Use of TVS Diodes across HV Mosfets

Thread Starter

SiCEngineer

Joined May 22, 2019
187
I have been seeing some research papers that place TVS diodes across MOSFETs for transient suppression. However when looking at the data sheets they seem to have a maximum voltage rating of around 100V ("Vr"). But the data sheet also says they are used in automotive and other sensitive applications, where I expect voltages would be more than 100V.

They also say they have protection "IEC-61000-4-2 ESD 15kV(Air), 8kV (Contact)" ... so I am unsure if I am misunderstanding the data sheets - what exactly is the maximum voltage that can be placed upon these devices? Is it the VR or is it the 8/15kV that is stated? I have only a basic idea of how these devices work so I could be easily misunderstanding, but for normal diodes the VR is the max voltage rating.

Data sheet: https://www.littelfuse.com/products/tvs-diodes/automotive-and-high-reliability-tvs/smcg-hr.aspx
 

kubeek

Joined Sep 20, 2005
5,719
Maybe would be better to show those research papers?
"IEC-61000-4-2 ESD 15kV(Air), 8kV (Contact)" and other mentioned ESD norms are mostly marketing wank, trying to sell it to you as a cure-all to help with any problems related to those. Vr means the voltage below which the tvs is sure to stay off, and above and somewhere between min and max Vbr it should conduct 1mA. Above that it acts basically like a big power zener diode clamping the voltage.

You can place basically any voltage you want, provided that the instanteneous power dissipation in the device is less than 1500W for a 1ms pulse, see figure 2.
 

RPLaJeunesse

Joined Jul 29, 2018
92
"IEC-61000-4-2 ESD 15kV(Air), 8kV (Contact)" are indeed marketing numbers, but developed under a very prescribed circumstance, so not what I'd call "wank". As voltages these are a bit misleading because they really refer to charge, as in charge stored in a capacitor. The capacitor value is called out in the test specification (IEC-61000-4-2) and is intended to represent what capacitance might be seen in a typical static discharge event. The stored charge (Q=C*V) indicated by the 15kV/8kV value is what the ESD supression device can tolerate without failure. It is not ever meant to be the voltage seen across your ESD supression device.

While I can't speak for hybrid engine or electric vehicles the standards for conventional 12V powered vehicle equipment design do call out voltages above 100V (and below -100V) as realistic. But these are typically very short duration low power inductive kicks that can be readily protected against with a modest inductor in series with the power feed. Sometimes a good broadband capacitor across the input is all that's needed.
 

mtripoli3

Joined Mar 1, 2016
35
Hi,
I don't know if it's applicable for what you're doing or not, there is a device I have been using that is a little "sleeper" , not well known. It incorporates all the necessary components to drive a relay, etc. These cost no more than a BSS138 which I also use.

Have a look:

ON Semi driver IC

Good luck!
 
Top