Fig. 4-3 Voltage and Current for a Switch-Mode Power Supply
ith a switch-mode power supply, the ac voltage is converted to dc through diode rectifiers. The regulated dc output is attained by means of high frequency (e.g., 20-100 kHz) switching. Although not shown, a high frequency isolation transformer is used. This transformer is much lighter and smaller than the one used in the linear power supply. The current draw is controlled by the voltage appearing across the low voltage capacitor. The resulting current waveform appears like that shown in Fig. 4-3, in which the current has a zero value until some minimum capacitor voltage is reached, and then rises to a peak value and back to zero again. As contrasted with the linear power supply, the current appearing on the ac side lasts for only a portion of each half-cycle. As a result, the characteristic is nonlinear and the device is a source of harmonics. The term crest factor, the ratio of peak to rms current, is often associated with harmonics. For a pure sine wave, the crest factor is 1.414. Common switch-mode power supplies can have crest factor values exceeding 5.
Measurements [from Grady] indicate the current waveform for a color television has a THDI of 121% and a third harmonic content of 84%. Similar measurements for a desktop computer with a laser printer indicate a THDI as high as 140% and a third harmonic content of 91%.