EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: saronno on November 06, 2019, 05:38:39 pm
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I am trying to repair a nvidia gtx 780 and after checking the main rails
it came out this X inductor (it's named L17) is open. Obviosly, I checked if there was a short and yes, a shorted capacitor (part of the 12V rail filter powering up the phase responsible for the Vcore of the GPU)
was responsible for the killing of the X inductor.
So far, so good. But, what about this X code. What is the meaning?
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It's a fuse (why they write L near it .. I don't know) .... but this code X doesn't help a lot in order to find the correct value and type of the little beast.
Can anyone guess what kind of fuse and hwo many ampere it's the correct value here? The GPU is a GTX 780
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https://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_437_datasheet.pdf.pdf (https://www.littelfuse.com/~/media/electronics/datasheets/fuses/littelfuse_fuse_437_datasheet.pdf.pdf)
R = 3.5A
X = 8A
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It depends ... http://www.mouser.com/pdfdocs/TEConnectivitysurfacemountfuses.pdf (http://www.mouser.com/pdfdocs/TEConnectivitysurfacemountfuses.pdf)
here (pag 94) X is 12A and R is 8A.
It's not so easy unfortunately.
Voltage rate is easy .... above 12v.
But hwo much ampere has to be guess from consumption ... GTX 780 has a tdp of 250W and core voltage of GPU is in the order of 1.0-1.20 V.
A lot of current ....
These are PWM voltage regulator. So current is absorbed (from 12v rail) just a fraction of time. If it was costant you wold have 20 ampere more or less from R+X+R fuse.
But it's not. And of course, it's important also how they are sincronized. If one if on when the other is off ...
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Maybe you could work out the current rating by determining the current requirements of each of the circuits which are fed from this input.
For example, RS5 (R005) appears to be a 5 mOhm current sense resistor for one of these circuits. I expect that the datasheet for the associated IC would tell you the corresponding current limit.
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Maybe you could work out the current rating by determining the current requirements of each of the circuits which are fed from this input.
For example, RS5 (R005) appears to be a 5 mOhm current sense resistor for one of these circuits. I expect that the datasheet for the associated IC would tell you the corresponding current limit.
Good idea.
Another way (or just to check the value obtained as you suggested) is from power ..
(https://imgur.com/JeiHfio.png)
The upper R take the 12V from PCI-Express.
The middle X from 8 pin connector.
The lower R from 6 pin connector.
PCI-Express is rated 75 watt max if I remember correctly.
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http://www.ti.com/lit/ds/sbos576b/sbos576b.pdf (http://www.ti.com/lit/ds/sbos576b/sbos576b.pdf)
This is the component responsible for the current.
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Unfortunately that doesn't look helpful.
Page 27 of the PDF states that the maximum shunt voltage is 163.8mV. That corresponds to a current of 32.76A across a 5 mOhm shunt.
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Anyway looking at some datasheets I found on littlefuse, it could be 8A or 12A ... probably the second.
The TDP in 250 W .... 12v x 20A ... 240W .... R fuse on the same series is 8A ... so 8+12 = 20A (R+X fuse)
Of course, the current is not always present since this is powering a switching regulator ... but probably
(with 8 phases) 80% of the time .... so they should roughly be 10A(R) and 15A(X)
But TDP include also RAM and other thing so 8A + 12A sound good to me.
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You need to consider average current rather than instantaneous current when dealing with a high frequency switchmode regulator.
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In this case I don't think there is a big difference since it's a polyphase regulator and one phase or the other drains current
about 80% of the time from the 12V rail ...