Output capacitor oscillations

Most of the evaluation boards of such ESR-sensitive parts are shipped out to customers with only aluminum electrolytic or tantalum capacitors at their outputs. But what really happens is that the customer happily connects the eval board (rather expectantly) into his or her system, and completely forgets there are a bunch of ceramic capacitors all over the system board (for local decoupling at different points). In effect, the switcher can lose that valuable zero in its control loop and break into oscillations (see Figure 3-5). More so if the connecting leads are short. 

These cause dynamic issues to the switching power supply, and usually the only solution to that is to have enough bulk capacitance present on the 12V output rail. Luckily, since the main feedback loop is derived from the primary 5V/3.3V rails of the power supply, there is no minimum ESR requirement for the 12V rail output capacitance, and we can freely add several electrolytic capacitors in parallel. However, modern core processors can place very fast transient load demands on the primary regulated rail, too, and for that we need a whole bunch of ceramic capacitors sitting right at the point of load. In that case we must ensure the converter is designed to accept ceramic loads. Otherwise it will break up into oscillations. 

The output is a square-wave oscillation with a fixed frequency set by the LC time constant. The switching signal is driven by a sine-wave resonance between the inductor and the capacitor. 

Transient — Subcycle disturbance in the AC waveform evidenced by a sharp, brief discontinuity of the waveform. This may be of either polarity and may be additive or subtractive from the nominal waveform. Transients occur when there is a sudden change in the voltage or the current in a power system. Transients are short-duration events, the characteristics of which are predominantly determined by the resistance, inductance, and capacitance of the power system network at the point of interest. The primary characteristics that define a transient are the peak amplitude, the rise time, the fall time, and the frequency of oscillation. Figure 1.12 shows a transient voltage waveform at the output of a power transformer as the result of switching-in of a motor containing power factor correction capacitors. 

Frequency-Modulated Output When the sensing capacitor is externally connected to a 555 timer 1C chip to form an oscillation circuit as shown in Fig. 6, the oscDlation frequency can be used to measure the flow velocity [10]. Here, both inputs of trigger and threshold in 555 timer are connected together. The respcaise of the output voltage Uo to the input voltage Vc on the capacitor is illustrated in Fig. 7. 

A diode array consists of a series of photodiode/ capacitor parallel circuits positioned side by side on a silicon chip. Each such circuit is connected via its own solid-state switch controlled by a shift register, to a common output line. The shift register is controlled over a quartz oscillator (timer). The readout cycle, corresponding to the illumination time, falls in the 100 ms range. A schematic diagram of the diode array is given in Figure 7. 

Low-frequency oscillation in an amplifier usually involves the RF chokes, especially when chokes are used in both the output and input circuits. Oscillation near the fundamental frequency involves the normal resonant circuits, and brings up the question of neutralization. When a parasitic self-oscillation is found on a very high frequency, the interconnecting leads of the tube, tuning capacitor, and bypass capacitors are typically involved. 

Figure 18.3(a) provides a simplified depiction of a differential capacitor used for sensitive displacement measurements. The motion of the input rod flexes the common plate, which increases the capacitance of one capacitor and decreases the capacitance of the other. In one measurement technique, the two capacitors are made part of an impedance bridge (such as a Sobering bridge), and the change in the bridge output is an indication of displacement of the common plate. In another technique, each capacitor is allowed to serve as tuning capacitor for an oscillator, and the difference in frequency between the two oscillators is an indication of displacement. 

Attaching the capacitor with clip leads as shown by the dashed lines, the sound changes to definite clicks, separated by a second or so. This is complete oscillation, which is more effective because the capacitor has delayed the current stoppage until there is enough time for completing the operation. If the capacitor is used, and if output wires (not shown) were attached between the NO contact and the negative wire, then the output signal would be a "square wave."... 

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