Discharge circuits and loads Page: 12 of 62
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DISCHARGE CIRCUITS AND LOADS
RL/Zo, there is a transfer efficiency of about 80%. The real reason, n
general terms, that the line impedance is matchedJ to load impedance is to
control the voltage left on the line at the end of the discharge cycle. If
an 80 to 85% efficiency is acceptable, there can be a large mismatch.
When the load impedance equals Zo, the impedance of the transmission
line, the energy stored in the line is one-h-if CoVo2 and it is all dis-
charged into the resistive load RL in the same time r. It can be shown
(Fig. 2) that the energy deposited into the load is the integral of the in-
stantaneous voltage times the current over the discharge time. The dis-
charge time is twice the line capacitance times thr line impedance. This
is a useful relationship to keep in mind.
The pulse risetime problem in short-pulse duration discharge circuits
is due to tile parasitic capacitance across the load. The parasitic capaci-
tance times the impedance of the transmission line gives a fairly good
measure uf what the ultimate risetime will be if the capacitance is fairly
large. That is normally the property of the real-world system that de-
grades the voltage risetime or the load. The current risetime is aeter-
mined primarily by the inductance in the switch-load interface area.
B. Pulse-Forming Network Systems
In the case of pulse-forming network (PFN) systems, Fig. 3 shows an
example of a system having a unidirectional switch, a type E PFN (of four
sections) discharging into an impedance equal to the PFN characteristic
Inpedance. The PFN discharge current in each section is shown to Illus-
trate the different current pulses flowing through the different capaci-
tances in the line. The last capacitor in the line sees twice the peak
current. The rms current through each is about the same, but the peak
current in the final capacitor is definitely higher.l This means that
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Sarjeant, W.J. Discharge circuits and loads, report, October 15, 1980; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1061299/m1/12/: accessed April 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.