# Choosing TVS diode

We have a a project where there a system powered by 13s Li battery - 54.6V max voltage of pack. The buck regulator we're choosing for this has an absolute max of 76V. For this we're considering two TVS diodes

824521481 Vbr=56.1V Vcl=77.4V

SMBJ48CD-M3/H Vbr=54.1V Vcl=76.3V

In the first option quite small current will flow through the TVS, but max clamping voltage is greater than 76V rating of buck regulator. In second case there is a possibility of more than 1 mA current flowing through diode when battery is fully charged but clamping voltage is much closer to max rating of buck regulator.

Which of the two options is better? Or will both of them not work and we've to look at a higher voltage rating for the buck?

Edit 1 for more context: This is a small add-on circuit with 2G GSM modem (2A peak) being powered from the battery of an EV that has a motor size of 0.5 to 2kW and battery capacity of 4 to 8 kWh. So at 48V from battery during the GSM peak current also there is sub 300 mA current draw. We are looking at protecting the circuit from any transients or surges from the motor operation, charger connection or BMS battery cut-off during any fault. We are not yet sure of what kind of transients to expect in such a system.

• Can you explain the scenario where the diodes have to clamp the voltage? Is this a motor driver that you are talking about? Is it possible for the battery to get disconnected during charge? I am having a hard time seeing how a system with a 13S battery can get to 76 V. Also, how much input current does the regulator need? If the input current is low you may be able to protect it with an RC. Edit your question to include all this information. Don't reply with a comment. Jun 7, 2021 at 6:32
• I don't think there is any way you can use the first one, the 824521481. Vbr is 56.1 +/- 5%. So the min is 56.1 - 0.05*56.1 = 53.295. Jun 7, 2021 at 6:36
• Yes if the BMS disconnects you have to disable regen very quickly. The voltage rise rate can be calculated using I = Cdv/dt. dv/dt = I/C. I is the regen current going into the battery before the BMS disconnects, and C is the total capacitance on VBAT. You don't have a lot of time. But a TVS doesn't buy you that much, either, because they can only absorb transient over-voltages. If you try to continue regen after battery disconnect your TVS will blow for sure. The motor controller really needs to be "in the loop" and disable regen BEFORE the BMS disconnects. Jun 7, 2021 at 22:32
• The motor controller generally has large capacitors. The regen current is not infinite. When the battery disconnects you may have some ms of time to halt regen before a too high voltage is reached. You can figure it out using dv/dt = I/C. One product I worked on had a braking resistor to dissipate extra energy. But another option is to simply disable braking. It could be based on voltage. Once the controller sees 60V, it should disable regen braking, for example. However, this would only work if the controller samples the voltage pretty frequently. Jun 8, 2021 at 15:40
• @mkeith A comparator feeding a break input could be used instead of high frequency sampling, and react faster. Jun 9, 2021 at 13:31