Calculate R C of RC Snubber Circuit How to?

Hello everyone! Today, we will demonstrate the working principle of a RC snubber circuit with R and C value calculation and simulation in Proteus. A snubber circuit is a crucial component used in power electronics to suppress voltage spikes and protect switching devices like MOSFETs and IGBTs from damage or burn out. These voltage spikes, known as transients, occur when the current through an inductive load is suddenly interrupted. Without a snubber circuit, these spikes can exceed the device's voltage rating and cause failure.

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Why Snubber Circuits Are Important

When a MOSFET or IGBT switches off, the inductor resists the sudden change in current due to its stored energy, generating high voltage spikes. A snubber circuit, typically consisting of a resistor $R_s$ and capacitor $C_s$, absorbs this energy and limits the voltage spike, ensuring the switching device remains within its safe operating limits.

There is also RCD Snubber Circuit, learn the differences and applications in the RC Snubber Vs RCD Snubber circuit blog post.

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Circuit Explanation

Let me walk you through the circuit setup. Here, I am using an IRF540 MOSFET. The source is grounded, and a PWM signal is applied to the gate. The drain is connected to a 1 mH inductor, which is connected to a 12V power supply through a 270-ohm resistor for current limiting.

Initially, we’ll simulate the circuit without the snubber. Notice how the voltage spikes appear across the MOSFET during switching. These transients can exceed the MOSFET's rated voltage of 100V, potentially causing it to fail.

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Simulation Without Snubber

Let's place a probe across the drain and source terminals and run the simulation.

The graph below shows voltage spikes exceeding 12V during switching. This happens because the energy stored in the inductor has no path to dissipate, leading to high transient voltages.

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Adding the Snubber Circuit

To mitigate these spikes, we add a snubber circuit with a resistor $R_s$ and capacitor $C_s$ across the MOSFET's drain and source terminals. This circuit provides a path for the inductor's energy to dissipate, limiting the voltage spikes. This is as shown below:

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Calculating Snubber Circuit Component Values

Tip:  Use the online RC Snubber Circuit Calculator to determine the components value easily.

Step 1: Calculate the Current
First, calculate the current through the resistor R1. Given:

• Vcc = 12 V
• R1 = 10 Ω

The current $I$ is given by:

$$I = \frac{\text{Vcc}}{\text{R1}} = \frac{12}{10} = 1.2 \, A$$

Step 2: Energy Stored in the Inductor
Now, calculate the energy stored in the inductor L. Using the formula:

$$E_L = 0.5 \times L \times I^2$$

Given:

• L = 1 mH = 0.001 H
• I = 1.2 A

Substitute the values:

$$E_L = 0.5 \times 0.001 \times (1.2)^2 = 0.5 \times 0.001 \times 1.44 = 720 \, \mu\text{J}$$

Step 3: Snubber Capacitance (Cs)
Now, calculate the snubber capacitance using the formula:

$$C_s = \frac{2 \times E_L}{\Delta V^2}$$

Assume the voltage spike $\Delta V$ is between 24 V and 50 V. Let’s take $\Delta V = 24 \, \text{V}$:

$$C_s=\frac{2\times 720\times 10^{-6}}{24^2}=\frac{1440\times 10^{-6}}{576}\approx 2.5\mu \text{F}$$

Step 4: Snubber Resistance (Rs)
The snubber resistance is calculated using:

$$R_s=\frac{\mathrm{\Delta }V}{I}$$

Substitute:

$$R_s=\frac{24}{1.2}=20\mathrm{\Omega }$$

Step 5: Power Dissipation in Rs
Finally, calculate the power dissipation in the snubber resistor:

$$P_{R_s}$$

Substitute:

$$P_{R_s}=(1.2{)}^2\times 20=1.44\times 20=28.8W$$

Summary of Results:

• Snubber Capacitance $C_s=2.5\mu \text{F}\approx 2.2\mu \text{F}$(standard value)
• Snubber Resistance $C_s=20$$$\mathrm{\Omega }$$$\approx 22$$$\mathrm{\Omega }$$$$(standard value)
• Power Dissipation in Rs $P_{R_s}=28.8W$

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Simulation With Snubber

Now, let’s rerun the simulation with the snubber circuit. Observe how the voltage spikes are significantly reduced, staying within safe limits. The snubber capacitor absorbs the energy stored in the inductor, while the resistor dissipates it as heat.

 

Video demonstration

 

 Watch the following RC Snubber Circuit Working principle Simulation Video and Design Calculation of R and C.

 

 

Key Takeaways

The snubber circuit protects the MOSFET from transients, increasing its lifespan and ensuring safe operation. For the IRF540, we made sure the spikes stayed below 50V—well within its 100V rating. Calculations and careful component selection are critical for an effective snubber design.

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Conclusion

If you found this demonstration helpful, please like, share, comment below and subscribe for more tutorials on power electronics. Check out the description for related links, including guides on building inverters and designing snubber circuits. Thank you for reading.

Related Links

•  MOSFET RCD Snubber Circuit Design Calculators Online
•  How to control Inrush Current in Switchmode Power Supplies (SMPS)
•  How to Design Output Stages in PWM Switching Power Supplies
•  How to Choose PWM Switching Power Supply Topology
  

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