Snubber Circuit Complete Guide – Working Principle – Design – Used in Power System

Introducing Snubber Circuits: In the world of power systems, the need to protect sensitive electronic components from the detrimental effects of voltage spikes, transient surges, and rapid switching events is of paramount importance. To achieve this, engineers and researchers have developed and implemented a highly effective safeguarding solution known as the “Snubber Circuit.”

In this comprehensive article, we delve into the world of snubber circuits, exploring their working principles, applications in power systems, circuit design methodologies, and the crucial role they play in mitigating the risks posed by power surges and switching phenomena.

What is Snubber Circuit? An Overview

Snubber circuits play a critical role in protecting diodes used in switching circuits. They act as a defense against overvoltage spikes that can occur during the reverse recovery process of the diode. A commonly used snubber circuit for power diodes includes a capacitor and a resistor connected in parallel with the diode (Figure 1).

As the reverse recovery current decreases, the capacitor’s property comes into play, holding the voltage across it, which approximates the voltage across the diode. Simultaneously, the resistor dissipates some of the energy stored in the inductor, forming the Irr loop.

To calculate the dv/dt (rate of voltage change) across the diode, the equation: dv/dt = VS / RS is used, where VS is the applied voltage across the diode. Diode manufacturers provide the dv/dt rating in their datasheets, and using this information along with RS, the appropriate value of the snubber capacitor Cs can be chosen. The selected dv/dt value must always be equal to or lower than the dv/dt value specified in the datasheet.

Snubber Circuit in Power Electronics

Snubber circuits play a crucial role in power electronics by mitigating the losses and energy dissipation that occur during power semiconductor switching. In simple power switching networks with resistive or inductive loads, switching between on and off states results in high voltage and current levels, leading to substantial power dissipation and energy loss.

To address this issue, snubber circuits are connected across the semiconductor switches. When the switch opens to interrupt current flow, the snubber diverts the current, slowing down the increase in switch voltage. This controlled voltage rise reduces switching energy loss and allows for higher operation frequencies with lower losses. However, the energy stored in the charged snubber must be discharged when the switch closes. This discharge is limited by a small resistor to prevent excessive current flow.

While snubbers were once a common solution, modern power electronics have introduced alternative methods to reduce losses, such as capacitors placed in parallel to transistors. These alternatives, along with snubbers, are primarily used to reduce ringing in power electronics applications. As technology evolves, the role of snubber circuits may evolve, but their effectiveness in reducing switching losses remains essential in optimizing power electronic systems.

The Role of RC Snubber Circuits in SCR dv/dt Protection

The RC snubber circuit is an effective method to protect a Silicon-Controlled Rectifier (SCR) from false triggering caused by high dv/dt (rate of voltage change). The circuit comprises a series combination of resistance (R) and capacitance (C) connected in parallel with the SCR.

When a reverse voltage is applied during the commutation process, the forward current through the SCR reduces, but due to inductance, a voltage spike occurs as the current falls quickly to zero. The capacitor C, behaving like a short circuit, protects the SCR by limiting the dv/dt across it.

However, the purpose of resistance R in the circuit is to control the discharge current of capacitor C when the SCR is fired. Without R, the discharge current could be high, potentially damaging the SCR due to large di/dt (rate of current change).

By including resistance R, the discharge current is reduced, providing protection against excessive di/dt. The proper selection of R, C, and load current parameters ensures that dv/dt during charging remains within the SCR’s specified rating and that the discharge current during SCR turn ON is within safe limits. Together, the RC snubber circuit effectively safeguards the SCR from false triggering and ensures reliable operation.

Snubber Types:

1. Passive Snubbers: Passive snubbers employ passive electronic components to reduce voltage transients and prevent excessive energy dissipation during switching. The primary components used in passive snubbers are resistors, capacitors, inductors, and diodes. Three common types of passive snubbers are:
   • a. RC Snubber: This configuration involves a series combination of a resistor and a capacitor in parallel with the semiconductor switch. The RC snubber effectively dampens voltage spikes and limits dv/dt across the switch.
   • b. RCD Snubber: An extension of the RC snubber, the RCD snubber includes a diode in series with the resistor and capacitor. This diode ensures efficient energy dissipation during the commutation process and enhances the snubber’s performance.
   • c. Diode Snubber: In this simple arrangement, a diode is connected in parallel with the switch to provide a path for the inductive energy during commutation. The diode snubber helps reduce switching losses and protects the switch from voltage transients.
2. Active Snubbers: Active snubbers, on the other hand, utilize semiconductor switches to build circuits that actively control voltage spikes and transients. These circuits offer advanced control and flexibility in reducing switching stress. The key feature of active snubbers is their ability to provide dynamic control of energy dissipation and voltage clamping during switching events, enhancing the overall efficiency of power systems.

1. RC Snubber Circuit

RC Snubber is a simple electronic circuit composed of a small resistor (R) and a tiny capacitor (C) connected in series. Its primary purpose is to suppress the rapid rise in voltage (dV/dt) across a thyristor, preventing accidental activation of the thyristor. By restricting the rate of voltage rise, the RC snubber ensures that the thyristor remains in the off state.

This type of snubber finds common use in both DC and AC loads, particularly in applications involving inductive loads like electric motors. When the switch is opened, the capacitor’s inability to change voltage instantaneously allows a transient current to flow through it for a brief period, causing a gradual rise in voltage across the switch.

Determining the voltage rating for RC snubbers can be challenging due to the transient nature of waveforms. However, it can be identified based on the application and the power rating of the snubber components.

RC snubbers can be constructed as a single component or individually. An example of an RC Snubber Circuit can be seen in a Boost Converter with RC snubber, where it is used to reduce high-frequency noise caused by resonance in the input loop when the high-side switch is turned ON or OFF. The addition of a simple RC network to the switch helps to diminish high-frequency noise and improves the overall performance of the circuit.

2. RCD Snubber Circuit

RCD Snubber, also known as RCD clamp, is an electronic circuit used to clamp voltage spikes without altering their frequency or ringing characteristics. This type of snubber is widely employed due to its low impedance nature, allowing efficient voltage spike clamping.

The RCD snubber comprises three components: a resistor, a capacitor, and a diode. The resistor releases energy from the leaking stored power, while the capacitor provides a low ripple DC source, and the diode conducts in a single direction.

An example of an RCD Snubber circuit is seen in the Forward Converter used for low-power DC/DC conversions. In this design, the transformer is reset using an RCD network, providing an economical solution that remains a popular choice for cost-effective designs.

When the switch is off, a diode and a capacitor clamp the voltage at a level determined by the input voltage and duty ratio. The capacitor is reset by a resistor, but the drawback is that the resistor loses power, leading to thermal challenges and decreased efficiency, especially in offline converters where the resistor’s power dissipation is proportional to the input line. Despite this drawback, RCD snubbers remain a valuable choice for voltage spike suppression in numerous power electronic applications.

3. Diode Snubber Circuit

A diode snubber is a straightforward solution utilized in circuits where DC current is flowing and an inductive load, such as a relay coil or electric motor, is present. This snubber diode is connected in parallel with the inductive load and is designed not to conduct under normal conditions. However, when the external driving current is interrupted, the inductor’s current flows through the diode.

The diode acts as a pathway for the inductor’s energy release, facilitated by both the diode’s voltage drop and the inductor’s resistance. A drawback of using a basic rectifier diode as a snubber is that it permits some current to pass for a brief duration, causing the inductor to operate slightly longer than desired. Engineers must consider this delay when designing circuits, particularly when dealing with actuators.

For effective diode snubber circuits, it is crucial that the diode switches quickly to forward conduction mode when the driving current is interrupted. Common diodes, including “slow” power silicon diodes, generally perform well in this regard, quickly turning on in contrast to their delayed reverse recovery durations. Diode snubbers are particularly useful in mitigating issues with electromechanical components like relays and motors.

Snubber Circuits for Thyristor Protection:

In snubber circuits for thyristor protection, a parallel thyristor and a series combination of capacitance (Cs) and resistance (Rs) are used. To prevent unauthorized dv/dt (rate of voltage change) triggering of the SCR, a capacitor Cs connected in parallel with the device is sufficient.

When the switch (S) is closed, a sudden voltage emerges across the circuit. Thanks to the capacitor Cs’s short-circuit-like behavior, the voltage across the SCR is zero initially. As time passes, the voltage across Cs gradually rises, ensuring that the dv/dt across both Cs and the SCR remains lower than the device’s maximum dv/dt rating.

To maintain acceptable dv/dt levels, the characteristics of Rs, Cs, and the load circuit are designed to create an underdamped circuit. Designing snubber circuit characteristics can be complex, and often engineers must fine-tune intended snubber parameters in the final completed power circuit to achieve optimal performance for the power electronics system.

Advantages of Snubber Circuits:

1. Voltage Spike Suppression: Snubber circuits effectively suppress voltage spikes and transients during switching events, protecting sensitive electronic components from damage caused by rapid voltage changes.
2. Reduced Switching Stress: By controlling the rate of voltage and current changes, snubbers reduce the switching stress on semiconductor devices, leading to improved reliability and extended device lifespan.
3. Improved Efficiency: Snubbers can help improve the overall efficiency of power systems by minimizing energy losses during switching and commutation processes.
4. EMI Reduction: Snubber circuits can reduce electromagnetic interference (EMI) caused by high-frequency ringing and voltage transients, resulting in quieter and more reliable electronic systems.
5. Flexibility and Customization: Snubber circuits can be tailored to specific applications, allowing engineers to optimize performance for various loads, switching frequencies, and operating conditions.

Disadvantages of Snubber Circuits:

1. Energy Dissipation: Passive snubber circuits may dissipate energy in resistors, leading to power losses and potential thermal challenges, reducing the overall efficiency of the system.
2. Complexity: Designing and optimizing snubber circuits can be complex, requiring careful consideration of component values, switching characteristics, and load requirements.
3. Component Cost: Some snubber circuits may require additional components, such as resistors, capacitors, and diodes, which can add to the overall cost of the system.
4. Circuit Tuning: Achieving optimal snubber performance may require fine-tuning of component values and circuit characteristics, making the design process more time-consuming.
5. Trade-offs: Designing snubber circuits involves trade-offs between damping excessive voltage transients and ensuring fast switching times, which can impact overall system performance.

Despite these challenges, snubber circuits remain essential tools in power electronics, providing valuable protection against voltage spikes and improving the reliability and efficiency of electronic systems. Properly designed snubber circuits help optimize power system performance and extend the lifespan of sensitive electronic components.

FAQs about Snubber Circuit

Sources of Snubber Circuit:

1. Power Electronics: Converters, Applications, and Design” by Ned Mohan, Tore M. Undeland, and William P. Robbins
2. “Power Electronics Handbook” edited by Muhammad H. Rashid
3. “Power Electronics: Circuits, Devices, and Applications” by Muhammad H. Rashid
4. “Switchmode RF Power Amplifiers” by Andrei Grebennikov
5. “Power Electronics: Principles and Applications” by Joseph Vithayathil