Documentation sections

H-Bridge MOSFET Selection

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

Slug h-bridge-selection. Calculate H-bridge MOSFET requirements including peak current, conduction losses, and minimum current rating for DC motor drivers.

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 h-bridge-selection
Name	Unit	Range	Default	Description
motorCurrentnumber	A	at least 0.1	2	Motor Rated Current
motorVoltagenumber	V	at least 1	12	Motor Supply Voltage
inrushMultipliernumber	×	1 … 15	6	Inrush Multiplier
pwmFrequencynumber	Hz	at least 100	20000	PWM Frequency
mosfetRdsonnumber	Ω	at least 0.001	0.01	MOSFET RDS(on)
dutyCyclenumber	%	0 … 100	75	The share of each period the supply drives the motor: the supply delivers V·I·D (sign-magnitude PWM, TI SLVA504A).
motorCurrentMinnumber	A	at least 0	0	The lowest running current in service; 0 for the nominal.
motorCurrentMaxnumber	A	at least 0	0	The highest running current in service; 0 for the nominal. The peak current and the FETs' rating are taken at it.
motorVoltageMinnumber	V	at least 0	0	The lowest supply in service; 0 for the nominal.
motorVoltageMaxnumber	V	at least 0	0	The highest supply in service; 0 for the nominal. The FETs' drain-source rating is taken at it.
tSwitchnumber	ns	0 … 100000	100	The hard-switched FET's rise and fall times added: ½·V·I·(t_r + t_f)·f (TI SLVA504A eq. 2).
tDeadnumber	ns	0 … 100000	100	The time both FETs of a half bridge are off at each edge, when the recirculating FET's body diode carries the current (TI SLVA504A eq. 3).
vBodynumber	V	0 … 5	1	Body Diode Forward Voltage
qgnumber	nC	0 … 10000	30	Per FET, at the gate drive voltage: the two FETs that switch each period draw 2·Q_g·V_g·f from the driver (IR AN-1070).
vGatenumber	V	1 … 20	10	Gate Drive Voltage

Outputs

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

Outputs of h-bridge-selection
Field	Unit	Description
peakCurrent	A	Peak Current
mosfetCurrentRatingheadline	A	Min. MOSFET Current Rating
conductionLoss	W	Conduction Loss (per FET)
switchingLoss	W	Switching Loss (per FET)
totalLoss	W	Total Bridge Loss (est.)
peakCurrentWorst	A	The inrush multiplier times the highest motor current
peakCurrentWorstCurrent	A	Motor Current at the Worst Peak
mosfetCurrentRatingWorst	A	1.5 times the worst-case peak current
vdsRating	V	The rating sold at or above 1.5 times the highest supply
vdsRatingVoltage	V	Supply at the Drain-Source Rating
totalLossWorst	W	2·I²·R_DS(on) + ½·V·I·(t_r + t_f)·f where it is largest
totalLossWorstVoltage	V	Supply at the Worst Bridge Loss
totalLossWorstCurrent	A	Motor Current at the Worst Bridge Loss
efficiencyWorst	%	Efficiency, Worst Case
efficiencyWorstVoltage	V	Supply at the Worst Efficiency
efficiencyWorstCurrent	A	Motor Current at the Worst Efficiency
lossConduction	W	2·I²·R_DS(on): two FETs carry the motor current at any instant
lossDeadTime	W	V_F·I·2·t_d·f, the body diode through the two dead times (TI SLVA504A eq. 3)
lossGateDrive	W	2·Q_g·V_g·f, dissipated in the gate driver (IR AN-1070)
lossTotal	W	Total Loss with Dead Time and Gate Drive
outputPower	W	V·I·D from the supply, less the losses
efficiencyLossModel	%	1 − (bridge loss + dead time + gate drive)/(V·I·D), at the nominal supply and current

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": "h-bridge-selection",
    "inputs": {
      "motorCurrent": 2,
      "motorVoltage": 12,
      "inrushMultiplier": 6,
      "pwmFrequency": 20000,
      "mosfetRdson": 0.01,
      "dutyCycle": 75,
      "motorCurrentMin": 0,
      "motorCurrentMax": 0,
      "motorVoltageMin": 0,
      "motorVoltageMax": 0,
      "tSwitch": 100,
      "tDead": 100,
      "vBody": 1,
      "qg": 30,
      "vGate": 10
    }
  }'

import rftools

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

result = client.calculate("h-bridge-selection", {
    "motorCurrent": 2,
    "motorVoltage": 12,
    "inrushMultiplier": 6,
    "pwmFrequency": 20000,
    "mosfetRdson": 0.01,
    "dutyCycle": 75,
    "motorCurrentMin": 0,
    "motorCurrentMax": 0,
    "motorVoltageMin": 0,
    "motorVoltageMax": 0,
    "tSwitch": 100,
    "tDead": 100,
    "vBody": 1,
    "qg": 30,
    "vGate": 10,
})
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": "h-bridge-selection",
    "inputs": {
      "motorCurrent": 2,
      "motorVoltage": 12,
      "inrushMultiplier": 6,
      "pwmFrequency": 20000,
      "mosfetRdson": 0.01,
      "dutyCycle": 75,
      "motorCurrentMin": 0,
      "motorCurrentMax": 0,
      "motorVoltageMin": 0,
      "motorVoltageMax": 0,
      "tSwitch": 100,
      "tDead": 100,
      "vBody": 1,
      "qg": 30,
      "vGate": 10
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "h-bridge-selection",
	  "inputs": {
	    "motorCurrent": 2,
	    "motorVoltage": 12,
	    "inrushMultiplier": 6,
	    "pwmFrequency": 20000,
	    "mosfetRdson": 0.01,
	    "dutyCycle": 75,
	    "motorCurrentMin": 0,
	    "motorCurrentMax": 0,
	    "motorVoltageMin": 0,
	    "motorVoltageMax": 0,
	    "tSwitch": 100,
	    "tDead": 100,
	    "vBody": 1,
	    "qg": 30,
	    "vGate": 10
	  }
	}`)
	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": "h-bridge-selection",
  "values": {
    "peakCurrent": 12,
    "conductionLoss": 0.04,
    "switchingLoss": 0.024000000000000004,
    "totalLoss": 0.10400000000000001,
    "mosfetCurrentRating": 18,
    "peakCurrentWorst": 12,
    "peakCurrentWorstCurrent": 2,
    "mosfetCurrentRatingWorst": 18,
    "vdsRating": 20,
    "vdsRatingVoltage": 12,
    "totalLossWorst": 0.10400000000000001,
    "totalLossWorstVoltage": 12,
    "totalLossWorstCurrent": 2,
    "efficiencyWorst": 99.31111111111112,
    "efficiencyWorstVoltage": 12,
    "efficiencyWorstCurrent": 2,
    "lossConduction": 0.08,
    "lossDeadTime": 0.008,
    "lossGateDrive": 0.012000000000000002,
    "lossTotal": 0.12400000000000001,
    "outputPower": 17.876,
    "efficiencyLossModel": 99.31111111111112
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:h-bridge-selection",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "TI SLVA504A, Calculating Power Dissipation for a H-Bridge or Half Bridge Driver (A. Gopalan, 2012, revised 2021), Table 1-1 and eqs. 2-3; International Rectifier AN-1070 (J. Cerezo), P_GATE",
    "assumptions": [
      {
        "code": "sign-magnitude-synchronous",
        "text": "An H-bridge driven by sign-magnitude PWM with synchronous recirculation: two FETs carry the motor current at any instant, one is hard-switched each period, and the supply delivers V·I·D (TI SLVA504A, Table 1-1)."
      },
      {
        "code": "dead-time-body-diode",
        "text": "Through each dead time the recirculating FET's body diode carries the current, V_F·I·2·t_d·f (TI SLVA504A eq. 3); its edges across V_F (eq. 4) and reverse recovery are left out, as SLVA504A leaves them."
      },
      {
        "code": "gate-drive-in-the-driver",
        "text": "The two FETs that switch each period draw 2·Q_g·V_g·f from the gate driver (IR AN-1070), counted in the loss model's total and efficiency, not in the bridge loss."
      },
      {
        "code": "average-motor-current",
        "text": "No motor inductance is modelled, so the motor current is drawn at its average, without ripple."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "motorCurrent": {
          "min": 0.1,
          "max": null,
          "unit": "A"
        },
        "motorVoltage": {
          "min": 1,
          "max": null,
          "unit": "V"
        },
        "inrushMultiplier": {
          "min": 1,
          "max": 15,
          "unit": "×"
        },
        "pwmFrequency": {
          "min": 100,
          "max": null,
          "unit": "Hz"
        },
        "mosfetRdson": {
          "min": 0.001,
          "max": null,
          "unit": "Ω"
        },
        "dutyCycle": {
          "min": 0,
          "max": 100,
          "unit": "%"
        },
        "motorCurrentMin": {
          "min": 0,
          "max": null,
          "unit": "A"
        },
        "motorCurrentMax": {
          "min": 0,
          "max": null,
          "unit": "A"
        },
        "motorVoltageMin": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "motorVoltageMax": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "tSwitch": {
          "min": 0,
          "max": 100000,
          "unit": "ns"
        },
        "tDead": {
          "min": 0,
          "max": 100000,
          "unit": "ns"
        },
        "vBody": {
          "min": 0,
          "max": 5,
          "unit": "V"
        },
        "qg": {
          "min": 0,
          "max": 10000,
          "unit": "nC"
        },
        "vGate": {
          "min": 1,
          "max": 20,
          "unit": "V"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "motorCurrent": 2,
      "motorVoltage": 12,
      "inrushMultiplier": 6,
      "pwmFrequency": 20000,
      "mosfetRdson": 0.01,
      "dutyCycle": 75,
      "motorCurrentMin": 0,
      "motorCurrentMax": 0,
      "motorVoltageMin": 0,
      "motorVoltageMax": 0,
      "tSwitch": 100,
      "tDead": 100,
      "vBody": 1,
      "qg": 30,
      "vGate": 10
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "motorCurrent"
      ],
      "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 pageH-Bridge MOSFET Selection, at these values