High-efficiency linear direct-current power supply with transformer secondary tap capable of realizing zero-crossing automatic switching

Abstract

The invention discloses a high-efficiency linear direct-current power supply capable of realizing zero-crossing automatic switching of a secondary tap of a transformer, which is characterized in that a 220V power supply is subjected to voltage reduction through a transformer T, terminals 4, 3, 2 and 1 of the transformer respectively output different alternating-current voltages, the terminals 4 and 1 are connected with an input end of a bridge rectifier B, the terminals 3 and 2 are respectively connected with a working ground through reverse diodes D6 and D5, and the working ground is connected with an output end of the bridge rectifier B; a transistor T1, resistors R1, R2, R3 and R4, a potentiometer P1 and a voltage stabilizing diode D7 form a grid trigger circuit of a one-way silicon controlled rectifier Th1, when small and medium loads are loaded, reverse breakdown of the D7 is just achieved by adjusting the voltage of the P1 and the sliding end, T1 is conducted, the silicon controlled rectifier Th1 is cut off, and a terminal 4-2 or a terminal 1-3 of a transformer T supplies power to the loads; when the load is heavy, the Uo drops, the result is that the sliding end voltage of the P1 cannot enable the D7 to be reversely broken down, the D7 is cut off, the T1 is cut off, the Th1 is conducted, and the transformer terminal 4-1 or 1-4 provides a power supply for the load.

Description

Technical Field

[0001] The present invention relates to a technology of a high-efficiency linear DC power supply in which the secondary tap of a transformer can be automatically switched at zero crossing. Since the transformer tap is automatically switched and switched at the zero crossing point, current impact, noise and interference can be reduced. Of course, the power loss of the power supply circuit is also very small. Background Art

[0002] In power electronic equipment and systems, power supply is a very important component, of which linear power supply and switching power supply are two common types of power supply.

[0003] Although the linear power supply looks bulky, the power of the linear power supply is generally determined by the transformer and the adjustment tube. Although the power of the linear power supply is relatively low, it will not introduce additional interference, so the electromagnetic interference of the linear power supply is relatively small, and the ripple coefficient will also be very low, almost negligible.

[0004] The voltage regulation rate of a linear power supply is generally higher, and the internal design is relatively simple. If there is a problem and needs to be repaired, it is also very convenient. As long as the technicians who have a basic understanding of electronic technology generally know how to repair it, the maintenance cost of a linear power supply is generally much lower than that of a switching power supply.

[0005] Linear power supplies have better lightning resistance, because the transformer inside a linear power supply is usually composed of two coils and an iron core, and the voltage applied to both ends of the coil usually does not change suddenly, so it generally has a strong suppression against instantaneous high voltage. If accidentally struck by lightning, linear power supplies can generally survive, while switching power supplies will generally be burned.

[0006] The number of turns and power of the power frequency transformer coil are two parameters often mentioned in the circuit. The number of turns refers to the number of turns wound in the coil, usually represented by N. The power refers to the amount of energy transmitted and consumed in the circuit, usually represented by P. In the circuit, there is a close connection between the number of turns and power.

[0007] The effect of the number of coil turns on circuit power depends on multiple factors, such as voltage and current. In some cases, increasing the number of coil turns can increase the efficiency of the circuit and improve power output. This is because in this case, the inductance of the coil will increase, thereby reducing the flow of current and converting more power energy into magnetic field energy.

[0008] When designing a circuit, the number of coil turns required is determined based on the power required. If the power required is high, more coil turns are required to achieve the desired effect. On the other hand, if the power required is low, fewer coil turns will suffice.

[0009] In general linear power supply design, the number of turns of the primary and secondary coils of the industrial frequency transformer is usually fixed, including the design of the secondary multi-tap transformer. The number of turns of each tap is actually fixed, and the output voltage and output power of each tap are different. After the power supply design is completed, the maximum current (or maximum power) that the power supply (or a certain tap) can withstand is also fixed accordingly. Then, the only way to choose the load is to connect the corresponding tap for the most efficient application. At this time, the circuit connection is often fixed and will not change.

[0010] However, if an overweight load needs to use the power supply corresponding to this tap, the output voltage of the power supply will definitely drop a lot, the power loss of the power supply will increase sharply, and of course the output efficiency will also drop a lot. This is not a high-efficiency power supply.

[0011] Therefore, a linear DC power supply can be designed more efficiently. The power supply can automatically select the tap voltage of the secondary of the power transformer according to the load current. For example, a stable load is originally powered by the tap of an 8V secondary power supply with high efficiency. If a heavier load wants to temporarily use the power supply, the larger load current will cause the output voltage to drop, resulting in greater heat loss (copper loss and iron loss are the main causes of heating of motors and transformers).

[0012] At this time, a thyristor can be used to conduct at the zero-crossing moment to switch the 8V tap to a 10V tap. As mentioned above, increasing the number of coil turns can increase the efficiency of the circuit and improve power output. Therefore, the overweight load can work stably under the 10V tap and the efficiency is increased. If the overweight load returns to normal load, at the zero-crossing moment, the thyristor will be cut off and the original tap power supply will be restored.

[0013] The output voltage of about 9V of this power supply is most suitable as the input voltage of the commonly used 5V voltage regulator circuit. Obviously, due to the automatic switching of the transformer taps and switching at the zero point, the current shock, noise and interference can be reduced. Of course, the power loss of the power supply circuit is also very small. Summary of the invention

[0014] The technical problem to be solved by the present invention is to provide a design technology for a high-efficiency linear DC power supply which has a simpler structure, lower cost, more reliable use, and a transformer secondary tap that can automatically switch at zero crossing, which is different from conventional designs.

[0015] To achieve the above-mentioned purpose, the present invention provides a high-efficiency linear DC power supply with a transformer secondary tap that can automatically switch at zero crossing, which includes a 220V AC power supply, an industrial frequency transformer circuit, a small and medium power full-bridge rectifier circuit, a relatively high power full-bridge rectifier circuit, a unidirectional thyristor gate trigger circuit, a unidirectional thyristor circuit, a filter circuit, and a load circuit. The 220V AC power supply is stepped down by the industrial frequency transformer circuit T, and terminals 4, 3, 2, and 1 of the secondary of the industrial frequency transformer T respectively output different AC voltages. A rectifier stack B constitutes the relatively high power full-bridge rectifier circuit, and rectifier diodes D1, D2, D5, and D6 constitute the small and medium power full-bridge rectifier circuit. Terminal 4 of the secondary of transformer T is connected to one end of rectifier bridge stack B, terminal 3 of the secondary of transformer T is connected to a working ground through a reverse rectifier diode D6, terminal 2 of the secondary of transformer T is connected to a working ground through a reverse rectifier diode D5, and terminal 1 of the secondary of transformer T is connected to rectifier bridge stack B. The 3 ends of the rectifier bridge B are connected to the 4 ends of the rectifier bridge B through the resistor R3, the resistor body of the potentiometer P1, the resistor R4, and the AK pole of the unidirectional thyristor Th1 in sequence, and the 2 ends of the rectifier bridge B are connected to the 4 ends of the rectifier bridge B through the resistor R2 and the CE pole of the transistor T1 in sequence. The C pole of the body tube T1 is connected to the G pole of the unidirectional thyristor Th1, the A pole of the unidirectional thyristor Th1 is connected to the working ground, the sliding end of the potentiometer P1 is connected to the 4 ends of the rectifier bridge stack B through the reverse voltage-stabilizing diode D7 and the resistor R1, the positive pole of the voltage-stabilizing diode D7 is connected to the base of the transistor T1, and the 2 ends of the rectifier bridge stack B are connected to the working ground through the filtering circuit, that is, the forward electrolytic capacitor C1, and the positive end of the electrolytic capacitor C1 outputs a DC voltage Uo, and the DC voltage Uo is connected to the working ground through the load circuit RL.

[0016] The secondary of the industrial frequency transformer circuit, i.e., the transformer T, is tapped according to 0V-2V-8V-10V, corresponding to terminal 1, terminal 2, terminal 3, and terminal 4, respectively.

[0017] The relatively high-power full-bridge rectifier circuit, i.e., the rectifier bridge stack B, is of model B80C2200.

[0018] The models of the rectifier diodes D5 and D6 of the small and medium power full-bridge rectifier circuit are both IN5401. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Attached Figure 1 It is used to provide a further understanding of the present invention and constitutes a part of this application. Figure 1 It is a schematic diagram of a DC power supply in which the secondary tap of the transformer can be automatically switched when it passes through zero. DETAILED DESCRIPTION

[0020] The embodiments of the present invention are further described below with reference to the accompanying drawings.

[0021] Silicon controlled rectifier (SCR) is a high-power electrical component, also known as thyristor. It has the advantages of small size, high efficiency and long life. In the automatic control system, it can be used as a high-power driver to control high-power equipment with low-power controls. It has been widely used in AC and DC motor speed control systems, power control systems and follow-up systems.

[0022] There are two types of thyristors: unidirectional thyristors and bidirectional thyristors. Bidirectional thyristors are also called three-terminal bidirectional thyristors, or TRIACs for short. Bidirectional thyristors are structurally equivalent to two unidirectional thyristors connected in reverse. This type of thyristor has a bidirectional conduction function, and its on-off state is determined by the control electrode G. Adding a positive pulse (or negative pulse) to the control electrode G can make it conduct in the forward (or reverse) direction. The advantage of this device is that the control circuit is simple and there is no reverse withstand voltage problem, so it is particularly suitable for use as an AC contactless switch. Bidirectional thyristors are generally triggered by a photocoupler with a zero-crossing detection circuit.

[0023] A unidirectional thyristor can generally only conduct in one direction, and its working characteristics are as follows: when a positive voltage is applied between the anode A and the cathode K, and the required positive trigger voltage is applied between the control electrode G and the cathode K, the thyristor is triggered to conduct, and at this time, A and K are in a low-resistance conduction state. After the unidirectional thyristor is turned on, even if the controller G loses the trigger voltage, as long as the positive voltage is maintained between the anode A and the cathode K, the unidirectional thyristor continues to be in a low-resistance conduction state.

[0024] Only when the voltage on anode A is removed or the voltage polarity between anode A and cathode K changes (AC zero crossing), the unidirectional thyristor will be converted from a low-resistance on state to a high-resistance off state. Once the unidirectional thyristor is cut off, even if a positive voltage is reapplied between anode A and cathode K, a positive trigger voltage must be reapplied between control electrode G and cathode K before the unidirectional thyristor can be turned on again. The on and off states of the unidirectional thyristor are equivalent to the closed and open states of the switch, and it can be used to make a contactless switch.

[0025] Zero crossing control The zero-crossing control circuit is a circuit that controls the output of AC voltage. The output of the controlled voltage can protect motors, electrical appliances and other equipment from damage due to overcurrent. In an AC circuit, the waveforms of current and voltage are sinusoidal waves, and the cycle is a complete waveform. The zero-crossing control circuit controls the output timing of the circuit so that the circuit switches at the zero point of the AC voltage sine wave upward or downward, thereby controlling the output voltage of the circuit.

[0026] Zero-crossing control can protect electrical equipment. In an AC circuit, the magnitude of the current will change with the magnitude of the voltage. Therefore, electrical equipment may be damaged due to excessive voltage or excessive current. The zero-crossing control circuit can protect electrical equipment by controlling the output of the AC voltage to avoid instantaneous changes in current and voltage.

[0027] It can reduce the power loss of the power grid. There are certain resistance, inductance and capacitance in the AC circuit, which will cause a certain phase difference between voltage and current, resulting in power loss. The zero-crossing control circuit reduces the phase difference between voltage and current by controlling the output of the circuit, thereby reducing the power loss of the power grid and improving the utilization rate of power.

[0028] The output power of the electrical equipment can be controlled. The zero-crossing control circuit can adjust the voltage by controlling the output timing of the circuit, thereby controlling the output power of the electrical equipment.

[0029] In short, the zero-crossing control circuit is a common circuit. It can protect electrical equipment, reduce the power loss of the power grid, control the output power of electrical equipment, etc. With the continuous advancement of science and technology, the application scope of the zero-crossing control circuit is becoming more and more extensive.

[0030] If you want to achieve zero-crossing control, you need a zero-crossing detection circuit. The basic principle of the zero-crossing detection circuit is to use circuits such as comparators or triggers to compare the input AC signal with the zero level. When the level of the input signal exceeds the zero level, a high-level signal is output and the trigger is activated; otherwise, a low-level signal is output and the trigger loses its activation state.

[0031] In this way, as long as the signal crosses the zero point, the trigger will change state, thereby achieving the purpose of detecting the signal.

[0032] DC power supply with transformer secondary tap that can automatically switch over zero The core of this power supply circuit is to use a unidirectional silicon controlled rectifier SCR as a switching switch, and switch when the sinusoidal AC power source passes through the zero point (0V voltage). Although this design realizes zero-crossing control, it does not use a dedicated zero-crossing detection circuit, and has a simpler structure and higher cost performance.

[0033] Zero-crossing control is controlled by transistor T1. When T1 is cut off, the trigger current at the gate of the thyristor passes through zero and turns on. When T1 is turned on, the gate of the thyristor loses the trigger current, and the thyristor AK is automatically cut off when the power supply passes through zero. The detailed schematic diagram is as follows: Figure 1 shown.

[0034] It can be seen that the circuit includes a 220V AC power supply, an industrial frequency transformer circuit, a small and medium power full-bridge rectifier circuit, a relatively large power full-bridge rectifier circuit, a thyristor gate trigger circuit, a unidirectional thyristor circuit, a filter circuit, and a load circuit.

[0035] The secondary design of the power frequency transformer T has 4 taps, namely 10V tap (No. 4), 8V tap (No. 3), 2V tap (No. 2), and 0V tap (No. 1). The secondary complete coil (between No. 4 and 1) can output 10V AC voltage, between No. 4 and 2 can output 8V AC, and between No. 3 and 1 can output 8V AC; this article hopes to output 9V DC voltage, so the tap No. 1 to 4 coils can be used to output 10V AC power as the input voltage of the larger power full-bridge rectifier circuit, and the No. 4 to 2 and No. 3 to 1 coils can output 8V AC power as the input voltage of the small and medium power full-bridge rectifier circuit. Of course, both methods hope to output a stable 9V DC voltage.

[0036] When the load is small or medium, the thyristor Th1 is cut off. Diodes D1, D2, D5, and D6 form a small and medium power full-bridge rectifier circuit, the cathode of diode D5 is connected to tap 2 of the secondary of transformer T, and the cathode of diode D6 is connected to tap 3 of the secondary of transformer T; diodes D1, D2, D3, and D4 form a large power full-bridge rectifier circuit.

[0037] Small and medium load RL is the typical application of this design power supply (i.e. normal application, not heavy load application, Figure 1 As shown), when the power supply is required to provide small and medium load current, the thyristor Th1 is in the cut-off state (normal state), and the secondary rectifier ring of the power frequency transformer only passes through diodes D1, D2, D5 and D6. During the positive half cycle of the AC power, the load current channel is: transformer T secondary tap 4, diode D1, load, diode D5 (diode D6 reverse voltage is cut off, which is easy to understand. Since the positive poles of D5 and D6 are short-circuited, the potential of the transformer secondary terminal No. 3 is higher than the positive short-connection point potential of D5 and D6), transformer secondary tap 2; during the negative half cycle of the AC power, the load current channel is: transformer T secondary tap 1, diode D2, load, diode D6 (diode D5 reverse voltage is cut off. Similarly, since the positive poles of D5 and D6 are short-circuited, the potential of the transformer secondary terminal No. 2 is higher than the positive short-connection point potential of D5 and D6), transformer secondary tap 3.

[0038] In the above two cases, the AC voltage of the transformer secondary tap is 8V, and the 2V tap is not used. After the 8V AC voltage is filtered by capacitor C1, a 9V DC voltage is output.

[0039] Transistor T1, resistors R1, R2, R3, R4, potentiometer P1 and voltage stabilizing diode D7 constitute a zero-crossing detection circuit.

[0040] When the load is small or medium, the power supply normally outputs a 9V DC regulated power supply. By adjusting the potentiometer P1, the voltage at its sliding end is just enough to cause the Zener diode D7 to reversely break down, the transistor T1 is turned on, and the gate of the unidirectional thyristor Th1 is not triggered to cut off the current. The load current is the small or medium load current channel described above.

[0041] When the load is large, the thyristor Th1 turns on and the load current channel changes. If the power supply is required to carry a larger load, the power supply needs to provide a larger load current, and the output voltage Uo will drop (less than 9V). As a result of the drop, the sliding end voltage of the potentiometer P1 cannot cause the voltage-stabilizing diode D7 to break down in reverse, which is equivalent to D7 being reversely cut off, and the transistor T1 being cut off, eliminating the short-circuited state of the gate of the thyristor Th1. The output DC voltage Uo less than 9V provides a trigger current for the unidirectional thyristor Th1 through the resistor R2, and Th1 is immediately turned on.

[0042] As long as this happens, the 10V AC voltage between the secondary taps 4 and 1 of the transformer T will be rectified by the full bridge D1~D4, and the diodes D5 and D6 will be reverse biased and cut off (the reason is the same as above, the potentials of the transformer secondary terminals 3 and 2 are higher than the positive short-circuit potentials of D5 and D6). During the positive half cycle of the mains, the load current channel is: transformer T secondary tap 4, diode D1, load, diode D3, transformer secondary tap 1; during the negative half cycle of the mains, the load current channel is: transformer T secondary tap 1, diode D2, load, diode D4, transformer secondary tap 4.

[0043] Note that the grounding point of the load RL has not changed. Figure 1 The connection method shown is one of the beauties of this design.

[0044] Analysis of Zero-Crossing Turn-On and Zero-Crossing Cut-Off of Unidirectional SCR Zero-crossing control technology has many advantages. This design also uses zero-crossing control technology to make the thyristor Th1 turn on when the power supply crosses zero and turn off when it also crosses zero, but no dedicated zero-crossing detection circuit is used.

[0045] See also Figure 1 Since the voltage across the Zener diode D7 is always at the lowest value when the secondary of the power transformer is at the zero crossing point, the Zener diode D7 can be reversely cut off by adjusting the potential of the sliding end of the potentiometer P1. At the same time, the thyristor Th1 is always turned on at this zero crossing moment. This can prevent large current pulses and other interference pulses that occur when the thyristor is turned on. Therefore, this circuit does not need to further install an interference suppressor.

[0046] As for how the thyristor achieves zero-crossing cutoff, the principle is even simpler. If the load RL is changed to a typical small or medium load, the DC power supply output voltage Uo will inevitably rise by more than 9V, and the sliding end voltage of the potentiometer P1 will rise, causing the reverse breakdown of the voltage-regulating diode D7, turning on the transistor T1, and the gate trigger current of the unidirectional thyristor Th1 will disappear. Then, when the industrial frequency input voltage is at the lowest end (zero crossing point), the voltage between "A (anode) ~ K (cathode)" of the thyristor Th1 disappears, and the two conditions for the conduction of the unidirectional thyristor disappear, so Th1 is naturally cut off when it passes through zero.

[0047] Debugging and production When debugging the DC power supply, pay attention to the output voltage Uo value corresponding to the sliding end voltage of the potentiometer P1, which can cause the Zener diode D7 to exit the reverse breakdown and enter the reverse cut-off state. The basic idea is that when the load is heavy, the voltage value of the output voltage Uo just makes the Zener diode D7 exit the reverse breakdown state, and the one-way thyristor TH1 is turned on; then when the load is light, the Uo value increases, D7 reverse breaks down, and TH1 is cut off.

[0048] Also, pay attention to avoid too frequent switching of the secondary tap of the power supply, that is, when the load is slightly heavier, the secondary tap will immediately switch to number 4~1, causing the output voltage to suddenly rise to more than 9V, and then switch back to the original tap, forming oscillation. The reason is still the adjustment of the sliding end position of the potentiometer P1.

[0049] However, the probability of this problem is not high. After the one-way thyristor TH1 is turned on due to heavy load, the voltage at the sliding end of the potentiometer P1 will be further reduced, thereby further ensuring that the one-way thyristor TH1 is in the on state. Only when the load changes from heavy load to light load, the output voltage Uo increases significantly, causing the Zener diode D7 to break down in the reverse direction, causing the one-way thyristor TH1 to be turned off, and the circuit returns to the initial state. Conversely, only when the load is heavy, the output voltage Uo decreases significantly, causing the Zener diode D7 to be reversely turned off, causing the one-way thyristor TH1 to be turned on. A slightly heavier load will not cause the Zener diode D7 to be reversely turned off, but remain in the original state.

[0050] When actually making this DC power supply, the industrial frequency transformer T can use a 12V power transformer, and the secondary of the transformer can be tapped as follows: 0V-2V-8V-10V. When the load current is 1.5A, a 2A transformer is sufficient; if the load current is 2A, a 3A transformer is required.

[0051] In short, this high-efficiency linear DC power supply with automatic switching of the transformer secondary tap at zero crossing has the characteristic of automatically controlling the input voltage (secondary voltage) with the load current, and uses the zero crossing point as a safety point to implement the conduction and cutoff of the thyristor, but does not use a dedicated zero-crossing detection circuit, making the circuit safe, reliable, simple, and with fewer components. It is indeed economical, affordable, and has a high cost performance.

[0052] Another feature is that the switching between the light-load rectification circuit and the heavy-load rectification circuit does not need to be realized through a physical switch, but is automatically realized by utilizing the inherent reverse cutoff characteristics of the rectification diode, making the circuit simpler.

[0053] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A high-efficiency linear DC power supply with a transformer secondary tap that can automatically switch over zero, characterized in that: The linear DC power supply includes a 220V AC power supply, an industrial frequency transformer circuit, a small and medium power full-bridge rectifier circuit, a relatively high power full-bridge rectifier circuit, a unidirectional thyristor gate trigger circuit, a unidirectional thyristor circuit, a filter circuit, and a load circuit. The 220V AC power supply is stepped down by the industrial frequency transformer circuit T. Terminals 4, 3, 2, and 1 of the secondary of the industrial frequency transformer T respectively output different AC voltages. The rectifier stack B constitutes the relatively high power full-bridge rectifier circuit. Rectifier diodes D1, D2, D5, and D6 constitute the small and medium power full-bridge rectifier circuit. Terminal 4 of the secondary of the transformer T is connected to terminal 1 of the rectifier bridge stack B. Terminal 3 of the secondary of the transformer T is connected to the working ground through a reverse rectifier diode D6. Terminal 2 of the secondary of the transformer T is connected to the working ground through a reverse rectifier diode D5. Terminal 1 of the secondary of the transformer T is connected to terminal 3 of the rectifier bridge stack B. Transistor T1, resistor R1, resistor R2, Resistor R3, resistor R4, potentiometer P1, and voltage regulator diode D7 constitute the unidirectional thyristor gate trigger circuit, unidirectional thyristor Th1 constitutes the unidirectional thyristor circuit, 2 ends of rectifier bridge stack B are connected to 4 ends of rectifier bridge stack B through resistor R3, the resistor body of potentiometer P1, resistor R4, and AK pole of unidirectional thyristor Th1 in sequence, 2 ends of rectifier bridge stack B are connected to 4 ends of rectifier bridge stack B through resistor R2 and CE pole of transistor T1 in sequence, and C pole of transistor T1 The G pole of the unidirectional thyristor Th1 is connected, the A pole of the unidirectional thyristor Th1 is connected to the working ground, the sliding end of the potentiometer P1 is connected to the 4 ends of the rectifier bridge stack B through the reverse voltage-stabilizing diode D7 and the resistor R1, the positive pole of the voltage-stabilizing diode D7 is connected to the base of the transistor T1, and the 2 ends of the rectifier bridge stack B are connected to the working ground through the filtering circuit, that is, the forward electrolytic capacitor C1, and the positive end of the electrolytic capacitor C1 outputs a DC voltage Uo, and the DC voltage Uo is connected to the working ground through the load circuit RL.

2. A high-efficiency linear DC power supply with a transformer secondary tap capable of automatic zero-crossing switching according to claim 1, characterized in that: The secondary of the industrial frequency transformer circuit, i.e., the transformer T, is tapped according to 0V-2V-8V-10V, corresponding to terminal 1, terminal 2, terminal 3, and terminal 4, respectively.

3. A high-efficiency linear DC power supply with a transformer secondary tap capable of automatic zero-crossing switching according to claim 1, characterized in that: The relatively high-power full-bridge rectifier circuit, i.e., the rectifier bridge stack B, is of model B80C2200.

4. A high-efficiency linear DC power supply with a transformer secondary tap capable of automatic zero-crossing switching according to claim 1, characterized in that: The models of the rectifier diodes D5 and D6 of the small and medium power full-bridge rectifier circuit are both IN5401.

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