineering.com/ap300-measurement-techniques.html?showall=&start=5

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

    SEPIC schematic with loop injection resistor (R32)
    http://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?
     
    Last edited: Mar 2, 2013
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