8.2.2.10 Feedback Loop Design Consideration

Refer to Figure 18, the UCC28742 converter feedback network is composed of TL431, optocoupler and several resistors and capacitors. R_FB1 and R_FB2 set up the converter output regulation point. A series-resistor R_FB3 is necessary to limit the current into FB and to avoid excess draining of C_VDD during this type of transient situation, although connecting the emitter directly to the FB input of the UCC28742 is possible. However, an unload-step response may unavoidably drive the optocoupler into saturation which will overload the FB input with full VDD applied. The value of R_FB3 is to limit the excess I_FB to an acceptable level when the optocoupler is saturated. The R_FB3 value is chosen to allow the current into the FB pin to reach the 30 µA. the maximum I_FB control level. This will be met if the voltage at I_FB can reach 1V at no load conditions. To improve transient response R_FB3 can be bypassed with C_FB3.

R_FB4 can be used to set a nominal operating current of the optocoupler to improve the current transfer ratio and bandwidth of the optocoupler. For low standby power this operating current level should be kept small since it must be supplied from VDD operating voltage. The value of R_FB4 is determined empirically due to the variable nature of the specific optocoupler chosen for the design. The ratio of R_FB4 to R_FB3 is typically in a range of 1/10 to 1/4 with typical value of R_FB4 in 4 kΩ to 25 kΩ, and R_FB3 in 25 kΩ to 200 kΩ. A good starting point is to select R_FB4 around 4 kΩ and R_FB3 around 30 kΩ for a design.

The shunt-regulator compensation network, Z_FB, is determined using well-established design techniques for control-loop stability. Typically, a Type-II compensation network is used. An effective approach is to set Z_FB to be a capacitor, Z_FB = C_FB to form an integrator, and adding a bypass capacitor R_FB3 will extend the frequency response of the optocoupler CTR.

Referring again to Figure 18, the shunt-regulator (typically a TL431) current is at about 1 mA even when almost no optocoupler diode current flows. Since even a near-zero diode current establishes a forward voltage, R_OPT is selected to provide regulator bias current such as for TL431. The optocoupler input diode must be characterized by the designer to obtain the actual forward voltage versus forward current at the low currents expected. At the full-load condition of the converter, I_FB is around 0.5 µA, I_CE may be around (0.4 V / R_FB4), and CTR at this level is about 10%, so the diode current typically falls in the range of 25 µA to 100 µA. Typical opto-diode forward voltage at this level is about 0.97 V which is applied across R_OPT. If R_OPT is set equal to 1 kΩ, this provides 970 µA plus the diode current for I_OPT.

As output load decreases, the voltage across the shunt-regulator also decreases to increase the current through the optocoupler diode. This increases the diode forward voltage across R_OPT. CTR at no-load (when I_CE is higher) is generally a few percent higher than CTR at full-load (when I_CE is lower). At steady-state no-load condition, the shunt-regulator current is maximized and can be estimated by and Equation 31. I_OPTNL, plus the sum of the leakage currents of all the components on the output of the converter, constitute the total current required for use in to estimate secondary-side standby loss.

The shunt-regulator voltage can decrease to a minimum, saturated level of about 2 V. To prevent excessive diode current, a series resistor, R_TL, is added to limit I_OPT to the maximum value necessary for regulation. Equation 32 provides an estimated initial value for R_TL, which may be adjusted for optimal limiting later during the prototype evaluation process.