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# Motherboard CPU Voltage Regulation FET swap

So here's my motherboard's CPU voltage regulation schematic:

Now that looks like a decent 3-phase setup, but unfortunately those nice FETs are not the ones that made it to production. Instead all FETs are much weaker mdu1516 FETs, and half of the high-side FETs were removed.

mdu1516: 14 mOhm resistance when Vgs = 4.5V
mdu2654: 4.5 mOhm resistance when Vgs = 4.5V
mdu2657: 11.3mOhm resistance when Vgs = 4.5V
mdu1512: 5.0 mOhm resistance when Vgs = 4.5V

Now these resistance values are rough estimates since this is a MOSFET and not a resistor, but since all that differs between these transistors is likely width/length ratios, it's safe to say that the production high side is 2.48 times as resistive as planned, and the low side is 3.11 times as resistive. If we take the worst case scenario of 14 mOhm resistance, then pulling 89W at 1.371V (what I need for 4.4ghz) is the following:

I = V / R
I = 89W / 1.371V
I = 65A total
I = 21.7A per phase

P = I^2 * R
P = 21.7A^2 * 14mOhm
P = 6.57W per phase
P = 19.7W total due to high-side power FETs alone!

This is a ridiculous amount of power draw just from power FETs. Yes it's a worst case scenario, but it shows that power losses in the voltage regulation is extremely high. Adding in the inductors could make this well over 20W. Using the planned high side power FETs is a much more reasonable 8W worst case scenario, thus I am looking into removing the current FETs and putting the planned FETs in. I have some questions though:

1. Does any other component need to be changed to accommodate the new FETs?

2. Currently turbo turns off when the CPU is drawing high amounts of power at normal room temps with the CPU in the mid 80s. It takes several minutes of high power draw before this occurs. It will turn back on after a few seconds, then off again a few seconds later with this cycle repeating indefinitely. This does not happen in a cold environment, or if a screw up die contact so that the CPU runs hotter and can hit TJmax. My thinking is that a motherboard component is triggering an overheat signal, cutting power draw by disabling turbo to save the motherboard. The only component close to the FETs with a heat sensor are the MOSFET gate drivers. By putting in bigger FETs, will I be putting more strain on the gate driver and exacerbate this issue?

3. To simplify purchasing can I buy matching high-side and low-side FETs? I was thinking of getting all mdu2654 or all mdu1512 FETs for both high side and low side.

4. Does anyone have an mdu2654 datasheet? I'm mainly interested in the gate capacitance. I found current ratings from a screenshot, but the picture lacked the capacitance numbers. I am concerned with overloading the gate driver.

5. The FETs are far enough away from the CPU socket that I can shield the socket from my heat gun when replacing the FETs. The same cannot be said for the inductors. Downsizing the inductors and adding caps to spread out the increased ripple current can also significantly reduce voltage regulation power draw, and improve output voltage stability in the process. My thinking is that this may melt the CPU socket, but it might not since the socket was likely put on in the first place using a oven hot enough to melt lead-free solder, so maybe the socket can take the heat. Is replacing the inductors a possibility, or is melting the socket too likely?

I only checked the MOSFET part numbers and not the gate driver parts. It is possible that they also cut back on the gate drivers, but for now I am assuming that they did not.

Oh and some system info:

Clevo P150EM with HEAVILY modified cooling (better than m18x cooling)
mediocre overclocker 3920xm. Power limits and current limits are set over 200.
4.2ghz in P95 tops out at 81C, 76W power draw. No throttling. 1.291V VID
4.3ghz in P95 tops out at 86C, 81W power draw. Turbo turns off for 3 seconds roughly every few minutes. 1.321V VID.
4.4ghz in P95 tops out at 88C, 89W power draw. Turbo turns off for 3 seconds roughly every 20 seconds. 1.371V VID.
4.5ghz in P95 throttles terribly and would probably hit 100C if it did not.

The voltage regulation cooling is tied to the CPU cooling. Reducing voltage regulation draw should also reduce CPU temps slightly, hopefully keeping the CPU temp under 90C at 4.4ghz. Note that laptops only have around 60% of the VCC and GND pins as a desktop and have no load-line compensation, thus vdroop is immense and the CPU never sees anywhere near the VID input.

I have run a 2920xm sandy bridge in the system in the past and pulled 105W for a short burst. I have pulled mid 90s on the ivy bridge. BIOS power limits do not seem to be an issue. Again this seems motherboard overheating based.

Currently I am thinking of just putting all MDU1512 chips in, filling the empty high-side FET spot as well. This will cut high-side power draw by 80%. Maybe I will put MDU1511 chips in for the low side, but I am concerned they will be too hard on the gate drivers.

If anyone has any input I would really appreciate it. Even if you aren't sure but have your own story of something you did I would really like to hear it. Or if you see something I could do to improve the CPU overclock please let me know that too. I also have the CPU VRM schematic if that would be helpful.
Quote:
Originally Posted by Khenglish

So here's my motherboard's CPU voltage regulation schematic:

I admire your skills. Most people would just but a motherboard with better voltage regulation.

How did you get those schematics?
 My System (15 items)
CPUMotherboardGraphicsRAM
FX6300 Black M5A99X EVO R2.0 Nvidia GTS450 Team Vulcan PC3 12800
Hard DriveOptical DriveCoolingOS
Samsung 840 PRO Asus DRW-1608P (x2) Custom Water Cooling Win7 (Ult), Win 8.1 & Win Server 2012 R2
MonitorKeyboardPowerCase
2 X Samsung 915N Ducky Shine III, Blue Cherry/Blue LEDs PCP&C 1kw Lian Li PC-71 (W/Window)
MouseAudio
Logiteck G400s none
 My System (15 items)
CPUMotherboardGraphicsRAM
FX6300 Black M5A99X EVO R2.0 Nvidia GTS450 Team Vulcan PC3 12800
Hard DriveOptical DriveCoolingOS
Samsung 840 PRO Asus DRW-1608P (x2) Custom Water Cooling Win7 (Ult), Win 8.1 & Win Server 2012 R2
MonitorKeyboardPowerCase
2 X Samsung 915N Ducky Shine III, Blue Cherry/Blue LEDs PCP&C 1kw Lian Li PC-71 (W/Window)
MouseAudio
Logiteck G400s none
Clevo provides all motherboard circuit schematics for their laptops right on their website. I assume it's so resellers can do repairs.
Thanks.
I was looking for a desktop board, Asus M5A99X EVO R2.
 My System (15 items)
CPUMotherboardGraphicsRAM
FX6300 Black M5A99X EVO R2.0 Nvidia GTS450 Team Vulcan PC3 12800
Hard DriveOptical DriveCoolingOS
Samsung 840 PRO Asus DRW-1608P (x2) Custom Water Cooling Win7 (Ult), Win 8.1 & Win Server 2012 R2
MonitorKeyboardPowerCase
2 X Samsung 915N Ducky Shine III, Blue Cherry/Blue LEDs PCP&C 1kw Lian Li PC-71 (W/Window)
MouseAudio
Logiteck G400s none
 My System (15 items)
CPUMotherboardGraphicsRAM
FX6300 Black M5A99X EVO R2.0 Nvidia GTS450 Team Vulcan PC3 12800
Hard DriveOptical DriveCoolingOS
Samsung 840 PRO Asus DRW-1608P (x2) Custom Water Cooling Win7 (Ult), Win 8.1 & Win Server 2012 R2
MonitorKeyboardPowerCase
2 X Samsung 915N Ducky Shine III, Blue Cherry/Blue LEDs PCP&C 1kw Lian Li PC-71 (W/Window)
MouseAudio
Logiteck G400s none

Check the MOSFET specifications. If gate charge is the same/very similar in the one you plan to swap in, and the pinout is the same (to be assumed if it's the same manufacturer and family), the swap is just remove and drop the new one in.

As for adding the remaining ones, I wouldn't go that way. There is likely to be a part of the driving circuitry missing. You could always post a picture though.

 Does computer things (22 items) Portable AIO Server (11 items) AGP bencher (14 items)
CPUMotherboardGraphicsRAM
Ryzen R7 1700 Gigabyte GA-AX370-Gaming 5 Sapphire HD 6950 2GiB 2x8GB KFA2 HOF DDR4-3600
Hard DriveHard DriveHard DriveHard Drive
Crucial MX100 256GB Seagate 600 Series 240GB Seagate 7200.14 2TB Samsung F3 1TB
CoolingCoolingCoolingCooling
EKWB Supreme HF XSPC Rasa GPU EK XT360 EK 4.0
OSMonitorMonitorKeyboard
W10 Pro LG IPS235 LG E2250V KUL ES-87
PowerCaseMouseAudio
SF Leadex II 650W Lian Li PC-A05NB Logitech G9 Xonar DX
AudioAudio
SMSL SA-S3+Technics CB-250 Sennheiser HD555
CPUMotherboardRAMHard Drive
AMD A10-5700 Gigabyte F2A75M-HD2 G.SKILL Ares 2133 CL9 Hitachi 5K750
Hard DriveCoolingOSMonitor
Momentus .7 200GB Noctua NH-L9a Server 2012 R2 Standard AUO B156HW01
PowerCaseOther
CPUCPUCPUMotherboard
Core2Duo E6400 Core2Quad Q6600 Pentium Dual Core E5200 AsRock 4COREDUAL-SATA2 R2.0
GraphicsRAMHard DriveOptical Drive
A dumpload of ancient AGP cards Kingston Value DDR2-667 CL4 2T @CL3 1T Seagate 160GB 7200.10 LG IDE DVD-ROM
CoolingCoolingOSMonitor
Ghettomade CPU waterblock 49cc 2stroke engine copper radiator WinXP SP2 32bit ProView 17"
PowerCase
Tacens Radix V 550W Ghetto aluminium bench
 Does computer things (22 items) Portable AIO Server (11 items) AGP bencher (14 items)
CPUMotherboardGraphicsRAM
Ryzen R7 1700 Gigabyte GA-AX370-Gaming 5 Sapphire HD 6950 2GiB 2x8GB KFA2 HOF DDR4-3600
Hard DriveHard DriveHard DriveHard Drive
Crucial MX100 256GB Seagate 600 Series 240GB Seagate 7200.14 2TB Samsung F3 1TB
CoolingCoolingCoolingCooling
EKWB Supreme HF XSPC Rasa GPU EK XT360 EK 4.0
OSMonitorMonitorKeyboard
W10 Pro LG IPS235 LG E2250V KUL ES-87
PowerCaseMouseAudio
SF Leadex II 650W Lian Li PC-A05NB Logitech G9 Xonar DX
AudioAudio
SMSL SA-S3+Technics CB-250 Sennheiser HD555
CPUMotherboardRAMHard Drive
AMD A10-5700 Gigabyte F2A75M-HD2 G.SKILL Ares 2133 CL9 Hitachi 5K750
Hard DriveCoolingOSMonitor
Momentus .7 200GB Noctua NH-L9a Server 2012 R2 Standard AUO B156HW01
PowerCaseOther
CPUCPUCPUMotherboard
Core2Duo E6400 Core2Quad Q6600 Pentium Dual Core E5200 AsRock 4COREDUAL-SATA2 R2.0
GraphicsRAMHard DriveOptical Drive
A dumpload of ancient AGP cards Kingston Value DDR2-667 CL4 2T @CL3 1T Seagate 160GB 7200.10 LG IDE DVD-ROM
CoolingCoolingOSMonitor
Ghettomade CPU waterblock 49cc 2stroke engine copper radiator WinXP SP2 32bit ProView 17"
PowerCase
Tacens Radix V 550W Ghetto aluminium bench
Yeah I'm trying to get the MDU2654 datasheet, but am having problems since it's EOL. I did find its current ratings and equivalent resistances though and they are slightly better than the MDU1512, but slightly worse than the MDU1511. Thus I assume that the MDU1512 has a slightly lower gate capacitance than the 2654, but it's possible that the 2654 is a shorter device and thus has a lower gate capacitance despite being able to supply more current.

As for missing circuitry for the second high-side FET, it's in parallel to the existing FET, so there is no extra circuitry required. They may have changed resistor and capacitor values associated with the gate driver. I can check resistor values with my multimeter, but not capacitance (it has a mode for measuring capacitance, but it does not work).

So by putting in all 1512 chips I suspect I will exceed the original planned gate capacitance for the high-side, but be slightly lower than planned on the low-side. I could just put in a second MDU1516 to be safe on the high-side, or maybe just a single 1512, or leave the 1516 and add a 1512. I think it is OK to mix chip types like that if they are in parallel (right?).

So do you think I can do two 1512 low-side, and 1 1512 with 1 1516 high-side per phase? That should be very close to the original specs in terms of current capability and gate capacitance. I really need a 2654 and 2657 datasheet to be sure though.
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