TVS Diodes for Transient Voltage Suppression

Transient voltage and current interference are primary causes of damage to electronic circuits and equipment, often leading to significant losses. This interference typically originates from sources such as the startup and shutdown of electrical equipment, AC power grid instability, lightning strikes, and electrostatic discharge (ESD). While transient events are nearly ubiquitous, high-performance circuit protection devices known as Transient Voltage Suppressors (TVS) can effectively mitigate them.

A Transient Voltage Suppressor (TVS) diode is a protection device developed based on Zener diode technology. Its circuit symbol is the same as a standard Zener diode, and its physical appearance is similar to a regular diode. When a TVS diode experiences a high-energy transient, it rapidly decreases its impedance—in as fast as 1 picosecond (1x10^-12 s)—to absorb a large current. This clamps the voltage across its terminals to a predefined level, thereby protecting downstream circuit components from damage.

TVS Characteristics and Parameters

Figure 1: TVS Characteristic Curves

TVS Characteristics

Observing a TVS diode with a curve tracer produces the waveform shown on the left in Figure 1. This curve shows a typical PN junction avalanche breakdown, similar to that of a standard Zener diode.

However, this curve only tells part of the story. The graph on the right, typically viewed on a dual-trace oscilloscope, shows the current and voltage waveforms of a TVS diode during a high-current surge. Curve 1, the current waveform, shows the current rising abruptly to its peak before decaying exponentially. This surge could be caused by an event like lightning or overvoltage. Curve 2, the voltage waveform, shows that as the current surges, the voltage across the diode rises but is clamped at a maximum value, Vc. This value is slightly higher than the breakdown voltage (VBR), thereby protecting the downstream circuit.

TVS Parameters

Figure 2: TVS Characteristics and Parameters

Breakdown Voltage (VBR): The voltage at which the TVS impedance drops sharply as it enters an avalanche breakdown state.

Test Current (IT): The current at which the breakdown voltage (VBR) is measured. Typically, IT is 1 mA.

Reverse Standoff Voltage (VRWM): The maximum rated DC operating voltage for the TVS. Below this voltage, the TVS remains in a high-impedance state.

Maximum Reverse Leakage Current (IR): The maximum current that flows through the TVS when the reverse standoff voltage (VRWM) is applied.

Maximum Peak Pulse Current (IPP): The maximum surge current that the TVS can withstand, reflecting its surge suppression capability.

Maximum Clamping Voltage (VC): When the TVS is subjected to a high-energy transient, a large current with a peak value of IPP flows through it. The voltage across its terminals rises from VRWM to VC and does not increase further. After the surge, IPP decays exponentially, and the voltage across the TVS returns to its normal state. The ratio of VC to VBR is known as the clamping factor (Cf = VC / VBR), which is typically between 1.2 and 1.4.

Peak Pulse Power (PP): TVS diodes are categorized by their peak pulse power rating, with common values being 500W, 600W, 1500W, and 5000W. The peak pulse power is calculated as PP = VC * IPP. A higher PP rating means the TVS can handle a larger IPP. The maximum allowable pulse power also depends on the pulse waveform, duration, and ambient temperature.

TVS diodes can handle instantaneous peak pulse currents of several hundred amperes with a response time as fast as 1 picosecond. Under conditions of 25°C and a 1/120 second pulse, they can also handle forward surge currents of 50-200 amperes. Generally, the surge handling capability is specified for non-repetitive pulses. In applications where repetitive pulses occur, designers must consider the pulse duty cycle. The typical specified maximum duty cycle is 0.01%. Exceeding this limit can cause cumulative heating and lead to device failure.

TVS operation is highly reliable and does not suffer from aging issues, even after withstanding thousands of high-energy pulses. Tests show that after 10,000 pulses, the maximum allowable pulse power often remains above 80% of its original value.

Due to their small size, high power handling, fast response, and low cost, TVS diodes are widely used for overvoltage protection in applications such as home appliances, electronic instruments, computer systems, and communication equipment. They are effective for protecting RS232, RS485, and CAN bus ports, ISDN lines, I/O ports, ICs, audio/video inputs, AC/DC power supplies, and for suppressing noise from motors and relays.

How to Select a TVS Diode

1. Determine the DC or continuous operating voltage of the circuit to be protected. For AC circuits, calculate the peak voltage (RMS voltage * 1.414).
2. Select a TVS with a reverse standoff voltage (VRWM) that is equal to or greater than the circuit's operating voltage. This ensures the TVS draws negligible current under normal conditions. If the operating voltage exceeds VRWM, the TVS will enter avalanche mode and interfere with circuit operation.
3. Determine the required peak pulse power (PP). Analyze the expected transient's waveform and duration to select a TVS with a PP rating sufficient to suppress it.
4. The maximum clamping voltage (VC) of the TVS must be lower than the maximum voltage the protected circuit can withstand.
5. Choose between a unidirectional or bidirectional TVS. Bidirectional TVS diodes are used for AC lines or in circuits where transients can be both positive and negative. Unidirectional TVS diodes are sufficient for DC circuits that only need protection from transients of a single polarity. During a reverse-bias surge, the unidirectional TVS enters avalanche breakdown; during a forward-bias surge, it conducts like a standard diode.
6. If the expected surge current (IPP) is known, you can use VC to calculate the required power rating. If the power range is uncertain, it is generally safer to select a TVS with a higher power rating.