Engine Management System Noise - MC9S12XDT

[ernestsnaith]

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.

Erny

Added p/n to subject.

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

[lachlanbastow]

I just came across this thread and was wondering if progress was made?

[StephenRussell]

What you see is not necessarily what your electronics gets.

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What you are seeing on the scope is likely not exactly what is happening on the power supply leads.  As you noted there is a lot of noise in the air, and a scope probe connected to nothing picks up noise. 

In similar situations I have seen a lot of noise pickup from the scope probe ground connection.  You can get a feeling for this by trying to get a shorter ground connection or using a differential probe with the shortest possible leads.

Often you can get some idea of where the problems are generated by disabling parts of the circuit to see what changes the observed noise

What to Do....

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Everything will be easier if you can cut down on the the generated noise in the system.   Likely noise generators are the inductive energy stored in the coils that operate the injectors and the ignition arcs.

When you shut off the current to and inductive load, such as a solenoid or relay coil the energy stored in the coil has to go somewhere. If there is a completely open circuit the inductor and stray circuit capacitance will form a resonant circuit and you will see a decaying sine wave at this frequency across the coil.

A reverse connected diode across the coil, or a zener diode across the coil will help.  A "snubber" made of a capacitor with a series resistor across the coil will dissipate the energy in the resistor.

The spark circuit is a worse problem.  You have the inductance of the coil, and in addition you have the spark itself, which probably has negative dynamic resistance.  This negative resistance can cancel out the real resistance in any parasitic circuits that are lying around, enabling them to oscillate.

Resistive sparkplug cables are one thing that helps this situation.  More bypassing and possibly more snubber circuits may help.

[Xbot]

my two cents... place the snubber circuit as close to the inductive load and did stephen already mention a freewheeling diode? that should also be included.

if you get it right kindly share it with us. :smileyhappy: