Basics Of Power Supplies; Linear Regulators; Parallel And Series Operation - Rohde & Schwarz Hameg Instruments HM8040-3 Manual

Basics of power supplies

Linear regulators

Linear regulators offer a very constant output
voltage even if the mains or the load may vary
considerably. The remaining ripple will be very low
with any good instrument, about 1 mV
and may be neglected. Also, linear regulators are
free from EMI as generated by SMPS.
A conventional mains transformer isolates the
instrument from the mains. A recitifi er provides
an unregulated dc voltage buffered by capacitors
which also serve as energy storage elements. A
transistor in series with the output is controlled
by a regulation loop which compares (a fraction of)
the output voltage with a highly stable reference
voltage. This analog loop is very fast and will re-
store the output voltage quickly to its desired value
whenever the input voltage or the load vary.

Parallel and series operation

Supplies must be specifi ed for these operating
modes which is the case with all HAMEG supplies.
The output voltages to be combined are normally
independent of each other. The outputs of one or
more supplies can then be connected in series
or parallel.
Series operation
As you see (Fig. 2) the voltages are added. The total
voltage generated thus may exceed the 42 V safety
limit. In such case touching live parts may create
Danger! – High voltage
Line transformer
AC
voltage
Fig. 1: Linear regulator
or less
rms
rectifi er
B1
TR1
C1
REF
B a s i c s o f p o w e r s u p p i e s
Fig. 2: Serial operation
danger of life! It is assumed that only qualifi ed
and extensively instructed personnel is allowed to
operate such power supplies and their loads. All
outputs carry the same current, hence the current
limit adjustments of all supplies should be set to
the same value. If in any of the series connected
power supplies the current limit will be reached
this supply will enter the current limiting mode, so
the total output voltage will decrease and become
unregulated!
Parallel operation
In order to increase the total available current
the outputs of supplies can be paralleled. The
output voltages of the supplies involved are
adjusted as accurately as possible to the same
value. In this mode it is possible that one or more
supplies enter the current limit mode. The output
voltage remains in regulation as long as still at
least supply is in the voltage control mode. It is
recommended but not absolutely necessary to
fi ne adjust the voltages such that the individual
current contributions remain nearly equal. Of
course, the maximum available output current
is the sum of the individual supplies' maximum
currents.
actuator
analog controller
OPVA
reference voltage
Subject to change without notice
output
C2
DC
voltage
GND
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