Online Flyback RC Snubber Calculator

Biricha Flyback Snubber Design Tool

Enter the required parameters and this online calculator will calculate the snubber R and C values for the Flyback converter.

Enter output voltage (Vout)

V

Enter the transformer turns ratio (in the form of N:1)

to 1

e.g. if your transformer has a turns ratio of 3 to 2, type in 1.5 and the tool will do the rest

Enter maximum input voltage (Vin)

V

Voltage stress across the primary side switch Q2 (Vds)

V

Voltage stress across the secondary diode/switch Q1/D1 (Vd1)

Enter power supply switching frequency (Fs)

Hz

e.g for 100kHz you can type 100000 or 100e3

Low Side Switch (Q2) Snubber Calculations (R_snub2 & C_snub2)

Measure and enter ringing frequency on the rising edge of switch node voltage at point X (Fr0_rising) without snubber

Hz

e.g. for 27MHz you can type 27e6. Note: measure with respect to primary ground.

Enter the datasheet value of Q2 output capacitance (Coss)

F

e.g. for 100pF please type in 100e-12

Recommended first pass snubber capacitor value (C_snub2)

F

Select a capacitor value close to this. After selecting this first pass snubber capacitor value solder this capacitor in place of C_snub2 and temporarily short R_snub2 with a 0R resistor. Then please follow the instructions below.

Enter actual value of C_snub2 soldered

F

e.g. for 400pF please type in 400e-12

Note: If snubber capacitance is too big, power dissipation will be large; if it is too small the circuit will not work well Start with a low value (typically ~4 x Coss or 4 x CT) and iterate until the snubber power loss is acceptable. This power will be lost when you solder a snubber resistor.

Estimated power dissipation in the final snubber resistor, R_snub2, as a result of the value of C_snub2 entered above

W

For reference typical maximum dissipation for 1 oz FR4 PCB with minimal pad size (check manufacturer datasheet) 0603 package ~ 0.050W to 0.100W 0805 package ~ 0.050W to 0.125W 1206 package ~ 0.125W to 0.250W 1210 package ~ 0.25W to 0.500W

Re-measure and enter the new ringing frequency after the addition of C_snub2

Hz

Estimated effective Coss of the switch

F

Estimated effective leakage inductance (L_lkg)

H

Recommended Snubber Resistance (R_snub2)

ohms

Note 1: Use as close a value as possible Note 2: To finalize your design, look at the ringing again. Ideally there should be only 1 or no undershoot. If this is the case, you may be able to reduce C_snub2 and check to see if you can reduce your losses

Secondary Side Switch (Q1/D1) Snubber Calculations (R_snub1 & C_snub1) Note: Often you do not need this after soldering the primary side snubber. Make sure the primary side snubber is in the circuit and functioning before adding this secondary side snubber.

Measure and enter ringing frequency on the falling edge of switch node voltage at point X (Fr0_falling) without snubber

Hz

e.g. for 90MHz you can type 90e6. Note: measure with respect to secondary ground.

Enter the datasheet value of Q1 output capacitance (Coss) or total/junction capacitance of D1 (CT)

F

Recommended first pass snubber capacitor value (C_snub1)

F

Select a capacitor value close to this. After selecting this first pass snubber capacitor value solder this capacitor in place of C_snub1 and temporarily short R_snub1 with a 0R resistor. Then please follow the instructions below.

Enter actual value of C_snub1 soldered

F

Note: If snubber capacitance is too big, power dissipation will be large; if it is too small the circuit will not work well Start with a low value (typically ~4 x Coss or 4 x CT) and iterate until dissipation in your resistor is acceptable. This power will be lost when you solder a snubber resistor.

Estimated power dissipation in the final snubber resistor, R_snub1, as a result of the value of C_snub1 entered above

W

For reference typical maximum dissipation for 1 oz FR4 PCB with minimal pad size (check manufacturer datasheet) 0603 package ~ 0.050W to 0.100W 0805 package ~ 0.050W to 0.125W 1206 package ~ 0.125W to 0.250W 1210 package ~ 0.25W to 0.500W

Re-measure and enter the new ringing frequency after the addition of C_snub1

Hz

Estimated effective Coss of the switch

F

Estimated effective leakage inductance (L_lkg)

H

Recommended Snubber Resistance (R_snub1)

ohms

Note 1: Use as close a value as possible Note 2: To finalize your design, look at the ringing again. Ideally there should be only 1 or no undershoot. If this is the case, you may be able to reduce C_snub2 and check to see if you can reduce your losses

Your design is complete! 🙂