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Notes ‐ v3.1 PCB

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Nunu edited this page Mar 3, 2025 · 10 revisions

Changes

After testing here is the list of required changes:

  • desolder 24V input and output ESD diodes - they start conducting at 18V (out of spec), and get really hot quickly at these voltages, if your input/output voltage will not exceed 12V they should be fine

Recommended changes:

  • replace one of the voltage regulator resistors to 1K (can be 0603 or 0805 size)
    • this will raise the Vmcu to 5V from 3.7V and thus will increase the voltage at which the MOSFET is driven (leading to increased efficiency), but also make the LEDs brighter
    • the cost of this operation is the increase of the quiescent current of the voltage regulator from 25uA to around 0.7mA
  • replace input reverse polarity protection diode (the one next to CHARGE led) with a 2A fuse/polyfuse
    • this will improve efficiency and should also protect against reverse polarity if the source is capable of providing enough current to trip the fuse
    • note that MCU power has its own reverse polarity protection
    • 2A polyfuse trips after a few seconds of 2.5A current and after tripping current is still around 0.7A - this is not ideal - as this can still cause issues with power source, but should protect the wiring. given the that vectatus is meant to be used as a portable, emergency power converter this fits the needs for this project. if however vectatus is meant to be used as unsupervised, standalone device then a FAST, SINGLE USE 2A FUSE SHOULD BE USED INSTEAD

vectatus-pcb

Load testing

After testing Vectatus with 55W 12V light bulb to test the limits and thermals I found out that:

  • I was able to drive this high load efficiently, the SEPIC inductor got hot first, and then MOSFET got slightly warm
  • I am still testing the safe maximum power operating conditions for these boards
  • I was a bit worried about the ceramic capacitors used and their ability to drive loads, but surprisingly they work really well
  • By rough current measurement I measured 90-95% conversion efficiency during tests (this number might be lower due to measurement error)
  • As I increased the load I burned the SEPIC output diode SS54
    • The diode failed short because the reverse voltage of the diode must be greater than Vin+Vout in this case it was 18V in and 12V out giving 30V, the SS54 is rated at 40V, but this was enough to end the diode
    • After this state shorted diode, causes the MOSFET to short drawing excessive amounts of current from input

Given above following improvements are needed:

  • upgrade the SEPIC diode to one with higher reverse voltage rating (like SS56)
  • install resetable polyfuse on input to protect from MOSFET failing short condition

Voltage regulator tests

I was able to cause a failure to the MCU voltage regulator circuit. By doing repetitive on-off arcing with beefy power supply the 2 ohm input resistor failed open. This resistor was there to act as a fuse and it did, but perhaps it was too sensitive.

Of course after shorting the resistor I can confirm that the voltage regulator is working and was not harmed.

Given above:

  • replace the 2 ohm resistor with either a ferrite bead (500mA) or 0 ohm resistor.
  • I used 100mA polyfuse and it also works

SEPIC diode

The SS54 (5A rated) used seems to be sufficient up to 2A of output current at which it gets really hot, but the thermal layout of the PCB is enough to dissipate this heat to some degree for continuous operation.

If higher output currents are desired a SS1045 diode (10A rated) could be an upgrade candidate (efficiency might slightly increase as well due to lower voltage drop).

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