Analysis of the principle of a self-excited voltage-stabilized power supply

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Analysis of the principle of a self-excited voltage-stabilized power supply [Copy link]

1. Principles of four typical DC voltage stabilizing circuits 1. Voltage stabilizing diode voltage stabilizing circuit The voltage stabilizing diode, also known as the Zener diode, is a semiconductor device that has a very high resistance until the critical reverse breakdown voltage. At this critical breakdown point, the reverse resistance is reduced to a very small value. In this low resistance region, although the current flowing through the diode changes greatly, the voltage across its two ends changes very little, and this phenomenon has good repeatability, thus playing a role in voltage stabilization. Because of this characteristic, the voltage stabilizing tube is mainly used as a voltage stabilizer or voltage reference element. Among them, Us is the unregulated input DC voltage, U. is the stabilized DC voltage, Rs is the current limiting protection resistor of Dz, and also plays the role of voltage adjustment, D2 is the voltage regulator diode, and R is the load resistor. Its working principle is: this circuit mainly uses the voltage regulation characteristics of the voltage regulator diode, that is, the voltage drop across Dz remains basically unchanged after reverse conduction. When Us increases, the current on Rs increases, but U., that is, the voltage across D remains constant, so that the increase in Us is all dropped on Rs to keep U. unchanged, and vice versa. In practical applications, the characteristics of R and D2 play a key role in the entire voltage regulation process. The working range of this voltage stabilizing circuit is limited by the maximum power consumption of the voltage stabilizing tube, and Iz cannot exceed a certain value. The key is: under the conditions that Us, R and U. are all given, the selection of Rs value should ensure that when the input voltage is the maximum value Usmax, the stable current Iz and the power consumption allowed by the voltage stabilizing tube do not exceed the specified maximum value; when the input voltage is the minimum value, it can ensure that Iz is not lower than the minimum stable current. 2. Parallel transistor voltage stabilizing circuit Transistor is a solid semiconductor device that can be used for detection, rectification, amplification, switching, voltage stabilization, signal modulation and many other functions. As a variable switch, the transistor controls the outflowing current based on the input voltage, so the transistor can be used as a current switch. Where T is the adjustment tube, D2 is the reference voltage regulator tube, Rs is the current limiting resistor of Dz, and R. is the load. The output voltage of this voltage regulator circuit is approximately equal to the voltage regulation value of the voltage regulator tube Dz (in fact, the T emitter junction voltage must be added, generally 0.3V for germanium tubes and 0.7V for silicon tubes). This is because when the power supply is working, the T emitter junction is turned on, the emitter voltage is connected to the base voltage, and the base voltage is stabilized at a fixed value by Dz. This circuit can be regarded as T amplifying the voltage regulation effect of Dz by B times, which is equivalent to connecting a voltage regulator tube with a voltage regulation value of Dz and a voltage regulation effect of B times D2. The voltage stabilization performance of the parallel voltage stabilization circuit has been improved, and the circuit is not complicated. Its advantages are: it has overload self-protection performance, and the adjustment tube will not be damaged when the output is disconnected; when the load changes slightly, the voltage stabilization performance is relatively good; and it has good adaptability to instantaneous changes. However, the parallel voltage stabilization circuit also has relatively large disadvantages: low efficiency, especially when the load is light, almost all the electric energy is consumed in the current limiting resistor and the adjustment tube; the output voltage adjustment range is very small; the stability is not easy to be very high. These inherent disadvantages are difficult to improve, so the series voltage stabilization circuit is now widely used. It is a simple series transistor voltage stabilization circuit. The adjustment tube T is connected in series with the load resistor R. When the output voltage of the circuit fluctuates due to changes in power supply or power consumption, it can be adjusted in time to keep the output voltage basically stable, so it is called an adjustment tube. The voltage regulator tube Dz provides a reference voltage for the adjustment tube, so that the base potential of the adjustment tube remains unchanged. R. is the protection resistor of D2, which limits the current passing through D2 and plays a role in protecting the voltage regulator tube. Analysis of the principle of a self-excited voltage regulator (four typical DC voltage regulator circuits) The voltage regulation process of the circuit is as follows: If the input voltage Us increases, the output voltage U. increases. Since U.=U. is fixed, the base-emitter voltage Uo=UU: of the adjustment tube will decrease, and the base current I. will decrease accordingly, while the tube voltage drop U.As a result, U increases, thus offsetting the increase in Us, making U basically stable. If the load current I increases, causing the output voltage U to decrease, since U is fixed, U will increase, and U decreases, which also makes U basically stable. From the above analysis, we can see that the adjustment tube is like an automatic variable resistor: when the output voltage increases, its "resistance" increases, sharing the increased voltage; when the output voltage decreases, its "resistance" decreases, making up for the decreased voltage. In either case, the circuit maintains a stable output voltage. This voltage stabilizing circuit can also output a large current, and has a low output resistance and good voltage stabilizing performance; the circuit is also easy to make, but it also has disadvantages such as the output voltage cannot be adjusted. 4. Switching voltage stabilizing circuit The heat sink has greatly reduced volume and weight, and has the advantages of small size and high efficiency. This switching circuit has been widely used in various electronic devices. The switching voltage regulator control mode is divided into two types: width modulation and frequency modulation. In practical applications, the width modulation is used more. In the switching power supply integrated circuits currently developed and used, most of them are also pulse width modulation. Although the linear voltage-stabilized power supply based on the above-mentioned linear voltage-stabilized circuit has a simple circuit structure and reliable operation, it has the disadvantages of low efficiency (only 30%-50%), large size, large copper and iron consumption, high operating temperature and small adjustment range. In order to solve the disadvantage of high power consumption of linear voltage-stabilized power supply, a switching voltage-stabilized power supply has been developed. The conversion rate of the switching voltage regulator can reach more than 60%~85%, and the basic circuit block diagram of the switching voltage-stabilized power supply can be eliminated as shown in Figure 4. After the AC voltage is rectified and filtered by the rectification circuit and the filtering circuit, it becomes a DC voltage with a certain pulsating component. This voltage enters the high-frequency converter and is converted into a square wave of the required voltage value. Finally, this square wave voltage is rectified and filtered to become the required DC voltage. The control circuit is a pulse width modulator, which is mainly composed of samplers, comparators, oscillators, pulse width modulation and reference voltage circuits. This part of the circuit has been integrated and made into various integrated circuits for switching power supplies. The control circuit is used to adjust the switching time ratio of high-frequency switching elements to achieve the purpose of stabilizing the output voltage. Commonly used methods for realizing switch control include self-excited switching regulators, pulse width modulation switching regulators and DC conversion switching regulators. The switching type voltage regulator circuit is small in size and high in conversion efficiency, but the control circuit is relatively complex. With the rapid development of self-shutoff power electronic devices and power integrated circuits, switching power supplies have been increasingly widely used. 2. Analysis of the principle of a self-excited regulated power supply The self-excited DC regulated power source has the advantages of small size, light weight, high efficiency, strong adaptability to changes in grid voltage and frequency, long output voltage retention time, and is conducive to computer information protection. Therefore, it is widely used in various terminal equipment and communication equipment dominated by electronic computers. It is an analysis of the principle of a self-excited DC regulated power supply in today's electronic information industry. Working principle of switching power supply The input voltage is AC220v, 50HzThe AC power is filtered and then rectified by the rectifier bridge to become DC. The primary side of the high-frequency transformer generates a low-voltage high-frequency voltage by controlling the on and off of the switch tube in the circuit. It is coupled to the secondary side through a low-power high-frequency transformer, and then rectified and filtered to obtain a DC voltage output. In order to stabilize the output voltage, TL431 sampling is used, the error is amplified by the optical coupler, and the on and off time (i.e. duty cycle) of the switch tube is controlled by PWM to keep the output voltage stable. Analysis of the principle of a self-excited voltage-stabilized power supply (four typical DC voltage-stabilized circuits) 1. Design of switching power supply The circuit diagram of the switching power supply is shown in Figure 2. In this power conversion circuit, a single-ended flyback converter is used. The reason for the single-ended converter is that the magnetic core of its high-frequency transformer only works in the first quadrant. According to the different wiring methods of the secondary switch rectifier tube of the transformer, the single-ended converter can be divided into two types: forward and flyback. The switching states of the primary main power switch tube and the secondary rectifier tube are opposite (when the switch tube is turned on, the rectifier tube of the secondary is turned off), which is called single-ended flyback. When the primary side is added to the high-level excitation The pulse turns on Q1, and the DC input is at both ends of the primary side of the high-frequency transformer. At this time, because the secondary side is negative at the top and positive at the bottom, the rectifier is cut off; when the drive pulse is low, Q1 is cut off, the polarity of the two ends of the primary side is reversed, so that the two ends of the secondary winding become positive at the top and negative at the bottom, then the rectifier diode is forward-conducted, and then the magnetic energy of the secondary side of the transformer is released to the load. Therefore, the single-ended flyback converter only stores energy when the primary side Q1 is turned on, and releases it to the load when it is turned off. Therefore, the high-frequency transformer plays a role of voltage isolation and inductive energy storage element during the switching process. The electromagnetic interference filter connected to the input end of the AC power supply is composed of common mode choke L1, C2 and C3. The midpoint of C2 and C3 should be grounded to suppress common mode interference. C1 is used for filtering and filtering out series mode interference. It has a large capacitance. In view of the fact that the leakage inductance of the high-frequency transformer will generate a spike voltage at the moment of the switch tube BU508A being turned off, a clamping circuit is formed by C8, R3 and D1. The function of C9 is to filter out the spike voltage of the collector of the switch tube and determine the automatic restart frequency. C9 and R4 together compensate the control loop. At the same time, C9 and R4 also play the role of rapid reset of the primary side, which can effectively protect the switch tube from being damaged. Analysis of the principle of a self-excited voltage-stabilized power supply (four typical DC voltage-stabilized circuits) 2. The switch control part of the switching power supply The core of the switching power supply is the switch control part. The main working process is to control the on and off time (i.e. the size of the duty cycle) of the main power switch tube Q1 through the voltage level of points B and C in Figure 2. When Q1 is turned off, point A is high, C5 discharges Q1, and the potential of point B increases rapidly, so that the base potential of the switch tube Q1 is higher than the emitter, so Q1 is saturated and turned on, and C5 is charged. At this time, the current is the sum of the primary current of the transformer and the current when Q1 is turned on, so the current flowing through R5 is very large, the potential of point C increases, and the saturated conduction causes the potential of point A to drop, and Q1 is turned off. The function of D2 and D3 is to prevent the potential at point C from being too high when Q1 is turned on. Otherwise, the discharge time of C5 will be too long, making the turn-off time toff of Q1 too long, while the turn-on time ton of Q1 remains unchanged, so that the frequency becomes lower. If point C rises too high when Q1 is turned on, Q1 will be turned off. At this time, D2 and D3 are forward-conducted, the potential at point C is reduced, and the discharge time of C5 is very short, which can make Vb>Vc, and toff is also very small, so that the frequency can be very high. 3. PWM adjustment part When Q1 is turned on, the winding N2 is positive at the top and negative at the bottom, and C10 absorbs the peak voltage just after discharge to prevent the diode D10 from being damaged by forward conduction. D10 is forward-conducted, which increases the potential of point B, thereby making Q1 saturated and turned on faster. At the same time, Q2 is turned on, and then Q3 is also turned on, the voltage at point B drops, and the current of the primary coil is reduced to cutoff. At this time, the N2 side is positive at the bottom and negative at the top, D4 and D5 are turned on, the base of Q4 becomes high potential, Q4 is turned on, the potential at point C decreases, the cut-off time becomes shorter, and the TL431 feedback current reduces the current flowing into the base of Q4, the potential at point C decreases slowly, and the cut-off time becomes longer. When Q1 is turned on, the TL431 feedback current determines how fast the potential at point C rises to achieve the purpose of voltage regulation. C12 is used to protect Q3. When the reverse peak voltage is too high at the cut-off, Q3 is damaged. Feedback control is to compare the sampled voltage with the reference voltage, convert it into current, and then adjust ton and toff through current amplification to control the duty cycle to achieve the purpose of voltage regulation. R12 is the minimum load of the output voltage, which prevents the voltage from being too high when the load is unloaded, and is used to improve the voltage regulation rate when the load is light. C17 can appropriately reduce the high-frequency gain of the error amplifier. The reference voltage of TL431 is compared with the output voltage Vo, and an error voltage is formed at R14, so that the diode of IC1 produces different currents. R14 is the current limiting resistor of the IC1 diode. The frequency of error amplification should be determined by R13, R16, VR and C17. The RC absorption network composed of C14 and R10 can eliminate high-frequency self-excited oscillation and reduce radio frequency interference. 4. High-frequency converter part Since the power provided by the primary side of the high-frequency transformer per unit time is proportional to the square of ton and the frequency, proportional to the square of the input primary DC voltage, and inversely proportional to the number of turns of the primary winding, if the transformer consumption is not considered, the secondary power of the transformer can be obtained by the law of conservation of energy, that is, the output power is independent of the number of turns of the secondary side of the transformer and the load, and is only determined by the power provided by the primary side.Therefore, to obtain different output powers, we can only change the power of the primary side of the high-frequency transformer. Changing ton has the greatest impact on the output power, but it is not suitable for large changes due to the limitation of the magnetic flux reset conditions. To change the DC voltage of the input primary side, we can only change the parameters such as the filter inductance and filter capacitance of the previous circuit. We can also add a potentiometer in the front to change the DC voltage, and the frequency is limited by the conditions of the power switch tube itself. Therefore, changing the number of turns of the primary winding is a better method. The width of the primary coil winding should not be too long, but divided into multiple layers. Each layer is controlled by a switch. Different winding turns are connected to different switches to control the power on the primary side well, so as to obtain different output powers. However, the primary magnetic flux of the high-frequency transformer must be reset within the toff time, and the secondary magnetic flux must be reset within the ton time. If the magnetic flux does not return to the starting point of the cycle at the end of the switch working cycle, the magnetic flux in the transformer core will gradually increase, causing the core to saturate and damage the power switch tube. To meet the flux reset condition of the single-ended converter, the time between Ton and Toff must be appropriate and not too long, otherwise the frequency of the switch tube will be lowered. It is also related to the number of turns of the primary and secondary windings of the high-frequency transformer.