21
Subject to change without notice
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
rms
which is mostly neglegible. 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 amplifier controls the series pass
transistor. This analog amplifier is generally quite fast and is able
to keep the output voltage within tight limits.
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.
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
).
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.
AC
voltage
switching
transistor
rectifier
filter
output
DC
voltage
GND
mains
transformer
OPVA
control
TR
D
T
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.
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.
Parallel operation
Basics of power supplies
T
T
2
D
Q
2
D
Q
1
D
I
AC
voltage
mains
transformer
rectifier
actuator
analog control
output
reference voltage
REF
DC
voltage
GND
C1
OPVA
C2
B1
TR1
Power supply 1
Output 1
+
-
Power supply 1
2
Output
+
-
Power supply 2
1
Output
+
-
U
3
U
2
U
1
U
= U + U + U
total
1
2
3
I
= I = I = I
total
1
2
3
I
2
I
3
I
1
+
-
+
-
+
-
U
= U = U = U
1
2
3
total
I
= I + I + I
total
1
2
3
U
3
U
2
U
1
I
2
I
3
I
1
Power supply 1
Output 1
Power supply 1
2
Output
Power supply 2
1
Output
AC
voltage
mains
rectifier
rf-transformer
rectifier
filter
output
potential seperation
DC
voltage
GND
switching
transistor
screening
OPVA
OC
B
control
