Testing Mosfets in Parallel

Sounds like they have no gate resistors. Bad news... edge of oscillation already.

The problem with the linear thing is that current sharing will be poor, so fet temps will be all over the place. You could pulse them at a modest duty cycle, enough to warm them up, and use the new Fluke IR viewer.

Can you probe the board and measure voltage drops in traces? That would tell you if all the fets are conducting, and (approximately) how equally.

John

Or do the magnetic thing, with an array of chip magnetometers at appropriate points.

Why not just test the board functionally, and ignore internals? If the designers made it untestable, that's what they deserve.

John

You could keep your cost down to a couple hundred dollars. Go to your local auto supply store. Buy a truck or car battery and a couple dozen 2.5 ohm 50 watt automobile ballast resistors.

Hi John, Tell us more of this Fluke IR viewer, like a part number. Can you measure junction temps? Regards, Harry

Well, there is the Fluke web site. They have a nice looking thermal imager now for $9995 or something, about a third of the price of competing products. I *really* need an excuse to buy one.

John

I assume this is a switched MOSFET design, and not a linear one (if it's linear, send it back to the drawing board, now!).

Folks often overlook the ease of measuring voltage drops along PCB traces if high currents are flowing. So, run your switch at full load current, get out your 5-1/2 digit multimeter set on its 200mV scale, lsb resolution only one thin uV.

Measure away.

So at 2' you can resolve 0.27" spot size. Your intent is to scan an operating PCB and resolve temps on SMD. Sounds great, let's see some color scans before we buy. Cheers, Harry

Either redesign the circuit, or change the installed parts to lateral "audio" MOSFETs, like the Hitachi -> Renesas parts, "Power MOSFETs for General Amplifier" on their website.

Hey, Win, clean out your picovolt.com mailbox! I'm getting lots o' bouncing action from it, and eBay will hose your account if that address is the one associated with it. :)

Apologies for the misuse of public bandwidth...

-- jm

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It's not, my Harvard email is the one. I thought that was clear. How would anyone even know about my old home email address, which I haven't used in over two years? Yes, I do have the picovolt.com domain, but I long ago stopped using its mailbox, because it became inundated with over 100MB of spam per day! I mean, HELP! So now I struggle with one single spam-blasted mailbox. With all my tools to help fight spam, half of my 300 daily Harvard email messages are correctly determined as spam and shunted aside, leaving me only 75 spam emails per day to hand identify. I then have 75 real messages to address. When I come in to work each weekend... And after a few weeks of vacation, sheesh! Over 5000 messages, and 2500 to study. Anyway, at home I seek some peace. No email for 25 months now. :-)

Ah, that explains it! Thanks; now I have to wonder where I got the picovolt.com address. Probably by taking your eBay handle too literally. I wasn't sure you got my reply on the Turbo option question, which I cc'ed to both harvard.edu and picovolt.com.

Anyway, all six packages shipped out today; let me know if you did not get the tracking # notification from UPS. Sorry for the threadjacking!

-- jm

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"John Larkin" a écrit dans le message de news: snipped-for-privacy@4ax.com...

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measure

Hey, so I volonteer for you to buy me one :-)

Maybe an array of hall effect sensors would work.

Functionally testing the board requires a 200 amp 12 V supply and a 24 KW load which still doesn't really tell you if there is a FET missing or stuffed with something else that is not conducting.

Regarding the "deserve" thing. My performance is rated on how much warranty return I prevent and prettty much nothing else.

Ok, so I only need one for about two hours maybe once every three months. I'll pitch in $1K for 10% usage. Get another nine dependable folk and you get yours free, just have to manage it's whereabouts. Got to see some scans first. Regards, Harry

Even when engineering feeds you a bad or untestable design?

John

Ah. Humor.

John

I hope that 24 KW was a typo.