Pass/fail tests don't tell you why a system passed or failed. TLP, now used for system-level ESD, can help.
The IV curve of a protection device shows some interesting things about the clamp structure's behavior. You can see at what voltage the device will begin to exhibit low impedance (i.e., turn-on or trigger voltage). You can also see the protection clamp's damage threshold, which is generally observed as a sharp secondary snapback in the IV curve. This snapback hints at when the protection element begins to sustain damage. Most TLP generators also allow for monitoring the DUT's leakage current.
When you examine the leakage current data of the protection device alongside the IV curve, you can isolate and confirm the current pulse at which the ESD protection element would be damaged by an ESD pulse (IF or the failure current). The other important metric that the TLP data gives us is the ability to measure the dynamic resistance (RDYN). This is typically defined as the slope of the IV curve, generally from 4 A to 16 A (as shown below). Protection device manufacturers use TLP data to characterize the dynamic resistance (RDYN) of their ESD clamps. You can think of RDYN as a measure of how well the transient voltage suppression (TVS) device is clamping -- ignoring the turn-on voltage, of course.
A TLP IV curve shows turn-on voltage, dynamic resistance (RDYN), and secondary breakdown.
One advantage of the TLP measurement technique is that the square pulse removes much of the variability in the system-level ESD test. Figure 3 shows the waveform of a +8 kV ESD pulse per the IEC61000-4-2 (black curve) plotted alongside a 16 A TLP pulse (red curve). You can think of the system-level waveform as the superposition of two waveforms: the initial first spike (a metal-to-metal interaction) and the discharge from the body resistance of a human body. This first peak varies greatly depending upon the inductance of the test setup/PCB. The TLP test essentially ignores the effects of this first peak. It approximates the energy due to the body resistance. For system-level protection, the first peak must be considered in the analysis. For now, however, we will ignore the first peak.
A +8kV IEC61000-4-2 waveform comparison to 16 A, 100 ns TLP waveform shows how TLP pulse lacks the initial peak.
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