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TIDA-00699: Load dump issue with SMBJ26A

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Replies: 4

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Part Number: TIDA-00699

We've run into an issue working with load dumps and was wondering if anyone else had seen the same thing.

The issue surrounds D2 & D3 as shown in TIDA-00699 figure 9. Apparently, these TVSs are supposed to withstand 36 Volt load dump pulses as shown in figure 43.

Unfortunately, every time we test this, D2, the SMBJ26A makes a loud pop and fuses short. The question is why.

After lots of checking and rechecking our setup and coming up empty, I started to look at SMBJ26A itself.

On Semiconductor offers this part. The data sheet ( www.onsemi.com/.../SMBJ9V0CA-D.pdf ) doesn't directly address a square 200ms 36V pulse. When I extrapolate figure 1 to 200ms, I come up with about 50-70 watts of peak power. But that's not for a square pulse, but rather one like in the datasheet's figure 3. So that 50-70 watts for 200ms seems way optimistic.

Lacking PSpice, I tried using Littlefuse PSpice models and LTSpice to subject those diodes to a square 200ms 36V pulse in series with a 0.5 ohm resistor. LTSpice reports that D2 is well into conduction, 2.5 Amps at 31.5 volts, about 80Watts.  That would explain the failure modulo the fact that LTSpice isn't really PSpice.

So:

  • Has anyone else had issues (or success) with these TVSs and load dumps?
  • Did anyone simulate the square 200ms 36V using PSpice?
  • Advice?

  • Hi Hedley

    I am sorry to hear this, but glad you brought this to our attention. As shown in the design guide test data, we were able to successfully withstand the load dump pulse. However, this was done a few years back.

    I extrapolated that figure to around 60 W of peak power for 200 ms square pulse.

    Since this is not our device, I will need to do a bit more digging. I will get back to you soon with more info.

    Luis

  • In reply to Luis Cordova:

    Hi Hedley,

    Could you provide the exact pulse waveform used, and your test setup?

    Luis

  • In reply to Luis Cordova:

    Sorry for the delay on this.

    I owe you and TI a tremendous apology. Simple and short, I messed up. The load dump generator was actually producing well over 36 volts. 

    I'm gonna go crawl under a rock someplace now.

    Hedley

  • In reply to hedley davis:

    Hi Hedley,

    No worries, we've all been there :)

    Thank you for the clarification!

    Luis

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