There are already many kinds of ESD protection devices on the market, but the most commonly used can be divided into three categories: polymer, varistor, suppressor and diode.

The most difficult part of choosing the right ESD protection device is how to make sure which device can provide maximum protection.

The system supplier usually compares the ESD protection device with the ESD rating (or nominal value) in the data manual. In fact, it is impossible to see how strong the protection system is from these ratings, depending on other diode parameters.

In addition to protecting the ESD nominal value of the device, the current value (residual current) of the voltage (clamping voltage) and the ASIC terminal is also a key factor. The function of the ESD protection device is achieved by shorting most of the current to the ground and clamping the voltage of the ASIC terminal to the value below the pulse voltage.

Determining clamping voltage and residual current is not an easy task. The clamping voltage quoted in most ESD protection data sheets (if the manual contains this information) is easily misleading. There is no residual current in the data handbook because it is related to the layout of the system and has nothing to do with the device itself.

The dynamic resistance (Rdyn) of the protective circuit, as an alternative parameter, helps to compare the device, because the device with a lower resistance can divert a large proportion of the current. Unfortunately, this dynamic resistance is usually not seen in the data sheet of the device.

Polymer device

Although polymer devices are attractive for high frequency applications, their sub - skin capacitance values are only 0.05~1.0pF, but such low capacitance can also bring a few minor side effects. Unlike diodes, polymer devices require breakdown voltage when the terminal voltage reaches the trigger voltage, which is much higher than the clamping voltage. Typical polymer ESD devices will not break down before 500V.

Once it is broken, it will jump to the highest clamping voltage of 150V. When the charge is released, the polymer will return to the high impedance state. But this process may take hours or even days, so they are not attractive to consumer applications.

These devices are difficult to accurately characterize in manufacturing, and their data manuals usually contain only typical parameter values without providing minimum and maximum guaranteed values. In addition, because they are physically flexible devices, their performance will decrease as the number of ESD pulses increases.

Varistor and suppressor

Varistors and suppressors are nonlinear variable resistors.

Although they are relatively inexpensive, the suppressors usually have high trigger voltage, high clamping voltage and high impedance characteristics, so that most of the energy will reach the protected devices, not to the ground.

The clamping voltage range of the typical low capacitance suppressor is 150~500V. The typical dynamic resistance of the low capacitance suppressor is 20~40. Because of its high impedance characteristics, almost all ESD impulse current will be transmitted to "protected" devices instead of shunting to the ground.

Semiconductor diode

Another way to protect ESD is to use semiconductor diodes.

The ESD protection diodes are characterized by low clamping voltage, low impedance, fast lead time and better reliability. Usually semiconductor diodes can provide the best ESD protection, and now the diode has been able to achieve the equivalent capacitance of the 1pF, so it has become the best choice for reliable ESD protection and good signal integrity.

Analysis of ESD electrostatic protector

Any protection element must appear as a high impedance circuit at the protected input during normal operation. The load applied to it must be as small as possible, so that the normal input signal can hardly be affected. However, at the moment of overvoltage, the same device must be the main channel of energy, transferring energy from the input end of the protected device.

In addition, the protection device (standing-off) voltage must be higher than the maximum signal voltage allowed by the protected terminal. Similarly, the clamping voltage must be low enough to prevent the protected device from being damaged, because the voltage on the input end will be the clamping voltage of the protective device during the transient period.

We often see the ESD electrostatic protector, the ESD electrostatic impedance, the ESD electrostatic release, the patch pressure sensitive resistor and so on, but the most important reference coefficient of the protective device should be the following three items:

1. fast response time

2. low clamp voltage

3. high electric vagrancy capacity

No matter how excellent the product is introduced, the actual comparison should be done carefully in the selection of ESD electrostatic protection components and the use of IEC61000-4-2 tests to verify it.

The current industry practice is to release the clamped voltage according to the pulse of 8us rising time and 20us duration, and the real ESD pulse should be the duration of 1ns rising and the duration of 60ns.