Key Takeaways
- DCM = inductor current drops to zero and stays there each cycle
- Light load (large R) → small average current → easier to enter DCM
- In DCM, VOUT ≠ D×VIN; output voltage is load-dependent
Lesson 3
DCM: Current Reaches Zero at Light Load
When load current is small, inductor current drops to zero and stays there within each cycle — this is Discontinuous Conduction Mode (DCM).
Increase load resistance from 10Ω to 100Ω (lighter load). Average load current drops from 0.6A to 0.06A. But inductor ripple ΔI=0.3A stays the same (it depends only on L, VIN, VOUT, f).
Here's the problem: average current is only 0.06A, but ripple is 0.3A. If current oscillates as a triangle, minimum would be 0.06 − 0.15 = −0.09A — but inductor current cannot go negative (the diode blocks reverse current)!
What actually happens: when current drops to zero, the diode turns off, clamping inductor current at zero. It stays at zero for a while until the next switch-on. This zero-current interval is the hallmark of DCM.
In DCM, VOUT is no longer simply D×VIN — it becomes load-dependent. With the same duty cycle, DCM output voltage tends to be higher. This makes control more complex.
Light-Load DCM
R=100Ω (light load). Inductor current reaches zero each cycle — typical DCM waveform.
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 shows clear 'zero-current plateaus' — the DCM signature
- Output voltage 'out' significantly above 6V (in DCM, output rises well above D×VIN)