ACS712 realiability & usability - Mains voltage power inverter  - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: January 13, 2015, 11:23:35 pm
content: HiI've just came across interesting problem to answer: Imagine synchronous buck like converter working on mains voltage (350V dc bus) supplying current to an inductive load. I want to measure the output current out of the buck. Shall I use ACS712ELCTR-05B-T there? I see a few problems with that:http://www.allegromicro.com/~/media/Files/Datasheets/ACS712-Datasheet.ashxIsolation. 2.1kVrms is stated. In the table a few pages below, I see a note about UL isolation compliance:"Working Voltage for Basic Isolation VWFSI For basic (single) isolation per UL standard 60950-1, 1st Edition 354 VDC or Vpk"which is not bad,...Now consider the voltage swing on the buck output. 350Vpp, 20kHz, fairly high switching times, which gives nasty kilovolts per microsecond. (350V, 100ns -> 3,5kV/us average)They claim, the ACS712 typical applications are:motor control, load detection and management, switchmode power supplies, and overcurrent fault protection. The device is not intended for automotive applications.Where the hell is some spec, how many kV/us can that sucker withstand?Standard optocouplers are rated at about 10kV/us, better ones about 20kV/us, but twice that isolation capabilities (4 to 6kVrms...) I suspect, the ACS712 should be worse than average optocpl, in the du/dt spec.My target isn't automotive, though I have a feeling, using this part in a mains powered switchmode design without having the du/dt spec is just wrong. Would it survive that? If yes, for how long? And asking like that is also just wrong for such design.What do you think? Should I use tha ACS there? Did you use this part in such an application?Thanx for your opinions,Y.

author: Paul Price
author_rank: Super Contributor
timestamp: January 14, 2015, 02:55:45 am
content: Did you ever think of emailing/calling the mfg of this part about your application? I would bet you would get the info you need fast.

author: Yansi
author_rank: Super Contributor
timestamp: January 14, 2015, 03:16:00 am
content: Yeah, definitely the right thing to do. My experience is that they (any big company) just don't care about "customers" like me, who buy 10pcs a year or less. They just do not respond me. Big companies are usually overloaded with cases from their more valuable customers, which is understandable. Maybe I am also a little bit paranoic?I will try to contact them, but 1% change the will respond. So let them surprise me.If anyone can make any thought about this, still I would appreciate that.

author: Niklas
author_rank: Frequent Contributor
timestamp: January 14, 2015, 12:50:02 pm
content: About the voltage swing on the buck. Have you considered to put the sensor on the load side of the inductor? I have used 3 ACS712 to measure the output currents of a frequency inverter with 450V supply voltage with good results.

author: Yansi
author_rank: Super Contributor
timestamp: January 14, 2015, 02:02:28 pm
content: No I haven't, bcs the load itself is the inductor. Just imagine how a VFD (variable frequency drive) works. There is no side of the load, which is not on a swing.To get rid of that swing, I should place on each phase a reconstruction filter (low pass), which is not allowable, for the size and price reasons and other complications, which I do not want in this simplistic-minimalistic design for my drillpress in my small shop.I've just thought of possible methods of current sensing, hall sensor as one of them.

author: prasimix
author_rank: Unknown
timestamp: January 14, 2015, 02:18:18 pm
content: What's about current transformer like Talema? Check for some examplehere.

author: max_torque
author_rank: Super Contributor
timestamp: January 14, 2015, 02:36:22 pm
content: Can someone smart please explain to me the difference between static, DC isolation voltage capability and a devices dynamic withstanding capability?

author: Yansi
author_rank: Super Contributor
timestamp: January 14, 2015, 02:46:31 pm
content: Quote from: prasimix on January 14, 2015, 02:18:18 pm"Frequency range from 20kHz to 200kHz", no such current flows into the load. Chopping frequency is about 20k, but VFD output is 0-400Hz typicaly. The amount of HF ripple flowing through the winding is negligible, well under <10% of the LF. And of course, you must measure the LF current, 0-400Hz.50Hz current transformers exist too, but you need to measure current in wide range of frequency, almost zero to 400Hz typically. This is almost undoable with standard iron core current transformer. It would just stop working, below certain frequency.

author: Niklas
author_rank: Frequent Contributor
timestamp: January 15, 2015, 12:33:15 am
content: Quote from: Yansi on January 14, 2015, 02:02:28 pmTrue. Missed that part about the external inductor and focused just on the wording "buck converter" when I first read the text.The VFD we were adopting for a specific purpose was a bit special. No fixed PWM frequency, up to 400 kHz switching frequency, internal LC filter and almost pure sine wave output. Passed EMC tests with plastic housing and unshielded cable.

author: Yansi
author_rank: Super Contributor
timestamp: January 15, 2015, 01:55:35 am
content: Was it then PWM, or something more like sigma-delta modulated outputs? I assume you used some technike like in class-D audio amplifiers, didn't you?Aaaawww.. those EMC tests.. At my work, I do not design stuff (more like a software/hardware debug type work), I design only as a hobby, mostly... With no access to this kind of measurement stuff. Would be interesting to see, how many of my "products" could pass the test, and if even remotely.

author: qwaarjet
author_rank: Regular Contributor
timestamp: January 15, 2015, 05:36:40 am
content: I've used these in measuring current out of a buck converter in the 200V range with the occasional 1kv transient without problem if that helps.

author: Niklas
author_rank: Frequent Contributor
timestamp: January 15, 2015, 07:13:33 pm
content: Quote from: Yansi on January 15, 2015, 01:55:35 amThe microcontroller used a DAC to generate 3 analog phase voltage reference signals to a control circuitry. Both the phase voltages and the current in the output filter inductors were fed back to the same circuitry, which also generated the control signals to the MOSFETs. The switching frequency adjusted itself to component variations, motor load and speed, with a top frequency of about 400 kHz. The microcontroller used the phase currents, measured by 3 ACS712, and the phase voltages to do the motor control calculations and adjust the signals to the DAC.

author: Yansi
author_rank: Super Contributor
timestamp: January 15, 2015, 07:35:27 pm
content: That is clear enough for me. I thought if you mind explaining me a little bit your "control circuitry". Bcs that seems more interesting. Was that self-oscillating class D stage, consisting of only comaprator? If you know the circuit with LM311 or so, mostly used by hobbyists to make simple but good performing class D AF stages. Or how did your output stage worked? How did the control voltage (from DACs) made its way to the output? Could you please explain, as long as it's not confidential?