Electrical – TVS diodes in series causing a short

power supply

On my board, I am using bidirectional TVS diodes for transient protection. In the schematic, the diodes are configured in bidirectional method so cathode/anode (D1) then anode/cathode (D2). During soldering, I accidentally soldered D2 the opposite way which resulted in the schematic shown.

When I powered on the board using a +5V DC supply, the power supply shorted. Even with D2 flipped, I don't understand why the supply would short as the reverse breakdown voltage is a lot higher than +5V. Flipping D2 of course fixed the problem. I am just curious as to what was the reason for the short?

schematic

simulate this circuit – Schematic created using CircuitLab

Best Answer

The min breakdown voltage @1mA is about 10% more than the standoff voltage for which it is rated in the pn.

So you have two unidrectional TVS's in series with a standoff of 26V+ 14V= 28V and a diode forward drop Vf for each <1V.

Conclusion

Maybe you put them in backwards. Cathode bar is +ve.

Why you chose these is a mystery .