Mosfet bridge configuration problem - Page 1
Forum section: electronics / projects

author: mattko
author_rank: Contributor
timestamp: December 16, 2017, 01:02:17 pm
content: Hello!I'm having an issue with my mosfets (again) and I would like your opinion and suggestions about this matterThe circuit is three phase MOSFET bridge, powering a BLDC motor. Everything is controlled with a 3.3V processor.I'm having trouble with cross-conduction at the time one of the MOSFETs is allready not conducting and the other one starts to conduct. Dead time is set to 1.5us, which should be plenty.My understanding is that Gate-drain capacitance voltage of the P-MOSFET is basically 0V, and when N-MOSFET starts conducting, phase voltage goes to 0V, and Gate-drain capacitance have to be charged to 5V, which takes some time. And in this time, both transistors are conducting, which we don't want. Mosfets threshold voltage is a maximum of 1V, so it doesn't take much to open them.I never heard of this before, so I'm curious how to solve it. I tried googling, but without luck.I attached pictures of waveforms from oscilloscope, circuit, Lt_spice circuit and Lt_spice simulation results. Lt_spice simulations are very close to the real waveforms. I choose similar MOSFETs as in the real circuit.Your opinion is highly valuedPN MOSFET datasheet:https://www.diodes.com/assets/Datasheets/DMC1030UFDBQ.pdf

author: capt bullshot
author_rank: Super Contributor
timestamp: December 16, 2017, 01:21:57 pm
content: Yes, you're right with your conclusion. It's the Drain to Gate capacitance that makes up your problem.There's a cure for it: You need to drive the gates from a very low impedance source (<= 10 Ohm), and you need to do this for both of the MOSFETs, 4k7 for the N-ch MOSFET is way too large. There are special gate driver ICs to accomodate for this.Another way to cope with the problem would be to deliberately slow down the turn on of both MOSFETs by using larger gate resistors (100 Ohm ... 1k) to charge the gate and lower value (10 Ohm ... 100 Ohm) to discharge (use a diode to decouple the discharge / turn-off resistor from the charge / turn-on resistor), may be useless if you drive inductive loads.

author: mattko
author_rank: Contributor
timestamp: December 16, 2017, 03:07:36 pm
content: Ooops. The gate resistance is not 4.7k OhmIts actually 100 Ohms, I forgot to change itThanks for your reply.

author: slugrustle
author_rank: Frequent Contributor
timestamp: December 16, 2017, 04:57:16 pm
content: Another issue could be the NPN transistor. I don't see a resistor limiting current into the base, so it may be entering saturation. I may be wrong, but I believe the storage time adds to the turn-off time for a BJT if you operate it in saturation.The bottom of page 4 of this MMBT4401 datasheet has all the switching delay times listed:https://www.diodes.com/assets/Datasheets/ds30039.pdfA baker clamp can help with this, or use an N-channel MOSFET with fast switching times in place of the NPN. Finally, gate driver ICs are purpose made for quickly turning MOSFETs on and off; they may be worth a look.

author: jmelson
author_rank: Super Contributor
timestamp: December 17, 2017, 03:27:26 am
content: You should set up the logic that controls the upper and lower transistor to provide a dead time. So, you turn off whichever transistor is conducting first, wait a small time and then turn on the other transistor. With a strong gate driver, I can do this with only a 200 ns dead time on some of my servo amps.Jon