Filter capacitors

When harmonic filter circuits are connected to a power system with saturated reactors (Figure 27.2(e)) resonance may occur between the reactor and the filter capacitors to give rise to overvoltages. 

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... 

These ac current loops should be routed before any other traces in the power supply. The three major components that make up each loop the filter capacitor, the power switch or rectifier, and the inductor or transformer must be located adjacent to one another. The components must also be oriented such that the current path between them is as short as possible. A good example of a layout of the power section of a buck (or step-down) converter can be seen in Figure 3-60. 

Each of the high current grounds must be short and have wide traces. As a general rule, the common terminal of the filter capacitors should be the only points where the other grounds couple into the high current AC grounds. The only exception may be the control ground. 

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). 

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 ... 

The components connected between the emitter-follower and the currentsensing filter capacitor can be thought of as a resistor divider. An additional 0.17 V needs to appear at pin 7 (through a 1K resistor) so the amount of current that must be contributed to that node is 0.17 V/1K which is 170 pA. The capacitive coupling of the PNP to pin 7 essentially centers the oscillator waveform upon the current ramp. So,... 

The approximate value of the bulk input filter capacitor is found from... 

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

This causes the control-to-output characteristic to add gain and phase above the location of this zero. This can be a problem with respect to the stability of the supply. Unfortunately, many capacitor manufacturers do not present the value of the ESR for their capacitors. Typically, the zero caused by the output filter capacitor falls in the following range ... 

As one can see, the choice of the type of output filter capacitor can influence the control-to-output characteristic, sometimes detrimentally. 

The zero attributed to the output filter capacitor is still present in the control-to-output characteristics. Its location is found in Section B.2.1 and Equation B.9. 

The location of the error amplifier s pole is used to counteract the effects of the output filter capacitor s zero caused by the ESR. The error amplifier s pole should be located by... 

Another major source of noise is the loop consisting of the output rectifiers, the output filter capacitor, and the transformer secondary windings. Once again, high-peak valued trapezoidal current waveforms flow between these components. The output Alter capacitor and rectifier also want to be located as physically close to the transformer as possible to minimize the radiated noise. This source also generates common-mode conducted noise mainly on the output lines of the power supply. 

This circuit is a bridge rectifier followed by a filter capacitor to produce a DC voltage with ripple at Vin. Connected to Vin is a linear regulator made from a Zener voltage reference and an NPN pass transistor. We will first run a Transient Analysis to see the operation of the circuit at room temperature (27°C). To set up a Transient Analysis, select PSpice and then New Simulation Profile from the Capture menus, enter a name for the profile and then click the Create button. By default the Time Domain (Transient) Analysis type is selected. Fill in the parameters as shown in the Time Domain dialog box below ... 

A zero is caused by the output filter capacitor, and the ESR of the outpi filter capacitor ... 

Because the ESR of the output filter capacitor creates a zero, it is essential to measure this term. The ESR measurement of output filter capacitor C6 is shown in Fig. 4.37. The ESR is approximately equal to 100 m 2 at the bandwidth of the converter. 

Section 8.2.3 also considers the effect of an abrupt parametric fault in one phase of the inverter s load. In this case study, the load of the three-phase inverter is an RL-network in delta configuration often used in studies of three-phase PWM voltage source inverters. The study may be extended by replacing the RL-network by a sophisticated BG-model of an induction motor (see for instance [31, Chap. 8]). Studies of the three-phase diode bridge rectifier typically assume a resistive load in parallel to a filter capacitor. 

Differential mode noise caused by the di/dt during the IGBT s turn-off and the parasitic inductance of busbar and filter capacitor is shown as curve 1 in Figure 6. 

Circuit enhancements for biopotential measurements, (d) Baseline restoration circuit the high-pass filter capacitor Cl is discharged by field-effect transistor F when activated manually or automatically by a baseline restoration pulse, (e) Electrical isolation transformer coupled using the transformer T (top) or optical using the diode D and the photodetector P (bottom). Note that the isolator separates circuit common on the amplifier side from the earth ground on the output side, (f) Electrical protection circuit resistance R limits the current, reverse-biased diodes D limit the input voltage, and the spark gap S protects against defibrillation pulse-related breakdown of the isolation transformer T. 

The amplifier circuit should also have extensive filtering of unwanted electromagnetic interference. To eliminate RF interference, filter capacitors should be used in the front end of the amplifier as well as at various stages of the amplifier. Very high frequencies can be... 

Consider a diode half-wave rectifier with a filter capacitor C and a resistor R as shown in Fig. 10.21(a). When the input voltage V) is positive, the diode conducts and charges the capacitor C. When the diode moves into cutoff, the capacitor discharges through the rest of the cycle untQ the time at which V) exceeds the voltage at the capacitor. The output waveform for the rectifier with a capacitor is shown in Fig. 10.21 (b). The ripple volts e W is given by... 

It bleeds the charge from the filter capacitor when the rectifier is turned off. 

Input supply filter capacitor ground loop... 

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