Introduction to commonly used capacitors

　　Introduction to commonly used capacitors

　　1. Ceramic Capacitors

　　Ceramic capacitors have strong insulation performance, high insulation resistance, can be used in high-voltage circuits, and have good heat resistance characteristics; ceramic materials have a wide range of temperature coefficients, and capacitors with different temperature coefficients can be produced to suit different applications. The loss tangent of ceramic capacitors has a very small relationship with frequency, and can be widely used in high-frequency circuits of switching power supplies. The capacitance of ceramic capacitors is relatively small, generally only a few picofarads to a few tenths of a microfarad. Ceramic capacitors are generally wafer-shaped, cylindrical or laminated. Low mechanical strength and easy breakage are the disadvantages of ceramic capacitors.

　　2. Film capacitors

　　Film capacitors are made of materials such as polystyrene, polytetrafluoroethylene, polypropylene or polycarbonate. Film capacitors are divided into polar organic film and non-polar organic film capacitors. Polar organic film capacitors have the advantages of large capacitance to volume ratio, high temperature resistance, and high voltage strength. Non-polar organic film capacitors have the advantages of small loss tangent, large insulation resistance, small dielectric absorption coefficient, and negative temperature coefficient. If a metal film is uniformly plated on one side of an organic film and then rolled to form a capacitor, it is called a metalized organic film capacitor. Film capacitors have cylindrical, flat, laminated block shapes and so on. In the product model of film capacitor, C means capacitor, and B means polystyrene. It is small in size, light in weight, and has a "self-healing" function.

　　Film capacitors include six types of film capacitors including polystyrene capacitors, polytetrafluoroethylene capacitors, polypropylene capacitors, polyester capacitors, and polycarbonate capacitors. Polystyrene film capacitors have the following characteristics:

　　a) Wide withstand voltage range, 30V~15KV. The working voltage of ordinary polystyrene capacitors is 100V, and the working voltage of high-voltage polystyrene capacitors can reach 10KV~15KV;

　　b) The insulation resistance is high, generally greater than or equal to 100000000000 ohms, so the leakage current is small. It can still maintain 95% of the charge after being charged for 1000 hours, while the low-quality capacitors are charged for 200 hours after being charged. Just put it all out

　　c) The tangent value of the loss angle is large, and metallized polystyrene capacitors should not be used in high-frequency circuits;

　　d) Capacitors have a wide capacity range, and can produce 100PF~100uF capacitors;

　　e) The accuracy of the capacitor is high, and it can produce 0.3%~0.1% high-precision capacitors;

　　f) The temperature coefficient is small, the performance is stable, acid and alkali resistance, moisture resistance, etc., pay attention to the marked withstand voltage and capacitance when using it.

　　3. Electrolytic capacitor

　　Electrolytic capacitors have adopted many new technologies in the production process. Expand the etching rate of electrode foil, improve the chemical stability and thermal stability of the electrolyte film, develop high-conductivity electrolyte with good heat resistance, and use high-airtight heat-resistant sealing linings, etc. At present, the factors that affect the basic performance of aluminum electrolytic capacitors, such as the operating temperature, high-frequency impedance, and service life, are electrolyte. The sealing gasket of the capacitor plays an important role in controlling the drying of the electrolyte, so the gasket material has a great influence on the life of the aluminum electrolytic capacitor. The following parameters should be considered when choosing aluminum electrolytic capacitors: rated capacity, nominal temperature (generally 105°C), withstand voltage and leakage current. All aluminum electrolytic capacitors that can withstand various tests are rarely damaged in practical applications.

　　In this circuit, the peak collector current is determined by the selected emitter resistance and the clamping Zener voltage, and the maximum current limit during turn-off is no longer dependent on the transistor gain.

　　The whole working process of the circuit of the figure is as follows. Close the switch, the current flowing through R makes Q1 turn on, the positive feedback generated by P2 will strengthen the turn-on process, and Q1 enters the saturation state of full conduction.

　　The drive current loop established from P2 through R3, Q1 base-emitter junction, R1, and back through D2 is like a forward diode. The larger drive current value makes the transistor fully saturated during the entire turn-on period.

　　When Q1 is turned on, the collector current will increase with time, and its size is determined by the primary inductance, the converted secondary inductance L and the load.

　　As the current increases, the voltage across R1 and the base voltage of Q1 also increase until the Zener diode turns on.

　　Choose the value of R1 reasonably so that the drive current is cut off when the collector current reaches the maximum control value. At this time, the magnetic core just enters the saturation point, that is, the point S1' in the figure. Since H cannot be further increased when the clamping current is reached, the voltage across all windings must drop to zero. In the end, only a very small flyback is required to turn off Q1 and turn on Q2 and Q2 to continue to complete the work cycle. The collector limit current no longer depends on the transistor gain.

　　When the converted load current is lower than the magnetizing current, the diodes D5 and D6 connected to the collector provide a path for the reverse current. These two diodes work in a cross-connected manner. For example, when Q1 is turned off, the magnetizing current will quickly make the collector of Q1 positive. At the same time, the collector of Q2 quickly becomes negative until D6 is turned on. The dislocation effect provided by D6 makes the collector voltage of Q1 twice of Vcc. In order to reduce the shadow of leakage inductance, P1 and P3 should be wound with double strands.

　　In order to reduce radio frequency interference and secondary side breakdown stress, a buffer circuit is required in addition to D5 and D6.

　　In order to ensure a good switching effect, sufficient regenerative feedback is required, and P2 in the circuit shown should provide a driving voltage of at least 4 volts.

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