Readme for QMXPS00, April 3, 2025   KE7ER
This project is hereby open-source, can be used by anybody in anyway for any reason.


A kicad project for a small auxiliary board for the QMX+ to implement replacement 
VCC and VDD switching power supplies using commercially available IC's.
IC's can be either TI LMR50410YQ or the 3Peak TPP361061 (both around 2mhz).
There are perhaps a dozen other possible IC's that could be used with the same 
pinout.  And a hundred in the Sot23 footprint if pinout can be adjusted.

Should be more robust than the firmware SMPS's.
Behaves well when +12v (could be down to +7v) varies wildly for some reason.
Behaves well when output is suddenly shorted and then the short is removed.
Correct function can be easily verified before installing it on a QMX+.


These files are the kicad project for creating rev00 boards
  qmxps.kicad_pcb
  qmxps.kicad_prl
  qmxps.kicad_pro
  qmxps.kicad_sch

These are the gerbers from the above project that were sent to OSHPark
  qmxps00_fab.zip

An updated schematic, adjusted to how my build item was actually stuffed
  qmxps01.pdf


There is some additional circuitry at the top of the PCB that I cut away.
It was intended as an experiment in using a 2A version of these switchers 
as a modulator for the QMX final.  Unfortunately, response is not fast enough.

The 5v switcher supplies the VCC rail, it also powers the 3.3v switcher.
The higher current allows the 5v switcher to operate in continuous mode,
and quiets any birdies from the 3.3v switcher since the voltage drop is small.

I am ignoring the switcher for TX_BIAS.  I'd think that should just be 
PFET Q502 feeding either +12v or VCC to R514 and R516.

Having no switchers getting mothered by the STM32F may make the firmware  
easier to maintain.  



Given the relatively low currents involved, the two inductors could be larger.
The IC's operate at around 2mhz but have 500khz variants for better efficiency.
At 500 khz the inductors should be around 22uH to stay in continuous mode.

The 2mhz birdies are few and fairly weak, could be reduced further:
Use ferrite beads instead of 0.1 ohm resistors would reduce 
Add inductor at Vin to each switcher (with big cap at the switcher),
since current drawn from the source is a square wave.
Increase inductors to 22uH so sawtooth current through them is more constant.
Stuff L shaped header pins into ground holes around periphery, fold copper foil
around the top and solder it to the bottom ground plane.

I have successfully been using it with a 6 pin socket mounted to the new SMPS 
board, mates with a 6 pin header at JP105 soldered to my QMX.  However, if the
ground pins fail this will blow the QMX+, would be safer to have a ground strap
soldered between the SMPS board and the QMX+.  Ideally, all 6 header pins
would be soldered to the SMPS board.

Next rev of this board will have:
  Larger inductors than the now 3x3mm, with pads large enough to do rework.
  Inductors (1uH?) added at input to each switcher to reduce noise on supply.
  Remove optional linear regualtor and unused power input options 
Could be paletized, built at PCBWay or JLCPCB, total parts cost around $1 in China
For example: https://jlcpcb.com/parts/componentSearch?searchTxt=TPP361060

With the same schematic but different board layout, this design could replace
the two plug in SMPS modules on the QMX and old QMX+.
If the two SMPS's must be separated, then run a wire between them for VCC.
Or the 3.3v switcher could operate from +12V, though that could increase noise there.

Significant QMX+ board mods are required to disable the old switchers: 
  Lift the drain lead of Q106 and Q107 to disable the old SMPS's, 
  Add 200k in parallel with R112 and R113 so the firmware thinks it's seeing  
   a bit more than 3.3v and 5v (it hangs if it sees a wee bit less) 
  Short drain to source across Q111 so the 3.3v linear can drive ADC_3V3.
Adding the 200k resistors in parallel with R112 and R113 could be avoided if the
resistive dividers on the new board are adjusted slight for increased voltage.

The mods could be reduced to just shorting LIN_REG_EN to ground (perhaps through 1k)
if QMX firmware is adjusted to not use the old SMPS's when LIN_REG_EN is low at boot.
I don't think most QMX+ users should be encouraged to use this board unless that
firmware mod is available.

If the firmware gets adjusted to suit this board, it could be mass produced 
and easily installed by all current QMX/QMX+ owners.
I could build a first run of autostuffed boards through PCBWay, 
hope somebody else can take over on turning the crank for additional boards.
Long term, I'd like to see something like this integrated into the main board
of future QRP Labs products..

Operating at around 2mhz, most bands will have no additional noise.  A few will 
have one additional noise source at an integer multiple of the switching frequency.
The 6m band might have two weak ones, as it is 4mhz wide.  If switcher noise is 
an issue, there are reasonably cheap SMPS chips (such as the clone TPS54160
up on Chipmall) that could sync to a PWM output from the STM32F so the birdies 
can be adjusted in software.  I feel any birdies can be reduced sufficiently by 
using the ferrite beads and shielding the board with foil, no need for syncing.
After all, many QMX users have an external switcher for power with little complaint.


Jerry, KE7ER   April 3, 2025