Ernest Snaith

Engine Management System Noise - MC9S12XDT

Discussion created by Ernest Snaith on Apr 27, 2008
Latest reply on Sep 18, 2012 by lachlanbastow
I have designed an engine management system that controls engine igniton and fuel injection as part of a university project. I am experiencing a big noise problem caused by the igniton MOSFET's.
At present i have seperated the ignition MOSFETS and their drivers from the main PCB, housing the MC9S12XDT microcontroller with opto isolators. I have been running both sides from seperate power supplies. In this configuration the system works well.
As soon as a single power supply is used every PCB track has a large amount of noise at the time of ignition; spikes are many volts. What i find strange is this noise is still present when two power supplies are used with common grounds. Therefore i see only 3 coupling paths between both sections; ground connection, opto isolation and radiated coupling.
The frequency of the noise oscillation seems to be around 50 MHz and i dont think the opto isolator would couple at this frequecy. A lot of noise is radiated from the coils, i have placed a resisitor accross the scope leads and the noise is picked up many feet away from them but the noise is not present in the main PCB when seperate power supplies are used with unconnnected grounds. Does that rule out that coupling path?
Can anyone suggest why the noise is present when the only change made is connecting the two grounds. I wouldnt have thought this would be possible as there would need to be a loop. Would the radiant coupling paths be more of a problem if both sections have a connected ground?
Batteries are used as the power supplies and are well decoupled on entry to the PCB's. I have used zener diodes, MOV's and ferrite beads in an attempt to conditon the power supply to the main PCB.
Any suggestions would be greatly appreciated.
Added p/n to subject.

Message Edited by NLFSJ on 2008-04-28 02:40 PM