In the previous post, I introduced the ESD Fundamentals blog series and in case you haven’t seen it, I’ve attached a link here. In the following posts, we will go over the different factors to consider when selecting an ESD protection diode for your system.
Nowadays, most circuit components will offer some kind of device-level ESD protection in compliance with the Charge Device Model (CDM) or Human Body Model (HBM) standards. Now if you were to take a datasheet and see the below section (figure 1), it would be tempting to assume that the 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) becoming charged and discharged 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 created a more rigorous standard called IEC 61000-4-2. As we can see from 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 can be generally found on datasheets: contact voltage discharge (ESD directly discharged onto the device) and air gap voltage discharge (ESD discharged onto device through a gap of air). The IEC 61000-4-2 standard specifies four levels of voltage ratings with level 4 being the highest (Figure 3). For most applications, level 4 IEC ESD protection (8kV contact/15kV air gap) is sufficient. However, in applications or environments where ESD strikes are expected to have stronger voltages or are expected to happen more frequently, higher contact voltage and airgap voltage ratings may be required (Figure 4). TI’s TPD1E1B04, for example has an IEC 61000-4-2 rating of 30kV/30kV.
Figure 3: IEC 61000-4-2 Standard Levels
Figure 4: Typical examples of ESD generation (source: Phil Storrs PC Hardware)
In conclusion, 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 to ensure that the diode itself will survive repeated exposure to ESD. Now that we know how to compare the robustness of ESD protection diodes, our next topic will be on clamping voltage: how well a diode can protect sensitive circuitry.
For more information regarding IEC 610000-4-2 testing, please refer to the following application report.