Calculators / Boost converter

Boost converter power stage

Duty cycle, inductor and ripple current, peak switch current against the IC's current limit, the output current that limit allows, output capacitance and ripple, and what the input rail has to supply. The equations are from TI's SLVA372D and are shown under the results.

Related: buck converter power stage · power budget for a multi-rail circuit

Operating point

Use the minimum switching frequency. For efficiency, read the curve in the converter's datasheet, or estimate — SLVA372D suggests 80 % as a not-unrealistic worst case for a boost converter.
Inductor
Ripple as a share of the inductor (input) current, IOUT × VOUT / VIN; SLVA372D suggests 20–40 %. It sets the recommended inductance. Leave Your inductor blank to use that value, or enter the part you have.
Switch and rectifier
The minimum value of the integrated switch's current limit, from the datasheet.
Output capacitor
Use the effective capacitance at the output voltage: ceramic capacitors lose much of their value under DC bias.
Duty cycle (VIN min)
—%
Recommended inductance
—µH
Inductor ripple (VIN min)
—A p-p
Peak switch current
—A
Max output current
—A
Input current (VIN min)
—A
Min output capacitance
—µF
Output ripple
—mV p-p
Diode loss
—W
Conduction mode
—
Total loss (from efficiency)
—W

At both input voltages

The same inductor at the minimum and the typical input.

InputVIN (V)DutyΔIL (A p-p)Input current (A)Peak switch (A)Max load (A)Mode

Switch current and available load across the input range

Peak switch current against the current limit, and the load that limit allows against your load, from VIN min towards VOUT.

Equations (TI SLVA372D)

D= 1 − VIN(min) × η / VOUT ΔIL= VIN(min) × D / (fS × L) IMAXOUT= (ILIM(min) − ΔIL/2) × (1 − D) ISW(max)= ΔIL/2 + IOUT(max) / (1 − D) L= VIN × (VOUT − VIN) / (ΔIL × fS × VOUT) [VIN typical] ΔIL(est)= (0.2 to 0.4) × IOUT(max) × VOUT / VIN PD(diode)= IOUT(max) × VF COUT(min)= IOUT(max) × D / (fS × ΔVOUT) ΔVOUT(ESR)= ESR × (IOUT(max) / (1 − D) + ΔIL/2)

How the calculation works

The method follows TI's application note Basic Calculation of a Boost Converter's Power Stage (SLVA372D), written for a boost IC with an integrated switch running in continuous conduction. It starts from the duty cycle at the minimum input voltage, because that is where the switch current is highest:

  • D = 1 − VIN(min) × η / VOUT
  • ΔIL = VIN(min) × D / (fS × L)
  • IMAXOUT = (ILIM(min) − ΔIL/2) × (1 − D)
  • ISW(max) = ΔIL/2 + IOUT(max) / (1 − D)

The efficiency is in the duty cycle because the converter also has to deliver the energy it dissipates, which gives a more realistic duty cycle than VIN/VOUT alone. If IMAXOUT is below your load, the IC's current limit trips before full load: choose an IC with a higher limit, or, if the shortfall is small, a larger inductor, which lowers the ripple. ISW(max) is the peak current the switch, the inductor and the diode must all withstand, so the inductor's saturation current has to exceed it.

Inductor

If the datasheet recommends an inductor or a range, use it and enter it under Your inductor. Otherwise the note estimates the ripple as 20–40 % of the inductor current and sizes the inductor at the typical input voltage:

  • ΔIL(est) = (0.2 to 0.4) × IOUT(max) × VOUT / VIN
  • L = VIN × (VOUT − VIN) / (ΔIL × fS × VOUT)

Rectifier and output capacitor

The diode carries the output current on average, so IF = IOUT(max) and its conduction loss is IF × VF. With a synchronous rectifier every equation here still applies except that one. The output capacitance for a ripple target, and the extra ripple from the capacitor's ESR, are

  • COUT(min) = IOUT(max) × D / (fS × ΔVOUT)
  • ΔVOUT(ESR) = ESR × (IOUT(max) / (1 − D) + ΔIL/2)

This page adds the two ripple terms for the reported ripple, which is the conservative reading. SLVA372D gives these capacitor equations for converters with external compensation; with an internally compensated IC, stay within the datasheet's recommended L and C.

Continuous or discontinuous conduction

Every equation above assumes the inductor current never reaches zero. That holds while the average inductor current, IOUT / (1 − D), is larger than half the ripple, ΔIL / 2. Erickson and Maksimović state the same boundary for the boost as K = 2L / (R × TS) against Kcrit(D) = D × (1 − D)². At light load the converter enters discontinuous conduction, where the duty cycle and ripple follow different relations and these numbers no longer apply. The results show the load below which that happens for the inductor in use.

What this page does not model

Losses other than the diode's are taken as one number through the efficiency you enter; this page does not build a loss model for the switch or the inductor. It does not size the input capacitor (the datasheet's minimum is the starting point) or the control loop. For a step-down stage with every loss mechanism modelled, see the buck converter calculator.

References

  1. Texas Instruments, Basic Calculation of a Boost Converter's Power Stage, SLVA372D.
  2. R. W. Erickson and D. Maksimović, Fundamentals of Power Electronics, Springer, chapter 5, “The Discontinuous Conduction Mode”.

This boost feeds other rails?

The editor solves the whole power tree at once: every stage's input current, efficiency and dissipation, with load-dependent efficiency, tolerance corners and thermal estimates.

Open the editor

These tools and articles give theoretical estimates for educational purposes. Real results depend on component tolerances, parasitics and thermal conditions.