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Basics of power supplies
Linear power supplies
Linear regulated power supplies excel by their highly constant
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
which is mostly neglegible. Further they
rms
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 ripp-
le. 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 amplifier controls the
series pass transistor. This analog amplifier is generally quite
fast and is able to keep the output voltage within tight limits.
mains transformer
rectifier
AC
B1
voltage
TR1
C1
REF
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.
st st st st st
1 1 1 1 1
Off-line SMPS
Off-line SMPS
Off-line SMPS
Off-line SMPS
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
mains
switching
rectifier
transistor
rf-transformer
B
AC
voltage
screening
potential seperation
GND
OC
nd
nd
nd
nd
nd
2 2 2 2 2
Secondary SMPS
Secondary SMPS
Secondary SMPS
Secondary SMPS
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
All manuals and user guides at all-guides.com
actuator
analog control
output
DC
OPVA
C2
voltage
reference voltage
GND
).
2
rectifier
filter
output
DC
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
mains
transformer
rectifier
D
TR
AC
voltage
GND
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.
D Q
1
D I
T
2
D Q
Parallel and series operation
It is mandatory that the power supplies used are definitely
specified for these operating modes. This is the case with all
HAMEG 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
In this mode the output voltages add, the output current is the
same for all supplies. As the sum of all voltages may well
surpass the 42 V limit touching of live parts may be fatal! Only
qualified and well instructed personnel is allowed to operate
such installations.
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.
Parallel operation
Parallel operation
Parallel operation
Parallel operation
Parallel operation
switching
transistor
filter
output
T
DC
voltage
control
GND
OPVA
T
2
21
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