Basics Of Power Supplies; Linear Power Supplies; Switched-Mode Power Supplies (Smps); Parallel And Series Operation - Hameg HM7042-5 User Manual

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
gible. Further they are free from EMI emission in contrast
to SMPS. A conventional mains transformer isolates the line
from the secondary which is recti ed 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 ampli er, the second input is connec-
ted mostly to a fraction of the output voltage, the output of
this ampli er controls the series pass transistor. This analog
ampli er is generally uite fast and is a le to keep the out-
put voltage within tight limits.
mains transformer recti er
B1
AC
voltage
TR1
C1
REF
3.2

Switched-mode Power Supplies (SMPS)

SM S operate with very much higher ef ciencies 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 recti ed, the uffer capacitor re uired is
of fairly small capacitance value because the energy stored
is proportional to the voltage squared (E = 1/2 x C x U
mains switching
recti er transistor rf-transformer
B
AC
voltage
screening
potential
seperation
GND
OC
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which is mostly negle-
rms
actuator
analog control
output
OPVA
C2
voltage
reference voltage
GND
).
2
recti er lter
output
voltage
GND
control
OPVA
B a s i c s o f p o w e r s u p p l i e s
2 nd Secondary SMPS
These still require a 50 or 60 Hz mains transformer, the
secondary output voltage is recti ed, smoothed and then
chopped.
mains transformer
AC
voltage
TR
The capacitance values needed here for ltering the 100
resp. 120 Hz ripple are higher due to the lower voltage.
All SM S feature a very much higher ef ciency 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 fre uency, however, the ef ciency
drops apppreciably above appr. 250 kHz as the losses in all
components rise sharply.
D Q
DC
D I
T
2
3.3

Parallel and Series Operation

It is mandatory that the power supplies used are de nitely
speci ed 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
DC
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.
switching
recti er transistor lter
D
T
control
GND
OPVA
1
T
D Q
2
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output
DC
voltage
GND