Transfer function of PFC modulator/powerstage

[shiphigh]

Hello,

I am writing with reference to the LT1509 PFC/PWM controller.

****LINK TO LT1509 DATASHEET.

http://www.linear.com/pc/downloadDocument.do?

navId=H0,C1,C1003,C1142,C1138,P1516,D1119


I believe that the equation for control_to_output (voltage loop) at the bottom (left) of page 7 of LT1509 datasheet is wrong.

I believe this because my input current is not sinusoidal and I set it partly using this equation.

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First of all, please see an equation on page 7 of LT1509 datasheet
(2nd equation up from bottom left hand side of page).

This equation has "f/1" in the numerator. This is clearly
erroneous. "f/1" is just "f" so I feel that this is also a good clue that there are errors here.
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As you know, the way to get a good sinusoidal input current is to correctly set the compensation components around the voltage and current error amplifiers.

The idea is to get suitable crossover frequency and phase margin for both voltage and current loops.

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However, the following (nice looking) loop characteristics gave a non-sinusoidal input current….

Voltage loop gain crossover = 11Hz,
Voltgae loop phase margin = 45 degrees,
Current loop crossover = 3162Hz,
Current loop phase margin = 45 degrees.

*****Picture of my poor sine current...

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The following (poor) loop characteristics gave a good sinusoidal input current…..

Voltage loop gain crossover = 10Hz,
Voltgae loop phase margin = 4 degrees,
Current loop crossover = 2400Hz,
Current loop phase margin = 15 degrees.

****picture of my good sine current...

(-the current is of course poor during the start up transients)
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The fact that the loop with the poor characteristics gave the best sinusoidal current shows me that the
Control_to_output function provided in the LT1509 datasheet is wrong.
(-equation at bottom of page 7 of LT1509 datasheet)

I am sure my calculations for the voltage and current error
amplifier gains and phases are correct as I checked them against page 257 of Christophe Basso's book (switch mode power supplies)


Does any reader know what is the correct control_to_output transfer function for the voltage loop of the PFC section of the LT1509 PFC/PWM controller?

Thankyou for reading.