JP2009273324A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply that prevents the degradation of efficiency caused by load change and reduces noise. <P>SOLUTION: The switching power supply includes: a smooth capacitor C0 that smoothes the output of a diode bridge DB; switching elements Q1, Q2 connected in series between both ends of the smooth capacitor C0; a transformer T of which the primary winding n1 is connected in series between the source/drain of the switching element Q2 together with an inductance L1 and a capacitor C1; a pair of diodes D1, D2 connected in reverse parallel with secondary windings n21, n22 of the transformer T; a capacitor C2 connected in series to the secondary winding n21 via the diode D1; a driving circuit DR in which switching is controlled so that a switching operation period and a switching stop period are alternatively repeated; and a frequency adjustment circuit CT1 in which the switching frequency in the switching operation period is controlled so that a voltage Vdc across the capacitor C2 becomes a desired fixed value. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a switching power supply apparatus.

  As a conventional switching power supply device, there is one shown in FIG. 11 (see Patent Document 1). This conventional device creates a desired DC voltage from a DC power source obtained by rectifying and smoothing an AC power source AC with a diode bridge DB and a smoothing capacitor C0, each of which is composed of a field effect transistor, and is formed between both ends of the smoothing capacitor C0. A transformer T in which a primary winding n1 is connected in series with an inductor L1 and a capacitor C1 between a source and a drain of a pair of switching elements Q1, Q2 connected in series to the low side switching element Q2, and a secondary of the transformer T A pair of diodes D1 and D2 connected in antiparallel to the windings n21 and n22, a smoothing capacitor C2 connected in series to one secondary winding n21 via the diode D1, and switching elements Q1 and Q2 A drive circuit DR that alternately turns on and off (switches) and a voltage across the capacitor C2 ( A feedback control circuit FC that controls the drive circuit DR so that the output voltage matches the target value, and feedback-controls the switching frequency of the switching elements Q1 and Q2. A constant DC voltage can be applied to the load L connected in parallel with C2.

  The feedback control circuit FC has an error amplifier (not shown) that amplifies and outputs a voltage corresponding to the difference between the output voltage and the reference voltage (target value), and the drive circuit DR is supplied from the feedback control circuit FC. Based on the feedback signal, the switching of the switching elements Q1, Q2 is controlled. Here, when the output voltage becomes larger than the reference voltage, the oscillation frequency (switching frequency) of the drive circuit DR increases based on the error voltage output from the feedback control circuit FC. As a result, the resonance impedance of the resonance circuit formed on the primary side of the transformer T increases, and the resonance current to the primary winding n1 of the transformer T decreases, so the output voltage decreases. On the other hand, when the output voltage becomes smaller than the reference voltage, the oscillation frequency of the drive circuit DR decreases based on the error voltage output from the feedback control circuit FC, and the resonance of the resonance circuit formed on the primary side of the transformer T Since the impedance decreases and the resonance current to the primary winding n1 of the transformer T increases, the output voltage rises.

  The switching power supply as described above has an advantage that a large amount of power can be generated with high efficiency and low noise. However, since it is necessary to increase the switching frequency in order to suppress the increase in output voltage in the light load state, the number of switching increases due to the increase in switching frequency, and the switching loss increases accordingly. There is a problem of end up.

Therefore, in order to solve such a problem, the switching operation period in which switching is performed and the switching stop period in which switching is stopped are alternately repeated, and when the light load state occurs, the output voltage is decreased by increasing the switching frequency. Instead, there is provided a switching power supply apparatus that performs control (hereinafter referred to as “intermittent switching control”) to reduce the output voltage by changing the ratio between the switching operation period and the switching stop period. .
Japanese Patent No. 2734296

  According to the latter conventional example, it is possible to suppress a decrease in efficiency at a light load, but it is inevitable that the efficiency gradually decreases until switching from the switching frequency control to the intermittent switching control.

  In order to reduce the size of the transformer, it may be possible to increase the switching frequency. However, if the switching frequency is higher than the standard frequency (= 150 kHz) of the noise terminal voltage, although it is originally low noise, the fundamental noise As a result, the standard value of the noise terminal voltage cannot be satisfied, and the size of the filter circuit is increased, resulting in an increase in the size of the entire power supply device.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a switching power supply device capable of suppressing a reduction in efficiency due to load fluctuations and reducing noise.

  In order to achieve the above object, the invention of claim 1 is directed to a transformer, a capacitor that forms a resonance circuit with the primary winding of the transformer, and one or more switching units that intermittently pass a current flowing through the primary winding of the transformer. Switching for controlling switching of the switching element so that the element, the rectifying / smoothing means for rectifying and smoothing the AC output generated on the secondary side of the transformer, and the switching operation period for switching and the switching stop period for stopping switching are alternately repeated It is characterized by comprising control means and frequency adjusting means for controlling the switching frequency during the switching operation period so that the DC output rectified and smoothed by the rectifying and smoothing means becomes a desired constant value.

  According to a second aspect of the present invention, in the first aspect of the invention, when the input voltage input to the primary side of the transformer is relatively high, the ratio of the switching operation period to the switching stop period is reduced, and the input voltage is relatively When it is low, a duty ratio adjusting means for increasing the ratio is provided.

  According to a third aspect of the present invention, in the first aspect of the invention, there is provided duty ratio adjusting means for controlling a ratio of the switching operation period to the switching stop period, and is input when the output power is smaller than a predetermined power value or on the primary side of the transformer. When the input voltage is higher than a predetermined voltage value, the frequency adjusting means controls the switching frequency to the maximum value, and the duty ratio adjusting means controls the ratio so that the DC output is a desired constant value, and When the output power is equal to or higher than the predetermined power value or the input voltage is equal to or lower than the predetermined voltage value, the duty ratio adjusting means controls the ratio to the maximum value, and the frequency adjusting means sets the DC output to a desired constant value. Thus, the switching frequency is controlled.

  According to a fourth aspect of the present invention, in the third aspect of the invention, when the output power is smaller than the predetermined power value or the input voltage is higher than the predetermined voltage value, the frequency adjusting means sets the maximum value of the switching frequency to the predetermined frequency. It is characterized in that it is varied with a predetermined fluctuation range as the center.

  According to a fifth aspect of the present invention, in the invention according to any one of the first to fourth aspects, the switching control means sets a frequency at which the switching operation period and the switching stop period are alternately repeated with a predetermined frequency as a center. It is characterized by being varied within a variation range.

  The invention of claim 6 is the invention of any one of claims 1 to 5, wherein the switching control means sets the frequency for alternately repeating the switching operation period and the switching stop period to a frequency outside the audible sound frequency range. It is characterized by that.

  The invention of claim 7 is the invention according to any one of claims 1 to 6, wherein the switching control means starts the switching stop period at a timing when the switching element is turned off.

  According to an eighth aspect of the present invention, in the invention according to any one of the first to seventh aspects, the switching control means outputs a drive signal having a signal level that does not drive the switching element during the switching stop period, and the drive signal Is provided as a communication signal.

  According to the first aspect of the present invention, the switching control means controls the switching so that the switching operation period and the switching stop period repeat alternately, the switching stop period is periodically provided, and the switching operation is intermittently performed. As a result, the quasi-peak value and average value of the noise terminal voltage can be reduced and the generated noise can be suppressed, and the switching loss is reduced because the switching frequency is reduced. As a result, the circuit efficiency is improved. Furthermore, since the switching frequency fluctuates in accordance with fluctuations in the load and the input voltage, the frequency at which noise is generated also fluctuates. As a result, the noise is distributed with respect to the frequency, so that further noise reduction is achieved. In addition, since it is not necessary to provide a large filter circuit for noise reduction, the entire power supply apparatus can be reduced in size.

  By the way, when the input voltage is relatively high, it is necessary to increase the switching frequency in order to keep the output constant. However, if the switching frequency is increased, the number of times of switching may increase and switching loss may increase. According to the invention of claim 2, when the input voltage is relatively high, since the ratio of the switching operation period to the switching stop period is reduced, the output can be kept constant while suppressing the increase of the switching frequency, Since switching frequency is reduced and switching loss is reduced, the circuit efficiency is improved.

  According to the invention of claim 3, not only when the input voltage is higher than the predetermined voltage value, but also when the output power is lower than the predetermined power value, the ratio of the switching operation period becomes small, so the number of switching times decreases and the switching loss is reduced. Is reduced and the circuit efficiency is improved. In addition, since the switching frequency is controlled to the maximum value when the load is light, the current peak flowing through the switching element is reduced and the loss due to the circuit resistance can be reduced, so that the circuit efficiency can be further improved. Further, when the output power is equal to or higher than the predetermined power value or when the input voltage is equal to or lower than the predetermined voltage value, the ratio of the switching operation period is maximized and the switching frequency is lowered, so that high output can be achieved. There is also an effect.

  According to the fourth aspect of the present invention, the maximum value of the switching frequency varies with a predetermined fluctuation range around the predetermined frequency, and noise caused by the switching frequency is dispersed with respect to the frequency. The peak value and the average value are reduced, and noise can be reduced.

  According to the fifth aspect of the present invention, the frequency at which the switching operation period and the switching stop period are alternately repeated is changed with a predetermined fluctuation range around the predetermined frequency, and the switching operation period and the switching stop period are Since the noise caused by the alternating frequency can be dispersed with respect to the frequency, the quasi-peak value and the average value of the noise are reduced, and the noise can be reduced.

  According to the sixth aspect of the present invention, since the frequency for alternately repeating the switching operation period and the switching stop period is set to a frequency outside the audible sound frequency range, the beat sound generated when the transformer vibrates in the audible sound frequency range. Can be prevented.

  According to the seventh aspect of the present invention, loss and noise can be reduced compared to the case where the switching stop period is started when the switching element is on.

  According to the invention of claim 8, during the switching stop period, the switching control means outputs a driving signal having a signal level that does not drive the switching element, and transmits the driving signal to the outside as a communication signal. There is an effect that a communication signal can be transmitted to the outside without adding a communication port.

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
A circuit configuration diagram of the present embodiment is shown in FIG. This switching power supply device creates a desired DC voltage from a DC power source obtained by rectifying and smoothing an AC power source AC with a diode bridge DB and a smoothing capacitor C0 made of an electrolytic capacitor, each of which includes a field effect transistor. , A pair of switching elements Q1, Q2 connected in series between both ends of the smoothing capacitor C0, and a transformer T in which a primary winding n1 is connected in series with an inductor L1 and a capacitor C1 between the source and drain of the low-side switching element Q2. A pair of rectifying elements (diodes) D1 and D2 connected in reverse parallel to the secondary windings n21 and n22 of the transformer T, and an electrolytic capacitor connected in series to one of the secondary windings n21 via the diode D1. The smoothing capacitor C2 and switching elements Q1 and Q2 are alternately turned on and off. A switching operation period (switching) and a switching stop period in which switching is stopped are repeated at a predetermined cycle, and a drive circuit (switching control) that changes the switching frequency in the switching operation period based on a control signal input from the frequency adjustment circuit CT1. Means) DR, a feedback control circuit FC that compares the voltage Vdc (output voltage) across the capacitor C2 with a desired target value (reference voltage), and outputs a feedback signal obtained by amplifying a voltage corresponding to the difference between the two, feedback Based on a feedback signal from the control circuit FC, a frequency adjustment circuit CT1 that controls the switching frequency in the switching operation period so that the DC output (voltage across the capacitor C2) Vdc becomes a desired constant value is provided. Parallel It is adapted to apply a constant DC voltage to the connected load L. Here, in the present embodiment, rectifying and smoothing means for rectifying and smoothing the AC output generated on the secondary side of the transformer T is configured from the diodes D1 and D2 and the capacitor C1. A resonance circuit is constituted by the primary winding n1 of the transformer T, the inductor L1, the capacitor C1, and the like. The inductor L1 may be formed by a leakage inductance due to the primary winding n1 of the transformer T.

  In this power supply device, the switching operation period and the switching stop period are alternately provided (hereinafter referred to as intermittent switching operation), and the switching frequency in the switching operation period is controlled so that the DC output becomes a constant value. FIGS. 2A and 2B are waveform diagrams showing gate-source voltages Vgs1 and Vgs2 of the switching elements Q1 and Q2. FIG. 2A shows a period during which the load is relatively light (light load) or a transformer. The voltage between the gate and the source when the input voltage Vin input to the primary side of T is relatively high (at high voltage) is shown, and FIG. 5B shows a period when the load is relatively heavy (at heavy load) or The gate-source voltage during a period when the input voltage Vin is relatively low (during low voltage) is shown.

  As shown in FIG. 2, the drive circuit DR alternately includes a switching operation period TA for performing a switching operation and a switching stop period TB for stopping the switching operation. In the present embodiment, the switching operation period with respect to the switching stop period TB is provided. The TA ratio is a constant value. The drive circuit DR changes the switching frequency during the switching operation period based on the control signal input from the frequency adjustment circuit CT1.

  The feedback control circuit FC compares the DC output voltage Vdc rectified and smoothed by the rectifying and smoothing means (comprising the diodes D1 and D2 and the capacitor C2) with a predetermined target value (reference voltage), and according to the difference between the two. A feedback signal obtained by amplifying the voltage is output to the frequency adjustment circuit CT1.

  The frequency adjustment circuit CT1 creates a control signal for changing the switching frequency in the switching operation period TA so that the DC output voltage Vdc becomes a desired constant value based on the feedback signal input from the feedback control circuit FC, This control signal is output to the drive circuit DR.

  As described above, in the present embodiment, the drive circuit DR alternately provides the switching operation period TA and the switching stop period TB to perform the switching operation intermittently. As a result, the switching stop period TB is periodically provided. Therefore, compared to the case where the switching operation is always performed, the quasi-peak value and the average value of the noise terminal voltage are reduced, and the generated noise can be suppressed. Further, since the number of times of switching is reduced, the switching loss is reduced and the circuit efficiency is improved.

  Further, the voltage obtained by rectifying and smoothing the input voltage varies depending on the size of the load, and the output voltage increases when the load becomes light, and the output voltage decreases when the load becomes heavy. Further, the output voltage also fluctuates according to the voltage fluctuation of the input voltage. When the input voltage becomes relatively high, the output voltage becomes large. When the input voltage becomes relatively low, the output voltage becomes small. In the frequency adjusting circuit CT1, the switching frequency is changed according to the output voltage. As shown in FIG. 2 (a), the switching frequency is increased at a light load or at a high voltage, and as shown in FIG. 2 (b). The switching frequency is lowered at the time of load or low voltage. Therefore, the switching frequency fluctuates in accordance with fluctuations in the load and input voltage. As a result, the frequency at which noise is generated also fluctuates, so that the noise is dispersed with respect to the frequency. Therefore, since the quasi-peak value and average value of noise can be reduced and noise can be further reduced, there is no need to provide a large filter circuit for noise reduction, and the power supply apparatus as a whole can be reduced in size. be able to.

(Embodiment 2)
A circuit configuration diagram of the present embodiment is shown in FIG. In the present embodiment, in the switching power supply device described in the first embodiment, duty ratio adjustment that controls the ratio of the switching operation period TA to the switching stop period TB according to the input voltage Vin input to the primary side of the transformer T. A circuit CT2 is provided. In the drive circuit DR, the ratio of the switching operation period TA to the switching stop period TB is changed based on the control signal input from the duty ratio adjustment circuit CT2. Since the configuration other than the duty ratio adjustment circuit CT2 is substantially the same as that of the first embodiment, the same components are denoted by the same reference numerals and the description thereof is omitted.

  4A and 4B are waveform diagrams showing the gate-source voltages Vgs1 and Vgs2 of the switching elements Q1 and Q2. FIG. 4A shows a period in which the input voltage Vin is relatively high (at the time of high voltage). FIG. 4B shows the gate-source voltage during a period when the input voltage Vin is relatively low (when the voltage is low).

  In the present embodiment, the drive circuit DR alternately provides a switching operation period TA in which a switching operation is performed and a switching stop period TB in which the switching operation is stopped, and the switching operation period TA based on a control signal from the frequency adjustment circuit CT1. And the ratio of the switching operation period TA to the switching stop period TB is changed according to the control signal input from the duty ratio adjustment circuit CT2. Here, in the duty ratio adjusting circuit CT2, when the input voltage Vin is relatively high, the ratio of the switching operation period TA to the switching stop period TB is decreased as shown in FIG. When the voltage is relatively low, the ratio of the switching operation period TA to the switching stop period TB is increased as shown in FIG.

  By the way, when the input voltage Vin becomes relatively high, it is necessary to increase the switching frequency in order to keep the output voltage constant. However, since the number of times of switching increases by increasing the switching frequency, the switching loss increases. As a result, the efficiency may decrease.

  On the other hand, in the present embodiment, when the duty ratio adjustment circuit CT2 detects the input voltage Vin and the input voltage Vin becomes relatively high, the ratio of the switching operation period TA to the switching stop period TB is reduced. The output voltage can be maintained at a constant value while suppressing an increase in the frequency, and as a result, the number of times of switching is reduced, so that switching loss can be reduced and circuit efficiency can be improved.

(Embodiment 3)
FIG. 5 shows a circuit configuration diagram of this embodiment. Since the basic configuration of the present embodiment is substantially the same as that of the switching power supply device of the second embodiment shown in FIG. 3, the same components are denoted by the same reference numerals and description thereof is omitted.

  The duty ratio adjustment circuit CT2 described in the second embodiment detects the input voltage Vin and changes the ratio of the switching operation period TA to the switching stop period TB according to the magnitude of the input voltage Vin. The duty ratio adjustment circuit CT2 changes the ratio of the switching operation period TA to the switching stop period TB based on the feedback signal from the feedback control circuit FC. Further, the frequency adjustment circuit CT1 controls the switching frequency in the switching operation period based on the feedback signal from the feedback control circuit FC as in the first and second embodiments.

  6A and 6B are waveform diagrams showing the gate-source voltages Vgs1 and Vgs2 when the output power is larger than a predetermined power value (for example, the rated power W0), and FIG. Shows the gate-source voltage during a light period (light load) or a period when the input voltage Vin is relatively high (high voltage), and FIG. 5B shows a period when the load is relatively heavy (heavy load). Alternatively, the voltage between the gate and the source during a period when the input voltage Vin is relatively low (when the voltage is low) is shown. FIGS. 7A and 7B are waveform diagrams showing the gate-source voltages Vgs1 and Vgs2 when the output power is smaller than a predetermined power value (for example, the rated power W0). FIG. The gate-source voltage at the time of high or low voltage is shown, and FIG. 5B shows the gate-source voltage at the time of heavy load or low voltage. FIG. 8 shows the relationship between the switching frequency f and the duty ratio DT (ratio of the switching operation period to the switching stop period) with respect to the output power. In the figure, a represents the switching frequency f and b represents the duty ratio DT. . The duty ratio DT is expressed as DT = ta / (ta + tb), where ta is the time width of the switching operation period TA and tb is the time width of the switching stop period TB.

  In the present embodiment, the feedback control circuit FC amplifies a voltage corresponding to the difference between the DC output voltage Vdc and a predetermined target value (reference voltage), and uses the frequency adjustment circuit CT1 and the duty ratio adjustment circuit CT2 as feedback signals. Is output. When the output power is smaller than the predetermined power value (rated power W0) or when the input voltage Vin is higher than the predetermined voltage value V0, the frequency adjustment circuit CT1 is based on the feedback signal from the feedback control circuit FC. The switching frequency is controlled to the maximum value fmax, and the duty ratio adjustment circuit CT2 controls the duty ratio to a smaller value as the input voltage Vin is higher or the output power is smaller (load is lighter). Therefore, as the input voltage Vin becomes higher or the output power becomes smaller (as the load becomes lighter), the duty ratio becomes smaller, so that the number of times of switching can be reduced. As a result, the switching loss is reduced and the circuit efficiency is reduced. Can be improved. Further, since the switching frequency is switched at the maximum value fmax at the time of this light load, the current peak flowing through the switching elements Q1 and Q2 can be reduced and the loss due to the circuit resistance can be reduced, so that the circuit efficiency can be improved.

  On the other hand, when the output power increases (whether the load becomes heavy) or the input voltage Vin decreases, the duty ratio adjustment circuit CT2 increases the duty ratio accordingly, and the output power corresponding to the predetermined power value ( The duty ratio is controlled to the maximum value DTmax when the rated power (W0) is reached or when the input voltage Vin decreases to the predetermined voltage value V0. When the output power becomes larger than the predetermined power value (rated power W0) or the input voltage Vin becomes lower than the predetermined voltage V0, the frequency adjustment circuit CT1 maintains the predetermined output voltage at the maximum duty ratio DTmax. Thus, the switching frequency is lowered to a small value based on the feedback signal from the feedback control circuit FC. Therefore, when the output power is equal to or higher than the predetermined power value W0, or when the input voltage Vin is equal to or lower than the predetermined voltage value V0, the ratio of the switching operation period TA is maximized and the input voltage Vin becomes lower, or Since the switching frequency can be lowered as the output power increases, the output power can be increased and higher output can be achieved.

  As described above, according to the present embodiment, high output can be achieved, high efficiency can be achieved at light load, and a single feedback (only output voltage feedback) can be realized. Thus, the output voltage on the secondary side can be easily stabilized.

  In this embodiment, when the output power is lower than the predetermined power value or when the input voltage Vin is higher than the predetermined voltage value, the frequency adjustment circuit CT1 controls the switching frequency to the maximum value fmax. The maximum value of the frequency may be changed with a predetermined fluctuation range around the predetermined frequency. When the output power is lower than the predetermined power value or when the input voltage Vin is higher than the predetermined voltage value, the output is achieved by changing the ratio of the switching operation period TA to the switching stop period TB even if the switching frequency is changed. The voltage can be kept constant. Therefore, since the maximum setting of the switching frequency can be changed by adjusting the duty ratio, noise caused by the switching frequency can be dispersed with respect to the frequency, and the quasi-peak value and average value of the noise are reduced. Further noise reduction can be achieved.

  Further, in each of the above-described embodiments, the drive circuit DR may change the frequency at which the switching operation period TA and the switching stop period TB are alternately repeated with a predetermined fluctuation range around the predetermined frequency. Here, even if the frequency at which the switching operation period TA and the switching stop period TB are alternately repeated is changed, the output voltage can be kept constant unless the ratio of the switching operation period TA is changed. Therefore, in the drive circuit DR, the frequency at which the switching operation period TA and the switching stop period TB are alternately repeated can be changed, and noise caused by the frequency that alternates between the switching operation period and the switching stop period is affected by the frequency. Therefore, the quasi-peak value and average value of noise are reduced, and noise can be further reduced.

  Furthermore, in each of the above-described embodiments, it is preferable that the frequency at which the drive circuit DR repeats the switching operation period TA and the switching stop period TB alternately is a frequency outside the audible sound frequency region. In the conventional switching power supply device, the output voltage is made constant by changing the switching frequency in a continuous switching operation without a switching stop period. However, in order to reduce the power consumption at light loads, the switching operation is switched to the intermittent switching operation. There is a need. When the voltage obtained by full-wave rectification of the commercial power supply is used as the input in the power output state near the switching of the operation, the smoothed input voltage fluctuates in synchronization with the frequency (100 Hz / 120 Hz) twice the commercial power supply frequency. Therefore, the intermittent switching operation is performed in the vicinity of the maximum voltage fluctuation voltage, but the input voltage is low and the output voltage is reduced in the vicinity of the lowest voltage fluctuation voltage, so that the continuous switching operation is performed instead of the intermittent switching operation. Therefore, two operations of an intermittent switching operation and a continuous switching operation are repeated at a frequency twice as high as the commercial power supply frequency, and a beat sound of the transformer is generated. In addition, the beat sound of the transformer is generated when the core of the transformer and the winding of the coil vibrate at an audible sound frequency by changing the magnetic flux. In addition, in the voltage output state near the switching operation, even when the output power fluctuates at the audible sound frequency, the transformer continuous switching operation and the intermittent switching operation are switched at the audible sound frequency. Become. Therefore, as in each of the above-described embodiments, the drive circuit DR always has the switching operation period TA and the switching stop period TB, and the frequency at which the both periods are switched is outside the audible sound frequency range (for example, 16 kHz or more). Therefore, it is possible to prevent the beat sound of the transformer.

  In each of the above-described embodiments, when the drive circuit DR switches from the switching operation period TA to the switching stop period TB, it is desirable to start the switching stop period TB at a timing when the switching element Q1 or Q2 is turned off. That is, when any switching element Q1 or Q2 is on and power is supplied to the secondary side of the transformer T, the diode connected to the secondary side of the transformer T when the switching stop period TB is started Switching stops when a current flows through D1 or D2, and a recovery current flows through the diode D1 or D2, resulting in power loss and noise. On the other hand, in the steady switching state, power is supplied to the secondary side of the transformer T after the switching element Q1 or Q2 is turned on, and the secondary side of the transformer T is resonated by the resonance operation of the primary side resonance circuit. Since the switching frequency is set so that the switching element Q1 or Q2 is turned off after the power supply to the power supply is stopped, the power supply to the secondary side of the transformer T is performed at the timing when the switching element Q1 or Q2 is turned off. The current is not flowing through the diode D1 or D2 connected to the secondary side of the transformer T. Therefore, if the switching stop period TB is started at the timing when the switching element Q1 or Q2 is turned off, the switching is stopped when the energization of the diode D1 or D2 is stopped, thereby reducing power loss and noise due to the recovery current. be able to. In addition, in order to prevent the switching elements Q1 and Q2 from being turned on at the same time or the like, if there is a minute rest period (dead time) in which both the switching elements Q1 and Q2 are turned off, switching is performed during this rest period. The stop period TB may be started.

(Embodiment 4)
FIG. 9 shows a circuit configuration diagram of the fourth embodiment. In the present embodiment, in the switching power supply described in the first embodiment, the drive circuit DR outputs a drive signal having a signal level that does not drive the switching elements Q1 and Q2 during the switching stop period TB, and the switching element Q1. , Q2 is provided with a communication circuit 1 (transmission means) that discriminates a drive signal having a signal level that does not drive Q2 as a communication signal and transmits the communication signal to the outside. Since the configuration other than the drive circuit DR and the communication circuit 1 is substantially the same as that of the first embodiment, the same components are denoted by the same reference numerals and the description thereof is omitted.

  The input voltage and output voltage of this device will increase the ripple component when the capacitance value of electrolytic capacitors C0 and C2, which are long-lived components, decreases due to aging, so the drive circuit DR monitors the input voltage Vin and the output voltage Vdc. By doing so, an increase in the ripple component is detected. In addition, during the conduction period of the switching elements Q1 and Q2, the drive circuit DR monitors the voltage at the connection point of the switching elements Q1 and Q2, so that an excess current is generated from the on-resistance of the switching elements Q1 and Q2 and the voltage due to the switch current. It is possible to detect the occurrence. Further, in the non-conduction period (particularly when switching from the switching operation period to the switching stop period), if overvoltage occurs in the switching elements Q1 and Q2 due to deterioration of components such as the inductor L1 and the capacitor C1 constituting the resonance circuit, circuit efficiency However, in the drive circuit DR, the voltage applied to the switching elements Q1 and Q2 is monitored, so that the overvoltage is applied to the switching elements Q1 and Q2. Can be detected in advance. Therefore, in the drive circuit DR, by monitoring the input voltage, the output voltage, and the switch voltage, an increase in the ripple component of the input voltage or the output voltage, an overcurrent flowing through the switching elements Q1 and Q2, and the switching elements Q1 and Q2 When the applied overvoltage is detected, it is determined that an abnormality has occurred in the power supply circuit, and the switching elements Q1 and Q2 can be stopped as necessary, and each abnormal state is indicated based on the signal content of the monitored signal. A communication signal is generated and output during the switching stop period TB. Here, the communication signal output from the drive circuit DR is a signal level that does not drive the switching elements Q1 and Q2, and the voltage amplitude of the switching elements Q1 and Q2 is, for example, as shown in FIG. A signal S1 (for example, 1 V or less) below the threshold level of the driving voltage is transmitted, or a signal S2 having a frequency (for example, several GHz or more) whose frequency greatly exceeds the switching frequency as shown in FIG. It may be transmitted during a period (for example, several pulses) or a pulse signal S3 having a small amplitude level and having a reverse polarity as shown in FIG.

  The communication circuit 1 includes a signal amplitude / frequency identification circuit 2 and an interface circuit 3. In the signal amplitude / frequency identification circuit 2, the threshold level of the drive voltage of the switching elements Q1 and Q2 and the frequency range of the switching frequency are set in advance, and the signal level of the output signal from the drive circuit DR is set to the drive voltage. Compared to the threshold level, the frequency of the output signal is compared to the frequency range of the switching frequency. Here, in the signal amplitude / frequency identification circuit 2, if the voltage level of the output signal is equal to or lower than the threshold level, or the polarity is reverse, the output signal is determined to be a communication signal and output to the interface circuit 3. In addition, if the frequency of the output signal is outside the predetermined frequency range (if it is higher than the frequency range), it is determined as a communication signal and output to the interface circuit 3. In the interface circuit 3, the communication signal input from the signal amplitude / frequency identification circuit 2 is transmitted to the outside, and a signal indicating a circuit abnormality detected by the drive circuit DR can be transmitted to the outside.

  As described above, in the switching power supply device of the present embodiment, during the switching stop period, the drive circuit DR outputs a communication signal that does not operate the switching elements Q1 and Q2, and transmits the communication signal to the outside. Therefore, a communication signal can be transmitted to the outside without using an expensive microcomputer having a separate communication port. Note that the communication circuit 1 described in the present embodiment may be applied to the switching power supply device described in the second or third embodiment, and a communication signal can be transmitted to the outside during the switching stop period.

It is a circuit block diagram which shows Embodiment 1 of this invention. (A) (b) is a wave form diagram which shows the gate-source voltage of a switching element same as the above. It is a circuit block diagram which shows Embodiment 2 of this invention. (A) (b) is a wave form diagram which shows the gate-source voltage of a switching element same as the above. It is a circuit block diagram which shows Embodiment 3 of this invention. (A) (b) is a wave form diagram which shows the gate-source voltage of a switching element same as the above. (A) (b) is a wave form diagram which shows the gate-source voltage of a switching element same as the above. It is a figure which shows the relationship between output power same as the above, and a switching frequency or a duty ratio. It is a circuit block diagram which shows Embodiment 4 of this invention. (A)-(c) is a wave form diagram which shows the gate-source voltage of a switching element same as the above. It is a circuit block diagram which shows a prior art example.

Explanation of symbols

DR drive circuit (switching control means)
CT1 Frequency adjustment circuit (frequency adjustment means)
C0 smoothing capacitor C1 capacitor C2 capacitor (rectifying and smoothing means)
D1, D2 diode (rectifying and smoothing means)
DB Diode bridge L1 Inductor Q1, Q2 Switching element T Transformer n1 Primary winding n21, n22 Secondary winding Vdc Voltage across terminal

Claims (8)

  A transformer, a capacitor that forms a resonance circuit with the primary winding of the transformer, one or more switching elements that interrupt current flowing in the primary winding of the transformer, and an AC output generated on the secondary side of the transformer is rectified and smoothed Rectifying and smoothing means for switching, switching control means for controlling switching of the switching element so that a switching operation period for performing switching and a switching stop period for stopping switching are alternately repeated, and DC output rectified and smoothed by the rectifying and smoothing means What is claimed is: 1. A switching power supply comprising: frequency adjusting means for controlling a switching frequency during a switching operation period so that a desired constant value is obtained.   Duty ratio adjusting means for reducing the ratio of the switching operation period to the switching stop period when the input voltage input to the primary side of the transformer is relatively high and increasing the ratio when the input voltage is relatively low The switching power supply device according to claim 1, further comprising:   A duty ratio adjusting means for controlling the ratio of the switching operation period to the switching stop period is provided, and the frequency adjustment is performed when the output power is smaller than the predetermined power value or when the input voltage input to the primary side of the transformer is higher than the predetermined voltage value The means controls the switching frequency to the maximum value, and the duty ratio adjustment means controls the ratio so that the DC output is a desired constant value, and the output power is equal to or higher than a predetermined power value or the input When the voltage is less than or equal to a predetermined voltage value, the duty ratio adjusting means controls the ratio to the maximum value, and the frequency adjusting means controls the switching frequency so that the DC output is a desired constant value. The switching power supply device according to claim 1.   When the output power is smaller than a predetermined power value or when the input voltage is higher than the predetermined voltage value, the frequency adjusting means varies the maximum value of the switching frequency with a predetermined fluctuation range around the predetermined frequency. The switching power supply device according to claim 3.   5. The switching control unit according to claim 1, wherein the switching control unit varies a frequency at which the switching operation period and the switching stop period are alternately repeated with a predetermined fluctuation range centering on the predetermined frequency. 6. The switching power supply device described in 1.   The switching power supply according to any one of claims 1 to 5, wherein the switching control means sets a frequency that alternately repeats a switching operation period and a switching stop period as a frequency outside the audible sound frequency range. apparatus.   The switching power supply device according to any one of claims 1 to 6, wherein the switching control unit starts the switching stop period at a timing when the switching element is turned off.   The switching control means includes a transmission means for outputting a drive signal having a signal level that does not drive the switching element during a switching stop period and transmitting the drive signal to the outside as a communication signal. 8. The switching power supply device according to any one of 1 to 7.

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