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I'm using a SID1182 driver IC, datasheet , and it says to use a series of TVS diodes for 900V. Is this 900V the breakdown voltage or the clamping voltage? The datasheet doesn't specify and I'm not too familiar with TVS diodes. enter image description here

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  • \$\begingroup\$ Where is this diagram from? And what are you trying to do with the driver, what is your goal? \$\endgroup\$ Jul 21, 2022 at 18:56
  • \$\begingroup\$ Hi. The diagram is from the datasheet of the driver IC. I'm just trying to get it setup with a sic mosfet. I'm not really sure what the function of the tvs diodes are in this schematic. The datasheet doesn't go into detail about which ones to choose or what their purpose is. I may end up switching to a simpler driver board with better documentation. \$\endgroup\$
    – Thomas C.
    Jul 21, 2022 at 20:21
  • \$\begingroup\$ Fig.8 in mouser.com/datasheet/2/328/… is a plot; can you find the correct link? \$\endgroup\$ Jul 21, 2022 at 22:13

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The point here is to make sure that for each cycle of switching on and off generating a voltage spike on the drain, the transistor works in the Safe Operating Area (SOA) so below the absolute maximum rated (should be VDSS). Once you know the max allowed VDSS, you can define the max clamping voltage you want to get with the TVS in series.

This application note can help you to manage the theoretical calculations and figure out how it works: TVS clamping protection mode

My recommendation as a starting point is to try to perform simulations. In this kind of applications, the current of the spike is not very high so you can first try with 400W SMA diodes (SM4TY datasheet) and put them in series to reach VDSS minus tolerance (10%). As the current is low refers to max VBR to calculate this value.

If you are operating at a temperature significantly higher than 25°C, you must calculate the max VBR using the alphaT coefficient (VBR is increasing with the T°). The formula is in the notes of the datasheet

Then you can try your simulations with it. Electro thermal models are available here: SM4TY electro thermal models This link should be direct. If not click on CAD resources tab. Electro-thermal simulations will take into account the self-heating of the TVS when they sink the current of the spike to get more accurate clamping voltage/current.

If the resulting clamping voltage is too high, reduce the overall VBR by selecting a lower VBR for one TVS in series (you do not need to select the same TVS voltages). If the resulting spike power exceed the TVS absolute max ratings move to bigger TVS 600W SMA : SMA6TY and so on up to 3000W (I can’t believe you will reach this point)

I don’t know your application but pay attention to the clamping voltage: too high your transistor is at risk, too low you may lose power conversion efficiency (if your application is related to power conversion).

I hope it helps

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