Mosfet common industry practice? - Page 1
Forum section: electronics / beginners

author: tonyh88
author_rank: Regular Contributor
timestamp: January 02, 2016, 10:19:40 pm
content: Hi,I'm pretty new to circuit design with mosfet and I juste need your opinion on what is the most common way to drive an N channel mosfet with a microcontroller to drive a dc motor. I'm mostly talking about circuit protection here because I saw a bunch of misleading information on the internet and I want to know what is the common approach to this. In my case the load will be a 5V dc pump drawing around 2 amps.I know I need to use a logic level mosfet to fully turn it on (low RDSon) directly from the uC pin otherwise I would need to use opto isolation or a transistor or anything to isolate between the uC and the mosfet to drive it with a higher voltage.Here are my basic questions:1) Does a Mosfet need a serie limiting resistor on it's gate to protect the uC output pin from drawing too much current? Nobody seems to agree on this one from the various articles I read. A mosfet gate should be high input impedance so IMO this should not be necessary to limit current but maybe I'm wrong.2) A pull down resistor between gate and ground is necessary to turn off the mosfet completely. Typical values seem to be between 10K and 100K so it is high enough resistance for the gate voltage not to be lowered to much and prevent to fully turn on the mosfet and also low enough resistance to discharge the mosfet gate capacitance fast enough. Did I get this right?3) I saw some protection zener between the gate and ground sometime? Overkill?4) Is it better to always use opto isolation or when should you use it?Thanks, if one of my questions wasn't clear let me know.

author: Christopher
author_rank: Frequent Contributor
timestamp: January 02, 2016, 10:32:23 pm
content: My general thing is 100k resistor and a 15V zener across the gate, with a couple of hundred ohms in series before these two if it's nothing quick.Opto-isolation of the gate pin isn't really need unless you have to... This can bring un-wanted delays.You can use a smaller FET or a driver IC such as Micrel MIC4414 to drive it if you need fast going edges with logic-level inputs (level-shifting). The problem with mosfets is the slower they turn on, the longer they act like a resistor.

author: SeanB
author_rank: Super Contributor
timestamp: January 02, 2016, 10:40:29 pm
content: Series resistor is generally around 100R, right by the gate, so that the device does not oscillate with long traces to the driver. With a 5V drive from a MCU you do not really need a zener clamp or a pull down resistor, provided the device is connected direct to the MCU pin and not via a wire or a socket. It really is there so the gate does not float. If your MCU can be powered off with the mosfet still powered put it in, as the pin of the MCU will be set as input during reset, so will float.Switching slowly is not really a problem, you want to turn the pump on and off only, no speed control. A catch diode across the pump, along with a RC snubber, around 10R and 100n in series, will help with spikes from the motor affecting the power supply. Place a 1000uF 10V capacitor so that lead lengths between mosfet, capacitor and pump are short. This is more for supply decoupling, so noise is not an issue on the MCU supply.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: January 02, 2016, 11:05:49 pm
content: Quote from: tonyh88 on January 02, 2016, 10:19:40 pmNot necessarily. Some mosfets, like the IRF3205, have a low enough Rdson and low enough Gate threshold voltage so as not to need heatsinking when driven by 5V to the Gate at low frequencies. The IRF3205 would probably work fine without extra components in your application.But if you are talking about making a commercial product for sale in mass quantities, sure, design with a logic level mosfet. But for a one-off system driving your 10 Watt pump from a 5V mcu output pin (like an arduino maybe?) .... it's not strictly necessary.QuoteIf the mosfet fails shorted, then the microcontroller pin may be shorted to ground or to the positive rail. It is common practice to use a series resistor between the microcontroller and the mosfet Gate. The Gate is like a capacitor, it may draw high current at the beginning of the drive pulse, and it may need a high current to charge quickly enough at high frequencies. This is why Gate drivers are often used, especially for high frequencies.QuoteYes.QuoteIt depends on your application. Since you are driving an inductive load (the motor) it might be worth including a gate protection zener in your design, as well as a reverse-biased diode across the motor itself.QuoteAgain, it depends. If you are concerned about the possibility that a shorted mosfet could take out your microcontroller, then an optoisolator/gate driver circuit is a good idea. If low parts count and simplicity are important, then maybe it is overkill for your application. Since you are just driving a 5 volt, 2 amp motor... personally I would not bother with optoisolators or gate drivers. If it were something like a big washing machine motor powered by rectified mains voltage, that would be a different story.QuoteHere's a short video I shot a few days ago, showing an Arduino ProMini driving an ordinary mosfet (IRF3205) with its PWM output, fading an automotive brake light bulb that draws about 2 amps when fully ON. I used a 330R series resistor for the Gate and a 100K Gate pulldown. The PWM frequency is about 500 Hz. The mosfet does not heat up enough to need a heatsink in this application. Driving your pump motor will be a little different, depending on whether you are controlling its speed with PWM or simply turning it on and off, but still you probably won't even need a heatsink.

author: tonyh88
author_rank: Regular Contributor
timestamp: January 03, 2016, 12:44:37 am
content: thanks for the very good response everyone and for the suggestions!

author: Audioguru
author_rank: Super Contributor
timestamp: January 03, 2016, 03:34:06 am
content: If the old fashioned incandescent light bulb draws 2A from 12V when it is fully hot, then it draws 20A or more the moment 12V is applied to it because its filament is a positive coefficient resistor with a very wide change of temperature.

author: Christopher
author_rank: Frequent Contributor
timestamp: January 03, 2016, 02:06:04 pm
content: You can add a PTC resistor in series with the load to help with inrush limiting,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2016, 02:38:13 pm
content: If you want a protected output, I recommend using a "protected FET". These have control circuitry within them, but otherwise behave like ordinary (though somewhat slow) MOSFETs. Stuff like this:http://www.digikey.com/product-detail/en/VNN3NV04PTR-E/497-13077-2-ND/3087988Note that they are specified for >= 5V operation, so you'll need a level shift if your microcontroller is lower voltage.Quote from: Christopher on January 03, 2016, 02:06:04 pmUh... NTC. But this only works the first time; subsequent turnings-on, before the NTC has had a chance to cool back down, will still see full surge, so it's not very good. A current limited switch is safest.Tim