Choosing a Mosfet for a Constant Current H-bridge - Page 1
Forum section: electronics / projects

author: dcnsolutions
author_rank: Newbie
timestamp: October 07, 2023, 12:05:17 am
content: Hello all!I am making a Frequency Specific Microcurrent (FSM) device (similar to a TENS machine) as my first analog electronics project.I borrowed a CellRegulogie5 for my wife and found health benefits as she has various medical issues (product info -https://proceptsystem.biobedded.de/pdf/CellRegulogie5_HandbuchVers01.pdf- it is in German, stats on page 9)However, it is very expensive and I like the challenge of trying to make my own.The machine works by outputting constant current between 100-600uA in symmetrical biphasic DC pulses of set frequencies (10hz - 10khz). To keep the constant current for human skin resistance, the HV is around 25-30V.I have uploaded videos I made using my DCScope. Its not the best resolution/quality, but it gives me an idea of what the machine is doing - if anyone is interested the links are:http://www.dcnsolutions.net/FSM/20191114_105749.mp4http://www.dcnsolutions.net/FSM/20191114_105827.mp4http://www.dcnsolutions.net/FSM/20191114_105905.mp4In order to replicate this I am using an embedded Arduino Mega and custom PCB (see Board.jpg, Board2.jpg) powered with a rechargeable LiPo.Each channel has a Mosfet H bridge (CH1.png). The electrodes are connected to CH_OUT directly. The drains of Q3 and Q5 are designated CH_CTRL. This is where I had the idea of running CH_CTRL through another IRFZ24N Mosfet (CC Mosfet) to control the constant current. This is done by attenuating the output of a AD9833 Function generator via MCP4131 digipots (Attenuation.png). I thought I could exploit the Mosfet's threshold voltage between between off and on to vary the amount of current.The output of the AD9833 is attenuated relative to ground, fed through MCP6004 rail-to-rail opamps to tune the signal (via R57) and offset by the 3.2V threshold voltage (see thresh.png) to make a pulsed DC waveform. This then drives the CC Mosfet that controls the H-bridge via CH_CTRL (label CH5 in the picture).When I designed the PCB I wanted to keep my options open for changing configurations, so I have 4 pairs of test points J40-J43:Sense (J40) - 120RFB (J41) - closedSIG (J42) - closedReturn (J43) - openThe above configuration worked perfectly in my initial tests... until I realized that I was testing with no load resulting in improper grounding.I tried the next configuration which resulted in no output at all:Sense (J40) - 120RFB (J41) - closedSIG (J42) - 10kReturn (J43) - openI think this will work if I change the CC Mosfet to a different model than the IRFZ24N but I'm not sure what to look for (other than a max voltage rating of at least 30V, the max current under 1mA, 10 and 10k hz operating frequency etc.)Thanks in advance - any advice and feedback is greatly appreciated

author: jonpaul
author_rank: Super Contributor
timestamp: October 09, 2023, 07:10:56 am
content: PDF link seems to have malware warninghttp://proceptsystemfix itj

author: dcnsolutions
author_rank: Newbie
timestamp: October 10, 2023, 11:37:58 pm
content: I don't get any malware warning from that link, but it was http not https. I have changed it to https.