As for the choice of DC power supply, is it a transistor linear DC power supply or a SCR DC power supply or a switching power supply? This must be adopted reasonably according to the specific situation. These three kinds of circuits are widely used both at home and abroad, each with its own characteristics.
SCR DC power supply, with its powerful output power, can not be replaced by transistor linear DC power supply and switching power supply. Transistor linear DC power supply is widely used because of its high accuracy and superior performance; switching power supply reduces its volume and weight in varying degrees by eliminating heavy power frequency transformers, and is also widely used in many situations where the output voltage and current are relatively stable.
ONE. The circuit structure of SCR DC regulated power supply is as follows:

SCR is a device to control voltage. Because the conduction angle of SCR can be controlled by circuit, the conduction angle of SCR varies with the output voltage Uo. The voltage Ui added to the primary transformer also changes.

That is to say, only a part of 220V AC power is added to the primary transformer after being controlled by SCR. When the output voltage Uo is higher, the conduction angle of SCR is larger, and most of the local voltage is "released" by SCR (as shown in the figure above), so the voltage added to the primary transformer, i.e. Ui, is higher. Of course, the output voltage after rectification and filtering is also higher.
When the output voltage Uo is very low, the conduction angle of SCR is very small. Most of the local voltage is "jammed" by SCR (as shown in the figure below). Only the very low voltage is added to the primary transformer, that is, the Ui is very low. Of course, the output voltage after rectification and filtering is very low.

TWO. The main circuit of the transistor linear DC power supply is as follows:

In fact, the transistor linear DC power supply is a series of high-power transistors (actually many in parallel) at the output end of the SCR DC power supply. As long as the control circuit outputs a small current to the base of the transistor, it can control the output current of the transistor, so that the power supply system can stabilize the voltage on the basis of the SCR power supply. Secondly, the voltage stability of the transistor linear DC power supply is better than that of the switching power supply or the controllable silicon DC power supply by 1-3 orders of magnitude. However, power transistors (also known as regulators) generally occupy 10 volts of voltage. Each ampere of output current consumes 10 watts more power inside the power supply. For example, the loss of 500V 5A power supply on the power transistor is 50 watts, which accounts for 2% of the total output power. Therefore, the efficiency of transistor linear DC power supply is slightly lower than that of SCR DC power supply.

THREE. The main circuit of the switching power supply is as follows:

From the circuit, it can be seen that after rectifying and filtering, the city power transformer will be transformed into 311V high voltage, and after orderly operation of K1-K4 power switch, it will be transformed into pulse signal added to the primary stage of high frequency transformer, and the pulse height will always be 311V. When K1 and K4 are turned on, 311V high-voltage current flows into primary transformer through K1 and outflows through K4, forming a forward pulse in primary transformer. Similarly, when K2 and K3 are turned on, 311V high-voltage current flows into primary transformer through K3 and outflows through K2, forming a reverse pulse in primary transformer. In this way, a series of forward and reverse pulses are formed in the secondary transformer, and DC voltage is formed after rectification and filtering. When the output voltage Uo is high, the pulse width will be wide. When the output voltage Uo is low, the pulse width will be narrow. Therefore, the switch is actually a device to control the pulse width.

In the absence of special volume requirements, transistor linear DC power supply is generally provided to users, which is mainly:
1. The transistor linear DC power supply has good accuracy (1-3 orders of magnitude better than switching power supply or controllable silicon power supply). It is suitable for many occasions, and the general users will not raise performance, accuracy and technical indicators.
2. Easy to maintain, because most users are familiar with transistor linear DC power supply maintenance personnel, there are also spare parts in this regard. Maintenance tools, a multimeter can basically solve the problem, more careful electricians can also start.
3. After maintenance, there are no sequelae, the fault can be completely eliminated and the performance can be completely restored. As long as a power supply is properly used and repaired in time, it will not be a problem if it is used for 10 years.

In the absence of special volume requirements, switching power supply is not particularly respected to users, which is mainly:
1、At present, all kinds of PWM integrated chips used in switching power supply are mainly designed from the point of view of small range of output voltage and stable output current.
But the so-called PWM chip is a kind of pulse width modulator. When the output voltage is high and the output current is large, the switching-on time of the internal power supply is longer and the switching-off time is shorter.

When the output power is small, the pulse width is narrower:

However, the pulse width can not be unlimited narrowing, the range of pulse width change, that is, the adjustment range is only 10%-90%. This characteristic determines that this kind of PWM chip is not suitable for a so-called continuous adjustable power supply starting from zero voltage. For example, a 500V5A switching power supply has the widest control pulse when its output reaches 500V5A, such as:

When the output voltage drops to 50V5A, the width of the control pulse decreases to 10% of the widest pulse, such as:

It's down to the narrowest. If the output voltage and current continue to decline, the control pulse is required to continue to narrow, but the PWM circuit can not meet, then the circuit becomes intermittent work, such as:

Pulse sometimes does not exist, bursts of power supply will emit noise, ripple and other will become larger, electrical performance will become worse, so-called "low-end instability", in fact, has become substandard products.
2、Switching power supply has polluted power grid and radiation interference. If a radio is inserted near the high-power switching power supply, the radio will not be able to radio and will interfere with the TV signal. Some units of instruments and meters appear strange interference, and this power grid pollution is not irrelevant. There are strict regulations in the national standards for such interference and radiation.
3、Maintenance is difficult and the risk of scrapping the whole machine is high.
Because switching power supply operates at high frequency, the higher the frequency, the smaller the main transformer. However, with the increase of frequency, the negative effects of various distribution parameters are also obvious. Therefore, the smaller the distribution parameters, the better, the more exquisite the process design, the shortest the lead and the closer the components are. Because of the dense components, it is difficult to maintain. In addition, because of the difference between the circuit and the linear power supply, the technical quality of the maintenance personnel is high, and the multimeter is no longer helpful. It is necessary to use the oscilloscope to observe the working state of each point of the circuit.
More importantly, as switching power transistors work under high voltage, once damaged, they are generally four, that is, all the bad light, emit a loud explosion sound, and further burn down the pulse transformer which generates the control signal, thus affecting the printed circuit board, almost burning a piece, as long as there is such, the whole power source newspaper. The risk of scrapping is high.

Fortunately, after years of manufacturing and R&D practice, we have solved many technical problems. At present, the two switching DC power supply (standard and intelligent) developed and manufactured in the range of 800V1000A work very steadily, and the failure rate is very low. The shortcomings of the switched DC power supply listed above have been greatly improved.