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The LT5176 buck boost converter outputs a short 90-91 volt spike at at the output in the 2nd second after startup, applying power to Vin.

Schematic

LT5176 buck boost schematic More details at Why does this buck boost with dual MOSFETs fails output at certain input voltages at light load? (board is populated with single MOSFETs: 4 pieces)

Measurement setup

  • 100 kHz LMG95 (Zes-Zimmer precision power meter)
  • Input resistance: 1MOhm, 20pF
  • 50 cm cable
  • clip probe tip LM5176 measurement setup

Regression

  • this overvoltage spike happens at all input voltages (33-55V)
  • this overvoltage spike happens at different loads (1ma, 10ma, 100ma, 1000ma)
  • also happens unloaded

Questions

Is it normal to measure such a voltage spike at the output? In case this is expected behaviour, why? Which design/IC would not suffer from a short overvoltage spike on startup?

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    \$\begingroup\$ 100 kHz scope? Show oscillograms. Show how you connected it. \$\endgroup\$ – winny May 1 '18 at 9:07
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    \$\begingroup\$ Show your probe and precisely where the spring ground clip was earthed. If you didn't use a spring ground clip then do so and report back. Show oscilloscope pictures and don't do reposts. If you feel you haven't had enough attention it's because you didn't provide the rudimentary detail needed to solve this. \$\endgroup\$ – Andy aka May 1 '18 at 9:14
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    \$\begingroup\$ @winny Not even a scope. And surely far too much inductance \$\endgroup\$ – PlasmaHH May 1 '18 at 9:18
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    \$\begingroup\$ Right. You can toss out all results coming from that thing. You need an oscilloscope with a probe and low inductance connection. \$\endgroup\$ – winny May 1 '18 at 9:59
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    \$\begingroup\$ Instruction video for you: m.youtube.com/watch?v=-4q8geE5ef8 \$\endgroup\$ – winny May 1 '18 at 10:04

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