Key Takeaways
- CCM = inductor current always > 0, never reaches zero
- In CCM, VOUT = D × VIN, depends only on duty cycle
- Volt-second balance across one cycle: energy in = energy out
Lesson 1
CCM Steady State: Inductor Current Never Reaches Zero
Under heavy load, inductor current remains positive at all times — this is Continuous Conduction Mode (CCM).
Recall the complete Buck from the previous chapter: VIN=12V, D=50%, f=10kHz, L=1mH, C=100μF, R=10Ω. In steady state, output voltage≈6V, load current≈0.6A.
Observe the inductor current waveform: it's a triangular wave oscillating around a positive average value. The key point — it never drops to zero. This state of continuously flowing inductor current is called CCM (Continuous Conduction Mode).
Steady-state analysis in CCM is clean: when the switch is ON, voltage across the inductor is VIN−VOUT (positive), current ramps up linearly; when OFF, it's −VOUT (negative, freewheeling through diode), current ramps down. Steady state requires net inductor current change over one cycle to be zero.
This gives us the classic formula: (VIN−VOUT)×D×T = VOUT×(1−D)×T → VOUT = D×VIN. This is the Buck voltage transfer function in CCM — simple, elegant, dependent only on duty cycle.
Standard CCM Buck
L=1mH, R=10Ω. In steady state, inductor current ripple≈0.3A, oscillating between ~0.45–0.75A, always positive.
After reading this section, run the simulation and observe the waveforms. To explore further, open the example in a standalone page.
Watch Items
- i_l1 waveform: triangular wave with minimum well above zero — confirms CCM
- out waveform: steady state ≈6V (= D × VIN = 0.5 × 12)