Switching power supply

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Switching power supply in a DVD player

A switch mode power supply ( SNT , also SMPS of English switched-mode power supply ) or switching power supply is an electronic assembly that converts an unregulated input voltage into a constant output voltage. In contrast to transformer power supplies and series voltage regulators , a switched-mode power supply is highly efficient. Switching power supplies belong to the group of power converters .

In contrast to conventional power supply units with a large mains frequency transformer , the conversion in the switching power supply takes place at a higher frequency, since transformers require less magnetic core volume for the same power at high frequencies. For this purpose, the mains voltage is mostly rectified , filtered , chopped into a higher-frequency voltage and, after the transformation in the high-frequency intermediate circuit, rectified again to the desired voltage.

Technical background

Conventional power supplies contain a power transformer for galvanic isolation and voltage transformation. The maximum power that can be transmitted with transformers via the soft iron core increases roughly proportionally to the mass at a constant frequency. If the transformer is operated at a higher frequency, the same iron volume can transmit more power. The increase in specific power is roughly proportional to the mass. The following applies accordingly: The mass (iron or ferrite core and copper windings) of the transformer can be significantly reduced at a higher frequency for the same power, which makes the power supply unit lighter.

The transformer cores of switched-mode power supplies are made of ferrite ( ferromagnetic ceramic ) or iron powder to reduce hysteresis and eddy current losses . At higher frequencies, because of the skin effect, the windings are made as flat copper tape or by means of high-frequency braids (thin wires connected in parallel against each other). A transformer suitable for transmitting 4000 watts weighs, for example:

  • at 50 Hz about 25 kg
  • at 125 kHz, however, only 0.47 kg.

The rapid current and voltage changes in switched-mode power supplies lead to the emission of high-frequency interference voltages , which require line filters , shields and output filters in order not to exceed the permissible interference fields.

Application and properties

Switching power supplies are mainly used because of the high power density that can be achieved to save mass and material.

Unlike conventional power supplies with a lower output, switch mode power supplies have a very high degree of efficiency. They can therefore be found in new designs for plug-in power supplies. Due to the lower copper losses in the power range below about 300 watts, switching power supplies have a higher degree of efficiency (often over 90%) than mains transformers and can be constructed more compact and lighter than conventional power supplies that contain a heavy transformer with an iron core.

The voltage is converted by a ferrite core transformer , which itself either serves as an inductive energy buffer, only with the flyback converter, or with another storage choke (discrete inductance ), which then works as an energy store. As much energy is stored in the air gap of the ferrite core of the transformer or the choke as is necessary for the current load. The regulation of the output voltage under load is possible with semiconductor components in the high-frequency intermediate circuit and avoids the losses of a series regulator in the output circuit.

After rectification, the output voltage is filtered with capacitors and chokes in order to generate the smoothest possible direct voltage . The exception are so-called electronic halogen transformers, which supply the lamp voltage directly at the output.

Usually, however, the switching frequency (ripple) should be removed as completely as possible from the output voltage ( EMC problem). The switching frequency is set in a frequency range that is less disruptive (e.g. above the upper hearing threshold and below the lower measurement limit of EMC measurements at 150 kHz). Interfering frequencies occur at and above the switching frequency (working frequency and harmonics ). Suppression chokes are used to reduce interference and its radiation via the supply lines.

Ferrite cores are also often pushed over the cables, but these are only effective at very high frequencies ( VHF range).

Switching power supplies also cause harmonics on the supply side due to the rectifier at the input, which are kept as low as possible, as they lead to increased reactive losses in the power supply network ( harmonic reactive power ). The increasing use of incorrectly designed switched-mode power supplies causes interference frequencies in the power grid if the switched-mode power supplies have not been sufficiently suppressed with filters as prescribed.

For this reason, switched-mode power supplies (current consumption below 16 A) with an input power of 50 W or 75 W (depending on the device class) have required a power factor correction ("PFC") since January 1, 2001 (EN 61000-3-2 ) have. This provides an additional, network-controlled switching stage (active PFC ) on the input side for an almost sinusoidal current curve . Often, however, only a large line choke is connected upstream, which provides at least approximately a sinusoidal input current (passive PFC).


Scheme of a switched-mode power supply (flyback or forward converter) with galvanic isolation.

Regulated switched-mode power supplies deliver constant output voltages or currents. The constancy of the output variable is achieved by controlling the energy flow into the power supply unit and thus for the connected consumers - there is a closed control loop .

Exceptions are unregulated electronic halogen transformers - these supply an alternating voltage around 45 kHz following fluctuations in the mains voltage.

The following processes take place in the switched-mode power supply:

  • Rectification of the AC mains voltage
  • Smoothing of the resulting DC voltage
  • "Chopping up" the DC voltage
  • Transformation of the resulting alternating voltage
  • Rectification of alternating voltage
  • Screening the DC voltage

With the help of the control circuit it is achieved that as much energy flows into the switched-mode power supply as is to be passed on to the consumer. The regulation required for this takes place via pulse duration or pulse phase control.

Switching power supplies have a ferrite core transformer to achieve voltage transformation and galvanic separation of the output and input side. An optocoupler is required to galvanically isolate the control loop from the mains . Alternatively, the switching signals can also be transmitted to the power transistors via auxiliary transformers in order to achieve potential separation. In this way, the entire control electronics are disconnected from the network. In the figure above, the separation is achieved by a transformer and an optocoupler in the regulation and control circuit.

In the illustration, a switching transistor works in the primary circuit of the transformer, which is why this type of primary switched-mode power supply is called. Primary switched mode power supplies have ferrite core transformers that are operated at a high frequency (the operating frequency of the switched mode power supply, typically 15 ... 300 kHz) and are therefore very small.

If the switching transistor works in the secondary circuit of the transformer, one speaks of secondary switched-mode power supplies. These have a transformer operated with mains frequency and therefore have no mass advantage over conventional power supplies. Here only the linear voltage regulator is replaced by a voltage converter, which improves the efficiency.

Transistors ( MOSFET , bipolar transistors , IGBT ) can be used as switches . Thyristors ( GTO or with quenching circuit) are also used for high outputs .

Schottky diodes are mostly used as rectifiers on the secondary side in order to achieve the lowest possible forward voltage and to ensure the necessary fast blocking times.

On the secondary side, electrolytic capacitors with low series resistance behavior = ESR are used as capacitors . Often several electrolytic capacitors are connected in parallel or electrolytic capacitors with a higher nominal voltage are used that have a lower ESR in this operating mode. The most common aging and heat-related failure of the power supply units consists in the electrolytic capacitors drying out or overstressing them if they are undersized.

Touch current

Y interference suppression capacitor with certification mark

A tingling sensation can be felt on electrically conductive, touchable parts of devices with protective insulation . This touch current is created by the Y capacitors built into the device for interference suppression and by parasitic capacitances to the supply network. Both are features of devices with a switched-mode power supply.

According to VDE regulation 0701/0702, the touch current may not exceed 0.5 mA, which is one of the prerequisites for affixing the CE mark .

The frequency and waveform of these interference voltages are often different from the mains voltage. The voltage against earth can be measured with high-resistance measuring devices and is usually higher than low voltage . However, it collapses when touched and is therefore considered harmless.

With fully insulated devices operated on 240 V in the USA, there is often no significant voltage, since the phases of the single-phase three-wire network (split-phase electric power) are symmetrical to earth and the capacitive voltage divider formed by the device is set to 0, apart from interference voltages and component tolerances Hold V. When operating on a phase with 115 V, the voltage division is only half as high as the contact voltage on 230 V.

In televisions , satellite television receivers and other devices with a signal input, this voltage is applied to the signal inputs (analog and digital interfaces such as USB, antenna inputs) against earth. To keep this voltage away from the sensitive input of the device, the signal lines should be the first when connecting and the last when disconnecting the device.

In the case of protective earthed devices , no significant voltage may be applied to accessible metal parts, as this would indicate a defect in the protective earth. However, hum loops can occur which are based on equalizing currents due to very small voltage differences (usually <1 V).

Advantages and disadvantages compared to conventional power supplies

Plug-in power supplies in size comparison. Left switching power supply with 20 watts, right conventional with 3.6 watt output power
Plug-in switching power supply with only two transistors without ICs (except optocoupler)
  • High efficiency possible even with low nominal power and changing loads
  • Good controllability and thus a large tolerance range for input voltage and mains frequency. A switched-mode power supply can be designed for use with very different line voltages (e.g. 85–255 V, 47–63 Hz).
  • In devices designed for this purpose, the input voltage can also be a direct voltage.
  • Low weight and low volume due to smaller transformers and smaller secondary-side filter capacitors (high operating frequency)
  • Lower copper volume
  • Lower standby consumption possible
  • Cheaper than linearly regulated power supplies without a DC link
  • Often no thermostats or thermal fuses are required, as overcurrent protection is sufficient for the properties of the circuit.
  • Due to the switching operation with high frequencies, measures to improve the EMC behavior (interference emission) are necessary. Switching power supplies can be sources of electromagnetic interference.
  • Deformation of the mains current (current pulses) due to the reactive power for the charging processes of the input-side electrolytic capacitors . Switching power supplies cause a distortion of the supply voltage, cf. Total Harmonic Distortion . Remedy: Power Factor Correction (engl. Power Factor Correction , shortly PFC); since 2001 prescribed for SMPS with less than 16 A input current, but 50 watt or 75 watt input power (depending on the device class)
  • Poor control behavior compared to the conventional series regulator with very fast load changes or with very low loads
  • Higher circuit complexity, more components and therefore statistically higher failure probability
  • Problematic with low loads (tendency to oscillate). Solution: preload
  • More heavily loaded passive components. For the high ripple currents , suitable capacitors with low ESR are required, which in turn have an influence on efficiency, ripple of the output voltage and EMC behavior.

Areas of application

Circuit types

In addition to the rectification of the mains voltage , switched-mode power supplies consist of a galvanically isolated DC voltage converter ; they are also counted among the primary clocked converters. The usual topologies are flyback converters , single-ended flux converters and push-pull flux converters for increasing powers . A complete list of the different topologies is compiled under DC voltage converters.

Secondary switched-mode power supplies are an outdated technology for general use. They consist of a conventional transformer power supply with a downstream step-down converter instead of the series regulator . They do not achieve the high efficiency of primary clocked circuits.

In the case of switched-mode power supplies that, like PC power supplies, generate several output voltages, the switching voltage regulator can be installed on the secondary side, since it can monitor the output voltages there directly. The switching transistors on the primary side are controlled by the switching voltage regulator (also switching power supply controller) via galvanic isolation such as transformers (pulse transformers) or optocouplers.


  • Ulrich Tietze, Christoph Schenk, Eberhard Gamm: Semiconductor circuit technology . 13th, revised edition. Springer, Berlin 2010, ISBN 978-3-642-01621-9 .
  • Otmar Kilgenstein: Switching power supplies in practice. Types of switching regulators, their properties and components, executed and measured examples . 3. Edition. Vogel, Würzburg 1992, ISBN 3-8023-1436-0 .
  • Ulrich Schlienz: Switching power supplies and their peripherals. Dimensioning, use, EMC . 4th edition. Vieweg + Teubner , Wiesbaden 2009, ISBN 978-3-8348-0613-0 .

Web links

Wiktionary: Switching power supply  - explanations of meanings, word origins, synonyms, translations
Commons : Switching Power Supplies  - Collection of images, videos, and audio files

Individual evidence

  1. Valter Quercioli: Pulse width modulated (PWM) power supplies . Elsevier, Amsterdam 1993
  2. https://www.heise.de/ct/hotline/Kribbelndes-Notebook-1486059.html Kribbelndes Notebook from c't issue 9/12 (ea)