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It is more reasonable to use bipolar TVS for communication circuits; Choosing TVS array for multi line protection is more advantageous.
(7) Temperature considerations.
The transient voltage suppressor can operate between -55 ℃ and+150 ℃. If TVS needs to operate at a changing temperature, its reverse leakage current ID increases with the increase; The power consumption decreases with the increase of TVS junction temperature, approximately linearly decreasing by 50% from+25 ℃ to+175 ℃. The breakdown voltage VBR increases by a certain coefficient with the increase of temperature. Therefore, it is necessary to consult relevant product information and consider the impact of temperature changes on its characteristics.
The best way to deal with the damage caused by instantaneous pulses to components is to divert the instantaneous current away from the sensing element. The TVS diode is connected in parallel with the protected circuit on the circuit board. When the instantaneous voltage exceeds the normal operating voltage of the circuit, the TVS diode generates an avalanche, providing an ultra-low resistance path for the instantaneous current. As a result, the instantaneous current is diverted through the diode, avoiding the protected component and maintaining the cut-off voltage of the protected circuit until the voltage returns to normal. After the instantaneous pulse ends, the TVS diode automatically returns to a high resistance state, and the entire circuit enters normal voltage. Many components may experience degradation in their parameters and performance after multiple impacts, but as long as they operate within a limited range, diodes will not be damaged or degraded.
From the above process, it can be seen that when selecting TVS diodes, attention must be paid to the selection of the following parameters:
1. Minimum breakdown voltage VBR and breakdown current IR.
VBR is the minimum breakdown voltage of TVS, and below this voltage, TVS will not experience avalanche at 25 ℃. When a specified 1mA current (IR) flows through the TVS, the voltage applied to the two poles of the TVS is its minimum breakdown voltage V BR. According to the degree of dispersion between the VBR of TVS and the standard value, VBR can be divided into two types: 5% and 10%. For a 5% VBR, V WM =0.85VBR； For a 10% VBR, V WM =0.81VBR。 In order to meet the IEC61000-4-2 international standard, TVS diodes must be able to handle a minimum of 8kV (contact) and 15kV (air) ESD shock. Some semiconductor manufacturers have adopted higher shock resistance standards in their products. For certain portable device applications with special requirements, designers can select components as needed.
2. Maximum reverse leakage current ID and rated reverse cut-off voltage VWM.
VWM is the voltage that a diode can withstand under normal conditions. This voltage should be greater than or equal to the normal operating voltage of the protected circuit, otherwise the diode will continuously cut off the circuit voltage; 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. When the rated reverse cut-off voltage VWM is applied between the two poles of TVS, it is in a reverse cut-off state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID.
3. Maximum clamping voltage VC and maximum peak pulse current IPP.
When a pulse peak current IPP with a duration of 20ms flows through the TVS, the maximum peak voltage appearing at both ends is VC. VC and IPP reflect the surge suppression ability of the TVS. The ratio of VC to VBR is called the clamping factor, which is generally between 1.2 and 1.4. VC is the voltage provided by the diode in the off state, which is the voltage passing through the TVS during ESD surge state. It cannot exceed the maximum voltage that the protected circuit can withstand, otherwise the component is at risk of damage.
4. The rated pulse power of Pppm is based on the maximum cut-off voltage and the peak pulse current at this time.
For handheld devices, a 500W TVS is generally sufficient. The maximum peak pulse power consumption PM is the maximum peak pulse power consumption value that TVS can withstand. At a specific maximum clamping voltage, the greater the power consumption PM, the greater its ability to withstand surge currents. At a specific power consumption PM, the lower the clamping voltage VC, the greater its ability to withstand surge currents. In addition, peak pulse power consumption is also related to pulse waveform, duration, and ambient temperature. Moreover, the transient pulses that TVS can withstand are non repetitive, and the pulse repetition frequency (duration to interval time ratio) specified by the component is 0.01%. If there are repeated pulses in the circuit, the accumulation of pulse power should be considered, which may damage the TVS.
5. The capacitance C is determined by the TVS avalanche junction cross-section and is measured at a specific frequency of 1MHz. 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, C is an important parameter for selecting TVS in data interface circuits. For circuits with higher data/signal frequencies, capacitors in diodes cause greater interference to the circuit, resulting in noise or signal attenuation. Therefore, the capacitor range of the selected component needs to be determined based on the characteristics of the circuit. High frequency circuits generally choose capacitors that are as small as possible (such as LCTVS, low capacitance TVS, capacitors not exceeding 3pF), while circuit capacitors with low requirements for capacitors can be selected with a capacitance higher than 40pF.
Transient voltage suppression diode characteristic curve:
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