TOP-Switch-GX loop stability design
Hi to all,
I try to calculate the transfer function (closed loop) of the system. I hasn't problem to calculate the feedback compensation transfer function (used optoisolated + TL431 solution); but I have a problem with the total transfer function (flayback or power stage and feedback).
In CCM mode the our flyback has:
2 complex pole (in low frequency, low to 1kHz)
1 zero dx (in high frequency >> 1kHz)
1 zero sx (in high frequency > 1kHz)
The problem are the two complex pole; in the PI-Expert 8 I can't understand where are those poles?. They look like real pole 1/[1+(2*d*s)/w+s^2/w^2] with d>1, but in our application d is <<1.
Somebody can help me, or indicate the correct transfer function for CCM flayback application with TOP-Switch-GX. Are correct the two conjugated complex pole?
How the PI-Expert 8 manage them?
Thanks
F. Torrero
Comments
Thanks for the answer and for your help.
The flayback transfer function, that in the PI Expert is called magnitude and phase response of Power Stage, calculated in CCM mode and DCM is indicated in the attached file. In CCM you can see that there are two complex poles, in our application are complex.
I try to calculate and plot the flyback transfer function that I attach in CCM mode, but is very different from the PI Expert plot. Why?
The transfer function is calculated with the following parameters:
Vin = 314 V (input rectified voltage)
Np = 36 (primary turns)
Ns = 10 (secondary turns)
R = 4.8 Ohms (load)
C = 3000 uF (output capacitor)
RC = 12.5 mOhms (ESR for output capacitor)
D = 0,233 (duty cycle)
Lsec = 20,8 uH (secondary transformer inductance)
In the Power Stage transfer function (magnitude) of PI Expert there isn't the two complex poles.
Thanks
F. Torrero
Hi to all,
I read this post and I have the same problem. I can't understand the PI Expert transfer function with bode magnitude and phase diagram, there is a lot of difference between the PI Expert function and my calculated and plotted function.
I look forward an answer about a Power Integration Member.
Thanks very much.
Robert Pozzo
Bode plots use both magnitude and phase angle information. So, complex number is not a problem.
The total transfer function will be the combination of flyback converter including output filter, relationship of duty cycle to control pin current and feedback circuit with TL431 and optocoupler.
While flyback converter and feedback circuit are straight forward. TopSwitch-GX has PWM gain (duty cycle to control pin current) of 23%/mA according to the datasheet. There is a control pin dynamic impedance Zc of 15 ohm which forms a pole with control pin capacitor (47 microfarad). This 47 microfarad capacitor and a 6.8 ohm resistor ( in series) form a zero. Moreover, there is a 7kHz pole fixed internally.