Switching power supply

Switching Power Supply - electronic circuit, where the input voltage is rectified, filtered, cut into high-frequency bursts for transmission through a small-sized transformer. The block becomes controllable, with flexibly adjustable parameters. The mass of the heaviest part of the source, the transformer, decreases. In the English literature, such devices are called Switching Mode Power Supply( SMPS).

The appearance of switching power supplies

The size of the transformers worried Tesla. The scientist, repeating the experience of the experience, has established: high frequencies of current are safe for humans, provoke large losses in the cores of transformers. The result of the dispute was the adoption of a frequency of 60 Hz for the construction of the Niagara hydroelectric station. We started with Nikola Tesla, because this is the first person who realized that you will not receive rapid oscillations mechanically. Therefore, it is necessary to use oscillatory circuits. This is how the Tesla transformer appeared( September 22, 1896), with the help of which the scientist decided to transmit messages and energy over a distance.

The essence of the invention is described in the section on the Tesla coil, we give brief information. The transformer consists of two parts connected in series. The primary winding of the first was connected to a source of alternating voltage of relatively low frequency. Due to the low transformation ratio, the capacitor connected to the secondary winding was charged to a high potential. The voltage reached the threshold, the arrester penetrated, connected in parallel to the capacitor. The oscillatory process of the discharge through the primary winding of the second transformer into the external circuit began. Tesla was receiving radio voltages with an amplitude of millions of volts.

The first step in creating a pulsed power supply, where the voltage of relatively low frequency is converted into pulses. A similar design was created in 1910 by Charles Kettering, equipping the ignition system of cars. Pulse power supplies appeared in the 60s. The idea of ​​minimizing the size of transformers( after Nikola Tesla) was put forward by General Electric in 1959 in the person of Joseph Murphy and Francis Starcher( U.S. Patent 3,040,271).The idea did not immediately find a hot response( there was no suitable element base); in 1970, Tektroniks launched a line of oscilloscopes with a new power source.

Two years later, inverters are used in electronics( Patent US3697854 A), the main thing - the first domestic models appear! Patents link to each other, it is impossible to understand who first proposed to use the idea in personal computers. In the USSR, development began in 1970, due to the appearance in the sale of high-frequency high-power germanium transistor 2Т809А.As stipulated in the literature, the Muscovite, candidate of technical sciences L. N. Sharov was the first to succeed in 1972.Later, a 400-watt pulse power supply unit by A.I. Ginzburg, S.A. Eranosyan appeared. EU computers are equipped with a novelty in 1976 by a team led by J. A. Mkrtchyan.

The first switching power supplies, known to the domestic consumer on digital TVs and VCRs, often broke down; modern products lack the disadvantage - they have been working continuously for years. The moment of the beginning of the 90s provides the following information:

1. Specific power: 35 - 120 W per cubic decimeter.
2. Inverter working frequency: 30 - 150 kHz.
3. efficiency: 75 - 85%.
4. Failure time: 50–200 thousand hours( 6250 working days).

The merits of

switching power supplies Linear power supplies are bulky, efficiency is lame. Efficiency rarely exceeds 30%.For pulsed power supplies, the average numbers are in the range of 70–80%, there are products that are very out of order. For the better, of course. The following information is given: The efficiency of a pulsed power supply reaches 98%.At the same time, the required filtering capacity of capacitors is reduced. The energy stored over a period drops dramatically with increasing frequency. It depends directly on the capacitor capacitance, quadratically on the voltage amplitude.

Raising to a frequency of 20 kHz( compared to 50/60) reduces the linear dimensions of the elements by 4 times. Flowers in comparison with the expectations in the radio. Explains the reason for equipping receivers with small capacitors.

Switching Power Supply Device

The input voltage is rectified. The process carries a diode bridge, rarely a single diode. Then the voltage is cut into pulses, here the literature cheerfully proceeds to the description of the transformer. Readers are probably plagued by the question - how does the chopper( a pulse shaping device) work. On the basis of the microcircuit, which is powered directly by the 230 volt mains voltage. Less often, a stabilitron( parallel-type stabilizer) is specially installed.

The microcircuit generates pulses( 20–200 kHz) of relatively small amplitude that control the thyristor or other semiconductor power switch. The thyristor cuts the high voltage pulses, according to a flexible program generated by the oscillator chip. Since the input has a high voltage, protection is needed. The generator is guarded by a varistor, whose resistance drops sharply when the threshold is exceeded, closing a harmful jump to the ground. From the power switch, the pulse packets arrive at a small-sized high-frequency transformer. Linear dimensions are relatively low. For a computer power supply with a capacity of 500 W fits children's palm.

The resulting voltage is rectified again. Schottky diodes are used, thanks to the low voltage drop of the metal-semiconductor transition. The rectified voltage is filtered, fed to consumers. Due to the presence of multiple secondary windings, the values ​​of different polarity and amplitude are obtained quite simply. The story is incomplete without mentioning the feedback loop. Output voltages are compared with a standard( for example, a zener diode), the pulse generator mode is adjusted: the transmitted power( amplitude) depends on the frequency, the duty cycle. Products are considered relatively unpretentious, can function in a wide range of supply voltages.

The technology is called inverter, used by welders, microwave ovens, induction hobs, cell phone adapters, iPad. A computer power supply works in a similar way.

Switching Power Supply Circuit Design. Nature provided 14 basic implementation topologies for switching power supplies. With inherent advantages, unique characteristics. Some are suitable for creating low-power power supplies( below 200 W), others show the best qualities when powered by 230 volts( 50/60 Hz).And to choose the desired topology, be able to present the properties of each. Historically, the first three are called:

• Buck - buck, deer, dollar.
• Boost - acceleration.
• Polarity inverter - polarity inverter.

Three topologies refer to linear regulators. The type of devices is considered to be the predecessor of pulse power supplies, not including advantages. The voltage is applied through the transformer, straightened, cut into the power key. The regulator is managed by the feedback, whose task is to generate an error signal. The type of devices was a multi-billion dollar turnover in the 60s, could only lower the voltage, and the consumer's common wire was connected to the mains.

Buck

topology So there were "deer".Originally intended for DC voltage, the input signal was cut into pulses, then the packs were straightened and filtered to obtain average power. Feedback controlled the duty cycle, frequency( pulse width modulation).Similar is being done today by computer power supplies. Almost immediately, power density values ​​of 1–4 W per cubic inch( subsequently up to 50 W per cubic inch) were achieved. Charmingly, it has become possible to get a multitude of output voltages unleashed from the input.

The disadvantage is the loss at the moment of the switching of the transistor, the voltage changes polarity, remains below zero until the next pulse. The indicated part of the signal, bypassing the diode, closes to ground without reaching the filter. The existence of optimal switching frequencies at which costs are minimized is found. The range of 25 - 50 kHz.

Boost Topology

The topology is called a ring throttle, put forward key. It is possible to increase the input voltage to the desired rating. The circuit works as follows:

1. At the initial moment of time, the transistor is open, the choke is stored with the energy of the voltage source through the collector, emitter pn-junctions, ground.
2. Then the key is locked, the capacitor charging process starts. The choke gives off energy.
3. At some point, the feedback amplifier is working, the load is energized. The capacitor is unable to give energy in the direction of the power switch, prevents the diode. The charge takes the payload.
4. A voltage drop will cause the feedback circuit to trip again, and the choke will begin to accumulate energy.

Polarity Inverter

topology The polar inverter topology is similar to the previous scheme, the choke is located behind the key. Works as follows:

1. At the initial moment of time, the key is open, the positive half-wave voltage fills the choke with energy. Further, the energy is powerless to pass - prevents the diode.
2. The transistor is closed, an emf is generated in the choke, called a parasitic one. It is directed oppositely to the initial one, the diode freely passes, recharging the capacitor.
3. The feedback circuit operates, the pulse-width modulator re-opens the transistor. The process of discharging the capacitor to the load begins, the throttle is again filled with energy.

Topology In this case, we observe the parallelism of the processes of energy storage / expenditure. All three considered schemes demonstrate the following disadvantages:

1. There is a DC link between the input and the output. In other words, there is no galvanic isolation.
2. It is impossible to get several voltage ratings from one circuit.

Minuses are eliminated by push-pull push-pull, late( top).Both use chopper with advance technology( forward).In the first case, a differential pair of transistors is used. It becomes possible to use one key for half the period. For control, a special shaping scheme is needed, alternately swinging these swings, and heat removal conditions are improved. The cut voltage is bipolar, it feeds the primary winding of the transformer, the secondary voltage is much in accordance with the requirements of consumers.

In the delayed topology, one transistor is replaced by a diode. The circuit is often operated with low-power power supplies( up to 200 W) with a constant output voltage of 60–200 V.