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I've had some transistors fail on a PCB, but I can no longer source them as they have long reached EOL and are now obsolete.

The part numbers are as follows (linked to the original data sheets):

I've contacted RS Components' tech department in the UK to ask them which MOSFETs I can replace these with but they're not getting back to me.

There are thousands of transistors listed on their website; I don't know where to start.

What do I need to be looking for when choosing new parts to replace these faulty transistors?

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  • \$\begingroup\$ Links are broken. \$\endgroup\$
    – jippie
    Mar 2 '16 at 15:04
  • \$\begingroup\$ A description of the PCB and a schematic if possible will make this question answerable; the devices were chosen for some subset of all the performance parameters that the devices exhibit. \$\endgroup\$ Mar 2 '16 at 15:05
  • \$\begingroup\$ @jippie I've replaced the links with another datasheet's site's links - try now, thanks. \$\endgroup\$ Mar 2 '16 at 15:07
  • \$\begingroup\$ @PeterSmith It is a Drive Control PCB. The board is an Aisin 88610-4020 (88611-4020-B) used in an in industrial embroidery machine. \$\endgroup\$ Mar 2 '16 at 15:11
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The four main parameters that you need to worry about are:

  • \$V_{GS\_MAX}\$ or maximum gate voltage. New device should meet or exceed rating of old device
  • \$R_{DS\_ON}\$ or Drain-to-source resistance at various \$V_{GS}\$. You will want to compare the old devices' \$R_{DS\_ON}\$ @ \$V_{GS} = \$ some voltage vs the new one at a similar \$V_{GS}\$. You want \$R_{DS\_ON\_NEW} \le R_{DS\_ON\_OLD}\$
  • \$V_{DS\_MAX}\$ which is the maximum voltage across drain and source, and effectively how much voltage the transistor can block before breaking down. New device should be \$\ge\$ old device.
  • \$I_{D\_MAX}\$ or maximum drain current, which is how much current the transistor can carry when it is fully on without breaking down. New device should be \$\ge\$ old device.

The FETs you are trying to replace are marketed as "High Speed" (whatever that means, its a fairly relative term especially if they are very old), so you may also want to compare switching speeds of the old ones to any new options.

Use a parts distributor's (like digikey, mouser, farnell, newark, etc.) parametric search tool to find a suitable replacement. Further refinements might include package form factor, total power rating (which may not actually be equal to \$V_{DS\_MAX} * I_{D\_MAX}\$ as you might expect), and price.

Most of these parameters are published on the first page of the datahseet, if not, don't use the numbers from the "Absolute Max" section, use the values from the "Recommended Operating Values" or whatever the case may be.

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    \$\begingroup\$ I don't agree that these are the key parameters for this application (of which we know nothing); the parts may have been chosen for a particular value of Coss for all we know. \$\endgroup\$ Mar 2 '16 at 15:08
  • \$\begingroup\$ Ok, these are all "Power MOSFETs", so I would argue instead that these are the most important parameters. However, having taken a gander at the datasheets, they seem to have been marketed as "High Speed". I'll update my answer accordingly. \$\endgroup\$ Mar 2 '16 at 15:11
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    \$\begingroup\$ A missing parameter is the Vgs threshold at which the FET turns on (Vth, I'm told). That should match the application. Lowering it can be disastrous, leaving the FET partially conducting when it should be off, even though all currently stated conditions are met. \$\endgroup\$
    – fgrieu
    Mar 2 '16 at 18:24
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    \$\begingroup\$ @fgrieu good point, however do you mean Vth by chance? \$\endgroup\$ Mar 2 '16 at 18:27

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