29
Basics of power supplies
Fig. 2.2: Rear panel of the HM8143
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20
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3 Basics of power
supplies
3.1 Linear power supplies
Linear regulated power supplies excel by their highly con-
stant output voltage, low ripple and fast regulation, even
under high line and load transients. Good power supplies
feature a ripple of less than 1 mV
rms
which is mostly negle-
gible. Further they are free from EMI emission in contrast
to SMPS.
A conventional mains transformer isolates the line from the
secondary which is rectified and supplies an unregulated
voltage to a series pass transistor. Capacitors at the input
and output of the regulator serve as buffers and decrease
the ripple. A high precision reference voltage is fed to one
input of an amplifier, the second input is connected mostly
to a fraction of the output voltage, the output of this am-
plifier controls the series pass transistor. This analog am
-
plifier is generally quite fast and is able to keep the output
voltage within tight limits.
3.2 Switched-mode power supplies (SMPS)
SMPS operate with very much higher efficiencies than li
-
near regulated power supplies. The DC voltage to be con-
verted is chopped at a high frequency rate thus requiring
only comparatively tiny and light ferrite chokes or transfor-
AC
voltage
mains
transformer
rectifier
actuator
analog control
output
reference voltage
REF
DC
voltage
GND
C1
OPVA
C2
B1
TR1
Fig. 3.1: Linear power supply
mers with low losses, also, the switching transistor is swit-
ched fully on and off hence switching losses are low. In
principle regulation of the output voltage is achieved by
changing the duty cycle of the switch driving waveform.
Primary SMPS
The line voltage is rectified, the buffer capacitor required is
of fairly small capacitance value because the energy stored
is proportional to the voltage squared (E = 1/2 x C x U
2
).
Secondary SMPS
These still require a 50 or 60 Hz mains transformer, the se-
condary output voltage is rectified, smoothed and then
chopped. The capacitance values needed here for filte
-
ring the 100 resp. 120 Hz ripple are higher due to the lower
voltage.
All SMPS feature a very much higher efficiency from appr.
70 up to over 95 % compared to any linear supply. They
are lighter, smaller. The capacitors on the output(s) of a
SMPS may be quite small due to the high frequency, but
the choice depends also on other factors like energy re-
Fig. 3.3: Socondary switched-mode power supply
AC
voltage
switching
transistor
rectifier
filter
output
DC
voltage
GND
transformer
OPVA
control
TR
D
T
GND
mains
Fig. 3.2: Primary switched-mode power supply
