Since the Low ripple functionality is now also implemented on the firmware side we can test it on the real equipment. 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.

Entering low ripple mode of operation require monitoring of the max. output power. The maximum output power is limited by the first of the following conditions that is met:

• The pre-regulator regulation element (switching mosfet) capability is limited with max. allowed continuous current (SOA_PREG_CURR value) for the applied input voltage (SOA_VIN value). The set output current cannot exceeds this value in any moment.
• The post-regulator regulation element (pass mosfet) could dissipate finite power (SOA_POSTREG_PTOT value) while load is connected. Therefore output power cannot exceed product of voltage difference (SOA_VIN and output voltage) and output current.

For example, when output voltage is set to 12 V and SOA_VIN = 50 V, SOA_PREG_CURR = 0.8 A andSOA_POSTREG_PTOT = 20 W, the max. output current in low ripple mode cannot exceed 0.526 A because SOA_VIN-VOLT = 50-12 = 38 V andImax=SOA_POSTREG_PTOT/38 = 20/38 = 0.526 A.

In annother example if the output voltage is set to 36 V with the same SOA values as in example above the max. output current cannot exceed 0.8 A (limited by SOA_PREG_CURR) regardless of the fact that max. power dissipation of 20 W allows output current of up to 1.428 A or 20/(50-36).

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.