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Hello guys,

today I tested my circuit. Its a simple Reverse Polarity Protection Circuit with a Buck-Converter. As I supplied voltagae to my circuit, everything was well (3.5V output). Going higher with the voltage was okay, too. Until I got to ~30V. Then my power supply gone into current limitation.

I think my TVS Diode (which I choosed too small..) gone to Breakdown Volatge and caused a short circuit (is that possible?). I Supplied voltage only to the pins of the Buck but its still not working, so I think something destroyed the buck but exactly what?

EDIT: With "the buck dont work" I mean, when I supply voltage to my circuit, my power supply goes into current limitation because the circuit wants to drain a lot of current. So if I dont supply from J1 and J5 and supply the IC pins of the buck directlyy its the same failure.


Datasheets: Buck

PMOS

Bipolar TVS

Schottky

Zener

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  • \$\begingroup\$ not going to work unless you connect bat+ \$\endgroup\$ – Jasen Feb 28 '18 at 10:35
  • \$\begingroup\$ I only connect Bat+ and Bat- \$\endgroup\$ – Fatih Y Feb 28 '18 at 10:44
  • \$\begingroup\$ not what the digram shows. \$\endgroup\$ – Jasen Feb 28 '18 at 10:44
  • \$\begingroup\$ Now you know it. \$\endgroup\$ – Fatih Y Feb 28 '18 at 10:46
  • \$\begingroup\$ C1 is 6 to 10 times bigger than the examples in the data-sheet \$\endgroup\$ – Jasen Feb 28 '18 at 10:57
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  1. TVS can fail in shorted mode, it is actually the preferred failure mode as it still protects the circuit against overvoltage.
  2. TVS is (without additional components) usually not suitable to clamp sustained overvoltage. It is intended for clamping voltage spikes (short duration) If you check your datasheet, the 600 W applies to pulse only (it is actually more energy limit than power limit). Sustained power absolute maximum is 3 W only, with 30V drop it is 100 mA only.
  3. It is well possible that your C2 capacitor failed as well. What is its voltage rating? Typical overvoltage failure mode of MLCCs is short circuit.
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  • \$\begingroup\$ 2. Shouldnt the diode behave like a zener diode at the breakdown voltage? The breakdown voltage is min. 26.7V to 29.5V and the supply voltage was max. 31V. To 3. the voltage rating is 50V \$\endgroup\$ – Fatih Y Feb 28 '18 at 11:05
  • \$\begingroup\$ 2. Yes, exactly. So about 30 V drop and whichever current your power supply allows to flow. Calculate I*U and you have dissipated power (likely too much). \$\endgroup\$ – Martin Feb 28 '18 at 11:09
  • \$\begingroup\$ 3. OK, it was just my guess. It would be good if you can describe "not working" buck in greater details in your question. Measure voltages at various points etc. \$\endgroup\$ – Martin Feb 28 '18 at 11:10
  • \$\begingroup\$ Do you think the the TVS caused a short circuit? 3. Read the EDIT in my question, I tried to explain it better \$\endgroup\$ – Fatih Y Feb 28 '18 at 11:15
  • \$\begingroup\$ Yes, overloaded TVS very likely fails into the short circuit. About your edit (just to be sure) -- when testing the buck only, you disconnect the circuit with MOSFET and TVS from the buck part, right? \$\endgroup\$ – Martin Feb 28 '18 at 11:22

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