Buck converter design
20 minutes · Advanced
Simulations and quizzes aren't read aloud; pause when you reach them and try them out.
In the Power Supplies course we used ready-made buck modules. In this course you'll learn to design your own converter: choosing the IC, the inductor, the capacitors, the feedback and the PCB. The first lesson covers the basic calculations for the most widely used topology, the buck (step-down) converter.
How does a buck work?
- While the switch (high-side MOSFET) is on, the inductor is connected to the input; the inductor current rises and the inductor stores energy.
- When the switch turns off, the inductor current can't be cut; it carries on through the low-side MOSFET (synchronous) or a diode and falls.
- The output capacitor filters this triangular current; a smooth DC voltage remains at the output.
The basic equations
D = Vout ÷ Vin
The ripple ΔI in the inductor current is usually chosen to be 20–40% of the load current. Small ripple means a large (and expensive, slow-responding) inductor; large ripple strains the capacitors and the switch.
Question: 12 V → 5 V, 2 A, 400 kHz. What inductor value gives 30% ripple?
D = 5 / 12 = 0.417
ΔI = 0.3 × 2 = 0.6 A
L = (12 − 5) × 0.417 / (400,000 × 0.6) ≈ 12 µH → standard value 10 or 15 µH
Try it yourself
Raise the frequency: a smaller inductor is enough for the same ripple. Lower the load current: beyond a certain point the inductor current falls to zero each period (discontinuous conduction, DCM). Increase the capacitor's ESR: see that most of the output ripple comes from the ESR.
Buck design bench
Duty cycle D
41.7%
Inductor ripple ΔI
442 mA
Peak current
2.22 A
Output ripple
13 mV
Choosing the IC
- Converters with built-in MOSFETs (TPS5430, MP1584, LM2596, AP63203): for 1–5 A; they work with a few external parts. Ideal for learning and most projects.
- Controllers (LM5116, TPS40057): drive external MOSFETs; for currents of 10 A and above.
- Synchronous or asynchronous: in an asynchronous design the low-side switch is a Schottky diode (0.4 V drops across it). A synchronous design uses a low-side MOSFET; at low output voltages the efficiency rises noticeably.
The datasheet's 'application' section
A good converter datasheet has an "Application Information" or "Design Procedure" section: it explains step by step how to choose the inductor, capacitors and feedback resistors, and even draws a recommended PCB layout. Always start your design there. Online tools such as TI WEBENCH also do these calculations automatically.Setting the output voltage
A voltage divider is connected from the output to the feedback (FB) pin. The IC works to make the FB pin equal to its internal reference voltage (e.g. 0.8 V):
Vout = Vref × (1 + Rtop ÷ Rbottom)
Question: Vref = 0.8 V, Rbottom = 10 kΩ, and the output should be 5 V. What is Rtop?
5 = 0.8 × (1 + Rtop / 10k) → Rtop = 52.5 kΩ → 52.3 kΩ (1% series)
Test yourself
1. Roughly what is the duty cycle in a 24 V → 3.3 V buck?
2. If the frequency doubles, the inductor for the same ΔI is?
3. What is the typical inductor ripple target as a fraction of the load current?
4. Vref = 1.25 V, Rbottom = 10 kΩ, Rtop = 30 kΩ. The output?
This was the free first lesson of the course
The remaining 7 lessons of Switch-Mode Power Supply Design, with their simulations and quizzes, unlock when you buy the course.
See the full course →