A TVS diode is used to protect electronic circuits from short-duration voltage spikes. It is commonly found in power supplies, communication equipment, automotive electronics, industrial controls and other electronic products.
Choosing a TVS diode is not simply a matter of selecting the highest power rating. The device needs to match the normal operating voltage of the circuit and provide suitable protection during a transient event.
The first specification to check is the standoff voltage, often called VRWM.
This is the maximum continuous voltage that the TVS diode can withstand without entering its breakdown region.
The standoff voltage should be higher than the highest normal operating voltage of the circuit.
For example, if a DC power rail normally operates around 12 V, a TVS diode designed for a much lower working voltage could conduct during normal operation. That would cause unwanted current and may affect the circuit.
The breakdown voltage is the voltage at which the TVS diode begins to conduct significantly.
It is usually specified as VBR in the datasheet.
There is normally a range rather than one exact value. The buyer should check the minimum and maximum values and make sure they are suitable for the circuit.
The breakdown voltage should be above the normal operating voltage but low enough to provide effective protection.
Clamping voltage is another important specification.
When a transient pulse occurs, the TVS diode conducts and limits the voltage seen by the protected circuit. The voltage remaining during this event is the clamping voltage.
A lower clamping voltage generally provides better protection, but the complete relationship between working voltage, breakdown voltage and clamping voltage needs to be considered.
The protected device must be able to withstand the remaining voltage.
TVS diodes are often rated by their peak pulse power.
This indicates how much transient energy the device can handle under a specified test waveform.
Common ratings may be expressed in watts, but the number alone does not tell the whole story. The datasheet will normally specify the pulse waveform and duration used for the rating.
A TVS diode intended for a small signal interface may need a very different rating from one used on a power input.
TVS diodes are available in unidirectional and bidirectional versions.
A unidirectional TVS diode behaves similarly to a Zener diode in one direction and provides forward conduction in the opposite direction.
A bidirectional TVS diode provides protection for voltage transients in both directions.
For a DC power supply, a unidirectional device is often suitable. For AC lines or differential signals where both positive and negative transients need to be controlled, a bidirectional device may be more appropriate.
The actual circuit should always be checked before selecting the polarity.
The correct TVS diode depends heavily on where it will be installed.
For example, different requirements may apply to:
DC power inputs
USB interfaces
CAN bus
RS-485
Ethernet
Automotive systems
Industrial control equipment
Battery systems
A TVS diode designed for a power rail may not be suitable for a high-speed communication interface because its capacitance could affect signal quality.
For high-speed signal applications, junction capacitance is important.
A TVS diode with excessive capacitance can load the signal line and affect signal integrity.
This is especially important for USB, Ethernet and other high-speed interfaces.
For these applications, look for a protection device specifically designed for high-speed data lines rather than choosing a general-purpose power TVS diode.
The package affects both electrical and thermal performance.
Small surface-mount packages are convenient for compact PCB designs, while larger packages may provide higher power handling.
The PCB layout is also important. A TVS diode should normally be placed close to the connector or power entry point where the transient enters the circuit.
Long PCB traces between the protection device and the protected circuit can add unwanted inductance and reduce protection effectiveness.
The expected transient environment should also be considered.
A circuit inside an office device may experience different transients from equipment installed in an industrial environment or vehicle.
The designer should consider the expected surge voltage, pulse duration, repetition and energy before selecting the TVS diode.
For automotive and industrial applications, the relevant testing standards may also influence the required device specifications.
Before purchasing a TVS diode, compare the actual datasheet values rather than relying only on the product name.
Important parameters include:
VRWM
VBR
VC
IPP
Peak pulse power
Capacitance
Polarity
Package
Operating temperature
These specifications should match the circuit's normal voltage and expected transient conditions.
For production projects, the supplier is also worth considering.
Buyers should check whether the supplier can provide consistent specifications, product traceability and suitable quality documentation. For large-volume applications, it is useful to confirm production capacity and long-term availability before approving the component.
A good TVS diode is one that protects the circuit without interfering with normal operation. The best choice therefore comes from matching the device's working voltage, clamping performance, pulse capability and electrical characteristics to the actual application.
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