Since the Low ripple functionality is now also implemented on the
firmware side we can test it on the real equipment. Idea about bypassing switching pre-regulator comes from void_error (thank again for that). When "Low ripple" mode of operation is selected the power pre-regulator is bypassed by setting so-called Duty cycle of the SMPS controller to 100 %. That will disable switching frequency and therefore that otherwise hard to filter component disappear entirely from the channel output. In that case the remaining noise should mainly come from the low power bias switching pre-regulator (LM5574) and the Arduino Due board.
Max. output current for pre-regulator mosfet (
SUD19P06) is limited in firmware to 0.8 A. That value comes from its SOA chart:
Maybe I should add some reserve and lower it to 10-20 %. Your input here is welcome. 50 V point is chosen since that is DC voltage at the power board input (LRS-150-48 module). As max. power dissipation for post-regulator's mosfet mounted on heatsink a 20 W is chosen. In real life I'm going to put that heatsink near 60 mm fan that should easily handle 40 W for two channels.
This time as a measurement point a 4mm to BNC adapter is used and no additional capacitors are connected on the output (only output capacitor is 15u in parallel with 470n+1R). Let's starts with grounded probe input:
When power is switched off:
OUTPut OFF means that SMPS pre-regulator is completely shut down and post-regulator's mosfet bias is switched off (Arduino Due is off course active):
Connected load is 16R4 power resistor that with output voltage set to 7 V consume about 0.43 A. That is within limits for low ripple mode of operation, but first how it looks when LRIPple if OFF:
and finally when LRIPple is ON:
It works!
While in this mode of operation and set parameters note that dissipation on post-regulator mosfet is about 18.5 W! In the same time pre-regulator mosfet is cold since with Rds(on) of 60 m? is dissipate 25 mW.
If you compare measured noise with LRIPple OFF (SMPS is turned on) with that from e.g. post
#310 you can see that this one are much more dirty. That is possibly due to the different method of signal probing. I also found that grounding or lack of it can influence greatly appearance on the scope of that noise. The Arduino shield board has 10n capacitors connected from power output terminals to PE (protective earth line), adding the same combination at the input of the pre-regulator (after common-mode choke) can also make a change at least when AC/DC adapter module as in this example is used.