SNUBBERS

SNUBBERSSNUBBERS

is a device used to suppress is a device used to suppress ("snub") voltage transients in ("snub") voltage transients in electricalelectrical systems, pressure systems, pressure transients in transients in fluidfluid systems, or excess systems, or excess force or rapid movement in force or rapid movement in mechanicalmechanical systems. systems.

Electrical systemsElectrical systems

Snubbers are frequently used in Snubbers are frequently used in electrical systems with an electrical systems with an inductiveinductive load where the load where the sudden interruption of sudden interruption of currentcurrent flow often leads to a sharp rise flow often leads to a sharp rise in in voltagevoltage across the device across the device creating the interruption. creating the interruption.

This sharp rise in voltage is a This sharp rise in voltage is a transient and can damage and transient and can damage and lead to failure of the controlling lead to failure of the controlling device. A spark is likely to be device. A spark is likely to be generated (arcing), which can generated (arcing), which can cause electromagnetic cause electromagnetic interference in other circuits. interference in other circuits. The snubber prevents this The snubber prevents this undesired voltage by undesired voltage by conducting transient current conducting transient current around the device.around the device.

RC snubbersRC snubbers

WHY???WHY???

Requirements:Requirements:

The voltage & current remain must remain The voltage & current remain must remain within the safe operating area (SOA) of the within the safe operating area (SOA) of the device.device.

To keep the rate of change of the device To keep the rate of change of the device voltage & current low enough to provide voltage & current low enough to provide correct & reliable operation during switch correct & reliable operation during switch transition.transition.

To limit the dissipation that occurs in the To limit the dissipation that occurs in the device during switching.device during switching.

Function of Snubber Function of Snubber CircuitsCircuits

• • Protect semiconductor devices by:Protect semiconductor devices by: • • Limiting device voltages during turn-off transientsLimiting device voltages during turn-off transients • • Limiting device currents during turn-on transientsLimiting device currents during turn-on transients • • Limiting the rate-of-rise (didt) of currents through theLimiting the rate-of-rise (didt) of currents through the semiconductor device at device turn-onsemiconductor device at device turn-on • • Limiting the rate-of-rise (dvdt) of voltages across theLimiting the rate-of-rise (dvdt) of voltages across the semiconductor device at device turn-offsemiconductor device at device turn-off • • Shaping the switching trajectory of the device as itShaping the switching trajectory of the device as it turns on/offturns on/off

2 basic kinds of snubbers:2 basic kinds of snubbers:

1.1. To control the rate of rise of the switch To control the rate of rise of the switch voltage. (turn-off snubbers)voltage. (turn-off snubbers)

2.2. The second controls the rate of rise of The second controls the rate of rise of switch current. (turn-on snubbers)switch current. (turn-on snubbers)

Both use small energy storage Both use small energy storage elements – a capacitor and inductor.elements – a capacitor and inductor.

TURN-ON SNUBBER CKTsTURN-ON SNUBBER CKTs

Turn on Snubber:Turn on Snubber:

A snubber inductor is placed in series A snubber inductor is placed in series with the transistor. (Ls) is used to with the transistor. (Ls) is used to limit the transistor current.limit the transistor current.

Practical Turn-on snubber:Practical Turn-on snubber:

We need to add a components to allow the transistor to survive the We need to add a components to allow the transistor to survive the switch transition.switch transition.

If nothing were added to the ckt., the transistor would receive an If nothing were added to the ckt., the transistor would receive an impulse of voltage when it turned off, because the inductor current impulse of voltage when it turned off, because the inductor current would be forced to change in a very short time.would be forced to change in a very short time.

Solution: Solution: We add resistor to provide alternative path for the inductor current We add resistor to provide alternative path for the inductor current

when the transistor turns off.when the transistor turns off. We add diode to keep the resistor from conducting during the turn-We add diode to keep the resistor from conducting during the turn-

on transition.on transition.

Note: the resistance Rs must be large enough to completely Note: the resistance Rs must be large enough to completely discharge Ls during the off-state. To minimize the transistor’s discharge Ls during the off-state. To minimize the transistor’s voltage stress, it shld. Not be made any larger thatn necessary.voltage stress, it shld. Not be made any larger thatn necessary.

TURN-OFF SNUBBER CKTsTURN-OFF SNUBBER CKTs

A More Practical Snubber:A More Practical Snubber:

A capacitor by itself is not a sufficient turn-off A capacitor by itself is not a sufficient turn-off snubber because when the transistor is turned on snubber because when the transistor is turned on again, Cs will discharge through Q. the resulting again, Cs will discharge through Q. the resulting transistor current can be very large and lead to transistor current can be very large and lead to failure .failure .

Solution: Solution: We add a resistor to limit the discharge current when the We add a resistor to limit the discharge current when the

transistor is turned on.transistor is turned on. We add a diode to allow charging current to bypass the We add a diode to allow charging current to bypass the

resistor during the turn-off transition.resistor during the turn-off transition.

TradeoffTradeoff

Is necessary in determining the value Is necessary in determining the value of Rs. It must be small enough to of Rs. It must be small enough to ensure the capacitor is fully ensure the capacitor is fully discharged during the shortest time discharged during the shortest time that Q might be on but large enough that Q might be on but large enough to prevent the discharge current to prevent the discharge current from exceeding the transistor rating.from exceeding the transistor rating.

DISSIPATION IN SNUBBER DISSIPATION IN SNUBBER CIRCUITSCIRCUITS

At the beginning of a turn-on/turn-off At the beginning of a turn-on/turn-off transition, the energy storage element transition, the energy storage element in the corresponding snubber ckt. in the corresponding snubber ckt. Contains no stored energy. When the Contains no stored energy. When the transition is over, however, the transition is over, however, the element is left with a nonzero value of element is left with a nonzero value of stored energy. This energy is stored energy. This energy is dissipated during the next switch dissipated during the next switch transition. transition.

RC SNUBBERRC SNUBBER

A simple snubber comprising a small resistor A simple snubber comprising a small resistor (R) in (R) in seriesseries with a small capacitor (C) is often with a small capacitor (C) is often used. This combination can be used to used. This combination can be used to suppress the rapid rise in voltage across a suppress the rapid rise in voltage across a thyristorthyristor..

are also often used to prevent arcing across are also often used to prevent arcing across the contacts of relays and switches and the the contacts of relays and switches and the electrical interference and welding/sticking of electrical interference and welding/sticking of the contacts that can occur. the contacts that can occur.

Types of Snubber CircuitsTypes of Snubber Circuits

1. Unpolarized series R-C snubbers1. Unpolarized series R-C snubbers • • Used to protect diodes and thyristorsUsed to protect diodes and thyristors 2. Polarized R-C snubbers2. Polarized R-C snubbers • • Used as turn-off snubbers to shape the turn-on switchingUsed as turn-off snubbers to shape the turn-on switching trajectory of controlled switches.trajectory of controlled switches. • • Used as overvoltage snubbers to clamp voltages applied Used as overvoltage snubbers to clamp voltages applied

toto controlled switches to safe values.controlled switches to safe values. • • Limit dvdt during device turn-offLimit dvdt during device turn-off 3. Polarized L-R snubbers3. Polarized L-R snubbers • • Used as turn-on snubbers to shapte the turn-off switchingUsed as turn-on snubbers to shapte the turn-off switching trajectory of controlled switches.trajectory of controlled switches. • • Limit didt during device turn-onLimit didt during device turn-on

DIODE SNUBBERSDIODE SNUBBERS

BJT SNUBBERS BJT SNUBBERS

OVER VOLTAGE SNUBBEROVER VOLTAGE SNUBBER

OTHER DEVICES OTHER DEVICES

COMBINATION OF TURN-COMBINATION OF TURN-ON AND TURN-OFF ON AND TURN-OFF

SNUBBERSSNUBBERS

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