Buck Converter Designer
Design step-down switching regulators.
What is a Buck Converter?
A Buck Converter (step-down converter) is a DC-to-DC power converter which efficiently reduces voltage using switching rather than heat dissipation. Unlike linear regulators, which can waste 50% or more of your energy as heat, a properly designed buck converter can achieve efficiencies over 95%.
It works by rapidly switching a transistor (usually a MOSFET) on and off, "chopping" the input voltage. An inductor and capacitor then smooth this square wave back into a steady, lower DC voltage.
Calculating Components (Formulas)
Designing a buck converter requires solving for two main components: the Inductor (L) and the Output Capacitor (Cout).
1. Duty Cycle
D = Vout / Vin * Efficiency
(Ideal D = Vout/Vin)
2. Inductor Selection (L)
We size the inductor to limit the ripple current (ΔIL) to a fraction of the maximum load (typically 30%).
L = (Vout × (Vin - Vout)) / (ΔIL × fsw × Vin)
Where fsw is the switching frequency (e.g., 300kHz).
3. Capacitor Selection (C)
The output capacitor dampens the voltage ripple (ΔVout). The formula depends on whether the capacitor's ESR (Equivalent Series Resistance) dominates the ripple.
Cmin = ΔIL / (8 × fsw × ΔVout)
Practical Applications
Buck converters are the standard for power delivery in modern electronics:
- USB Chargers: Converting 12V car battery power to 5V for phones.
- Motherboards: Stepping down 12V from the PSU to 1.0V for the CPU core (VRMs are multiphase buck converters).
- LED Drivers: Constant-current buck converters are used to drive high-power LEDs efficiently.
FAQ
Buck Converter vs Linear Regulator (LDO): Which is better?
Use a Linear Regulator (like LM7805) for very low power or noise-sensitive analog circuits where efficiency doesn't matter. Use a Buck Converter when the current is high (>0.5A) or the voltage drop is large (e.g., 24V to 5V), where a linear regulator would overheat instantly.
What is switching frequency?
It is the speed at which the MOSFET turns on and off. Higher frequencies (e.g., 1MHz) allow for smaller physical inductors and capacitors but increase switching losses and heat in the MOSFET. Lower frequencies (e.g., 50kHz) are more efficient but require large, bulky components.
What happens if the inductor saturates?
If the current exceeds the inductor's saturation rating, its inductance drops to near zero. It effectively becomes a short circuit, causing massive current spikes that will likely blow up the MOSFET or the controller chip.