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Message started by somisetty on May 5^th, 2010, 10:35pm

Title: transfer function...
Post by somisetty on May 5^th, 2010, 10:35pm

hi

can anyone tell me how to find the transfer function..the procedure for the given circuit attached..

is the gain in this case a ratio of vcomp/(vref-vout) ?

thanks a lot...

Title: Re: transfer function...
Post by sheldon on May 6^th, 2010, 4:20am

Somi,

If you have access, you might want to try looking at Chapter 3 of
"Switch-Mode Power Supplies" by Christophe Basso. Chapter 3 provides
detailed discussion of the controller transfer function. The topic is
also covered in "SMPS simulation with SPICE3" by Steve Sandler
as I remember.

Best Regards,

Sheldon

Title: Re: transfer function...
Post by raja.cedt on May 10^th, 2010, 9:51pm

hi,
that is the gain from Vcomp/Vout...because Vref is bias, so how can you take into consideration while calculating gain?

thanks,
Rajasekhar.

Title: Re: transfer function...
Post by buddypoor on May 11^th, 2010, 2:13am

More than that, since the opamp has no dc feedback, the ref. voltage will be amplified by a factor of 1E4---1E5 (dc gain). Even if Vref=0 there will be still an offset problem due to lack of resistive feedback - unless, the circuit is part of an overall stabilizing feedback loop.

Title: Re: transfer function...
Post by raja.cedt on May 11^th, 2010, 7:47am

ya buddy poor,
what you have said is correct..i guess this is a part of an controller, so it can stabilize overall _ve feedback..thanks for mention that

Thanks,
Rajasekhar.

Title: Re: transfer function...
Post by AdrianSN on May 23^rd, 2010, 11:05am

This is an error amplifier used in controllers. Here is how to derive the transfer function:

First, bring R2, C1 and C2 under one component, Z2. R2 is in series with C2 and both in parallel with C1.

Z2 = ((R2+(sC2)^-1)^-1+sC1)^-1

The amplifier becomes a differential amplifier in AC, with the components in the non-inverting input removed. The transfer function is:

V_COMP=REFERENCE*(1+Z2/R1)-V_OUT*Z2/R1

The differential amplifier transfer function is explained here:
http://masteringelectronicsdesign.com/the-differential-amplifier-transfer-function/.