8.2.2.9 Feedback Network Biasing

Achieving very low standby power while maintaining high-performance load-step transient response requires careful design of the feedback network. Optically coupled secondary-side regulation is used to provide the rapid response needed when a heavy load step occurs during the no-load condition. One of the most commonly used devices to drive the optocoupler is the TL431 shunt-regulator, due to its simplicity, regulation performance, and low cost. This device requires a minimum bias current of 1 mA to maintain regulation accuracy. Together with the UCC28740 primary-side controller, careful biasing will ensure less than 30 mW of standby power loss at room temperature. Where a more stringent standby loss limit of less than 10 mW is required, the TLV431 device is recommended due to its minimum 80-µA bias capability.

Facilitating these low standby-power targets is the approximate 23-µA range of the FB input for full to no-load voltage regulation. The control-law profile graph (see Figure 15) shows that for FB-input current greater than 22 µA, no further reduction in switching frequency is possible. Therefore, minimum power is converted at f_SW(min). However, the typical minimum steady-state operating frequency tends to be in the range of several-hundred Hertz, and consequently the maximum steady-state FB current at no-load will be less than I_FBMAX. Even so, prudent design practice dictates that I_FBMAX should be used for conservative steady-state biasing calculations. At this current level, V_FBMAX can be expected at the FB input.

Referring to the Design Procedure Application Example in Figure 18, the main purpose of R_FB4 is to speed up the turnoff time of the optocoupler in the case of a heavy load-step transient condition. The value of R_FB4 is determined empirically due to the variable nature of the specific optocoupler chosen for the design, but tends to fall within the range of 10 kΩ to 100 kΩ. A tradeoff must be made between a lower value for faster transient response and a higher value for lower standby power. R_FB4 also serves to set a minimum bias current for the optocoupler and to drain dark current.

It is important to understand the distinction between steady-state no-load bias currents and voltages which affect standby power, and the varying extremes of these same currents and voltages which affect regulation during transient conditions. Design targets for minimum standby loss and maximum transient response often result in conflicting requirements for component values. Trade-offs, such as for R_FB4 as discussed previously, must be made.

During standby operation, the total auxiliary current (used in Equation 8) is the sum of I_WAIT into the IC and the no-load optocoupler-output current I_CENL. This optocoupler current is given by Equation 30.

Equation 30.

For fast response, the optocoupler-output transistor is biased to minimize the variation of V_CE between full-load and no-load operation. Connecting the emitter directly to the FB input of the UCC28740 is possible, however, an unload-step response may unavoidably drive the optocoupler into saturation which will overload the FB input with full VDD applied. 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. The value of R_FB3 is chosen to limit the excess I_FB and R_FB4 current to an acceptable level when the optocoupler is saturated. Like R_FB4, the R_FB3 value is also chosen empirically during prototype evaluation to optimize performance based on the conditions present during that situation. A starting value may be estimated using Equation 31.

Equation 31.

Note that R_FB3 is estimated based on the expected no-load VDD voltage, but full-load VDD voltage will be higher resulting in initially higher I_CE current during the unload-step transient condition. Because R_FB3 is interposed between V_E and the FB input, the optocoupler transistor V_CE varies considerably more as I_CE varies and transient response time is reduced. Capacitor C_FB3 across R_FB3 helps to improve the transient response again. The value of C_FB3 is estimated initially by equating the R_FB3C_FB3 time constant to 1 ms, and later is adjusted higher or lower for optimal performance during prototype evaluation.

The optocoupler transistor-output current I_CE is proportional to the optocoupler diode input current by its current transfer ratio, CTR. Although many optocouplers are rated with nominal CTR between 50% and 600%, or are ranked into narrower ranges, the actual CTR obtained at the low currents used with the UCC28740 falls around 5% to 15%. At full-load regulation, when I_FB is near zero, V_FB is still approximately 0.4 V and this sets a minimum steady-state current for I_CE through R_FB4. After choosing an optocoupler, the designer must characterize its CTR over the range of low output currents expected in this application, because optocoupler data sheets rarely include such information. The actual CTR obtained is required to determine the diode input current range at the secondary-side shunt-regulator.

Referring again to Figure 18, the shunt-regulator (typically a TL431) current must be at least 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 the minimum 1-mA regulator bias current. 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 Equation 30 and Equation 32. 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 Equation 7 to estimate secondary-side standby loss.

Equation 32.

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 33 provides an estimated initial value for R_TL, which may be adjusted for optimal limiting later during the prototype evaluation process.

Equation 33.

The output-voltage sense-network resistors R_FB1 and R_FB2 are calculated in the usual manner based on the shunt-regulator reference voltage and input bias current. Having characterized the optocoupler at low currents and determined the initial values of R_FB1, R_FB2, R_FB3, R_FB4, C_FB3, R_OPT and R_TL using the above procedure, the DC-bias states of the feedback network can be established for steady-state full-load and no-load conditions. Adjustments of these initial values may be necessary to accommodate variations of the UCC28740, optocoupler, and shunt-regulator parameters for optimal overall performance.

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. The compensation design procedure is beyond the scope of this datasheet.