Snapback TVS diode behaviour - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: November 25, 2024, 02:33:56 am
content: Hi,To protect a MIPI-DSI interface, I’m considering using this TVS diode: PESD4USB3UTBS-Q ( datasheet:https://assets.nexperia.com/documents/data-sheet/PESD4USB3UTBS-Q.pdf).It has a working voltage of 3.3V, but since it’s a snapback type, my understanding is as follows: once it reaches the breakdown voltage of 9V (typical), it snaps back to 3.5V (as indicated in Table 5, page 4). However, looking at Figure 5 (page 5), it seems that the voltage first drops to approximately 1.2V, and then, at the specified 5A current, it clamps to 3.3V.What I am trying to figure out is:1) Does the diode recover immediately after the ESD pulse ends, allowing the signal to resume normal operation? Or does it remain clamped, as I’ve experienced with a similar diode I used in a previous project (which I posted about here before)? What are the conditions to ensure it does not remain clamped?2) How can I determine the specific conditions required for the diode to recover and allow the signal to return to its normal working voltage, without staying triggered? I am asking because for example, being used on a fast MIPI-DSI signal it might switch to high-level (1.2V to 1.8V) too fast hence not allowing enough time for the TVS to recover out of clamp mode).3) Additionally, although the datasheet states that it is a unidirectional TVS diode, Table 5 (page 5) suggests it exhibits the same behavior for negative voltages, with identical voltage values. Could someone clarify this please?Thank you as always

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 25, 2024, 04:17:02 am
content: You would need to measure it.The data on these is usually very poor, and incomplete.The AOZ8S205BLS data is slightly better with I(Hold) at least drawn on a graph, but alas, not specified anywhere.

author: Psi
author_rank: Super Contributor
timestamp: November 25, 2024, 07:17:36 am
content: ah yes, the magical snapback diode.I wish you luck figuring out the specs.

author: selcuk
author_rank: Frequent Contributor
timestamp: November 25, 2024, 08:31:09 am
content: I've not seen this snap-back diode before. Interestingly, it seems like a transistor circuit more than a diode. Is there a reason to use it instead of a regular TVS. Like this one having the same capacitance and leakage but not a defined dynamic resistance:ULC3311CDN

author: Marco
author_rank: Super Contributor
timestamp: November 25, 2024, 08:34:17 am
content: Probably just a thyristor with a Zener on the trigger input. Aren't all the high speed interfaces mentioned in the datasheet AC coupled?

author: ricko_uk
author_rank: Super Contributor
timestamp: November 25, 2024, 03:17:36 pm
content: Thank you all,the high speed signals are mentioned in the datasheet but when I used it in the past with a 400KHz signal once triggered it remained triggered (I posted here back then).I better stick tot he classic TVS even if it has a higher Vclamp.Thank you

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 25, 2024, 09:47:29 pm
content: Quote from: ricko_uk on November 25, 2024, 03:17:36 pmYes, such a thyristor needs to fall below a holding voltage, or current, to release.Spice gives you a feel for what to measure, taking the Nexperia equivalent circuit and whacking in some low values gives the attached.Trigger/release here is ~ 150mA for 10R VBE, as both Rbe need to conduct to trigger.If your circuit is stiff enough to hold-up above the clamp voltage/current point, it will not release.

author: ricko_uk
author_rank: Super Contributor
timestamp: November 25, 2024, 10:31:12 pm
content: Thank you @PCB.Wiz, I appreciate you doing that simulation.

author: Benta
author_rank: Super Contributor
timestamp: November 25, 2024, 11:06:46 pm
content: It's basically a Silicon Bidirectional Switch (SBS), except it's unidirectional.https://electronicsarea.com/sbs-silicon-bidirectional-switch/When the input voltage reaches a certain point, it triggers and short-circuits the signal. It only resets when current flow is interrupted.Also a lot like a DIAC.Great for trigger circuits (SCRs etc.), but I fail to see the advantage in a USB interface.