Asus P4C800-E VRM issues

[c_corr]

This board had the initial fault in VRM with high side Mosfet from PWM1 (phase 1) getting very hot. It also had the VRM caps quicly heating up in 10-15 seconds after power on.

This board uses mext ICs and Mosfets in VRM:
- ADP3180 PWM controller - in 3 phase mode
- ADP3418 as Modfet drivers
- High side Mosfets STD38NH02L (from ST)
- Low side Mosfets STD90NH02L

Some of the symptoms were very similar with the "Dead Asus P5P800-VM" in this forum.
I had the sawtooth voltage on Vcore too this was probably the cause of the Vcore caps getting hot.

Vcore caps repalced as they were old anyway.
I also replaced in the affcted phase 1 the Mosfet driver ADP3180 and the High side Mosfet with a higher spec STD86N3LH5 (bought a number of these from RS Components to fit as low side Mosfets too if needed).
Had to repalce the ADP3180 PWM controller to fix the sawtooth voltage on Vcore.
VRM phases (ADP3180) run at 132Khz.

Tested. the board with a Pentium 4 3 GHz and the stock CPU cooler from Intel.

VRM seem to work ok now but I think it may be still heating up more than usual. I know these boards and the CPU usually produce lots of heat.
With the stock CPU cooler installed and the fan running - VRM caps go quite warm after 3 min of operation displaying the Bios menus.

I am unclear if the Vcore ripple I see when in Bios screen is normal or not. I have no other similar generation mobo to measure Vcore for comparison.
Waveforms look normal at the Mosfet driver ICs input and all three high side mosfets.
I see a bit of ringing on the Vgs of all three low side Mosfets - I am not sure if this is normal wonder if this can cause the possible extra heating.


Is the Vcore ripple when in Bios screen looking ok on the waveform pic?
Is the ringing on the Vgs of all three low side Mosfets normal? Or can this cause extra heat dissipation?

Waveform pics are quick screen photos hope they are clear enough.

[c_corr]

I used these Panasonic FK seriers caps for recapping the VRM Vcore line. Hope they are the right type for the VRM needs.
https://uk.farnell.com/panasonic/eeu...-20/dp/1744873

Plan to replace the remaining two phases high side Mosfets with the higher spec STD86N3LH5 and check if any improvement.

[DynaxSC]

Screens look good, the ringing is normal. Pls be more specific, "quite warm" is not a very exact information. Measure the temperatures, test the vrm under full load. This is also a quite old construction, so vrm efficiency (thermal losses) might be not as good as today. The heat might come also from the coils resistance. Have you checked if the replacement mosftes have lower Rdson (this is crucial for thermal lossess) ?

Regarding the Panasonic caps you should consult Panasonic design guide (available on registration), the FK datasheet itself is crap, has completely no information about application scenarios. For pwm vrms important for the caps is very low internal impedance, and capability to work with quite high frequencies (low internal inductive component).

[c_corr]

Thank you for the very good suggestions. Did not know about the Panasonic design guide. This is the first VRM I repaired (I usually repair 230V SMPS) and was not sure about the small ringing on the waveform.
"Quite warm" is indeed not the best description to use will remeber this.I have not measured with a thermometer but I estimated 45 degrees.
You are correct about the efficiency of the older VRMs, nothing seems to be wrong anymore with that VRM. The heat generated by CPU seems to slowly move through the PCB copper layers into the VRM caps and coils that are nearby.
Board tested meantime with WIN XP, AGP video card and some demanding games from those times and works ok - it needed an additional fan to blow air in the VRM.and chipset heatsink.
This mainboard model seems to be known for producing extra heat.

I measured the inductance and Q factor of each coil (with a DE-5000) to make sure there were no shortcircuited windings and they all tested the same.

I think I should be on the safe side with the mosfets.
Original low side Mosfets STD90NH02L were N-channel 24V - 0.0052Ω - 60A.
Modern replacement low side I used STD86N3LH5 is automotive-grade N-channel 30 V, 0.0045 Ω, 80 A.

High side Mosfet that failed was STD38NH02L N-channel 24V - 0.011Ω - 38A. Replaced all 3 high side Mosfets with the above mentioned STD86N3LH5 as it has lower Rds(on) than the orginal.

Might be useful to say I found these needed ICs for this repair on AliExpress site (they hapended to be the real deal):
- ADP3180 PWM controller
- ADP3418 Mosfet drivers
I normally use only proper grade components as RS comp or Farnell in SMPS I fix but ADP3180 was nowhere else easily available.