# Which parameters should be compared to to replace N-MOSFETS with different package but with same pin functionality?

Since NTZD3154N is used a lot in my design, and I have BSS138PS only for used for one circuit, I want to replace BSS138PS with NTZD3154N in the following DP++ Auxiliary Conversion Block:

Note: This replacement will make the assembly process easier.

Since this is a very important circuit, I'm afraid if this change problematic, since this circuit was tested with BSS138PS only and it works.

As I checked in Datasheet, the test condition for BSS138PS where its guaranteed to work is when VGS=5 V.

But in fact, since this circuit was tested and works, I can deduce that It worked with VGS=2.7 V, Since DP0_AUX/P have 3.3 V voltage level, and VG for Q5A and Q5B is 5V, then VGS could reach: 5 V - 3.3 V = 2.7 V. this VGS could also work for NTZD3154N.

Is my analysis correct?

Circuit Explanation:

When DP0_CONFIG1 (pin 13 in DP) = LOW -> DC Coupling -> Q2,Q3B,Q4A are ON -> DP0_AUXN and DP0_AUXP are passed as DDC_DATA and DDC_CLK, and they are used for HDMI. (the connector is DP and a passive DP to HDMI adapter will be required)

When DP0_CONFIG1 (pin 13 in DP) = High-> AC Coupling -> Q2,Q3B,Q4A are OFF -> DP0_AUXN and DP0_AUXP are passed as Auxiliary signals through C294 and C295, and they are used for Display-Port.

By comparing the two transistors using alltransistors.com, I got the following:

BSS138PS:

Maximum Power Dissipation (Pd): 0.28 W

Maximum Drain-Source Voltage |Vds|: 60 V

Maximum Gate-Source Voltage |Vgs|: 20 V

Maximum Gate-Threshold Voltage |Vgs(th)|: 1.5 V

Maximum Drain Current |Id|: 0.32 A

Maximum Drain-Source On-State Resistance (Rds): 1.6 Ohm

NTZD3154N:

Maximum Power Dissipation (Pd): 0.25 W

Maximum Drain-Source Voltage |Vds|: 20 V

Maximum Gate-Source Voltage |Vgs|: 7 V

Maximum Gate-Threshold Voltage |Vgs(th)|: 1 V

Maximum Drain Current |Id|: 0.54 A

Maximum Drain-Source On-State Resistance (Rds): 0.55 Ohm

Here is the difference in size (NTZD3154N is the smaller one):

• So i would suggest: If it is pin-compatible - which implies the parts beeing footprint compatible: Test it. You can do datasheet comparisons on the essential parameters (Vdsmax, Vgs, RDson, etc) first - then go on an change the device on a dev-board e.g. and test it again. You will have to do testing anyway. So just go for it. Apr 4 at 17:51
• @ElectronicsStudent, Actually, I was mistaken, they have different sizes, but the same functionality and order for all the pins. (that's the reason I'm doing the replacement). to make all the transistor the same and to save more area. I fixed the question, and you can see the picture above. Apr 5 at 4:57
• @greybeard, Yes, all of them are BSS138PS, please be aware that each transistor has two parts, A and B. I divided them into block. and I also updated the picture. you're right, alltransistors.com has some mistakes and I fixed them in my summary) Apr 5 at 7:40
• (I had an inkling counting the terminals.) Seeing no pair used as a differential pair: Does placing 4 or 8 parts, paying 4 dual FETs or 8 standard ones make a difference one way or the other? Apr 5 at 7:46
• In my way of looking at it, it's always better to use dual instead of separate two transistors, in terms of assembly, area etc.. specially when there are a lot of it. Apr 5 at 12:05