Boost Converter - Duty Cycle and Inductor Sizing

A boost converter calculator solves the steady-state design equations for a step-up DC-DC switching converter. It takes a lower DC input voltage and produces a higher DC output voltage by storing energy in an inductor while a transistor switch is closed and then releasing that stored energy into the output capacitor and load while the switch is open. This page works in continuous conduction mode (CCM) and returns duty cycle, output voltage, average currents, inductor value, and output capacitor value from your inputs.

• • Sizing a battery-to-USB boost regulator: Pick the inductor and capacitor values needed to step a 3.7 V lithium cell up to 5 V at 1 W for a USB device.
• • Verifying a switching power supply design: Confirm the duty cycle and ripple targets a textbook problem needs before choosing a controller IC and an inductor.
• • Teaching continuous conduction mode: Walk students through Vout = Vin / (1 - D) and the ripple budgets used in power-electronics courses.
• • Comparing boost and buck topologies: Estimate how much current the inductor must carry when stepping a supply up to a higher rail for an LED string or sensor front end.

A boost converter is one of the three basic PWM DC-DC switching topologies along with the buck (step-down) and buck-boost converters. The output voltage is always above the input, so the inductor carries more average current than the load, and the diode only conducts during the switch off-time. The calculator treats the converter as lossless, useful when switch on-resistance, diode forward drop, and inductor DCR are small.

When you need to convert the load power into the load resistance used here, Ohm's Law Calculator handles the V = IR relationship directly.

The boost converter calculator applies volt-second balance to the inductor in continuous conduction mode and charge balance to the output capacitor. Both balances come from averaging the inductor voltage and capacitor current over one switching period.

• Vin: DC supply voltage feeding the boost converter input.
• Vout: Average DC output voltage at the load.
• D: Switch duty cycle, the fraction of each switching period that the transistor is on.
• f: Switching frequency in hertz, set by the controller IC.
• Pout: Output power delivered to the load in watts.

Once the duty cycle is known, the load current Iout = Pout / Vout and the equivalent load resistance R = Vout / Iout follow directly. The average inductor current is Iout / (1 - D), because the same energy that flows out of the converter in (1 - D) of the period flows in during the full period. The required inductance L = (Vin * D) / (f * dIL) and output capacitance C = (Iout * D) / (f * dVout) keep the ripple inside the budget you set.

According to Texas Instruments application note SLVA372, applying volt-second balance to the boost inductor in continuous conduction mode gives the steady-state transfer function Vout/Vin = 1/(1 - D).

To estimate the RC time constant of the output capacitor, Capacitor Charge Time Calculator reuses the same RC math on a different topology, and the boost converter calculator on this page returns the matching L and C values for a chosen switching frequency.

Four concepts describe how the calculator handles a switching converter. None of them require a derivation; they describe what each one means inside a design.

If the inductor current is allowed to touch zero the converter enters discontinuous conduction mode (DCM), and the simple transfer function no longer holds. The calculator stays in CCM because that is where the closed-form formulas come from and where the textbook treatment begins. If your load is so light that DCM becomes likely, lower the inductor ripple fraction or raise the switching frequency so the average inductor current stays comfortably above half of its peak-to-peak ripple.

Both the boost converter and a Wheatstone bridge rest on the same idea of balancing two halves of a circuit, so Wheatstone Bridge Calculator is a useful companion when you explain the topology in class.

Six numbered steps walk through a typical design pass. The example uses 3.7 V in, 5 V out, 250 kHz, and 1 W.

1. 1 Enter the input voltage: Type the DC supply voltage that powers the boost converter. A single lithium cell is 3.7 V nominal, a 12 V lead-acid rail is 12 V, and a USB-C PD rail can be 5 V, 9 V, or 15 V.
2. 2 Enter the target output voltage: Type the DC voltage the load needs. The boost converter calculator returns a duty cycle D = 1 - Vin / Vout once Vout is greater than Vin.
3. 3 Enter the switching frequency: Pick the frequency from the controller datasheet. Most boost controllers run between 100 kHz and 1 MHz; faster switching shrinks the inductor but raises switching losses.
4. 4 Enter the load power: Type the output power the load draws in watts.
5. 5 Set the ripple fractions: Choose the inductor ripple fraction (0.2 to 0.4 is common) and the output ripple fraction (0.01 to 0.05 for analog rails).
6. 6 Read the required inductance and capacitance: Use the calculated L and C values as a starting point. Real designs pick the next standard E-series value above the calculated minimum.

For a 3.7 V lithium cell that must power a 5 V, 1 W USB load at 250 kHz with 30 percent inductor ripple and 2 percent output ripple, the calculator returns D = 26 percent, L = 47 uH, and C = 2.08 uF. A real board would pick a 56 uH inductor and a 4.7 uF X7R ceramic for headroom.

Five reasons students, hobbyists, and practising engineers prefer this tool over solving the boost-converter equations by hand each time.

• • Saves design time: Returns duty cycle, inductor value, and output capacitor value in one pass, so you can move from the spec to a bill of materials quickly.
• • Catches invalid designs early: Flags duty cycles at or above 1, switching frequencies at zero, and requested step-downs that a boost converter cannot perform.
• • Connecting theory to numbers: The same transfer function taught in power-electronics classes becomes a concrete L and C value for a specific Vin, Vout, and load.
• • Making ripple budgets visible: Entering an inductor ripple fraction and an output ripple fraction turns an abstract specification into concrete component values.
• • Supports homework and lab prep: Students can confirm their hand calculations before building a circuit, which makes troubleshooting easier when the real measurements disagree.

Treat the output as a starting point rather than a final answer. Real converters also need to budget for switch on-resistance, diode forward drop, inductor DCR, and the controller current limit. Once the basic design is in place, fine-tune the inductor and capacitor on the bench using an oscilloscope to confirm the ripple stays inside the agreed limits.

Five factors drive the design numbers returned by the calculator. The first three are inputs the calculator already accepts; the last two are important real-world caveats.

• • The model assumes continuous conduction mode (CCM). At light load the converter may slip into discontinuous conduction mode (DCM), where the Vout = Vin / (1 - D) relation no longer holds.
• • The model uses an ideal lossless converter. Switch on-resistance, diode forward drop, inductor DCR, and switching losses are not modelled, so reserve headroom on the duty cycle and on the input current limit of the chosen controller.

If the duty cycle returned is above about 80 percent, the converter is close to the limit of CCM operation and you should raise the switching frequency or pick a controller that can handle a higher duty cycle. Below 10 percent the step-up ratio is small and you may want to revisit whether a boost converter is the right topology at all.

According to Texas Instruments (SLVA477), the boost output capacitor required to hold a peak-to-peak ripple voltage dVout is C = (Iout * D) / (f * dVout).

According to MIT OpenCourseWare 6.622 Power Electronics, the boost converter only stays in continuous conduction mode while the average inductor current is above half the peak-to-peak ripple current.

Boost converters are also the standard topology for active power factor correction front ends, so Power Factor Calculator covers the related AC-side problem of PF, kVAR, and capacitor kVAR sizing.