一 Principle of operation of TVS diode

TVS diodes operate similarly to common voltage regulator diodes, but they conduct when the breakdown voltage exceeds their nominal value and exhibit higher current conduction capability. When its two ends are subjected to reverse transient high energy impact, TVS diodes can respond at a speed of 10^-12 seconds, quickly transform impedance, and absorb up to several kilowatts of surge power, thus ensuring that the voltage between the two poles is clamped at a safe value, to protect the precision components in the electronic circuits from being damaged by the surge pulse.

The core operating principle of the TVS diode is that when a circuit is subjected to an instantaneous overvoltage, the TVS device is able to rapidly reduce its internal impedance, thereby allowing a large amount of current to pass and limiting the voltage to a safe predetermined level. In reverse application conditions, when a TVS device is subjected to a high-energy transient overvoltage pulse, its operating impedance is immediately reduced to a very low level, which occurs in a very short period of time, with a clamping response time of up to 1 ps (10^-12 seconds). This fast response allows TVS devices to effectively absorb transient pulse power with a capacity of thousands of watts.

In addition, TVS diode allow the passage of considerable forward surge currents, which under certain conditions (e.g., temperature T=25°C, time T=10ms) can reach values of 50 to 200 amperes. Bidirectional TVS diode are capable of absorbing transient large pulses of power in both the forward and reverse directions, which makes them particularly suitable for AC circuits. In contrast, unidirectional TVS diode are typically used in DC circuits.

二 Electrical Characterization of TVS diode

  1. unidirectional TVS diode V-I characteristics are shown in its forward characteristics similar to ordinary voltage regulator diodes, and in the reverse characteristics, when the breakdown voltage is reached, there will be an approximate right-angle hard breakdown phenomenon, which is a typical PN junction avalanche device characteristics. When the circuit is subjected to transient overvoltage pulses, the current of the unidirectional TVS will increase dramatically, while the reverse voltage will rise to the clamping voltage value and maintained at this level, thus protecting the circuit from damage.
  2. The V-I characteristics of a bidirectional TVS diode are like two unidirectional TVS diodes combined back to back. This means that it has the same avalanche breakdown and clamping characteristics in both forward and reverse directions. There is a symmetrical relationship between the forward and reverse breakdown voltages of a bi-directional TVS, i.e., the ratio of the forward breakdown voltage to the reverse breakdown voltage is between 0.9 and 1.1. When the disturbing voltage applied to the terminals exceeds the clamping voltage Vc, the bi-directional TVS is able to suppress this voltage immediately and keep it at a safe level. This feature makes bi-directional TVSs ideal for protecting AC circuits.

三 Main electrical parameters of TVS diode

  1. breakdown voltage V (BR): this is an important parameter of the TVS diode, refers to the specified test current I (BR), the measured voltage across the device. When this voltage is reached, the TVS diode will break down, thus transforming into a low impedance state and allowing high current to pass through.
  2. Maximum Reverse Peak Pulse Current IPP: This parameter describes the maximum peak pulse current that the TVS diode can withstand during reverse operation. the product of IPP and the maximum clamping voltage Vc(MAX) determines the maximum transient pulse power of the TVS diode. It is critical to properly select the TVS diode to ensure that its transient pulse power rating, PPR, is greater than the maximum transient surge power that may occur in the protected device or line.
  3. Maximum Reverse Working Voltage VRWM: This is the value of the voltage across the device terminals when the TVS diode is operated in reverse at a specified IR. Typically, the value of VRWM is between 0.8 and 0.9 times V(BR). At this voltage, the power consumption of the TVS diode is very small, and it should be used to ensure that the VRWM is not lower than the normal operating voltage of the protected device or line.
  4. Maximum clamping voltage Vc(max): This is the maximum value of voltage that can be reached across the TVS diode under the action of the peak pulse current Ipp. This parameter is essential to protect the components in the circuit from over-voltage damage.
  5. Reverse Pulse Peak Power PPR: This parameter depends on the peak pulse current IPP and the maximum clamping voltage Vc(max), and is also affected by the pulse waveform, the pulse time and the ambient temperature.PPR is an important index to measure the TVS diode protection capability.
  6. Capacitance CPP: The capacitance of the TVS diode is determined by the area of the wafer and the bias voltage. With zero bias, the capacitance value decreases as the bias voltage increases. The size of the capacitance directly affects the response time of the TVS diode and therefore requires special attention in certain applications.
  7. Leakage Current IR: When the maximum reverse operating voltage is applied to the TVS diode, the TVS diode will have a leakage current IR. In high impedance circuits, this leakage current is an important consideration as it may affect the overall performance of the circuit.

四 Classification of TVS diode

TVS diodes can be classified into various types according to different classification criteria:

  1. Categorized by polarity:
    Unipolar TVS diodes: provide protection in one direction only and are suitable for DC circuits.
    Bipolar TVS diodes: provide protection in both positive and negative directions, suitable for AC circuits.
  2. According to the use of classification:
    General purpose TVS diodes: applicable to a variety of circuit protection occasions.
    Specialized TVS diodes: TVS diodes designed for specific applications or industries, such as for automotive electronics, medical equipment and so on.
  3. According to the package and internal structure classification:
    Axial leaded diodes: traditional package form, peak power up to 400W, 500W, 600W, 1500W and 5000W, suitable for high power applications such as power feeders.
    Dual in-line TVS arrays: suitable for multi-channel protection and easy to mount on printed circuit boards.
    SMD TVS Diodes: Surface Mount Technology (SMT) packages for high density mounting applications such as portable devices.
    High Power Modules: Specially designed for high power applications such as power supply protection.

Different types of TVS diodes are suitable for different circuit protection needs, and choosing the right TVS diode is critical to ensure circuit stability and safety.

五 Selection principles for TVS diode

When selecting a TVS diode, there are several key factors to consider to ensure effective circuit protection:

  1. Polarity selection: If the circuit may be subjected to spike pulse voltage (surge voltage) impact from two directions, bipolar TVS diodes should be selected. Otherwise, you can choose unipolar TVS diodes.
  2. Clamping voltage Vc: the selected TVS diode Vc value should be lower than the maximum voltage of the protected component. Vc is the diode voltage in the cut-off state, that is, the ESD shock state through the TVS diode voltage. This voltage should not exceed the tolerable limit voltage of the protected circuit to avoid damage to the device. Normal operating state: TVS diode should not be in the normal operating state should not be in the breakdown state, preferably in the maximum reverse operating voltage VR below. Selection should take into account the requirements of both VR and VC.
  3. Peak pulse power consumption PPR: If the exact inrush current IPP is known, Vc and Ipp can be used to determine the power. If it is not possible to determine the range of IPP, should choose a TVS diode with higher power. ppr is the maximum peak pulse power dissipation value that the TVS diode can withstand. The greater the power dissipation PM, the greater the inrush current tolerance for a given maximum clamping voltage.
  4. Pulse repetition frequency: TVS diode can withstand the transient pulse is not repeated, its specified pulse repetition frequency of 0.01%. If repetitive pulses occur within the circuit, the accumulation of pulse power should be considered to avoid damage to the TVS diode.
  5. Current-limiting resistor: for the protection of small-current loads, you can add a current-limiting resistor in the circuit to reduce the interference current, so that it may be possible to select a TVS diode with less peak power for protection.
  6. Capacitance C: TVS diode capacitance is determined by its avalanche junction cross-section, measured at a specific frequency (eg, 1 MHz). the size of C is proportional to the current carrying capacity of the TVS diode, but too much capacitance will make the signal attenuation. Therefore, in data interface circuits, a suitable capacitance range needs to be selected according to the characteristics of the loop.
  7. IEC61000-4-2 standard: TVS diodes must be able to handle ESD shocks of at least 8 KV (MB, contact) and 15 kV (BM, air) to meet the IEC61000-4-2 international standard. For portable device applications with special requirements, designers can select devices with higher standards as needed.

More Information

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