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　　What is the use of the rectifier circuit in the switching power supply? The working principle and significance of three-phase bridge rectifier circuit

　　What is the use of the rectifier circuit in the switching power supply? The working principle and significance of the three-phase bridge rectifier circuit - the rectifier circuit is the main part of the switching power supply. The rectifier circuit has the forms of single-phase half-wave, single-phase full-wave, single-phase bridge, voltage doubler rectification and multi-phase rectification. These rectifier circuits can be used in switching power supply circuits, but the operating frequency of the switching power supply rectifier circuit is much higher than the rectifier circuit of ordinary linear regulated power supply.

　　The working principle of the three-phase bridge fully controlled rectifier circuit:

　　1. In the three-phase bridge type fully controlled rectifier circuit, two thyristors must be turned on at any time, and one of the two thyristors is a common cathode group and the other is a common anode group. Only if they can be turned on at the same time can a thyristor be formed. Conductive loop.

　　2. The three-phase bridge fully controlled rectifier circuit is a series connection of two sets of three-phase half-wave rectifier circuits. Therefore, like the three-phase half-wave rectifier circuit, the requirement for the trigger pulse of the common cathode group is to ensure that the thyristors Kpl, Kp3 and Kp5 conduct in sequence. pass, so the phase difference between their trigger pulses should be 120. The requirement for the trigger pulse of the common anode group is to ensure that the thyristors Kp2, Kp4 and Kp6 are turned on in sequence, so the phase difference between their trigger pulses is also 120.

　　3. Since the common cathode thyristor is triggered in the positive half cycle and the common anode group is triggered in the negative half cycle, the phases of the trigger pulses of the two thyristors connected to the same phase should be 180 degrees apart.

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　　4. Three-phase bridge type fully controlled rectifier circuit every 60? There is a thyristor that needs to be commutated. From the commutation of the previous thyristor to the trigger of the next thyristor, the sequence of trigger pulses is: 1, 2, 3, 4, 5, 6, 1, and so on. The phase difference between two adjacent pulses is 60.

　　5. Since the thyristor can be turned on again after the current is interrupted, there must be a trigger pulse for a pair of thyristors in the two groups that should be turned on at the same time. In order to achieve this goal, two methods can be adopted; one is to make the width of each pulse greater than 60 (must be less than 120), generally 80 to 100, which is called wide pulse triggering. The other method is to trigger a certain thyristor number while simultaneously sending a pulse to the previous thyristor number, so that the two thyristors of the common cathode group and the common anode group that should be turned on have trigger pulses, which is equivalent to two narrow thyristors. Pulse equivalently replaces a wide pulse greater than 60. This method is called double pulse triggering.

　　6. The voltage of the rectified output is the voltage on the load. The rectified output voltage should be the waveform after subtracting the two-phase voltages. In fact, they all belong to the line voltage. The wave heads uab, uac, ubc, uba, uca, and ucb are all part of the line voltage and are the envelope of the above line voltage. . The intersection point of the phase voltage and the intersection point of the line voltage are at the same angular position, so the intersection point of the line voltage is also the natural commutation point. It can also be seen that the three-phase bridge fully controlled rectified voltage pulses six times in one cycle. The pulsation frequency is 650=300 Hz, which is twice as large as the three-phase half wave.

　　7. The voltage that the thyristor withstands. In the three-phase bridge rectifier circuit, only the components of the two arms are turned on at any moment, and the components of the remaining four arms are subject to changing reverse voltages. For example, during period (1), Kp1 and Kp6 are turned on. At this time, Kp3 and Kp4 bear the reverse line voltage uba=ub-ua. Kp2 withstands the reverse line voltage ubc=ub-uc. Kp5 withstands the reverse line voltage uca=uc-ua. The maximum reverse voltage experienced by the thyristor is the peak line voltage. When increasing from zero, it can also be analyzed that the maximum forward voltage that the thyristor withstands is also the peak line voltage.

　　Why do switching power supplies use rectifier circuits?

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　　The rectifier circuit is the main part of the switching power supply. The rectifier circuits include single-phase half-wave, single-phase full-wave, single-phase bridge, voltage doubler rectification and multi-phase rectification. These rectifier circuits can be used in switching power supply circuits. The operating frequency of the switching power supply rectifier circuit is much higher than that of the ordinary linear regulated power supply rectifier circuit.

　　1. Constant power rectifier

　　Among ordinary current-limiting rectifiers, there are constant-voltage rectifiers and constant-current rectifiers. In a constant voltage rectifier, its output voltage remains unchanged; in a constant current rectifier, its output current remains unchanged. If the load current exceeds the current limit value, the rectifier output voltage will drop rapidly as the current increases, or even the rectifier overshoots. flow and shut off.

　　In a constant current limiting rectifier, the three current values of rated current, limit and overcurrent value are quite close. The power rectifier can give rated power within the changing range of AC input voltage and DC output voltage. The difference between the constant power rectifier and the ordinary current-limiting rectifier is that it has three different output stages, that is, a constant power stage is inserted into the constant voltage stage and the constant current stage. The working conditions of the constant voltage stage and constant current are different from those of ordinary Current-limiting rectifiers are exactly the same. The constant power stage is not available in ordinary current-limiting rectifiers. With the constant power stage, the output power of the rectifier can be kept unchanged.

　　When the output current of an ordinary current-limiting rectifier exceeds the limit value, the output voltage will drop significantly, and the output power cannot be guaranteed to remain unchanged. However, in a constant power rectifier, when the output current exceeds the limit value, the output voltage will also decrease, but the rate of decrease is not as fast as that of the current-limiting rectifier. The output power can still be maintained and the normal operation of electronic equipment can be maintained. Therefore, in the design of a switching power supply using a constant power rectifier, only the maximum load of the electronic device and the redundancy of the rectifier are considered to determine the specific output power of the switching power supply, and subsequently determine the adjustment range of the output voltage and output current.

　　2. Current doubler rectifier

　　The current doubler rectifier consists of the secondary side of a high-frequency transformer, two inductors, two rectifier diodes and an output capacitor. The characteristic of the current doubler rectifier is that the secondary winding of the high-frequency transformer has no center tap. The two filter inductors are wound on the same magnetic core and have the same inductance. In this way, the current flowing through the secondary winding of the transformer and the two inductors is only half of the output load current, which greatly simplifies the structural design and size of the high-frequency transformer and the filter inductor. The output current of the current doubler rectifier is the output current of the two filter inductors. The sum of the currents of the two filter inductors cancels each other out, so the current doubler rectifier can obtain a DC output with very small pulse current.

　　3. Synchronous rectifier

　　High-speed data processing systems and notebook computers require low-voltage ultra-large-scale high-speed integrated circuit ICs, making the rectification loss of the power supply the main loss. For example, in the past, DC/DC converters used silicon Schottky diodes as the output rectifier diodes. When the DC converter is working normally, the forward voltage drop of the silicon Schottky diode is 0.4V~0.6V, and the output voltage of the DC/DC converter is about 5V; when the output current is large, the silicon Schottky The power consumption is very large, and the efficiency of the DC/DC converter is greatly reduced. Now the power supply voltage of high-speed data processing systems has dropped to about 3V, or even 1.5V~1.8V. Obviously, when silicon Schottky diodes are used as output rectifiers, the efficiency is even lower.

　　Research shows that approximately 22% of the power is dissipated in silicon Schottky diodes. In order to improve efficiency, MOSFET devices with low on-resistance are now used for rectification. Due to the small forward voltage drop of MOSFET, MOSFET has been successfully used in rectification circuits, which greatly improves the efficiency of the converter and does not exist due to Schott The dead zone voltage caused by the base barrier voltage. MOSFET is a voltage-controlled device, and its volt-ampere characteristics when turned on are linear. When using a power MOSFET as a rectifier, the gate voltage must be synchronized with the phase of the rectified voltage to complete the rectification function, so it is called synchronous rectification. The MOSFET switching device used for synchronous rectification is called synchronous rectifier SR. The advantage is that the on-resistance is small, which can be on the order of milliohms, the forward voltage drop is small, the efficiency of the power converter is high, and it also has the advantages of high blocking voltage and small reverse current, so it is suitable for high power and low output applications. It is widely used in voltage power converters.

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