JP2015008599A - Switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply device that keeps a holding time in a light load condition within a holding time in a heavy load condition.SOLUTION: The switching power supply device includes: a discharge path forming section 7 for forming a discharge path; a voltage monitoring section 5 for, in response to a stop of input of an AC input voltage, changing a condition of an output end from a first condition to a second condition at the timing when a first delay time elapses; and a load condition detection section 8 for, if the AC input voltage stops in a heavy load condition, outputting a first level signal until a second delay time set longer than the first delay time elapses and outputting a second level signal after the second delay time elapses, and if the AC input voltage stops in a light load condition, on the other hand, keeping on outputting the second level signal. The discharge path forming section 7 does not form the discharge path in the first condition or even in the second condition if the first level signal is input, and forms the discharge path in the second condition and if the second level signal is input.

Description

  The present invention relates to a switching power supply device, and more particularly to a switching power supply device that generates a DC output voltage based on an AC input voltage input from the outside and supplies the DC output voltage to a load.

  As a conventional switching power supply device, for example, the one described in Patent Document 1 is known. As shown in FIG. 3, the switching power supply 10 </ b> B includes a primary side rectifying / smoothing unit 11 that rectifies and smoothes an AC input voltage input via the power plug PL <b> 11, and a voltage output from the primary side rectifying / smoothing unit 11. A transformer T11 having a switching element FET11 for generating a switching voltage, a primary winding N11, a secondary winding N12 and an auxiliary winding N13, and the switching voltage being supplied to the primary winding N11, and a secondary Secondary side rectifying / smoothing unit 12 for rectifying and smoothing the induced voltage induced in winding N12, output detection unit 13 for feeding back the output from secondary side rectifying / smoothing unit 12 to the primary side, Accordingly, a switching control unit 14 for controlling the switching of the switching element FET11 and a voltage monitor for monitoring the AC input voltage. Auxiliary power supply unit 16 having a unit 15 and a capacitor C13 charged by an induced voltage induced in the auxiliary winding N13 and supplying a voltage corresponding to the electric charge accumulated in the capacitor C13 to the switching control unit 14 as a power supply voltage And a discharge path forming part (discharge circuit) 17 for forming a discharge path.

  In the switching power supply device 10B, when the input of the AC input voltage is stopped for some reason during the steady operation, the capacitor C14 of the voltage monitoring unit 15 is discharged through the resistor R14, and the transistor TR11 is turned off. When the transistor TR11 is turned off, the transistor TR12 of the discharge path forming unit 17 is turned on by the charge accumulated in the capacitor C11 of the primary side rectifying and smoothing unit 11, and a discharge path is formed in the discharge path forming unit 17. When the discharge path is formed, the capacitor C13 of the auxiliary power supply unit 16 is discharged through the discharge path, the power supply voltage of the switching control unit 14 rapidly decreases, and the switching control unit 14 stops. When the switching control unit 14 stops, the switching operation of the switching element FET11 stops, and the output from the secondary side rectifying / smoothing unit 12 to the load stops. Thus, according to this switching power supply apparatus 10B, the output to the load can be stopped in a relatively short time after the AC input voltage is stopped.

  FIG. 4 shows another conventional switching power supply apparatus 10C. As shown in the figure, the switching power supply device 10C includes a primary side rectifying / smoothing unit 1, switching means Q2, a transformer T1, a secondary side rectifying / smoothing unit 2, an output detecting unit 3, a PWM control circuit (switching). A control unit 4, a voltage monitoring unit 5, an auxiliary power supply unit 6, and a discharge circuit (discharge path forming unit) 7.

  The primary side rectifying / smoothing unit 1 includes a diode bridge BD1 that rectifies an AC input voltage input from a commercial AC power supply ACV, a smoothing capacitor C1 that smoothes the rectified AC input voltage, and a diode bridge BD1 and a smoothing capacitor C1. And a power factor correction circuit connected to. The power factor correction circuit is provided to improve the power factor, and includes a coil L, a switching element (MOSFET) Q1, and a rectifier diode D1.

  The switching means Q2 includes a switching element (FET) that generates a switching voltage by switching the DC voltage output from the primary side rectifying and smoothing unit 1. The switching means Q2 performs a switching operation under the control of the PWM control circuit 4.

  The transformer T1 has a primary winding N1, a secondary winding N2, and an auxiliary winding N3. One end of the primary winding N1 (for the transformer T1, the side marked with ● is the one end side and the opposite side is the other end side; the same shall apply hereinafter) is connected to the primary side rectifying and smoothing unit 1, while the primary The other end of the winding N1 is connected to the switching means Q2. A switching voltage is supplied to the primary winding N1.

  The secondary side rectifying / smoothing unit 2 includes a rectifier diode D2 having an anode connected to the other end of the secondary winding N2, one end connected to the cathode of the rectifier diode D2, and the other end connected to one end of the secondary winding N2. And a smoothing capacitor C2 connected to the. The secondary side rectifying / smoothing unit 2 rectifies the induced voltage induced in the secondary winding N2 by the rectifying diode D2 and smoothes it by the smoothing capacitor C2, thereby generating a DC output voltage to be supplied to the load, The DC output voltage is output.

  The output detection unit 3 outputs an output voltage detection circuit for detecting the DC output voltage output from the secondary side rectifying / smoothing unit 2, and a signal relating to the amount of the DC output voltage detected by the output voltage detection circuit. 4 and a photocoupler PC1 that outputs to 4.

  The PWM control circuit 4 controls the switching operation of the switching means Q2 so that the DC output voltage is constant. In the PWM control circuit 4, the terminal voltage of the smoothing capacitor C <b> 1 is input to the VH terminal via the starting resistor R <b> 5 during startup, and the power supply voltage generated by the auxiliary power supply unit 6 is input to the Vcc terminal during steady operation.

  The voltage monitoring unit 5 includes a rectifier diode D4 having an anode connected to one input terminal of the primary side rectifying / smoothing unit 1, a resistor R3 having one end connected to the cathode of the rectifier diode D4, and one end being the other end of the resistor R3. And a transistor Q3 having a base connected to one end of the parallel circuit and an emitter connected to the other end of the parallel circuit. ing. The transistor Q3 is turned on during the steady operation, and is turned off when the input of the AC input voltage is stopped. In other words, the collector of the transistor Q3 (the output terminal of the voltage monitoring unit 5) is in a low level state (first state) during steady operation, while it is in a high level state (second state) when the input of the AC input voltage is stopped. State).

  By the way, in the switching power supply device 10C, when the input of the AC input voltage is stopped during the steady operation, the switching voltage is generated from the voltage across the smoothing capacitor C1. For this reason, when a certain amount of time elapses after the AC input voltage is stopped, the switching voltage is generated in a state where the voltage across the smoothing capacitor C1 is greatly reduced, which may cause malfunction of the switching means Q2 and the like. In order to prevent this malfunction, the voltage monitoring unit 5 is provided in order to detect the input stop of the AC input voltage at an early stage.

  Further, in the switching power supply device 10C, the voltage monitoring unit 5 is provided with a parallel circuit composed of the resistor R4 and the capacitor C4, and the timing at which the transistor Q3 is turned off is delayed, so that even after the AC input voltage is stopped, A time for holding the DC output voltage (hereinafter, holding time) is secured. For example, if the load includes a microcomputer and non-volatile memory, even if the input of AC input voltage is stopped when the load is operating (in a heavy load state), the data in the microcomputer is not stored during this holding time. It can be saved in a non-volatile memory.

  When the load is heavy, the holding time is relatively short because the power consumption of the load is large, and when the load is in a light load state (standby state), the power consumption of the load is small and is relatively long. Become.

  The auxiliary power supply unit 6 includes a rectifier diode D6 having an anode connected to the other end of the auxiliary winding N3, a capacitor having one end connected to the cathode of the rectifier diode D6 and the other end connected to one end of the auxiliary winding N3. And C7. The auxiliary power supply unit 6 outputs the terminal voltage of the capacitor C7 charged by the induced voltage induced in the auxiliary winding N3 to the Vcc terminal of the PWM control circuit 4 as a power supply voltage.

  The discharge circuit 7 includes a transistor Q4 whose base is connected to the collector of the transistor Q3 of the voltage monitoring unit 5 and whose emitter is grounded, one end connected to one end of the capacitor C7 of the auxiliary power supply unit 6, and the other end The resistor R9 is connected to the base of Q4, and the resistor R10 has one end connected to one end of the capacitor C7 and the other end connected to the collector of the transistor Q4. In the discharge circuit 7, when the input of the AC input voltage is stopped, the transistor Q3 of the voltage monitoring unit 5 is turned off, so that the transistor Q4 is turned on and a discharge path for discharging the capacitor C7 of the auxiliary power supply unit 6 is formed.

  Next, with reference to FIG. 5, the operation of the switching power supply device 10 </ b> C when the input of the AC input voltage is stopped during the steady operation will be described.

In the following description, the load is intended to include a microcomputer and a nonvolatile memory, when the heavy load state, and T _A the retention time necessary for saving the data in the microcomputer in the nonvolatile memory after input stop the AC input voltage To do. Further, the voltage across the smoothing capacitor C1 at the time of steady operation and V _1, the voltage across the smoothing capacitor C1 is required to hold the DC output voltage after the input stops the AC input voltage is V _2.

In the switching power supply apparatus 10C, as the time and holding time T _A to a voltage across the smoothing capacitor C1 at the time of heavy load state is from V ₁ to V ₂ is the same level, to decide the capacity of the smoothing capacitor C1 Yes. Further, in the switching power supply apparatus 10C, the transistor Q3 after a lapse of the holding time T _A is to be turned off, and setting the time constant of the parallel circuit composed of a capacitor C4 and a resistor R4 and to have some margin.

As shown in FIG. 5, when the input of the AC input voltage is stopped at time t ₁ (FIG. 5A), the capacitor C4 of the voltage monitoring unit 5 starts to discharge with a preset time constant via the resistor R4. (FIG. 5 (b)), the transistor Q3 at time _{t 5} later than the time _{t 3} which is the end of the holding time _{T a} is turned off (FIG. 5 (c)).

When the transistor Q3 is turned off at time t _5, the transistor Q4 of the discharging circuit 7 is turned on discharge path is formed (FIG. 5 (d)), PWM control circuit 4, the power supply voltage Vcc is rapidly lowered (Fig. 5 (E)) At time t ₅ ′, the PWM control circuit 4 stops. As a result, at time t ₅ ′, the switching operation of the switching means Q2 (discharge of the smoothing capacitor C1) stops, and the DC output voltage to the load stops (FIG. 5 (f)).

After all, in the switching power supply apparatus 10C, since the DC output voltage to the load at time t _{5 'later} than the time t ₃ which is the end of the holding time T _A is to be stopped, reliably secure the retention time T _A can do.

Utility Model Registration No. 2551390

Incidentally, when the load is a light load, for example because the time or the like for saving the data in the microcomputer in the non-volatile memory is not required, it is not necessary to hold time T ₂ holds more time T _A of the light load state , retention time T ₂ of the light load state rather it is preferable equal to or less than the retention time T _a.

However, in the conventional switching power supply device 10C, the time from when the input of the AC input voltage is stopped until the output of the DC output voltage to the load is stopped is the capacitor C4 of the voltage monitoring unit 5 and since the cause determined uniformly by the time constant of the resistor R4, when you try to ensure the retention time T _a at the time of heavy load state, there is a problem that the retention time T ₂ at light load condition becomes unnecessarily long.

  Therefore, in the conventional switching power supply device 10C, the holding time in the light load state cannot be made equal to or less than the holding time in the heavy load state.

  This invention is made | formed in view of the said situation, The place made into the subject is providing the switching power supply device which can make the holding time at the time of a light load state below the holding time at the time of a heavy load state. It is in.

  In order to solve the above-described problems, a switching power supply according to the present invention includes (1) a transformer and switching means connected to a primary side of the transformer, and a DC output voltage based on an AC input voltage input from the outside. A switching power supply that generates and supplies a DC output voltage to a load connected to the secondary side of the transformer, the switching control unit controlling the switching of the switching means, and the induced voltage induced in the auxiliary winding of the transformer And an auxiliary power supply unit that outputs a voltage corresponding to the electric charge accumulated in the capacitor as a power supply voltage to the switching control unit, and monitors the AC input voltage and stops the input of the AC input voltage. A voltage monitoring unit that changes the state of the output terminal from the first state to the second state at the timing when one delay time has elapsed, and the load is heavy When the AC input voltage stops at the time of the state, the first level signal is output until the second delay time set longer than the first delay time elapses, and after the second delay time elapses, the second level signal is output. A load state detection unit that outputs a level signal and continues to output a second level signal when the AC input voltage stops when the load is in a light load state, and a state of the output end of the voltage monitoring unit and a load state detection unit A discharge path forming unit that stops the switching control unit by forming a discharge path of the charge accumulated in the capacitor based on a signal from the discharge path forming unit when the output terminal is in the first state or the output The discharge path is not formed when the first level signal is input even when the end is in the second state, and the discharge path is formed when the output terminal is in the second state and the second level signal is input. To do.

  According to this configuration, the discharge path forming unit forms the discharge path at the timing when the second delay time has elapsed when the load is in a heavy load state, and is set shorter than the second delay time when in a light load state. Since the discharge path is formed at the timing when the first delay time has elapsed, the holding time in the light load state can be made equal to or less than the holding time in the heavy load state.

  The “heavy load state” in the present specification refers to a state in which a relatively large amount of power is consumed, such as a state in which the microcomputer is operated, and the “light load state” in the present specification refers to, for example, A state in which almost no power is consumed, such as a standby state.

  In the switching power supply device according to (1), for example, (2) the voltage monitoring unit is a switching unit in which an output end is connected to the discharge path forming unit, and the load state detecting unit is connected to one end of the discharge path forming unit. A voltage comparison unit that compares a fluctuation voltage that varies in accordance with a current flowing through the predetermined reference voltage with a predetermined reference voltage, and switches the output between the first level signal and the second level signal when the magnitude relationship between the fluctuation voltage and the reference voltage changes. The reference voltage is set to be smaller than the fluctuation voltage in the heavy load state and larger than the fluctuation voltage in the light load state, and the voltage comparison unit is in the heavy load state. When the input of the AC input voltage is stopped, it can be configured to include a variable voltage adjustment circuit that reduces the variable voltage to the reference voltage in the second delay time.

  In the switching power supply device of (2), for example, (3) the fluctuation voltage adjusting circuit has a first capacitor charged by a current flowing through the switching means and a first resistor for discharging the first capacitor connected in parallel. It can consist of a first parallel circuit.

  In the switching power supply of (2) or (3), for example, (4) the voltage monitoring unit connects in parallel a second capacitor that is charged by an AC input voltage and a second resistor that discharges the second capacitor. When the second parallel circuit is included, the first delay time can be determined by the time constant of the second parallel circuit.

  In the switching power supply devices of (2) to (4) above, for example, (5) the voltage comparison unit has a variable voltage input to the inverting input terminal, a reference voltage input to the non-inverting input terminal, and output from the output terminal. You may have the comparator from which a signal is output.

  In the switching power supply device of (5), for example, (6) the voltage comparison unit includes a third resistor connected in series to a low potential line connecting the primary side rectifying / smoothing unit and the switching unit, and an anode serving as the switching unit and A first rectifier diode connected to the connection point of the third resistor and having a cathode connected to the inverting input terminal of the comparator; a second rectifier diode having an anode connected to the auxiliary winding; and a cathode of the second rectifier diode; A voltage dividing circuit connected between the non-inverting input terminal of the comparator and a Zener diode having a cathode connected to the voltage dividing circuit and an anode grounded may be further included.

  ADVANTAGE OF THE INVENTION According to this invention, the switching power supply device which can make holding time at the time of a light load state below the holding time at the time of a heavy load state can be provided.

1 is a block diagram of a switching power supply device according to the present invention. It is a graph which shows operation | movement of the switching power supply device which concerns on this invention when the input of alternating current input voltage stops. It is a block diagram of the conventional switching power supply device. It is a block diagram of another conventional switching power supply device. It is a graph which shows operation | movement of another conventional switching power supply device when the input of alternating current input voltage stops.

  Hereinafter, preferred embodiments of a switching power supply according to the present invention will be described with reference to the accompanying drawings. Note that, among the components shown in FIG. 1, the components given the same reference numerals as those in FIG. 4 are the same as those described in the related art, and thus the description thereof is partially omitted here.

[Constitution]
FIG. 1 shows a switching power supply apparatus 10A according to an embodiment of the present invention. As shown in the figure, the switching power supply device 10A has a configuration in which a voltage comparison unit (load state detection unit) 8 is added to the conventional switching power supply device 10C shown in FIG.

  The voltage comparison unit 8 includes a comparator Amp, resistors R1, R2, R6 to R8, rectifier diodes D3 and D5, capacitors C3 and C5, and a Zener diode ZD1.

  The resistor R1 is connected in series to a low potential line connecting the primary side rectifying / smoothing unit 1 and the switching means Q2. The rectifier diode D3 has an anode connected to a connection point between the switching means Q2 and the resistor R1, and a cathode. Is connected to the inverting input terminal (−) of the comparator Amp.

  The capacitor C3 and the resistor R2 are connected in parallel. One end of a parallel circuit (fluctuation voltage adjusting circuit) including the capacitor C3 and the resistor R2 is connected to the cathode of the first rectifier diode D3 and the inverting input terminal (−) of the comparator Amp, and the other end is grounded. Here, the voltage between both ends of the capacitor C3 is maintained at a constant voltage while repeating charging and discharging during a steady operation, but decreases when the charging / discharging balance is lost due to the input stop of the AC input voltage.

  In the present embodiment, when the fluctuation voltage is larger than the reference voltage, the first delay time set in advance (the time from when the capacitor C4 starts discharging until the transistor Q3 is turned off) is longer. The capacitor C3 and the resistor R2 are selected so that the fluctuating voltage falls below the reference voltage at the timing when the second delay time set in (2) has elapsed.

  In the comparator Amp, a fluctuation voltage (terminal voltage of the capacitor C3) that fluctuates according to the current flowing through the switching means Q2 is input to the inverting input terminal (−), and a reference voltage is input to the non-inverting input terminal (+). The voltage and the reference voltage are compared. When the load is in a heavy load state, the current flowing through the switching means Q2 increases. For this reason, the capacitor C3 is charged by a relatively large current, and as a result, the fluctuation voltage becomes large. On the other hand, when the load is in a light load state, the current flowing through the switching means Q2 becomes small. For this reason, the capacitor C3 is charged by a relatively small current, and as a result, the fluctuation voltage becomes small. Here, the magnitude of the fluctuation voltage is not the magnitude of the pulsating flow, but the magnitude of the direct current component excluding the pulsating flow.

  The reference voltage is set to be smaller than the fluctuation voltage in the heavy load state and larger than the fluctuation voltage in the light load state. Specifically, the Zener diode ZD1 and the resistors R6 to R8 (voltage dividing circuit) are selected according to the reference voltage.

  Since the output terminal of the comparator Amp is connected to the base of the transistor Q4 via the resistor R9 of the discharge circuit 7, when the fluctuation voltage is larger than the reference voltage, a low level signal (first level) is supplied to the base of the transistor Q4. Signal) and a variable voltage is smaller than the reference voltage, a high level signal (second level signal) is input to the base of the transistor Q4. For this reason, in the discharge circuit 7, even if the transistor Q3 of the voltage monitoring unit 5 is turned off by stopping the input of the AC input voltage, a discharge path is not formed unless a high level signal is input from the comparator Amp.

  The anode of the diode D5 is connected to the other end of the auxiliary winding N3 via the rectifier diode D6 of the auxiliary power supply unit 6 (for the transformer T1, the side marked with ● is the one end side and the opposite side is the other end side. The same applies hereinafter), and the cathode is connected to the non-inverting input terminal (+) of the comparator Amp through a voltage dividing circuit composed of resistors R6, R7, and R8. The Zener diode ZD1 has a cathode connected to a connection point between the resistor R7 and the resistor R8, and an anode grounded. One end of the resistor R6 is connected to the connection point between the resistor R7 and the non-inverting input terminal (+) of the comparator Amp, and the other end is grounded.

  One end of the capacitor C5 is connected to the connection point between the diode D5 and the resistor R8, and the other end is grounded.

[Operation]
Next, with reference to FIG. 2, the operation of the switching power supply device 10A when the input of the AC input voltage is stopped during the steady operation will be described separately for the heavy load state and the light load state.

In the following description, the load is intended to include a microcomputer and a nonvolatile memory, when the heavy load state, and T _A the retention time necessary for saving the data in the microcomputer in the nonvolatile memory after input stop the AC input voltage To do. Further, the voltage across the smoothing capacitor C1 at the time of steady operation and V _1, the voltage across the smoothing capacitor C1 is required to hold the DC output voltage after the input stops the AC input voltage is V _2.

Further, in this embodiment, as the time and holding time T _A to a voltage across the smoothing capacitor C1 at the time of heavy load state is from V ₁ to V ₂ is the same extent, determine the capacitance of the smoothing capacitor C1 It shall be.

(In heavy load condition)
When the load is in a heavy load state, as shown in FIG. 2, when the input of the AC input voltage is stopped at time t ₁ (FIG. 2 (a)), the capacitor C4 of the voltage monitoring unit 5 is preset via the resistor R4. Discharge starts at the set time constant (FIG. 2B).

The time constant of the parallel circuit of the capacitor C4 and the resistor R4 are set so that capacitor C4 is the time from the start of the discharge until transistor Q3 is turned off (first delay time) is shorter than the holding time T _A Has been. Specifically, at time t ₂ before the time t ₃ which is the end of the holding time T _A, the transistor Q3 is set to OFF (FIG. 2 (c)).

The voltage comparator 8, the capacitor C3 begins to discharge through the resistor R2 at time t _1, thereby the voltage across the capacitor C3 (the voltage change) starts to decrease (Figure 2 (d)). In the voltage comparison unit 8, the time (second delay time) until the fluctuation voltage input to the inverting input terminal (−) of the comparator Amp falls below the reference voltage input to the non-inverting input terminal (+) is as follows. It is set to be longer than the holding time T _A. Specifically, at time t ₄ later than the end time that is t ₃ of the retention time T _A, it is set so that fluctuations voltage falls below the reference voltage (Figure 2 (d)).

Therefore, the voltage comparator 8, until the variable voltage at time t ₄ is less than the reference voltage, a low-level signal from the output terminal of the comparator Amp is output, when the variable voltage falls below the reference voltage at time t _4, The polarity of the output signal of the comparator Amp changes to a high level (the upper diagram in FIG. 2 (e)).

In the discharge circuit 7, at time t ₂ ~t _4, since the low-level signal from the comparator Amp voltage comparators 8 is input, the transistor Q4 also transistor Q3 of the voltage monitoring unit 5 has been turned off it is turned off Thus, no discharge path is formed (the upper diagram in FIG. 2 (f)). On the other hand, the time t ₄ later, the transistor Q3 of the voltage monitoring unit 5 are turned off, and the high level signal from the comparator Amp voltage comparators 8 is input, the discharge path transistor Q4 is turned ON form (FIG. 2 (f) upper diagram).

In the PWM control circuit (switching control unit) 4, since the discharge path is not formed in the discharge circuit 7 during the time t _{2 to} t ₄ , the power supply voltage Vcc is maintained (the upper diagram in FIG. 2G). As a result, from time t _{2 to} t ₄ , the switching operation of the switching element FET 11 is continued, and the power stored in the smoothing capacitor C 1 of the primary side rectifying and smoothing unit 1 is used to generate the DC output voltage to the load. The output continues (FIG. 2 (h)).

On the other hand, when the discharge path in the discharge circuit 7 at time _{t 4} is formed, the power supply voltage Vcc in the PWM control circuit 4 is rapidly lowered (Fig. 2 (g) shown above), the PWM control circuit 4 at time _{t 4} ' Stops. As a result, at time t ₄ ′, the switching operation of the switching means Q2 (discharge of the smoothing capacitor C1) stops, and the DC output voltage to the load stops (FIG. 2 (h)).

After all, in the switching power supply apparatus 10A, since the DC output voltage to the load at time t _{4 'is} later than the time t ₃ which is the end of the holding time T _A is to be stopped, the retention time of the heavy load state T ₁ (= T _A ), and the holding time T _A can be reliably ensured.

(At light load)
If the load is a light load state, when the input is stopped for the AC input voltage at the time t ₁ (FIG. 2 (a)), the voltage monitoring unit 5, the transistor Q3 in the same manner as when a heavy load state time t ₂ is turned off (FIG. 2 (c)).

The voltage comparator 8, since the reference voltage is set to be larger than the variation voltage at light load conditions (FIG. 2 (d)), from the output terminal of the comparator Amp, already at the time of time t ₁ A high level signal is output (the lower diagram in FIG. 2 (e)).

In the discharge circuit 7, at time t _2, transistor Q3 of the voltage monitoring unit 5 is turned off, and since the high level signal from the comparator Amp is input, the transistor Q4 is turned on, and the discharge path is formed (FIG. 2 (F) Below).

The PWM control circuit 4, the power supply voltage Vcc starts to rapidly decrease (Fig. 2 (g) below) at time _{t 2,} the PWM control circuit 4 is stopped at time _{t 2} '. As a result, at time t ₂ ′, the switching operation of the switching means Q2 (discharge of the smoothing capacitor C1) stops, and the DC output voltage to the load stops (FIG. 2 (h)).

After all, in the switching power supply apparatus 10A, since the DC output voltage to the load in the time that is t ₃ time t _{2 'before} the end of the holding time T _A is to be stopped, the retention time of the light load state T ₂ (= t ₂ ′ −t ₁ ) can be made shorter than the holding time T ₁ (= t ₃ −t ₁ ) in the heavy load state.

Therefore, the switching power supply apparatus 10A according to the embodiment of the present invention, while reliably ensuring the holding time T _A, it is possible to shorten the holding time T ₂ of the light load state.

  As mentioned above, although preferable embodiment of the switching power supply device which concerns on this invention was described, this invention is not limited to the said embodiment.

For example, in the above embodiment, by setting shorter than the holding time T _A of the first delay time, although shorter than the holding time T ₁ of the heavy load condition holding time T ₂ of the light load state , by setting the same extent as the holding time T _a of the first delay time, it can be to the same extent as the holding time T ₁ of the heavy load condition holding time T ₂ of the light load state.

In the above embodiment, as the time and holding time T _A to a voltage across the smoothing capacitor C1 at the time of heavy load state is from V ₁ to V ₂ is the same extent, determine the capacitance of the smoothing capacitor C1 and is but the time until the voltage across the smoothing capacitor C1 is composed of V ₁ to V ₂ is to be longer than the holding time T _a, may be determined capacitance of the smoothing capacitor C1.

  As the switching power supply device according to the present invention, the flyback converter has been described as an example in the above embodiment, but the present invention can also be suitably applied to other converters such as a forward converter and a half-bridge converter.

DESCRIPTION OF SYMBOLS 1 Primary side rectification smoothing part 2 Secondary side rectification smoothing part 3 Output detection part 4 Switching control part (PWM control circuit)
5 Voltage monitoring unit 6 Auxiliary power supply unit 7 Discharge path forming unit (discharge circuit)
8 Voltage comparator

Claims (6)

A load having a transformer and switching means connected to the primary side of the transformer, generating a DC output voltage based on an AC input voltage input from the outside, and connecting the DC output voltage to the secondary side of the transformer A switching power supply for supplying to
A switching control unit for controlling switching of the switching means;
An auxiliary power supply unit having a capacitor charged by an induced voltage induced in the auxiliary winding of the transformer, and outputting a voltage corresponding to the electric charge accumulated in the capacitor to the switching control unit as a power supply voltage;
A voltage monitoring unit that monitors the AC input voltage and changes the state of the output end from the first state to the second state at a timing when a first delay time has elapsed when the input of the AC input voltage is stopped;
If the AC input voltage is stopped when the load is in a heavy load state, a first level signal is output until a second delay time set longer than the first delay time elapses, and the second delay A load state detection unit that outputs a second level signal after time has elapsed, and continues to output the second level signal when the AC input voltage stops when the load is in a light load state;
A discharge path forming unit that stops the switching control unit by forming a discharge path of the charge accumulated in the capacitor based on the state of the output terminal of the voltage monitoring unit and a signal from the load state detection unit; Prepared,
The discharge path forming unit does not form the discharge path when the output terminal is in the first state or when the first level signal is input even when the output terminal is in the second state, and the output terminal is in the second state. The switching power supply device is characterized in that the discharge path is formed when the second level signal is input in a state. The voltage monitoring unit has the output terminal connected to the discharge path forming unit,
The load state detection unit compares a fluctuation voltage that varies according to a current flowing through the switching means, one end of which is connected to the discharge path formation unit, and a predetermined reference voltage, and A voltage comparison unit that switches the output between the first level signal and the second level signal when the magnitude relationship changes;
The reference voltage is set to be smaller than the fluctuation voltage in the heavy load state and larger than the fluctuation voltage in the light load state,
The voltage comparison unit includes a fluctuation voltage adjustment circuit that reduces the fluctuation voltage to the reference voltage in the second delay time when the input of the AC input voltage is stopped in the heavy load state. The switching power supply device according to claim 1.   The variable voltage adjustment circuit includes a first parallel circuit in which a first capacitor charged by a current flowing through the switching unit and a first resistor for discharging the first capacitor are connected in parallel. Item 3. The switching power supply device according to Item 2. The voltage monitoring unit includes a second parallel circuit in which a second capacitor charged by the AC input voltage and a second resistor for discharging the second capacitor are connected in parallel.
4. The switching power supply device according to claim 2, wherein the first delay time is determined by a time constant of the second parallel circuit. 5.   The voltage comparison unit includes a comparator in which the variable voltage is input to an inverting input terminal, the reference voltage is input to a non-inverting input terminal, and the output signal is output from an output terminal. The switching power supply device in any one of 2-4. The voltage comparison unit
A third resistor connected in series to a low potential line connecting the primary side rectifying and smoothing unit and the switching means;
A first rectifier diode having an anode connected to a connection point of the switching means and the third resistor, and a cathode connected to an inverting input terminal of the comparator;
A second rectifier diode having an anode connected to the auxiliary winding;
A voltage dividing circuit connected between a cathode of the second rectifier diode and a non-inverting input terminal of the comparator;
A Zener diode having a cathode connected to the voltage dividing circuit and an anode grounded;
The switching power supply device according to claim 5, further comprising:

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