Key Takeaways
- Light load → DCM → Vout > D×VIN
- Fixed D cannot guarantee constant output under varying load
Lesson 1
Load Lightens → Enters DCM → Output Rises
Change load from 10Ω to 100Ω (lighter load), the same D=50% Buck output rises from 6V to above 8V — open-loop is helpless.
In real systems, load changes frequently: a phone CPU switches from power-save to high-performance mode, current jumps from 100mA to 2A; or the reverse, from full load to nearly idle.
Ch2 already mentioned: when load lightens, inductor current may reach zero, entering DCM. In DCM, Vout ≠ D×VIN — output rises significantly. Here R changes from 10Ω to 100Ω, output rises from 6V to ~8V, over 30% deviation.
D is fixed, VIN hasn't changed, only the load changed — yet output deviated. This is the fragility of 'open-loop'.
Light Load: R=100Ω, Output Rises
D=50%, VIN=12V, R=100Ω. Enters DCM, Vout≈8V instead of expected 6V.
After reading this section, run the simulation and observe the waveforms. To explore further, open the example in a standalone page.
Watch Items
- out steady state well above 6V (~8V) — deviation caused by lighter load
- i_l1 shows zero-current plateaus — confirms DCM entry