TVS Diodes
A TVS diode (Transient Voltage Suppressor) is a semiconductor device designed to protect electronic circuits from short-duration voltage transients and electrical surges. TVS diodes respond very quickly and can divert transient current away from sensitive components, helping limit the voltage to a safer level.
What Is a TVS Diode?
A TVS diode is a specialized diode designed to suppress short-duration voltage transients.
Normal circuit operation may involve a relatively stable voltage, but electronic equipment can also experience sudden voltage spikes caused by switching, inductive loads, electrostatic discharge, lightning-related transients, or other electrical disturbances.
A TVS diode is normally connected across the circuit that requires protection. Under normal voltage conditions it has little effect on the circuit. When a sufficiently large transient occurs, the TVS conducts strongly and diverts current away from the protected circuitry.
Why Voltage Transients Are Dangerous
Electronic components such as microcontrollers, MOSFETs, communication ICs, sensors, and other semiconductor devices can have relatively low maximum voltage ratings.
A short voltage spike that lasts only microseconds or milliseconds can damage a component even when the normal operating voltage is completely safe.
For example:
Normal voltage:
─────────────── 12 V ───────────────
Transient:
───────────┐
│
│ Voltage spike
│
───────────┴────────────────────────
A properly selected TVS diode can clamp such a transient and reduce the voltage seen by the protected circuit.
TVS Diode Symbol
Protected Line
│
│
─┴─
/ \
/ TVS \
\ /
\___/
│
GND
The schematic symbol is based on the Zener-diode concept because TVS devices operate using controlled avalanche or breakdown behavior.
The exact symbol and markings depend on whether the TVS device is unidirectional or bidirectional.
How a TVS Diode Works
A TVS diode normally remains in a high-impedance state while the circuit voltage is within its normal operating range.
When a transient raises the voltage beyond the device's specified breakdown region, the TVS begins conducting heavily.
Normal voltage
│
▼
TVS mostly OFF
│
▼
Circuit operates normally
Transient voltage
│
▼
TVS conducts strongly
│
▼
Transient current diverted
│
▼
Voltage clamped
The TVS absorbs or diverts the transient energy for the duration of the event, subject to its pulse-energy and current ratings.
TVS Diode Protection Concept
Protected Circuit
│
│
Input ──────────────┼──────────────
│
TVS
│
GND
The TVS is connected in parallel with the protected circuit.
During a transient, the TVS provides a low-impedance path for the surge current, limiting the voltage across the protected circuit.
Stand-Off Voltage (VRWM)
One of the most important TVS specifications is the stand-off voltage, commonly represented as VRWM.
VRWM is the maximum continuous working voltage that can normally be applied to the TVS without causing significant conduction.
The selected TVS should have a stand-off voltage appropriate for the normal operating voltage of the protected circuit.
Choosing a stand-off voltage that is too low can cause the TVS to conduct during normal operation.
Breakdown Voltage (VBR)
The breakdown voltage, commonly represented as VBR, is the voltage range at which the TVS begins entering its avalanche or breakdown conduction region under specified test conditions.
The breakdown voltage is normally higher than the stand-off voltage.
VRWM < VBR
The exact values and test currents must be obtained from the manufacturer's datasheet.
Clamping Voltage (VC)
The clamping voltage, commonly represented as VC, is the voltage measured across the TVS while it is conducting a specified peak pulse current.
This is one of the most important specifications when determining how much voltage the protected circuit may experience during a transient.
The clamping voltage depends on the current through the device.
A simplified relationship is:
Higher surge current
│
▼
Higher clamping voltage
Peak Pulse Current
TVS diodes are designed to handle short-duration pulses rather than continuous high current.
The maximum peak pulse current depends on the device and the specified waveform.
The manufacturer's datasheet may specify the peak pulse current for a standardized test waveform.
The actual transient in the application may have a different waveform, so the TVS must be selected according to the real transient environment.
Peak Pulse Power
TVS devices are commonly specified by their peak pulse power capability.
A simplified estimate is:
P ≈ VC × IPP
where:
- VC = clamping voltage
- IPP = peak pulse current
This relationship provides a useful estimate, but the actual device rating must be interpreted using the manufacturer's specified test waveform and conditions.
Transient Energy
A transient contains energy that the protection system must handle.
The energy depends on the voltage, current, and duration of the event.
A TVS with an inadequate pulse-energy capability may overheat or fail during a surge.
For repeated or high-energy surges, additional protection components may be required.
Unidirectional TVS Diodes
A unidirectional TVS behaves similarly to a Zener diode in reverse bias.
In the forward direction it behaves approximately like a conventional diode.
Normal polarity: TVS remains largely non-conductive Positive transient: TVS enters avalanche and clamps voltage Negative transient: Forward conduction occurs ```Unidirectional TVS devices are commonly used on DC power rails and other circuits where the polarity of the normal signal is known.
Bidirectional TVS Diodes
A bidirectional TVS is designed to suppress positive and negative voltage transients with approximately symmetrical behavior.
This makes bidirectional devices useful on:
- AC lines
- Differential communication lines
- Audio lines
- Signal lines that swing in both directions
The exact electrical behavior depends on the specific device.
Unidirectional vs Bidirectional TVS
| Characteristic | Unidirectional | Bidirectional |
|---|---|---|
| Positive transient | Clamps through avalanche | Clamps through avalanche |
| Negative transient | Forward conduction | Clamps in the opposite direction |
| Typical applications | DC power rails | AC and bipolar signal lines |
| Polarity | Important | Less polarity-dependent |
TVS Diodes on DC Power Rails
A common application is protecting a DC power input.
+12 V Input
│
├────────────── Circuit
│
TVS
│
GND
The TVS should have a stand-off voltage above the normal operating voltage but a clamping voltage low enough to protect the downstream electronics.
TVS Diodes in Automotive Electronics
Automotive electrical systems can experience substantial voltage transients from switching inductive loads and other electrical events.
TVS devices can be used to protect:
- Electronic control units
- Sensor circuits
- Communication interfaces
- Power inputs
- Automotive accessories
Automotive applications require components specifically selected for the vehicle's voltage range, transient environment, temperature range, and reliability requirements.
TVS Diodes for ESD Protection
Specialized low-capacitance TVS devices are commonly used to protect electronic interfaces from electrostatic discharge.
Examples include:
- USB interfaces
- Ethernet interfaces
- HDMI interfaces
- CAN interfaces
- RS-485 interfaces
- General-purpose communication lines
For high-speed interfaces, the TVS capacitance is particularly important because excessive capacitance can degrade signal integrity.
TVS Diodes on Signal Lines
Protection devices can be connected between a signal line and ground to provide a path for transient currents.
Signal ─────────────── Circuit │ TVS │ GND
The protection device must be selected so that it does not interfere with the normal signal while still responding quickly enough to the expected transient.
TVS Diodes and Inductive Loads
Inductive loads such as motors, relays, solenoids, and transformers can generate voltage transients when their current is interrupted.
The stored magnetic energy can produce a voltage spike.
Inductive Load
│
│
Switch ──────── OFF
│
▼
Inductive voltage spike
│
▼
TVS protection
```
A suitable TVS can provide a controlled path for the transient energy and
reduce the voltage stress on switching components.
TVS vs Zener Diode
| Characteristic | TVS Diode | Zener Diode |
|---|---|---|
| Primary purpose | Transient protection | Voltage reference, regulation, and low-power clamping |
| Pulse handling | Designed for transient pulses | Usually lower-energy operation |
| Typical application | Surge and ESD protection | Voltage reference and simple regulation |
| Response | Very fast | Very fast |
| Capacitance | Depends on device | Depends on device |
A TVS diode can be considered a specialized high-energy transient suppression device, while ordinary Zener diodes are commonly used for voltage regulation, references, and lower-energy limiting applications.
TVS vs MOV
| Characteristic | TVS Diode | MOV |
|---|---|---|
| Response speed | Very fast | Fast |
| Typical use | Electronic circuit protection | Surge protection |
| Size | Often compact | Varies |
| Capacitance | Can be important in signal applications | Can be significant |
| Typical application | Power rails and signal interfaces | Mains and larger surge environments |
The appropriate protection technology depends on the voltage, current, energy, repetition rate, response time, and circuit requirements.
TVS Diode Response Time
TVS diodes respond very rapidly to voltage transients.
This makes them particularly useful for protecting modern semiconductor devices from fast events such as ESD and switching spikes.
The complete protection system response also depends on PCB layout and the inductance of the current path.
PCB Layout for TVS Protection
A TVS diode can only protect a circuit effectively if the transient current has a low-inductance path.
The TVS should normally be placed close to the connector or point where the transient enters the circuit.
Connector │ ├──── TVS ─── GND │ └──────────── Protected IC
Long traces between the connector, TVS, and ground can introduce inductance that increases the voltage seen by the protected device during a very fast transient.
TVS Diode for USB Protection
USB interfaces require specialized protection devices because the data lines operate at relatively high speeds.
A suitable low-capacitance TVS array can protect the data lines without introducing excessive capacitive loading.
The exact protection device depends on the USB standard and signal speed.
TVS Diodes for Communication Interfaces
Communication interfaces can be exposed to electrostatic discharge and electrical transients through cables and external connectors.
TVS protection is commonly used for interfaces such as:
- RS-232
- RS-485
- CAN
- Ethernet
- Industrial sensor interfaces
The protection device must have suitable working voltage and sufficiently low capacitance for the interface.
Choosing the Stand-Off Voltage
The TVS stand-off voltage should be higher than the highest normal continuous voltage that the protected circuit is expected to experience.
If the stand-off voltage is too low, the TVS can conduct during normal operation.
If it is excessively high, the TVS may allow a damaging transient voltage to reach the protected component before significant clamping occurs.
The correct selection therefore requires balancing:
- Normal operating voltage
- Transient voltage
- Protected component maximum voltage
- TVS clamping voltage
Choosing the Clamping Voltage
The clamping voltage must be low enough that the protected circuit remains within its safe voltage range during the expected transient.
For example, if a sensitive component has a maximum allowable transient voltage, the TVS must be selected so that its actual clamping voltage under the expected surge current remains below that limit.
Do not compare clamping voltage without also considering the specified peak pulse current.
TVS Dynamic Resistance
A TVS does not behave like an ideal voltage clamp.
Its voltage increases as the current through it increases.
A simplified representation is:
VC ≈ VBR + IPP × RD
where RD represents an effective dynamic resistance.
The actual device behavior is nonlinear and should be taken from the manufacturer's datasheet and transient curves.
TVS Capacitance
TVS devices have capacitance.
For power-line protection, this capacitance may have little practical importance.
For high-speed communication lines, however, excessive capacitance can attenuate signals or distort their edges.
Low-capacitance TVS devices are therefore available for high-speed interfaces.
TVS Diode Testing
A digital multimeter can perform a basic check for an obvious short or open failure, but it cannot fully verify the surge-protection characteristics of a TVS diode.
A healthy TVS may appear similar to a conventional diode during a basic multimeter test.
The actual breakdown and clamping characteristics require controlled test equipment.
Testing a TVS for a Short Circuit
A TVS that has experienced a severe surge may fail short-circuit.
With the device disconnected from the circuit, measure resistance or use an appropriate diode-test function.
A very low resistance in both directions can indicate a failed shorted device.
In-circuit measurements can be misleading because other components may provide parallel current paths.
Testing a TVS for an Open Circuit
A damaged TVS can also fail open.
A basic multimeter may not always distinguish a healthy high-voltage TVS from an open device because the meter's test voltage is normally far below the TVS breakdown voltage.
Therefore, an open-looking reading does not automatically prove that a TVS is faulty.
Testing the Actual Clamping Voltage
Testing the actual clamping voltage requires a controlled transient source or suitable laboratory test equipment.
The test must reproduce an appropriate voltage waveform and current while remaining within the device's specified pulse conditions.
This type of testing should only be performed with equipment designed for the required energy and voltage levels.
Common TVS Diode Faults
| Fault | Possible Symptoms |
|---|---|
| Short circuit | Power supply short, blown fuse, circuit unable to start |
| Open circuit | Protection function lost |
| Excessive leakage | Unexpected current or loading of the protected line |
| Incorrect clamping voltage | Insufficient protection or unwanted conduction |
| Thermal damage | Discoloration, cracking, intermittent operation |
Why TVS Diodes Fail
A TVS diode can fail when the energy of the transient exceeds its capability or when it is exposed to repeated surges beyond its design limits.
Possible causes include:
- Excessive surge current
- Excessive pulse duration
- Repeated transients
- Incorrect voltage selection
- Insufficient thermal management
- Incorrect circuit placement
A failed TVS should therefore be treated as evidence that the circuit may have experienced a significant electrical event.
Replacing a TVS Diode
A replacement TVS should not be selected solely by matching its physical package or nominal voltage.
Compare:
- Unidirectional or bidirectional type
- Stand-off voltage
- Breakdown voltage
- Clamping voltage
- Peak pulse current
- Peak pulse power
- Capacitance
- Package
- Temperature range
For signal interfaces, also compare capacitance and signal-frequency requirements.
TVS Selection Example
Suppose an electronic circuit operates from a nominal 12 V DC supply. The protection device must remain essentially inactive during normal operation but clamp an abnormal positive voltage transient.
The selection process should consider:
- Maximum normal supply voltage
- Expected transient voltage
- Maximum voltage tolerated by the protected circuit
- Required TVS stand-off voltage
- Breakdown voltage
- Clamping voltage at the expected surge current
- Pulse duration
- Peak pulse current
- Required pulse-power capability
The final device should be selected from the manufacturer's transient curves and specifications rather than from the nominal voltage alone.
TVS Diodes and Fuses
A TVS can be combined with a fuse or other current-limiting device.
Supply ── Fuse ──+──── Protected Circuit
│
TVS
│
GND
During a severe and sustained surge, the TVS may conduct enough current to cause the upstream protective device to open.
This can provide a coordinated protection strategy in which the TVS absorbs the transient while the fuse handles an excessive sustained fault.
TVS Diodes and Series Resistance
In some circuits, series resistance or another current-limiting element can work together with a TVS to control surge current.
The complete protection network may therefore contain:
- TVS diode
- Series resistor
- Fuse
- Inductor or common-mode choke
- Filtering capacitors
The appropriate combination depends on the source and nature of the transient.
TVS Diode Selection Checklist
- ✔ Determine the normal operating voltage
- ✔ Determine the maximum continuous voltage
- ✔ Determine the expected transient waveform
- ✔ Select suitable stand-off voltage
- ✔ Check breakdown voltage
- ✔ Check clamping voltage at the expected current
- ✔ Check peak pulse current
- ✔ Check peak pulse power
- ✔ Check pulse duration
- ✔ Check repetition rate
- ✔ Check capacitance for signal lines
- ✔ Select unidirectional or bidirectional type
- ✔ Check package and thermal requirements
- ✔ Confirm the manufacturer's datasheet
Common TVS Selection Mistakes
- Choosing a TVS only by its nominal voltage
- Ignoring the stand-off voltage
- Ignoring clamping voltage
- Ignoring the surge current waveform
- Ignoring pulse duration
- Using a high-capacitance TVS on a high-speed data line
- Using the wrong unidirectional/bidirectional configuration
- Placing the TVS too far from the connector
- Ignoring PCB ground-path inductance
- Using an ordinary Zener when a high-energy TVS is required
Important Safety Notes
- TVS testing can involve dangerous voltages and high transient currents.
- Do not intentionally create high-energy electrical surges without appropriate laboratory equipment.
- Disconnect power before replacing a TVS diode.
- Discharge capacitors before servicing power circuits.
- A failed TVS may indicate that another component or external source caused a severe electrical transient.
Key Points
- TVS diodes protect circuits against short-duration voltage transients.
- They are normally connected in parallel with the protected circuit.
- Important specifications include VRWM, VBR, VC, peak pulse current, and pulse power.
- Unidirectional and bidirectional TVS devices are available.
- Low-capacitance TVS devices are important for high-speed communication lines.
- PCB layout has a major effect on the effectiveness of transient protection.
- A multimeter can detect some obvious TVS failures but cannot fully verify its clamping performance.
- TVS replacement requires matching the complete protection characteristics, not just the package or nominal voltage.