Application of high frequency switching power supply in microwave station

0 Introduction
With the rapid development of electronic technology, the application of switching power supplies is becoming more and more extensive. The continuous improvement of switching frequency has further optimized the performance of switching power supplies, with higher integration, lower power consumption, simpler circuits, and more reliable operation, which is the development direction of switching power supplies. At present, high-frequency switching power supplies have been widely used in various microwave stations of radio and television in our province. Our station now uses the Norwegian POWEC resonant high-frequency switching power supply with advanced technology and reliable quality produced by Guangzhou Jishiyu Communication Co., Ltd.

1 Composition of high-frequency switching power supply

1.1 Main circuit
The whole process of input from AC grid and output from DC includes:
(1) Input filter: Its function is to filter out the noise in the grid and prevent the noise generated by the machine from being fed back to the public grid.
(2) Rectification and filtering: The AC power of the grid is directly rectified into smoother DC power and stable DC power is provided to the power factor correction circuit.
(3) Power factor correction: It is located between rectification and filtering and inverter, in order to eliminate the harmonic current pollution of the grid caused by the rectification circuit and reduce reactive power loss to improve the power factor.
(4) Inversion: The rectified DC power is converted into high-frequency AC power, which is the core part of the high-frequency switching power supply. The higher the frequency, the smaller the ratio of volume, weight and output power.
(5) Output rectification and filtering: According to the load requirements, a stable and reliable DC power supply is provided.
1.2 Control circuit
On the one hand, the sample is taken from the output end, compared with the set standard, and then the inverter is controlled to change its frequency or pulse width to achieve output stability. On the other hand, according to the data provided by the test circuit and identified by the protection circuit, the control circuit is provided to take various protection measures for the whole machine.
1.3 Detection circuit
In addition to providing various parameters in the protection circuit, it also provides various display instrument data for the on-duty personnel to observe and record.
1.4 Auxiliary power supply
Provides various power supplies (AC and DC voltage power supplies of various levels) required for the operation of all circuits of the switching rectifier itself.

2 High-frequency switch control voltage regulation principle

The switch K is repeatedly turned on and off at a certain time interval. When the switch K is turned on, the input power supply E is provided to the load RL through the switch K and the filter circuit. During the entire switch-on period, the power supply E provides energy to the load RL; when the switch K is turned off, the input power supply E interrupts the energy supply. It can be seen that the input power supply provides energy to the load intermittently. In order to enable the load to obtain continuous energy supply, the switching voltage stabilizer must have a set of energy storage devices to store part of the energy when the switch is turned on and release it to the load when the switch is turned off. In Figure 2, the circuit composed of inductor L, capacitor C2 and diode D has this function. Inductor L is used to store energy. When the switch is turned off, the energy stored in inductor L is released to the load through diode D, so that the load obtains continuous and stable energy. Because diode D makes the load current continuous, it is called a freewheeling diode. The average voltage EAB between AB can be expressed by the following formula:

Where TON is the time when the switch is turned on each time, and T is the working cycle of the switch on and off (that is, the sum of the switch on time TON and the off time TOFF). It can be seen from the formula that by changing the ratio of the switch on time and the duty cycle, the average voltage between AB will also change accordingly. Therefore, by automatically adjusting the ratio of TON and T as the load and input power supply voltage change, the output voltage V0 can be maintained unchanged. Changing the ratio of the on time TON and the duty cycle is to change the duty cycle of the pulse. This method is called "time ratio control" (abbreviated as TRC).
According to the TRC control principle, there are three methods
: (1) Pulse Width Modulation (abbreviated as PWM), the switching cycle is constant, and the duty cycle is changed by changing the pulse width.
(2) Pulse Frequency Modulation (abbreviated as PFM), the on-pulse width is constant, and the duty cycle is changed by changing the switch operating frequency.
(3) Mixed modulation, the on-pulse width and the switch operating frequency are not fixed, and can be changed at the same time. It is a mixture of the above two methods.

3 Characteristics of high-frequency switching power supply
(1) Light weight and small size. High-frequency technology is used to remove the power frequency transformer. Compared with the phase-controlled rectifier, the size and weight of the switching rectifier are only 1/10 of the phase-controlled rectifier when the output power is the same.
(2) Low power consumption and high efficiency. The power devices used in the switching power supply generally have low power consumption. The overall efficiency of the switching power supply with power factor compensation can reach 88%, and the better ones can reach more than 91%.
(3) High power factor. The power factor of the phase-controlled rectifier changes with the change of the conduction angle of the thyristor. Generally, it can be close to 0.7 or more when fully turned on, and only about 0.3 when the load is small. The power factor of the calibrated open-magnetic power supply is generally above 0.93, and is basically not affected by load changes (for loads above 20%).
(4) The voltage regulation accuracy can be as high as 0.2% (phase-controlled 1%).
(5) Low noise. In the phase-controlled rectifier equipment, the audible noise generated by the power frequency transformer and filter inductor when working is relatively large, generally greater than 60 dB. The audible noise of the switching power supply is only about 45 dB without a fan.
(6) Easy maintenance, because the switching power supply is modular and can be replaced during operation without affecting the work (the phase-controlled power supply needs to be shut down for processing).
(7) N+1 matching is implemented, with high reliability. Stable, reliable, long life, and N+1 redundant configuration is adopted.
(8) Convenient expansion.
(9) It can be controlled by a microcomputer, with a remote interface, to form an intelligent power supply device, which is convenient for centralized monitoring.
(10) Low requirements for AC input. When the three phases are seriously unbalanced, the rectifier system can still output and provide stable DC power.

4 Classification of high-frequency switching power supplies
(1) According to the excitation method, it can be divided into: self-excited and externally excited
The power conversion circuit of the self-excited switching power supply generates oscillation by itself after the power is turned on, that is, the circuit relies on the positive feedback process of the circuit itself to achieve power conversion. The self-excited circuit appeared the earliest. Its characteristics are simple circuit and fast response speed, but the switching frequency changes greatly, the output ripple value is large, and it is not easy to make accurate analysis and design. It is usually only used in low-power situations, such as home appliances and instrument power supplies. Separately excited switching power supplies require external excitation signal control to make the conversion circuit work and complete the power conversion task. Separately excited switching power supplies are characterized by constant switching frequency and small output ripple, but the circuit is more complex, the cost is higher, and the response speed is slower.
(2) According to the switching devices used in the switching power supply, it can be divided into: bipolar transistor switching power supply, power MOS tube switching power supply, IGBT switching power supply, thyristor switching power supply, etc.
Power MOS tubes are used in switching power supplies with a switching frequency of more than 100 kHz, and thyristors are used in high-power switching power supplies.
(3) According to the control method of the switching power supply, it can be divided into: pulse width modulation (PWM) switching power supply, pulse frequency modulation (PFM) switching power supply, and hybrid modulation switching power supply.
(4) According to the structural form of the power conversion circuit of the switching power supply, it can be divided into: buck type, inverter type, boost type and transformer type. In the transformer type, according to the form of the switching tube output circuit, it can be divided into: single-ended switching power supply and double-ended switching power supply. Single-ended switching power supplies can be divided into single-ended forward type and single-ended flyback type. Double-ended switching power supplies can be divided into push-pull type, half-bridge type and full-bridge type.

5 Application of POWEC power supply
(1) The POWEC power supply system adopts zero voltage resonant switching technology, as shown in Figure 3:

In Figure 3, the resonant frequency of C1, C2 and L is always lower than that of Q1 and Q2, thereby ensuring that the switch tubes Q1 and Q2 are turned on under zero voltage conditions, eliminating switching losses and parasitic capacitance.
(2) The POWEC power supply can work normally in harsh ambient temperatures of -25℃ to 55℃, and can work for two hours at 70℃. The average time between failures is up to one million hours.
(3) The POWEC power supply adopts a hardware current balancing method that is independent of the control module. Even if the control module fails, the system can maintain a good load balancing, with an error of less than 5% of the standard current, greatly improving the stability and reliability of the system; in terms of battery measurement, it provides a voltage measurement port and a temperature measurement port; in terms of battery charging, in addition to floating charging, it also provides three types of equalization charging, namely manual equalization charging, timed equalization charging and automatic equalization charging, as well as temperature compensation function and battery current limiting function. In addition, it also provides output overvoltage, overcurrent protection, rectifier module overtemperature protection, and a settable low voltage cut-off device to protect the battery or load.

6 Conclusion
The high-frequency switching power supply is a modular design with high reliability and intelligence. It is small in size, low in power consumption, high in efficiency, low in noise, high in voltage stabilization accuracy, safe and reliable, and easy to use and maintain. It has been widely used in various microwave stations.