Synchronous recfitication project (some basic questions) - Page 1
Forum section: electronics / projects

author: omglol
author_rank: Regular Contributor
timestamp: December 25, 2015, 10:15:54 pm
content: I am making synchronous rectifier for an automotive alternator (12V). I have some basic questions that (I believe) many of users here know how to answer!1) I will be using CASR 50-NP (http://www.farnell.com/datasheets/1639867.pdf) current transducers to measure phase currents and DC current. The transducer has output voltage between 0.325V and 4.625V, with 0A at 2.5V. The MCU I will be using has ADC with 3.3V inputs, so I need to scale the current transducer output. I've read "3v tips'n'tricks" by Microchip (http://ww1.microchip.com/downloads/en/DeviceDoc/chapter%208.pdf>> TIP #16) and they are proposing design with voltage divider + op-amp as voltage follower to do the job. Is this good practice? Can someone verify my design (attachment)?2) I would also like to have digital signal for frequency of the alternator (shaft). I will be calculating this from voltage frequency of one of the three phases. I was thinking of creating 1.65V source with an op-amp, and then somehow compare phase voltage with this 1.65V. Whenever the phase voltage is greater than 0V the output should be 3.3V, otherwise it should be 0V. I also need to add some hysteresis to that. What solution do you suggest?Thank you very much for your answers!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 26, 2015, 07:17:31 am
content: You're going to use a microcontroller to do this..??!My crystal ball is telling me a cloudy future for you... no, literally, clouds of smoke...The preferred approach is something likehttp://www.linear.com/product/LT4320but you have some other issues to consider:- Maximum operating frequency is quite high (~kHz), making this part unsuitable- Three phases needs two of these (and I doubt you'll find an IC that'll do three phases / 6 FETs in one)- Robustness is needed. Automotive conditions are harsh: high temperature, high current, high voltage (in a load dump event), and transients from switches, relays, ignition, etc.As heat and efficiency matter very little in the application, I would be more than happy to use bog standard general purpose junction diodes for rectification. They're usually press-fit into the alternator case itself.(Unless you're talking about a non-automotive application of an automotive alternator.)Tim

author: Marco
author_rank: Super Contributor
timestamp: December 26, 2015, 10:14:32 am
content: Would be easier to replace each diode with something likethis. With faster and higher voltage opamp obviously, need to do some math to keep clear of opamp offset voltage.

author: omglol
author_rank: Regular Contributor
timestamp: December 26, 2015, 03:42:19 pm
content: Its a research project and no, I am not going to use anything else. Also, it will be only bench tested and not mounted in an actual car. Can anybody answer to my questions?

author: prasimix
author_rank: Unknown
timestamp: December 26, 2015, 04:19:19 pm
content: Maybe you can reuse sync rectifier that Liv used in hispower supplies...LTspice model is in the attachment ...

author: Marco
author_rank: Super Contributor
timestamp: December 26, 2015, 05:55:54 pm
content: Quote from: omglol on December 25, 2015, 10:15:54 pmWhat micro are you using? Do you really care about the higher measurement range? (It's not relevant to rectification.)

author: omglol
author_rank: Regular Contributor
timestamp: December 26, 2015, 06:00:25 pm
content: Micro type does not matter. If I want to measure 150A I need to scale 5v to 3.3v, as simple as that ...

author: Marco
author_rank: Super Contributor
timestamp: December 26, 2015, 06:55:05 pm
content: Quote from: omglol on December 26, 2015, 06:00:25 pmIf the micro has an integrated input buffer there is no need for any additional buffering, the transducer already has an integrated output buffer.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 26, 2015, 06:56:45 pm
content: This is a research project? What... what is your thesis? How are you hard set on this methodology? This sounds all kinds of horrible.Tim

author: dom0
author_rank: Super Contributor
timestamp: December 26, 2015, 08:03:50 pm
content: You people are aware that you can do a self-controlled active rectifier with just a few MOSFETs and resistors (and some Z-diodes, for higher voltages)?At least for 50 Hz it's a fine solution. Gate capacitance will ruin it for kHzes, though.

author: omglol
author_rank: Regular Contributor
timestamp: December 26, 2015, 08:33:22 pm
content: Horrible? Why?

author: Marco
author_rank: Super Contributor
timestamp: December 26, 2015, 09:00:52 pm
content: Quote from: dom0 on December 26, 2015, 08:03:50 pmOnly for a resistive load, not going to work with a smoothing capacitor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2015, 12:49:56 pm
content: Quote from: omglol on December 26, 2015, 08:33:22 pmSmoke?