switching high DC current - for charging an electric car - Page 1
Forum section: electronics / projects

author: jmibk
author_rank: Regular Contributor
timestamp: February 23, 2020, 05:43:06 pm
content: A friend of mine has an Citroen Saxo electric car from 1998, that was operated by some NiCd batteries till now.Last year he bought some tesla battery bricks and yesterday we installed them into the car - everything works fine - so no problem there. Charging and balancing works as well as suggested.A little problem is with the charger. He has 3 switchmode powersupplies with 3kW each, that are connected in series. These are connected directly to the car battery. Now when they start up, one doesn't switch on - my idea is because the voltage comming from the batteries. If we disconnect the battery, start up all power supplies and after that plugging the battery back onto the supplies, everything is fine.So I have the idea to use an arduino for controlling this situation by reading all voltages of the supplies with the ADCs and switching the battery onto the PSUs when a threshold voltage is reached by all supplies. See the drawn schematic here.This has also the advantage to get data of the charging process, voltage and current.So my problem is, that Im not much into power electronics like that, so I hope for some hints and help to plan the switching system for this thing.I think low side switching is more simple than high side, the split ground (battery and supply) is no problem here. Continuous charging current is 60A, Voltage is around 180V (I don't know the actual value, but its near 180V). Its clear for me to get the lowest RDSon for lowest power dissipation and fast switching from ON to OFF (not staying in the "linear" area of the mosfet).There are 100+ mosfet devices on the market, that claims 200V VDS and over 100A Drain current at 100 degrees, but I don't trust them to switch that current. I have also some in our workshop (I have to see for the details), they have some screw terminals on them, they look beefy.Does anyone have some idea for switching parts for this current and voltage? I think the use of a mosfet gate driver is recommended to switch the device(s).Second thing is the DCDC converter to get the 5V (and maybe 12V for mosfet switcher). There are some buck regulators on the market (XLSemi has some nice parts there). My idea is to get the operating voltage from the lowest PSU only, its easier to get 5V out of the 50V than 150V when using the whole charging voltage. I'm correct here?Thanks for any comment!

author: jmibk
author_rank: Regular Contributor
timestamp: February 23, 2020, 06:15:46 pm
content: IXFN210N30X3 should do the trick, but whatsvthe deal with devices like IXFT120N30X3HV, that is much smaller or the IXFT150N30X3HV, that is available as a surface mount device?If I use the last one and calculate the voltage drop at 60A, its around .5V. The power loss is 30 watts, that is impossible to dissipate on a smt part that is mounted on a pcb.The TO247 can be mounted on a heatsink, so it is possible to get the heat away. The big part i mentioned first of all three should do the trick easily when mounted on a propper heatsink.Are this calculations right or am I missing something important?

author: Benta
author_rank: Super Contributor
timestamp: February 23, 2020, 06:28:06 pm
content: If the problem is battery voltage feeding back, just place a big diode/rectifier in the + connection to the battery. Much simpler.

author: jmibk
author_rank: Regular Contributor
timestamp: February 23, 2020, 06:34:57 pm
content: The idea with the simple diode was also there but the mosfet version has some useful advantages like getting values during charging or switching on or off the charging process.

author: Benta
author_rank: Super Contributor
timestamp: February 23, 2020, 07:20:31 pm
content: Quote from: jmibk on February 23, 2020, 06:34:57 pmControlling the PSUs would be a better option. I'm pretty certain they have control inputs.

author: trobbins
author_rank: Frequent Contributor
timestamp: February 25, 2020, 01:39:35 am
content: Are the PSU's rated for floating output operation, and to a working voltage of at least 200V (or whatever the max charge voltage can get to)?You should do a simple thermal estimate of max junction temp of FET, using max ambient, and max thermal resistances of FET package, and interface and heatsink, and relate that back to Rds-on and the min gate drive voltage available. Imho I would keep max Tj under about 110C.Program in hysteresis, and keep gate-source resistor value as low as practical, and add a safety zener, and perhaps add a bit of gate series resistance at the gate terminal as a stopper. Perhaps also use a FET with significant voltage margin, and add a safety zener from drain to source.Were you planning on including a battery fuse or fuse isolator, or CB as close to the battery terminals as possible for this accessory circuit?

author: amyk
author_rank: Super Contributor
timestamp: February 25, 2020, 03:50:31 am
content: I would go with a contactor over MOSFETs, the switching action is far more decisive and it won't be damaged by ESD.