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Mosfet not working properly

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soumen21

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I have a mosfet FDS7088N7 whose gate is driven by TPS2321. The board is working fine when working on placed on a backplane. While the board is working a I get a Vgs of 10.5V. I replaced FDS7088N7 with FDMS7672. On replacement the power supply board is not working when kept in the backplane. When the board is not working the Vgs voltage is 3.3V. Can anyone let me know what is wrong with the replacement ? This mosfet in question is used in a load sharing circuit.
 

Your replacement MOSFET has a distinctly larger RDS(on)=5mΩ @ VGS=10V , compared to FDS7088N7 RDS(on)=3mΩ @ VGS=10V , i.e. +67%. Perhaps your load sharing circuit doesn't like it?
 
When the board is not working the Vgs voltage is 3.3V.
Click to expand...
What's the voltage at the other FET terminals? Sounds like the gate voltage is shorted, e.g. due incorrectly soldered part.
 

TPS2321 suggest many mosfet whose Rdson is greater than the FDMS7672. Is Rds on only factor or there was other parameters to be considered ? Will FDMS7660 solve my problem. It has an Rds on of 2.8 Ohms at 10V.
Please refer
**broken link removed**
 

soumen21 said:
Will FDMS7660 solve my problem. It has an Rds on of 2.8 Ohms at 10V.
Click to expand...

I'd hope so. You mentioned this is a "load sharing circuit", so I suppose there are several of these output MOSFETs in parallel, right? They should all be the same (from the same type, same producer). If this isn't possible, they should at least own the same ratings concerning voltage, current, thermal resistance, and, most important, Rds, to allow for a fair current distribution between all of them. Very important are individual (and identical) gate series resistors (Rg in Figure 1 of the a.m. application note). Check if they are really equal -- may be that one for the exchanged MOSFET has changed its value due to an overload?

Also, measure and compare the individual Vgs voltages in order to find a possible circuit error, as FvM suggested above.
 

You have pin number 1 twice for gate and drain of the MOSFET.

P.S.: The pinning looks messed up anyway. There should be 3 source terminals rather than 1.
 
Last edited:

The schematics shown is with FDS7088N7.
Pinouts of FDS7088N7 - Gate - Pin 4
Source - Pins 1,2,3,5,6,7,8
Drain - Bottom of the Chip

In the board I have externally wired FDMS7672 removing FDS7088. FDMS7672 have 3 source pins.

---------- Post added at 15:42 ---------- Previous post was at 15:41 ----------

I will measure and post the values.
 

The circuit is working now. It was an issue with Rds(on). I replaced FDMS7672 with FDMS7670 whose Rds(On) is less then FDMS7672. It worked.
However, I plan to use FDMS7660 in my circuit once I receive the samples.
Thanks a lot.
 

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