23
Subject to change without notice
O p e r a t i n g c o n t r o l s
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 un
-
regulated 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 connec
-
ted mostly to a fraction of the output voltage, the output of
this amplifier controls the series pass transistor. This analog
amplifier is generally quite fast and is able to keep the out
-
put voltage within tight limits.
3.2 Switched-mode Power Supplies (SMPS)
SMPS operate with very much higher efficiencies than linear
regulated power supplies. The DC voltage to be converted
is chopped at a high frequency rate thus requiring only
comparatively tiny and light ferrite chokes or transformers
with low losses, also, the switching transistor is switched
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.
1
st
Off-line 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
).
AC
voltage
mains
transformer
rectifier
actuator
analog control
output
reference voltage
REF
DC
voltage
GND
C1
OPVA
C2
B1
TR1
AC
voltage
mains
rectifier
rf-transformer
rectifier
filter
output
potential
seperation
DC
voltage
GND
switching
transistor
screening
OPVA
OC
B
control
GND
2
nd
Secondary SMPS
These still require a 50 or 60 Hz mains transformer, the
secondary output voltage is rectified, smoothed and then
chopped.
The capacitance values needed here for filtering 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 required
for buffering or ac ripple from the load (e.g. motors). In
principle the size of the major components decreases with
increasing operating frequency, however, the efficiency
drops apppreciably above appr. 250 kHz as the losses in all
components rise sharply.
3.3 Parallel and Series Operation
It is mandatory that the power supplies used are definitely
specified for these operating modes. This is the case with
HM7043-5 power supplies. As a rule, the output voltages
to be combined are independent of each other, hence, it is
allowed to connect the outputs of one supply with those
of another or more.
Series Operation
The current limit of the outputs in series should be adjusted
to the same value. If one output reaches the current limit
the total voltage will break down.
AC
voltage
switching
transistor
rectifier
filter
output
DC
voltage
GND
transformer
OPVA
control
TR
D
T
GND
mains
T
T
2
Q
2
Q
1
I
B a s i c s o f p o w e r s u p p l i e s
Summary of Contents for HM7042-5
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