Page 122 - Power Quality in Electrical Systems

Switch Mode Power Supplies 105

i (t)
sw
T

90%

T D
10%
t
t r
(a)
Spectral
envelope

–20 dB/decade

–40 dB/decade

f
F 1 F 2
(b)
Figure 7.6 (a) The switching waveform of the drain current. The
switching period is T, the pulse width is T D , and the risetime of the
current pulse is t r . (b) The spectral envelope of the drain current.

Example 7.1: Spectrum of boost converter current. We’ll find the spectrum
of the drain current for a boost converter operating at a switching fre-
quency f sw 500 kHz, a duty cycle D 0.5, and with a switch risetime
and falltime t 25 nanoseconds. With a switching frequency of 500 kHz,
r
the switching cycle T 2000 nanoseconds, and a pulse width of T
d
DT 1000 nanoseconds, we find the two corner frequencies as follows:
1 1
f 318 kHz
1
9
pT d 1p211000 10 2
1 1
f 12.7 MHz
2
9
pt r 1p2125 10 2