request Help to troubleshoot GaN mosfet circuit error - Page 1
Forum section: electronics / projects

author: jf_NH
author_rank: Newbie
timestamp: August 16, 2020, 08:51:24 pm
content: HiTHis is my first post as I found this forum Yesterday through eevBLog youtube channelMy question is rather simple but need some context:I want to learn GaN mosfet specifics and for that embarked in the following project.(in insight maybe a bit crazy...but I like challenge)Starting from this:https://www.digikey.com/product-detail/en/transphorm/TDINV3000W050-KIT/TDINV3000W050-KIT-ND/8322117Transphorm has published the eagle files for this project so I started from them and I simplified it in the following ways:1-I kept the power and driver sections unchanged2 Instead of the inverter logic I just put 4 pwm input that I power with 4 channels function generator.3 I removed all the filtering in the main circuit So Ideally I should have an alternating square wave as output ( and I want to see all the non ideal specially at higher frequencies)4 I removed all the sensor required by the original microcontroller logic5 instead of bootstap circuits On the high side as in Transphorm's I used 4 insulated 12v power supply one for each mosfet.6 I added a 0.1 Ohm current sensing resistor between lowside mosfet source and power groundUltimately I want to go to high frequency switching where my interest lies)( reason for the insulated power supply on the low side mosfetI build the pcb and started testing with either no main DC voltage or relatively low voltage DCThe problem I am facing now is the followingFirst I want to test each mosfet separatelyFor that, I connect a 10 Ohm resistance between the source of the mosfet to be tested and the power groundMy dc source is connected to the drain of the mosfet to be tested1 Without the main DC powered I sent a pwm signal at very low frequency ( 0.1 Hz) so I have time to see things and perform manual measurement as needed,I track the gate to source voltage on a scope and it rises as expected to 12V.With Main DC OFF, I measure the Drain to source resistance with a multimeter and I go from "Infinite out of range" to 0.1 Ohm for 0.05 Ohm theoretical as per the data sheet.So it seems it works??...no so fast...My problem is that when I power the main dc to any voltage ( between 3V to 20V so far) the gate voltage measured with respect to source is still a beautiful flat 12V...unchanged compared to the waveform without DCbut NO current flow in my resistor so it seems the mosfet is not closing....and yes I checked all the connection and continuity :the main test circuit is just a power supply a resistance and a switch.So apparentlyThe Mosfet close when no voltage is applied to its drain and fail to close when some voltage is applied to itI repeat this procedure for both high side and low side mosfet (all 4) .So I obviously made a design mistake somewhere in "trying" to copy and simplify the transphorm design but I cannot see it.My question is what kind of testing could be recommended to find this error??I can publish my design if someone want to have a look at it but I want to learn and am not asking for someone to fix my design for me....and again it is just ( or should just be..) a straight "simplified" copy of Transphorm'sAny advice will be welcome..Thanks in advanceJF