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| Quick TVS diode question. Vclamp < Vbreakdown |
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| Psi:
I see TVS diodes sometimes where the Imax clamping voltage is lower than the 1mA breakdown voltage. Which, all things being equal, doesn't seem to make sense unless something else is going on. I did some googling but didn't find a satisfactory answer as to why this occurs. Do thermal effects of the power dissipation change its properties and lower the breakdown point? eg, are Vclamp and Vbreakdown exactly the same property just measured at active vs inactive temperature. |
| niconiconi:
Some high-end ESD protection diodes for high-speed data lines use the thyristor principle to create negative resistance - once triggered, they remain in conduction until the voltage drops to a level much lower than the initial trigger voltage. These are marketed as "snapback TVS diodes". There's also a related technology called Thyristor Surge Suppressor (TSS), commonly used for telecom applications to clamp huge surges in telephone lines. Large TSS can compete with Gas Discharge Tubes (GDT). But if you are talking about ordinary TVS diodes, I don't know the answer. |
| Psi:
Here's an example of what i mean. It doesn't seem to be a snapback, or it doesnt say that anywhere. The graph in fig 3 on page 4 looks normal, ie no inversion/hysteresis like in your example graph. But the clamp is still less than the breakdown which seems to go against what fig3 shows https://assets.nexperia.com/documents/data-sheet/PESD5V0C1USF.pdf |
| T3sl4co1l:
Yeah, snapback: https://assets.nexperia.com/documents/leaflet/Nexperia_TrEOS_ESD_protection_for_USB_Type-C_leaflet.pdf --- Quote from: niconiconi on July 24, 2022, 09:50:45 am ---Some high-end ESD protection diodes for high-speed data lines use the thyristor principle to create negative resistance - once triggered, they remain in conduction until the voltage drops to a level much lower than the initial trigger voltage. These are marketed as "snapback TVS diodes". --- End quote --- Just to clarify -- snapback isn't a thyristor (4-layer) mechanism, it's... punch-through, I believe? So, a 3-layer structure, like a BJT without a base connection, and with a light enough doped base that it fully depletes at some point, effectively shorting from C to E. (I might have this wrong, but ESD protection features are notoriously secret-sauce, so that's not necessarily my fault. :P ) When this is tuned for just the right breakdown voltage, it can happen that a fair amount of current is already flowing (avalanche/zener mode) by the time the negative resistance effect pulls in, and the negative resistance effectively compensates for the bulk resistance of the device, hence flattening the curve -- or at least not having such a severe "snapback" as for thyristor devices. They can be made in much lower voltages than avalanche types (which are zeners below 5V, so, basically useless for surge purposes!), and are the only type that offers effective protection for systems 3.3V and below. they're also faster, as it only depends on the motion of carriers, rather than diffusion of minority carriers (thyristors are fast enough for surge, which they excel at, but aren't suitable for ESD, AFAIK). Tim |
| Benta:
It seems that the "clamping voltage" is the rise above the "breakdown voltage" related to current. |
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