In the first installment of this series, I argued for the need for discrete electrostatic discharge (ESD) protection. Now, I’ll review the different factors you need to consider when selecting an ESD protection diode for your system.
Most circuit components do offer some kind of device-level ESD protection in compliance with charge device model (CDM) or human body model (HBM) standards. If you were to look at the data sheet spec shown in Figure 1, it would be tempting to assume that this component would be robust enough to survive ESD strikes. Perfect! Done! No discrete ESD protection required, right?
Figure 1: Example Datasheet with HBM and CDM ESD Ratings
Well …not so fast. The CDM simulates an integrated circuit (IC) charging and discharging, while the HBM simulates a human being discharging onto the IC in a controlled ESD environment. These standards are useful in ensuring that components will survive manufacturing and assembly in factories where there are protocols to minimize ESD exposure. However, they do not accurately represent what a component will experience in an end-user scenario. I don’t know about you, but I don’t wear an ESD strap when I use my toaster in the morning.
To accurately model ESD strikes in real user scenarios, the International Electrotechnical Commission (IEC) created a more rigorous standard called IEC 61000-4-2. As you can see in Figure 2, this IEC pulse has a faster rise time, a longer duration, a higher peak pulse current and significantly more energy than the CDM and HBM pulses.
Figure 2: Comparison of different ESD models
The IEC 61000-4-2 standard includes two different ratings for ESD that you can generally find on data sheets: contact voltage discharge (ESD directly discharged onto the device) and air-gap voltage discharge (ESD discharged onto the device through a gap of air). The IEC 61000-4-2 standard specifies four levels of voltage ratings, with level 4 being the highest (Table 1).
Table 1: IEC 61000-4-2 Standard Levels
For most applications, level 4 IEC ESD protection (8kV contact/15kV air gap) is sufficient. However, applications or environments where ESD strikes are expected to have stronger voltages or may occur more frequently require higher contact-voltage and air-gap voltage ratings (Table 2). TI’s TPD1E1B04, for example, has an IEC 61000-4-2 rating of 30kV/30kV.
Table 2: Typical examples of ESD generation (source: Phil Storrs PC Hardware)
If a device is only rated for HBM and CDM ESD, it most likely does not have enough robustness to survive continued normal operation in real-world scenarios. Therefore, when selecting ESD protection diodes to protect these devices, it is critical to select a diode that has a sufficient IEC 61000-4-2 rating to ensure that the diode itself will survive repeated exposure to ESD.
Now that you know how to compare the robustness of ESD protection diodes, in the next installment of this series I’ll move on to the clamping voltage, which indicates how well a diode can protect sensitive circuitry.
I did not know that the use of vacuum solder remover can generat 8 kV ESD Voltage. Now I wonder how many systems I destroyed by the vacuum solder remover, even without knowing it.
I once managed to destroy 3 microcontrollers via ESD within 2 days before I noticed that it was my pants that generated the load. On the third day I wore jeans, and the series of destruction stopped. I think it was a combination of trousers with a carpet in an early company.
Matthew; The HBM rating is specified to survive a single event... is there any kind of guidance as to what kind of ESD event is 'survivable', on a repeated basis, without degradation of the ESD diodes, or the device functions on the affected pin?
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