MOSFET (N) On resistance not as expected (Solved!!!!) - Page 1
Forum section: electronics / projects

author: AMS
author_rank: Contributor
timestamp: June 29, 2017, 06:26:17 pm
content: Hi AllI am currently trying to use an N channel MOSFET as a switch (by switching the GND to a module which uses about 200mA). The MOSFET that I am using is DMG6968U and based on the datasheet it should have around 25-30 mohm ON resistance (at much higher currents that I use) however the ON resistance as soon as there is any load on it jumps to around 10-12 OHM which cause a huge voltage drop. The VGS is at 5v. I m really baffled by this I have replaced the part to see if this one was faulty but the new one is also the same. Anyone knows what is happening.

author: DBecker
author_rank: Frequent Contributor
timestamp: June 29, 2017, 07:04:11 pm
content: Try the same part from a different supplier.I was once burnt by counterfeit MOSFETs, back in the MOSFET shortage around 2010. We kept blowing up the power section of a motor controller. It appeared that we were being struck by a transient or poor current control. We redesigned the gate driver cascade, changed the current sense components, then redesigned the sense circuit, then rewrote all of the firmware related to the control. Finally, after repeatedly blowing up the power section, we tested the next batch. At 40 amps the on resistance was several times worse than the real part.

author: abraxa
author_rank: Frequent Contributor
timestamp: June 29, 2017, 07:15:23 pm
content: Quote from: AMS on June 29, 2017, 06:26:17 pmWhen? At Id = 0 or under full load? Maybe there's something between the FET's source pin and GND that raises the source pin's potential against GND, reducing VGS in turn.Alternatively, could it be that VGS drops at the moment the switched-on module draws power and charges its caps?You could of course also test the part by itself in a simple resistor circuit. If it passes, something's wrong with your setup - otherwise the part is faulty as DBecker suggested.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: June 29, 2017, 07:19:16 pm
content: Try checking the markings on the top of the part as well, the datasheet shows what it should be. It's possible you were shipped the wrong part, Digikey did that to me recently. The bag was labeled correctly, but instead of 0603 Vishay 10k resistors, it had 0603 1k resistors from some other brand, different markings from what Vishay uses, etc. Like somebody grabbed the wrong strip but threw it in the bag and labeled it with what I had ordered. The only reason I knew was because my LED was insanely bright, so I started making measurements.

author: AMS
author_rank: Contributor
timestamp: June 29, 2017, 07:43:17 pm
content: Thank you for the replies.QuoteI tried it with a simple resistor across a 5v supply (the MOSFET switching the GND) with no luck the VGS is at 8ohm for 200mA. The part should not be fake as I bough it from Farnell and the markings also match the data sheet.at Id=0 the Rds is 0.2 but as soon as you increase current it got up rapidly .

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 29, 2017, 08:30:50 pm
content: Please post the circuit.Tim

author: AMS
author_rank: Contributor
timestamp: June 29, 2017, 08:59:48 pm
content: I have attached the schematic The resistor is 50ohm and the battery is 5v

author: sleemanj
author_rank: Super Contributor
timestamp: June 29, 2017, 09:05:07 pm
content: What is your measurement method.

author: AMS
author_rank: Contributor
timestamp: June 29, 2017, 09:07:39 pm
content: Quote from: sleemanj on June 29, 2017, 09:05:07 pmmeasuring the drop across drain-source and dividing by the current measurements

author: smoothVTer
author_rank: Regular Contributor
timestamp: June 29, 2017, 09:15:07 pm
content: Any chance you could post a picture of your physical circuit with battery, switch, DMM, and all connections in place?

author: sleemanj
author_rank: Super Contributor
timestamp: June 29, 2017, 09:15:54 pm
content: I would double check everythingMeasure resistor out of circuitMeasure vdrop of r in circuit (on) and calc current to confirm current measurementMeasure vds and calc rdsMeasure vgs to confirm expectationsPost your measurements.

author: Belrmar
author_rank: Regular Contributor
timestamp: June 29, 2017, 09:37:58 pm
content: i have cheked the datasheet and everything you did looks fine, have you tested other mosfet on the same setup? try to solder another mosfet on the pcb and check, but it does not look as your fault

author: AMS
author_rank: Contributor
timestamp: June 29, 2017, 10:01:19 pm
content: Thank you all. I solved the problem. the problem was a resistance caused by cheap contacts to the PCB that holds the MOSFET they were adding about 4-5 Ohms to the circuit (Cheap breadboard which the PCB of the misfit was sitting on) Thank you all for your responses. I changed the board and now it works perfectly.

author: AMS
author_rank: Contributor
timestamp: June 29, 2017, 10:02:33 pm
content: Quote from: sleemanj on June 29, 2017, 09:15:54 pmthank you sleemanj your post made me double check everything and find the stupid breadboard.