Documentation sections

Buck Converter Design Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug buck-converter. Design a synchronous buck converter: calculate duty cycle, inductor, output and input capacitors, peak current, and ripple. Free, instant results.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of buck-converter
Name	Unit	Range	Default	Description
vinnumber	V	1 … 100	12	Input Voltage
voutnumber	V	0.5 … 99	5	Output Voltage
ioutnumber	A	0.01 … 50	1	Output Current
fswnumber	kHz	10 … 5000	500	Switching Frequency
deltaILnumber	%	5 … 80	30	Inductor current ripple as % of output current. Typical: 20-40%.
deltaVoutnumber	%	0.1 … 10	1	Output voltage ripple as % of Vout. Typical: 0.5-2%.
deltaVinnumber	%	0.1 … 10	1	Input voltage ripple as % of Vin, which the input capacitor holds. Typical: 0.5-2%.
vinMinnumber	V	0 … 100	0	The lowest input the converter must work from; 0 for the nominal. The parts are sized and the stresses taken over the range.
vinMaxnumber	V	0 … 100	0	The highest input the converter must work from; 0 for the nominal.
coutEachnumber	μF	0.1 … 10000	22	One output capacitor; the bank is as many in parallel as the ripple target needs at the worst ripple current.
rdsOnHighnumber	mΩ	0.1 … 10000	12	The high-side FET's on-resistance at its operating temperature.
rdsOnLownumber	mΩ	0.1 … 10000	6	The low-side (synchronous) FET's on-resistance at its operating temperature.
tSwitchnumber	ns	0 … 10000	20	The high side's rise and fall times added: the switching loss is ½·V_in·I_out·(t_r + t_f)·f_sw.
qgnumber	nC	0 … 10000	23	Both FETs' gate charge at the drive voltage: the gate drive takes Q_g·V_g·f_sw.
vGatenumber	V	1 … 20	5	Gate Drive Voltage
tDeadnumber	ns	0 … 1000	20	The time both FETs are off at each edge, when the low side's body diode carries the current.
vBodynumber	V	0 … 2	0.7	Body Diode Forward Voltage
dcrnumber	mΩ	0 … 100000	20	The inductor's winding resistance.
coreLossnumber	mW	0 … 100000	50	From the inductor's datasheet at this ripple and frequency.
esrOutnumber	mΩ	0 … 100000	3	Output Capacitor ESR (each)
esrInnumber	mΩ	0 … 100000	3	Input Capacitor ESR

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of buck-converter
Field	Unit	Description
dutyCycle	%	Duty Cycle
inductorUHheadline	μH	Minimum Inductance
coutUF	μF	Output Capacitance
cinUF	μF	Input Capacitance
ilPeak	A	Peak Inductor Current
dutyMin	%	At the maximum input voltage
dutyMax	%	At the minimum input voltage
inductorMinWorstUH	μH	For the ripple target where the ripple is largest, at the maximum input voltage
inductorChosenUH	μH	The E12 value at or above the minimum inductance; every current below is the chosen part's
rippleNomA	A	Peak to peak, at the nominal input, with the chosen inductor
rippleWorstA	A	Inductor Ripple, Worst Case
rippleWorstVin	V	Input at the Worst Ripple
ilPeakWorst	A	The inductor's saturation current must be at least this
ilPeakWorstVin	V	Input at the Worst Peak
coutCount	none	How many of the chosen output capacitor, in parallel
coutTotalUF	μF	Output Capacitance (bank)
outputRippleMV	mV	Peak to peak: the bank's charge ripple plus its ESR ripple, added (an upper bound)
outputRippleVin	V	Input at the Worst Output Ripple
outputRipplePct	%	Output Ripple, % of Vout
cinMinWorstUF	μF	Minimum Input Capacitance over the Input Range
cinMinWorstVin	V	Input at the Largest Input Capacitance
cinRmsA	A	Input Capacitor RMS Current, Worst Case
cinRmsVin	V	Input at the Worst Input RMS Current
q1RmsA	A	High-Side FET RMS Current, Worst Case
q1RmsVin	V	Input at the Worst High-Side Current
q2RmsA	A	Low-Side FET RMS Current, Worst Case
q2RmsVin	V	Input at the Worst Low-Side Current
vdsRating	V	The rating sold at or above 1.5 times the maximum input
cinVoltageRating	V	The rating sold at or above 1.5 times the maximum input
coutVoltageRating	V	The rating sold at or above 1.5 times the output
lossDcr	W	Inductor DCR Loss
lossCore	W	Inductor Core Loss
lossHsConduction	W	High-Side Conduction Loss
lossLsConduction	W	Low-Side Conduction Loss
lossHsSwitching	W	High-Side Switching Loss
lossGateDrive	W	Gate Drive Loss
lossDeadTime	W	Dead-Time Loss
lossCapEsr	W	Capacitor ESR Loss
lossTotal	W	Total Loss
outputPower	W	Output Power
efficiency	%	At the nominal input and load, from the loss model

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "buck-converter",
    "inputs": {
      "vin": 12,
      "vout": 5,
      "iout": 1,
      "fsw": 500,
      "deltaIL": 30,
      "deltaVout": 1,
      "deltaVin": 1,
      "vinMin": 0,
      "vinMax": 0,
      "coutEach": 22,
      "rdsOnHigh": 12,
      "rdsOnLow": 6,
      "tSwitch": 20,
      "qg": 23,
      "vGate": 5,
      "tDead": 20,
      "vBody": 0.7,
      "dcr": 20,
      "coreLoss": 50,
      "esrOut": 3,
      "esrIn": 3
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("buck-converter", {
    "vin": 12,
    "vout": 5,
    "iout": 1,
    "fsw": 500,
    "deltaIL": 30,
    "deltaVout": 1,
    "deltaVin": 1,
    "vinMin": 0,
    "vinMax": 0,
    "coutEach": 22,
    "rdsOnHigh": 12,
    "rdsOnLow": 6,
    "tSwitch": 20,
    "qg": 23,
    "vGate": 5,
    "tDead": 20,
    "vBody": 0.7,
    "dcr": 20,
    "coreLoss": 50,
    "esrOut": 3,
    "esrIn": 3,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "buck-converter",
    "inputs": {
      "vin": 12,
      "vout": 5,
      "iout": 1,
      "fsw": 500,
      "deltaIL": 30,
      "deltaVout": 1,
      "deltaVin": 1,
      "vinMin": 0,
      "vinMax": 0,
      "coutEach": 22,
      "rdsOnHigh": 12,
      "rdsOnLow": 6,
      "tSwitch": 20,
      "qg": 23,
      "vGate": 5,
      "tDead": 20,
      "vBody": 0.7,
      "dcr": 20,
      "coreLoss": 50,
      "esrOut": 3,
      "esrIn": 3
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "buck-converter",
	  "inputs": {
	    "vin": 12,
	    "vout": 5,
	    "iout": 1,
	    "fsw": 500,
	    "deltaIL": 30,
	    "deltaVout": 1,
	    "deltaVin": 1,
	    "vinMin": 0,
	    "vinMax": 0,
	    "coutEach": 22,
	    "rdsOnHigh": 12,
	    "rdsOnLow": 6,
	    "tSwitch": 20,
	    "qg": 23,
	    "vGate": 5,
	    "tDead": 20,
	    "vBody": 0.7,
	    "dcr": 20,
	    "coreLoss": 50,
	    "esrOut": 3,
	    "esrIn": 3
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "buck-converter",
  "values": {
    "dutyCycle": 41.66666666666667,
    "inductorUH": 19.444444444444446,
    "coutUF": 1.5,
    "cinUF": 4.050925925925925,
    "ilPeak": 1.15,
    "dutyMin": 41.66666666666667,
    "dutyMax": 41.66666666666667,
    "inductorMinWorstUH": 19.444444444444446,
    "inductorChosenUH": 22,
    "rippleNomA": 0.2651515151515152,
    "rippleWorstA": 0.2651515151515152,
    "rippleWorstVin": 12,
    "ilPeakWorst": 1.1325757575757576,
    "ilPeakWorstVin": 12,
    "coutCount": 1,
    "coutTotalUF": 22,
    "outputRippleMV": 3.808539944903582,
    "outputRippleVin": 12,
    "outputRipplePct": 0.07617079889807164,
    "cinMinWorstUF": 4.050925925925925,
    "cinMinWorstVin": 12,
    "cinRmsA": 0.4930066485916346,
    "cinRmsVin": 12,
    "q1RmsA": 0.6473853750425616,
    "q1RmsVin": 12,
    "q2RmsA": 0.7659967058327322,
    "q2RmsVin": 12,
    "vdsRating": 20,
    "cinVoltageRating": 25,
    "coutVoltageRating": 10,
    "lossDcr": 0.02011717554331191,
    "lossCore": 0.05,
    "lossHsConduction": 0.005029293885827978,
    "lossLsConduction": 0.0035205057200795834,
    "lossHsSwitching": 0.060000000000000005,
    "lossGateDrive": 0.0575,
    "lossDeadTime": 0.014000000000000002,
    "lossCapEsr": 0.0007467429981634526,
    "lossTotal": 0.21091371814738294,
    "outputPower": 5,
    "efficiency": 95.95246189909342
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:buck-converter",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Erickson & Maksimovic, \"Fundamentals of Power Electronics\" 3rd ed.",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vin": {
          "min": 1,
          "max": 100,
          "unit": "V"
        },
        "vout": {
          "min": 0.5,
          "max": 99,
          "unit": "V"
        },
        "iout": {
          "min": 0.01,
          "max": 50,
          "unit": "A"
        },
        "fsw": {
          "min": 10,
          "max": 5000,
          "unit": "kHz"
        },
        "deltaIL": {
          "min": 5,
          "max": 80,
          "unit": "%"
        },
        "deltaVout": {
          "min": 0.1,
          "max": 10,
          "unit": "%"
        },
        "deltaVin": {
          "min": 0.1,
          "max": 10,
          "unit": "%"
        },
        "vinMin": {
          "min": 0,
          "max": 100,
          "unit": "V"
        },
        "vinMax": {
          "min": 0,
          "max": 100,
          "unit": "V"
        },
        "coutEach": {
          "min": 0.1,
          "max": 10000,
          "unit": "μF"
        },
        "rdsOnHigh": {
          "min": 0.1,
          "max": 10000,
          "unit": "mΩ"
        },
        "rdsOnLow": {
          "min": 0.1,
          "max": 10000,
          "unit": "mΩ"
        },
        "tSwitch": {
          "min": 0,
          "max": 10000,
          "unit": "ns"
        },
        "qg": {
          "min": 0,
          "max": 10000,
          "unit": "nC"
        },
        "vGate": {
          "min": 1,
          "max": 20,
          "unit": "V"
        },
        "tDead": {
          "min": 0,
          "max": 1000,
          "unit": "ns"
        },
        "vBody": {
          "min": 0,
          "max": 2,
          "unit": "V"
        },
        "dcr": {
          "min": 0,
          "max": 100000,
          "unit": "mΩ"
        },
        "coreLoss": {
          "min": 0,
          "max": 100000,
          "unit": "mW"
        },
        "esrOut": {
          "min": 0,
          "max": 100000,
          "unit": "mΩ"
        },
        "esrIn": {
          "min": 0,
          "max": 100000,
          "unit": "mΩ"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vin": 12,
      "vout": 5,
      "iout": 1,
      "fsw": 500,
      "deltaIL": 30,
      "deltaVout": 1,
      "deltaVin": 1,
      "vinMin": 0,
      "vinMax": 0,
      "coutEach": 22,
      "rdsOnHigh": 12,
      "rdsOnLow": 6,
      "tSwitch": 20,
      "qg": 23,
      "vGate": 5,
      "tDead": 20,
      "vBody": 0.7,
      "dcr": 20,
      "coreLoss": 50,
      "esrOut": 3,
      "esrIn": 3
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vin"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageBuck Converter Design Calculator, at these values