Help, test to do b efore install Mosfets

Thread Starter

ren1976

Joined Oct 3, 2012
13
Hello, is there a good "check list for dummies" to do before install the mosfets onto the board to avoid the destruction of them or the component of the board? I don't like too much the explosions and components are not gratis...;)

Joking...but, seriously , are there some common steps to be sure that everything is ok or some tricks to limit the current trough them (mosfets) to avoid serious damage?
 

jpanhalt

Joined Jan 18, 2008
10,087
One of my very first projects was a mosfet PWM control of a winch made from a 6-volt Ford long-shaft starter running at 12V.

I did a lot of testing with dummy loads and even a few mosfets soldered to a strip with the motor under very light load before I built this prototype:

1595451499296.png

It is a bit overdone (not engineered), but worked in the house. In the field, one small addition, the resistor shown by red dot, was necessary. I didn't have the input of this forum, and that resistor was necessary for guaranteed turn off.

Bottom line, I wasted maybe 4 to 6 mosfets, but it worked well thereafter. I didn't use any modeling, like LTSpice, so I can't comment on whether that would have revealed the problem revealed in a field test.

No fires (except for a tantalum capacitor I stupidly installed backwards early on), and only a few mosfets perished. Be careful and think about what you are doing.

The second design was more compact. I still soldered the mosfets to the heat sink copper buses.
 

dl324

Joined Mar 30, 2015
11,264
is there a good "check list for dummies" to do before install the mosfets onto the board to avoid the destruction of them or the component of the board
I have never tested a MOSFET for correct operation before using it or soldering it into a circuit. If these devices came from a reliable source and have been handled and stored appropriately, there's no reason to believe that they're defective.

MOSFETs are static sensitive devices and handling them improperly can damage them. If you're lucky, they die outright. Otherwise, the devices either don't function correctly or die before you'd expect.

In the past year or so, I've managed to damage a couple of 2N2000. None of them failed outright; they sort of worked, but not the way I expected. I think all of them were exhibiting high leakage current so they never turned off completely.
 

Thread Starter

ren1976

Joined Oct 3, 2012
13
I have never tested a MOSFET for correct operation before using it or soldering it into a circuit. If these devices came from a reliable source and have been handled and stored appropriately, there's no reason to believe that they're defective.

Thanks , i totally agree for this, mosfets are new, sealed and bought from certified reseller, i don't whant to test them. What i mean are the test on the circuits...i repaired a board, in this situation a amp, changed optocoupler ,driver, some diodes and resistors, burned by the mosfets explosion. So, to be sure that the work is ok, before install the mosfest i' have to do a preliminary test? Are some "standard" test or just pray and install?
 

jpanhalt

Joined Jan 18, 2008
10,087
Thanks , i totally agree for this, mosfets are new, sealed and bought from certified reseller, i don't whant to test them. What i mean are the test on the circuits...i repaired a board, in this situation a amp, changed optocoupler ,driver, some diodes and resistors, burned by the mosfets explosion. So, to be sure that the work is ok, before install the mosfest i' have to do a preliminary test? Are some "standard" test or just pray and install?
Sorry, I thought you were designing, not repairing. Mosfet can go bad during use, particularly if they are overload and/or in a poorly designed device. More likely, something else in the supply went bad (like a driver) that lead to their destruction.

You might try bring the equipment up to voltage with a light load and see if anything gets hot before adding a full load.
 

dl324

Joined Mar 30, 2015
11,264
So, to be sure that the work is ok, before install the mosfest i' have to do a preliminary test? Are some "standard" test or just pray and install?
You need to determine why the components you replaced failed in the first place. If you haven't corrected the fault that caused them to fail, they could fail again. If you can't see any reason for their failure, you turn on power. If the components are damaged again, then the fault hasn't been corrected.
 
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