How to use the buck converter calculator
- Enter the input voltage, the output voltage (lower than the input) and the load current.
- Enter the switching frequency in kilohertz.
- Choose the ripple current as a percent of the load and the output ripple voltage you accept. The page starts with example values.
- Read the duty cycle, the inductor and the output capacitor, the ripple and the peak inductor current.
Formula
Duty cycle D = Vout / Vin
Ripple current = ripple percent x load current
Inductor L = Vout x (1 - D) / (frequency x ripple current)
Output capacitor C = ripple current / (8 x frequency x ripple voltage); peak current = load + ripple / 2
These are the textbook equations of an ideal buck converter in continuous conduction. They give a starting point for the inductor and capacitor values, which are then adjusted for the parts you can buy.
12 V to 5 V at 1 A
At 100 kHz with 30 percent ripple, the duty cycle is 5 / 12 = 0.41667 and the ripple current is 0.3 A. The inductor is 5 x (1 - 0.41667) / (100,000 x 0.3) = 97.222 uH. For 50 mV of output ripple the capacitor is 0.3 / (8 x 100,000 x 0.05) = 7.5 uF. The peak inductor current is 1.15 A, and the converter stays in continuous conduction down to a load of 0.15 A.
For 24 V to 3.3 V at 2 A and 300 kHz with 30 mV of ripple, the duty cycle is 0.1375, the inductor is 15.812 uH, the capacitor is 8.3333 uF and the peak current is 2.3 A. A 48 V to 12 V converter at 5 A, 200 kHz, 40 percent ripple and 100 mV needs 22.5 uH and 12.5 uF.
Choosing the ripple
A larger inductor gives less ripple current but costs more and responds slowly. Many designs aim for ripple of 20 to 40 percent of the load current, which is a design choice and not a rule. The ripple must also stay below the point where the peak current exceeds the saturation rating of the inductor.
The RLC impedance and resonance calculator helps with filter parts, and the voltage divider calculator sets feedback resistors. A timer based oscillator is covered by the 555 timer calculator.
Where the ideal model stops
A real converter has switch resistance, diode or synchronous rectifier drops, inductor winding resistance and capacitor equivalent series resistance. The capacitor ESR often sets the real output ripple, so the capacitor you pick may need to be larger or have low ESR. The input capacitor and the control loop are not covered here. Test the design on the bench before you rely on it.
Assumptions and limits
- The converter is ideal and in continuous conduction mode, with no losses.
- The ripple current is peak to peak and given as a percent of the load current.
- The output ripple comes from the capacitance alone. Equivalent series resistance is not included.
- The input capacitor, the feedback loop and the choice of switch are outside this page.
Frequently asked questions
How do I calculate the inductor for a buck converter?
Use L = Vout x (1 - D) / (f x ripple current), where D = Vout / Vin. For 12 V to 5 V at 100 kHz and 0.3 A of ripple that is 97.2 uH.
How do I size the output capacitor?
Use C = ripple current / (8 x f x ripple voltage). With 0.3 A, 100 kHz and 50 mV of ripple that is 7.5 uF.
What ripple current should I pick?
It is a design choice. A common starting point is around 30 percent of the load current, which the page uses as an example.
Does this include losses?
No. It is the ideal model, so real duty cycles are a little higher and real ripple depends on the capacitor ESR.