SLUAAJ0 February   2024 TPS51397A , TPS54308 , TPS54320 , TPS54350 , TPS54538 , TPS54620 , TPS54622 , TPS54821 , TPS54824 , TPS563300 , TPS566231 , TPS566235 , TPS566238 , TPS568230 , TPS56C215 , TPS62933 , TPS62933F , TPS62933O

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Comparison of Feedback Sense Methods with Second Stage Filter
  6. 3Transfer Function Derivation of PCM Converter with Second Stage Filter and Hybrid Sense
  7. 4Overall Loop Model
  8. 5Zero and Pole Analysis
  9. 6Stability Design Method
  10. 7Design Example and Experimental Validation with TPS62933F
  11. 8Summary
  12. 9References
  13.   A Appendix

2 Comparison of Feedback Sense Methods with Second Stage Filter

Figure 2-1 to Figure 2-3 show the schemes of power designs with second stage filter, correspond to the power designs with first stage sense, second stage sense and hybrid sense. The advantages and disadvantages of each design are summarized in the following:

GUID-20231017-SS0I-4SJV-C78X-9LXWNVHJFGNG-low.svg Figure 2-1 Scheme of Converter Second Stage Filter Design with First Stage Sense
GUID-20231017-SS0I-XT12-WGJ7-WBXDKRKNLGJT-low.svg Figure 2-2 Scheme of Converter Second Stage Filter Design with Second Stage Sense
GUID-20231017-SS0I-1WC6-X6SQ-SZZXRHCLMMZB-low.svg Figure 2-3 Scheme of Converter Second Stage Filter Design with Hybrid Sense
  • With first stage sense, the feedback sense point is Vo1 and the voltage drop on DCR of L2 can’t be compensated, so load regulation performance is worse. But the stability is good as the double poles of second stage filter are not included in the control loop.
  • With second stage sense, the voltage drop on DCR of L2 can be compensated. But the double poles of second stage filter can have obvious impacts on the loop response. When the values of L2 and C2 become larger, the double poles frequency of second stage filter reduces and can be closer to bandwidth, which can cause less phase margin and possible instability. That limits the maximum value of second stage filter components selection and the ability to reduce output ripple.
  • With hybrid sense, the feed-forward capacitor Cff is connected with Vo1 and upper feedback resistor R1 is connected with Vo2. The AC disturbance of Vo1 can be coupled to VFB and reduce the portion of Vo2 AC disturbance in total feedback. That can help to reduce the effects of second stage filter on loop stability. And the load regulation performance is also good since the DC regulation is based on feedback from Vo2. So the loop stability and output accuracy can be made sure at the same time with the hybrid feedback sense.

Due to the obvious advantages of hybrid sense, the application design method in this application note is based on this sense approach.