Leakage capacitors

The MOSEET has three regions of operation. The cutoff region occurs for V g < Up. In this region, the drain-to-source current is the reverse saturation current of the back-to-back source and drain junctions. This leakage current is small but nonzero and allows charge to leak off capacitors which are isolated by cutoff MOSFETs. Because this is how bits are stored in dynamic memory (DRAM) ceUs, DRAMs must be regularly refreshed to retain their memory. 

Irrespective of the class of an insulation system and its quality, it will have some leakage current through its dielectric circuit on application of a high voltage. For all practical piiiposes. therefore, we can consider an insulation system as an imperfect capacitor. 

Tliis is the prime cause of noise and distortion in an audio system. The capacitive coupling (conduction) between the power and the communication lines gives rise to such an effect. It is associated more with the voltage of the system and particularly when it is capacitor compensated. Even without the power capacitors, the leakage (coupling) capacitances between the HV or EHV power lines, particularly 132 kV and above, and the overhead communication lines play an important role and give rise to this phenomenon. Systems lower than 132 kV do not cause such a situation as a result of the insignificant capacitive effect. 

The elements thus formed are then inserted into a sheet steel container, vacuum dried and impregnated, with suitable non-PCB dielectric, which may be an oil dielectric or epoxy resin. The capacitor shell is then hermetically sealed, in oil dielectrics, to avoid any leakage of dielectric during operation. 

For total impregnation of the capacitor unit, a nonoil dielectric such as flexible epoxy is used lo eliminate leakage... 

This is required to check the leakage of the capacitor dielectric during long operations. It is carried out by heating the unit uniformly on all sides, at least 20 C higher than the maximum ambient temperature of the capacitor unit, for at least 2 hours. There should be no leakage. 

Consider the leaky parallel plate capacitor shown in Figure A-4-1.3. If the capacitor is momentarily charged and allowed to discharge through resistor / L, so that the charging current Iq = 0, the leakage current... 

Real world values do not allow a capacitor of this large a value. The largest value capacitor that will pass the ac leakage current test is 0.05 pF. This is 50 percent of the calculated capacitor value, so the inductor must be increased 200 percent in order to maintain the same corner frequency. The inductance then becomes 900 pH and the resultant damping factor is 2.5 which is acceptable. The resulting schematic is shown in Figure 3-77. 

Real-world values do not allow a capacitor of this large a value. The largest value capacitor that will pass the ac leakage current test is 0.05qF. This is 27... 

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