Hello,
We tried to design a PoE Power Supply with the TPS 23753A like proposed in the Application Note SLVA305C. In order to calculate the feedback etc. we transfered the equations of the A.N. into a Mathcad sheet.
A problem occured in: "2.10.3 Compensate Opto-Isolator or Inner Loop" (page 14):
Equation 35 said:
Z_OUT(w)=Z_cout1(w)||Z_cout2(w)||R_load
so
Z_Out(w)=1/(i*w*Cout2+i*w*Cout1+1/R_load)
Is this correct so far?
By Equation 36 I get
MPF(w)=I_MPS(w)*Z_OUT(w).
with
I_MPS(w)=K.MPS*(1-(iw/w_RHPZ))
with
w_RHPZ=RHPZ*2*pi
Putting this into Equation 38 should return a C_CTL of 50.3nF. In my case it did not. The squared term got bigger than 1 so the number in the squareroot was negative, giving me a complex number as a result.
The values used for the Calculation were the ones used in the A.N.:
K_CTL=2
CTR_2mA=0.85
F_0=5.5kHz
G_mo (2 pi F_0 ) = 0.75
R_CTL=2k
R_OB=402R
C_out1=47µF
C_out2=2*47µF
RHPZ=21.6kHz
K_MPS=4.66A/V
R_load=1.56R
So either did i make a mistake probably somewhere in the MPF function or i confused a value or there's something strange in the A.N?
Can you help me with that?
Thank you in advance.
Benedikt Bugl
