Documentation sections

Class D Amplifier Efficiency

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

Slug class-d-efficiency. Estimate Class D amplifier efficiency from MOSFET conduction losses and quiescent current at a given output power.

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 class-d-efficiency
Name	Unit	Range	Default	Description
outputPowernumber	W	at least 0.1	50	Output Power
supplyVoltagenumber	V	at least 1	36	Supply Voltage
loadImpedancenumber	Ω	at least 1	8	Load Impedance
rdsonnumber	mΩ	at least 1	50	Per MOSFET in milliohms
mosfetCountnumber	none	at least 2	4	Half bridge 2, full bridge (BTL) 4. One FET of each half bridge carries the load at a time, so N/2 conduct
quiescentCurrentnumber	mA	at least 0	30	Quiescent Current
switchingFreqnumber	kHz	50 … 5000	400	The PWM carrier frequency.
filterCutoffnumber	kHz	20 … 500	40	The LC filter's Butterworth cutoff, which sizes L and C from the load (TI SLAA701B).
outputPowerMinnumber	W	at least 0	0	The lowest power in service, where the fixed losses weigh most; 0 for the nominal.
outputPowerMaxnumber	W	at least 0	0	The highest continuous sine power in service; 0 for the nominal.
supplyVoltageMinnumber	V	at least 0	0	The lowest supply in service, which must still swing the maximum power; 0 for the nominal.
supplyVoltageMaxnumber	V	at least 0	0	The highest supply in service; 0 for the nominal. The parts' voltage ratings are taken at it.
tSwitchnumber	ns	0 … 10000	10	The FETs' rise and fall times added: ½·I·V_s·(t_r + t_f)·f_sw per half bridge (IR AN-1070).
cossnumber	pF	0 … 100000	200	½·C_oss·V_s²·f_sw per half bridge (IR AN-1070).
qgnumber	nC	0 … 10000	10	Per FET: 2·Q_g·V_driver·f_sw per half bridge (IR AN-1070).
vGatenumber	V	1 … 20	12	Gate Drive Voltage
dcrnumber	mΩ	0 … 100000	25	Each output inductor's winding resistance.
coreLossnumber	mW	0 … 100000	100	Each output inductor's, from its datasheet at this ripple and frequency.

Outputs

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

Outputs of class-d-efficiency
Field	Unit	Description
efficiencyheadline	%	Efficiency
conductionLoss	W	I²·R_DS(on)·N/2: the FETs that carry the load current at once
quiescentLoss	W	Quiescent Loss
totalLoss	W	Total heat dissipated — use for heatsink sizing
totalInput	W	Total Input Power
lossSwitching	W	½·I·V_s·(t_r + t_f)·f_sw per half bridge, with I the load current averaged over the audio cycle
lossOutputCap	W	½·C_oss·V_s²·f_sw per half bridge
lossGateDrive	W	2·Q_g·V_driver·f_sw per half bridge
lossDcr	W	(I² + ΔI²/12)·DCR per output inductor, with the idle ripple
lossCore	W	Filter Inductor Core Loss
lossTotal	W	Conduction, quiescent, switching, output capacitance, gate drive and the filter inductors
loadPower	W	Power into the Load
efficiencyLossModel	%	P_out/(P_out + every loss in the loss model), at the nominal supply and power
filterInductorUH	μH	Butterworth, L = R_L·√2/ω₀, per output
filterInductorChosenUH	μH	The E12 value at or above the filter inductance
filterCapUF	μF	Butterworth, C = 1/(ω₀·R_L·√2), per output to ground
filterCapChosenUF	μF	The E12 value at or above the filter capacitance
filterCutoffBuilt	kHz	Filter Cutoff, Chosen Parts
filterCount	none	One LC filter per half-bridge output
fetCount	none	MOSFETs
efficiencyWorst	%	Efficiency with Switching and Filter Losses, Worst Case
efficiencyWorstVoltage	V	Supply at the Worst Efficiency
efficiencyWorstPower	W	Output Power at the Worst Efficiency
lossTotalWorst	W	For the heatsink
lossTotalWorstVoltage	V	Supply at the Worst Loss
lossTotalWorstPower	W	Output Power at the Worst Loss
headroomWorst	V	The swing the supply gives (V_s bridged, V_s/2 for a half bridge) less the peak load voltage √(2·P·Z_L); below 0 the output clips
headroomWorstVoltage	V	Supply at the Least Headroom
headroomWorstPower	W	Output Power at the Least Headroom
rippleA	A	Peak to peak at idle (50 % duty), where it is largest: V_s/(4·L·f_sw)
rippleWorstA	A	Filter Inductor Ripple, Worst Case
rippleWorstVoltage	V	Supply at the Worst Ripple
peakCurrentWorst	A	The load's peak current √(2·P/Z_L) plus half the ripple, through the FETs and the inductors
peakCurrentWorstVoltage	V	Supply at the Worst Peak Current
peakCurrentWorstPower	W	Output Power at the Worst Peak Current
inductorRmsWorst	A	Filter Inductor RMS Current, Worst Case
inductorRmsWorstPower	W	Output Power at the Worst Inductor Current
vdsRating	V	The rating sold at or above 1.5 times the highest supply
capVoltageRating	V	The rating sold at or above 1.5 times the highest supply
vdsRatingVoltage	V	Supply at the Voltage Ratings

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": "class-d-efficiency",
    "inputs": {
      "outputPower": 50,
      "supplyVoltage": 36,
      "loadImpedance": 8,
      "rdson": 50,
      "mosfetCount": 4,
      "quiescentCurrent": 30,
      "switchingFreq": 400,
      "filterCutoff": 40,
      "outputPowerMin": 0,
      "outputPowerMax": 0,
      "supplyVoltageMin": 0,
      "supplyVoltageMax": 0,
      "tSwitch": 10,
      "coss": 200,
      "qg": 10,
      "vGate": 12,
      "dcr": 25,
      "coreLoss": 100
    }
  }'

import rftools

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

result = client.calculate("class-d-efficiency", {
    "outputPower": 50,
    "supplyVoltage": 36,
    "loadImpedance": 8,
    "rdson": 50,
    "mosfetCount": 4,
    "quiescentCurrent": 30,
    "switchingFreq": 400,
    "filterCutoff": 40,
    "outputPowerMin": 0,
    "outputPowerMax": 0,
    "supplyVoltageMin": 0,
    "supplyVoltageMax": 0,
    "tSwitch": 10,
    "coss": 200,
    "qg": 10,
    "vGate": 12,
    "dcr": 25,
    "coreLoss": 100,
})
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": "class-d-efficiency",
    "inputs": {
      "outputPower": 50,
      "supplyVoltage": 36,
      "loadImpedance": 8,
      "rdson": 50,
      "mosfetCount": 4,
      "quiescentCurrent": 30,
      "switchingFreq": 400,
      "filterCutoff": 40,
      "outputPowerMin": 0,
      "outputPowerMax": 0,
      "supplyVoltageMin": 0,
      "supplyVoltageMax": 0,
      "tSwitch": 10,
      "coss": 200,
      "qg": 10,
      "vGate": 12,
      "dcr": 25,
      "coreLoss": 100
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "class-d-efficiency",
	  "inputs": {
	    "outputPower": 50,
	    "supplyVoltage": 36,
	    "loadImpedance": 8,
	    "rdson": 50,
	    "mosfetCount": 4,
	    "quiescentCurrent": 30,
	    "switchingFreq": 400,
	    "filterCutoff": 40,
	    "outputPowerMin": 0,
	    "outputPowerMax": 0,
	    "supplyVoltageMin": 0,
	    "supplyVoltageMax": 0,
	    "tSwitch": 10,
	    "coss": 200,
	    "qg": 10,
	    "vGate": 12,
	    "dcr": 25,
	    "coreLoss": 100
	  }
	}`)
	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": "class-d-efficiency",
  "values": {
    "conductionLoss": 0.625,
    "quiescentLoss": 1.08,
    "totalLoss": 1.705,
    "totalInput": 51.705,
    "efficiency": 96.70244657189826,
    "lossSwitching": 0.3241138738165582,
    "lossOutputCap": 0.10368,
    "lossGateDrive": 0.19200000000000003,
    "lossDcr": 0.31539351851851855,
    "lossCore": 0.2,
    "lossTotal": 2.8401873923350767,
    "loadPower": 50,
    "efficiencyLossModel": 94.62494829693381,
    "filterInductorUH": 22.507907903927652,
    "filterInductorChosenUH": 27,
    "filterCapUF": 0.703372121997739,
    "filterCapChosenUF": 0.82,
    "filterCutoffBuilt": 33.82449516143777,
    "filterCount": 2,
    "fetCount": 4,
    "efficiencyWorst": 94.62494829693381,
    "efficiencyWorstVoltage": 36,
    "efficiencyWorstPower": 50,
    "lossTotalWorst": 2.8401873923350767,
    "lossTotalWorstVoltage": 36,
    "lossTotalWorstPower": 50,
    "headroomWorst": 7.715728752538098,
    "headroomWorstVoltage": 36,
    "headroomWorstPower": 50,
    "rippleA": 0.8333333333333334,
    "rippleWorstA": 0.8333333333333334,
    "rippleWorstVoltage": 36,
    "peakCurrentWorst": 3.9522005725994043,
    "peakCurrentWorstVoltage": 36,
    "peakCurrentWorstPower": 50,
    "inductorRmsWorst": 2.511547405559045,
    "inductorRmsWorstPower": 50,
    "vdsRating": 60,
    "capVoltageRating": 63,
    "vdsRatingVoltage": 36
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:class-d-efficiency",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "International Rectifier AN-1070 (J. Cerezo), Class D Audio Amplifier Performance Relationship to MOSFET Parameters; Infineon AN-1198 (I. Ferstenberg, 2013), Bridging Class D Amplifiers",
    "assumptions": [
      {
        "code": "fets-in-conduction-path",
        "text": "One FET of each half bridge carries the load current at any instant, so N/2 FETs conduct: one in a half bridge (N = 2), two in a full bridge (N = 4) (IR AN-1070; Infineon AN-1198)."
      },
      {
        "code": "no-switching-or-gate-loss",
        "text": "Switching loss 0.5·I·V·(t_r + t_f)·f_sw, output-capacitance loss and gate-drive loss 2·Q_g·V·f_sw (IR AN-1070) are not included, so the efficiency is an upper bound."
      },
      {
        "code": "loss-model-adds-switching-and-filter",
        "text": "The loss model adds, per half bridge, AN-1070's switching ½·I·V_s·(t_r + t_f)·f_sw with I the load current averaged over a sine, its ½·C_oss·V_s²·f_sw and its gate drive 2·Q_g·V_driver·f_sw, and per output the filter inductor's DCR at the idle ripple and its core loss; it reports its own efficiency beside the upper bound. Reverse recovery is left out: AN-1070 scales it by a factor it does not give."
      },
      {
        "code": "butterworth-output-filter",
        "text": "Each output has a second-order Butterworth LC filter, L = R_L·√2/ω₀ and C = 1/(ω₀·R_L·√2) with R_L = Z_L/2 for a bridged load, its capacitor to ground, each part chosen at or above in E12 (TI SLAA701B, eqs. 5 and 6)."
      },
      {
        "code": "waveforms-at-the-sine-peak",
        "text": "The waveforms are drawn at the peak of the nominal power's sine, AD modulation: the leg's duty (1 + m)/2 for a modulation depth m, the inductor current about the load's peak current."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "outputPower": {
          "min": 0.1,
          "max": null,
          "unit": "W"
        },
        "supplyVoltage": {
          "min": 1,
          "max": null,
          "unit": "V"
        },
        "loadImpedance": {
          "min": 1,
          "max": null,
          "unit": "Ω"
        },
        "rdson": {
          "min": 1,
          "max": null,
          "unit": "mΩ"
        },
        "mosfetCount": {
          "min": 2,
          "max": null,
          "unit": ""
        },
        "quiescentCurrent": {
          "min": 0,
          "max": null,
          "unit": "mA"
        },
        "switchingFreq": {
          "min": 50,
          "max": 5000,
          "unit": "kHz"
        },
        "filterCutoff": {
          "min": 20,
          "max": 500,
          "unit": "kHz"
        },
        "outputPowerMin": {
          "min": 0,
          "max": null,
          "unit": "W"
        },
        "outputPowerMax": {
          "min": 0,
          "max": null,
          "unit": "W"
        },
        "supplyVoltageMin": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "supplyVoltageMax": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "tSwitch": {
          "min": 0,
          "max": 10000,
          "unit": "ns"
        },
        "coss": {
          "min": 0,
          "max": 100000,
          "unit": "pF"
        },
        "qg": {
          "min": 0,
          "max": 10000,
          "unit": "nC"
        },
        "vGate": {
          "min": 1,
          "max": 20,
          "unit": "V"
        },
        "dcr": {
          "min": 0,
          "max": 100000,
          "unit": "mΩ"
        },
        "coreLoss": {
          "min": 0,
          "max": 100000,
          "unit": "mW"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "outputPower": 50,
      "supplyVoltage": 36,
      "loadImpedance": 8,
      "rdson": 50,
      "mosfetCount": 4,
      "quiescentCurrent": 30,
      "switchingFreq": 400,
      "filterCutoff": 40,
      "outputPowerMin": 0,
      "outputPowerMax": 0,
      "supplyVoltageMin": 0,
      "supplyVoltageMax": 0,
      "tSwitch": 10,
      "coss": 200,
      "qg": 10,
      "vGate": 12,
      "dcr": 25,
      "coreLoss": 100
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "outputPower"
      ],
      "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 pageClass D Amplifier Efficiency, at these values