Introduction
As switching power supply technology continues to mature, its application areas have been further broadened. Compared with traditional series continuous regulated power supplies, switching power supplies have been improved in terms of efficiency, electromagnetic pollution, size and reliability. On the other hand, the latest solid-state FM broadcast transmitters have increasingly high requirements for power supplies, and the maturity of switching power supply technology, the continuous updating of components, and the application of highly reliable control chips are fully capable of meeting the requirements of FM broadcast transmitters. The current solid-state FM broadcast transmitter components such as exciters and power amplifiers commonly use switching power supplies as energy support. The future of digital control and management for switching power supply has put forward higher requirements, intelligent, digital, small size and high reliability will be the development direction of FM broadcast transmitter switching power supply.
Switching power supply
Power supply is the power heart of the whole FM broadcast transmitter. Considering the electromagnetic compatibility between the transmitter room equipment, the overall efficiency of the transmitter, the reliability of the power supply and routine maintenance, the switching power supply is undoubtedly the best choice for solid FM broadcast transmitter power supply. The excellent characteristics of switching power supply is mainly reflected in the following aspects. First: smaller size. It can be integrated with the power amplifier assembly. Hundreds of kHz switching frequency makes the filtering impedance component size reduced to a minimum, which not only reduces the weight of the transmitter and reduce the size, easy to transport and routine maintenance. Second: higher efficiency. Including the application of new devices such as power switching MOSFETs, switching power supply multiple circuit topology combination of switching technology is an important guarantee to reduce losses and improve the efficiency of the power supply system. Third: less electromagnetic pollution. Transmitter power supply built-in electromagnetic interference (EMI) filter circuit and related high spike pulse absorption circuit is an important guarantee that the power supply current harmonics meet the requirements, it can not only improve the power supply to the grid load characteristics, reduce the serious pollution brought to the grid, but also reduce harmonic interference with other network equipment. Fourth: reliability is further improved. A variety of protective measures against lightning, induction or reverse overvoltage and the use of three-proof paint (moisture, salt and mold resistant) printed circuit boards can be minimized the chance of failure.
Switching power supply applications
A switching power supply is a form of power supply that operates by continuously controlling power switching tubes at a certain frequency so that power can be supplied to a converter or load through energy storage elements (such as inductors and capacitors). The average output voltage or current can be controlled simply by changing the duty cycle, switching frequency or associated phase. The switching frequency range of switching power supplies is from 20 kHz to several MHz, and for power supply applications with power greater than 90 W, switching power supplies are usually converted in two stages. That is, the power factor correction (PFC) control converter and the DC/DC converter. Particular mention should be made here of the power factor correction circuit. It is set up to ensure that the input voltage and current work in phase. The result is a power factor close to 1. The apparent power is fully converted to active power, and thus the system efficiency is improved. Without the PFC correction circuit, the input current would be fed into the switching power supply as narrow pulse width high peak pulses causing severe harmonic interference components. These harmonic components not only do not provide any energy to the load, but also cause transformers and other equipment to heat up. There are two types of power factor correction circuits, active and passive. Most switching power supplies for FM broadcast transmitters use active power factor correction circuits, which are composed of two major parts: an AC/DC converter with active power factor correction and a separate DC/DC converter.
Discussion of related issues in practical applications
Switching power supplies have a greater chance of failure during the use of FM broadcast transmitters for a number of reasons. Transmitter room environmental factors (such as ventilation, temperature and humidity), power control cabinet lightning problems, switching power supply itself design and device problems, staff misuse problems, etc. are the hidden danger of failure. If you want the equipment to work properly, in addition to mastering the necessary professional knowledge, continuous accumulation of experience is also necessary. The failure rate can often be minimized through the observation and analysis of the fault display of the attached protection circuitry built into the switching power supply. Switching power supply due to the use of large-capacity energy storage capacitors, in the work of generating a large inrush current, making the switching tube in the AC voltage close to the peak when the shutdown. Instantaneous changes in the input AC voltage itself can also lead to the same result. Therefore, in the actual circuit of the switching power supply, a thermistor with negative temperature characteristics is often used in series with the bridge rectifier block before. When the power switch is closed, the thermistor temperature is low, a high resistance state, the inrush current is suppressed. As the current flow thermistor temperature rises, the resistance value drops to zero, and the input voltage is added to the load at full voltage. However, this basic protection mechanism in actual use is slightly inadequate. If the power switch is disconnected for a few seconds and then re-closed, the thermistor does not have sufficient time to cool, the input amplitude of the AC voltage close to the peak at this time, will produce a larger than normal inrush current, both this current in the sense resistor to produce a voltage higher than 6V, because the LT1249 chip has not been charged, can not play a protective role. This is the direct cause of the power switch MOSFET breakdown short-circuit damage. This was confirmed in Dalian at the beginning of the strong wind and rainstorm disaster caused by a number of FM broadcast transmitter power failure.
Varistors connected in parallel at both ends of the AC circuit input can also absorb electrical surges. Under the condition of constant ambient temperature, the varistor resistance value increases with the applied voltage and decreases sharply. Therefore, it has superior efficacy in absorbing surges. In order to prevent the opening and closing of the amplifier power supply caused by surge voltage, the use of varistors connected to the power line between the phases, thus playing a role in the protection of power supply equipment.
Grounding is the most basic and simple security measures. Transmitter cabinet, amplifier box shell, power supply shell, panel and door have been connected to each other and connected to the grounding end of the transmitter, the transmitter is installed in place, the grounding end of the machine (located on the bottom plate of the power supply part of the transmitter) should be bent to connect with the room ground reliably, in order to avoid unfortunate events due to leakage. It is also required to ground all points in the circuit that require grounding, so as to ensure that the current that needs to be grounded and the high-frequency current leaking from the transmitter can flow smoothly into the earth.
Concluding remarks.
Although there are various circuit topology combinations for switching power supplies, with different options for different occasions due to load type, power requirements, and control methods, the PFC control unit and PWM control unit in the switching power supply are the core, which is an important guarantee for FM broadcast transmitters to obtain high quality signal transmission and emission. In addition, in the process of using the equipment, should fully understand the working state of the equipment and fault phenomenon, and constantly accumulate lessons learned, so as to help master the fault characteristics of the switching power supply, improve the FM broadcast transmitter maintenance level, to ensure that the equipment is in normal working condition.