#power supplies
Transformer and pulse power supplies.
Transformer power supplies
A classic power supply is a transformer power supply.
In general, it consists of a step-down transformer or autotransformer, whose primary winding is designed for line voltage.
Then a rectifier is installed, converting alternating voltage to direct voltage (pulsating unidirectional).
In most cases, the rectifier consists of one diode (half-wave rectifier) or four diodes forming a diode bridge (full-wave rectifier).
Sometimes other circuits are used, for example, in rectifiers with voltage doubling. After the rectifier, a filter is installed to smooth out oscillations (pulsations). Usually, it is just a large-capacity capacitor.
The circuit can also include filters for high-frequency interference, surges, short-circuit protection, voltage and current stabilizers
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Schematic of the simplest transformer power supply with a full-wave rectifier
Transformer dimensions
There is a formula that is easily derived from the basic laws of electrical engineering (and even Maxwell's equations):
( 1/n ) ~ f * S * B
where n is the number of turns per 1 volt (the left side of the formula contains the EMF of one turn, which is, according to Maxwell's equation, the derivative of the magnetic flux, the flux is something in the form of sin ( f * t ), in the derivative f is taken out of the brackets), f is the frequency of alternating voltage, S is the cross-sectional area of the magnetic circuit, B is the magnetic field induction in it.
The formula describes the amplitude of B, not the instantaneous value.
The value of B is limited in practice from above by the occurrence of hysteresis in the core, which leads to losses due to remagnetization and overheating of the transformer.
If we assume that f is the network frequency (50 Hz), then the only two parameters available for selection when developing a transformer are S and n. In practice, the heuristic n = (from 55 to 70)/S in cm2 is accepted.
Increasing S means increasing the dimensions and weight of the transformer.
If we follow the path of decreasing S, then this means increasing n, which in a small transformer means decreasing the wire cross-section (otherwise the winding will not fit on the core).
Increasing n and decreasing the cross-section means a strong increase in the active resistance of the winding.
In low-power transformers, where the current through the winding is small, this can be neglected, but with increasing power, the current through the winding increases and, with high winding resistance, dissipates significant thermal power on it, which is unacceptable.
The above considerations lead to the fact that at a frequency of 50 Hz, a transformer of high power (tens of watts) can be successfully implemented only as a device of large dimensions and weight (by increasing S and the wire cross-section with a decrease in n).
Therefore, modern power supplies take a different path, namely by increasing f, i.e. switching to pulse power supplies.
Such power supplies are several times lighter (and the main part of the weight falls on the shielding cage) and are significantly smaller in size than classic ones.
In addition, they are not demanding on the input voltage and frequency.
Advantages of transformer power supplies
- Simple design
- Reliability
- Availability of the element base
- Absence of generated radio interference (unlike pulsed ones, which create interference due to harmonic components)
- Disadvantages of transformer power supplies
- Large weight and dimensions, especially at high power
- Metal consumption
- A compromise between reduced efficiency and output voltage stability: a stabilizer is required to ensure stable voltage, introducing additional losses.
Pulse Power Supplies
Pulse power supplies are an inverter system. In pulse power supplies, the alternating input voltage is first rectified.
The resulting DC voltage is converted into rectangular pulses of increased frequency and a certain duty cycle, either fed to a transformer (in the case of pulse power supplies with galvanic isolation from the power supply network) or directly to the output low-pass filter (in pulse power supplies without galvanic isolation).
Small-sized transformers can be used in pulse power supplies — this is explained by the fact that with an increase in frequency, the efficiency of the transformer increases and the requirements for the dimensions (cross-section) of the core required to transmit equivalent power decrease.
In most cases, such a core can be made of ferromagnetic materials, unlike the cores of low-frequency transformers, for which electrical steel is used.
In pulse power supplies, voltage stabilization is ensured by negative feedback.
Feedback allows maintaining the output voltage at a relatively constant level, regardless of fluctuations in the input voltage and the load value. Feedback can be organized in different ways.
In the case of pulsed sources with galvanic isolation from the power supply network, the most common methods are the use of communication via one of the output windings of the transformer or using an optocoupler.
Depending on the magnitude of the feedback signal (depending on the output voltage), the duty cycle of the pulses at the output of the PWM controller changes.
If isolation is not required, then, as a rule, a simple resistive voltage divider is used. Thus, the power supply maintains a stable output voltage.
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Schematic diagram of the simplest single-ended pulse power supply
Advantages of pulse power supplies
- Comparable in output power with linear stabilizers, the corresponding pulse stabilizers have the following main advantages:
- less weight due to the fact that with increasing frequency it is possible to use transformers of smaller sizes with the same transmitted power. The weight of linear stabilizers consists mainly of powerful heavy low-frequency power transformers and powerful radiators of power elements operating in linear mode;
- significantly higher efficiency (up to 90-98%) due to the fact that the main losses in pulse stabilizers are associated with transient processes at the moments of switching the key element. Since the key elements are in one of the stable states most of the time (i.e. either on or off), energy losses are minimal;
- lower cost, due to the mass production of a unified element base and the development of high-power key transistors. In addition, it should be noted that the cost of pulse transformers is significantly lower with comparable transmitted power, and the possibility of using less powerful power elements, since their operating mode is key;
- comparable reliability with linear stabilizers. (Power supplies for computing equipment, office equipment, and household appliances are almost exclusively pulsed).
- a wide range of supply voltage and frequency, unattainable for a comparable priced linear one. In practice, this means the ability to use the same pulse power supply for portable digital electronics in different countries of the world — Russia/USA/England, which have very different voltage and frequency in standard sockets.
- the presence in most modern power supplies of built-in protection circuits against various unforeseen situations, such as short circuits and lack of load at the output
Disadvantages of pulse power supplies
- The main part of the circuit operates without galvanic isolation from the network, which, in particular, somewhat complicates the repair of such power supplies;
- All pulse power supplies without exception are a source of high-frequency interference, since this is due to the very principle of their operation. Therefore, additional interference suppression measures are required, which often do not allow eliminating interference completely. In this regard, it is often unacceptable to use pulse power supplies for some types of equipment.
- In distributed power supply systems: the effect of harmonics multiple of three. With the presence of effective power factor correctors and filters in the input circuits, this drawback is usually not relevant.