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TIDA-00699: Load dump issue with SMBJ26A

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?