what is exactly gate driver and when to use it with mosfets - Page 1
Forum section: electronics / beginners

author: nour
author_rank: Frequent Contributor
timestamp: December 09, 2015, 07:23:16 am
content: I want to design ESC for driving BLDC, I haven't yet decided about which MOSFET to use, but I will use AVR MCU to control the MOSFETsI have heard about something called gate driver to drive MOSFET transistor, I wounder what is gate driver and when to use it?also would it be safe to use the digital pins connected directly from the MCU to MOSFET's gatebecause I have read somewhere that in some cases the MOSFET gate require a lot current to be fully charged which will be of course much higher than the MCU output capabilities <--- would someone confirm this and explain it pleaseat the end what is the best way to connect MCU to MOSFETthanks

author: Simon
author_rank: Unknown
timestamp: December 09, 2015, 07:36:24 am
content: Mosfet gates have capacitance, this means they do not switch instantly because the mosfet is voltage controlled and the voltage has to rise in the capacitance of the gate as it charges. In low power and/or slow applications this is not a problem and an MCU pin can switch the mosfet directly, but when you are turning the gate on and off at high speed you need to charge and discharge that gate capacitance as fast as possible. Enter the gate driver that can be as little as 2 BJT transistors that will provide more current capability and lower resistance than the MCU pin the resistance of the MCU output driver and the capacitance of the gate form an RC circuit that will have a frequency limit, by lowering R you increase the speed the mosfet can switch at. The BJT's used often can source or sink 2 amps in order to achieve high speed switching. While you gate voltage is rising and falling your mosfet channel resistance will vary between open and closed and thi causes heat dissipation because when fully on with low channel resistance there is minimum voltage drop on the mosfet channel so low power dissipation, when the mosfet fully off no currenyt flows so no dissipation, anywhere in between dissipates power that you don'y want to because it overheats the mosfet.

author: ConKbot
author_rank: Super Contributor
timestamp: December 09, 2015, 07:51:05 am
content: There is a few situations where a mosfet gate driver is needed. First thing to remember is that a mosfet gate appears capacitive to whatever is driving them. And in a switching application a mosfet can be crudely modeled as a voltage controlled resistor. When the resistor is high resistance and no current is flowing, no power is being dissipated. When the mosfet is low resistance (Rdson) very little power is being dissipated. As its transitioning from high to low resistance (or low to high) you have both voltage across the mosfet and current though it, so lots of power is being dissipated. This is called the switching loss, and you can minimize it by pumping a lot of current into the gate to charge the capacitor as fast as possible. A gate driver can let you do this. The Miller effect comes into play here too, but for a basic idea of why to use gate driver, that can be saved for later. A microcontroller IO pin typically does 10-20 mA, a gate driver can peak at 9A or more on some variants.The second is voltage level translation. if you have a 12V ESC, and a 5v microcontroller, even if you use a P-channel fet on the top of each half-bridge, you need to drive the gate to 12V eventually to turn it off. Or if youre using a N-channel FET, youre going to need to drive it to 16+ volts to turn it on fully (D and S should be pretty equal in voltage when its on, so 12V + 4v for Vgs) Yes you can do this with transistors, but this is just making your own gate driver. Common half-bridge driver ICs have logic level translation built in, and ones meant to drive an N-channel FET for the top transistor in an H-bridge have a 'bootstrap' capacitor which is charged when the MOSFET is off, and used as a charge pump to generate the 4-9v Vgs.If youre just switching at a few KHz, and you want to use a discreet transistor driver, go ahead. Just mind your coding (including breakpoints during debugging, and any glitches on turn on) so that both transistors in a half bridge dont get turned on at the same time.Some gate drivers will have protection against that, to make it harder to explode your ESC via software.

author: nour
author_rank: Frequent Contributor
timestamp: December 09, 2015, 07:56:40 am
content: ok, that was the working principles just very good and more than enoughbut now how I can decide if my application switching rate is high that it would require gate driverdoes the datasheet mention such a thing? or its all about trail and error?

author: nour
author_rank: Frequent Contributor
timestamp: December 09, 2015, 08:02:54 am
content: Quote from: ConKbot on December 09, 2015, 07:51:05 amdo you have idea about BLDC motors how much switching can it reach (I am talking about max values) because I really don't know?I am willing to make some experiments with cdroms & HDD BLDC motors (I have so many) but after burning some of them in the learning processI will buy real motors.I want to build Quad-copter form scratch just for fun even it may cost me much less to buy the parts ready made but where is the fun in doing that

author: tron9000
author_rank: Frequent Contributor
timestamp: December 09, 2015, 09:24:41 am
content: I think this is what was happening in the electronic load I was designing and simulating.Cos I had an op-amp directly driving a MOSFET (albeit through a 22R resistor), I was getting quite a large overshoot in current and took a few u-seconds to stabilise.Once I connected a npn transistor to the output of the op-amp and then connected a 100R resistor to the emitter to ground (and collector to Vcc), then connected the emitter to the gate of the MOSFET, and repeat the same simulation I got alot less overshoot in current and quicker settling time.https://www.eevblog.com/forum/projects/i-should-know-this-but/msg700550/#msg700550

author: lm3baker
author_rank: Contributor
timestamp: December 10, 2015, 12:05:31 am
content: Quote from: nour on December 09, 2015, 07:23:16 amThis is a big problem when switching high voltages, inductive loads, or both.Imagine a 5V micro controller connected directly to the mosfet gate switching a 100V inductive load.When the load is switched off, the voltage at the drain rises rapidly, and is coupled into the gate via the gate-drain capacitance - aka. the miller effect. (refer to this simple MOSFET model,http://www.silvaco.com/tech_lib_TCAD/simulationstandard/2000/jan/a1/kaist_fig1.gif) The micro controller pin sees a low-impedance source driving into it, possibly exceeding the maximum current loading of the pin's output buffer. After a few thousand switching cycles, it is going to be.BLDC motors are an inductive load, btw.