Project for EV charger - Page 1
Forum section: electronics / general

author: pisoiu
author_rank: Regular Contributor
timestamp: September 23, 2022, 06:34:33 am
content: Hi all,I am working on a project for an EV charger. I have enough experience regarding digital electronics and low voltage part, but mains voltage is a bit new to me, especially from a regulatory and standards point of view, so I need a bit of help.Problem #1. I need to measure voltage and current on each phase. Current is simple, I use a current transformer. However with the voltage I have a problem, please see pic #1. An isolation transformer for measurement is not an option due to costs reasons. A voltage divider between L and N will feed a differential input ADC. But the ADC is supplied with VCC and GND from a SMPS which powers all low voltage circuits. From what I know, all GND must be connected to PE (protective earth). Following the indicated schematic, I create a low impedance path between N and PE, which is also not ok. If somebody installs L and N reversed by mistake, boom. If N is interrupted somewhere before device, also high voltage can come into ADC. It will trip an external RCCB but until then, ADC may blow. My sollution is to connect lower resistor of the divider to PE instead of N and measure that way. In some situations, if there is an AC voltage on N, that will induce errors in measurement, but I can live with few volts error. I only have to keep resistors value high enough to avoid high differential currents which can be sensed by RCCB.Problem #2, feeding the voltage divider output to ADC, see pic #2.I have 2 options I consider, one is MCP3914 from Microchip, and ADS131M08 from TI,-MCP3914: analog inputs are specified (abs maximum ratings) at +2v/-2v with respect to AGND, recommended is +1V/-1V with respect to AGND and full scale is 1,2V (ref voltage), that means +0.6V/-0.6V on AIN+ and 0v on AIN- in my case. Assume N=0V connected to AIN- and L sine wave scaled down connected to AIN+. What I do not understand is how ADC is processing AIN+ or AIN- voltages below its negative reference, AGND. Will I be able to measure full wave in these conditions?-ADS131M08 is quite similary, but valid measuring range is -1.25V/+1.25V. Same question, negative half of sine wave going on AIN+ is below GND, how can it be correctly measured since there is no negative power supply on chip?Thanks.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 23, 2022, 01:21:17 pm
content: There are energy monitoring chips like that HLW8032 that send the data on a one way UART link which would be trivial to isolate.

author: pisoiu
author_rank: Regular Contributor
timestamp: September 23, 2022, 06:53:21 pm
content: Yes, they are trivial to use because typical application of them, energy counter, do not involve signals coming out from the DC side, where GND must be connected to earth. I looked in the datasheet of HLW8032, N is IC GND, PE is not even connected anywhere. That is not my case. I have signals coming out of the box from DC side (CP-control pilot) which is referenced to GND/PE. Same GND/PE is the GND of my ADC. I cannot use N as system ground.

author: Faringdon
author_rank: Super Contributor
timestamp: September 23, 2022, 07:03:40 pm
content: You can choose to connect secondary side of smps to whatever you like as long as the isolation barrier voltage is not exceeded.Why not just have a Full wave rect bridge...then connect line and neutral to it...then measure the peak voltage of its output.......whatever you use to measure it coudl be powered by a linear regulator or from an isolated SMPS.Once you have the reading, you can transmit it digitally via a digital isolator to wherever you like.Or else you can use a diff amp with several megohms input resistances, and do it that way.