Introduction of overvoltage protection device ESD electrostatic protection device
Time:2023-08-15
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The most commonly used ESD protection devices on the market can be divided into three categories: polymers, varistors/suppressors, and diodes. The biggest challenge in selecting the appropriate ESD protection device is how to easily determine which device can provide the maximum protection. System suppliers generally compare the quality of ESD protection devices through the ESD rated value (or nominal value) in the data manual. In fact, it is impossible to see how strong the device protection system is from these ratings, and the key depends on other diode parameters. In addition to the nominal ESD value of the protective device, the voltage value (clamping voltage) and the amount of current (residual current) at the ASIC end are also key factors. The function of ESD protection devices is achieved by short circuiting most of the current to ground and clamping the voltage at the ASIC end to a value lower than the pulse voltage.
Determining the clamping voltage and residual current is not an easy task. The clamping voltage referenced in most ESD protection data manuals (if this information is included in the manual) is easily misleading. The term residual current has never been included in the data manual because it is related to the system layout and not to the device itself. The dynamic resistance (Rdyn) of the protective circuit as an alternative parameter helps to compare devices, as devices with lower resistance can divert a larger proportion of current. Unfortunately, this dynamic resistance value is usually not found in the data manuals of protective devices. Next, Uen Semiconductor will talk about these types of protective devices:
Polymer devices
Although polymer devices are quite attractive for high-frequency applications because their sub pico capacitance values are only 0.05-1.0pF, such low capacitance can also bring some slight side effects. Unlike diodes, polymer devices require the terminal voltage to reach the triggering voltage before breakdown, which is much higher than the clamping voltage. A typical polymer ESD device will not breakdown before 500V. Once broken down, it will quickly jump to a clamping voltage of up to 150V, and when the charge is released, the polymer will return to a high resistance state. However, this process may take several hours or even a day, so they are not attractive for consumer applications. These devices are difficult to accurately characterize in * *, and their data manuals typically only contain typical parameter values without providing minimum and maximum guaranteed values. In addition, as they are physically flexible devices, their performance will decrease with the increase in the number of ESD pulses they are subjected to.
Varistors and suppressors
Varistors and suppressors are nonlinear variable resistors. Although they are relatively inexpensive, suppressors typically have high triggering voltage, high clamping voltage, and high impedance characteristics, allowing most energy to reach the protected device instead of being diverted to ground. The clamping voltage range of a typical low capacitance suppressor is between 150 and 500V. The typical dynamic resistance of a low capacitance suppressor is 20-40 Ω. Due to its high impedance characteristics, almost all ESD surge currents are transmitted to the "protected" device rather than being diverted to ground.
Semiconductor diode
Another ESD protection method is to use semiconductor diodes. ESD protection diodes are characterized by low clamping voltage, low impedance, fast conduction time, and better reliability. Usually, semiconductor diodes can provide the best ESD protection, and now diodes can achieve an equivalent capacitance of 1pF, making them the best choice for reliable ESD protection and good signal integrity.
Analysis of ESD electrostatic protection components
Any protective component must appear as a high impedance circuit at the protected input during normal operation. The capacitive load it applies must be as small as possible, so that it has almost no impact on the normal input signal. However, at the moment of overvoltage, the same device must become the main channel of energy, transferring energy from the input of the protected device. In addition, the standing off voltage of the protective device must be higher than the maximum signal voltage allowed at the protected end. Similarly, the clamping voltage must be low enough to prevent damage to the protected device, as the voltage on the input terminal will be the clamping voltage of the protective device during transient events.
We often see the introduction of ESD electrostatic protectors, ESD electrostatic impedors, ESD electrostatic dischargers, SMD varistors, and other products, but in fact, the most critical reference coefficients for protecting devices should be the following three:
1. Quick response time
2. Low clamping voltage
3. High voltage surge withstand capacity
No matter how excellent the product is introduced, when selecting ESD electrostatic protection components, practical comparisons should still be carefully made and verified using IEC61000-4-2 testing. The current industry practice is to announce the clamping voltage based on pulses with a rise time of 8us and a duration of 20us, while the true ESD pulse should have a rise time of 1ns and a duration of 60ns.
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