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.

Circuit Connection

 

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 V_BR should not exceed the abs max rating for the IC it is protecting.

Clamping voltage (VCL)
Voltage V_CL is always defined for a given peak pulse current (I_PP).

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 (V_CL) of the selected diode. TVS diode data sheets typically list the V_CL for a given IPP in the part selection table. For other peak pulse currents, however, this V_CL may not be valid. To accommodate other IPP values, the TVS diode manufacturer gives a formula in the data sheet to calculate the V_CL. Use this formula to calculate the V_CL for a given I_PP.

4.      Confirm that the calculated V_CL is less than the specified abs max rating for the pin.