Solid State Discharge Tube (SSD), usually referring to surge protection devices based on semiconductor thyristor structure, such as surge suppression thyristors, is an important transient overvoltage protection device. Its core parameters determine its protection performance and application range. The following is an original summary and analysis of its core parameters, striving for logical clarity:
1. Nominal operating voltage/DC off state voltage:
Meaning: The maximum direct current voltage that a device can withstand continuously without misoperation or breakdown in normal working condition (before triggering).
Meaning: This is the basis for selection, which must be greater than the normal operating voltage of the protected circuit and leave a certain margin (usually 10% to 20%). Choosing too low may lead to false triggering, while choosing too high may result in insufficient protection.
Unit: Volt.
2. Breakdown voltage/transition voltage:
Meaning: The minimum voltage threshold required for a device to transition from a high blocking state to a low blocking conducting state. For devices such as SIDACtor, it is also known as "breakdown voltage".
Meaning: It determines the voltage level at which the device begins to provide protective action. This voltage needs to be lower than the withstand voltage of the protected equipment, but higher than the peak normal operating voltage of the line (including allowable fluctuations).
Characteristics: There is usually a tolerance range (such as ± 5% or ± 10%). Different models have specific standard value series (such as 90V, 110V, 230V, etc.).
Unit: Volt.
3. Maximum current carrying capacity/surge current bearing capacity:
Meaning: The maximum peak current that a device can safely discharge without damage in a single or multiple cycles (depending on the specification) under a specified waveform (most commonly 8/20 μ s current wave).
Meaning: It directly reflects the device's ability to withstand high-energy surges (such as lightning induction and switch surges). This is a key parameter for evaluating the protection level, which must be selected based on the expected surge level of the application environment.
Unit: Ampere (usually labeled as A or kA, such as 100A, 5kA).
4. Clamping voltage/conduction residual voltage:
Meaning: The voltage drop present at both ends of a device when it is triggered to conduct and discharge amplified current.
Meaning: This is the actual residual voltage applied to the protected equipment. A high-quality solid discharge tube should have low clamping characteristics, which means that when discharging a large current, the voltage at both ends can still be maintained at a relatively low level, effectively protecting the downstream circuit. Usually, the clamping voltage is slightly higher than the breakdown voltage.
Unit: Volt (usually labeled at a specific current, such as @ 100A).
5. Response time:
Meaning: The time required from the overvoltage reaching the breakdown voltage threshold to the device fully conducting and clamping the voltage to a lower level.
Meaning: The response speed determines whether the device can act in a timely manner to provide protection before sensitive electronic devices are damaged. The response time of solid discharge tubes is usually in the nanosecond range (ns), much faster than gas discharge tubes (GDT, microsecond range), but slightly slower than TVS diodes (picosecond range). Special attention should be paid to ESD protection for extremely high speeds.
Unit: nanoseconds.
6. Leakage current:
Meaning: A small current flowing through a device at normal operating voltage (below breakdown voltage).
Meaning: In an ideal state, the device should exhibit extremely high impedance when not triggered. A large leakage current means an increase in power consumption, which may affect the normal operation of the protected circuit, especially at high temperatures where the leakage current will significantly increase. Usually requires very small (microampere level or even nanoampere level).
Unit: microampere or nanoampere.
7. Junction capacitance:
Meaning: The inter electrode capacitance of a device in an non-conductive state.
Meaning: It is particularly important in the protection applications of high-frequency signal lines such as Ethernet, USB, and video cables. Excessive junction capacitance can degrade the integrity of high-speed signals (causing signal attenuation and distortion). The type of low junction capacitor needs to be selected based on the signal frequency.
Unit: Pifa.
8. Temperature characteristics:
Meaning: The characteristic of key parameters such as breakdown voltage and leakage current changing with environmental temperature.
Meaning: The device needs to ensure performance within the specified operating temperature range. Usually:
The breakdown voltage may have a positive or negative temperature coefficient (depending on the design), and its drift with temperature should be considered.
The leakage current increases exponentially with temperature, and the leakage current at high temperatures is an important consideration.
Unit: Usually using temperature coefficient (%/° C) or providing a range of parameter values at a specific temperature.
9. Packaging form:
Meaning: The physical appearance and pin arrangement of the device.
Meaning: It affects the heat dissipation capacity, installation method (through-hole or patch), space occupation, and applicable surge level (usually the larger the package, the better the heat dissipation, and the stronger the flow capacity may be). Common packaging options include DO214AA/AB/AC (SMB/SMC/SOD123), SOT23, SOT323, etc.
Summary and selection logic:
When choosing a solid discharge tube, a core logical chain must be followed:
1. Voltage matching: Firstly, select devices with appropriate nominal operating voltage/DC off state voltage (margin) based on the normal operating voltage of the protected circuit. Then ensure that its breakdown voltage is below the tolerance limit of the equipment.
2. Protection capability: Evaluate the expected surge threat level (energy) and select models with sufficient maximum current carrying capacity.
3. Protection effect: Pursuing the lowest possible clamping voltage while meeting the current requirements to provide the best level of protection.
4. Speed requirement: Consider the sensitivity of the protected equipment and ensure that the response time is fast enough.
5. Minimizing side effects:
For signal/data lines, attention must be paid to whether the junction capacitance is within an acceptable range to avoid affecting signal quality.
In low-power or high-temperature applications, it is necessary to strictly investigate the leakage current, especially at high temperatures.
6. Environmental adaptability: Confirm that the temperature characteristics and operating temperature range of the device meet the requirements of the application environment.
7. Physical implementation: Select appropriate packaging based on PCB design and space.
Understanding and comprehensively considering these core parameters is the key to selecting solid discharge tubes correctly and providing reliable and effective transient overvoltage protection for electronic devices.