Buck converter power mosfet question [SOLVED] - Page 1
Forum section: electronics / beginners

author: Lucky-Luka
author_rank: Unknown
timestamp: February 23, 2021, 10:07:07 am
content: Hi allI attach an image of an Arduino controlled buck circuit that I found on a YT video.Is the bjt needed to drive the power mosfet?Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2021, 10:22:31 am
content: Yes, but the schematic is wrong anyway, or at least, very poorly performing.Tim

author: Lucky-Luka
author_rank: Unknown
timestamp: February 23, 2021, 10:30:15 am
content: So, is the bjt needed? Why? And why is the circuit poorly performing?

author: langwadt
author_rank: Super Contributor
timestamp: February 23, 2021, 11:09:49 am
content: p-channel symbol put in backwards and with an n-channel part number and that's just the beginning

author: Lucky-Luka
author_rank: Unknown
timestamp: February 23, 2021, 12:33:08 pm
content: Quote from: langwadt on February 23, 2021, 11:09:49 amThis is NOT the main problem of the circuit and doesn't answer my questions...

author: langwadt
author_rank: Super Contributor
timestamp: February 23, 2021, 01:13:26 pm
content: Quote from: Lucky-Luka on February 23, 2021, 12:33:08 pmmaybe not main problem, just some of many. What famous youtuber made that abomination?

author: langwadt
author_rank: Super Contributor
timestamp: February 23, 2021, 01:24:24 pm
content: ohh my..

author: filssavi
author_rank: Frequent Contributor
timestamp: February 23, 2021, 01:44:00 pm
content: Will it work, provided you have a large enough supply of blasphemies and transistors to blow up...I encourage you to experiment with it, as such a buck converter is. Really great and safe way of learning power electronics (you really can’t get hurt with 6-12 Volts as long as you limit the current)My only advice Is to follow a good reference (eg.https://www.ti.com/lit/an/slva477b/slva477b.pdf) instead of a random YouTube video

author: Peabody
author_rank: Super Contributor
timestamp: February 23, 2021, 02:53:54 pm
content: I don't know anything about power electronics, but the NPN is needed to protect the Nano from the 6-12V supply and make sure the mosfet can turn off. If you drive the mosfet gate directly from the Nano, then the gate pullup resistor would provide up to 12V to the Nano's GPIO pin, which exceeds the absolute maximum rating for that situation. Also, there is a protection diode in the Nano from the GPIO pin to the 5V pin, which means the mosfet gate would never go above 5.6V, and the mosfet would therefore never turn off.Edit: the mosfet in this circuit has to be a P-channel. The schematic symbol shows a P-channel, but it is oriented backwards, and the part number is an N-channel.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2021, 03:46:19 pm
content: There's also no complement to IRFZ44N so it's not clear at all what the schematic author actually meant.Maybe relevant:https://hackaday.io/project/172203-how-to-use-mosfets-irfz44n-irf9540The "load" will be the buck converter circuit. I do encourage taking the time to understand how the buck converter circuit behaves. At least with a 100mH choke (of any breadboarding size), the resistance will be so high that it is very unlikely to cause destruction. Unless that was a typo and 100uH was meant, again, who knows.The fundamental problems with the circuit as shown, assuming the most likely corrections, are:1. The gate drive is very slow, so the MOSFET will take perhaps a microsecond to turn on and off. This will dissipate a lot of energy, so must be done infrequently -- perhaps a few kHz switching frequency. This is impractical -- but at least not a bad thing for demonstration purposes, it is easier.2. There is no current limiting. If the MOSFET turns on and stays on, short-circuit current will be limited only by Rds(on) and inductor DC resistance.3. There may not be any control at all. If the CPU crashes, it won't be reading the ADC and updating PWM, or worse still if it goes off the rails and sets the output pin to full-on.For a beginner, I suppose there can't be any solution for all of these problems -- that's the point, these are more than basic problems. The best I can suggest, then, is to take the time to understand the circuit, how it behaves, and what some strategies are to deal with the above problems.I would personally encourage an analog or hardware based solution -- it's more likely to be "correct" than software is! -- but, I also recall how many years of learning I took before I could design the correct analog circuitry, so, I will leave that as something for you to explore in due time.Tim

author: KT88
author_rank: Frequent Contributor
timestamp: February 23, 2021, 07:39:00 pm
content: The main issue is that the N-MOS transitor requires a positive gate voltage. Without a proper bootstrap circuit the 'Buck' will only be a chopped linear(-isch) regulator - if it even somewhat regulates...A P-MOS transistor would work with this kind of gate driver although the 10kOhm resistor would be too waek - something <1k might work. Note that the control scheme would be inverted if you exchange the N-MOS with a P-MOS....Adding a push-pull stage would improve the efficiency. But you aways can trade efficiency for simplicity...CheersAndreas

author: Lucky-Luka
author_rank: Unknown
timestamp: February 23, 2021, 08:17:41 pm
content: Ok, thank you (almost) all. Now the situation is more clear.