SLYT800B december   2020  – december 2020 LM25180 , LM25183 , LM25184 , LM5180 , LM5181

 

  1. 1Introduction
  2. 2A PSR Flyback DC/DC Converter with Multimode Control
  3. 3Output Capacitor Sizing for Voltage Ripple
  4. 4Small-signal Stability Review
  5. 5Output Capacitor Sizing for Small-signal Stability
  6. 6Conclusion
  7. 7References
  8. 8Related Web Sites

5 Output Capacitor Sizing for Small-signal Stability

From Figure 6, the loop gain generally presents as a slope of –20 dB per decade up to and beyond fC, simplifying the determination of required output capacitance. From Equation 7, Equation 9 gives a simplified loop gain expression in DCM:

GUID-A1AC0B3D-8E65-4E64-841C-09FB9973BFE1-low.png

where RC and CC denote the compensation resistance and capacitance, respectively.

Equation 9 further simplifies to yield an expression for COUT based on a target crossover frequency at unity gain (Equation 10):

GUID-F09324B5-5410-46B7-ABD4-67D8990B191A-low.png
GUID-2F67C36E-F469-45F7-A0F7-B6B86B159016-low.png Figure 6 Simulated Bode Plots at Input Voltages of 14 v (Dashed Lines, BCM) and 42 v (Solid Lines, DCM)

Using parameters from the LM25184 circuit shown in Figure 1, Equation 11 gives the output capacitance for a target crossover frequency of 35 kHz (10% of the switching frequency):

GUID-F60EBBD4-A21B-48CF-BE27-B176D0AF5356-low.png

overestimation given the attenuating contributions from the PSR sample-and-hold and the current-mode control high-frequency pole (although offset by the RHPZ gain) that may exist at a high fC. The result also aligns with a generally satisfactory load transient response.