voltage across it. In a switching supply, however, the input ac is rectified directly (Figure 9) and the filter capacitor is allowed to charge to a much higher voltage (the peaks of the ac line). Since the energy stored in a capacitor = 0.5CV2, while its volume (size) tends to be proportional to CV, storage capability is better in a switching supply.
Although its advantages are impressive, a switching supply does have some inherent operating characteristics that could limit its effectiveness in certain applications. One of these is that its transient recovery time (dynamic load regulation) is slower than that of a series regulated supply. In a linear supply, recovery time is limited only by the speeds of the semiconductors used in the series regulator and control circuitry. In a switching supply, however, recovery is limited mainly by the inductance in the output filter. This may or may not be of significance to the user, depending upon the specific application.
Also,
Reliability has been another area of concern with switching supplies. Higher circuit complexity and the relative newness of switching regulator technology have in the past contributed to a diminished confidence in switching supply reliability. Since their entry into this market, Agilent has placed a strong emphasis on the reliability of their switching supplies. Field failures have been minimized by such factors as careful component evaluation, MTBF life tests, factory
Typical Switching Regulated Power Supplies
Currently, switching supplies are widely used by Original Equipment Manufacturers (OEMs). This class of switching supply provides a high degree of efficiency and compactness,
Figure 10 shows one of Agilent’s higher power, yet less complex, OEM switching supplies. Regulation is accomplished by a pair of push-pull switching transistors operating under control of a feedback network consisting of a pulse width modulator and a voltage comparison amplifier. The feedback elements control the ON periods of the switching transistors to adjust the duty cycle of the bipolar waveform (E) delivered to the output rectifier/filter. Here the waveform is rectified and averaged to provide a dc output level that is proportional to the duty cycle of the waveform. Hence, increasing the ON times of the switches increases the output voltage and vice-versa.
The waveforms of Figure 10 provide a more detailed picture of circuit operation. The voltage comparison amplifier continuously compares a fraction of the output voltage with a stable reference (EREF) to produce the VCONTROL level for the turn-on comparator. This device compares the VCONTROL input with a triangular ramp waveform (A) occurring at a fixed 40KHz rate. When the ramp voltage is more positive than the control level, a turn-on signal (B) is generated. Notice that an increase or decrease in the VCONTROL voltage varies the width of the output pulses at B and thus the ON time of the switches.
Steering logic within the modulator chip causes switching transistors Q1 and Q2 to turn on alternately, so that each switch operates at one-half the ramp frequency (20KHz).
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