SIEMENS
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Siemens KT 10.1 · 2016
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Overview
(continued)
Technical information and configuration
Power supplies general
Primary pulsed switched-mode power supplies:
The term SMPS (Switch Mode Power Supply) or primary
switched-mode regulator is often used in the literature.
Block diagram: Single-ended forward converter
The primary switched-mode regulators are available in many dif-
ferent circuit versions. The most important basic circuits are sin-
gle-ended forward converters, flyback converters, half-bridge
converters, full-bridge converters, push-pull converters and res-
onance converters.
The general principle of operation of the primary switched-mode
regulator is shown in the block diagram of the single-ended for-
ward converter:
The non-stabilized supply voltage is first rectified and filtered.
The capacitance of the capacitor in the DC link determines the
storage time of the power supply on failure of the input voltage.
The voltage at the DC link is approximately 320 V DC for a 230 V
supply. A single-ended converter is then supplied with this
DC voltage and transfers the primary energy through a trans-
former to the secondary side with the help of a pulse width reg-
ulator at a high switching frequency. The switching transistor has
low power losses when functioning as a switch so that the power
balance lies between > 70% and at least 90%, depending on the
output voltage and current.
The volume of the transformer is small in comparison with a
50 Hz transformer due to the high switching frequency because
the transformer size, taking into account the higher switching
frequency, is smaller. Using modern semiconductors, clock
frequencies of 100 kHz and above can be achieved. However,
switching losses increase at excessively high clock frequencies
so that in each case a compromise has to be made between
high efficiency and the largest possible clock frequency. In most
applications, the switching frequencies lie between approxi-
mately 20 kHz and 250 kHz depending on the output power.
The voltage from the secondary winding is rectified and filtered.
The system deviation at the output is fed back to the primary
circuit through an optocoupler. By controlling the pulse width
(conducting phase of the switching transistor in the primary
circuit), the necessary energy is transferred to the secondary
circuit and the output voltage is regulated. During the non-
conducting phase of the switching transistor, the transformer is
demagnetized through an auxiliary winding. Exactly the same
amount of energy is transferred as is removed at the output. The
maximum pulse width for the pulse duty factor for these circuits
is < 50%.
Advantages:
•
Small magnetic components (transformer, storage reactor,
filter) thanks to the high operating frequency
•
High efficiency thanks to pulse width regulation
•
Compact equipment units
•
Forced-air cooling is not necessary up to the kW range
•
High storage times are possible in case of power failure by
increasing the capacitance in the DC link
•
Large input voltage range possible
Disadvantages:
•
High circuit costs, many active components
•
High costs for interference suppression
•
The mechanical design must be in accordance with HF
criteria
Primary switched-mode power supplies have taken over from
the other switching modes in recent years. This is due, in partic-
ular, to their compact size, minimal weight, high efficiency and
excellent price/performance ratio.
Summary
The most important characteristics of the circuit types described
above are summarized in the table.
Comparison criteria for basic circuit versions
Control
Unstabi-
lized
mains
Single-ended-forward
Load
stabilized
U
OUT
G_KT01_EN_00180
Comparison
criteria
Connection methods
Primary-
switched
mode
Secondary-
switched
mode
Transformer
with in-phase
regulation
Magnetic
stabilizer
Input voltage
range
Very large
Average
Very small
Large
Regulation
speed
Average
Average
Very fast
Slow
Storage time
after power
failure
Very long
Long
Very short
Long
Residual
ripple
Average
Average
Very low
Average
Power loss
Very small
Small
Large
Very small
Size
Very small
Average
Very large
Large
Weight
Very light
Average
Heavy
Very heavy
Interference
suppression
overhead
Very large
Average
Low
Average
© Siemens AG 2016