Characteristics of TVS diode and its application in circuit design

Transient voltage suppressor is a high-performance protective device in the form of a diode, with extremely fast response time and considerable surge absorption capability. When the two ends of TVS are subjected to reverse transient overvoltage pulses, it can quickly convert the high impedance between the two ends into low impedance to absorb instantaneous high current and clamp the voltage at a predetermined value, effectively protecting the components in the circuit from damage. This article describes the main characteristic parameters and selection precautions of TVS devices, and also provides the application methods of TVS in circuit design.

Characteristics and main parameters of TVS devices

1.1 Device characteristics of TVS

Under the specified reverse application conditions, TVS exhibits a high impedance state towards the protected circuit. When the instantaneous voltage exceeds its breakdown voltage, TVS will provide a low impedance path and divert the instantaneous current flowing to the protected component to the TVS diode through a high current method, while limiting the voltage at both ends of the protected component to the clamping voltage of TVS. After the overvoltage condition disappears, the TVS returns to a high impedance state. Compared with ceramic capacitors, TVS can withstand a voltage of 15 kV, but ceramic capacitors have weaker ability to withstand high voltage. A 5 kV impact will cause about 10% of ceramic capacitors to fail, and at 10 kV, the damage rate will reach as high as 60%.

1.2 Main parameters of TVS devices

(1) Minimum breakdown voltage VBR

When a specified current flows through the TVS, the voltage at both ends of the TVS is called the minimum breakdown voltage, and in this region, the TVS exhibits a low impedance path. At 25 ℃, below this voltage, TVS will not experience avalanche breakdown.

(2) Rated reverse turn off voltage VWM

VWM is the voltage that TVS can withstand under normal conditions, and this voltage should be greater than or equal to the normal operating voltage of the protected circuit. But it also needs to be as close as possible to the normal operating voltage of the protected circuit, so as not to expose the entire circuit to overvoltage threats before TVS operation. According to the dispersion between the VBR of TVS and the standard value, VBR can be divided into two types: 5% and 10%. For a VBR of 5%, VWM=0.85 VBR leg; For 10% VBR, VWM=0.81VBR。

(3) Maximum peak pulse current IPP

IPP is the maximum pulse peak current that TVS allows the device to pass through under specified pulse conditions when operating in reverse state.

(4) Clamping voltage Vc

When the pulse peak current Ipp flows through the TVS, the maximum voltage value appearing at both ends is called the clamping voltage Vc. Vc and Ipp reflect the surge suppression capability of the TVS. The ratio of Vc to VBR is usually referred to as the clamping factor (coefficient), and its value is generally between 1.2 and 1.4. In practical use, Vc should not exceed the maximum allowable safe voltage of the protected circuit, otherwise the protected device may face the possibility of damage.

(5) Maximum peak pulse power consumption PM

PM is usually the product of the maximum peak pulse current Ipp and the clamping voltage Vc, which is the maximum peak pulse power consumption. It is the maximum peak pulse power consumption value that TVS can withstand. At a given maximum clamping voltage, the greater the power consumption PM, the greater its ability to withstand surge currents. In addition, peak pulse power consumption is also related to pulse waveform, pulse duration, and ambient temperature. Moreover, the transient pulses that TVS can withstand cannot be repeatedly applied.

(6) Capacity C

The capacitance of TVS is determined by the cross-sectional area of its silicon wafer and bias voltage, measured at a specific frequency of 1 MHz. The size of C is directly proportional to the current carrying capacity of TVS. If C is too large, it will cause signal attenuation. Therefore, capacitor C is an important parameter for selecting TVS in data interface circuits.

(7) Leakage current IR

IR is the leakage current of the TVS transistor when the maximum reverse operating voltage is applied to the TVS. When TVS is used in high impedance circuits, this leakage current IR is an important parameter.

2 Precautions for selecting TVS

When selecting TVS, the following main factors should generally be considered based on the specific situation of the circuit:

Firstly, bidirectional TVS can absorb instantaneous high pulse power in both positive and negative directions and clamp the voltage to a predetermined level. Therefore, if the circuit is likely to withstand surge voltage from two directions, a bidirectional TVS should be selected. Bidirectional TVS is generally suitable for AC circuits, while unidirectional TVS is generally used for DC circuits. In addition, the clamping voltage Vc shall not exceed the maximum allowable safe voltage of the protected circuit.

Secondly, the maximum peak pulse power consumption PM must be greater than the maximum transient surge power that occurs in the circuit. However, in practical applications, surges may repeatedly occur. In this case, even if the individual pulse energy is much smaller than the pulse energy that TVS devices can withstand, if repeatedly applied, these individual pulse energies may accumulate and exceed the pulse energy that TVS devices can withstand in some cases. Therefore, in circuit design, it is necessary to carefully consider and select suitable TVS devices to ensure that the accumulation of repeated pulse energy within the specified interval time does not exceed the rated pulse energy of the TVS device.

Thirdly, in practical applications, the maximum reverse operating voltage of the TVS tube must be selected correctly. The general principle is to select the maximum reverse operating voltage of the TVS tube at 1.4 times the AC voltage. The maximum reverse operating voltage of the TVS tube is selected based on 1.1 to 1.2 times the DC voltage.

Application of TVS in Circuit Design

In practical application circuits, the best way to deal with the damage caused by instantaneous pulses to devices is to divert the instantaneous current away from sensitive devices. To achieve this goal, the TVS is connected in parallel with the protected circuit on the circuit board. In this way, when the instantaneous voltage exceeds the normal operating voltage of the circuit, TNS will undergo avalanche breakdown, providing an ultra-low impedance path for the instantaneous current. As a result, the instantaneous current is diverted through TVS, avoiding the protected device and maintaining the cut-off voltage of the protected circuit until the voltage returns to normal. After that, when the instantaneous pulse ends, the TVS diode automatically returns to the high resistance state, and the entire circuit enters the normal voltage state.