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Updating Circuit Theory: Ground Bounce

Series

By Ian Darney

Ground bounce is due to supply current being switched off. All the energy stored in the supply wiring departs into the environment via the surface of the conducting structure, leaving charge on that surface, charge which manifests itself as a very high, short duration, voltage between the structure and any conductor routed along it. A test rig is used to demonstrate this, and a circuit model is used to simulate the response. 

The construction of a switching unit which can deliver a current of about 400mA and then switch that current OFF is described. This can be done at a repetition rate which allows the waveform to be monitored on a general-purpose oscilloscope. A signal generator set to give a square wave is used to control the times at which switching occurs.

The test rig consists of a twin-conductor cable routed along a copper pipe. The terminals are short-circuited at the far end. It is configured as two loops. In the culprit loop, current flows along the structure and back via one conductor. Current flowing in this loop develops a voltage between the open-circuit terminals at the near end of the victim loop.

The output of the switching unit is used to switch the supply current ON and OFF. The voltage between the terminals of the culprit loop is monitored by an ocsilloscope. This waveform shows that a high transient pulse is created at switch OFF

The test is repeated to show that the electromagnetic coupling creates a similar voltage spike in the victim loop.

A circuit model of the setup is then developed. This uses using time-step analysis to replicate the observed waveforms. It is reasoned that the voltage spike is created by current flowing out of the conducting surfaces of the scope and the signal generator, into the environment.

The significant feature of the modelling technique is that is uses the relationships of Circuit Theory to carry out the analysis. So it can be carried out by any Electrical Engineer. There is no need to invoke any of the relationships of Full Field Modelling.

Enough detailed information is provided for any circut designer to replicate the experiment.

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