[CLOSE]Designing a H-Bridge with only N MOSFETS - Page 1
Forum section: electronics / projects

author: remopong
author_rank: Contributor
timestamp: May 16, 2024, 12:42:03 pm
content: Hi,So I was looking for designing a H-Bridge with only N MOSFETS when I encountered a little issue..After setting this in CircuitJS, see picture (I know it's far from being a reference..) my mosfets died..They are BUZ31L and I was wondering why ?In the center of the picture is a motor I'd like to power forward and backward with speed modulation that I thought I would apply on the MOSFETS (PWM instead 5V)Any help would be verry appreciated ^^Thanks !

author: CaptDon
author_rank: Super Contributor
timestamp: May 16, 2024, 12:57:06 pm
content: Everything imaginable is wrong with that circuit. Clearly you don't understand mosfet's or H-Bridge topology and driver circuits. Just google it. There are some versions of H-Bridges that do use a diode in place of one or two of the mosfet's but they are not the norm. With +48 as your supply you will need the specialized 'hi-side driver I.C.'s' which also mandate a max pulse width of about 95% and a min pulse width of 5% to keep the bootstrap capacitors fully charged.

author: remopong
author_rank: Contributor
timestamp: May 16, 2024, 01:17:14 pm
content: Hi, thanks for such a quick answer..I deeply know that I woulnd be effective or anything but not that i would destroy my mosfet in 1s (legitimate time of the 48V being present)I've googled it a bit but not quite get the idea of how it really works..When trying all this I've worked with a DRV8844 and I think it fried too even if the pcb was corresponding to the datasheet but that's an other story..So technically, I can't just make my H bridge without a specialized IC ?If not, and if I understand well, I should use something like a FAN73711MX ?Thanks

author: Retirednerd2020
author_rank: Regular Contributor
timestamp: May 16, 2024, 02:33:39 pm
content: My advice is to take a few days and look at some online information about how to drive fets, fet properties (data sheets) and then maybe some projects for motor drivers just to get a flavor, and really get familiar with the care and feeding of the FETs. You have a bit of a distance to travel before you start the actual design I think. Come back here and ask specific questions like "what is that gate capacitance I see in the data sheet?" "What is shoot-through and how do I avoid it?" "How do I figure out FET losses and temperature rise?" And many others.

author: H.O
author_rank: Frequent Contributor
timestamp: May 16, 2024, 06:52:07 pm
content: Yeah, the first thing to understand is that a MOSFET is controlled by the voltage between its gate and its source - NOT between its gate and "ground".In an H-bridge like this the source of the top MOSFETs basically swings between the two rails of the power supply (lets call them GND and Vcc).When the lower switch in one "leg" of the bridge is ON the source of the upper switch in the same leg is at ground potential (more or less). When the upper switch is ON its source is at Vcc potential.If the datasheet says the MOSFET needs a Vgs of 10V to be fully on, and with the above in mind, what voltage needs to be on the gate (relative to GND) of the upper switch to actually keep it fully turned on if Vcc is 48V?

author: Andree Henkel
author_rank: Regular Contributor
timestamp: May 17, 2024, 09:24:55 am
content: In Case you want to make H-Bridge only of N-CH MOSFET you need:Charge Pump to generate higher Voltage than the Bridge is steering.the higher voltage is needed to turn the upper FET on, by applying it to the gate of that FET.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 17, 2024, 10:07:43 am
content: There are specialized 'high side MOSFET driver IC', good to control H bridges of same MOSFET type, some are also acting as isolators, to stop the HV entering into the LV microcontroller if a MOSFET fails as a short circuit to its gate.Random example: LM5100 /LM5101 High Voltage High Side and Low Side Gate Driverhttps://www.ti.com/lit/ds/symlink/lm5101.pdf

author: brumbarchris
author_rank: Regular Contributor
timestamp: May 17, 2024, 10:29:30 am
content: Remopong,I recommend you to capture this thread for posterity, and to look at it throughout your upcoming EE career, every 5 or every 10 years. You will then be amazed of what you were doing "in the beginning".I did that with stuff I used to "work on" in college and I can't stop laughing now of that earlier "me". A flagship of my errors is connecting just the RS-232 RX and TX between two different devices (not also the GND) and then wondering why it only works when I touch one of the devices with my hand. It was magic!20 years down the line, I still remember that and laugh about it...Cristian

author: H.O
author_rank: Frequent Contributor
timestamp: May 18, 2024, 07:08:30 am
content: Ahh, the memories.I remember learning that a transistor could be used as a switch and tried to control a 24V, 150W halogen lamp with a BC547 that I had salvaged from something I took apart. I was stoked when the lamp turned on on my command. Needless to say the smile on my young face went away quite soon.

author: remopong
author_rank: Contributor
timestamp: May 20, 2024, 10:37:45 am
content: Hi, first of all, thanks to you all for all those precious answers.I have heard you all and I'll work on my side to understand better what I'm doing with MOSFET and the bridges themselves...I may comeback in bit with better questions if I still have some or not but I'll close this subject as I need to work on my side to understand it all better.Thanks Andree Henkel for the explanation about the charge pump, I understood what you meant on the spot but indeed I didn't knew that. I had some trouble with that charge pump on the drv8844 that may be the reason why I destroyed some of them as well..Thanks brumbarchris, I will save this post for later of course !Thanks H.O for the informations about the gate and its source.Thanks RoGeorge, I will have a look at what you sent !Thanks Retirednerd2020, I will have a look more efficiently on the datasheetsSorry I couldn't answer earlier.Thanks again