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MOSFET low side switching, Opto-Isolated Driver, isolated Amp Votage sensing !
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MasterT:

--- Quote from: castingflame on August 31, 2018, 01:42:11 am ---
--- Quote ---Data sheet FQP30N06L says Gate-Source Voltage Vgss +-20V absolute maximum
--- End quote ---


Well I dropped a clanger there. :-DD I have purchased several different MOSFETs while getting to grips with theory and practice. This MOSFET was initially not going to be driven by the opto but by the uController (via a 100R resistor). That would have been fine but now I have changed the Gate voltage from 5v to 24v!

While we are talking about this particular subject, I have seen 'Gate drivers' recommended for this application such as the
UCC27423 http://www.ti.com/lit/ds/symlink/ucc27424.pdf. What advantage would that bring to my current 'design' (and I use the term lightly!)? I have attached a design, stumbled across via google.

--- End quote ---

 It's good to bring specifically designed MOSFET driver into perspective.  IMHO, galvanic isolator is right things for safety, and there are many IC to choose. For example, https://www.vishay.com/product?docid=81808
Study data sheets, there are a lot useful info there.
castingflame:

--- Quote --- It's good to bring specifically designed MOSFET driver into perspective.  IMHO, galvanic isolator is right things for safety, and there are many IC to choose. For example, https://www.vishay.com/product?docid=81808
Study data sheets, there are a lot useful info there.
--- End quote ---


After more hours of study, looking at various components and designs I am left with an additional question. If I use an Isolated Driver for the MOSFET (which will also help when I get to PWM control), I  believe I will have a problem with my voltage sensing design as I will no longer have a common GND reference. Is this correct?


I am currently using a ACS712 (hall effect) as a current sensor. It provides a scaled voltage that is proportional to current.
I am wondering if I could use the same type of sensor as a voltage sensor, or another model of hall effect voltage sensor, as common GND reference would not be an issue with this design.

ACS712 data sheet https://www.sparkfun.com/datasheets/BreakoutBoards/0712.pdf


Any other more appropriate/better suited ideas to overcome this issue are welcome. Please remember to treat me gently and I am learning as I go.



Thanks, Paul
MasterT:

--- Quote from: castingflame on August 31, 2018, 08:11:44 am ---After more hours of study, looking at various components and designs I am left with an additional question. If I use an Isolated Driver for the MOSFET (which will also help when I get to PWM control), I  believe I will have a problem with my voltage sensing design as I will no longer have a common GND reference. Is this correct?

--- End quote ---
Right. Use isolated amplifier as voltage sensor. Like:
https://www.broadcom.com/products/optocouplers/industrial-plastic/isolation-amplifiers-modulators/isolation-amplifiers/acpl-c870
Or
http://www.ti.com/lit/ug/tidua58/tidua58.pdf


castingflame:

--- Quote ---Right. Use isolated amplifier as voltage sensor. Like:
https://www.broadcom.com/products/optocouplers/industrial-plastic/isolation-amplifiers-modulators/isolation-amplifiers/acpl-c870
Or
http://www.ti.com/lit/ug/tidua58/tidua58.pdf

--- End quote ---

Fantastic. I'll study those and try and put together my thoughts/design in KiCAD  :-+
castingflame:
Okay, so another basics question about my choice of Isolated Driver for the MOSFET.

I have 2 that I am looking at;

UCC27424 has a supply voltage of 4-V to 15-V.  Datasheet http://www.ti.com/lit/ds/symlink/ucc27424.pdf

and

VO3150A has a supply voltage of 15 V to 32 V   Datasheet https://www.mouser.co.uk/datasheet/2/427/vo3150a-244703.pdf



With just supply voltage in mind, I am a little confused as to which side of the circuit should power the Driver to remain isolated; In as much as Arduino 5v Supply  -OR-  The 24v MOSFET 'power' side. 

The example circuits I have found for each driver have not really helped with my confidence. Initially I just thought it was it 5v Arduino side until I saw the Supply Voltage of the VO3150A was 15 V to 32 V. I don't want to guess at this point!



Paul
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