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Forum Index : Electronics : Differential measurements at high voltage
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LadyN Guru  Joined: 26/01/2019 Location: United StatesPosts: 408 |
Need your guidance about Differential measurements at a high common mode voltage I am using the PCF8591 to measure the voltage drop across two N Channel MOSFETs that are in parallel and on the high side, so that the load is connected on the low side. The gate of both MOSFETs are driven using an optoisolator that is powered by a flyback PSU that provides the gate voltage riding on top of V+, the source rail of 150V DC. The PCF8591 has two pairs of differential pins, but my concern is that there's a 150V DC common mode voltage that I need to handle. The PCF8591 outputs the ADC reading over I2C. I have been looking at how to add around a 145V DC offset to the PCF8591 GND so that it sees 5V DC as it expects to and see vague descriptions of using high voltage transistors to provide a "standoff" but nothing concrete. Is there a way to: 1. Add around a 145V DC offset 2. Even if I managed to do this, what could be a way to read the I2C that has this pseudo ground (PGND)? One solution I can think of is to run the whole control circuit off this pseudo ground. |
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mackoffgrid Guru Joined: 13/03/2017 Location: AustraliaPosts: 460 |
I use Digital Isolator ADUM1250 in situations like that. Cheers Andrew |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
Your options are limited: 1: Add an op amp like a INA149 which has a common mode input of +-275v to the input of your measuring device, operating at ground reference. 2: Operate the PCF8591 on the high side rail supply common, take its input\output I2C signals via an digital isolator chip like the ADUM1201BRZ Cheers Mike |
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| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
Thank you Andrew, Mike! I get the idea that since the ADC will be at a high potential, the digital isolator chip will help remove the offset from its output so a uC at "true" GND can read it. The two specific digital isolator chips were (probably) recommended versus the more popular optos like CNY17 or PC817 because the AD ones can support the high freq. that I2C demands? However, I am still stuck at step 1 - how to operate the ADC against the high rail to begin with (assuming I stick with the PCF8591 instead of going with the INA149) If I gave you an idea that I am knowledgeable about how to go about it - I do not. I happened to read that high voltage transistors can be used to provide a "standoff" offset but came across nothing concrete that I could act on. Is there something I can read to get past that problem? |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
I think you will have to run the voltage measuring part referenced to the HV rail and use digital isolation to read the measurements. A resistive divider can scale down the 150vdc to something that the PCF8591 can manage. See this article on isolated I2C Here Cheers Mike |
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