Structural Parameters and Fault Analysis of DC Stabilized Power Supply
DC regulated power supply is an important part of electronic circuits. Whether it is in industrial electronic products or civilian electronic products, DC power supply is generally used to supply power to its control circuit. The performance of the regulated power supply is directly related to the performance of the entire electronic circuit. In simple terms, the DC stabilized power supply is to convert the 220 V alternating current in the grid whose direction changes at all times through the conversion circuit into direct current with constant direction.
The general DC regulated power supply is composed of four parts, namely the transformer part, the rectifier circuit part, the filter circuit part and the voltage stabilizing circuit part. Its function is to convert the input AC power into the DC power required by electronic equipment, and provide DC power for various electronic circuits.
1 AC Transformer Section
The function of the AC transformer is to change the larger AC voltage input by the grid into a smaller AC voltage, and the frequency of the AC voltage will not change. The AC transformer uses the law of electromagnetic induction. There is no electrical connection between the primary winding and the secondary winding, only magnetic coupling.
The selection of transformer parameters mainly considers the transformation ratio of the transformer and the efficiency of transformation: the transformation ratio of the transformer voltage is proportional to the number of turns of the primary and secondary windings; the efficiency of the transformer is the primary power / secondary power. The required output voltage is determined by the load voltage. Common faults of transformers include insulation reduction, coil short circuit, open circuit and so on. Insulation reduction is a condition that often occurs during the working process of transformers. Insulation reduction faults are actually the decrease in insulation resistance that causes the transformer current to increase, causing serious heat generation, and the increase in temperature causes further aging of the insulation layer, forming a vicious circle. A partial short circuit of the primary coil will reduce the output voltage, a partial short circuit of the secondary coil will increase the output voltage, and a serious short circuit will cause the transformer to heat up or even smoke and burn out. Short circuit faults can be measured with the voltage range of a multimeter. An open circuit in either the primary coil or the secondary coil will result in no output voltage in the circuit.
2 rectifier circuit part
The so-called rectifier circuit is a circuit that converts alternating voltage or current into pulsating DC voltage or current. The diode is used in the rectifier circuit. The diode has single-phase conductivity, that is, the diode is turned on when it is connected to the forward voltage (the anode is connected to the high potential, and the cathode is connected to the low potential), and the reverse voltage is connected (the anode is connected to the low potential, and the cathode is connected to the high potential). Potential) when the diode cuts off. Using this characteristic of the diode can form a half-wave rectification circuit or a full-wave rectification circuit. In a half-wave rectification circuit, the diode conducts only for half the cycle; in a full-wave rectification circuit, the diode conducts for the entire cycle. Compared with the half-wave rectifier circuit, the full-wave rectifier circuit has a smaller output voltage pulsation, a larger average value of the output voltage, and effective use of the energy of the power supply, so now the rectifier circuit is basically composed of two diodes. A full-wave rectifier circuit or a rectifier bridge composed of four diodes.
The selection of diode parameters in the rectifier circuit mainly considers the average working current of the diode and the highest reverse working voltage that the diode can bear. For the sake of safety, the selected parameters should be about twice as large as the calculated value. The common faults in the bridge rectifier circuit include diode virtual welding, reverse welding, short circuit and so on. When a diode in the bridge rectifier circuit is soldered or disconnected, the rectifier circuit becomes a half-wave rectifier circuit. When checking with an oscilloscope, you will see that the waveform of the output voltage only appears in half a cycle. If it is D1 or D3 virtual welding, the waveform only appears in the second half cycle of the power supply; if it is D2 or D4 virtual welding, the waveform only appears in the first half cycle of the power supply. If a diode is reversed, it will cause a short circuit fault. At this time, the current is very large, and both the diode and the transformer will be burned.
3 filter circuit part
Use the characteristic that the voltage across the capacitor of the energy storage element cannot be mutated to connect the capacitor in parallel with the load RL, or use the characteristic that the current passing through the inductance of the energy storage element cannot be mutated to connect the inductance and the load RL in series to form a filter circuit to filter out the upper rectifier The AC component of the output voltage and current in the circuit retains its DC component, making the waveform of the output voltage or current smoother and improving the pulsation of the output voltage. The capacitor filter circuit is the simplest. When the inductance of the inductance coil in the inductance filter circuit is large, an iron core is required, which is easy to cause electromagnetic interference.
The selection of capacitor parameters in the filter circuit mainly includes the discharge time constant RC and withstand voltage value. In order for the circuit to work reliably, RC is generally greater than or equal to 1.5 ~ 2.5T (T is the period of the transformer output voltage), and the withstand voltage value of the capacitor is generally Take 1.5 ~ 2U (U is the effective value of the output voltage of the transformer). Common faults in the filter circuit include breakdown, open circuit, and capacity reduction. You can use the ohm gear of the multimeter to judge the breakdown or open circuit fault of the capacitor. When the filter capacitor is broken down or short-circuited, it will cause the rectifier diode and the voltage transformer to burn out; when the filter capacitor is open-circuited or its capacity is reduced, the output voltage will drop a lot, which will cause the load input voltage to be too low to work normally.
