How a Buck Converter Steps Down Voltage: The Switching Cycle
- [Buck converter MOSFET] [stores] [energy in inductor during on-time]
- [Duty cycle] [determines] [output voltage as Vout = Vin x D]
- [Feedback loop] [adjusts] [duty cycle to regulate output voltage under load changes]
⚡ Switch ON Phase
MOSFET conducts. Current flows: Vin → MOSFET → Inductor → Load → GND. Inductor builds up magnetic field, storing energy. Output capacitor also charges.
🔁 Switch OFF Phase
MOSFET opens. Inductor maintains current via freewheeling diode (Schottky). Current path: Inductor → Load → GND → Diode → Inductor. Inductor releases stored energy to load.
📊 Feedback Control
Error amplifier compares Vout to reference. PWM controller adjusts duty cycle to compensate: more load → lower Vout → increase duty cycle → more energy delivered per cycle.
Buck Converter vs LDO Regulator: Efficiency Comparison
| Factor | Buck Converter | LDO Regulator |
|---|---|---|
| Efficiency | 85–95% | (Vout/Vin)×100% max — can be 40% |
| Heat Generated | Very low | High: P = (Vin–Vout)×I |
| Output Noise | Switching ripple present | Very clean (no switching) |
| Component Count | Higher (inductor, caps) | Lower (simpler circuit) |
| Cost | Higher ($1–5 module) | Lower ($0.10–0.50) |
| Best Use | 12V→5V at 2A+ | 5V→3.3V at <300mA, audio/RF |
| Example | LM2596, MP2307, XL4016 | L7805, AMS1117, LM1117 |
Frequently Asked Questions
How does a buck converter work?
A MOSFET switches on/off at high frequency (50 kHz–3 MHz). When ON, current through inductor builds magnetic energy. When OFF, inductor releases energy via freewheeling diode. Duty cycle (on-time%) controls output: Vout = Vin × D. Feedback loop auto-adjusts D to maintain regulated output.
What is the efficiency formula for a buck converter?
η = (Vout × Iout) / (Vin × Iin) × 100%. Example: 12V → 5V at 2A. Buck converter: η ≈ 92%. LDO same conversion: η = 5/12 × 100% = 41.7%. The LDO burns 7W as heat; the buck converter wastes only 0.9W.
What is the relationship between duty cycle and output voltage in a buck converter?
Vout = Vin × D. For 5V output from 12V: D = 5/12 = 41.7%. The feedback error amplifier continuously measures Vout and adjusts D up (more load) or down (less load) to maintain regulation.
When should I use a buck converter vs an LDO regulator?
Use buck converter: voltage drop > 2V AND current > 300mA. Use LDO: small drop (5V→3.3V), low current (<100mA), noise-sensitive circuits (audio, RF). The 5V→3.3V LDO is efficient (66%) and simple; 12V→3.3V LDO would waste 76% as heat.
How do I set the output voltage on an LM2596 module?
Adjust the blue trimmer pot on the module. Vout = 1.23 × (1 + R2/R1). Connect input, attach multimeter to output, turn pot clockwise to increase voltage until target is reached. Range: 1.25V–35V. Always verify output before connecting sensitive loads.
Conclusion
Buck converters are the workhorses of modern power electronics — laptop chargers, phone chargers, solar charge controllers, and EV battery management all rely on switching regulators for their efficiency. Understanding the switching cycle, inductor role, and duty cycle relationship lets you design reliable power supplies for any project.
📚 References & Sources
Related Resources
How Voltage Regulators Work
LDO regulators vs buck converters — comprehensive power guide
How Inductors Work
Inductors are the energy storage heart of every buck converter
MOSFET as a Switch
The switching MOSFET is the main switch in a buck converter
LiPo Battery Charging
CC-CV charging uses a switch-mode buck regulator