Hameg Instruments HM7042-5 Manual Del Usario página 21

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 rectifi ed 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 amplifi er,
the second input is connected mostly to a fraction of the output
voltage, the output of this amplifi er controls the series pass
transistor. This analog amplifi er is generally quite fast and is
able to keep the output voltage within tight limits.
mains transformer rectifi er
B1
AC
voltage
TR1
C1
REF
Switched-mode power supplies (SMPS)
SMPS operate with very much higher effi ciencies 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 rectifi ed, 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
rectifi er 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 rectifi ed, smoothed and then chopped.
The capacitance values needed here for fi ltering the 100 resp.
120 Hz ripple are higher due to the lower voltage.
All SMPS feature a very much higher effi 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
actuator
analog control
output
DC
OPVA
C2
voltage
reference voltage
GND
).
2
rectifi er fi lter
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
rectifi er
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 effi ciency drops apppreciably above appr. 250 kHz
as the losses in all components rise sharply.
D Q
1
D I
T
2
Parallel and series operation
It is mandatory that the power supplies used are defi nitely
specifi ed 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
qualifi ed 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
fi lter
T
control
OPVA
T
D Q
2
Sous réserve de modifi cations
output
DC
voltage
GND
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