Automotive filter against voltage spikes
I once helped diagnose a 10,000 horsepower electric_train speed controller.
The customers were experiencing field failures. No one was happy.
Turns out a consultant had been brought in to change some computer code.
And management has decided to instruct the consultant &quot;Make that PCB smaller. And Place that PCB closer to the 5,000 volt 2,000 amp buss&quot; so the consultant did so.
At 200 euros (?) an hour, why not do what management ordered.
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Turns out, with 1 microsecond switching times for the current, lots of &quot;eddy currents&quot; were upsetting the &quot;ground plane&quot; in the PCB.
And with 1 microsecond switching times for the voltage, lots of capacitively coupled &quot;displacement currents&quot; were upsetting numerous high_impedance (10,000 ohm) control_circuit nodes in the PCB.
Its easy IMHO to shield against 1uS edges. But the usual 1.4 mils (35 micron) copper foil is not thick enough.
The cure was to install an aluminum plate between the Buss and the PCB, tho initially the consultant balked at that, because &quot;there is no room&quot;. I gently suggested &quot;gutta percha is a valuable insulator.
The problem seems to go away, after that. There were no more questions.
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Switching 400 amps in 400 nanoSeconds, in a wire located 10cm away from a PCB with 10cm by 10cm Ground Plane, will be at risk for this much Ground upset:
Vinduce = [Mu0 * MUr * Area/(2 * PI * Distance)] * dI/dT
This simplified( give MUr = air/copper/FR-4) to
Vinduce = [2e-7 * Area/Distance] * dI/dT
Vinduce = 2e-7 henry/meter * 0.1 meter * 1e+9 amps/second
Vinduce = 2 * 10 = 20 volts.
Thus ground is not ground, anywhere in/on your PCB.