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Choosing the right MOSFET

Posted by obelisk79 
Choosing the right MOSFET
October 30, 2017 03:55AM
So if I wanted to choose MOSFETs for driving heatbed/hotend, what characteristics do I need to pay most attention to?

24V source, 3.3v gate
I was looking at these, but I'd like an opinion from some of you more experienced than I.
[www.arrow.com]
[www.arrow.com]
Re: Choosing the right MOSFET
October 30, 2017 04:06AM
Last time I looked, there were no mosfets suitable for switching the high current of a bed heater with 3.3V drive on a production controller board (i.e. with a guaranteed maximum Rds(on) @ 3V gate drive). That's why we shift the drive voltage to 5V on the Duets. For a 1-off hobby project, you would typically get away with a IRLB3036.

Also worth considering is that although you don't need a great deal of gate drive current in a 3D printer, the output current capacity of 3.3V processors is sometimes very low.

Edited 1 time(s). Last edit at 10/30/2017 04:07AM by dc42.

Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]

Large delta printer, and other 3D printer blog postings: [miscsolutions.wordpress.com]

Disclosure: I have a financial interest in sales of the Panel Due, Mini IR height sensor, and Duet WiFi/Duet Ethernet [www.duet3d.com].
Re: Choosing the right MOSFET
October 30, 2017 08:38PM
So doing some research on the matter, I'm thinking a Gate Driver may be the appropriate way to apply the rated voltage and avoid issues.

Thoughts dc42? Anyone else?

I find it somewhat odd that so many people are designing boards that drive MOSFETs with the 3.3v logic rather than utilize something simple like the above mentioned driver or similar solution.

Edited 2 time(s). Last edit at 10/30/2017 08:42PM by obelisk79.
Re: Choosing the right MOSFET
October 31, 2017 02:18AM
A gate driver is is certainly an adequate solution, but it's also overkill because you don't need very fast gate switching in a 3D printer, because the PWM frequencies are low. High switching speeds will lead to EMI generation and to ringing and slight avalanching of the mosfet due to inductance in the circuit. But you can control the switching speed easily by adjusting the value of the gate series resistor.

It's apparent that many of the popular 3D printer control boards have not been designed by competent electronics engineers, given that so many use badly-chosen mosfets.


Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]

Large delta printer, and other 3D printer blog postings: [miscsolutions.wordpress.com]

Disclosure: I have a financial interest in sales of the Panel Due, Mini IR height sensor, and Duet WiFi/Duet Ethernet [www.duet3d.com].
Re: Choosing the right MOSFET
October 31, 2017 04:18AM
Thanks for the information, now, shifting to 5v is what you're doing on the Duet series, but would there be any benefit to perhaps using a simple npn circuit to provide a 10v gate to get rated Rds? Or is that also overkill.

I'm an electronics technician by trade, however circuit design and EE is obviously not something that I have experience with. I appreciate the feedback.

Edited 1 time(s). Last edit at 10/31/2017 04:18AM by obelisk79.
VDX
Re: Choosing the right MOSFET
October 31, 2017 04:23AM
... I'm using the small BS170 MOSFETS for switching high current MOSFET's - mostly with 5V-Arduinos, but would work with 3,3V too ...


Viktor
Re: Choosing the right MOSFET
October 31, 2017 04:27AM
If you were supporting 12V operation only, then providing 12V gate drive would be straightforward to do. But if you want to support 24V operation too, then you would need extra components to ensure that the gate drive voltage doesn't exceed the rating of the the mosfet, which is typically 20V or lower. So it's easier to use 5V drive, because a 5V supply is already available - but then you need to choose mosfets for which 5V drive is adequate.

The first-generation Duets used small N-channel mosfets to level shift the gate drive signals to 5V. However, that didn't provide equal turn-on and turn-off times and it consumed more power than necessary. The second generation Duets use 74HCT logic gates to level shift the signals to 5V, and 180ohm gate series resistors to control the switching time.


Delta printer calibration calculator, mini IR Z probe, and colour touch screen control panel: [escher3d.com]

Large delta printer, and other 3D printer blog postings: [miscsolutions.wordpress.com]

Disclosure: I have a financial interest in sales of the Panel Due, Mini IR height sensor, and Duet WiFi/Duet Ethernet [www.duet3d.com].
Re: Choosing the right MOSFET
October 31, 2017 04:32AM
Yeah, I was just coming back here to say that I now understand why 5v is sufficient. I plan on supporting 24v and agree that I would need more components to make that work. I'll look into shifting the 3.3V to 5V for driving an appropriate MOSFET. Thanks for the feedback!
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