How to select a replacement MOSFET/transistor?

[dfear]

How to select a replacement MOSFET/transistor?

Regarding a recent post,

Is this the correct method to select a replacement transistor (when the one on the board is discontinued or unavailable to purchase)?

My method:
1) Match the transistor type (NPN or PNP).
2) Match the "Vceo" voltage.
3) Same or higher "Collector current".
4) Same or very similar sized package.
3) buy the complimentary transistor at the same time.

Anything else I should be considering?

 

[Ian Finch]

Read up some on mosfets and bipolar transistors as you may be a bit confused about the different semiconductor types, their attributes and internal features. You've actually referred to the characteristics of standard bipolar junction transistors there and you may also need to consider those more appropriate for mosfets, such as maximum VDS, VGS and ID. Those specs. accord with the maximum working drain-to-source voltage, gate to source voltage and drain current of mosfets.

Mosfet specifications - Google Search

 

[Mark Tillotson]

3) buy the complimentary transistor at the same time.

If its complimentary (free, gratis) why would you buy it 🙂 If you mean complEmentary (opposite) then that depends on whether the original device was used in a complementary circuit doesn't it? Many transistors don't have a complementary device anyway.

For power transistors the SOA graph is very important, you don't want secondary breakdown because you forgot to check this.

For different uses different parameters may matter more (Early voltage, capacitances, storage time)

You've forgotten hfe.... Often performance is limited by gain, so a hfe=600 transistor is going to need some searching for
a replacement that can do it justice. Note the final letter often denotes gain-class for a device so you need to include that.


And the Vcbo is the normal voltage to worry about, unless you use the transistor without the emitter connected(!)

For MOSFETs the relevant parameters are voltage, on-resistance, threshold, transconductance and total gate charge (and type - lateral v. vertical)

 

[avtech]

I find it useful to look at AllTransistors and see what they come up with before diving into the rabbit hole of spec matching 🙂

alltransistors.com/crsearch.php

 

[jaycee]

Depends how the part is used. Ft is also important in many cases, especially in a power amplifier - slower parts can bring stability issues, but then again, so can faster parts.