Buck converter using N-channel mosfet? Possible? - Page 1
Forum section: electronics / beginners

author: czbaterka
author_rank: Contributor
timestamp: March 20, 2025, 08:24:32 pm
content: I successfully made a simple Arduino based buck-converter based on this schematic:I wanted to create something simple, to understand the feedback system and poke around with my new scope and see visually what is hapenning in the system like this.Now, after I understood the basics of this type of DC-DC converter, I am wondering if it is possible to do similar buck converter, but using N-channel MOSFET? N channels are cheaper and have lower Rds on, so why would anyone use P-channel in some production buck, right?I guess that ICs like LM2596 are using N-channel MOSFETs right?I was not able to find any example of N-channel MOSFET based diy buck converter.

author: PGPG
author_rank: Frequent Contributor
timestamp: March 21, 2025, 12:59:51 am
content: Look through some buck converters datasheets. I believe some of them will have enough detailed IC inside schematic shown to let you understand how they are doing it. The key is to use capacitor to get voltage higher than input voltage and use this voltage to drive NMOS gate.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 21, 2025, 03:20:41 am
content: Yes it is possible.Here is an example of a high side driverYour Arduino will connect to Hin (suitably shifted to the appropriate Vcc level)See page 22 for more details.https://www.infineon.com/dgdl/Infineon-HV_Floating_MOS_Gate_Drivers-ApplicationNotes-v01_00-EN.pdf?fileId=5546d4626c1f3dc3016c47de609d140a

author: Konkedout
author_rank: Frequent Contributor
timestamp: March 21, 2025, 04:03:54 am
content: If you go to the Digikey website and search forpositive railDC buck controllers ("controllers" drive external MOSFETs) you will see that maybe 90% of them work with N channel. As you indicated those are better FETs.The thing of it is the Nch MOSFET needs a gate voltage going 5V-10V above the input voltage in order to switch efficiently when the high side MOSFET is ON. For that, most of them use a "bootstrap" circuit.https://www.onsemi.com/pub/collateral/an-6076.pdfWith that bootstrap circuit, the maximum duty cycle will be somewhere around 90%.

author: czbaterka
author_rank: Contributor
timestamp: March 21, 2025, 08:02:12 am
content: I've read littlebit more about high-side switching for N-channel MOSFETs today, but one thing I still dont get:Even tho I understand that I need to drive it with higher voltage than VDS (so as you stated, using bootstrap circuit or charge pumps), how I can do that when usually, VGS is very often limited to around +-16-20V? In my example, I am using 24VDC as input voltage (the one I am switching) and that would require me to use at least 29V to switch the mosfet which would destroy it? Or it means that when using Gate voltage of 29V, the VGS will be 24-29=-5?

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2025, 09:22:48 am
content: Quote from: czbaterka on March 21, 2025, 08:02:12 amVGSis just the potential difference between the gate and the source. In the case of 24V, the gate might be at 29V, but the source is at 24V, so VGSis only 5V.

author: MrAl
author_rank: Super Contributor
timestamp: March 21, 2025, 12:01:10 pm
content: Quote from: czbaterka on March 20, 2025, 08:24:32 pmHi,You can create a drive voltage in different ways and another way would be to make a simple step-up DC to DC converter. This would be easy if you have a microcontroller with several i/o pins as only one of them would be needed.You don't always have to follow the crowd either. If you do not need to have the output ground referenced you can use an N channel device with the source connected right to ground ... ground of the power input supply that is. The output will not have the same ground, but will now be the DC output of the converter. The positive output will be the positive of the input power supply. So, it's like a P channel converter with everything kind of inverted.Think about this first and see if you can figure this out. If you absolutely need to have the output have the same ground as the input though, that's not an option.

author: Peabody
author_rank: Super Contributor
timestamp: March 21, 2025, 02:19:39 pm
content: Doesn't the need to generate the high gate voltage depend on your regulated ouput voltage? If your input is 24V, and you need to generate 16V, you should be good to go - as long as you can switch the gate at 24V.

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2025, 02:54:02 pm
content: Quote from: Peabody on March 21, 2025, 02:19:39 pmNo, because for high efficiency, the full supply voltage needs to be connected to the input side of inductor and diode.

author: Konkedout
author_rank: Frequent Contributor
timestamp: March 21, 2025, 06:46:09 pm
content: Quote from: Zero999 on March 21, 2025, 02:54:02 pmZero999 is correct. With +24V in and the FET is turned on, you have +24V at the drain and (call it +24V in spite of a few mV of drop) at the source. If you have +30V at the gate, then Vgs = +30V-(+24V) = +6V.When the FET is turned off, you still have +24V at the drain but the gate and source will be around 0V. So in my example you have 30V swing at the gate relative to ground, but only 6V swing relative to the gate.

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2025, 11:05:41 pm
content: Quote from: czbaterka on March 20, 2025, 08:24:32 pmThen that mustn't have been the schematic you built, because it won't work.The drain and source connections are the wrong way round, to the internal body diode will conduct.If you swapped them (source to positive) then it will work, but the performance will be poor because the MOSFET's gate will take a long time to discharge through that 10k resistor, resulting in a slow turn-off time, during which a significant amount of power will be lost as heat in the MOSFET.

author: czbaterka
author_rank: Contributor
timestamp: March 22, 2025, 10:25:00 am
content: Quote from: Zero999 on March 21, 2025, 11:05:41 pmIts something I found online and yes, I am aware that the MOSFET's Drain and Source are flipped. I flipped it in my practical example back.You are also right with the slow discharge. It takes like 60uS to discharge it while using 5kHz and not discharge at all at around 10kHz.

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2025, 05:12:44 pm
content: Here's a simulation showing an example of bootstrapping.When the MOSFET is off, C1 charges through the load. When the MOSFET turns off D1 prevents it from discharging back into U1, so it floats up to just under 12V above the supply voltage.U1 is a similar voltage regulator to the LM317. It has a maximum voltage rating of 40V, but it's a floating regulator, so can handle 50.8V, because its output voltage is set to 12V. C2 and R6 help to filter the supply voltage and ensures the input voltage ramps up more quickly than the output, so it's not damaged when the power is first applied. In a practical circuit, it's a good idea to connect a diode (anode to output, cathode to input) to protect it from reverse polarity, from C3 discharging, when the power is removed.The top two plots are the input and output voltages. The output swings the full supply voltage, with hardly any loss.The middle two plots show the voltages on the gate and capacitor. The gate voltage swings from 0V to nearly 12V above the supply voltage. The capacitor's voltage just swings between U1's output voltage and the supply voltage + U1's output voltage.The bottom two traces show the gate an capacitor voltages, relative to the MOSFET's source, which is connected to the output. The capacitor's voltage remains close to U1's output voltage, charging and discharging slightly, as the MOSFET is turned on and off. The gate voltage, swings between 0V and 11V.

author: MrAl
author_rank: Super Contributor
timestamp: March 23, 2025, 01:08:14 pm
content: Here's a brief example of the buck circuit I was talking about with the NMOS transistor source connected directly to the input ground.Compare to the PMOS version.