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Chapter 5 of 5 · Chapter Overview

Loop Compensation and Stability

Bode plots, phase margin, Type-II compensation — making the Buck both accurate and stable.

The previous chapter's proportional feedback improved regulation but with limited accuracy — yet raising gain causes oscillation. This chapter starts from the high-gain oscillation phenomenon, introduces frequency-domain analysis (Bode plots, phase margin), then uses Type-II compensation to achieve both high accuracy and stability.

Start Lesson 1 Open complete Buck example

What you will learn in this chapter

  • Understand why high gain causes oscillation: LC double pole creates 180° phase shift
  • Master Bode plot fundamentals: gain margin, phase margin, crossover frequency
  • Understand Type-II compensator zero/pole placement strategy
  • Interpret step response in relation to frequency-domain phase margin

How to use this chapter

This chapter has multiple simulations: transient (observe oscillation/stability) and AC sweep (observe Bode plots). Start from L1 to follow the causal chain of gain, phase, and compensation.

1

Too Much Gain Causes Instability: Where Oscillation Comes From

Raising the error amplifier gain from 10 to 100 makes the output stop regulating — it oscillates violently.

 2

Bode Plots and Phase Margin: Stability Criterion in Frequency Domain

AC sweep the LC filter to understand -40dB/dec slope, 180° phase shift, and phase margin.

 3

Type-II Compensation: Stabilize the Loop with One Zero

Add R-C compensation around the error amplifier, using a zero to cancel the LC pole phase lag while keeping high DC gain.

 4

Compensation in Action: Transient Response Under VIN Step

Test Type-II compensation under a VIN step (12V→15V) and compare with Ch4's proportional feedback.