EMC filtering on power - Page 1
Forum section: electronics / projects

author: steaky1212
author_rank: Regular Contributor
timestamp: August 08, 2013, 10:10:14 am
content: Hi,I'm designing a project to go in a harsh industrial/automotive environment and was looking for some advice on EMC filtering.I'm going to have two boxes talking to each other over a 4 wire cable (+12V, 0V, CANH, CANL), and I'm looking at the daughter box at the moment.My CAN interface has 60R resistors going to ground through a 0.1uF capacitor, a common mode choke, and TVS diodes protecting the input to the CAN transceiver - so I am pretty confident in the CAN interface.But the only thing I have currently on the power side is a diode protecting against reverse voltage connection.I was thinking about having a TVS diode on the +12V to clamp to around 15-19V above 0V, and a common mode choke on the +12V and 0V close to the connecter to stop any common mode noise present on the cable getting into the system.Does anyone have any thoughts as common mode chokes are quite bulky and I would prefer something lower profile?Thanks in advance,Other information:The 12V is stepped down to 5V internally with a SMPS (switching frequency ~3MHz).Daughter unit draws less than 100mA.

author: ignator
author_rank: Regular Contributor
timestamp: August 08, 2013, 03:21:34 pm
content: Is this going to be for a product or one off experimenting on your own car?For a product, unless you know the quality of the power system, you need to have lots of protection and filtering of the power.For power you want to use something that can absorb joules of energy, more like a MOV. And put this down stream of a choke. Common mode chokes are more needed where you expect to have a high level of common mode noise on a power system that has the ground return a long distance from where the "box" is located (like your desktop PC). The reason for down stream, is it lets you use the resistance of the choke wire as a current limiter for large surges. Power system in any alternator driven system can have large overvoltage conditions. Not just spikes. Think of what happens when a short occurs, the regulator attemps to keep the voltage up, then the fault is cleared via the fuse opening (this is not instantaneous), realize the regulator has a delay in reducing the voltage. For avionics that operate with 12volt system, the test is 40volts for 100milliseconds. But the battery is your friend in helping if it's connected during the surge.It's been a few years since I looked at the can bus. It was not fit for avionics use, at least for essential or critical operation. It's not transformer coupled, 1Mhz, so low capacitance TVS might be able to be used. It's more the unintended short that can cause issues, as it is a transmission line, and requires some level of impedance cabling. Shielded Twisted pair and isolate the power from causing differential noise induction. I don't know what auto installation do with this bus to provide any protection. Multi-drop serial buses don't work very good at high speeds (multi xmitter receivers on the bus), as transmission line theory generally only covers single transmitter, cable, receiver (e.g. RS232, ethernet).My experience was mainly with ARINC429, and AFDX avionics busing.Hopefully others will add their experience.

author: Niklas
author_rank: Frequent Contributor
timestamp: August 08, 2013, 04:58:25 pm
content: I dont think a 15 Volt rating on the TVS diode is high enough, 20-22 Volts as I(5mA) would be better. The reason is that a lot of battery chargers and start help units can apply higher voltages than the nominal battery voltage. I don't have the exact voltages and times, but there is some sort of standard that specifies what voltages an ECU must sustain for certain amounts of time. Something like: 13 Volts(indefinite), X Volts(30 minutes), Y Volts(1 minute) etc. The defined time periods are there for the possibility of higher heat related losses at higher supply voltages and I think the maximum voltage with the shortest time was about 25 Volts. A lower limit of 6-7 Volts is also good as engine cranking can cause large voltage dips.That higher voltage rating can also affect the selection of switchmode controllers and their corresponding storage choke.The filter will also work better if you build it from cascaded sections instead of just on beefy stage. A motor ECU I opened up recently had an input stage as follows:3xTVS in parallell (SMC package)3xP-channel MOSFETs as reverse polarity protectionElectrolytic and ceramic caps (First bulk, approx 600 uF)Choke (not common mode, normal energy storage choke for DC/DC)Electrolytic and ceramic caps (Second bulk, approx 300 uF)Voltage regulators (switchmode)Small value resistors (2R2) or powder chokes (BLM18 or similar) on each sub block, ie CAN tranceiver, LIN tranceiver, EEPROM etc

author: steaky1212
author_rank: Regular Contributor
timestamp: August 08, 2013, 05:58:41 pm
content: First of all, thanks to both of you for taking the time to respond.I forgot to say, the daughter box (the one I'm designing atm) will be powered from the other box (call it a power box).My power box will have to deal with load dumps, cranking voltages etc etc.I'm going to be using an un-shielded cable to connect the two, so the protection should be to stop the noise that will undoubtedly be coupled across onto the cable.I just need to figure out if I need anything other than a TVS at the moment (TVS will clamp any transient voltages to about 19V, and protect against incorrect polarity).