Switching power supply

Abstract

To provide a switching power supply capable of suppressing an inrush power supply when connected to an AC power supply, and detecting a zero-cross point of the AC power supply regardless of a load.SOLUTION: A switching power supply includes: a rectifying unit configured to rectify an AC power supply; a smoothing capacitor configured to smooth a rectified voltage; a switching unit provided in parallel with the smoothing capacitor; a voltage detection circuit provided between the AC power supply and the rectifying unit; a first switch element provided between the AC power supply and the voltage detection circuit; a second switch element provided between the rectifying unit and the smoothing capacitor; and a control unit. When the AC power supply, the smoothing capacitor, and the switching unit are connected, the second switch is made conductive at a zero-cross point of the AC power supply.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a switching power supply. [Background technology]

[0002] In a device having a switching power supply that uses an AC power supply as a power source, a switch element may be provided to control the connection between the AC power supply and the switching power supply. In addition, a switching power supply may generally have capacitance such as an across-the-line capacitor (hereinafter, X-capacitor) or a primary smoothing capacitor near the AC power supply input terminal. When such a switching power supply and an AC power supply are connected with a switch element, an inrush current flows through the capacitance according to the instantaneous value of the voltage of the AC power supply at the moment the switch element is turned on, and an element in the circuit may break down due to an overcurrent. If the switch element can be turned on near the zero-crossing point where the absolute value of the instantaneous value of the AC power supply becomes small, the inrush current can be suppressed, but when the switch element is a relay, it is difficult to precisely control the timing of contact conduction. For this reason, a method has been proposed in which a semiconductor switch element such as a thyristor is provided in addition to a relay in the path through which the inrush current flows (Patent Document 1). In the switching power supply described in Patent Document 1, the zero-crossing point is detected by detecting the voltage of the primary smoothing capacitor, and the semiconductor switch element is turned on at the zero-crossing point to suppress the inrush current. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2008-160996 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the switching power supply described in Patent Document 1, the voltage of the primary smoothing capacitor that is detected to detect the zero-cross point does not drop to a value corresponding to the instantaneous value of the AC power supply when the load of the switching power supply is small. Therefore, when the load of the switching power supply is small, the zero-cross point may not be detected. If the zero-cross point cannot be detected, problems such as the inability to achieve low-noise operation in the switching power supply may occur.

[0005] An object of the present invention is to provide a switching power supply that can suppress inrush power when connected to an AC power supply and can detect the zero-cross point of the AC power supply regardless of the load. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, a switching power supply of the present invention includes a rectifier unit to which a single-phase AC power supply consisting of a first phase and a second phase is connected, and which performs full-wave rectification of the AC power supply, a smoothing capacitor to smooth an output voltage of the rectifier unit, a switching unit provided in parallel with the smoothing capacitor, a first switch element provided in the first phase or the second phase, a second switch element provided between the rectifier unit and a high potential side of the smoothing capacitor, a first voltage detection circuit provided between the first switch element and the rectifier unit, a second voltage detection circuit provided in the first phase or the second phase that is opposite to the phase in which the first switch element is provided, and a control unit, wherein the control unit detects a zero crossing of the AC power supply based on an output of at least one of the first voltage detection circuit and the second voltage detection circuit, and the control unit causes the first switch element to be conductive and then causes the second switch element to be conductive in synchronization with the zero crossing.

[0007] Moreover, a switching power supply of the present invention includes a rectifier unit to which a single-phase AC power supply consisting of a first phase and a second phase is connected, and which full-wave rectifies the AC power supply, a smoothing capacitor to smooth an output voltage of the rectifier unit, a switching unit provided in parallel with the smoothing capacitor, a first switch element provided in the first phase or the second phase, a second switch element provided in the first phase or the second phase that is opposite to the phase in which the first switch element is provided, a voltage detection circuit provided between the AC power supply and the second switch element, and a control unit, wherein the control unit detects a zero crossing of the AC power supply based on an output of the voltage detection circuit, and the control unit causes the first switch element to be conductive and then causes the second switch element to be conductive in synchronization with the zero crossing. Effect of the Invention

[0008] According to the present invention, there is provided a switching power supply that can suppress inrush power when connected to an AC power supply and can detect the zero-cross point of the AC power supply regardless of the load. [Brief description of the drawings]

[0009] [Figure 1] Schematic diagram of a switching power supply according to a first embodiment [Diagram 2] Schematic diagram of the operation waveform of the switching power supply of the first embodiment [Diagram 3] Schematic diagram of a switching power supply according to a second embodiment of the present invention. [Figure 4] Schematic diagram of the operation waveform of the switching power supply of the second embodiment DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [Example 1] The switching power supply 100 of the first embodiment is an ACAC inverter that supplies AC power of any frequency to an insulated heating body, and is characterized in that it can suppress the inrush current that flows through the smoothing capacitor C1 when the AC power supply 10 is connected to the switching power supply 100. The circuit configuration of the switching power supply 100 will be described below, and then the operation will be described.

[0011] First, the circuit configuration of the switching power supply 100 will be described with reference to Fig. 1. The switching power supply 100 has a rectifier unit DA1, a smoothing capacitor C1, a switch element SW1 (first switch element) and a switch element SW2 (second switch element), and a switching unit 110. The switching power supply 100 further has a voltage detection circuit 120 (first voltage detection circuit) and a voltage detection circuit 130 (second voltage detection circuit), and a control unit 140. The switch element SW2 is provided between the rectifier unit DA1 and the high potential side of the smoothing capacitor C1.

[0012] The switching power supply 100 supplies power to the load 150 by the switching operation of the switching unit 110, thereby causing the heating element S1 on the secondary side insulated from the AC power supply 10 to heat up. The switching power supply 100 adjusts the power supplied to the load 150 so that the temperature of the heating element S1 is constant. Note that, although the switching power supply 100 is used as an ACAC inverter in this embodiment, it may be used as a switching power supply other than an ACAC inverter. For example, it may be used as an ACDC converter by replacing the heating element S1 with a diode and a capacitor and changing the controlled object to the output voltage.

[0013] The AC power supply 10 is a single-phase AC power supply, and supplies AC power to the switching power supply 10 through a first phase and a second phase. The voltage VAC is an instantaneous voltage value between the first phase and the second phase. The effective voltage value and frequency of the AC power supply 10 are, for example, 100 Vrms and 60 Hz, respectively.

[0014] The rectifier DA1 is a diode bridge composed of four diodes, and full-wave rectifies the AC power supply 10 and outputs the result to the smoothing capacitor C1 and the switching unit 110. Although the rectifier DA1 is a diode bridge in this embodiment, a circuit that performs a similar function, such as a power factor compensation circuit (PFC) using switching elements such as field effect transistors (hereinafter referred to as FET), may also be used.

[0015] The smoothing capacitor C1 is provided to smooth the output of the rectifier DA1 or to prevent switching noise generated in the switching unit 110 from propagating to the AC power supply 10. The voltage VC1 is the voltage of the smoothing capacitor C1, and the current Irush is the current flowing through the smoothing capacitor C1. Generally, a capacitor with a large capacitance and a low allowable ripple current, such as an aluminum electrolytic capacitor, is often used as the primary smoothing capacitor of a switching power supply, but in this embodiment, a film capacitor with a small capacitance and a high allowable ripple current is used. This is because the load of the switching unit 110 is large and it is necessary to reduce the board size of the switching power supply 100. In this embodiment, the heating element S1 to which the switching power supply 100 supplies power has a large rated load (e.g., 1000 W or more), so the ripple current flowing through the smoothing capacitor C1 during rated operation becomes very large. Therefore, if a capacitor with a low allowable ripple current, such as an aluminum electrolytic capacitor, is used as the smoothing capacitor C1, it is necessary to connect many capacitors in parallel to satisfy the ripple current rating, which increases the board size. For the above reasons, in this embodiment, a film capacitor is used for the smoothing capacitor C1. Since the smoothing capacitor C1 has a low capacitance, when the switching unit 110 is operating at rated voltage, the voltage of the smoothing capacitor C1 does not have a smoothed voltage waveform, but is approximately equal to the waveform obtained by full-wave rectifying the AC power supply 10. The voltage waveform of the smoothing capacitor C1 will be explained using Figure 3. Note that if excessive board dimensions do not pose a problem, an aluminum electrolytic capacitor or the like may be used instead of a film capacitor.

[0016] The switch element SW1 is a relay having a mechanical contact, and is a switch element provided between the AC power supply 10 and the switching power supply 100 in order to control the connection / disconnection between the AC power supply 10 and the switching power supply 100. The switch element SW1 is controlled to be on / off by a SW1 control signal output from a control unit (not shown) of a device in which the switching power supply 100 and the load 150 are mounted. When the SW1 control signal is at H level, the switch element SW1 is turned on, and when the SW1 control signal is at L level, the switch element SW1 is turned off. A circuit that controls the switch element SW1 turns on the switch element SW1 when power supply to the switching power supply 100 is required. The voltage detection circuits 120 and 130 in which the switch element SW1 is conductive can detect the instantaneous voltage value of the AC power supply 10, and therefore the control unit 140 can detect the zero cross point, which is the time when the instantaneous voltage value of the AC power supply 10 becomes 0V. The control unit 140 detects the zero cross of the AC power supply 10 based on the output of at least one of the voltage detection circuits 120 and 130. In this embodiment, the switch element SW1 is provided in the first phase of the AC power supply 10, but there is no problem if it is provided in the second phase of the AC power supply 10 as long as it is closer to the AC power supply 10 than the voltage detection circuit 130. Also, the SW1 control signal may be output by the control unit 140, not by the control unit of the device in which the switching power supply 100 is mounted.

[0017] The switch element SW2 is a three-terminal thyristor, and is a switch element for controlling the connection / disconnection between the rectifier unit DA1 and the smoothing capacitor C1 and the switching unit 110. The switch element SW2 is controlled by a control current Ig2 output from the control unit 140, and is turned on when it is necessary to connect the AC power source 10 to the smoothing capacitor C1 and the switching unit 110. The switch element SW2 is turned on when the switch element SW1 is on and near the zero cross point of the AC power source 10 in order to suppress the inrush current to the smoothing capacitor C1. The control method of the switch elements SW1 and SW2 and the relationship between the inrush current to the smoothing capacitor C1 will be described with reference to FIG. 2. In this embodiment, the switch element SW2 is a three-terminal thyristor, but as long as the time required from receiving a control signal to becoming conductive is sufficiently shorter than that of the switch element SW1, it does not need to be a three-terminal thyristor. For example, a bidirectional three-terminal thyristor (hereinafter referred to as a triac) or a Pch FET may be used. In this embodiment, a P-type three-terminal thyristor is used, but an N-type three-terminal thyristor may also be used.

[0018] The switching unit 110 is a full-bridge switching circuit composed of FETs Q111, Q112, Q113, and Q114, and a FET control unit 115, and supplies power of any frequency to the load 150. The FETs Q111 and Q112 are controlled to be turned on and off complementarily with a dead time therebetween, and to make the on-time and the off-time excluding the dead time of both FETs the same. Similarly, the FETs Q113 and Q114 are controlled to be turned on and off complementarily with a dead time therebetween, and to make the on-time and the off-time excluding the dead time of both FETs the same. Furthermore, the on-time and off-time of Q111, Q112, Q113, and Q114 have the same value, and the dead time is a common value.

[0019] The switching unit 110 controls the output power by phase shift control that shifts the switch on / off phases of FETs Q111 and Q112 and FETs Q113 and Q114. When the phase shift amount is 0 rad (radians), FETs Q111 and Q113 switch on / off simultaneously, and FETs Q112 and Q114 switch on / off simultaneously. When the phase shift amount is 0 rad, FETs Q111 and Q114 or FETs Q112 and Q113 do not switch on simultaneously, so no current flows through the load 150 and the output power of the switching unit 110 is 0 W. When the phase shift amount is increased from 0 rad, the period during which FETs Q111 and Q114 are on simultaneously and the period during which FETs Q112 and Q113 are on simultaneously increases, so the output power of the switching unit 110 also increases. The output power of the switching unit 110 is maximum when the phase shift amount is π rad.

[0020] In this embodiment, a full-bridge circuit is used for the switching unit 110, but other switching circuits may be used as long as they can control the frequency and output power independently. For example, an active clamp power supply such as the switching power supply 200 of the second embodiment may be used.

[0021] The voltage detection circuit 120 is a circuit for detecting the voltage of the AC power supply 10, and is composed of resistors R121 and R122. The voltage detection circuit 120 outputs a voltage Vdt120 obtained by subtracting the forward voltage or reverse voltage of the diode constituting the rectifier unit DA1 from the instantaneous voltage value VAC of the AC power supply 10 and dividing the voltage by the resistors R121 and R122. When the switch element SW1 is conductive, a forward voltage is applied to the diode constituting the rectifier unit DA1 during a period in which the first phase of the AC power supply 10 has a higher potential than the second phase. Therefore, during a period in which the first phase of the AC power supply 10 has a higher potential than the second phase, the voltage Vdt120 obtained by subtracting the forward voltage of the diode constituting the rectifier unit DA1 from the instantaneous voltage value of the AC power supply 10 is output to the control unit 140. On the other hand, during a period in which the first phase of the AC power supply 10 has a lower potential than the second phase, the entire instantaneous voltage value VAC of the AC power supply 10 is applied as a reverse voltage to the diode of the rectifier unit DA1, so that the voltage Vdt120 is 0 V. When the switch element SW1 is not conductive, the entire voltage of the AC power supply 10 is applied to the switch element SW1, so that the output voltage Vdt120 of the voltage detection circuit 120 is 0 V regardless of the phase of the AC power supply 10.

[0022] The voltage detection circuit 130 is a circuit for detecting the voltage of the AC power source 10, similar to the voltage detection circuit 120, and is composed of resistors R131 and R132. The voltage detection circuit 130 outputs a voltage Vdt130 obtained by subtracting the forward voltage or reverse voltage of the diode constituting the rectifier unit DA1 from the instantaneous voltage value VAC of the AC power source 10 and dividing the voltage by the resistors R131 and R132. When the switch element SW1 is conductive, a forward voltage is applied to the diode constituting the rectifier unit DA1 during a period in which the second phase of the AC power source 10 has a higher potential than the first phase. Therefore, during a period in which the second phase of the AC power source 10 has a higher potential than the first phase, the voltage Vdt130 obtained by subtracting the forward voltage of the diode constituting the rectifier unit DA1 from the instantaneous voltage value of the AC power source 10 is output to the control unit 140. On the other hand, during a period in which the second phase of the AC power supply 10 has a lower potential than the first phase, the entire instantaneous voltage value VAC of the AC power supply 10 is applied as a reverse voltage to the diode of the rectifier unit DA1, so that the voltage Vdt130 is 0 V. When the switch element SW1 is not conductive, the entire voltage of the AC power supply 10 is applied to the switch element SW1, so that the output voltage Vdt130 of the voltage detection circuit 130 is 0 V regardless of the phase of the AC power supply 10.

[0023] In this embodiment, a circuit for detecting the instantaneous voltage value VAC of the AC power supply 10 is used to detect the zero-crossing points of the AC power supply 10, but it is not necessary to use a circuit for detecting the voltage value as long as the zero-crossing points can be detected. For example, a circuit for outputting a digital signal whose logic changes according to the phase of the AC power supply 10 or a circuit for outputting a pulse signal at each zero-crossing point may be used.

[0024] The control unit 140 is a circuit that detects the output voltages of the voltage detection circuits 120 and 130, and outputs a control current Ig2 for the switch element SW2 using the voltage Vcc as a power source. The control unit 140 determines that the zero cross point of the AC power source 10 is reached when the sum of the output voltage Vdt120 of the voltage detection circuit 120 and the output voltage Vdt130 of the voltage detection circuit 130 is equal to or less than a threshold value. In this embodiment, the control unit 140 and the FET control unit 115 are separate circuits, but either the control unit 140 or the FET control unit 115 may control both the FETs Q111 to Q114 and the switch element SW2. In this embodiment, the control unit 140 sets the reference potential of the voltage detection circuits 120 and 130 to the same potential as the low potential side of the smoothing capacitor C1, but another point may be set as the reference potential. For example, the reference potential of the control unit 140 may be placed on the secondary side insulated from the AC power source 10, and the switch element SW2 may be controlled via an element such as a photocoupler.

[0025] The inductive load 150 has a primary inductance P1 electrically connected to the switching unit 110, and a secondary load (heating body) S1 magnetically connected to the primary inductance P1. The primary inductance P1 and the heating body S1 are magnetically coupled, and the heating body S1 is supplied with power from the primary inductance P1 by the switching operation of the switching unit 110. When power is supplied from the primary inductance P1, a current flows through the heating body S1, and the heating body S1 generates heat due to Joule heat caused by the electrical resistance of the heating body S1 itself. Although the load 150 divided into the primary side and the secondary side is used in this embodiment, depending on the safety design of the device using the switching power supply, a configuration equivalent to reinforced insulation or double insulation in the safety standard is not essential between the primary inductance P1 and the heating body S1.

[0026] Next, a method of controlling the switch elements SW1 and SW2 and an inrush current flowing through the smoothing capacitor C1 will be described with reference to FIG.

[0027] 2, with time taken on the horizontal axis, is a diagram showing transitions of voltage, current, etc. at various parts of the switching power supply 100 when the switch elements SW1 and SW2 are turned on. From the top, these are the instantaneous voltage value VAC of the AC power supply 10, the SW1 control signal, the resistance value of the switch element SW1, and the sum Vdt120+Vdt130 of the output voltages of the voltage detection circuits 120 and 130. In addition, the diagram shows the control current Ig2 of SW2, the voltage VC1 of the smoothing capacitor C1, and the current Irush flowing through the smoothing capacitor C1.

[0028] When the SW1 control signal is at an L level, the contacts of the switch element SW1, which is a relay, are not conductive, the resistance value of the switch element SW1 is a very high value Roff (e.g., 1000 MΩ or more), and the AC power supply 10 and the switching power supply 100 are in a non-connected state. In addition, the sum Vdt120+Vdt130 of the output voltages of the voltage detection circuits 120 and 130 is 0 V. In addition, the control current Ig2 of the switch element SW2 is also 0 A, so the switch element SW2 is not on either, the voltage VC1 of the smoothing capacitor C1 is 0 V, and the current Irush is 0 A.

[0029] When the SW1 control signal switches to the H level, the mechanical contacts of the switch element SW1, which is a relay, begin to make contact, but the resistance value of the switch element SW1 remains at Roff until it makes complete contact. For example, the time it takes for the contacts of the switch element SW1 to make contact after the SW1 control signal switches to the H level is 15 ms.

[0030] When the contacts of the switch element SW1 are completely in contact and the resistance value of the switch element SW1 switches from Roff to Ron (i.e., when the switch element SW1 is conductive), the voltage of the AC power supply 10 can be detected by the voltage detection circuit 120 or 130. In FIG. 2, the voltage instantaneous value VAC at the moment when the switch element SW1 is conductive is greater than 0V, so the output voltage Vdt120 of the voltage detection circuit 120 is a voltage value proportional to the voltage instantaneous value VAC of the AC power supply 10. On the other hand, the output voltage Vdt130 of the voltage detection circuit 130 becomes 0V. Since the control unit 140 can detect the voltage instantaneous value VAC of the AC power supply 10 by the voltage Vdt120+Vdt130, the zero-cross point of the AC power supply 10 can be detected by the switch element SW1 being conductive. In FIG. 2, the forward voltage of the diode constituting the rectifier unit DA1 is ignored because it is sufficiently smaller than the maximum value of the voltage VAC.

[0031] When the switch element SW1 becomes conductive and the voltage detection circuits 120 and 130 become able to detect the voltage VAC of the AC power supply 10, the control unit 140 starts to flow the control current Ig2 in synchronization with the zero crossing point of the AC power supply 10. Unlike the switch element SW1 which is a relay, the switch element SW2 which is a three-terminal thyristor becomes conductive in a very short time (for example, several μs to several tens of μs) after the control current Ig2 starts to flow. Therefore, the AC power supply 10 and the smoothing capacitor C1 are connected almost simultaneously with the next zero crossing point when the switch element SW1 is turned on. Since the voltage applied to the smoothing capacitor C1 at the time when the smoothing capacitor C1 is connected to the AC power supply 10 is almost 0V, the current Irshu which flows when the smoothing capacitor C1 is connected to the AC power supply 10 becomes an extremely small value of several A or less.

[0032] In this embodiment, the control unit 140 continues to flow the control current Ig2 after it starts to flow, but this is for the sake of simplifying control, and as long as the switch element SW2 is continuously on, the transition of the control current Ig2 does not need to be step-like as in Fig. 2. For example, if the switch element SW2 is a three-terminal thyristor or triac, the control current Ig2 may be controlled to flow in a pulsed manner at each zero crossing point.

[0033] Next, the reason for controlling the switch elements SW1 and SW2 as shown in FIG. 2 and the reason for providing the voltage detection circuits 120 and 130 on the AC power supply 10 side relative to the rectifier unit DA1 will be described.

[0034] In general, the inrush current that flows when a capacitor is connected to a power source increases in proportion to the voltage of the power source and the capacitance of the capacitor. In a switching power supply, an inrush current flows through the primary smoothing capacitor, the magnitude of which corresponds to the instantaneous value of the AC power supply at the moment the switching power supply is connected to the AC power supply and the capacitance of the primary smoothing capacitor. In the case of a general commercial AC power supply voltage (for example, 0V to 141V) and a primary smoothing capacitor capacitance (several tens of μF to several thousand μF), the inrush current can be as large as 100A or more, so measures to prevent element failure are essential. As an example of a measure, there is a method of reducing the inrush current by installing a filter element such as a power thermistor or a harmonic choke coil in the path through which the inrush current flows, but filter elements that can suppress inrush currents exceeding 100A are often large.

[0035] Therefore, in this embodiment, the timing at which the switching power supply 100 is connected to the AC power supply 10 is controlled to suppress the absolute value of the instantaneous voltage value of the AC power supply at the moment the primary smoothing capacitor is connected to the AC power supply, thereby suppressing the inrush current. By controlling the switch elements SW1 and SW2 as in this embodiment, the absolute value of the instantaneous voltage value VAC of the AC power supply 10 when the switching power supply 100 is connected to the AC power supply 10 becomes a very small value, and the inrush current to the smoothing capacitor C1 is suppressed to several amperes or less.

[0036] If the purpose is to suppress the inrush current as described above, the switch element SW2 can be turned on in synchronization with the zero crossing point, and there is no need to provide the switch element SW1. However, in this embodiment, the switch elements SW1 and SW2 are used together from the viewpoint of safety. When the connection / disconnection between the AC power supply 10 and the switching power supply is controlled only by the switch element SW1, it is difficult to suppress the inrush current by the switch element SW1 alone, because the relay cannot precisely control the time it takes to become conductive due to the influence of individual variations in the elements. On the other hand, it is necessary to use a semiconductor switch element for the switch element SW2, which takes a short time to become conductive. Therefore, the switching power supply 100 uses the switch element SW1, which is a relay having mechanical contacts, and the switch element SW2, which is a semiconductor, together, and uses each of them as a protection means, thereby realizing both suppression of the inrush current and a protection circuit with higher safety.

[0037] The voltage detection circuits 120 and 130 are provided on the AC power supply 10 side of the rectifier unit DA1 in order to detect the instantaneous voltage value VAC of the AC power supply 10 even when the switch element SW2 is in a conductive state. Even if the voltage detection circuits 120 and 130 are connected to the output terminal side of the rectifier unit DA1 rather than the input terminal side, they can detect the instantaneous voltage value VAC of the AC power supply 10 as long as the switch element SW2 is in a non-conductive state.

[0038] However, when the voltage detection circuits 120 and 130 are provided between the rectifier unit DA1 and the switch element SW2, the control unit 140 detects the voltage of the smoothing capacitor C1, not the voltage of the AC power source 10, when the switch element SW2 is conductive. The smoothing capacitor C1 is charged by the AC power source 10 and discharged by the switching unit 110. Therefore, when the power stored in the smoothing capacitor C1 is larger than the output power of the switching unit 110, the voltage of the smoothing capacitor C1 does not drop completely, and a period occurs during which the instantaneous voltage value VAC of the AC power source 10 cannot be detected around the zero cross point. In other words, when the output power of the switching unit 110 is small while the switch element SW2 is conductive, the switching power supply 100 cannot detect the zero cross point of the AC power source 10. If the zero cross point cannot be detected when controlling the switching unit 110, the absolute value of the instantaneous voltage value of the AC power source 10 at the timing when the switching operation of the FETs Q111 to Q114 starts may become large. Since the load 150 of the switching unit 110 is inductive, when a current starts to flow to the load 150 when the absolute value of the instantaneous voltage of the AC power supply 10 is large, a large noise is generated from the switching unit 110 and the load 150 and propagates as noise to the terminals of the AC power supply 10. Therefore, when switching cannot be started from the zero crossing point, that is, when the output power of the switching power supply 100 is small, the noise generated by the switching power supply 100 may become large.

[0039] Therefore, in this embodiment, in order to be able to detect the zero crossing point even if the output power of the switching unit 110 is small, the voltage detection circuits 120 and 130 are provided on the AC power supply 10 side rather than the rectification unit DA1.

[0040] Therefore, according to this embodiment, it is possible to provide a switching power supply that can suppress inrush power when connected to an AC power supply and detect the zero-cross point of the AC power supply regardless of the load.

[0041] [Example 2] The switching power supply 200 of the second embodiment differs from the switching power supply 100 of the first embodiment in that it uses a switch element SW3 instead of the switch element SW2, that there is one voltage detection circuit, that the configuration of the switching unit is different, and that it has a capacitor CX. The circuit configuration of the switching power supply 200 and the control method of the switch elements SW1 and SW3 are described below. Note that the same reference numerals as those in the switching power supply 100 of the first embodiment are used to designate the same parts, and description thereof will be omitted.

[0042] First, the circuit configuration of the switching power supply 200 will be described with reference to Fig. 3. The capacitor CX is an X capacitor provided between the input terminals of the rectifier unit DA1 in order to prevent switching noise generated from the switching unit 210 from propagating to the AC power supply 10. The voltage applied to the capacitor CX is VCX, and the current flowing through the capacitor CX is ICX. The relationship between the control of the switch elements SW1 and SW2 and the voltage VCX and current ICX will be described with reference to Fig. 4.

[0043] The switch element SW3 is a triac, and is a switch element for controlling the connection / disconnection between the AC power source 10 and the capacitor CX and the rectifier unit DA1. The switch element SW3 is controlled by a SW3 control signal sent from the control unit 240 via an SW3 drive circuit 250, and is turned on when it becomes necessary to connect the AC power source 10 to the smoothing capacitor C1 and the switching unit 210. The switch element SW3 is turned on only when the switch element SW1 is on and near the zero cross point of the AC power source 10 in order to suppress an inrush current to the capacitor CX and the smoothing capacitor C1. The relationship between the control method of the switch elements SW1 and SW3 and the inrush current to the smoothing capacitor C1 and the capacitor CX will be described with reference to FIG. 4.

[0044] Switching unit 210 is an active clamp type switching circuit composed of FETs Q211 and Q212, capacitors C211 and C212, and FET control unit 215, and supplies power of any frequency to load 150. FETs Q211 and Q212 are turned on and off complementarily with dead time in between, and at any frequency. FET control unit 215 adjusts the power supplied by switching unit 210 to load 150 by changing the on-time ratio of FET Q211, and the output power of switching unit 210 increases as the on-time ratio of FET Q211 increases.

[0045] In this embodiment, an active clamp circuit is used for the switching unit 210, but other switching circuits may be used as long as they can control the frequency and output power independently. For example, a full-bridge power supply like the switching power supply 100 of the first embodiment may be used.

[0046] The voltage detection circuit 220 is a circuit for detecting the voltage of the AC power supply 10, and is composed of resistors R221 and R222. The voltage detection circuit 220 outputs a voltage Vdt220 obtained by subtracting the forward voltage or reverse voltage of the diode constituting the rectifier unit DA1 from the instantaneous voltage value VAC of the AC power supply 10 and dividing the voltage by the resistors R221 and R222. When the switch element SW1 is conductive, a forward voltage is applied to the diode constituting the rectifier unit DA1 during a period in which the second phase of the AC power supply 10 has a higher potential than the first phase.

[0047] Therefore, during a period in which the second phase of the AC power supply 10 is at a higher potential than the first phase, a voltage obtained by subtracting the forward voltage of the diode constituting the rectifier unit DA1 from the instantaneous voltage value of the AC power supply 10 is divided by resistors R221 and R222, and the voltage Vdt220 is output to the control unit 240. On the other hand, during a period in which the second phase of the AC power supply 10 is at a lower potential than the first phase, the instantaneous voltage value VAC of the AC power supply 10 is applied to the diode of the rectifier unit DA1 as a reverse voltage, so that the voltage Vdt220 is 0 V. When the switch element SW1 is not conductive, the voltage of the AC power supply 10 is applied to the switch element SW1, so that the output voltage Vdt220 of the voltage detection circuit 220 is 0 V regardless of the phase of the AC power supply 10.

[0048] Like the voltage detection circuits 120 and 130 of the first embodiment, the voltage detection circuit 220 may be a circuit that outputs a digital signal whose logic changes depending on the phase of the AC power supply 10, or a circuit that outputs a pulse signal at each zero crossing point.

[0049] The control unit 240 is a circuit that detects the zero cross point of the AC power source 10 from the output voltage Vdt220 of the voltage detection circuit 220 and outputs the SW3 control signal using the voltage Vcc as a power source. Unlike the control unit 140 of the first embodiment, the control unit 240 detects the voltage instantaneous value VAC of the AC power source 10 only during a period in which the second phase of the AC power source 10 has a higher potential than the first phase. Therefore, the control unit 240 determines that the AC power source 10 has a zero cross point when the output voltage Vdt220 of the voltage detection circuit 220 is decreasing and is equal to or lower than the threshold value. Note that, similar to the control unit 140 of the first embodiment, either the control unit 240 or the FET control unit 215 may control both the FETs Q211 and Q212 and the switch element SW3. Also, similar to the control unit 140 of the first embodiment, the reference potential of the control unit 240 may be set to another point.

[0050] The SW3 drive circuit 250 is composed of resistors R251 and R252 and a switch element SSR251, and is a circuit that controls the on / off of the switch element SW3 based on the SW3 control signal. When the switch element SSR251 is turned on, the switch element SW3, which is a triac, becomes conductive, and when the switch element SSR251 is subsequently turned off, the switch element SW3 continues to be conductive until the zero-cross point of the AC power supply 10. In this embodiment, the switch element SSR251 is a phototriac coupler, and is turned on when the SW3 control signal is at H level, and is turned off when the SW3 control signal is at L level.

[0051] Next, a method of controlling the switch elements SW1 and SW3 and an inrush current flowing through the smoothing capacitor C1 and the capacitor CX will be described with reference to FIG.

[0052] 4 is a diagram showing transitions of voltage, current, etc. at various parts of the switching power supply 200 when the switch elements SW1 and SW3 are turned on, with time taken on the horizontal axis. From the top, these are the instantaneous voltage value VAC of the AC power supply 10, the SW1 control signal, the resistance value of the switch element SW1, the output voltage Vdt220 of the voltage detection circuit 220, and the SW3 control signal. In addition, the diagram shows the voltage VC1 of the smoothing capacitor C1, the current Irush flowing through the smoothing capacitor C1, the voltage VCX of the capacitor CX, and the current ICX flowing through the capacitor CX.

[0053] As in the first embodiment, when the SW1 control signal is at an L level, the contacts of the switch element SW1, which is a relay, are not conductive, the resistance value of the switch element SW1 is a very high value Roff, and the AC power supply 10 and the switching power supply 200 are not connected. In addition, the output voltage Vdt220 of the voltage detection circuit 220 is 0 V. In addition, since the SW3 control signal is also at an L level, the switch element SW3 is not turned on, the voltages of the smoothing capacitor C1 and the capacitor CX are 0 V, and no current flows.

[0054] When the SW1 control signal switches to H level, the mechanical contacts of the switch element SW1, which is a relay, begin to make contact, but similarly to the first embodiment, the resistance value of the switch element SW1 remains at Roff until complete contact is made.

[0055] When the contacts of the switch element SW1 are completely in contact and the resistance value of the switch element SW1 is switched from Roff to Ron (when the switch element SW1 is turned on), the voltage instantaneous value VAC of the AC power supply 10 is 0V or more. Therefore, at this point, the output voltage Vdt220 of the voltage detection circuit 220 is 0V, and when the voltage instantaneous value VAC of the AC power supply 10 subsequently becomes less than 0V, the voltage detection circuit Vdt220 becomes a voltage proportional to the voltage instantaneous value VAC of the AC power supply 10. After the switch element SW1 becomes conductive, the output voltage Vdt220 of the voltage detection circuit 220 becomes 0V when the voltage instantaneous value VAC of the AC power supply 10 is 0V or more, and becomes a value proportional to the voltage instantaneous value VAC of the AC power supply 10 when it is less than 0V. In FIG. 4, as in FIG. 2, the forward voltage of the diode constituting the rectifier DA1 is ignored because it is sufficiently smaller than the maximum value of the voltage VAC.

[0056] When the switch element SW1 becomes conductive and the voltage detection circuit 220 becomes able to detect the voltage instantaneous value VAC of the AC power source 10, the control unit 240 changes the SW3 control signal to H level in synchronization with the zero cross point of the AC power source 10. When the SW3 control signal becomes H level, the SW3 drive circuit 250 turns on the switch element SW3, and the switch element SW3, which is a triac, becomes conductive in a very short time (for example, several μs to several tens of μs). Therefore, the AC power source 10 is connected to the smoothing capacitor C1 and the capacitor CX almost simultaneously with the first zero cross point after the switch element SW1 becomes conductive and the voltage instantaneous value VAC of the AC power source 10 becomes less than 0V. When the AC power source 10 is connected to the smoothing capacitor C1 and the capacitor CX, a voltage is applied to the smoothing capacitor C1 and the capacitor CX, but at that point, the voltage VC1 applied to the smoothing capacitor C1 and the voltage VCX applied to the capacitor CX are almost 0V. Therefore, the currents Irush and ICX that flow when the smoothing capacitor C1 and the capacitor CX are connected to the AC power supply 10 are very small, at a few amperes or less.

[0057] By controlling the switch elements SW1 and SW3 as described above, it is possible to reduce the inrush current that occurs when the AC power supply 10 and the switching power supply 200 are connected.

[0058] The reason why the switching power supply 200 of this embodiment employs an active clamp type circuit, unlike the switching power supply 100 of the first embodiment, is to reduce the size and cost of the switching power supply. The power generated by the switching element of the switching power supply generally tends to increase as the output power increases. Therefore, in a switching power supply with an output power of 1000 W or more, such as the switching power supply 200 or the switching power supply 100 of the first embodiment, measures are required to prevent the switching element from overheating.

[0059] As a means for preventing overheating of the switching element, there are a heat sink and a switching element with low on-resistance, but most of these means lead to an increase in size and cost of the switching power supply. Although the power generated by the switching element differs depending on the circuit method, the above means must be applied to all switching elements, so the fewer the number of switching elements used, the smaller the circuit and the lower the cost. In addition, reducing the number of switching elements used can be expected to reduce the cost of the control unit.

[0060] In switching power supplies that perform complex control, such as the switching power supply 100 and the switching power supply 200 of the first embodiment, a control element such as a general-purpose microcomputer may be used in the control unit. Since the price of a general-purpose microcomputer tends to increase as the number of pins and peripheral functions increases, a cheaper microcomputer can be used as the number of switch elements to be controlled decreases. In addition, in a circuit method having a high-side switch element, such as a full bridge circuit or an active clamp circuit, a circuit for driving the high-side switch element is required. Therefore, the fewer the number of switch elements used, the lower the cost of the drive circuit. Therefore, the switching power supply 200 can have a control unit that is cheaper than the switching power supply 100 of the first embodiment.

[0061] Also, unlike the switching power supply 100 of the first embodiment, the switching power supply 200 has only one voltage detection circuit, and the second switch element is located closer to the AC power supply side than the rectifier, but this is because the X capacitor CX is provided.

[0062] In a switching power supply, an X capacitor may be provided between the rectifier and the AC power supply to prevent noise voltage generated from the switching power supply from propagating to the AC power supply 10. Since an equivalent series resistance exists in the X capacitor, power loss occurs in the X capacitor connected to the AC power supply. If this power loss occurs when a device equipped with a switching power supply is in a low power consumption (sleep) state, the power consumption of the device during sleep may become excessive. For this reason, an X capacitor may be provided between the first switch element (relay) and the switching power supply, and the first switch element may be turned off during sleep.

[0063] However, if the second switch element is provided on the output terminal side of the rectifier as in the switching power supply 100 of the first embodiment, an inrush current flows into the X capacitor at the moment the relay is turned on even if the second switch element is not turned on. Therefore, in order to prevent an inrush current into the X capacitor, it is necessary to provide the second switch element on the AC power supply side rather than the X capacitor. If the second switch element is provided between the AC power supply and the rectifier, even if a second voltage detection circuit is provided as in the switching power supply 100 of the first embodiment, the second voltage detection circuit cannot detect the voltage of the AC power supply when the second switch element is not turned on. On the other hand, even if there is only one voltage detection circuit and the voltage can only be detected during a half cycle of the AC power supply, it is possible to detect the zero crossing point if the instantaneous voltage value and the slope are known as in this embodiment. Therefore, if it is no problem if the time for which the zero crossing point can be detected is half, the second voltage detection circuit is not necessary to control the second switch element as in this embodiment.

[0064] Therefore, even if an X capacitor is provided, by configuring a circuit as in this embodiment and controlling the switch element, it is possible to suppress the inrush current to both the smoothing capacitor and the X capacitor.

[0065] Therefore, according to this embodiment, it is possible to provide a switching power supply that can suppress inrush power when connected to an AC power supply and detect the zero-cross point of the AC power supply regardless of the load. [Explanation of symbols]

[0066] 100 Switching Power Supply DA1 rectifier 110 Switching section 120 Voltage detection circuit 130 Voltage detection circuit 140 Control section 150 Load

Claims

1. a rectification unit to which a single-phase AC power supply having a first phase and a second phase is connected and which full-wave rectifies the AC power supply; a smoothing capacitor for smoothing the output voltage of the rectifier; A switching unit provided in parallel with the smoothing capacitor; a first switch element provided in the first phase or the second phase; a second switch element provided between the rectifier and a high potential side of the smoothing capacitor; a first voltage detection circuit provided between the first switch element and the rectification unit; a second voltage detection circuit provided in a phase opposite to a phase in which the first switch element is provided, out of the first phase or the second phase; A control unit; having the control unit detects a zero crossing of the AC power supply based on an output of at least one of the first voltage detection circuit and the second voltage detection circuit; The switching power supply according to claim 1, wherein the control unit, after having made the first switch element conductive, makes the second switch element conductive in synchronization with the zero crossing.

2. 2. The switching power supply according to claim 1, wherein a reference potential of the first voltage detection circuit and a reference potential of the second voltage detection circuit are the same potential as a low potential side of the smoothing capacitor.

3. 3. The switching power supply according to claim 1, wherein the first voltage detection circuit and the second voltage detection circuit are capable of detecting the voltage of the AC power supply when the first switch element is conductive.

4. a rectification unit to which a single-phase AC power supply having a first phase and a second phase is connected and which full-wave rectifies the AC power supply; a smoothing capacitor for smoothing the output voltage of the rectifier; A switching unit provided in parallel with the smoothing capacitor; a first switch element provided in the first phase or the second phase; a second switch element provided in a phase opposite to a phase in which the first switch element is provided, among the first phase and the second phase; a voltage detection circuit provided between the AC power supply and the second switch element; A control unit; having The control unit detects a zero crossing of the AC power supply based on an output of the voltage detection circuit, The switching power supply according to claim 1, wherein the control unit, after having made the first switch element conductive, makes the second switch element conductive in synchronization with the zero crossing.

5. 5. The switching power supply according to claim 4, further comprising a capacitor between the input terminals of the rectifier section.

6. 6. The switching power supply according to claim 4, wherein a reference potential of the voltage detection circuit is the same potential as the low potential side of the smoothing capacitor.

7. 5. The switching power supply according to claim 4, wherein the voltage detection circuit is capable of detecting the voltage of the AC power supply when the first switch element is conductive.

8. the switching unit supplies power to an inductive load, the inductive load has a primary inductance electrically connected to the switching unit, and a secondary load magnetically connected to the primary inductance, 5. The switching power supply according to claim 1, wherein the secondary load generates heat when power is supplied from the AC power supply and the smoothing capacitor through the primary inductance by a switching operation of the switching unit.

Images