Documentation sections

Motor Driver Power Dissipation

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

Slug motor-driver-power. Calculate motor driver IC or discrete MOSFET power dissipation including conduction loss and switching loss at a given PWM frequency.

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 motor-driver-power
Name	Unit	Range	Default	Description
motorCurrentnumber	A	at least 0	2	Motor Current (RMS)
supplyVoltagenumber	V	at least 1	12	Supply Voltage
rdsonnumber	Ω	at least 0.001	0.015	RDS(on)
dutyCyclenumber	%	0 … 100	75	PWM Duty Cycle
switchingFreqnumber	Hz	at least 100	20000	Switching Frequency
gateChargenumber	nC	at least 0.1	30	From MOSFET datasheet
transitionTimenumber	ns	at least 0	50	Rise or fall time of the hard-switched FET, taken equal: TI SLVA504A's 0.5·V·I·(t_r + t_f)·f is then V·I·t_sw·f
motorCurrentMinnumber	A	at least 0	0	The lowest RMS motor current in service; 0 for the nominal.
motorCurrentMaxnumber	A	at least 0	0	The highest RMS motor current in service; 0 for the nominal. The FETs' current and loss are taken at it.
supplyVoltageMinnumber	V	at least 0	0	The lowest supply in service; 0 for the nominal.
supplyVoltageMaxnumber	V	at least 0	0	The highest supply in service; 0 for the nominal. The FETs' drain-source rating is taken at it.
tDeadnumber	ns	0 … 100000	100	The time both FETs of the switching 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

Outputs

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

Outputs of motor-driver-power
Field	Unit	Description
conductionLoss	W	I²·R_DS(on); two FETs conduct at any instant
switchingLoss	W	V·I·t_sw·f, in the one FET hard-switched each period
totalLossPerFet	W	The larger of I²·R (the high side that conducts all period) and I²·R·D + P_sw (the low side, which also switches)
totalLossBridgeheadline	W	2·I²·R + P_sw (TI SLVA504A); the gate drive is reported apart
efficiency	%	1 − (bridge loss + gate drive)/(V·I·D), against the power the supply delivers
gateDriveLoss	W	2·Q_g·V·f for the two FETs that switch each period, dissipated in the gate driver (IR AN-1070)
totalLossBridgeWorst	W	2·I²·R + V·I·t_sw·f where it is largest
totalLossBridgeWorstVoltage	V	Supply at the Worst Bridge Loss
totalLossBridgeWorstCurrent	A	Motor Current at the Worst Bridge Loss
totalLossPerFetWorst	W	Hottest FET, Worst Case
totalLossPerFetWorstCurrent	A	Motor Current at the Hottest FET
fetRmsCurrentWorst	A	The high side that conducts all period carries the motor current
vdsRating	V	The rating sold at or above 1.5 times the highest supply
vdsRatingVoltage	V	Supply at the Drain-Source Rating
efficiencyWorst	%	Efficiency with Dead Time, 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)
lossTotal	W	The bridge loss, the dead time and the gate drive
outputPower	W	V·I·D from the supply, less the losses
efficiencyLossModel	%	1 − (bridge loss + dead time + gate drive)/(V·I·D): the driver efficiency with the dead time counted

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": "motor-driver-power",
    "inputs": {
      "motorCurrent": 2,
      "supplyVoltage": 12,
      "rdson": 0.015,
      "dutyCycle": 75,
      "switchingFreq": 20000,
      "gateCharge": 30,
      "transitionTime": 50,
      "motorCurrentMin": 0,
      "motorCurrentMax": 0,
      "supplyVoltageMin": 0,
      "supplyVoltageMax": 0,
      "tDead": 100,
      "vBody": 1
    }
  }'

import rftools

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

result = client.calculate("motor-driver-power", {
    "motorCurrent": 2,
    "supplyVoltage": 12,
    "rdson": 0.015,
    "dutyCycle": 75,
    "switchingFreq": 20000,
    "gateCharge": 30,
    "transitionTime": 50,
    "motorCurrentMin": 0,
    "motorCurrentMax": 0,
    "supplyVoltageMin": 0,
    "supplyVoltageMax": 0,
    "tDead": 100,
    "vBody": 1,
})
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": "motor-driver-power",
    "inputs": {
      "motorCurrent": 2,
      "supplyVoltage": 12,
      "rdson": 0.015,
      "dutyCycle": 75,
      "switchingFreq": 20000,
      "gateCharge": 30,
      "transitionTime": 50,
      "motorCurrentMin": 0,
      "motorCurrentMax": 0,
      "supplyVoltageMin": 0,
      "supplyVoltageMax": 0,
      "tDead": 100,
      "vBody": 1
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "motor-driver-power",
	  "inputs": {
	    "motorCurrent": 2,
	    "supplyVoltage": 12,
	    "rdson": 0.015,
	    "dutyCycle": 75,
	    "switchingFreq": 20000,
	    "gateCharge": 30,
	    "transitionTime": 50,
	    "motorCurrentMin": 0,
	    "motorCurrentMax": 0,
	    "supplyVoltageMin": 0,
	    "supplyVoltageMax": 0,
	    "tDead": 100,
	    "vBody": 1
	  }
	}`)
	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": "motor-driver-power",
  "values": {
    "conductionLoss": 0.06,
    "switchingLoss": 0.024000000000000004,
    "totalLossPerFet": 0.069,
    "totalLossBridge": 0.144,
    "efficiency": 99.11999999999999,
    "gateDriveLoss": 0.014400000000000001,
    "totalLossBridgeWorst": 0.144,
    "totalLossBridgeWorstVoltage": 12,
    "totalLossBridgeWorstCurrent": 2,
    "totalLossPerFetWorst": 0.069,
    "totalLossPerFetWorstCurrent": 2,
    "fetRmsCurrentWorst": 2,
    "vdsRating": 20,
    "vdsRatingVoltage": 12,
    "efficiencyWorst": 99.07555555555557,
    "efficiencyWorstVoltage": 12,
    "efficiencyWorstCurrent": 2,
    "lossConduction": 0.12,
    "lossDeadTime": 0.008,
    "lossTotal": 0.1664,
    "outputPower": 17.8336,
    "efficiencyLossModel": 99.07555555555557
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:motor-driver-power",
    "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. 1-2; International Rectifier AN-1070 (J. Cerezo), P_GATE",
    "assumptions": [
      {
        "code": "sign-magnitude-synchronous",
        "text": "An H-bridge driven by sign-magnitude PWM with synchronous (high-side) recirculation: two FETs carry the motor current at any instant and one is hard-switched each period (TI SLVA504A, Table 1-1)."
      },
      {
        "code": "equal-rise-and-fall",
        "text": "The rise and fall times are equal, so SLVA504A's 0.5·V·I·(t_r + t_f)·f is V·I·t_sw·f. The dead-time diode conduction (SLVA504A eq. 3) is not included."
      },
      {
        "code": "gate-drive-at-supply-voltage",
        "text": "The gate charge is delivered at the supply voltage, 2·Qg·V·f, and is counted in the driver efficiency, as an integrated driver dissipates it; it is not a FET loss."
      },
      {
        "code": "loss-model-dead-time",
        "text": "The loss model adds the dead time, V_F·I·2·t_d·f (SLVA504A eq. 3), to the bridge loss and the gate drive; its efficiency is the driver efficiency with the dead time counted, and the two agree where t_d or V_F is 0."
      },
      {
        "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,
          "max": null,
          "unit": "A"
        },
        "supplyVoltage": {
          "min": 1,
          "max": null,
          "unit": "V"
        },
        "rdson": {
          "min": 0.001,
          "max": null,
          "unit": "Ω"
        },
        "dutyCycle": {
          "min": 0,
          "max": 100,
          "unit": "%"
        },
        "switchingFreq": {
          "min": 100,
          "max": null,
          "unit": "Hz"
        },
        "gateCharge": {
          "min": 0.1,
          "max": null,
          "unit": "nC"
        },
        "transitionTime": {
          "min": 0,
          "max": null,
          "unit": "ns"
        },
        "motorCurrentMin": {
          "min": 0,
          "max": null,
          "unit": "A"
        },
        "motorCurrentMax": {
          "min": 0,
          "max": null,
          "unit": "A"
        },
        "supplyVoltageMin": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "supplyVoltageMax": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "tDead": {
          "min": 0,
          "max": 100000,
          "unit": "ns"
        },
        "vBody": {
          "min": 0,
          "max": 5,
          "unit": "V"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "motorCurrent": 2,
      "supplyVoltage": 12,
      "rdson": 0.015,
      "dutyCycle": 75,
      "switchingFreq": 20000,
      "gateCharge": 30,
      "transitionTime": 50,
      "motorCurrentMin": 0,
      "motorCurrentMax": 0,
      "supplyVoltageMin": 0,
      "supplyVoltageMax": 0,
      "tDead": 100,
      "vBody": 1
    },
    "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 pageMotor Driver Power Dissipation, at these values