With the continuous maturity of switching power supply technology, its application areas have been further broadened. Compared with the traditional series continuous regulated power supply, the switching power supply has been greatly improved in terms of efficiency, electromagnetic pollution, volume and reliability. On the other hand, the latest solid-state FM broadcast transmitters have higher and higher requirements for power supply, while the maturity of switching power supply technology, the continuous update of components, and the application of high-reliability control chips can fully meet the requirements of FM broadcast transmitters. Currently, components such as exciters and power amplifiers in solid-state FM broadcast transmitters generally use switching power supplies as energy support. The future digital control and management put forward higher requirements for switching power supplies. Intelligent, digital, small size and high reliability will be the development direction of switching power supplies for FM broadcast transmitters.
switching power supply
The power source is the power heart of the entire FM broadcast transmitter. Taking into account the electromagnetic compatibility between the various equipment in the transmitter room, the overall efficiency of the transmitter, the reliability of the power supply and daily maintenance, switching power supplies are undoubtedly the best choice for the power supply of solid-state FM broadcast transmitters. The excellent characteristics of switching power supply are mainly reflected in the following aspects. First: smaller size. It can be integrated and assembled with the power amplifier. The switching frequency of several hundred kHz minimizes the volume of the filter impedance component, which reduces the weight and volume of the transmitter, and is convenient for transportation and daily maintenance. Second: higher efficiency. Including the application of new devices such as power switching tube MOSFET, the switching technology of switching power supply multiple circuit topologies is an important guarantee for reducing losses and improving the efficiency of the power supply system. Third: Less electromagnetic pollution. The electromagnetic interference (EMI) filter circuit and related high-spike absorption circuit in the transmitter power supply are important guarantees for the current harmonics of the power supply to meet the requirements. It can not only improve the load characteristics of the power supply to the power grid, but also reduce the serious impact on the power grid. Pollution can also reduce harmonic interference to other network equipment. Fourth: Reliability has been further improved. A variety of protection measures against lightning, induction or counter-overvoltage and the use of printed circuit boards coated with three anti-paint (anti-moisture, anti-salt and anti-mildew) can minimize the probability of failure.
Switching power supply application
Switching power supply is a form of power supply in which the power switch tube is continuously controlled at a certain frequency for on-off operation, so that it can provide power to the converter or load through energy storage elements (such as inductors and capacitors). As long as the duty cycle, switching frequency or relative phase is changed, the average output voltage or current can be controlled. The switching frequency of the switching power supply ranges from 20kHz to several MHz. For working occasions where the power of the power supply is greater than 90W, the switching power supply usually adopts a two-stage conversion method. That is, the power factor correction (PFC) controls the converter and the DC/DC converter. In particular, it should be mentioned here is the power factor correction circuit. It is set to ensure that the input voltage and current work in the same phase. As a result, the power factor is close to 1, the apparent power is all converted into active power, and the system efficiency is improved. If there is no PFC correction circuit, the input current will be input into the switching power supply in the form of a narrow pulse width and high peak value pulse, causing serious harmonic interference components. These harmonic components not only do not provide any energy to the load, but also cause the transformer and other equipment to heat up. Power factor correction circuits are divided into two types, active and passive. Most switching power supplies of FM broadcast transmitters use active power factor correction circuits, which are composed of an AC/DC converter with active power factor correction and an independent DC/DC converter. AC/DC converter mainly includes: EMI filter, slow start circuit, bridge rectifier, PFC controller, power drive circuit and converter circuit (consisting of power switch MOSFET, energy storage inductor L, fast recovery rectifier diode and filter capacitor And other composition.
The AC input is passed through the EMI filter circuit to filter out the differential and common electromagnetic interference signals, and then input to the slow start circuit, and after a delay, the full voltage is added to the bridge rectifier circuit, and the output DC voltage is provided to the power MOSFET. Drain. The PFC controller is a circuit composed of an 8-pin LT1249 power factor control chip and fewer peripheral components. The 8th pin outputs a drive signal with a switching frequency of 100kHz, which is added to the gate of the MOSFET power switch via the drive circuit, and the MOSFET converter starts to switch on and off with a certain duty cycle, and outputs the required DC voltage. The LT1249 integrated chip produced by Linear Technology has built-in oscillators, current multipliers, current amplifiers, error voltage amplifiers, voltage comparators and reference voltage sources. By averaging the set high-frequency pulse width modulation current, the LT1249 can achieve the lowest possible current distortion, and can work in continuous and discontinuous operating modes. In addition, the built-in current multiplier squaring the current from the error voltage amplifier can reduce the AC gain at light load, thereby maintaining low current distortion and high system stability. The PFC controller extracts sensing signals from the bridge rectifier, the converter, and the sensing resistance between them to implement multiple protection functions, such as peak current limit and overvoltage protection.
It is mainly composed of switching transformer, MOSFET power switch tube, rectifier components, sensing circuit (including voltage, current and temperature sampling), auxiliary power supply, UC3843PWM controller and related driving circuit. The DC voltage input from the previous stage is added to the drain of the parallel power switch MOSFET, and its gate input is provided by the drive circuit by the set frequency switching signal in the UC3843 control chip. After the voltage is boosted by the switching transformer, the required DC voltage is obtained by rectification and filtering. UC3843 control chip is a current mode PWM control regulator. It has the characteristics of optimized DC/DC converter, low starting current, automatic feedforward compensation, current limit, low voltage lockout, pulse suppression, high current drive and up to 500kHz switching frequency. From the analysis of the internal circuit of UC3843, the internal reference signal and the voltage sampling value of the transformer secondary after rectification and filtering are processed in the error amplifier. The processed error voltage and the voltage formed by the sense resistance are input to the PWM comparator, and its output is in the same way as the clock signal. Waveform processing is performed in the trigger circuit, and finally a switching frequency signal with the same frequency as the clock frequency is output.
Discussion of related issues in practical applications
Switching power supply has a greater chance of failure during the use of FM broadcast transmitters due to various reasons. Environmental factors (such as ventilation, temperature, and humidity) in the transmitter room, lightning protection problems in the power control cabinet, switching power supply itself design and device problems, and staff misoperation problems are all hidden dangers of failure. If you want the equipment to work normally, in addition to mastering the necessary professional knowledge, continuous accumulation of experience is also necessary. The failure rate can often be minimized by observing and analyzing the fault display of the auxiliary protection circuit built in the switching power supply. The switching power supply uses a large-capacity energy storage capacitor, which generates a relatively large surge current during operation, so that the switching tube is turned off when the AC voltage is close to the peak value. The instantaneous change of the input AC voltage itself can also cause the same result. Therefore, in the actual circuit of the switching power supply, a thermistor with negative temperature characteristics is often used in series in front of the bridge rectifier block. When the power switch is closed, the thermistor has a low temperature and a high resistance state, and the surge current is suppressed. As the temperature of the thermistor rises as the current flows, the resistance drops to zero, and the full input voltage is added to the load. However, this basic protection mechanism is slightly insufficient in actual use. If the power switch is turned off for a few seconds and then closed again, the thermistor does not have sufficient time to cool down. At this time, the input AC voltage with the amplitude close to the peak value will generate a larger surge current than normal. The current produces a voltage higher than 6V on the sense resistor, and because the LT1249 chip has not been powered on, it cannot play a protective role. This is the direct cause of the breakdown and short-circuit damage of the power switch MOSFET. This was confirmed in the power failures of many FM radio transmitters caused by strong storms and rain disasters in Dalian at the beginning of the year.
Varistor connected in parallel at both ends of the AC circuit input can also absorb electrical surges. Under the condition that the ambient temperature does not change, the resistance of the varistor decreases sharply as the applied voltage increases. Therefore, it has a superior effect on absorbing surges. In order to prevent the surge voltage caused by switching the power amplifier power supply, a varistor is connected between the power lines to protect the power supply equipment.
The grounding wire is the most basic and simple safety measure. The cabinet, power amplifier box shell, power supply shell, panel and door of the transmitter have been connected to each other and connected to the ground terminal of the transmitter. After the transmitter is installed in place, the ground terminal of the unit (located in the power supply part of the transmitter) should be connected to each other. The corners on the bottom plate are reliably connected with the ground of the machine room to avoid the occurrence of unfortunate incidents due to electric leakage. At the same time, it is also required to ground each point in the circuit that requires grounding, so as to ensure that the current that needs to be grounded and the high-frequency current leaked by the transmitter can smoothly flow into the ground.
Conclusion
Although the switching power supply has a variety of circuit topology combinations, there are different options due to different occasions such as load types, power requirements, control methods, etc., but the PFC control unit and PWM control unit in the switching power supply are the core, which is the frequency modulation broadcast transmitter. An important guarantee for quality signal transmission and emission. In addition, during the use of the equipment, it is necessary to fully understand the working status and failure phenomena of the equipment, and continuously accumulate experience and lessons. This will help to grasp the failure characteristics of the switching power supply, improve the maintenance level of the FM radio transmitter, and ensure that the equipment is in normal working condition. .
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