Power Supply repair - testing a Mosfet?

[solarbot]

Hi, I'm working on the repair of a power supply for my 3D printer which went short circuit last week.  I've traced one leg of the rectifer chip as being short, this is now replaced which gives a bit of hope as LED D40 comes on but I'm only measuring 5V at the 19V jack output.  The 24V input measures just fine from the external input.

My current suspicion is the power mosfet just below the rectifier circled red: CSD18501Q5A by TI - Pins 1,2,3 are source, 4 gate and 5,6,7,8 drain.  My multimeter in dioide mode measures 0.002v between gain and source, 0.514v source positive/drain negative and 1.658v source negative/drain positive.  It looks fried to me but what do the experts think.... perhaps I'm missing something?

Thanks

[Kleinstein]

The 0.002 V gate to source could be a blown MOSFET, but it could also be a blown gate driver, if here is no series resistor at the gate.
I would probably just lift / cut the gate pin of ther MOSFET to get an isolated reading.

[solarbot]

Thanks Kleinstein, with the chip off the board it operates normally..... so the hunt goes on!

[solarbot]

No sure where to look next.... the LM25088 regulator?.... any tips on how to test this on-board?  Unfortunatley the LM25088 seems to be out of stock eveywhere so hoping its not that.

[perieanuo]

Thanks Kleinstein, with the chip off the board it operates normally..... so the hunt goes on!

what do you mean by that?
from your pic, i bet pin HG from L25088 goes with his via to your problematic mosfet gate
if the mosfet is dead, how the power supply operates at full load?
 remove the mosfet, tell us if G-S short is comming from mosfet G-S or L25088 or whatever
i bet mosfet is dead, anyway you can "activate" him, once removed, with multimeter with 9V battery or just do your fav test or just solder another mosfet, a through-hole for testing can do it.and can remain like this for 2 years until the next voltage fluctuation kills him too
careful at equivalent mosfet rdson spec

[ErwinsCat]

one of the first things I usually do is finding the chips pinout and from the pinout see if they get correct voltages or not and before that see if any of those rails shorted this can help to isolate the issue.

[solarbot]

Thanks folks.

I have removed the CSD18501Q5A mosfet, when I test with my meter in dioide mode I get the following:

Drain Negative, Source Positive = 0.51V
Drain Positive, Source Negative = Open Loop
Drain Positive, Source Negative, Gate Positve = 1.88V - this changes to Open Loop when Gate is connected to Negative

Is this looking OK or Bad?

I will attempt to check the regulator pin voltages now.

[solarbot]

Hi again,

I've just measure the voltages across the CSD18501Q5A mosfet pads on the PCB which are as follows:

Drain to Gate = 19.6V
Drain to Source = 19.6V
Source to Gate = 0V

.... which looks correct to me so I guess the Mosfet is blown?

[vintagepc]

Not sure if it's just shadows or not but the left lower region of the component in your picture has a subtly different colour tint and shading to it than the rest - more yellowish.

Can you confirm that's not a crack in the epoxy casing of the part and it has indeed let out the magic smoke?

[solarbot]

Hi Vintagepc, are you referring to the mosfet?.... I've had a closer look and it seems to be OK visually.

[solarbot]

So I've just taken some measurements across the mosfet pads on the PCB relative to ground - with the mosfet removed.

Drain to GND = 24V
Source to GND = 4.8V
Gate to GND = 4.8V

I'm not sure what to make of these readings, any thoughts?

[aqibi2000]

Measure the resistance source to gate with no voltage applied, is it 0 ohms?

[solarbot]

Measuring the resistance between Source and Gate.....

On the PCB, no power applied, Mosfet removed = 0.44 Ohms
On the Mosfet, removed from PCB no power = Open Loop

[perieanuo]

looks like your pwm chip has left the building
anyway change the mosfet at least for testing purposes after you decide pwm ic is dead or he doesn't run from another cause, like vcc missing

[solarbot]

Thanks, I've just ordered replacements for the CSD18501Q5A Mosfet and LM25088 regulator - should know more soon and will report back - thanks for the help.

[solarbot]

So, changed the CSD18501Q5A Mosfet and LM25088 regulator and 2 electrolytics and it all works again - thanks for your help folks