Application of switching power supply in FM broadcast transmitter
With the continuous maturity of switching power supply technology, its application field has been further expanded. Compared with the traditional series continuous regulated power supply, the switching power supply has been greatly improved in efficiency, electromagnetic pollution, volume and reliability. On the other hand, the latest solid-state FM broadcast transmitters have higher and higher requirements for power supplies. With the maturity of switching power supply technology and continuous updating of components, the application of high reliability control chips can fully meet the requirements of FM broadcast transmitters. At present, 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 puts 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 supply is the driving heart of the entire FM radio transmitter. Considering 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, etc., the switching power supply is undoubtedly the best choice for the power supply of the solid-state FM broadcast transmitter. The excellent characteristics of the switching power supply are mainly reflected in the following aspects. First: the volume is smaller. It can be integrated with power amplifiers. The switching frequency of several hundred kHz minimizes the volume of the filter impedance component, thereby reducing the weight and volume of the transmitter, which is convenient for transportation and daily maintenance. Second: higher efficiency. Including the application of new devices such as power switch MOSFETs, the switching technology of multiple circuit topologies of switching power supplies is an important guarantee for reducing losses and improving the efficiency of power supply systems. Third: Fewer electromagnetic pollution. The electromagnetic interference (EMI) filter circuit and related high spike absorption circuit built in the transmitter power supply is an important guarantee 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 Pollution can also reduce harmonic interference to other network equipment. Fourth: Reliability has been further improved. Various protective measures for lightning protection, induction or counter-overvoltage, and the use of printed circuit boards coated with three anti-paints (moisture-proof, salt-proof and mildew-proof) can minimize the probability of failure. Switching power supply application The switching power supply is a form of power supply that continuously controls the power switching tube at a certain frequency for on-off operation so that energy can be provided 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 range of the switching power supply is from 20kHz to several MHz. For working situations where the power supply is greater than 90W, the switching power supply usually adopts a two-stage conversion method. That is, power factor correction (PFC) controls the converter and the DC / DC converter. Special mention should be made here of the power factor correction circuit. It is set to ensure that the input voltage and current work in phase. The result is that the power factor is close to 1, and the apparent power is all converted to active power, so the system efficiency is improved. If there is no PFC correction circuit, the input current will enter the switching power supply in the form of narrow pulse width and high peak pulse, causing serious harmonic interference components. These harmonic components not only do not provide any energy to the load, but also cause transformers and other equipment to generate heat. Power factor correction circuits are divided into active and passive types. The switching power supplies of FM broadcast transmitters mostly use active power factor correction circuit, which is composed of two parts: AC / DC converter with active power factor correction and independent DC / DC converter. AC / DC converter mainly includes: EMI filter, slow start circuit, bridge rectifier, PFC controller, power drive circuit and converter circuit (power switch MOSFET, energy storage inductor L, fast recovery rectifier diode and filter capacitor Etc. The AC input is filtered by the EMI filter circuit to remove the differential and common electromagnetic interference signals, and then input to the slow start circuit, and then the full voltage is added to the bridge rectifier circuit after a delay. 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 100 kHz, which is added to the gate of the MOSFET power switch tube through the drive circuit, and the MOSFET converter starts to perform on-off operation with a certain duty ratio and outputs the required DC voltage. Linear Technology's LT1249 integrated chip 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 squares the current from the error voltage amplifier to reduce the AC gain at light load, thereby maintaining low current distortion and high system stability. The PFC controller extracts the sensing signals from the bridge rectifier, the converter and the sensing resistor between them to achieve various protection functions, such as peak current limit and overvoltage protection. It is mainly composed of switching transformer, MOSFET power switch tube, rectifier element, sensing circuit (including voltage, current and temperature sampling), auxiliary power supply, UC3843PWM controller and related driving circuit. The DC voltage input by the previous stage is added to the drain of the parallel power switch MOSFET, and its gate input is provided by the drive circuit through the UC3843 control chip set frequency switching signal. After boosted by the switching transformer, rectified and filtered to obtain the required DC voltage. The 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 blocking, pulse suppression, high current drive and switching frequency up to 500kHz. From the internal circuit analysis of UC3843, the internal reference signal and the transformer secondary rectified and filtered voltage sample value are processed in the error amplifier. The processed error voltage and the voltage formed by the induced resistance are input to the PWM comparator, and its output is in accordance with the clock Waveform processing is performed in the trigger circuit, and finally a switching frequency signal with a frequency consistent with the clock frequency is output. Discussion on related issues in practical application Switching power supplies have a greater chance of failure during the use of FM broadcast transmitters for a variety of reasons. The environmental factors of the transmitter room (such as ventilation, temperature and humidity), the lightning protection of the power control cabinet, the design and components of the switching power supply, and the misoperation of the staff are all hidden dangers 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. Observation and analysis of the fault display of the auxiliary protection circuit provided in the switching power supply can often minimize the fault rate. The switching power supply uses large-capacity energy storage capacitors, which generates a large inrush current during operation, which makes the switching tube turn off when the AC voltage is close to the peak value. The instantaneous change of the input AC voltage itself will also lead to the same result. Therefore, in the actual circuit of the switching power supply, a thermistor with a negative temperature characteristic is often used in series in front of the bridge rectifier block. When the power switch is closed, the thermistor temperature is low, showing a high resistance state, and the inrush current is suppressed. As the temperature of the thermistor increases with current flow, 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 enough time to cool down. At this time, the AC voltage whose amplitude is close to the peak value will generate a larger inrush current than normal. The current generates a voltage higher than 6V on the sensing resistor. Since the LT1249 chip has not been powered up, it cannot play a protective role. This is the direct cause of the short-circuit damage of the power switch MOSFET breakdown. This was confirmed in the power failure of several FM broadcast transmitters caused by the severe storms and rain disasters in Dalian at the beginning of the year. Varistors connected in parallel across the input of the AC circuit can also absorb electrical surges. Under the condition that the ambient temperature is constant, 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 the power supply of the switching power amplifier, the varistor is connected between the power lines, so as to protect the power equipment. The ground wire is the most basic and simple safety measure. The transmitter cabinet, power amplifier box shell, power supply shell, panel and door are all connected to each other and to the ground terminal of the transmitter. After the transmitter is installed in place, the ground terminal of the machine (located on the (On the floor) The corners are reliably connected to the machine room ground to avoid unfortunate events due to electrical leakage. At the same time, 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 leaked by the transmitter can smoothly flow into the ground. Conclusion Although the switching power supply has a variety of circuit topological combinations, there are different choices due to different types of load, power requirements, control methods, etc., but the PFC control unit and PWM control unit in the switching power supply are the core, which is a high frequency for FM broadcast transmitters. An important guarantee for quality signal transmission and transmission. In addition, in the process of using the equipment, you should fully understand the working status and fault phenomena of the equipment, and constantly accumulate experience and lessons, which is conducive to mastering the fault characteristics of the switching power supply, improving the maintenance level of the FM broadcast transmitter, and ensuring that the equipment is in a normal working state. .
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