TVS Diode Burnt Root Cause?

Dear All,

The PCBA that I am working on has used SMBJ12A TVS diode from fair child in its 12V supply rails.

The loads drawn from that 12V rails are Solenoid Valves.

After running the PCBA for several weeks, in the final test, those TVS diodes were burnt in two PCBAs at the same location.

Could anybody please give me some hints for the failure mechanisms of TVS diodes in power rails?


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Could be either a current spike or a voltage over maximum rating.

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12V rail exceeding 13.3V?
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Shouldn,t the Diode be directely across the solenoid valve as close to it as pysically possible?


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Is this a regulated 12V or a 12V automotive power? Keep in mind that the vehicle power is normally higher (13.5-14.4V) and SMBJ12A is not suitable. Accounting for temperature, parts variation, etc. etc. you need the TVS with the breakdown at 18V at the least. Also, the SMB parts are flimsy. For automotive use, I'd recommend a solid throughole TVS.

Some idiot reversed the power supply polarity?

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Vladimir Vassilevsky

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Depends. How much wire is between the coil and the switch? We had a situation, moons ago, where smoke was rolling out of our boxes on the customer's floor. At a few $M a copy, a few banks were not happy. The engineer who designed the contactor drivers thought as you do.

The root cause was that the inductance in the wire caused secondary breakdown in the contactor drivers. Only Motorola parts went into secondary breakdown[*], holding the contactor closed, letting the magic smoke out of the inrush limiters. *Lots* of smoke.

[*] The immediate fix was to replace the drivers with Sprague parts. Sprague had power engineers throw a little logic in front of their power transistor. Motorola put a big-assed transistor on the output of their logic chip. There is more to a power transistor than "big".
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Thank you so much for your highlights on the use of TVS.

Actually it is not for automotive.

It is a pneumatic control device. The PCBA simply checks the air pressure, run the pump to charge air in the tank, and then drive check vales and LVM Ports to guide the air for certain application.

The 12V supply is from TI module type power supply: PTN78060WAH.

It already has reverse recovery diodes across every solenoid.

The TVS diode is additional protection for 12V rails.

By the way, regarding TVS selection as per your suggestion, is it always true that a higher TVS should be selected for such kind of power supply protection?

Let's say i need to protect 24 V rails, would SMBJ26A or SMBJ26A be a better choice than SMBJ24A?


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