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 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 volta-
ge 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.
mains transformer rectifier
B1
AC
voltage
TR1
C1
REF
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 compa-
ratively 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
mains switching
rectifier transistor
rf-transformer
B
AC
voltage
screening
potential
seperation
GND
OC
2 nd 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 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
actuator
analog control
output
OPVA
DC
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
AC
D
voltage
TR
GND
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
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 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
switching
transistor filter
output
T
voltage
control
GND
OPVA
T
2
21
Subject to change without notice
DC