Electronics

What is Snubber Circuit and How does it work?

What is meant by Snubber circuit?

  • Basically, a series-connected resistor and capacitor are linked across a device is called as a snubber circuit.
  • Snubbers are devices that are employed to inhibit phenomenon like voltage transients in electrical systems, pressure transients in fluid systems, or excess force or quick movement in mechanical systems
  • Circuit protection against voltage spikes, ringing, and oscillation effects are provided by snubbers.
  • Snubber performs the same function or clamps voltage spikes without changing the ringing frequency.
  • The rate of rise of forward voltage, or dv/dt, across the thyristor must be suppressed by a snubber circuit in order to prevent damage to the device.
RC Snubber Circuit

Purpose of Snubber Circuit

  • The primary goal of the Snubber circuit is to avoid unintentional SCR or thyristor triggering caused by a rapid rise in voltage, or high dv/dt.
  • We already know that if the SCR’s anode to cathode voltage rises quickly, erroneous triggering may result. Commonly referred to as dv/dt triggering

What are the functions of a snubber circuit

  • It is possible to reduce or get rid of voltage or current spikes.
  • Limitations for dI/dt or dV/dtshape the load line to keep it inside the safe operating area.
  • It distributes the switch’s power onto a resistor or another suitable load.
  • Overall switching losses may be reduced.
  • It can lower EMI by lessening voltage and current ringing.

Types of Snubbers

  • Snubbers circuits are divided into passive and active types.
  • Passive parts make up the passive snubber (resistor, inductor, capacitor & diode).
    • RC Snubber
    • RCD Snubber
    • Diode Snubber
  • The semiconductor switch was used to build the active snubbers circuit.

What is meant by RC Snubber?

  • A small resistor (R) and a tiny capacitor are connected in series to create an easy RC snubber (C). By restricting the rate of rise in voltage (dV/dt) across the thyristor to a value that will not activate it, this combination can be used to suppress the rapid rise in voltage across a thyristor and prevent the thyristor from being accidentally turned on.
  • Both DC and AC loads can be utilised with an adequately made RC snubber.
  • Common applications for this kind of snubber include inductive loads like electric motors.
  • Because a capacitor’s voltage cannot change instantly, a decreasing transient current will flow through it for a brief period of time when the switch is opened, causing the voltage across the switch to rise more gradually.
  • Determining the voltage rating can be difficult due to the nature of transient waveforms.
  • However, it can be identified by the application and the power rating of the snubber components alone.
  • RC snubbers can be constructed as a single component or individually.
RC Snubber element

Example of RC Snubber Circuit – Boost Converter with RC Snubber circuit

  • When the high-side switch is turned ON or OFF, the energy that has accumulated in the parasitic inductors causes resonance in the input loop.
  • Due to resonance frequencies for parasitic elements exceeding several hundred MHz at relatively low values, EMI characteristics decrease.
  • RC snubber circuits are widely employed to reduce this high-frequency noise.
  • To lessen high-frequency noise, a simple RC network might be added to the switch.
Boost Converter with RC Snubber circuit

What is meant by RCD Snubber ?

RCD Snubber
  • Some designers referred to this as an RCD clamp rather than an RCD snubber.
  • RCD clamps are widely used as it is a low impedance circuit. It does so because the voltage spikes will actually be clamped without changing the spike or ringing frequency.
  • The components of an RCD snubber are a resistor, capacitor, and diode.
  • Resistor will release energy from the leaking stored power, Capacitor provides a low ripple DC source and diodes function in only one direction.

Example of RCD Snubber circuit

Forward Converter with RCD Snubber
  • For low-power DC/DC conversions, the forward converter continues to be the backbone of the industry
  • An early attempt to enhance reset was the Forward Converter with an RCD Snubber Circuit. The transformer is reset using an RCD network.
  • This strategy has a low cost. It is still a wise choice for affordable designs.
  • A diode D and a capacitor C clamp the voltage at a level set by the input voltage and duty ratio when the switch is off. The capacitor is reset by a resistor R. The drawback of R is that it loses power. This dissipation leads to a difficulty with the thermal design in addition to decreasing efficiency.
  • This is especially true for offline converters because the resistor’s power dissipation is proportional to the input line

Diode Snubber

  • A simple rectifier diode is typically employed as a snubber when a DC current is flowing.
  • The snubber diode is linked in parallel with an inductive load (such as a relay coil or electric motor).
  • The diode’s installation prohibits it from conducting under normal conditions.
  • When the external driving current is interrupted, the inductor current takes its place and passes through the diode.
  • The energy held in the inductor is gradually released by a combination of the diode voltage drop and the resistance of the inductor.
  • A basic rectifier diode’s disadvantage when used as a snubber is that it permits current to pass for a time, which causes the inductor to run somewhat longer than is desirable.
  • This delay in the actuator dropping out must be taken into account when designing circuits.
  • The diode must quickly convert to forward conduction mode when the driving current is interrupted.
  • The majority of common diodes, including “slow” power silicon diodes, may turn on relatively quickly, in contrast to their delayed reverse recovery durations. This work well for getting around electromechanical components like relays and motors.

Snubber Circuits for Thyristor Protection

SCR Snubber Circuit
  • A snubber circuit is made up of a parallel thyristor and a series combination of capacitance Cs and resistance Rs.
  • To stop unauthorized dv/dt triggering of the SCR, a capacitor Cs connected in parallel with the device is adequate.
  • A sudden voltage emerges across the circuit when switch S is closed. As a result of the capacitor Cs‘s short circuit-like behavior, the voltage across the SCR is zero. Because voltage across Cs rises up slowly over time, dv/dt across Cs and therefore across SCR is lower than the device’s maximum dv/dt rating.
  • In order to keep dv/dt within acceptable bounds, Rs, Csand the load circuit characteristics normally create an underdamped circuit. Snubber circuit characteristics are designed in a very complicated way. In order to achieve a good performance of the power electronics system, intended snubber parameters are in actuality increased or decreased in the final completed power circuit.

Selecting criteria of Rs, Cs

         In actual usage, Rs, Cs, and the parameters of the load circuit should be chosen so that the dv/dt across Cs during its charging falls below the SCR’s stipulated dv/dt rating and the discharge current upon SCR turn-on is within reasonable bounds.

Why use Rs in series with Cs if Cs is sufficient to prevent dv/dt from accidentally turning on the device?

  • Cs charges to its maximum voltage Vs before the gate pulse activates the SCR.
  • The capacitor discharges through the SCR when the SCR is turned on, sending a current equal to Vs / (resistance of the local path formed by Cs and SCR).
  • Due to the relatively low resistance, the turn-on di/dt will typically be high, which could lead to SCR destruction.
  • A resistance Rs is connected in series with Cs to limit the amount of discharge current.
  • The initial discharge current Vs/Rs is now relatively small when the SCR is turned on, and the turn-on di/dt is decreased.

Response of Switching converter with and without RC Snubber

         To reduce the voltage spike on the switching device to a safe level, RC snubbers are frequently utilized in switching converters. In order to avoid more problems, the voltage spike should not only be clamped, but also the spike frequency or ringing behavior should be changed.

         The RC snubber reduces the voltage spike level while also altering the ringing frequency. The resistor operates as a discharge channel, and the capacitor serves as a charge storage device.

Circuit response with and without RC Snubber

Snubber circuits in power electronics

  • When a power semiconductor switch turns on or off in a simple power switching network with a resistive load, device voltage and current are high. The device loses energy and dissipates power.
  • In high-voltage switching circuits, peak power dissipation is 25% of maximum voltage and current. Inductive loads dissipate more.
  • However, a semiconductor-connected snubber reduces turn-off losses. When the switch opens, the uncharged snubber draws current. This diversion reduces switch voltage build-up, lowering current before switch voltage rises considerably. Slowly increasing switch voltage reduces switching energy loss.
  • Power electronics snubbers can operate at high frequencies and low switching losses. The charged snubber must release its energy. A tiny resistor limits the discharge current of the snubber when the switch is closed at the start of its conduction angle.

What is disadvantages of RC snubber over RCD snubber?

         The RC snubber has the drawback of increasing the amount of current the transistor must carry when it goes on without taking into account whether the switch node voltage is growing or lowering. To make an RCD snubber for higher power applications, you can connect a diode in series with the RC.

Explain the snubber circuit in terms of mechanical systems

  • Snubbers are used to restrict movement in pipe and equipment during unexpected circumstances including earthquakes, turbine trips, and safety/relief valve closure.
  • Snubbers constrain a component in irregular situations yet allow for free thermal mobility under normal circumstances.
  • Pipe deflection is possible using a hydraulic snubber under typical operating circumstances.
  • The snubber becomes activated and works as a restraint to prevent pipe movement when it is subjected to an impulse load.
  • A mechanical snubber applies the constraint force using mechanical means.

How do you test a Snubber circuit?

The charged snubber must release its energy. A tiny resistor limits the discharge current of the snubber when the switch is closed at the start of its conduction angle.The snubber capacitor is to lessen the parasitic inductance of electrical cable. It is a capacitor that is linked to a large-current switching node. Turn off the power, remove the external snubber capacitor from the circuit, and measure the capacitance between its two leads.

How inspection done in Snubber?

  • This may be the cause of resets or strange relay point shutdowns on a job site.
  • Before testing, unplug control power to the controller and relays.
  • If you receive a bad reading, switch test leads and try again.

Step 1: Measure capacitance with a capacitance metre with all control power off and relay output connectors disconnected. Set the capacitance dial. Measure capacitance between “common (COM)” and “Normally Open (NO)” on the board. The meter should read between.08μF and.120μF.

Repeat for each Relay Output. The board must be replaced if any Snubber Capacitors are out of range.

Step 2: Checking “Normally Closed (NC)” is more complicated. Turn off control power and remove the panel board first. Only power the board’s power terminal on a test bench. Power the board up and manually switch each relay on, then read the capacitance again, this time between “Common (COM)” and “Normally Closed (NC)” on the board, where you should again read between.08μF and.120μF on the meter. Turn off the power, remove the external snubber capacitor from the circuit, and measure the capacitance between its two leads.

How the Snubber circuit protects the MOSFET?

MOSFET Snubber Circuit
  • Power MOSFET the RC snubber is connected in parallel with a power MOSFET.
  • Cutting off a current in a circuit causes its voltage to increase sharply due to stray inductances.
  • The snubber damps this surge voltage to protect the power MOSFET as well as components in its vicinity.
  • A circuit used to prevent voltage spikes and voltage oscillations across a MOSFET during device turn-off.
  • The large peak current handling capability of the MOSFET and the fact that its switching speed can be easily controlled by controlling the gate current eliminates the need for a turn-on snubber in most cases.

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