1.Introduction

The high-frequency switching power supply first directly rectifies and filters the alternating current into direct current through the diode, and then inversely converts it into high-frequency alternating current through the switching tube. After being transformed and isolated by the high-frequency transformer, it is rectified by the fast recovery diode and filtered by the inductor and capacitor, then it's output. Its working principle is different from that of silicon controlled rectifier, which directly rectifies alternating current. Compared with traditional silicon controlled silicon rectifiers, high-frequency switching power supplies have the characteristics of small size, light weight, good waveform, and high efficiency. The details will be introduced later.

2.Compared with traditional silicon rectifiers, high-frequency switching power supplies have the following main advantages:

2.1 Small size and light weight

The frequency of high-frequency switching power supply is generally above 10KHZ. The frequency is high. The volume of the main transformer is generally only one-tenth or even a few-tenth of that of the 50HZ power frequency transformer of the same specification. Therefore, the high-frequency switching power supply is small in size and heavy in weight. Lightweight, more convenient for users to carry and site planning.

2.2 High control accuracy

Due to its high frequency, the high-frequency switching power supply controls the opening and closing of the switching tube with a period of tens of microseconds. The control accuracy is high and can generally be guaranteed to be less than 1% in the full range, while the silicon controlled rectifier directly controls the opening and closing of 50HZ. and shutdown, the accuracy can generally only be guaranteed to be between 1-3%.

2.3 The output waveform is good

The frequency of high-frequency switching power supply is high, and the DC output waveform is a high-frequency square wave of more than ten KHZ or even dozens of KHZ. The waveform is good and only a few filter capacitors and filter reactors are needed to ensure smooth output DC, while the frequency of silicon controlled rectifier Low, the output waveform is a low-frequency half-wave of 100HZ, the waveform is poor, and it is difficult to ensure smooth DC output no matter how much reactance and capacitance is added. Therefore, high-frequency switching power supplies are most suitable for use in fields that require waveforms such as electroplating, electrophoresis, and test aging.

2.4 Energy saving

High-frequency switching power supplies have more advantages than silicon-controlled rectifiers in low-voltage and high-current applications. The most obvious one is energy saving, which is mainly reflected in the following aspects:

(1) The power frequency transformer used in the main transformer of the silicon rectifier is not only bulky, but also very power-consuming. For a six-phase half-wave rectifier transformer, its loss is generally 5% of the total power when rated; and high-frequency switching power supplies The high-frequency transformer used in the main transformer is small in size and the loss is generally within 1%.

(2) The diodes of high-frequency switching power supplies use Schottky diodes, with a voltage drop of only 0.6V, while the loss of silicon rectifier diodes is generally above 0.9V. For low-voltage and high-current power supplies, the loss of the rectifier is the largest loss besides the transformer. The main loss, other primary side losses are particularly small due to the small current.

Therefore, the high-frequency switching power supply saves at least 5% more energy than the silicon-controlled rectifier when rated. As the voltage and current decrease, the energy-saving ratio will gradually increase. For example, at 70% full load, the energy-saving ratio will reach 10 % above.

Example: When a 3000A24V high-frequency switching power supply and a silicon-controlled rectifier are running at rated power, based on the calculation of at least 5% energy saving in loss, a high-frequency switching power supply can save at least W = (3000× 24/0.854)-(3000×24/0.9)=4706W≈4.706KW. According to 20 hours a day and 365 days a year, the energy that can be saved is Q=4.706×20×365=34354KW·H. According to one yuan per KW·H Calculated, it can save at least 34,000 yuan/unit every year.

2.5 High power factor and low harmonic content

The power factor of a silicon controlled rectifier is generally variable between 0-0.85. Its power factor is the highest at the rated voltage. When the output voltage is lower than the rated voltage, its power factor gradually decreases. As the voltage decreases, the proportion of reactive power will gradually increase. When the voltage is particularly low, that is, when the trigger a angle is large, the reactive power may be greater than the active power, so many Enterprises have to accept fines from the power department, but the power factor of high-frequency switching power supplies can be guaranteed to be above 0.95 at any time.

The silicon controlled rectifier directly achieves DC output by phase-controlled 50HZ alternating current. Its harmonic content is very large. The large harmonic content will cause great interference to the power grid, which not only affects the normal operation of the rectifier equipment and other equipment, but also seriously It will cause the power supply system to collapse. High-frequency switching power supplies use inverter technology to achieve adjustment by controlling the duty cycle of IGBT or MOS tubes. The harmonic content is very low. When designing and manufacturing high-frequency switching power supplies, Liyuan Company uses a number of technologies to reduce harmonics. For example, adding a noise filter (EMI) to the incoming line and adding a DC reactor on the DC high-voltage side have greatly reduced the interference of harmonics to the power grid, making the harmonic content of high-frequency switching power supplies far lower than that of silicon controlled rectifier.

Due to the low power factor and large harmonic content of silicon controlled rectifiers, in order to provide power factor and reduce harmonic content, a harmonic filtering power factor compensation cabinet is generally installed, but this will increase the user's investment cost, and high-frequency switching power supplies doesn't require any compensation device.

Due to the low efficiency, low power factor, and high harmonic content of silicon-controlled rectifiers, the required grid capacity must also increase accordingly, which directly leads to an increase in the capacity of power transformers, connecting wires, switches, etc., and the total investment cost of users will also increase.

2.6 Modular structure can be thermally maintained

Due to the limitation of electronic components, the power of a single machine of high-frequency switching power supply should not be too large. At present, the power of a single machine is mainly below 40KW. The production of high-power high-frequency switches generally adopts a modular structure and multiple machines in parallel. There are two definitions of modularity of high-frequency switching power supply. One refers to the modularization of power devices, and the other refers to the modularization of power supply units. The former is low in cost and simple to manufacture, but has poor stability, uneven current sharing, and cannot be maintained thermally; Although the latter is high in cost, it has high stability, good current sharing, and can be thermally maintained and replaced.

Therefore, high-power and high-frequency switching power supplies generally adopt a multi-module parallel structure. Liyuan high-power high-frequency switching power supply has a multi-module parallel structure, combined with the fully digital RS485 control system and current sharing system developed by Liyuan. It has fast control speed, high precision, and the current sharing coefficient is within 1%.

The high-power and high-frequency switching power supply can be designed as an N+X hot redundant backup system. Under normal circumstances, N sub-units operate and X sub-units are in standby. When one of the modules fails, the system automatically shuts down the faulty unit and starts the standby unit. The power supply maintains its original voltage and current. Since the possibility of simultaneous failure of two modules is extremely low, the high-frequency switching power supply can basically guarantee that it will never shut down.

Since the high-power and high-frequency switching power supply adopts a modular structure, it can be easily expanded in the future.

The silicon controlled rectifier is made of a single machine and does not have thermal maintenance function, nor can it be expanded.

3. Comprehensive performance comparison

4. Development Trend

From the above points, it can be seen that high-frequency switching power supplies have the characteristics of small size, light weight, good waveform, high control accuracy, small harmonic content, and high power factor than silicon-controlled rectifiers. Therefore, high-frequency switching power supplies are becoming more and more popular. It has been applied in many fields. Under the national policy of vigorously promoting energy conservation and emission reduction, it will also be an inevitable trend for high-frequency switching power supply to replace silicon controlled rectifiers.

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