Calculating gain and phasemargins for DCM flyback?

[treez]

Hello,
We are doing a sub 1W, offline, DCM Flyback SMPS with the Viper12 PWM controller.
We have a main, regulated output of 5V, 0.5W, and an auxiliary rail of some 18V, 100mW, which we use to power the viper chip itself.


Page 30 shows schematic with “R(pullup)” resistor in series with the opto transistor…
http://www.st.com/web/en/resource/technical/document/application_note/CD00077926.pdf


We wish to calculate the feedback loop gain and phase margins, and to do this we are using the equation 3-66 on page 292 of the Basso book (as attached). To use this equation we need to know the value for “R(pullup)”.

What is the value of the “R(pullup)”?…..is it 10K? (R2 in above st.com schematic), or is it 11k, (ie 10k plus the 1k inside the viper12A)?

The error amplifier schematic , showing R(pullup) is on page 296 (attached).

VIPER12A pwm controller datasheet…
http://dalincom.ru/datasheet/VIPER12A.pdf

 

[treez]

Also, page 12 of this document says shows a formula for the "control to output" transfer function of the DCM flyback...

https://www.st.com/web/en/resource/technical/document/application_note/CD00077926.pdf

..do you know why this equation does not include the turns ratio of the transformer?, that is surely needed in such an equation?

 

[Velkarn]

Also, page 12 of this document says shows a formula for the "control to output" transfer function of the DCM flyback...

https://www.st.com/web/en/resource/technical/document/application_note/CD00077926.pdf

..do you know why this equation does not include the turns ratio of the transformer?, that is surely needed in such an equation?

as you can see in eq.20 Gfly includes Vo and Ipk. i think this determines gain instead of turns ratio

- - - Updated - - -

 

[treez]

if using vout, then ipk needs to be referred to the secondary, but they don't do that.
The flyback equation is the buckboost feedback equation but adjusted for turns ratio.........they don't adjust for turns ratio...I wondr why?

 

[FvM]

if using vout, then ipk needs to be referred to the secondary

The opposite is true. By relating Vo to primary Ipk, you get the "gain" of the flyback converter, including the transformer ratio. Just a basic behavioral description.

Please notice that the gain ΔVo/ΔIpk is a small signal quantity, valid for a particular operation point. To derive the value in a calculation, winding ratio, transformer inductance, switching frequency and duty cycle (at least these) have to be put in.

 

[treez]

Referring again to the above document about DCM flyback, and for stability calculation...
https://www.st.com/web/en/resource/technical/document/application_note/CD00077926.pdf

...Do you agree that the gain and phase margins must be calculated by multiplying equation 23 (page 14) by equation 19 (page 12).?

Also that "RL" on page 13 actually should mean "load resistance" and not "inductor coil resistance", and that "delta V0" on page 12 should mean "change in vout" , and not "output voltage ripple"?

 

[Velkarn]

Referring again to the above document about DCM flyback, and for stability calculation...
https://www.st.com/web/en/resource/technical/document/application_note/CD00077926.pdf

...Do you agree that the gain and phase margins must be calculated by multiplying equation 23 (page 14) by equation 19 (page 12).?

no, i think you should use eq 23

 

[treez]

thanks, but equation 23 is just the error amplifier transfer function