Analysis of the principle of power supply for street lamp monitoring terminal Xinjun Dalian Street Lamp Management Office (116021) Abstract The circuit principle of Dalian street lamp monitoring terminal equipment is taken as an example to analyze the circuit principle and provide reference for monitoring terminal maintenance personnel to master and repair switching power supply. Key words street lamp monitoring power supply analysis street lamp monitoring terminal equipment power supply, generally using switching power supply, which is a pulse width modulation type regulated power supply without power frequency transformer, directly using rectification, high frequency conversion and pulse width modulation technology, smashing The cumbersome 50Hz transformer has the advantages of light weight, high efficiency, stable output and reliable performance, so it is widely used in microcomputers and other small high-tech equipment. The switching power supply is actually a converter, which converts 220V AC mains into DC power, converts the DC power into a high-frequency rectangular or approximately sinusoidal voltage through a transforming oscillator, and then performs high-frequency rectification and filtering to form a load. A low voltage DC power supply is required. It modulates the pulse voltage width of the converter output through the control circuit to achieve the purpose of stabilizing the output voltage. In the switching power supply, the converter is the core of the whole power supply, and its oscillation frequency is generally 20 kHz. Because it is much higher than the 50 Hz frequency of the mains, the volume and weight of the high-frequency transformer and filter components used are greatly reduced. Moreover, the triode components used in the converter all operate in the switching state, thereby reducing power loss and improving power supply efficiency. In the switching power supply, the power conversion circuit is commonly used in two forms, one is a single-tube self-excited conversion circuit, and the other is a double-tube half-bridge conversion circuit. Although the principle is different, the reason is similar. Taking the switching power supply of the street lighting monitoring terminal equipment in Dalian as an example, the circuit principle is analyzed to provide reference for the monitoring terminal maintenance personnel to master and repair the switching power supply. The schematic diagram of the switching terminal power supply of the monitoring terminal device is shown in Figure 1. The working principle is as follows:
1. After the input circuit 220V AC mains input switching power supply, a low-pass filter composed of capacitor C and inductor L is connected through the fuse F1. In the figure, C1, L1 and C2 form a differential mode anti-interference circuit. L1, C3, C4 and L2, C2 and C5 form a 2-level common mode anti-interference circuit. Its function is to suppress high frequency interference from the power grid and suppress the switching power supply. Reverse high frequency interference generated to avoid polluting the power grid. Such a combination has a good effect on the suppression of various radio frequency interferences. R0 is a thermistor, which has small thermal resistance and large cold resistance. It limits the inrush current during startup and avoids surge damage during power-on damage to circuit components. QZ, C6 and C7 form a bridge full-wave rectification and filtering circuit, which converts the mains AC voltage into DC voltage and provides 300V DC working power for the converter. 2. The converter circuit consists of a Q1, Q2 high-power triode, a high-frequency transformer T1, an auxiliary transformer T2, and a surrounding resistor, capacitor, and diode to form a double-tube half-bridge converter circuit, which functions to convert 300V DC voltage into a high frequency. The alternating rectangular wave voltage (this transformation is also called inverter), through the high-frequency transformer output winding and rectification and filtering, provides a stable, high-quality DC power supply for the load. The converter circuit is the core of the switching power supply. Its working principle is as follows: The device is powered, and the input circuit outputs 300V DC voltage, which is applied to the filter capacitors C6 and C7. Due to the action of the Q1 and Q2 base bias circuits, Q1 is first turned on, and C6 forms a discharge loop through the CE poles, T2, T1, and C8 of Q1. At the same time, the T2 winding biases the base of Q1, which further increases the base current of Q1, and the CE current of Q1 also increases, and Q1 further accelerates conduction. Due to the positive feedback of the T2 winding, Q1 quickly enters saturation. After Q1 enters the saturation state, the CE pole current does not increase any more. At the same time, with the discharge process of C6, the CE pole current of Q1 gradually decreases. With the positive feedback of the T2 winding, the base current of Q1 is continuously reduced. Q1 quickly transitions from saturation to off state, which causes the T1 primary winding to store energy and couple it to the secondary output energy. During the C1 discharge through C1, C7 is charged by the power supply through Q1. When the C6 discharge process ends, the control circuit causes C7 to start discharging. C7 forms a discharge loop through C8, T1, T2, Q2 and CE poles, and Q2 starts to conduct. Similarly, through the positive feedback of the T2 winding, Q2 is turned on. Quickly enter saturation. With the discharge process of C7, Q2 enters the off state from the saturated state. This process stores the "opposite" energy in the T1 primary winding and is coupled to the secondary to get the output. While C7 is discharging through Q2, C6 is also charged by the power supply through Q2. Thus, after the end of the C7 discharge, the discharge of Q1 by C6 is continued, thereby forming a cyclic process of the Q1 and Q2 switching cycles. Since Q1 and Q2 are turned on and the currents passing through the T1 primary winding are opposite in direction, the energy coupled to the secondary is converted into alternating voltages of different directions. The alternating voltage passes through the rectifying and filtering output circuit to form the DC working power required for the load. In order to avoid the common on-time of Q1 and Q2, causing short-circuit damage to Q1 and Q2, it is necessary to ensure that the base drive of Q1 and Q2 has a common cut-off time, that is, control the "dead zone" time of the drive. It is implemented by the control circuit. See Figure 2 for the base drive waveforms of Q1 and Q2, the voltage waveform between the CE poles, and the waveform versus time on the T1 winding.
Figure 2 Waveforms of Q1, Q2 and T1 windings Figure 3. The control and protection circuit consists of TL-494 integrated voltage regulator control chip, drive transistors Q3 and Q4 and surrounding resistors, capacitors and diodes. Its function is to stabilize the voltage, control and protect the power supply as required. The TL-494 integrated voltage regulator control chip is the core of the control circuit.
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