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Expression for SMPS transfer function is mixing log numbers and "normal" numbers in the same expression?

treez

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Hello,

Please assist with the expression for the transfer function of the modulator and error amplifier of this TL431 based feedback loop for a TOPswitch Flyback from Power integrations.

It looks like they are mixing in decibel numbers and “normal” numbers in the same expression?

The modulator and error amplifier circuit is shown as attached on page 466. Page 467 (Equn 8.35) shows the transfer function. However, Equn 8.35 on page 467 shows the parameter “G0”.

“G0” is shown in Equn 8.36 on page 467.

“G0” comprises the “Gpwm” parameter which is shown on page 455 (also attached)

The question is, “Gpwm” is a decibel number…… Why are they putting a decibel number into equation 8.35 on pg 467?

…..Equn 8.35 (pg 467) is an expression that needs to be calculated, (working through the complex numbers), and then that should have a log taken of its magnitude to express it in decibels…..so why is “Gpwm” in there as a decibel number already?

(by the way the pages are from a single SMPS book)
 

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treez

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Also, on the very same subject area, please may i also ask here this question?....

QUESTION: Adjustment needed to Modulator & Error amplifier transfer function of TL431/Opto based feedback compensation network (due to added NPN)
(book pages referred to are as in the above post)

Hi

We are calculating the Bode Plots for the 65W Offline DCM Flyback SMPS in DER-243 by Power Integrations……Please help to adjust the Modulator & Error amplifier transfer function? It needs modifying due to inclusion of an NPN (Q2) , as on page 7 of DER-243….

DER-243: 65W Flyback (Schematic on page 7)
https://ac-dc.power.com/design-support/reference-designs/design-examples/der-243-65-w-adapter/

The Modulator & Error Amplifier transfer function (without the NPN) is as on page 467 (equation 8.35) of the attached (Basso) book page. (some of this expression is also defined on page 455).
(Attached Page 466 shows the same TL431 based schem as in DER-243, but unfortunately the component designators are different.)

Anyway, would you agree that Equn 8.35 on page 467 should be modified by simply multiplying it by the gain of the NPN? (ie, a number between 70 and 100) (albeit perhaps a bit less than 70 due to the emitter degeneration resistor , R25, seen on page 7 of DER-243)
 

Akanimo

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Hi Treez,

Hello,

Please assist with the expression for the transfer function of the modulator and error amplifier of this TL431 based feedback loop for a TOPswitch Flyback from Power integrations.

It looks like they are mixing in decibel numbers and “normal” numbers in the same expression?

The modulator and error amplifier circuit is shown as attached on page 466. Page 467 (Equn 8.35) shows the transfer function. However, Equn 8.35 on page 467 shows the parameter “G0”.

“G0” is shown in Equn 8.36 on page 467.

“G0” comprises the “Gpwm” parameter which is shown on page 455 (also attached)

The question is, “Gpwm” is a decibel number…… Why are they putting a decibel number into equation 8.35 on pg 467?

…..Equn 8.35 (pg 467) is an expression that needs to be calculated, (working through the complex numbers), and then that should have a log taken of its magnitude to express it in decibels…..so why is “Gpwm” in there as a decibel number already?

(by the way the pages are from a single SMPS book)
There is no way to multiply the 'decibel' number as is presented in equation 8.35. I believe it's the absolute value of Spwm (|-163| = 163) according to equation 8.2 that they meant there. I'd advise that you check for some example designs in the book. If there are example designs, and there is same mix up, please dump the book.
 
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FvM

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There is no way to multiply the 'decibel' number as is presented in equation 8.35. I believe it's the absolute value of Spwm (|-163| = 163) according to equation 8.2 that they meant there. I'd advise that you check for some example designs in the book. If there are example designs, and there is same mix up, please dump the book.
Gpwm and G0 are gain values and don't have a dB unit as such. They are displayed in dB in certain context, e.g. when plotting a Bode diagram. Things can be often expressed clearer, but I refuse to held the author responsible for any lack of reading comprehension .
 
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treez

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Thanks, “Gpwm” is a value derived from taking the log of the magnitude of the Modulator Transfer Function. (as seen on page 455).

The Equation 8.35 on page 467 is an equation involving complex numbers. The idea is surely to evaluate this, then take a log of its magnitude…so surely we can’t put a value into equn 8.35 that is already a log’d value?...As, such, it seems right that instead of “Gpwm”, we should use “Spwm”, shown on page 455 as being 163.

|Spwm| = [antilog(Gpwm)] / 20
 

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