Page 152 - Uninterruptible Power Supplies

Static Uninterruptible Power Supplies
150 Chapter Five

Output
Transformer
Capacitor
Diode
Bridge
Mains 1000 uF
Input 400 Volt Switching
Transistor

Basic Elements of Switched Mode Power Supply

Figure 5.9

Computers, although demanding ac, in fact operate internally on 3 to
5 volts dc, with very onerous voltage/ripple variations allowable. To com-
ply with this requirement historically, large rectifier transformers with
smoothing circuits were employed. Such devices rapidly approached the
volume of early computer systems. Therefore, to reduce size and losses
400 Hz was a chosen input to the computer. This assisted in providing a
supply which significantly reduced the size of the system and also gave
good isolation from external supply variations.
Nowadays, the switch mode power supply has assumed a predomi-
nant role. This device is compact and has a relatively high efficiency
(see Fig. 5.9). However, this device does have a peaky waveform (see
Fig. 5.10). Measurements and wavetraces are available showing wave-
forms with 6:1 crest factor, but for most design considerations due to
the induction of impedance and capacitance effects by cable and distri-
bution effects a 3:1 crest factor is seldom exceeded.

Types of Loads

Computer loads in the past were frequency sensitive , modern design
has obviated this problem to a large extent,and typically such loads will
easily operate on 3 to 5 percent frequency variation.
Consideration of other loads has to be examined. For example, light-
ing loads can induce their own problems. The incandescent lamp has a

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