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PD Power Path Switching Validation W25

PD Power Path Switching Validation W25

Owned by Liam Sky
Last updated: Feb 06, 2025
2 min read

Description of the issue:

Sometimes PD will lock up when switching from Aux to DCDC (or the other way around I don’t remember, but it was one way). PD will switch and the LEDs indicate turning on and off, but no voltage on the output. The board will also buzz (fast clicking). Reproduce and fix this.

Unknowns / First Checks

  • What was the load? i.e. how much current were we drawing?

    • There is a known issue for drawing >3A current which has not been addressed

    • NO LOAD

  • Was the controller programmer connected?

    • Another known issue which caused 12V inputs to lock up and “buzz”

    • NO

Hypotheses on why:

  • LTC4418 automatically controls the switches so start there

  • An inrush of current could trigger an UV fault (disconnecting source)

  • Voltages are out of their threshold values (11V - 15V) when they reach the IC (including conversion to the internal 1V threshold)

  • During switching, large inductive spikes are tripping OV thresholds repeatedly

  • Is there larger than 3V on the output during startup?

  • 2x8.4uF on input and output which when switch closes, voltage will average across, which could be below the UV trip point

    • Use larger input CAP

  • UV threshold lowered? This would worsen the voltage averaging affect, allowing us to be below the other threshold if one was lower than the other (explains the one way effect)

    • Check resistor values and calculate changed UV and OV thresholds

  • Increase retry interval to attempt to avoid continual switching

Test Plan:

Setup

  • Connect 2 channels of power supply to the inputs of PD

  • Connect the E-load to the output of PD

  • Connect the oscilloscope probe to the power rails near the switcher and on the output

    • Where to make a good connection?

Idea

The input voltages and CV/CC draw of the E-load can be manipulated (within allowable thresholds) to try to recreate the issue (ONE WAY). Turn power supplies (on 12V) on and off slowly to reproduce issue. Once this issue is recreated (no voltage on the output) we should note down our setup.

At this point, if the issue is reproduceable, we can set triggers on the scope to look for inductive spikes or transient voltage droops due to inrush. Show Aryan the buzzing sound.

We can also set a trigger for the output when it falls and see the waveforms on the input that cause this.

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