Detailed explanation of core parameters of ESD static tube (TVS diode)
ESD static tube, also known as transient voltage suppression diode (TVS diode), is a key component that protects electronic devices from transient overvoltage damage such as electrostatic discharge (ESD), electrical fast transient (EFT), and surge. Its core parameters determine its protection performance and application scenarios. The main core parameters are as follows:
Breakdown voltage
Function: Define the critical point at which the TVS tube begins to significantly conduct and clamp the voltage. This is the voltage threshold at which the TVS transistor switches from a high resistance state (off) to a low resistance state (on).
Key points:
Nominal value: usually measured at a specific test current (such as 1mA), denoted as' V_ {BR} '.
Selection criteria: It must be higher than the maximum normal operating voltage (V_WM) of the protected circuit and leave sufficient margin (usually 15% -20%) to prevent accidental triggering of conduction during normal operation. For example, to protect 5V lines, VBR usually selects 6V or higher.
Clamp voltage
Function: Define the maximum voltage across the TVS tube when subjected to a specified peak pulse current (` I2 {PP} `). This is the highest voltage that the protected device can actually withstand.
Key points:
Core protection indicator: It must be lower than the maximum withstand voltage (V_MAX) of the protected device and leave a safety margin. This is the most direct parameter for evaluating the protection effect of TVS tubes.
Relationship with breakdown voltage: Generally, V_CLAMP is significantly higher than V_BR because as the current increases after conduction, the voltage across the TVS tube will rise (dynamic resistance effect).
Dynamic characteristics: measured under specific waveforms (such as 8/20 μ s) and peak currents, and must be selected with reference to these conditions.
Peak pulse current
Function: Define the maximum instantaneous current that TVS tubes can safely withstand without damage under specified pulse waveforms (such as standard ESD waveform IEC 61000-4-2, or surge waveform 8/20 μ s).
Key points:
Current endurance index: reflects the ability of TVS tubes to absorb transient energy.
Selection criteria: The selection should be based on the highest transient energy level that may be encountered in the application environment (such as ESD level Air 15kV, Contact 8kV). `The larger the value of I2 {PP} `, the stronger the impact that the TVS tube can withstand, usually resulting in higher volume and cost.
Waveform dependence: The 'I2 {PP}' values under different test waveforms cannot be directly compared.
Response time
Function: Define the delay time of TVS tube from detecting overvoltage to starting effective clamping.
Key points:
Speed indicator: Extremely fast response is the key to effectively suppressing nanosecond level transient events such as ESD.
Typical value: The response time of TVS tubes is usually very fast, less than 1 nanosecond (picosecond level), far faster than other protective devices (such as varistors MOV). This parameter is usually not the main selection bottleneck.
Reverse turn off voltage
Function: Define the maximum reverse DC voltage that a TVS tube can continuously withstand under normal operating conditions (without transient impact).
Key points:
Compatibility of operating voltage: It must be greater than or equal to the maximum continuous operating voltage (V_WM) of the protected circuit. For example, to protect the 12V power cord, VRWM should choose 12V or 14V, etc.
Relationship with breakdown voltage: 'V_RWM' is usually slightly lower than 'V_BR' (at 1mA), ensuring that the TVS is in a high impedance turn off state under normal operating voltage.
Reverse leakage current
Function: Defined as the small current flowing through the TVS tube under the reverse turn off voltage (V_RWM).
Key points:
Static power consumption index: It is particularly important in low-power or battery powered devices.
Typical values: typically at microampere level (μ A) or even nanoampere level (nA). The smaller the leakage current, the better, indicating that the TVS has less impact on the circuit during normal operation.
Junction capacitance
Function: Refers to the inherent parasitic capacitance of TVS tubes in an non-conductive state.
Key points:
Signal integrity impact: It has a significant impact on the performance of high-speed data lines such as USB 3. x, HDMI, Ethernet, and DisplayPort.
Selection criteria:
High speed signal line: TVS tubes with ultra-low junction capacitance (usually between 0.5pF and 3pF or even lower) must be selected to avoid signal attenuation, distortion, and errors.
Power line or low-speed signal line: The capacitance requirements are relatively loose (tens of pF to hundreds of pF or even higher), and lower cost models can be chosen.
Packaging and power dissipation
Function: The packaging form determines the physical size, installation method, and most importantly, the thermal performance (heat dissipation capacity) of the TVS tube.
Key points:
Associated with 'I2 {PP}': TVS tubes that withstand high currents (high I2 {PP} ') typically require larger packaging to effectively dissipate heat.
Miniaturization requirements: Portable devices require extremely small packaging (such as DFN1006/0402, SOD-882).
Power requirements: For situations where multiple or prolonged transient events may occur (such as surges), the thermal resistance and power dissipation capacity of the package need to be considered.
Summary and selection logic:
When choosing ESD static tubes (TVS diodes), it is necessary to systematically consider these core parameters:
Determine the work environment: Identify the types of threats that require protection (ESD, EFT, surge)? ）And grade (such as IEC 61000-4-2 grade).
Matching circuit voltage:
Ensure that 'V_ {RWM}'>='V_ {WM}' (maximum operating voltage).
Ensure that ` V_ {BR} `>` V_ {WM} ` (with margin).
Most critical: Ensure that under the worst transient impact, ` V_CLAMP `<` V_MAX ` (the protected device withstand voltage, with margin).
Evaluate transient energy: Select sufficient TVS tubes based on the threat level for 'I2 {PP}'.
Considering signal speed: High speed signal lines must choose models with ultra-low junction capacitance (` C_j `).
Pay attention to static effects: Low power applications need to focus on low reverse leakage current (` I2R `).
Physical limitations: Choose appropriate packaging based on PCB space and heat dissipation requirements.
Understand the interrelationships and trade-offs between these parameters (for example, low clamping voltage often means lower 'I2 {PP}' or higher cost; The ultra-low capacitance may limit the maximum 'I2 {PP}', which is the key to successfully selecting and applying ESD electrostatic tubes.