______________________________________________________________ Solid State Power Supplies
THE POWER TRANSFORMER
In some cases a power supply may not use a transformer. Therefore, the power
supply would be connected directly to the source line voltage. This type of connection is
primarily used because it is economical. However, unless the power supply is completely
insulated, it presents a dangerous shock hazard to anyone who comes in contact with it.
When a transformer is not being used, the return side of the AC line is connected to the
metal chassis. Use a transform to remove this potential shock hazard and to have the option
of stepping up or stepping down the input voltage to the rectifier.
Figure 4-2 shows the schematic diagram for the following:
View (A) shows a STEP-UP transformer.
View (B) shows a STEP-DOWN transformer.
View (C) shows a STEP-UP, CENTER-TAPPED transformer.
Only the center-tapped transformer will be discussed in this chapter. The primary purpose
of the center-tapped transformer is to provide two equal voltages to the conventional full-
Figure 4-2. Common Types of Transformers
RECTIFICATION is the changing of an AC voltage to a pulsating DC voltage (see
chapter 1). Let us look at how the process of RECTIFICATION occurs in a half-wave and
a full-wave rectifier.
Since a silicon diode will pass current in only one direction, it is ideally suited for
converting AC to DC. When AC voltage is applied to a diode, the diode conducts ONLY
ON THE POSITIVE ALTERNATION OF VOLTAGE; that is, when the anode of the
diode is positive in respect to the cathode. This simplest type of rectifier is the half-wave
rectifier. Figure 4-3, view (A), shows that the half-wave rectifier uses only one diode.
During the positive alternation of input voltage, the sine wave applied to the diode makes
the anode positive with respect to the cathode. The diode then conducts, and current (I)
flows from the negative supply lead (the secondary of the transformer), through the
milliammeter, through the diode, and to the positive supply lead. As indicated by the
shaded area of the output waveform in view (B), this current exists during the entire period
23 June 2005