Transient-voltage-suppression diode
Transients are temporary spikes or surges in voltage or current that can impact circuits in minor glitches to complete
failure. A voltage transient can be anywhere from a few millivolts to thousands
of volts, and they can last from nanoseconds to hundreds of milliseconds. Some
transients are repetitive, such as those caused by inductive ringing in a
motor, while other transients are more sporadic, such as ESD events.
The symbol of TVS diode is as shown below.
Reverse standoff voltage (VRM)
This voltage is also
known as maximum working peak voltage. This
is the normal operating threshold voltage of the device. The TVS diode will
appear as high impedance to the protected circuit when the applied voltage
across the diode is less than this threshold.
Reverse breakdown voltage (VBR)
This voltage is also known as breakdown voltage, this is the voltage threshold
at which the TVS diode begins to conduct a specified amount of current. Voltage
VBR should not
exceed the abs max rating for the IC it is protecting.
Clamping voltage (VCL)
Voltage VCL is
always defined for a given peak pulse current (IPP).
Peak pulse current (Ipp)
This current is the maximum surge current the TVS diode can withstand without
damage. The peak pulse current is defined based on the surge-current transient
waveform, which in most industrial applications is rated at 8/20µs with 8µs
representing rise time to peak value and 20µs representing pulse duration until
the current falls to 50% of peak value.
Follow
these steps to select a TVS diode
1.
Select a diode with a standoff voltage that is higher than
the normal operating voltage. Ensure that the TVS diode maximum clamping
voltage is less than the abs max rating of all the devices on the line to be
protected. It is important to consider operation during both the transient
event (a higher conduction current will result in a higher clamping voltage,
for example) and during normal operation when a transient event is not present.
2.
Verify that the specified peak current exceeds the expected
peak current. Ensure that the diode is specified to handle the required power
during a transient event. Diodes that are too small, or are not designed for a
given current, may fail and cause the circuit to be destroyed during a surge or
EFT event.
3.
Calculate the maximum clamping voltage (VCL) of
the selected diode. TVS diode data sheets typically list the VCL for
a given IPP in the part selection table. For other peak pulse currents,
however, this VCL may not be valid. To accommodate other IPP
values, the TVS diode manufacturer gives a formula in the data sheet to
calculate the VCL. Use this formula to calculate the VCL for
a given IPP.
4.
Confirm that the calculated VCL is less than
the specified abs max rating for the pin.
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