Documentation sections

RCD Clamp Snubber Calculator — Flyback Leakage-Inductance Clamp

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

Slug rcd-clamp-snubber. Design the RCD clamp that limits a flyback converter's drain voltage spike: clamp power including the reflected-voltage factor, the clamp resistor and capacitor, the peak drain voltage and the diode's reverse-voltage rating, from the leakage inductance, peak current and clamp voltage.

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 rcd-clamp-snubber
Name	Unit	Range	Default	Description
leakageInductancenumber	µH	at least 0.001	1	Primary leakage inductance of the transformer, measured with the secondary shorted; typically 1 to 3% of the magnetising inductance
peakCurrentnumber	A	at least 0.001	5	Primary current at the moment the switch turns off, at full load and minimum input voltage
switchingFrequencynumber	kHz	at least 0.001	100	Switching frequency of the flyback converter
reflectedVoltagenumber	V	at least 0.001	40	Output voltage (plus the rectifier drop) times the primary-to-secondary turns ratio n, which the primary sees while the secondary conducts
clampVoltagenumber	V	at least 0.001	80	Voltage the clamp capacitor holds above the input rail; AN-4147 suggests 2 to 2.5 times the reflected voltage
ripplenumber	%	0.1 … 99	5	Allowed ripple on the clamp capacitor as a share of the clamp voltage; 5 to 10% is usual
maxInputVoltagenumber	V	at least 0	72	Highest DC input voltage (for an off-line converter, the rectified peak of the highest mains voltage)
eSeriesnumber	none	0 … 96	0	Rounds every part to the nearest value in the series; the response and the export are the rounded design's.One of: 0 Ideal values · 12 E12 (10%) · 24 E24 (5%) · 48 E48 (2%) · 96 E96 (1%)

Outputs

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

Outputs of rcd-clamp-snubber
Field	Unit	Description
clampPowerheadline	W	Clamp Power
clampResistance	kΩ	V_sn²/P_sn, rounded to the part series when one is chosen
clampCapacitance	nF	V_sn/(ΔV_sn·R_sn·f_s), rounded to the part series when one is chosen
peakDrainVoltage	V	Peak Drain Voltage (V_in,max + V_sn)
diodeReverseVoltage	V	Diode Reverse Voltage (minimum rating)
leakagePower	W	Leakage-Energy Loss Alone (½·L·I²·f)
lossFactor	none	Reflected-Voltage Factor V_sn/(V_sn − nV_o)
diodeConductionTime	ns	Diode Conduction Time
rippleVoltage	V	Clamp Ripple Voltage
clampVoltageBuilt	V	The voltage the clamp settles to with the resistor as rounded: the energy balance V·(V − nV_o) = R·½·L·I²·f, the clamp voltage entered with the exact resistor
rippleVoltageBuilt	V	The ripple the rounded capacitor and resistor give at that clamp voltage, V/(R·C·f_s) (AN-4147 Eq. 5)
clampPowerBuilt	W	What the rounded resistor dissipates at the clamp voltage it settles to, V²/R; the export sizes the resistor from it
peakDrainVoltageBuilt	V	V_in,max plus the clamp voltage the rounded resistor gives

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": "rcd-clamp-snubber",
    "inputs": {
      "leakageInductance": 1,
      "peakCurrent": 5,
      "switchingFrequency": 100,
      "reflectedVoltage": 40,
      "clampVoltage": 80,
      "ripple": 5,
      "maxInputVoltage": 72,
      "eSeries": 0
    }
  }'

import rftools

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

result = client.calculate("rcd-clamp-snubber", {
    "leakageInductance": 1,
    "peakCurrent": 5,
    "switchingFrequency": 100,
    "reflectedVoltage": 40,
    "clampVoltage": 80,
    "ripple": 5,
    "maxInputVoltage": 72,
    "eSeries": 0,
})
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": "rcd-clamp-snubber",
    "inputs": {
      "leakageInductance": 1,
      "peakCurrent": 5,
      "switchingFrequency": 100,
      "reflectedVoltage": 40,
      "clampVoltage": 80,
      "ripple": 5,
      "maxInputVoltage": 72,
      "eSeries": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "rcd-clamp-snubber",
	  "inputs": {
	    "leakageInductance": 1,
	    "peakCurrent": 5,
	    "switchingFrequency": 100,
	    "reflectedVoltage": 40,
	    "clampVoltage": 80,
	    "ripple": 5,
	    "maxInputVoltage": 72,
	    "eSeries": 0
	  }
	}`)
	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": "rcd-clamp-snubber",
  "values": {
    "clampPower": 2.5,
    "clampResistance": 2.56,
    "clampCapacitance": 78.125,
    "peakDrainVoltage": 152,
    "diodeReverseVoltage": 152,
    "leakagePower": 1.25,
    "lossFactor": 2,
    "diodeConductionTime": 125,
    "rippleVoltage": 4,
    "clampVoltageBuilt": 80,
    "rippleVoltageBuilt": 4,
    "clampPowerBuilt": 2.5,
    "peakDrainVoltageBuilt": 152
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:rcd-clamp-snubber",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Koo, Design Guidelines for RCD Snubber of Flyback Converters, Fairchild AN-4147, Rev. 1.1.0 (2006), Eqs. (2)–(5)",
    "assumptions": [
      {
        "code": "constant-clamp-voltage",
        "text": "The clamp capacitor is large enough that its voltage stays at V_sn over a switching period."
      },
      {
        "code": "clamp-returns-to-input",
        "text": "The clamp resistor and capacitor return to the input rail, so the drain peaks at V_in,max + V_sn and ringing above the clamp is not modelled."
      },
      {
        "code": "design-point",
        "text": "The leakage inductance and peak current are those at the design point (AN-4147: minimum input voltage, full load); the clamp voltage at other operating points is not computed."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "leakageInductance": {
          "min": 0.001,
          "max": null,
          "unit": "µH"
        },
        "peakCurrent": {
          "min": 0.001,
          "max": null,
          "unit": "A"
        },
        "switchingFrequency": {
          "min": 0.001,
          "max": null,
          "unit": "kHz"
        },
        "reflectedVoltage": {
          "min": 0.001,
          "max": null,
          "unit": "V"
        },
        "clampVoltage": {
          "min": 0.001,
          "max": null,
          "unit": "V"
        },
        "ripple": {
          "min": 0.1,
          "max": 99,
          "unit": "%"
        },
        "maxInputVoltage": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "leakageInductance": 1,
      "peakCurrent": 5,
      "switchingFrequency": 100,
      "reflectedVoltage": 40,
      "clampVoltage": 80,
      "ripple": 5,
      "maxInputVoltage": 72,
      "eSeries": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "leakageInductance"
      ],
      "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 pageRCD Clamp Snubber Calculator — Flyback Leakage-Inductance Clamp, at these values