A soft start circuit

Abstract

A soft start circuit, thermistor R T For positive temperature coefficient, can be old color TV degaussing resistance, and transformer B in series, switch K1 parallel thermistor R T Both ends, the device is working properly, K1 closed short circuit, transformer B normal work, when you need to shut down, first let switch K1 open, thermistor resistance sharply to maintain the current, this is really shut down, so that the transformer B core remanence pole low, effectively reduce the next time the surge current, even when the power supply is at the peak of the sine wave around.

Description

Technical Field

[0001] This invention relates to soft-start technology for transformers, particularly power-on circuit technology for mains transformers. Background Technology

[0002] Before the advent of switching power supplies, household appliances and industrial equipment used traditional linear transformers to step down or step up AC mains voltage and isolate it. When necessary, the voltage was then rectified and filtered before being supplied to subsequent circuits or equipment. In high-performance circuits, linear voltage regulators were also used to remove ripple from the rectified DC power. AC mains voltage, also known as alternating current, commonly has voltages of 220V or 110V. In practical use, AC mains voltage fluctuates; its frequency is 50Hz or 60Hz, and its waveform is sinusoidal or cosine. The following description of AC mains will follow the same pattern.

[0003] Traditional linear power supplies have low conversion efficiency and have been gradually replaced by high-efficiency switching power supplies. However, in certain fields, traditional linear power supplies still have significant applications due to the large electromagnetic radiation and conducted interference of switching power supplies, such as audio power amplifiers, welding machines, and low-voltage electroplating.

[0004] Despite the excellent electromagnetic compatibility of high-quality switching power supplies, audio power amplifiers still largely rely on traditional linear transformers for power supply. Even Class D audio power amplifiers (also known as digital amplifiers), which operate in a switching state, stubbornly use linear transformers and rectifier and filter circuits for their power supply.

[0005] The most common and traditional type of linear transformer is the laminated transformer, commonly known in the industry as the "square-core transformer," originating from a southern Chinese dialect. Its core cross-section and shape are typically square. Another type is the toroidal transformer, whose core cross-section is still square, but the overall core shape is circular. A circle has the shortest perimeter for a given area, resulting in the shortest magnetic field lines, reduced losses, low internal resistance, and high efficiency, offering better performance than square-core transformers; this type is commonly known as the "toroidal transformer." One type of toroidal transformer has a circular or near-circular core cross-section. Because the circumference of each turn of the enameled wire in its coil is also minimized, its performance is extremely excellent. This is the R-type transformer, actually a special form of toroidal transformer. The overall shape of the R-type transformer's core is no longer circular, but rather a running track-like U-shape, for ease of winding.

[0006] A significant problem with linear transformers is the large inrush current at the moment of energization, which can be heard as a mechanical vibration. This necessitates a larger capacity mains control switch, increasing the cost of the circuit system. Furthermore, the large inrush current may trigger overcurrent protection devices in the power supply circuit, such as air circuit breakers, leading to system malfunctions. Toroidal transformers, in particular, exhibit an even larger inrush current at the moment of energization for the same power and no-load current, a drawback that severely restricts their widespread adoption. Many specialized fields have even employed power sequencers to ensure that equipment is energized in a specified time sequence, preventing the main switch from tripping due to simultaneous energization of numerous devices.

[0007] In the utility model patent application No. 200920178160.X, entitled "Surge Absorber for Toroidal Transformer" (hereinafter referred to as Patent Document 1), a solution is presented where mains power supplies the toroidal transformer through an NTC thermistor. Its working principle is relatively typical. A negative temperature coefficient thermistor, also known as an NTC thermistor, is a resistor whose resistance decreases as temperature increases. It is widely used in various electronic circuits, commonly for limiting start-up current. When powered on, the NTC thermistor is in a zero-power resistance state, with a relatively large resistance value, limiting the start-up current and keeping it below the desired value. The maximum peak value of the start-up current is calculated as the peak value of the mains power divided by the zero-power resistance value of the NTC. As current flows through the NTC thermistor, its temperature rises, its resistance decreases, and the heating power P = I... 2 R also decreases accordingly, thus reducing insertion loss under normal operating conditions. Even so, since the temperature coefficient of an NTC thermistor ranges from -2% to -6.5% per degree, its insertion loss during normal operation is still significant. This utility model patent uses a relay to short-circuit the NTC thermistor at a certain time after power-on to reduce its insertion loss. In fact, this circuit has been widely used in the audio industry since the 1980s in audio power amplifiers, both in early analog amplifiers and later Class D digital power amplifiers. Using a relay to short-circuit the NTC thermistor at a certain time after power-on to reduce insertion loss is a well-known and publicly disclosed technique. The surge current during power-on can shorten the lifespan of the NTC thermistor. Therefore, later soft-start circuits used high-power cement resistors (a type of wire-wound resistor) instead of NTC thermistors to increase circuit reliability.

[0008] The circuit in Patent Document 1, due to an incorrect connection of diodes D1 to D4, cannot achieve the rectified output through capacitor C1 (which provides a current-limiting constant current output), and therefore the circuit fails to function. Here, we will explain using a subsequent working circuit feature from Patent Document 1: a relay is used to short-circuit the NTC thermistor at a certain time after power-on. This feature is consistent with known technology from the 1980s. Its shortcoming is that it still cannot solve the problem of a large inrush current in linear transformers when powered on, especially if the mains voltage is not zero-crossing. Under 220V / 50Hz mains voltage, a high-quality toroidal transformer with a no-load current of only 30mA and a power rating of 500W records a peak inrush current of 1.1A to 2.7A upon power-on. This is related to the position of the mains voltage at the moment of power-on; if the peak voltage is below 30V, the inrush current is small; if the peak voltage is above 165V, the inrush current is large.

[0009] Previous literature has largely used resistive devices connected in series in the linear transformer circuit to limit the maximum inrush current at startup, but it overlooks a fundamental fact: the primary winding of a linear transformer connected to mains power has a very large inductance. For example, the toroidal transformer with a no-load current of only 30mA and a power rating of 500W, measured with an LCR bridge, has a primary inductance between 1.8 and 2.2H, while its calculated inductance is around 24H. According to current theoretical knowledge, the current in an inductor cannot change abruptly. When a relatively high DC voltage (simulating the instantaneous value of mains power-on) is applied, the magnetizing current should theoretically start from zero. It is important to note that the large number of turns and the tight winding of the enameled wire in the primary winding of the linear transformer have created a large distributed capacitance. This primary distributed capacitance can also cause inrush current, but the inrush current caused by this distributed capacitance is relatively small and accounts for a small proportion, insufficient to form the "inrush current at the moment of power-on" mentioned earlier.

[0010] In summary, existing soft-start circuits and related technologies have the following shortcomings:

[0011] (1) Using resistive devices to forcibly limit the surge current during power-on results in insertion loss and unsatisfactory lifespan. Furthermore, when using NTC thermistors in series, the resistance of the NTC thermistors increases at low temperatures, causing the power-on failure. A solution for low-temperature operation is provided in the proceedings of the 21st Annual Academic Conference of the China Power Supply Society, entitled "Application of Flyback Switching Power Supplies at Low Temperatures," authored by Zheng Lingxiao et al. However, the method described in the paper still suffers from insertion loss.

[0012] (2) The linear transformer can still be heard vibrating when it is powered on;

[0013] (3) Existing technology does not reduce the surge current of the linear transformer itself when it is powered on. Summary of the Invention

[0014] Therefore, the technical problem to be solved by this invention is to reduce the inrush current of the linear transformer itself when it is powered on, thereby reducing the inrush current flowing through the NTC thermistor or other current-limiting resistors during startup. The technical solution of this invention to solve the above-mentioned technical problem is as follows:

[0015] A soft-start circuit includes at least a linear transformer, a positive temperature coefficient thermistor, and a first switch. The linear transformer and the thermistor are connected in series and electrically connected to an alternating current. The first switch is connected in parallel across the thermistor via an electrical connection. When the linear transformer is required to work normally, the first switch is in a closed state; when the linear transformer is not required to work normally, the first switch is in an open state.

[0016] Preferably, it also includes a second switch, which is connected in series with the linear transformer and the thermistor in a series circuit. When the linear transformer is not required to work normally, the first switch is in the open state, and after a delay, the second switch is opened again; when the linear transformer is required to work normally, the second switch is in the closed state.

[0017] Preferably, it also includes a third switch and a first resistor. The first resistor and the thermistor are thermally encapsulated together. The third switch is connected in series with the first resistor and then in parallel with the linear transformer. When the linear transformer is not required to work normally, the third switch is in the closed state. When the linear transformer is required to work normally, the third switch is in the open state.

[0018] Preferably, the above-mentioned scheme of adding a third switch and a first resistor also includes a second switch, which is connected in series with the linear transformer and the thermistor in a series circuit. When the linear transformer is not required to work normally, the first switch is in the open state, and after a delay, the second switch is opened again; when the linear transformer is required to work normally, the second switch is in the closed state.

[0019] The beneficial effects of the soft-start circuit of the present invention are that it truly reduces the surge current of the linear transformer itself when it is powered on, eliminates the vibration sound emitted by the linear transformer when it is powered on, and eliminates the need to use resistor-type devices to forcibly limit the surge current when it is powered on. Attached Figure Description

[0020] Figure 1 This is a circuit schematic diagram of the first embodiment of the present invention;

[0021] Figure 2 This is a circuit schematic diagram of the second embodiment of the present invention;

[0022] Figure 3 This is a circuit schematic diagram of the third embodiment of the present invention;

[0023] Figure 4This is a circuit schematic diagram of the fourth embodiment of the present invention;

[0024] Figure 5 This is a measured static hysteresis loop cluster diagram of the core of transformer B in the first embodiment;

[0025] Figure 6 This is a schematic diagram showing the decrease of AC current in transformer B over time. Detailed Implementation

[0026] Example 1

[0027] See Figure 1 This is a schematic diagram of the soft-start circuit according to the first embodiment of the present invention. The soft-start circuit of this embodiment includes at least a linear transformer B and a positive temperature coefficient thermistor R. T This is the degaussing resistor from an older color television set, model MZ72 M10R. The first switch, K1, is a common AC power switch, which can be replaced by relay contacts. The linear transformer B is an 800W toroidal transformer with a nominal no-load current of 50mA, which was measured to be 43.7mA at AC220V / 50Hz. This is related to the thermistor R... T After being connected in series, it is electrically connected to AC power, and the first switch K1 is electrically connected in parallel to the thermistor R. T At both ends, when the linear transformer B needs to work normally, the first switch K1 is in the closed state; when the linear transformer B does not need to work normally, the first switch K1 is in the open state.

[0028] The working principle of this invention will be explained in detail here. As mentioned in the background section: When the primary inductance of a linear transformer is connected to the mains power, according to current known theory, the current in the inductor cannot change abruptly. When a relatively high DC voltage (simulating the instantaneous value of the mains power at random power-on) is applied, the excitation current should theoretically start from zero. Furthermore, the surge current generated by the distributed capacitance of the primary side is relatively small and accounts for a small proportion, insufficient to form the "surge current at the moment of power-on" mentioned earlier. Under 220V / 50Hz mains power, the first embodiment uses a high-quality toroidal transformer with an open-circuit current of 43.7mA and a nominal power of 800W. When powered on, the instantaneous value of the AC power at the moment of power-on is monitored with an oscilloscope, and only the surge current data corresponding to the instantaneous value above 130V is adopted, recording a peak surge current of 2.5A to 4.1A.

[0029] The inventors of this invention discovered that the surge current at the moment of energization is caused by the residual magnetism of the transformer core. The verification method involves using two transformers of the same specification, with no-load currents of the same or very close values. Their primary windings are connected in parallel with the same pure AC power supply in phase, and their secondary windings are connected to the same purely resistive load, reaching 80% of the rated power. Then, the AC power is simultaneously and randomly turned off. The primary windings are then connected to DC power supplies of the same voltage, with the leads being red and black wires respectively. For the first transformer, the red wire is connected to the positive DC power supply, and the black wire to the negative; for the second transformer, the connection is reversed, with the red wire connected to the negative and the black wire to the positive. If the residual magnetism of both cores is zero, then the surge current of the two transformers should be the same when the DC power supply is energized. However, actual measurements show a significant difference in the surge current between these two connections. Many companies and researchers have discovered this using similar methods, but have not provided a solution.

[0030] This indicates that when the AC power supply to the transformer is randomly interrupted, the iron core retains residual magnetism. In practical use, it is extremely difficult for electrical equipment to shut down at the exact moment the AC current crosses zero. While Chinese scientists have indeed designed overvoltage zero-crossing starting circuits, they haven't considered zero-crossing shutdown circuits. Furthermore, because linear transformers are generally inductive during operation, their voltage and current phases are different; the AC current phase lags behind the voltage. This complicates the design of zero-crossing shutdown circuits. Therefore, when electrical equipment is shut down, whether at the zero-voltage or zero-current crossing, there is a very high probability that it will not shut down at the exact moment of zero-crossing. Because zero-crossing protection cannot be achieved... Figure 5 It shows Figure 1 The measured static hysteresis loop cluster of the core of the linear transformer B shows that curve 101 represents the largest hysteresis loop. Limit tests demonstrate this. Figure 1 The core permeability of the linear transformer B is very high, and its hysteresis loop is close to a rectangle. Curve 102 is the hysteresis loop of this 800W toroidal transformer at an output power of 1200W. It can be seen that when the power is exceeded, the transformer core exhibits shallow saturation. Curve 103 is the hysteresis loop at an output power of 800W, and curve 104 is the hysteresis loop at an output power of 300W. From curves 101, 102, 103, and 104, it can be seen that when the excitation current in the coil is zero, the residual magnetism of the core is very large, with the value of 101 being greater than that of 102. From 101 to 104, the value decreases from large to small. To achieve zero residual magnetism, a reverse excitation current must be applied. Since curves 101 and 102 are close to rectangles, the reverse excitation current is extremely difficult to control. Curves 103 and 104 drop very quickly, making it difficult to achieve precise control instantaneously, i.e., to cut off the current when the residual magnetism is zero. The residual magnetism in the iron core when the power is off is the root cause of the instantaneous surge current when the power is turned on again.

[0031] This invention achieves near-zero residual magnetism by having the first switch K1 in the open state when the linear transformer B is not required to operate normally. This means the device, using the first embodiment of this invention, is preparing to shut down. This device could be an audio power amplifier or a low-voltage stage lighting fixture. At the moment of shutdown or after the shutdown command is issued, i.e., "when the linear transformer B is not required to operate normally," switch K1 is in the open state. At this time, the transformer B passes through the thermistor R... T Powered by AC power at 220V and 50Hz, in this embodiment, the thermistor R... T The demagnetizing resistor, model MZ72 M10R, has a zero-power series resistance of 10Ω. Due to its small resistance, the circuit continues to function normally at this moment. Meanwhile, the thermistor R... T The transformer generates heat, and its impedance rises rapidly within seconds, while the alternating current flowing through transformer B decreases over time. Figure 6 As shown. After 180 seconds, the thermistor R... T The resistance of the thermistor rises to about 27.5KΩ, and the measured AC current flowing through transformer B is 7.09mA. At this time, the thermistor R... T The voltage across transformer B is 194.9V, and the voltage across transformer B is 31.4V. Note that the thermistor R... T If the terminal voltage of transformer B is 194.9V, and the terminal voltage of transformer B is 31.4V, the voltage becomes 226.3V, exceeding the mains voltage of 220V. This is because the terminal voltage of transformer B leads the phase of its current. Over time, the thermistor R... T The resistance will increase further, and the current I0 flowing through transformer B will decrease further. After 360 seconds, this current drops to 2.16mA. At this point, the thermistor R... T The voltage across transformer B is 212.4V, and the voltage across transformer B is 9.56V. In this state, in the first embodiment, linear transformer B is connected to the thermistor R. T The power supply circuit's static power consumption is less than 475mW, less than 0.5W, meeting energy-saving requirements and can be considered to be in a shutdown state. Actual measurements of the first embodiment showed that after 5 minutes, the power consumption was below 250mW, and after 15 minutes, the power consumption was below 220mW. When the thermistor R... T When employing insulation measures, the leads are lengthened to prevent heat loss from a large area of ​​copper foil on the circuit board; in the thermistor R T With its square casing and white heat-shrink tubing, the steady-state power consumption drops to 165mW and does not decrease further.

[0032] When linear transformer B needs to work normally, the first switch K1 is in the closed state. At this time, since linear transformer B has been in a state of extremely low current excitation before the first switch K1 is closed, its residual magnetism is extremely small. At the moment the first switch K1 is closed, multiple measurements were taken and the peak surge current corresponding to the peak value of 305V or more at 220V AC was recorded. The maximum peak value was only 0.37A, which is about 9% lower than the traditional peak surge current of 4.1A. This directly reduces the surge current of the linear transformer itself when it is powered on, which has a good effect. Moreover, linear transformer B is very quiet when powered on and does not make any vibration noise.

[0033] To better achieve the purpose of the invention, the thermistor R T The residual current should be less than half of the no-load current of linear transformer B. In actual tests, it is better to keep it below one-quarter.

[0034] Another beneficial effect is that if the first switch K1 is used as the power switch of the device, the anti-sparking capacitor originally connected in parallel with the power switch can be eliminated. During normal operation, the first switch K1 is in the closed state, there is no insertion loss, and the invention can be turned on normally at low temperatures.

[0035] The first embodiment of the present invention still has shortcomings. When the device wants to be turned off, that is, when the first switch K1 is in the open state, the linear transformer B passes through the thermistor R. T Power is supplied, and after a few minutes, the thermistor R is used. T The impedance rises rapidly due to heat, causing the AC current flowing through linear transformer B to drop to the milliampere level. The terminal voltage of linear transformer B drops to less than one-seventh of the original mains voltage, i.e., below 31.4V. However, the device still has static power consumption. In many fields and work environments, many people hope to truly cut off the power supply to achieve electrical safety and zero power consumption. The present invention will provide a solution in the second embodiment, as follows.

[0036] Example 2

[0037] See Figure 2 This is a schematic diagram of the soft-start circuit according to the second embodiment of the present invention. The soft-start circuit of this embodiment includes at least a linear transformer B and a positive temperature coefficient thermistor R. T The first switch is K1. R T This is a degaussing resistor, model MZ75 5R, found in older color televisions. The first switch, K1, is a common AC power switch and can be replaced by relay contacts. Linear transformer B is a 1200W toroidal transformer with a nominal no-load current of 80mA; its measured current at AC220V / 50Hz is 81.2mA. This is related to the thermistor R. TAfter being connected in series, it is then connected to a 220V 50Hz AC power supply via a second switch K2. The first switch K1 is connected in parallel to the thermistor R. T At both ends,

[0038] When linear transformer B is required to operate normally, the first switch K1 is in the closed state; when linear transformer B is not required to operate normally, the first switch K1 is in the open state. It also includes a second switch K2, which is connected to linear transformer B and the thermistor R. T In a series circuit, when the linear transformer B is not required to work normally, the first switch K1 is in the open state, and after a certain delay, the second switch K2 is opened. When the linear transformer is required to work normally, the second switch K2 is in the closed state, as mentioned above, at which time the first switch K1 should be in the closed state.

[0039] Under 220V / 50Hz AC mains power, this embodiment uses a high-quality toroidal transformer with an open-circuit current of 81.2mA and a nominal power of 1200W. When powered on, the instantaneous value of the AC current is monitored with an oscilloscope. Only the surge current data corresponding to the instantaneous value above 130V is adopted, and the peak surge current is recorded from 4.6A to 11.1A.

[0040] Similar to Example 1, the surge current at the moment of energization is caused by the residual magnetism of the transformer core. For example... Figure 5 As can be seen from the working principle described in the first embodiment, the residual magnetism of the iron core when the power is off is still the fundamental reason for the instantaneous surge current when the power is turned on again.

[0041] This invention achieves near-zero residual magnetism by having the first switch K1 open when the linear transformer B is not required to operate normally. This indicates that the device, as described in the second embodiment of this invention, is preparing to shut down. This device is a low-voltage follow spot for stage use. At the moment of shutdown or after the shutdown command is issued, i.e., "when the linear transformer B is not required to operate normally," switch K1 is open, while switch K2 remains closed. At this time, the transformer B, through the thermistor R... T Powered by AC current with an effective value of 220V and a frequency of 50Hz, in Example 2, the thermistor R T The demagnetizing resistor, model MZ75 5R, has a zero-power series resistance of 5Ω. Due to its small resistance, the circuit still functions relatively normally at this instant. The thermistor R... T The terminal voltage is 17.6V the instant switch K1 is opened, and at the same time the thermistor R... T The heating power is 61W, its impedance rises rapidly within seconds, while the AC current flowing through transformer B decreases over time, similarly... Figure 6 As shown. After 180 seconds, the thermistor R... TThe resistance of the thermistor R rises to approximately 17.1 kΩ, and the measured AC current flowing through transformer B is 11.81 mA. At this point, the thermistor R... T The voltage across transformer B is 201.9V, and the voltage across transformer B is 25.3V. Note that the thermistor R... T If the terminal voltage of transformer B is 201.9V, and the terminal voltage of transformer B is 25.3V, the voltage becomes 227.2V, exceeding the mains voltage of 220V. This is because the terminal voltage of transformer B leads the phase of its current. Over time, the thermistor R... T The resistance will increase further, and the current I0 flowing through transformer B will decrease further. After 360 seconds, this current drops to 3.86mA. At this point, the thermistor R... T The voltage across transformer B is 211.2V, and the voltage across transformer B is 10.56V. In this state, in the second embodiment, linear transformer B is connected to the thermistor R. T The power supply circuit's static power consumption is less than 855mW, less than 1W, which does not meet energy-saving requirements. Although it can be considered to be in a shutdown state, there is still a significant power consumption. At this point, opening the second switch K2 achieves zero power consumption. Since the circuit's operating current is very small (3.86mA) when the second switch K2 is open, the residual magnetism of the linear transformer B is close to zero. When power is restored, the surge current of the linear transformer B at the moment of power-on has dropped to a very low level, achieving the invention's objective. In actual testing of the second embodiment, with the second switch K2 not initially open, the power consumption is below 600mW after 5 minutes and below 500mW after 15 minutes. If the second switch K2 is opened at this point, zero power consumption can be achieved, and the residual magnetism of the linear transformer B is even closer to zero.

[0042] When linear transformer B needs to operate normally, the first switch K1 is closed and the second switch K2 is closed. At this time, since linear transformer B was in a state where the residual magnetism was close to zero before the first switch K1 and the second switch K2 were closed, its residual magnetism is extremely small. At the moment the first switch K1 and the second switch K2 were closed, multiple measurements were taken and the peak surge current corresponding to a peak value of over 305V at 220V AC was recorded. The maximum peak value was only 0.83A, which is lower than the traditional peak surge current of 11.1A, down to below 7.5%. The effect is very good. Moreover, linear transformer B is very quiet when powered on, without any vibration noise, thus achieving the purpose of the invention.

[0043] Another beneficial effect is that since the second switch K2 is the power switch of the device in Embodiment 2, and it is turned off in the current state, the spark suppression capacitor that was originally connected in parallel with the second switch K2 as the power switch can be eliminated, or a small-capacity spark suppression capacitor can be used. In addition, since the peak surge current is controlled when the device is turned on, the current margin of the second switch K2 does not need to be large, thus reducing the cost.

[0044] Example 3

[0045] The first embodiment of the present invention still has shortcomings. When the device wants to be turned off, that is, when the first switch K1 is in the open state, the linear transformer B passes through the thermistor R. T Power is supplied, and after a few minutes, the thermistor R is used. T The impedance rises rapidly due to heat, causing the AC current flowing through linear transformer B to drop to the milliampere level. The terminal voltage of linear transformer B drops to less than one-seventh of the original mains voltage, i.e., below 31.4V. This voltage is still relatively high. The present invention will provide a solution in the third embodiment, as follows.

[0046] like Figure 3 As shown, in Figure 1 Based on the corresponding embodiment one, it also includes a third switch K3, a first resistor R1, and a thermistor R T The components are thermally sealed together. The third switch K3 is connected in series with the first resistor R1, and then in parallel with the linear transformer B. When the linear transformer B is not required to operate normally, the third switch K3 is closed; when the linear transformer B is required to operate normally, the third switch K3 is open. The first resistor R1 can be a PTC thermistor; here, an MZ23 type PCT thermistor is selected, with a nominal resistance of 500Ω and a Curie point of 80℃. Thermal sealing refers to two devices being encapsulated together using encapsulation material or a heat sink. Heating in either device will cause the other device to heat up simultaneously. Current flowing through the thermistor causes a temperature rise, meaning the thermistor's temperature increases. When the Curie point temperature is exceeded, the thermistor's resistance increases or rises sharply, thus limiting the current increase. The decrease in current leads to a decrease in the thermistor's heating power, allowing the thermistor's temperature to remain essentially constant at the Curie point temperature.

[0047] The principle is explained as follows: when the device is to be turned off, i.e., the first switch K1 is in the open state, the linear transformer B passes through the thermistor R. T When power is supplied, the first resistor R1 also heats up rapidly to 80℃ due to the high terminal voltage. The first resistor R1 and the thermistor R T The components are thermally sealed together, utilizing a thermistor R. T The impedance is affected by the heat generated by the thermistor R1. T The impedance rises rapidly, causing the AC current flowing through linear transformer B to drop to the milliampere level. Because of the parallel connection of the first resistor R1, the voltage across linear transformer B decreases further. At this point, due to the significant drop in voltage across the first resistor R1, its own heat generation is very slight; its temperature mainly comes from the thermistor R. T Thermistor R T The Curie point is slightly lower than the Curie point of the first resistor R1, where the thermistor R...T The Curie point is 75°C. The resistance of the first resistor R1 is about 1kΩ at this temperature. It is connected in parallel with the linear transformer B through the closed third switch K3. After the first switch K1 is opened, the terminal voltage of the linear transformer B drops to 5.5V after 180 seconds. Compared with the first embodiment where the terminal voltage of the linear transformer B is 31.4V, the terminal voltage of the linear transformer B is further reduced, and its remanence is further reduced to a minimum.

[0048] The heating of the first resistor R1 causes the static power consumption of the device to decrease rapidly. At the same time, because the terminal voltage of the linear transformer B is further reduced, its residual magnetism is further reduced. When the device is turned on again, that is, the first switch K1 is in the closed state and the third switch K3 is in the open state, the peak surge current is extremely small. The linear transformer B is very quiet and does not make any vibration noise, thus achieving the purpose of the invention.

[0049] Example 4

[0050] The third embodiment of the present invention still has shortcomings. When the device wants to be turned off, that is, when the first switch K1 is in the open state, the linear transformer B passes through the thermistor R. T Power is supplied, and after a few minutes, the thermistor R is used. T The impedance rises rapidly due to heat, causing the AC current flowing through the linear transformer B to drop to the milliampere level. Due to the installation of the third switch K3 and the first resistor R1, the terminal voltage of the linear transformer B drops to less than one-thirtieth of the original mains voltage, i.e., below 7.33V. However, the device still has static power consumption. In many fields and work environments, many people hope to truly cut off the power supply to achieve electrical safety and zero power consumption. This invention will provide a solution in this embodiment, as follows.

[0051] Similar to the second embodiment, the third embodiment adds a second switch K2 to the power supply circuit. When the linear transformer B is not required to work normally, the first switch K1 is in the open state, and after a delay, the second switch K2 is opened again. When the linear transformer B is required to work normally, the second switch K2 is in the closed state.

[0052] That is, when the terminal voltage of the linear transformer B is further reduced to the desired value in the third implementation, its residual magnetism is further reduced to a minimum. When the peak surge current of the next start-up reaches the desired value, the second switch K2 is opened, and zero power consumption of the circuit can be achieved.

[0053] Another beneficial effect is that, since the second switch K2 is the power switch of the device in Embodiment 2, and it is turned off in the current state, the spark suppression capacitor that was originally connected in parallel with the second switch K2, which is the power switch, can be eliminated or a small-capacity spark suppression capacitor can be used. In addition, since the peak surge current is controlled when the device is turned on, the current margin of the second switch K2 does not need to be too large, thus reducing the cost.

[0054] This invention is also applicable to linear transformer manufacturers, allowing for proper demagnetization of transformers before shipment, i.e., bringing the transformer's magnetism close to zero.

[0055] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art and hobbyists, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention, such as the thermistor R. T The MZ92 type motor starting thermistor is selected, or a thermistor specially developed for this invention is used. For example, the first resistor R1 is not the same as the thermistor R. T The components are packaged together to achieve the same purpose; after the first resistor R1 and the third switch K3 are connected in parallel, they are no longer connected in parallel with the linear transformer B, but rather with the linear transformer B and the thermistor R. T The two-terminal circuits connected in series can also achieve the purpose of the invention by connecting them in parallel; for example, the first switch K1, the second switch K2, and the third switch K3 can be replaced by relay contacts controlled by a microcontroller or other timing circuits, which can also achieve the purpose of the invention.

Claims

1. A soft-start circuit, comprising at least a linear transformer, a positive temperature coefficient thermistor, and a first switch, wherein the linear transformer and the thermistor are connected in series and electrically connected to an alternating current, and the first switch is connected in parallel across the thermistor via an electrical connection; when the linear transformer is required to operate normally, the first switch is in a closed state; when the linear transformer is not required to operate normally, the first switch is in an open state.

2. The soft-start circuit according to claim 1, characterized in that: It also includes a second switch, which is connected in series with the linear transformer and the thermistor in a series circuit. When the linear transformer is not required to work normally, the first switch is in the open state, and after a delay, the second switch is opened again; when the linear transformer is required to work normally, the second switch is in the closed state.

3. The soft-start circuit according to claim 1, characterized in that: It also includes a third switch and a first resistor. The first resistor and the thermistor are thermally encapsulated together. The third switch is connected in series with the first resistor and then in parallel with the linear transformer. When the linear transformer is not required to work normally, the third switch is in a closed state. When the linear transformer is required to work normally, the third switch is in an open state.

4. The soft-start circuit according to claim 2, further characterized in that: It also includes a third switch and a first resistor. The first resistor and the thermistor are thermally encapsulated together. The third switch is connected in series with the first resistor and then in parallel with the linear transformer. When the linear transformer is not required to work normally, the third switch is in a closed state. When the linear transformer is required to work normally, the third switch is in an open state.

5. The soft-start circuit according to claims 3 and 4, characterized in that: The first resistor is a PCT thermistor, and the Curie point of the PCT thermistor is lower than the Curie point of the first resistor.

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