Choosing a Mosfet for a Constant Current H-bridge

[dcnsolutions]

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.mp4
http://www.dcnsolutions.net/FSM/20191114_105827.mp4
http://www.dcnsolutions.net/FSM/20191114_105905.mp4

In 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) - 120R
FB (J41) - closed
SIG (J42) - closed
Return (J43) - open

The 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) - 120R
FB (J41) - closed
SIG (J42) - 10k
Return (J43) - open

I 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

[jonpaul]

PDF link seems to have malware warning

http://proceptsystem

fix it

j

[dcnsolutions]

I don't get any malware warning from that link, but it was http not https. I have changed it to https.