Physical Origins of Input Filter Oscillations in Current Programmed Converters

Addition of an input filter to a current-programmed converter can cause the controller to oscillate. Two instability mechanisms can typically occur: 1) the current programmed controller effective current feedback loop may become unstable, or 2) the controller effective input voltage feed forward loop, which becomes a positive feedback loop when an input filter is added, may oscillate. Design criteria are derived and interpreted here. The effective current feedback loop is unmodified by addition of the input filter provided that the usual duty-cycle-programmed design criteria are satisfied: the filter output impedance Z, must be much smaller in magnitude than the closed-loop low-frequency regulator input impedance R / M 2 and the reflected converter filter impedance Z e i / M 2 . In addition, the effective input voltage loop is stable provided that a third criterion is satisfied. This criterion is active only at high frequency and/or for operating points close to the discontinuous conduction mode boundary. When all three criteria are well satisfied, then the output voltage regulation loop gain is unchanged. Hence, input filters of current programmed converters can be designed in essentially the same manner as for duty-ratio programmed converters. Results are summarized in tabular form for the basic buck, boost, and buck-boost converters. Experimental measurements for a buck converter with different input filters support the theoretical predictions.