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I have a schematics that uses the BS270 FET transistor, but I want to know what to do if one is not available or one wants to adapt the schematics to a different transistor

What quantities should I look for in order to match a transistor to another? Gate, Source, Drain voltages, etc.

For example, I have 2SK246 Toshiba FET, and I have both datasheets open, but not sure how to go about to modify the schematics or which values to consider in order for the calculations

Edit the application is using the FET as a switching device for a LED with a Beaglebone

enter image description here

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    \$\begingroup\$ what is your application? \$\endgroup\$
    – next-hack
    Commented Sep 18, 2017 at 17:32
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    \$\begingroup\$ The problem with this question is it really depends on how the transistor is being used. First you have to fully understand the application and be aware of what paramaters are important for that mode of operation, then go from there. \$\endgroup\$
    – Trevor_G
    Commented Sep 18, 2017 at 17:33
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    \$\begingroup\$ VTC - Questions regarding looking for parts recommendations are off topic here. Without even seeing fit to provide a schematic in the posting there is not even a feasible way to consider this a design assistance question. \$\endgroup\$ Commented Sep 18, 2017 at 17:37
  • \$\begingroup\$ updated the question with a reference to the application \$\endgroup\$
    – lurscher
    Commented Sep 18, 2017 at 20:10

2 Answers 2

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First of all, you must be sure that maximum current, voltages, and power dissipation are ok for your particular use, to avoid the magic smoke.

Then, each application might have other requirements, in addition to current, voltage and power ratings.

It's very difficult to cover all the possible cases. I will cite some cases, which are totally not meant to be exhaustive, and should be considered just as examples.

For instance, in a digital application, you might want to have a look at:

  • The switching characteristics (i.e. the speed).
  • The on-state resistance (if used with a pull-up resistor).
  • The threshold voltage and if it is logic level compatible.

In an analog application, you might want to have a look at:

  • the transconductance.
  • the threshold voltage.
  • also the output characteristics.
  • The capacitances (for bandwidth/stability estimation and or compensation)

In power applications you might need:

  • The on-state resistance.
  • The switching characteristics (speed).
  • The input/output capacitances, and gate charge.
  • The threshold voltage.
  • The peak power as a function of duty cycle.

In other applications (e.g. if you use the MOSFET as an analog switch), you might need also to know the OFF-state characteristics.

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In a switching application (such as controlling an LED) the most important characteristics to match in a FET are the Gate turn-on voltage, Drain-Source resistance, and voltage ratings. Capacitance and Gate charge (QG) may also be important if it needs to switch quickly and/or the driver circuit is sensitive to capacitive loading.

Comparing the BS270 to the 2SK246, the first thing to note is that they are different types - one is a MOSFET and the other is a Junction FET. Next we see that, as usual for Junction FETs, the 2SK246 requires negative voltage on the Gate to turn it off. This means it is totally inappropriate for your circuit because it will be turned ON at 0V, whereas you need a transistor which is turned OFF at 0V and turns on at 3.3V.

So what should you look for?

  1. An N Channel enhancement mode MOSFET that is rated for Gate drive of 3.3V or less, eg. 2.5V. Note that this is not the threshold voltage, which is much lower.

  2. Low enough RDSON to drop minimal voltage at the expected load current. Equal to or less than the original FET should be fine.

  3. Gate-Source and Drain-Source voltage ratings safely above the supply voltage.

If the LED is only a visual indicator then switching speed isn't important, and if the other parameters are matched then the capacitances will probably be similar.

One thing that does concern me about the original FET is that it is not rated for 3.3V drive. Looking at the Gate transfer characteristics I see that it is barely turning on at 3V. While the graph suggests it can switch enough current to drive an LED at 20mA, process variations may cause some units to have higher threshold voltage and higher RDSON. This may explain why they suggest paralleling FETs to get 260mA. For this current it would be better to use a FET with much lower RDSON at lower drive voltage (eg. 0.2Ω at 2.5V).

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