SLUSBW3D March 2014 – December 2017 UCC28630 , UCC28631 , UCC28632 , UCC28633 , UCC28634
PRODUCTION DATA.
To support operation in CCM, and allow operation at fixed frequency over a large percentage of the load range, the UCC2863x uses fixed-point sampling rather than knee-point detection. When conventionally used, fixed-point sampling typically suffers from poorer regulation performance. This poor performance results from the voltage drops across the secondary-side parasitic resistance R_{SEC}, and the secondary-side leakage inductance from secondary-side to bias L_{LK(sec_bias)}, as a consequence of the fact that current remains flowing on the secondary-side when the device measures the output voltage. As shown in Figure 27, the secondary-side pin voltage that gets reflected to the bias winding is detailed in Equation 9.
Equation 9 can be expanded and rearranged into Equation 10.
Many elements contribute errors to the sensed secondary-side pin voltage, when measured across the bias winding:
Typically, the peak secondary-side winding current I_{SEC} is many times larger than the load current, and the secondary-side winding resistance is typically larger than the output capacitor esr. Thus, the last term in Equation 10 involving I_{LOAD} can typically be neglected.
The leakage inductance and secondary-side rectifier terms represent quasi-constant offset terms, so do not affect regulation to a significant extent. Thus, the quasi-constant offset terms can be accounted for in the calculation of the required scaling resistors to produce the desired setpoint voltage.
The remaining term that dominates the regulation error in Equation 10 is the drop across the secondary-side winding resistance and capacitor esr at the sample instant, \{I_{SEC} x(R_{SEC} + R_{C(esr)})\}. The controller internally adjusts the control loop reference in proportion to the primary peak current demand in order to null the I_{SEC} related error term in the sampled bias winding voltage. Since the peak secondary-side current I_{SEC(pk)} is the primary peak current I_{PRI(pk)} scaled by the transformer turns ratio, the internal control loop reference effectively varies in approximate proportion to I_{SEC}, resulting in dramatically improved regulation performance.
This improved regulation performance allows the use of primary-side regulation in a wider range of applications, and at unprecedented power levels, operating in both CCM and DCM.