HP HSTNS-PD43 PSU Hack & Feedback Circuit (Partial)

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I wanted to hack a high output server PSU for use during automotive ECU programming, and found this HSTNS-PD43 PSU to be a good candidate especially after finding a simple hack to boost output voltage and adjust the OVP setting.


For a better understanding of the PSU's feedback circuit I traced out the auxiliary PCB shown in the video (just the fully accessible side of the double sided/multi-layered PCB, as the backside is mostly blocked by the large main/high voltage cap) and put it in schematic form below.

HSTNS-PD43 Auxiliary PCB (Fully Accessible Side)
Photo


Schematic (Large*)
*Composite image of 8 monitor screenshots: I suggest open(click) the schematic thumbnail below, then right-click on opened schematic > "Save Image As" and reopen downloaded schematic in your preferred image viewing app, for easier viewing(zooming, jumping around etc.).


*Notes*
1. Schematic is in grid form (alphabetical rows / numeric columns). The primary side columns are marked with added "P" and are numbered right to left (to differentiate from secondary side which was completed first).

2. Main output voltage feedback points(vias) are marked "+12V+" and their simulation voltages reflect the set power rail voltage which can be other than 12V, hence the added volt meter(which is obviously not present on the PCB) to show the active set power rail voltage (e.g. 15.5V, 14V, 13.8V etc.).

3. Vias and hidden traces of which connections were confirmed, if not indicated with a visible trace, have been marked with "terminals"(line ending in a circle) and marked with the terminal's grid code, followed by the destination's device pin(s) or another grid code in parentheses - "( )" [e.g. C10(PIC-21/22) suggests, terminal in C10 connects with PIC pins 21/22].

4. Vias or trace ends who's destinations have not been determined are marked with a "?" followed by their grid locations (e.g. ?E13)

5. All caps are uniformly marked 1uF as they cannot be easily determined without desoldering and doing so would be a pain as they are so tiny. Also, the cap symbol on the schematic is for a polarized cap, but obviously those on this auxiliary PCB are NOT(please ignore the suggestion of polarization - my bad).

6. The first 1/3rd on the left is isolated from the 2/3rds on the right.

7. Ground(GND) points are marked with measured resistance values* with respect to the main output ground tab or in the primary section's case, the negative pin of the main high voltage cap(cap voltage @ 0.0V).
*e.g. 0R, 0R1, 0R2 for 0.0, 0.1, 0.2Ohm etc. - a few are higher and may not be "direct" links to ground.

[The Left Side]
1. TI UCD3138 (Digital Controller)
2. TI LMR12010 (Switching Regulator marked "SF7B")

[The Right Side]
1. Microchip dsPIC33FJ64GS606
2. TI UCC3895 (Phase-Shift PWM Controller)
3. TI LMR12010(Switching Regulator marked "SF7B")
4. Several OpAmps/Comparators

[Backside Of PCB]
There are more ICs and 3 more opto-couplers.
         

The 3 connectors in the 1st of 3 photos above from left to right are for AC input control(black), fan, and status LED. They were disconnected for better views of the backside.

[Opto-Coupler On Mainboard]
I even spotted one on the main PCB  "hiding out" between the big cap and the auxiliary PCB.


[Daughter Board]
Has numerous OnSemi FDMS039N08B N-Channel POWERTRENCH MOSFETs.


[Simulation Notes Regarding Voltage and OVP Hack]
Note: Only "passive" components(resistors, capacitors, and single diodes) were active during simulation(all "active" semiconductors were "excluded" from simulation as I do not have access to the software driving the controls, and therefore opamps/comparators would not function as they are intended).

1. In "normal" 12V operation, the 12V feedback voltage is reduced to about 2.7V at the initial comparator's negative input(LM324A pin-2, in D8), and the OVP related voltage to about 4V at LM393 pin-3(in D12).


2. When the hack resistors(12k & 4.3k as in the video) are connected, the 2.7V drops to about 1.8V, and the 4V drops to about 1.6V. Note: this is with the simulation power rail kept at 12.3V.


3. If the simulation power rail is adjusted to 15.5V output(as in the unit in the video), the 2.7V drops to 2.27V, and the 4V drops to 2.07V.


Higher valued resistors drop the FB voltages less so lower voltage boosting is possible.


If there are any issues with the schematic, I'd appreciate comments, and will update the schematic above in this first post so it is always the updated version.

Cheers~

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I've moved the questions from the first post to this 2nd post (for better visibility), and added some more below.

[Questions]
Despite having observed all of the above, I'm still baffled about the following:

1. What type of feedback compensator is used in this PSU's feedback circuit and where are its core traces/components on the schematic? Perhaps a "type 3" mutant not in textbooks?

2. How is the OVP being controlled and where are its core traces/components on the schematic?

3. What are the roles of the 2 switching regulators marked "SF7B" (Texas Instruments LMR12010) - 1 on the primary side and 1 on the secondary side?
* In low cost PSUs, a single switching regulator like the "SF7B" is used as the "brains" for control, and its FB(Feed Back) input's voltage divider resistors can be manipulated for voltage adjustment. But the PD43 PSU has relatively advanced ICs controlling this switching regulator(and 2 of them!), so I'm wondering how these 2 regulators are "specialized" for the digital controller on the left primary side, and for the PIC on the right secondary side.

Thanks...