Buck converter - Maximum VIN of IC vs Maximum clamping voltage of TVS diode

Question

For the circuit below which uses a LMR14050SDDAR buck converter from Texas Instruments. The absolute maximum VIN of the IC is 44V, and the capacitors are all of 50V. In practice VCC can be 36VDC continuous (worst case possible), so I choose a TVS diode which has 36V of Reverse Standoff (Typ) and 40V of Breakdown (Min). About the TVS diode choosen, it has a maximum clamping voltage of 58.1V @ Ipp of 25.8A. Will the buck converter be damaged easily this way due to short period peaks at the input? By voltage spikes in the order of microseconds? Do I really need a 60V+ buck converter to be safe? Regards.

Answer 1

Will the buck converter be damaged easily this way due to short period peaks at the input? By voltage spikes in the order of microseconds? Do I really need a 60V+ buck converter to be safe?

It's all about threats and victims.

• You've named the victims and we know where their weaknesses are.
• You've added a TVS that "might" offer a lot (or a little bit) of protection against an unspecified threat but,
• You haven't stated what the threat is (and we cannot guess).
• You need to ensure that your protection method (the TVS) doesn't become a victim.

The threat might be an indirect lightning strike that is modelled by a voltage pulse and a series resistance that can be as low as 2 Ω. That means that your TVS has to withstand a current pulse of maybe several thousand amps for maybe a hundred micro seconds. I'm talking about EN61000-4-5 and here are some of the pulses that can be used: -

So, you have to work out what specification corresponds with your threat. If EN 61000-4-5 indirect lightning is something you expect your circuit to survive AND you are happy with a 2 kV pulse (class 3) then the peak current into a short circuit is 1,000 amps and has a waveform shape like the lower one shown here: -

Picture above from here.

I've designed a surge suppressor capable of withstanding class 4 using several parallel TVS diodes so, it can be done. But, if your threat is ESD, then it's much easier in terms of finding a TVS that will survive.

One final word, you can pretty reliably model these surges using a simulator so, decide on your threat, check what the peak voltage is and what impedance it is sourced through. That dictates the pulse current your TVS has to withstand. Then check that the surge power rating isn't exceeded for the TVS and that the clamped terminal voltage doesn't exceed your victim's limit and you are usually good to go.

Answer 2

Yes, it might get damaged. Although the capacitors probably will not charge in a microsecond, but a surge can still be longer than capacitors time constant. you can alternatively get a lower voltage TVS.

Additionally, the TVS itself is not invincible, and if you don't take care, it might be burned by a surge.

However, to answer whether you need to take another DC/DC, we would have to understand your application, other constraints. For example, it can be acceptable to burn a DC/DC in a rare case. Sometimes investing a month in RnD (not to replace a DC/DC, but just as an example) may have a lot more consequences than a damaged circuit here and there.