Mosfet testing for short - youtube repair video - Page 1
Forum section: electronics / beginners

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 23, 2019, 07:15:18 pm
content: https://youtu.be/YQ7SXJqIpzsIn the 23:10 of the youtube video above, the presenter does a p-channel mosfet test for short, using his bench power supply on the circuit.From what i understood, he disconnected the charger and with his bench power supply he applied the nomimal 19v with 0.5A of current on the Drain of the suspected mosfet. The PSU (and not the DMM how he did that ?) showed about 0.2A and he concluded that the mosfet is not shorted. I suspect the he tried to switch ON the mosfet externally but i can not see the actual wiring.Can someone explain how this test works ? And the other thing i saw is that this laptop motherboard had two mosfet one n-channel that outputs to a p-channel. Why this combination is used ? I mean what is the purpose of this?PS: Is safe to touch a circuit of 19V - 2A ? In another video he used exactly that setting to his power supply to find a short - to - ground part (it was an smd capacitor) just from the heating !!

author: helius
author_rank: Super Contributor
timestamp: May 23, 2019, 07:35:55 pm
content: A MOSFET "shorting" is when the thin oxide layer between the channel and the gate is broken. This results in a low resistance between the gate and source, whereas in a functioning MOSFET the gate-to-source resistance should be over a megaohm. You can test this easily with the diode function of your multimeter.I don't understand the reasoning behind using large currents from a power supply to test this.

author: sdancer75
author_rank: Frequent Contributor
timestamp: May 24, 2019, 09:25:20 am
content: Quote from: helius on May 23, 2019, 07:35:55 pmHi,I know how to test a mosfet out of the circuit and all you have said its true, but it I was curious what exactly was trying to do this guy.Is that because (maybe) the mosfet was on the circuit and he wanted just to be sure that he had the right measurements ?Regards

author: exe
author_rank: Unknown
timestamp: May 24, 2019, 05:04:44 pm
content: I don't have time to watch the video (but I do have time to leave useless comment), so I can only speculate. From 19V I infer he's testing the power input (because laptop power bricks provide ~19V), may be bypassing some switching/ORing fets. So, looks fine to me.Of course, you don't want put 19V in arbitrary places of your circuit.Detecting broken parts by checking their temperature actually works. I even repaired an amplifier, found a broken opamp this way.

author: Seekonk
author_rank: Super Contributor
timestamp: May 24, 2019, 05:27:33 pm
content: I'm not watching someone on youtube. 95% of the time, the gate being a low resistance is all you need to know. In inverters where a number of transformers exist you go for the lowest gate resistance and pull that first. I often apply a limited 10A to an inverter and search for the lowest drain voltage for the odd FET failure or shorted TVS. Watch out for fingers on a removed FET. You can easily charge up the gate and the capacitance can hold the FET in conduction for a long time.

author: Shock
author_rank: Super Contributor
timestamp: May 24, 2019, 05:38:09 pm
content: Quote from: sdancer75 on May 23, 2019, 07:15:18 pmSorin has a lot of experience, he has probably worked out that 19V 2A (if that is what he used in the other video) is fine in certain situations. An example would be if the short is already taking high current and hard to locate. He isn't afraid of blowing the short open because he can trace backwards from the ICs and regulators to see where power is reaching. So context is everything.You misread the current in the above video. He set the power supply to 19.3V and current limit to 500mA. When he applied the 19.3V (via a mutimeter probe) on the output the rest of the circuit was pulling only 20mA. So he concluded from his previous measurement of low voltage that the fet itself was pulling down the voltage, not the rest of the circuit.Two fets setup together is probably for forward and reverse current protection.