How to Calculate Snubber Circuit Values in Power Electronics

To calculate snubber circuit values, first determine the maximum voltage spike and switching speed of your device. Then, select the snubber resistor and capacitor using formulas based on the device's energy dissipation and desired damping, such as R = V_peak / I_peak and C = I_peak × dt / dV where dt is switching time and dV is voltage change.

Syntax

A basic RC snubber circuit consists of a resistor (R) and capacitor (C) connected across the switching device or transformer winding.

Key formulas:

• Capacitance (C): C = I_peak × dt / dV where I_peak is peak current, dt is switching time, and dV is voltage spike allowed.
• Resistance (R): R = V_peak / I_peak to limit current and dissipate energy.

This controls voltage spikes and absorbs energy during switching.

c

/* Snubber calculation formulas */
// C = I_peak * dt / dV
// R = V_peak / I_peak

Example

This example calculates snubber values for a transistor switching 5A peak current with a 100ns switching time and a maximum allowed voltage spike of 50V.

c

#include <stdio.h>

int main() {
    const double I_peak = 5.0;      // Peak current in Amps
    const double dt = 100e-9;       // Switching time in seconds (100ns)
    const double dV = 50.0;         // Allowed voltage spike in Volts
    const double V_peak = 50.0;     // Peak voltage across snubber resistor

    // Calculate capacitance in Farads
    const double C = (I_peak * dt) / dV;
    // Calculate resistance in Ohms
    const double R = V_peak / I_peak;

    printf("Calculated Snubber Capacitance: %.2e F\n", C);
    printf("Calculated Snubber Resistance: %.2f Ohms\n", R);
    return 0;
}

Output

Calculated Snubber Capacitance: 1.00e-06 F
Calculated Snubber Resistance: 10.00 Ohms

Common Pitfalls

Common mistakes when calculating snubber values include:

• Choosing a capacitor value too large, causing excessive power loss and slow switching.
• Using a resistor too small, leading to high current and device stress.
• Ignoring the switching time dt, which affects energy absorption.
• Not considering the device's maximum voltage rating and energy dissipation capability.

Always verify snubber power ratings and test under real conditions.

c

/* Wrong approach: Using too large capacitor and too small resistor */
// C = 10e-6; // Too large, causes slow switching
// R = 1;     // Too small, high current

/* Correct approach: Use calculated values based on formulas */
// C = I_peak * dt / dV
// R = V_peak / I_peak

Quick Reference

Parameter	Formula	Description
Capacitance (C)	C = I_peak × dt / dV	Stores energy to limit voltage spike
Resistance (R)	R = V_peak / I_peak	Dissipates energy and controls current
I_peak	Measured or specified	Peak current through device
dt	Measured switching time	Time over which voltage changes
dV	Allowed voltage spike	Maximum voltage increase allowed

Key Takeaways

Calculate snubber capacitance using peak current, switching time, and allowed voltage spike.

Select snubber resistance to limit current and dissipate energy safely.

Avoid oversized capacitors and undersized resistors to prevent losses and device stress.

Always verify snubber ratings against device specifications and operating conditions.