Click on the following bookmarks for further details of a Transient Tool for emc analysis: Transient voltage
clamping The voltage waveform utilised by this tool has a linearly rising edge followed by
an exponential decay, and a defined source resistance. The circuit arrangement is shown below: The analysis provides total energy, peak power, peak current and peak voltage in the load and series resistor. The clamp analysis provides total energy, peak power and peak current. The load voltage can be plotted, and an example plot is shown below. The load voltage when a 1500V peak voltage with a 500ohm load is clamped by a 36V clamping device. A poor ground with 50mohm resistance raises the peak voltage to 50V. The rise time of the pulse is 1us, and the decay time 5us.
The waveform shown on the data entry form below covers many requirements, consisting of a linearly rising edge, followed by an exponential decay. The waveform is defined by the tool in terms of the risetime to 50% of the peak value, and the time from the pulse onset until decay to 50% of the peak. During analysis, the time constant of the decay is displayed for information. The circuit arrangement used by the inductor transient attenuator is shown below: A spot analysis is produced of peak current, voltage and power in the series resistor and load resistor, and also of total energy. Peak voltage and current at the inductor are also presented. A plot can be produced, which graphs the load voltage and inductor voltage against time.The performance of an inductive transient attenuator varies for a given inductance according to the pulse shape. For instance, an analysis of a 500V peak transient, of 8us rise and 20us decay times, shows a peak voltage of 48V at the load. Increasing the decay time to 200us, increases the peak load voltage to 86V, and also increases the total energy delivered to the load from 21.5mJ to 141mJ. The plot above shows the 200us decay time pulse, with the blue curve showing the inductor voltage, and the black curve the load voltage. The charged capacitor discharge tool uses the circuit arrangement shown on the form below: The capacitor C1 holds the initial charge, and the resistor Rs can
represent both source resistance, and any additional series resistance the user may wish
to add. Rl represents the load resistance, and C2 the charge balancing capacitor. The peak voltage has been further reduced to less than 20 volts, albeit at the expense of increased capacitance on the line.
EMI Suppression Filters consist of shunt capacitances and/or series
inductances. Their general response to the application of a step voltage is to produce a
delayed rising edge at a resistive load. Application of the step then shows the load response; expand the timescale until the
rising edge can be seen. Comparison of the edge with the digital signal period will show
if the signal could suffer attenuation. Note that inductor/capacitor combinations can
cause ringing when an edge is applied, especially when the load resistance is high. A spot
analysis at a defined time is also available.
High energy voltage transients, originating from lightning strikes
for instance, produce two problems for single suppression devices. A silicon transient
suppression diode, or metal oxide varistor switches rapidly, but may have peak voltage and
total energy ratings below those produced by the transient. A gas discharge tube however
has very much higher peak current capability, but also has a finite switching time. This
results in a significant voltage overshoot at the load, which can be damaging. In addition to specification of the basic circuit values, the gas discharge tube characteristics can be set, including the voltage overshoot against dV/dt. This data is accessed from a separate form called from the hybrid protection tool form, which is itself shown below. The voltage overshoot increases with the rate of voltage increase;
this increase is logarithmic however, which means that the actual switching time decreases
with decreasing risetime. A voltage overshoot figure can be set for each decade of
risetime rate between 10 The plot illustrates the complex behaviour of the hybrid circuit. The blue curve
shows the voltage across the gas discharge tube. The voltage rises up to the calculated
overshoot value of approaching 500 volts, and then collapses rapidly to the arc voltage of
20 volts. The arc voltage is maintained until there is insufficient current to maintain
the arc, after which the voltage at the gas tube rises again, and then decays.
Although emc is often concerned with minimising the effects of
externally generated noise upon a system, it should be remembered that all resistive
components produce a wide band non-periodic noise voltage, arising from thermal movement
of electrons within the resistance. The open-circuit rms noise voltage produced by a
resistance is given by: The magnitude of the thermal noise is independant of whether the resistor
is carbon, or wirewound etc., and depends only on temperature, the magnitude of the
resistance, and the system bandwidth. Intrinsic noise within a resistor is also often
called Johnson noise, after its discoverer. The Thermal Noise Tool allows a spot analysis to be made, and up to
5 sets of data to be plotted, as dBuV versus resistance (log scale) plots. For constant
bandwidth, the dBuVolt versus resistance graphs are straight line plots, with noise
increasing with increasing frequency. A typical value for a 100kohm resistance at room
temperature, in a 20kHz bandwidth system would be 15.18dBuV, or 5.74uVolts. As the distribution never reaches zero, there is a finite chance of
extremely high noise voltages. However as indicated in the diagram, the chance of the peak
voltage exceeding 5 times the rms value is exceedingly small. |

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