SGLS245E May 2020 – May 2020 UCC2813-0-Q1 , UCC2813-1-Q1 , UCC2813-2-Q1 , UCC2813-3-Q1 , UCC2813-4-Q1 , UCC2813-5-Q1
PRODUCTION DATA.
Refer to the PDF data sheet for device specific package drawings
For good transient response, the bandwidth of the finalized design must be as wide as possible. The bandwidth of a CCM flyback (f_{BW}) is limited to ¼ of the RHP-zero frequency, or approximately 1.9 kHz using Equation 33.
The gain of the open-loop power stage at f_{BW} is equal to –22.4 dB and the phase at f_{BW} is equal to –87°. First step is to choose the output voltage-sensing resistor values. The output sensing resistors are selected based on the allowed power consumption and in this case, 1 mA of sensing current is assumed.
The TL431 is used as the feedback amplifier. Given its 2.5-V reference voltage, the voltage-sensing dividers R_{FBU} and R_{FBB} can be selected with Equation 34 and Equation 35.
Next step is to put the compensator zero f_{CZ} at 190 Hz, which is 1/10 of the target crossover frequency. Choose C_{Z} as a fixed value of 10 nF and choose the zero resistor value according to Equation 36.
Next, place a pole at the lower of RHP-zero or the ESR-zero frequencies. Based previous analysis, the RHP zero is at 7.65 kHz and the ESR zero is at 6 kHz, so the pole of the compensation loop should be put at 6 kHz. This pole can be added through the primary side error amplifier. R_{FB} and C_{FB} provide the necessary pole. Choosing R_{FB} as 10 kΩ, C_{FB} is calculated by Equation 37.
Based on the compensation loop structure, the entire compensation loop transfer function is written as Equation 38.
where
The only remaining unknown value required in this equation is R_{LED}. The entire loop gain must be equal to 1 at the crossover frequency. R_{LED} is calculated accordingly as 1.62 kΩ.
The final closed-loop Bode plots are shown in Figure 36 and Figure 37. The converter achieves approximately 2-kHz crossover frequency and approximately 70° of phase margin.
TI recommends checking the loop stability across all the corner cases, including component tolerances, to ensure system stability.