snubber circuit design


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snubber circuit

snubber circuit diagram
Basic RC snubber circuit

Snubber Circuit Calculation

Snubber circuit for MOSFET

In the Snubber circuit, the RC series network is used to dampen the ringing generated by the circuit’s stray capacitance and inductance. Cumulative capacitance and inductance including inductive loads such as relay or solenoid coil produce a resonance frequency ringing. Due to inductance, a change in the load current has a high kick-back ringing voltage at the resonance frequency. The following formula can calculate ringing frequency

resonance frequency for stray incuctance and stray capacitance
Resonance Frequency of LC circuit

Here f0 is ringing frequency without adding a snubber capacitor.  C is stray capacitance. And L is parasitic inductance is load is inductive then load inductance should be added. The stray capacitance and stray inductance are usually very low. So, the ringing frequency is high. At high-frequency di/dt is high so the initial ringing amplitude is very high.

inductive reactive voltage
Inductor response

    To reduce the di/dt we decrease f0 by adding a capacitor in parallel with switching element. When thyristor goes off from conduction. Stored magnetic energy in inductive load is discharged through an infinite circuit impedance. Which generate a high voltage spike and parasitic capacitance initiate ringing. If we add a resistance and a capacitor in thyristor’s parallel. The new ringing frequency after adding a capacitor into circuit is fs. Cs is snubber capacitor.

snubber ringing frequency after adding snubber capacitor
Snubber circuit ringing frequency

How to calculate snubber resistor?

Resistor in snubber acts as the damping element. Damping factor can be determined by following formula

damping factor with known resistance capacitance and inductance
RLC Damping factor

ζ is damping factor were Rs is snubber resistor. L and C are final values after adding snubber capacitance. However sometimes only Rs is enough to damp unwanted ringing. But, mostly in MOSFET bridge circuits, resistance passes a continues current in off condition which is unwanted. here capacitance is used to block DC current and lower the ringing frequency.

A good damping factor can be determined by viewing a graphical example of step response of a RLC circuit.

chart for different damping factor and ringing amplitude response
step response of RLC damping circuit

Damping factor 0.7 and 1 are suitable values for a snubber. It is over damping when damping factor value is 2.  If we consider ζ = 1.

how to calculate snubber resistance
Snubber circuit Resistance equation

Snubber capacitor?

Now we have Rs value for RC snubber circuit from above equation. And we can calculate Cs from critical frequency of RC circuit equation.

critical frequency of RC circuit
Critical frequency of RC snubber circuit

To find snubber capacitance Cs value from the equation given under. By f0 and Rs values

how to calculate snubber capacitor
Snubber capacitor value

Assume for optimal design fc = f0 we can choose different frequencies than the critical frequency as per dedicated application.

Choosing Capacitor and Resistance values

only standard values of Capacitors and resistors are available commercially. For the snubber circuit, there is no need to be exact values. we can use round-about standard values of capacitors and resistors in the snubber circuit.

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