Output capacitor

The Series 1151 differential pressure transmitter manufactured by Rosemount (MinneapoHs, Minnesota) uses a capacitance sensor in which capacitor plates are located on both sides of a stretched metal-sensing diaphragm. This diaphragm is displaced by an amount proportional to the differential process pressure, and the differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4—20 m A d-c output. 

Smoothing circuit To obtain a near-constant voltage source for the inverter circuit a smoothing capacitance across the d.c. link is used to smooth the a.c. ripples present in the d.c. link after cotiversion. The capacitor retains the charge and provides a near-constant d.c. voltage output. 

Harmonic output of a capacitor unit The rating of a shunt capacitor unit... 

If 3000 kVAr capacitor banks are required for a system of 33 kV + 7.5%, -5% and the capacitors are rated for 34/v 3 kV then the output of the capacitors at nominal voltage will reduce to... 

Suitable lor systems svhere fre( uenl switching is not likely and the system is I ree frotn harmonic generating sources. They are the sell-healing type and their output may become reduced with lime (bectiuse of railures of capacitor elements as a result of ssvitching inrush currents and. system harmonics). If the system conditions are not conducive, an inductor coil may be provided to limit the harmonic effects (Section 23.9). Some manufacturers, as standard practice, provide an inductor coil inside the shell to contain the inrush current and also dampen the harmonics. 

This is another name for a heat run test. A voltage sufficient to cause an output of the capacitor unit equal to 1.5 times the rated output is applied at the rated frequency and test conducted for at least 48 hours. During the last six hours of the test the temperature rise and tan 5 are measured at least four times. The temperature rise during the last six hours should not exceed 1 C, otherwise the test duration must be extended until the temperature stabilizes to a rise of only l°C. The final test results... 

Unnecessary loading of the breakdown fuse through transient overvoltages can be avoided by connection to a r element which consists of a length choke and transverse capacitors. So-called iron core chokes are most conveniently used for series chokes, which are usual in power electronics. A damping element with a 61-fjF capacitor is advised at the input and output of the r element. 

Design output indicators select rectifiers and capacitors. 

A dc restoration circuit is needed following the output coupling capacitor to make the drive voltage referenced to the power switch s common. The supply voltage of the driver should be well bypassed so that its voltage does not droop during the drive pulse. 

To properly design the capacitance for the output stage, one should place enough capacitors in parallel so that each capacitor operates at about 70 to 80 percent of its maximum ripple current rating. The sum of the capacitors should equal the final calculated value, but each capacitor should have the value of Ctot/fi, where n is the number of capacitors in parallel. 

Ultimately, the designer must try-out the final design and check the output ripple voltage and the temperature rise of each capacitor. 

One last factor is the physical layout of the output stage when more than one output filter capacitor is used. The capacitors should be located radially symmetric from the output rectifier, and the printed circuit traces for the rectified voltage and the grounds should be of similar trace-widths and lengths. Any dissimilarity of these traces causes more series resistance and inductance to the... 

The major concern of both output and input filter capacitors is the ripple current entering the capacitor. In this application, the ripple current is identical to the inductor ac current. The maximum limits of the inductor current is 2.8 A for I peak and about one-half the maximum output current or 1.0 A. So the ripple current is 1.8 A p-p or an estimated RMS value of 0.6 A (about one-third of the p-p value). 

The best candidate capacitors are from AVX, which have very low ESR and thus can handle very high levels of ripple current. These capacitors are exceptional and not typical. One piece of these could handle the output demands. 

This capacitor experiences the same current waveform at the power switch, which is a trapezoid with an initial current of about 1A rising to 2.8 A with very sharp edges. This capacitor has much more rigorous operating conditions than the output filter capacitor. I will estimate the RMS value of the trapezoidal current waveform as a piecewise superposition of two waveforms, a rectangular 1A peak waveform and a triangular waveform with a 1.8 A peak. This yields an estimated RMS value of 1.1 A. The value of the capacitor is then calculated as ... 

The values for the output filter capacitors are to be determined by using Equation 3.36. 

Determining the minimum values of output filter capacitors. +5V output ... 

It is necessary to add a small leading edge spike filter to the current sensing output. To keep the time delay to a reasonable value, I will use a 1K ohm resistor and a 470 pF capacitor. 

As one can see, there is the familiar choke input filter (T-C) on the output, which is characteristic of the buck and all forward-mode converters. The configuration shown in Figure 4—10 is called a parallel resonant topology because the load impedance (the T-C filter acting as a damping impedance) is placed in parallel to the resonant capacitor. The input to the T-C filter stage... 

Let us examine how one determines the values of the inductor and capacitor. Several assumptions have to be made at the beginning of the design process since several of the tank circuit s characteristics are variable within the application. The first is to assume a value for the Q of the tank circuit. In the application, the Q varies greatly with the amount of load placed on the output of the supply. So, a good value to start with is... 

One caution Whenever the resonant capacitor is placed on a lower voltage output, the ripple current entering the capacitor increases. So, carefully check the ratings against the application. 

R8 charges the timing capacitor (C8) and R7 discharges the timing capacitor. First, one decides the maximum duty cycle of the converter. Since the output voltage is about 50 percent of the lowest input voltage, I will choose a 60 percent maximum duty cycle. From the data sheet ... 

This value is higher than that exhibited by the output capacitance of the MOSFETk so I will add an external resonant capacitor outside the MOSFET. 

The form of compensation is the 2-pole-2-zero method of compensation. This is to compensate for the effect of the double pole caused by the output filter inductor and capacitor. One starts by determining the control-to-output characteristic of the open-loop system. 

The estimated frequency of the zero caused by the tantalum output filter capacitor and its ESR is around 20 kHz, so... 

The location of the lower of the two error amplifier zeros (/epi) is placed at the location of the estimated zero of the output capacitor and its ESR. Hence,... 

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