Transcript SNUBBERS

SNUBBERS
is a device used to suppress ("snub")
voltage transients in electrical systems,
pressure transients in fluid systems, or
excess force or rapid movement in
mechanical systems.
Electrical systems
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Snubbers are frequently used in
electrical systems with an
inductive load where the sudden
interruption of current flow often
leads to a sharp rise in voltage
across the device creating the
interruption.
This sharp rise in voltage is a
transient and can damage and
lead to failure of the controlling
device. A spark is likely to be
generated (arcing), which can
cause electromagnetic
interference in other circuits. The
snubber prevents this undesired
voltage by conducting transient
current around the device.
RC snubbers
WHY???
Requirements:
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The voltage & current remain must remain
within the safe operating area (SOA) of the
device.
To keep the rate of change of the device voltage
& current low enough to provide correct &
reliable operation during switch transition.
To limit the dissipation that occurs in the device
during switching.
Function of Snubber Circuits
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• Protect semiconductor devices by:
• Limiting device voltages during turn-off transients
• Limiting device currents during turn-on transients
• Limiting the rate-of-rise (didt) of currents through the
semiconductor device at device turn-on
• Limiting the rate-of-rise (dvdt) of voltages across the
semiconductor device at device turn-off
• Shaping the switching trajectory of the device as it
turns on/off
2 basic kinds of snubbers:
1.
2.
To control the rate of rise of the switch
voltage. (turn-off snubbers)
The second controls the rate of rise of
switch current. (turn-on snubbers)
Both use small energy storage elements
– a capacitor and inductor.
TURN-ON SNUBBER CKTs
Turn on Snubber:
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A snubber inductor is placed in series with
the transistor. (Ls) is used to limit the
transistor current.
Practical Turn-on snubber:
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We need to add a components to allow the transistor to survive the switch
transition.
If nothing were added to the ckt., the transistor would receive an impulse of
voltage when it turned off, because the inductor current would be forced to
change in a very short time.
Solution:
 We add resistor to provide alternative path for the inductor current
when the transistor turns off.
 We add diode to keep the resistor from conducting during the turn-on
transition.
Note: the resistance Rs must be large enough to completely discharge
Ls during the off-state. To minimize the transistor’s voltage stress, it
shld. Not be made any larger thatn necessary.
TURN-OFF SNUBBER CKTs
A More Practical Snubber:
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A capacitor by itself is not a sufficient turn-off snubber
because when the transistor is turned on again, Cs will
discharge through Q. the resulting transistor current can
be very large and lead to failure .
Solution:
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We add a resistor to limit the discharge current when the
transistor is turned on.
We add a diode to allow charging current to bypass the resistor
during the turn-off transition.
Tradeoff
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Is necessary in determining the value of
Rs. It must be small enough to ensure the
capacitor is fully discharged during the
shortest time that Q might be on but large
enough to prevent the discharge current
from exceeding the transistor rating.
DISSIPATION IN SNUBBER CIRCUITS
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At the beginning of a turn-on/turn-off
transition, the energy storage element in
the corresponding snubber ckt. Contains
no stored energy. When the transition is
over, however, the element is left with a
nonzero value of stored energy. This
energy is dissipated during the next switch
transition.
RC SNUBBER
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A simple snubber comprising a small resistor (R)
in series with a small capacitor (C) is often
used. This combination can be used to suppress
the rapid rise in voltage across a thyristor.
are also often used to prevent arcing across the
contacts of relays and switches and the electrical
interference and welding/sticking of the contacts
that can occur.
Types of Snubber Circuits
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1. Unpolarized series R-C snubbers
• Used to protect diodes and thyristors
2. Polarized R-C snubbers
• Used as turn-off snubbers to shape the turn-on switching
trajectory of controlled switches.
• Used as overvoltage snubbers to clamp voltages applied to
controlled switches to safe values.
• Limit dvdt during device turn-off
3. Polarized L-R snubbers
• Used as turn-on snubbers to shapte the turn-off switching
trajectory of controlled switches.
• Limit didt during device turn-on
DIODE SNUBBERS
BJT SNUBBERS
OVER VOLTAGE SNUBBER
OTHER DEVICES
COMBINATION OF TURN-ON
AND TURN-OFF SNUBBERS
END
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