JP2022095331A - Switching power source device - Google Patents

Abstract

To suppress the occurrence of overshoot in output voltage at power source start, in an insulated type switching power source device.SOLUTION: A switching power source device according to an embodiment is an insulated type switching power source device having a switching transistor, a sense resistor, a transformer, a photocoupler, an output voltage feedback circuit, a current source, a voltage detection circuit, and a control circuit. The output voltage feedback circuit generates a photocoupler current corresponding to an output voltage higher than a target voltage. The voltage detection circuit, when the output voltage is lower than the target voltage, makes the current source connected with a cathode of the photodiode generate a current to generate a photocoupler current corresponding to an output voltage lower than the target voltage, and when the output voltage reaches a predetermined detection voltage, stops generation of the current by the current source. The control circuit controls an ON/OFF operation of the switching transistor on the basis of a sense voltage of the sense resistor and the photocoupler current.SELECTED DRAWING: Figure 1

Description

Embodiments of the present specification relate to a switching power supply device.

Conventionally, an isolated switching power supply device as a DC / DC converter using a transformer and a photocoupler is known. In this switching power supply device, the photocoupler current starts to flow when the output voltage on the secondary side reaches the set value, and switching control by the control circuit on the primary side is started.

Japanese Unexamined Patent Publication No. 6-046560

However, in a configuration in which switching control is started by the photocoupler current generated when the output voltage reaches the set value, if the output voltage rises sharply when the power supply is started, the output voltage exceeds the set value at the control start time. As a result, overshoot may occur in the output voltage.

An object of the present invention has been made in view of the above, and is to suppress the occurrence of overshoot of the output voltage at the time of starting the power supply in the isolated switching power supply device.

In order to solve the above-mentioned problems and achieve the object, the switching power supply device according to the embodiment includes a switching transistor, a sense resistor, a transformer, a photocoupler, an output voltage feedback circuit, a current source, and a voltage detection. It is an isolated switching power supply device including a circuit and a control circuit. The sense resistor generates a sense voltage according to the current flowing when the switching transistor is turned on. The transformer has a primary winding to which an input voltage is applied when the switching transistor is turned on, and a secondary winding to which a load is connected. The photocoupler has a phototransistor provided on the primary side and a photodiode provided on the secondary side, and a photocoupler current is generated in the phototransistor according to the current flowing through the photodiode. In the output voltage feedback circuit, when the output voltage obtained by rectifying and smoothing the voltage generated in the secondary winding exceeds a predetermined target voltage, the photocoupler current according to the difference between the output voltage and the target voltage. Is generated in the photodiode. The current source is connected to the cathode of the photodiode of the photocoupler. When the output voltage is lower than the target voltage, the voltage detection circuit generates a current in the current source to generate the photocoupler current corresponding to the output voltage lower than the target voltage in the photodiode. When the output voltage reaches a predetermined detection voltage, the generation of current by the current source is stopped. The control circuit controls the ON / OFF operation of the switching transistor based on the sense voltage and the photocoupler current.

According to the present invention, in an isolated switching power supply device, it is possible to suppress the occurrence of overshoot of the output voltage when the power supply is started.

FIG. 1 is a diagram showing an example of a configuration of a switching power supply device according to an embodiment. FIG. 2 is a diagram showing an example of the configuration of the output voltage detection circuit according to the embodiment. FIG. 3 is a diagram showing an example of the configuration of a switching power supply device not equipped with a photocoupler current adjusting circuit, unlike the switching power supply device according to the embodiment. FIG. 4 is a diagram showing an example of an operation waveform of each signal when the power supply is started in the switching power supply device of FIG. FIG. 5 is a diagram showing an example of an operation waveform of each signal when the power supply is started in the output voltage detection circuit according to the embodiment. FIG. 6 is a diagram showing an example of an operation waveform of each signal when the power supply is started in the switching power supply device according to the embodiment. FIG. 7 is a diagram showing another example of the configuration of the output voltage detection circuit according to the embodiment. FIG. 8 is a diagram showing another example of the configuration of the switching power supply device according to the embodiment.

Hereinafter, embodiments of the switching power supply device will be described in detail with reference to the drawings. In the following embodiments, the parts with the same reference numerals perform the same operation, and duplicate description will be omitted as appropriate. In the following embodiments, "connection" means "electrical connection".

FIG. 1 is a diagram showing an example of the configuration of the switching power supply device 1 according to the embodiment. The switching power supply device 1 is an isolated switching power supply circuit as a DC / DC converter. The switching power supply device 1 uses the input voltage _VIN of the DC voltage supplied from the pair of input terminals on the primary side, and the DC voltage set according to the load connected to the pair of output terminals on the secondary side. Is configured to supply the output voltage V _OUT of. As the load, any circuit element or circuit configuration can be appropriately used.

As shown in FIG. 1, the switching power supply device 1 includes a capacitor C _IN , a transformer 11, a diode D ₁ , a capacitor C _OUT , an µ switching transistor _MPW , a sense resistor _RS , a control circuit 13, and an output voltage feedback (FB) circuit. 15. It has a photocoupler PC ₁ and a photocoupler current adjusting circuit 17.

Capacitors C _IN are connected in parallel to the pair of input terminals on the primary side. The capacitor C _IN is a capacitive element that stores electric charges supplied from a pair of input terminals on the primary side. The negative input terminals of the pair of primary input terminals are connected to the ground potential. The positive input terminals of the pair of primary input terminals are connected to _one end of the primary winding T1 of the transformer 11 and the control circuit 13.

One end of the _primary winding T1 opposite to the positive input terminal is connected to the drain of the MIMO switching transistor _MPW . That is, the input voltage _VIN is applied to the primary winding T ₁ when the MIMO switching transistor _MPW is turned on. A rectifying smoothing circuit using a diode D ₁ and a capacitor C _OUT is connected to the secondary winding T ₂ of the transformer 11. Specifically, the anode of the diode D ₁ is connected to one end of the secondary winding T ₂ . A capacitor C _OUT is connected between the other end of the secondary winding T ₂ and the cathode of the diode D ₁ . The capacitor C _OUT is a capacitive element that stores the electric charge supplied from the secondary winding T ₂ .

The pair of output terminals on the secondary side are connected in parallel to the capacitor C _OUT . That is, the pair of output terminals on the secondary side are connected to the rectifying smoothing circuit by the diode D ₁ and the capacitor C _OUT . The negative output terminals of the pair of secondary output terminals are connected to the ground potential. The pair of output terminals on the secondary side are connected to an external load of the switching power supply device 1. In other words, the external load of the switching power supply 1 is connected to the _secondary winding T2 via the pair of output terminals on the secondary side.

The IGMP switching transistor _MPW is a switching element that operates in response to a PWM (Pulse Width Modulation) signal from the control circuit 13.

The sense resistor _RS is a resistance element that detects the drain current flowing through the MIMO switching transistor _MPW . In other words, the sense resistor _RS is a resistance element that generates a sense voltage according to the current flowing when the MIMO switching transistor _MPW is turned on. One end of the sense resistor _RS is connected to the source of the MIMO switching transistor _MPW , and the other end is connected to the ground potential.

The control circuit 13 is a circuit that controls the ON / OFF operation of the IGMP switching transistor _MPW based on the sense voltage from the sense resistance _RS and the photocoupler current from the photocoupler PC ₁ . The control circuit 13 is connected to the gate and source of the IGMP switching transistor _MPW , respectively. Further, the control circuit 13 is connected to the collector of the phototransistor of the photocoupler PC ₁ . The control circuit 13 generates a PWM signal based on the sense voltage generated in the sense resistor _RS and the photocoupler current from the phototransistor of the photocoupler PC ₁ , and the generated PWM signal is used in the MIMO switching transistor _MPW . It is a circuit configured to output to the gate. That is, the control circuit 13 can change the duty D at the time of output voltage control by modulating the PWM signal.

Here, the duty D at the time of output voltage control in the flyback type switching power supply device 1 can be expressed by the following equation (1). Here, N ₁ and N ₂ are the number of windings of the primary winding T ₁ and the secondary winding T ₂ of the transformer 11, respectively.

The output voltage FB circuit 15 rectifies and smoothes the voltage generated in the _secondary winding T2, and when the output voltage V _OUT exceeds a predetermined target voltage value V _OUT 1, the output voltage V _OUT and the target voltage value V This circuit is configured to generate a current corresponding to the difference from _OUT 1 (see FIG. 5) in the photodiode of the photocoupler PC ₁ . As shown in FIG. 1, the output voltage FB circuit 15 includes a resistor R ₁ , a resistor R ₂ , a resistor R ₃ , a resistor R ₄ , a resistor R _NF , a capacitor C _NF , and a shunt regulator IC.

The resistance R ₁ and the resistance R ₂ are connected in series. One end of the resistance R ₂ is connected to the output terminal on the positive side, and the other end of the resistance R ₂ is connected to one end of the resistance R ₁ . The other end of the resistor R ₁ is connected to the ground potential. The resistance R ₃ and the resistance R ₄ are connected in series. One end of the resistance R ₃ is connected to the output terminal on the positive side, and the other end of the resistance R ₃ is connected to one end of the resistance R ₄ . The resistor R _NF and the capacitor C _NF are connected in series. One end of the resistor R _NF is connected between the resistors R ₁ and R ₂ , the other end of the resistor R _NF is connected to one end of the capacitor C _NF , and the other end of the capacitor C _NF is connected to the other end of the resistor R ₄ . Will be done.

The anode of the shunt regulator IC is connected to the ground potential. The cathode of the shunt regulator IC is connected to the other end of the resistor _R4 . The reference of the shunt regulator IC is connected between the resistors _R1 and _R2 .

The photocoupler PC ₁ has a photodiode and a phototransistor. The photocoupler PC ₁ is a circuit element that generates a photocoupler current in a phototransistor according to a current flowing through a photodiode. The photodiode is provided on the secondary side and is connected in parallel with the resistor _R4 . Specifically _, the anode of the photodiode is connected between the resistors R3 and _R4 . The cathode of the photodiode is connected between the resistor _R4 and the cathode of the shunt regulator IC. The phototransistor is provided on the primary side. The base of the phototransistor is configured to input light from the photodiode. The emitter of the phototransistor is connected to the ground potential.

The photocoupler current adjusting circuit 17 is a circuit configured to generate a photocoupler current in the photocoupler PC ₁ according to an output voltage V _OUT lower than the target voltage value V _OUT 1. The optocoupler current adjusting circuit 17 has a current source and an output voltage detecting circuit 19 (voltage detecting circuit).

The current source has an _{µtransistor MSF} and a resistor _RSF . The current source generates a current in the resistor _RSF when the Now's transistor _MSF is turned on. One end of the current source is connected to the cathode of the photodiode of the photocoupler PC ₁ . Specifically, the drain of the MIMO transistor _MSF is connected to the cathode of the photodiode of the photocoupler PC ₁ . The source of the MIMO transistor _MSF is connected to one end of the resistor _RSF . The other end of the resistor _RSF is connected to the ground potential. That is, the other end of the current source (one end opposite to the cathode of the photodiode) is connected to the ground potential.

In this embodiment, a current source having an _{µtransistor MSF and} a resistance RSF is exemplified, but the present invention is not limited to this. For example, a bipolar transistor may be used instead of the NOTE transistor _MSF . In this case, in the description of the embodiment, the gate, source, and drain of the MIMO transistor _MSF can be read as the base, emitter, and collector of the bipolar transistor, respectively.

The output voltage detection circuit 19 is a circuit configured to control the ON / OFF operation of the NOTE transistor _MSF of the current source according to the output voltage V _OUT lower than the target voltage value V _OUT 1. Specifically, the output voltage detection circuit 19 turns on the NOTE transistor _MSF when the output voltage V _OUT 2 (see FIGS. 5 and 6) is lower than the detection voltage value V _OUT 2 (see FIGS. 5 and 6) relating to the predetermined output voltage. It is a circuit configured in. As will be described later, the detected voltage value V _OUT2 is smaller than the target voltage value V _OUT1 . Further, the output voltage detection circuit 19 is a circuit configured to turn off the nanotube transistor _MSF when the output voltage V _OUT reaches the detection voltage value V _{OUT 2} .

FIG. 2 is a diagram showing an example of the configuration of the output voltage detection circuit 19 according to the embodiment. As shown in FIG. 2, the output voltage detection circuit 19 includes an NaCl transistor M _S11 , a resistance R _S11 , a resistance R _S12 , and a resistance R _S13 . The drain of the MIMO transistor M _S11 is connected to the output terminal on the positive side via the resistor R _S11 . A resistor R _S12 is connected between the drain and the gate of the MIMO transistor M _S11 . A resistor _RS13 is connected between the gate and the source of the MIMO transistor _MS11 . The source of the nanotube transistor M _S11 is connected to the ground potential. Further, the drain of the HCl transistor M _S11 is connected to the gate of the nanotube transistor _MSF .

Here, unlike the switching power supply device 1 according to the present embodiment, the switching power supply device 3 not equipped with the photocoupler current adjusting circuit 17 will be described with reference to FIGS. 3 and 4.

FIG. 3 is a diagram showing an example of the configuration of a switching power supply device 3 in which the photocoupler current adjusting circuit 17 is not mounted, unlike the switching power supply device 1 according to the embodiment. As shown in FIG. 3, unlike the switching power supply device 1 according to the embodiment shown in FIG. 1, the switching power supply device 3 does not include the photocoupler current adjusting circuit 17. On the other hand, the switching power supply device 3 of FIG. 3 has the same configuration as the switching power supply device 1 according to the embodiment shown in FIG. 1, except that the photocoupler current adjusting circuit 17 is not mounted.

FIG. 4 is a diagram showing an example of an operation waveform of each signal when the power supply is started in the switching power supply device 3 of FIG.

When the switching power supply device 3 is started, the supply of the input voltage _VIN to the pair of input terminals on the primary side is started.

The control circuit 13 inputs an input voltage V _IN stabilized by the capacitor C _IN , and starts operation using the input voltage V _IN as a power supply voltage. The control circuit 13 inputs a sense voltage generated in the sense resistor _RS , generates a PWM signal based on the sense voltage, and outputs the generated PWM signal to the gate of the MIMO switching transistor _MPW . Further, the primary winding T ₁ inputs an input voltage V _IN stabilized by the capacitor C _IN , and transmits the excitation energy generated by the ON / OFF operation of the MIMO switching transistor _MPW to the secondary winding T ₂ . do. The pair of output terminals on the secondary side output the output voltage V _OUT from the rectification smoothing circuit by the diode D ₁ connected to the secondary winding T ₂ and the capacitor C _OUT .

The output voltage FB circuit 15 generates a current according to the difference between the output voltage V _OUT and the target voltage value V _OUT 1. For example, the shunt regulator IC generates a suction current according to the difference between the voltage division by the resistance R ₁ and the resistance R ₂ of the output voltage V _OUT and the reference voltage set according to the target voltage value V _OUT 1. Therefore, the magnitude of the suction current from the shunt regulator IC increases as the output voltage V _OUT increases.

When the voltage drop of the resistor _R4 generated by the suction current from the shunt regulator IC is smaller than the forward voltage of the photodiode of the photocoupler PC ₁ , the photocoupler current from the phototransistor is not generated. At this time, the duty D at the time of output voltage control of the switching power supply device 3 defined by the equation (1) becomes the duty D1 (maximum) due to the photocoupler current zero.

After that, when the output voltage V _OUT reaches the target voltage value V _OUT 1 in the switching power supply device 3, the voltage drop of the resistor R ₄ according to the suction current from the shunt regulator IC is the forward voltage of the photodiode of the photocoupler PC ₁ . Exceeds. As a result, the photodiode of the photocoupler PC ₁ emits light with an amount of light corresponding to the voltage drop of the resistor _R4 . Further, the phototransistor of the photocoupler PC ₁ inputs the base input current with the light of the photodiode, and generates a photocoupler current proportional to the amount of light emitted thereof. The phototransistor generates a larger photocoupler current as the output voltage V _OUT becomes larger than the target voltage value V _OUT 1.

The control circuit 13 inputs the sense voltage generated in the sense resistor _RS and the photocoupler current output as a suction current from the phototransistor of the photocoupler PC ₁ . The control circuit 13 generates a PWM signal based on the sense voltage and the voltage corresponding to the photocoupler current, and outputs the generated PWM signal to the gate of the MIMO switching transistor _MPW . Here, the control circuit 13 controls so that the larger the photocoupler current is, the shorter the period during which the MIMO switching transistor _MPW is ON. As an example, the duty D at the time of output voltage control of the switching power supply device 3 defined by the equation (1) is the duty D3 by the output voltage control by the shunt regulator IC. The duty D3 due to the output voltage control by the shunt regulator IC is smaller than the duty D1 due to the zero photocoupler current.

In this way, when the output voltage V _OUT reaches the target voltage value V _OUT 1, the switching power supply device 3 starts switching control based on the photocoupler current so that the output voltage V _OUT becomes the target voltage value V _OUT 1. Output voltage control is performed.

However, it takes time from the time when the output voltage V _OUT reaches the target voltage value V _OUT 1 until the output control by the photocoupler current is started. Due to the operation delay of the photocoupler PC ₁ , in the switching power supply device 3, as shown in FIG. 4, the output control by the photocoupler current starts when the output voltage V _OUT exceeds the target voltage value V _OUT 1. Overshoot OS at the output voltage occurs.

Therefore, the switching power supply device 1 according to the present embodiment is equipped with the photocoupler current adjusting circuit 17 as described above with reference to FIGS. 1 and 2. In the following description, the differences from the switching power supply device 3 of FIG. 3 described mainly with reference to FIG. 4 will be described.

FIG. 5 is a diagram showing an example of an operation waveform of each signal when the power supply is started in the output voltage detection circuit 19 according to the embodiment. FIG. 6 is a diagram showing an example of an operation waveform of each signal when the power supply is started in the switching power supply device 1 according to the embodiment.

The upper part of FIG. 5 illustrates the operation waveforms of the output voltage V _OUT and the gate voltage of the HCl transistor _MSF . In the upper part of FIG. 5, the vertical axis and the horizontal axis indicate voltage [V] and time [s], respectively. The lower part of FIG. 5 exemplifies the operation waveforms of the drain current of the NOTE transistor _MSF and the cathode current of the shunt regulator IC. In the lower part of FIG. 5, the vertical axis and the horizontal axis indicate the current [mA] and the time [s], respectively.

The output voltage detection circuit 19 controls the ON / OFF operation of the NOTE transistor _MSF according to the output voltage V _OUT lower than the target voltage value V _OUT 1. Specifically, as shown in FIG. 2, the gate voltage of the NOTE transistor _MSF gradually increases due to the voltage division of the resistors R _S11 and (resistance R _S12 + resistance R _S13 ) as the output voltage V _OUT increases. do. Therefore, as shown in FIG. 5, even if the output voltage V _OUT is lower than the target voltage value V _OUT 1, the gate voltage of the nanotube transistor _MSF increases as the output voltage V OUT increases. Further, since the resistor R _SF is provided on the source side of the NOTE transistor M _SF , the drain current of the NOTE transistor M _SF also increases (in linear proportion) with the increase of the output voltage V _OUT . As described above, when the output voltage V _OUT is lower than the detected voltage value V _OUT 2, the output voltage detection circuit 19 turns on the nanotube transistor _{MSF to generate a current in the resistance R SF .} Since the drain of the IGMP transistor M _SF is connected to the cathode of the photodiode of the photocoupler PC ₁ , the voltage drop of the resistor R ₄ is generated by the current generated by the current source including the Now's transistor M _SF and the resistor R _SF . .. That is, even if the output voltage detection circuit 19 has an output voltage V _OUT lower than the target voltage value V _OUT 1, as shown in FIG. 6, the output voltage detection circuit 19 converts the photocoupler current according to the output voltage V _OUT 1 to the photodiode of the photocoupler PC ₁ . Can occur in.

The control circuit 13 generates a PWM signal based on the sense voltage generated in the sense resistor _RS and the voltage corresponding to the photocoupler current output as the suction current from the phototransistor of the photocoupler PC ₁ , and is generated. The PWM signal is output to the gate of the MIMO switching transistor _MPW . At this time, the duty D at the time of output voltage control of the switching power supply device 1 defined by the equation (1) is the duty D2 by the output voltage detection so that the photocoupler current gradually increases as the output voltage V _OUT increases. It becomes. The duty D2 due to the output voltage detection is smaller than the duty D1 due to the zero photocoupler current, and is larger than the duty D3 due to the output voltage control by the shunt regulator IC. As a result, the output voltage V _OUT gradually increases.

Further, when the output voltage V _OUT further increases and reaches the detected voltage value V _OUT 2, in the output voltage detection circuit 19, the voltage between the gate and the source of the MIMO transistor M _{S 11} is increased by the divided voltage of the resistors RS 12 and the resistors _{RS 13} . To increase. As a result, the norm transistor M _S11 is turned on, and the nanotube transistor M _SF is turned off by the drain current of the nanotube transistor M _S11 .

In this way, when the output voltage V _OUT reaches the detected voltage value V _{OUT 2} , the NOTE transistor _MSF is turned off. That is, when the output voltage V _OUT reaches the detected voltage value V _{OUT 2} , the photocoupler current adjusting circuit 17 stops the generation of current by the current source. As a result, the generation of the photocoupler current in the photodiode of the photocoupler PC ₁ ends. After that, as described with reference to FIGS. 3 and 4, the output voltage control by the shunt regulator IC is started in the switching power supply device 1.

When the MIMO transistor _MSF is in the ON state after the operation of the shunt regulator IC is started, the photocoupler current includes a current generated by the operation of the shunt regulator IC and a current generated by the photocoupler current adjusting circuit 17. That is, when the IGMP transistor _MSF is in the ON state after the operation of the shunt regulator IC is started, the OFF time of the MIMO switching transistor _MPW may increase as the photocoupler current increases, and the output voltage V _OUT may decrease.

Therefore, in the switching power supply device 1 according to the present embodiment, the detected voltage value V _OUT 2 is equal to or less than the target voltage value V _OUT 1. Preferably, the detected voltage value V _OUT2 is smaller than the target voltage value V _OUT1 . Specifically, the detection voltage value V _{OUT 2} is set so that the output voltage detection circuit 19 can turn off the SOI transistor _MSF before the shunt regulator IC starts operating as the output voltage V _OUT rises. Is preferable.

As described above, the switching power supply device 1 according to the embodiment is equipped with a photocoupler current adjusting circuit 17. The optocoupler current adjusting circuit 17 has an µtransistor MSF in which a drain is connected to the cathode of the photodiode of the optocoupler PC ₁ and a resistor _RSF connected to the source of the _{µtransistor MSF} . Further, the photocoupler current adjusting circuit 17 detects an output voltage V _OUT of the target voltage value V _OUT 1 or less on the secondary side of the switching power supply device 1, and outputs voltage detection that controls the ON / OFF operation of the NOTE transistor _MSF . It further has a circuit 19.

According to this configuration, the photocoupler current can be generated in the photocoupler PC ₁ according to the output voltage V _OUT lower than the target voltage value V _OUT 1. Therefore, according to the technique according to the embodiment, switching control by the photocoupler current can be started when the output voltage V _OUT is equal to or less than the target voltage value V _OUT 1, so that overshoot occurs in the output voltage when the power supply is started. Can be suppressed.

(Modification example of output voltage detection circuit)
The output voltage detection circuit 19 according to the embodiment is not limited to the configuration shown in FIG. 2, and may have other configurations. FIG. 7 is a diagram showing another example of the configuration of the output voltage detection circuit 19 according to the embodiment. The output voltage detection circuit 19 of FIG. 7 has a resistance R _S21 , a resistance R _S22 , a resistance R _S23 , a resistance R _S24 , a resistance R _S25 , a shunt regulator IC _S1 and a comparator COMP _S1 .

The comparator COMP _S1 is a differential amplifier circuit (comparison circuit) that outputs a comparison result according to the potential difference between the inverting input terminal (−) and the non-inverting input terminal (+). One of the power supply terminals of the comparator COMP _S1 is connected to the output terminal on the positive side. The other end of the power supply terminal of the comparator COMP _S1 is connected to the ground potential. The inverting input terminal (-) of the comparator COMP _S1 is connected to the output terminal on the positive side via the resistor _RS21 . Further, the inverting input terminal (-) of the comparator COMP _S1 is connected to the ground potential via the resistor _RS22 . The non-inverting input terminal (+) of the comparator COMP _S1 is connected to the cathode and reference of the shunt regulator IC _S1 . A resistor _RS23 is connected between the cathode of the shunt regulator IC _S1 and the output terminal on the positive side. The anode of the shunt regulator IC _S1 is connected to the ground potential. The output terminal of the comparator COMP _S1 is connected to the gate of the comparator transistor _MSF via the resistor _RS24 . Further, the output terminal of the comparator COMP _S1 is connected to the ground potential via the resistance R _S24 and the resistance R _S25 .

According to this configuration, the output of the comparator COMP _S1 becomes “H” as the output voltage V _OUT increases, and the MIMO transistor _MSF connected to the output terminal of the comparator COMP _S1 via the resistor _RS24 is turned on. Can be done. As a result, the drain current of the NOTE transistor _MSF can be increased according to the output voltage V _OUT , and switching control by the photocoupler current can be started when the output voltage V _OUT is equal to or less than the target voltage value V _OUT 1.

Further, when the output voltage V _OUT increases to the detected voltage value V _OUT 2, the potential of the inverting input terminal (−) increases due to the voltage division of the resistors R _S21 and the resistors R _S22 . On the other hand, the potential of the non-inverting input terminal (+) is a constant voltage due to the shunt regulator IC _S1 , and when the output voltage V _OUT increases to the detected voltage value V _OUT 2, the potential of the inverting input terminal (-) is non-inverting. The output of the comparator COMP _S1 becomes “L” beyond the potential of the inverting input terminal (+), and the comparator transistor _MSF can be turned off. As a result, the output voltage detection circuit 19 turns on the thought transistor _MSF in response to the output voltage V _OUT lower than the target voltage value V _OUT 1, and before the shunt regulator IC starts operating as the output voltage V _OUT rises. , Immuno transistor _MSF can be turned off.

(Modification example of switching power supply)
In the above-described embodiment, the case where the duty D is changed by the PWM method is illustrated, but the present invention is not limited to this. FIG. 8 is a diagram showing another example of the configuration of the switching power supply device 1 according to the embodiment.

In the switching power supply device 1 of FIG. 8, the transformer ₁₁ further includes an auxiliary winding T3. Further, the switching power supply device 1 of FIG. 8 further includes a resistor R _SUB , a diode D _SUB 1, a capacitor C _SUB 1, and a resistor R _ST1 .

The auxiliary winding T ₃ is provided on the primary side of the transformer 11. A rectifying smoothing circuit with a diode D _SUB 1 and a capacitor C _SUB 1 is connected to the auxiliary winding T ₃ via a resistor R _SUB . Specifically, one end of the auxiliary winding T ₃ is connected to the ground potential, and the other end is connected to the resistor R _SUB . One end of the resistor R _SUB opposite to the auxiliary winding T ₃ is connected to the anode of the diode D _SUB 1. A capacitor C _SUB1 is connected between the cathode of the diode D _SUB1 and the ground potential. The capacitor C _SUB 1 is a capacitive element that stores the electric charge supplied from the auxiliary winding T ₃ . The cathode of the diode D _SUB 1 is connected between the resistor R _ST1 and the control circuit 13. The resistor R _ST1 is connected between the input terminal on the positive side and the control circuit 13.

In the auxiliary winding T ₃ , a voltage generated corresponding to the current flowing in the primary winding T ₁ is generated. To the power supply terminal of the control circuit 13, the power supply voltage VDD from the rectifying smoothing circuit by the diode D _SUB1 and the capacitor C _SUB1 connected to the auxiliary winding _T3 is supplied.

Even with this configuration, the photocoupler current can be generated in the photocoupler PC ₁ according to the output voltage V _OUT lower than the target voltage value V _OUT 1 in the same manner as in the above-described embodiment. Therefore, similarly to the above-described embodiment, switching control by the photocoupler current can be started when the output voltage V _OUT is equal to or less than the target voltage value V _OUT 1, so that overshoot occurs at the output voltage when the power supply is started. Can be suppressed.

Further, in the switching power supply device 1 shown in FIG. 8, the power supply voltage VDD of the control circuit ₁₃ is supplied from the auxiliary winding T3. Further, the control circuit 13 has a low switching frequency in a discontinuous mode when the load current of the output where the photocoupler current is large is small, as in the output voltage control by the photocoupler of the ringing choke converter (RCC), and the photocoupler current is high. It is assumed that the output voltage is controlled so that the load current of the output becomes smaller and the switching frequency becomes higher in the critical mode. When the switching power supply device 1 shown in FIG. 8 is not provided with the photocoupler current adjusting circuit 17, overshoot occurs at the rising edge of the output voltage V _OUT when the power supply is started, as in the switching power supply device 3 shown in FIG. .. In the output voltage control by the shunt regulator IC, when the high output voltage V _OUT due to overshoot is maintained and the photocoupler current becomes large, the duty D of the signal to the µ switching transistor _MPW by the control circuit 13 becomes small, and the switching frequency is further increased. Will be low. Therefore, the OFF time of the MIMO switching transistor _{MPW becomes long, the electric charge from the auxiliary winding T3 is not supplied to the capacitor CSUB1 as} the switching is stopped _, and the power supply voltage VDD drops.

Under these circumstances, according to the switching power supply device 1 shown in FIG. 8, since the occurrence of overshoot in the output voltage at the time of starting the power supply can be suppressed, it is possible to prevent an increase in the OFF time of the IGMP switching transistor _MPW and switch. It is also possible to suppress the occurrence of a decrease in the power supply voltage VDD due to the stoppage.

Although the embodiments of the present invention have been described above, the above embodiments are presented as examples and are not intended to limit the scope of the invention. The above-mentioned novel embodiment can be implemented in various other embodiments, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and the equivalent scope thereof.

1 Switching power supply 11 Transformer 13 Control circuit 15 Output voltage feedback (FB) circuit 17 Photocoupler current adjustment circuit 19 Output voltage detection circuit (voltage detection circuit)
C _IN , C _NF , C _OUT , C _SUB1 Capacitor COMP _S1 Comparator D ₁ , D _SUB1 Diode IC, IC _S1 Shunt Regulator M _PW CICS Switching Transistor M _S11 , M _SF NMOS Transistor PC ₁ Photocoupler _R1 , _R2 , R ₃ , R ₄ , R _NF , R _S11 , R _S12 , R _S13 , R _S21 , R _S22 , R _S23 , R _S24 , R _S25 , R _SF , R _ST1 , R _SUB resistance R _S sense resistance T ₁ 1st volume Wire T ₂ Secondary winding T ₃ Auxiliary winding

Claims (4)

Switching transistor and
A sense resistor that generates a sense voltage according to the current that flows when the switching transistor is turned on,
A transformer having a primary winding to which an input voltage is applied by turning on the switching transistor and a secondary winding to which a load is connected.
A photocoupler having a phototransistor provided on the primary side and a photodiode provided on the secondary side, and generating a photocoupler current in the phototransistor according to a current flowing through the photodiode.
When the output voltage obtained by rectifying and smoothing the voltage generated in the secondary winding is higher than the predetermined target voltage, the output that causes the photodiode to generate the photocoupler current according to the difference between the output voltage and the target voltage. In an isolated switching power supply with a voltage feedback circuit,
A current source connected to the cathode of the photodiode of the photocoupler,
When the output voltage is lower than the target voltage, a current is generated in the current source to generate the photocoupler current corresponding to the output voltage lower than the target voltage in the photodiode, and the output voltage is set in advance. A voltage detection circuit that stops the generation of current by the current source when the specified detection voltage is reached, and
An isolated switching power supply device including a control circuit that controls ON / OFF operation of the switching transistor based on the sense voltage and the photocoupler current. The current source is
A MOS transistor with a gate connected to the output of the voltage detection circuit, a source connected to the negative voltage end of the secondary side, and a drain connected to the cathode of the optocoupler.
A base is connected to the output of the voltage sensing circuit, an emitter is connected to the negative voltage end of the secondary side, and a collector is connected to the cathode of the optocoupler, including one of the bipolar transistors. The switching power supply device according to claim 1. The switching power supply device according to claim 1 or 2, wherein the detected voltage is smaller than the target voltage. The switching power supply device according to claim 1 or 2, wherein the current source increases in proportion to an increase in the output voltage.

Images