MOSFET Snubber Circuit Design Calculators Online

Inductive loads like flyback transformers, relays, or motors often cause high-voltage spikes when a transistor interrupts their current. These spikes can damage components and cause EMI issues. Snubber circuits are the most common solution, helping to manage these spikes effectively. This guide explains two common types of snubber circuits and includes design calculations to make implementation easier.

Voltage Rise Control RCD Snubber Circuit

This method limits the voltage rise when the transistor turns off. The goal is to control how fast the transistor’s drain or collector voltage reaches the rail voltage.

Calculator 

The following online calculator calculates the component values and power dissipation for the RCD Sunbber Circuit to control the voltage rise.

f: KHz
I_pk: A
V_rail: V
r_t: ns us ms

Theoretical Results:
C:
R:
P:

Key Formulas:

1. Capacitance to limit rise time:
   $C=\frac{I_{pk}\cdot \mathrm{\Delta }t}{V_{rail}}$

   • $I_{pk}$: Peak current
   • $\mathrm{\Delta }t$: Desired rise time
   • $V_{rail}$: Rail voltage

2. Resistor selection for RC time constant:
   $R=\frac{1}{f\cdot C\cdot 20}$

   • $f$: Switching frequency

   Note: See the derivation note below.
   

3. Power dissipated by the resistor:
   $P=\frac{1}{2}\cdot C\cdot V^2\cdot f$

Tips for Optimization:

• Lower resistance decreases rise time but reduces power dissipation in the resistor.
• Increasing capacitance slows rise time but increases power dissipation.
• Ensure the diode can handle peak current.
• Use components rated for peak voltage and thermal limits.

Voltage Clamping RCD Snubber Circuit

This circuit is designed to clamp voltage spikes to a safe level. It uses a rectifier, capacitor, and resistor to manage energy from the inductive load.

Calculator 

The following online calculator calculates the component values and power dissipation for the RCD Sunbber Circuit to control the voltage rise. 

f: KHz
I_pk: A
V_ssc: V
dV: V mV
L: uH mH H

Theoretical Results:
C:
R:
P:

Key Concepts:

• The capacitor limits voltage changes ($dV$), while the resistor dissipates the stored energy over each cycle.

Key Formulas:

1. Capacitance to limit voltage ripple:
   $C = \frac{L \cdot I^2}{dV \cdot (dV + 2V)}$

   • $L$: Inductance
   • $I$: Peak current
   • $dV$: Desired voltage ripple
   • $V$: Minimum circuit voltage

2. Resistor selection:
   $R = \frac{10}{f \cdot C}$

3. Power dissipated by the resistor:
   $P = \frac{f}{2} \cdot L \cdot I^2$

Tips for Optimization:

• Use a time constant 10x the switching period for stable operation.
• Adjust capacitor and resistor values to avoid overheating and overvoltage.
• Confirm all components can handle maximum voltage and power levels.

Practical Notes:

• RCD rate-of-voltage-rise snubbers focus on controlling the initial voltage rise.
• RCD clamping snubbers are ideal for limiting overall voltage spikes in circuits like power supplies.
• Choose snubber types based on your circuit’s requirements and constraints.

This simplified explanation, paired with the formulas, should help you design effective snubber circuits for your specific applications.

 Resistor Selection Formula Derivation:

1. Time Constant Definition

The RC time constant ($\tau$) is given by:

The time constant defines how quickly the capacitor charges or discharges. For snubber design, the goal is to ensure the capacitor charges or discharges within a fraction of the switching period.

2. Choosing the Time Constant

A practical rule of thumb is to set the time constant ($\tau$) to be about 1/10th of the switching period. This ensures the capacitor responds quickly enough to changes in the circuit without overly dampening the system.

The switching period ($T$) is the reciprocal of the switching frequency ($f$):

3. Relating Resistance to Time Constant

Using the time constant formula:

Substitute $\tau = \frac{1}{10f}$:

Solve for $R$:

4. Incorporating Practical Factor of Safety

To account for additional considerations like response time and energy dissipation, a factor of 2 is often introduced, making the denominator 20 instead of 10. This slightly increases the resistance to balance performance and thermal limits.

Thus, the final formula becomes:

Key Insight:

• The formula ensures the RC circuit responds fast enough relative to the switching frequency without dissipating excessive power or causing delays.
• The factor $20$ arises from practical tuning rather than strict theoretical derivation, reflecting real-world trade-offs in snubber design.