JP2012205365A - Switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply device which, when in a non-loaded or lightly loaded state, automatically switches output voltages to a standby voltage lower than a rated voltage.SOLUTION: A switching power supply device, which rectifies and then smooths an AC voltage to convert it into a prescribed DC voltage, comes with a transformer for the switching power supply device, the transformer being provided with primary and secondary windings. The secondary winding includes a secondary side rectification-smoothing circuit consisting of a diode and a capacitor and an error amplifier which sends out a signal for detecting the voltage of the secondary side rectification-smoothing circuit via a resistor and adjusting it to a prescribed DC voltage. The secondary winding includes a detection circuit for rectifying and then integrating a pulse voltage generated in the secondary winding to detect voltage, which circuit, when the integrated voltage is less than a predetermined voltage, changes a detection resistor value of the error amplifier to make it a standby voltage lower than a rated voltage.

Description

  The present invention relates to a switching power supply device, and more particularly to switching control for automatically reducing an output voltage when a predetermined output voltage is not required in a light load or no load state such as an AC adapter.

  The switching power supply device 1b shown in FIG. 6 is composed of a variable output voltage power supply circuit. A switching regulator is provided with a current detection circuit CD for detecting an output current, and the output current is detected by the current detection circuit CD. Yes. If the cable used to connect the power source and the load is disconnected and the output current becomes zero, a signal is sent from the current detection circuit CD, and the resistance of the detection resistor of the output voltage error voltage generation circuit EA By changing the value, the output voltage is controlled to be lower than the original voltage value.

JP 2000-92830 A

However, in Patent Document 1, since a current detection resistor is arranged on the output and the output current is monitored, a loss of the current detection resistor always occurs. In addition, in order to cover no load, light load to rated current with a single detection resistor, a detection voltage that can accurately detect at least the output current at light load is required, and the current detection resistance value is the aforementioned detection value. Is set, the resistance loss when the rated current flows is proportional to the square of the output current, so that there is a problem that it becomes large.

That is, even if the power consumption at no load / light load can be improved, the power consumption at the rated load may increase.

  Therefore, in the present invention, in view of the prior art, detection at the time of no load / light load is determined based on a pulse voltage signal generated in the secondary winding of the transformer, and the output current is not detected without detecting the output current. We propose to reduce the power consumption by switching the output voltage to a low voltage.

A switching power supply device that rectifies and smoothes an AC voltage and converts it into a predetermined DC voltage, and includes a transformer provided in the switching power supply device, and the transformer includes at least a primary winding and a secondary winding. The winding has a secondary side rectifying / smoothing circuit composed of a diode and a capacitor, and an error amplifier for detecting a voltage of the secondary side rectifying / smoothing circuit through a resistor and sending a signal for controlling to a predetermined DC voltage. The secondary winding includes a voltage switching circuit having a means for detecting the voltage by rectifying and integrating the pulse voltage generated in the secondary winding, and the voltage switching circuit has a predetermined integrated voltage. When the voltage becomes lower than the voltage, the detection resistance value of the error amplifier is changed to make the standby voltage lower than a predetermined DC voltage.
In addition, the switching power supply device is composed of an on / off type DC-DC converter, and the voltage switching circuit rectifies and integrates the pulse voltage generated in the secondary winding at the time of switching off to detect the voltage. It is characterized by.
Also, a switching power supply device that rectifies and smoothes an AC voltage and converts it to a predetermined DC voltage, the transformer provided in the switching power supply device, and the transformer includes at least a primary winding and a secondary winding, In the secondary winding, the voltage of the secondary side rectifying / smoothing circuit composed of a diode and a capacitor and the voltage of the secondary side rectifying / smoothing circuit are supplied to the load as an output voltage, and the voltage of the secondary side rectifying / smoothing circuit is supplied via a resistor. An error amplifier that detects and sends a signal for controlling to a predetermined DC voltage is provided. The secondary winding rectifies the pulse voltage generated in the secondary winding and integrates it with a resistor and capacitor to set the time constant. A first voltage switching circuit having a detection function; and a second voltage switching circuit having a time constant set shorter than that of the first voltage switching circuit. The first voltage switching circuit is connected to the secondary winding. Generated pulse voltage in advance The second voltage switching circuit is provided with a means for changing the detection resistance value of the error amplifier to a standby voltage lower than a predetermined DC voltage when the measured time becomes less than the first time constant. When the pulse voltage generated in the winding becomes less than a predetermined second time constant, the detection resistance value of the error amplifier is changed to a voltage lower than the predetermined DC voltage and higher than the standby voltage. A switching means is provided.
The switching power supply device is composed of an on / off type DC-DC converter, and the first voltage switching circuit and the second voltage switching circuit rectify the pulse voltage generated in the secondary winding when switching off. The voltage is detected by integration.
Further, the output voltage switching by the first voltage switching circuit is such that the output voltage reduced by the sum of the capacitance of the secondary side rectifying and smoothing circuit and the load capacitance connected to the output terminal and the load current is the second voltage switching. The time for reaching the switched voltage by the circuit is longer than the time constant of the first voltage switching circuit.

  According to the present invention, the no load / light load state is not detected by the current detection resistor, so the power loss and parts of the current detection resistor in the rated load state are eliminated, the output voltage switching function and the rated load from no load to light load are eliminated. It is possible to achieve an energy saving effect in the entire load state while achieving both efficiency improvement in the state.

It is a circuit block diagram of the switching power supply device of Example 1 of this invention. It is a timing chart which shows the signal of each part of Example 1 shown in FIG. It is a circuit block diagram of the switching power supply device of Example 2 of this invention. It is a timing chart which shows the signal of each part of Example 2 shown in FIG. FIG. 4 is an enlarged view of the timing chart shown in FIG. 3. It is a circuit block diagram of the conventional switching power supply device.

  Next, embodiments according to the present invention will be specifically described with reference to the drawings.

FIG. 1 is a diagram illustrating a circuit configuration of a switching power supply device 1 that is Embodiment 1 of the present invention.
FIG. 1 shows a flyback switching power supply circuit, and the voltage obtained by rectifying and smoothing an AC power supply by a rectifier DB1 and a smoothing capacitor C1 is turned on and off by a switching element Q1 connected via a primary winding P of a transformer T. , And output to the secondary winding S of the transformer T during the aforementioned off period. The output voltage rectified and smoothed by the diode D3 and the smoothing capacitor C5 is supplied to the load as the voltage of the secondary winding S, and the output voltage divided by the resistors R4 and R5 is input to the error amplifier IC2. A reference voltage Vr (not shown) built in the amplifier IC2 is compared with the divided output voltage, and the difference is fed back to the primary side control circuit IC1 via the photocouplers PC1-1 and PC1-2 as an error signal. Be controlled. The control circuit IC1 on the primary side performs pulse control of the on / off signal of the switching element Q1 based on the feedback signal of the photocoupler PC1-2 to control the output voltage to be constant.

Here, the control circuit IC1 is a control circuit having a function of intermittent oscillation in a no-load / light load state. The control circuit IC1 detects the drain current flowing through the MOSFET (Q1), which is a switching element, by the resistor R2, or compares the feedback signal voltage with a predetermined threshold voltage, and if it is less than the threshold value, or the drain current When the detected current is less than a predetermined value, the switching oscillation frequency is switched to intermittent oscillation for control.
In the present invention, the switching circuit 2 for switching the output voltage at no load / light load is connected between the terminals of the secondary winding S. The switching circuit 2 includes a diode D4, a capacitor C6, resistors R6 to 10, a Zener diode ZD1, and an NPN transistor Q2, and rectifies and integrates the secondary winding voltage of the transformer T to obtain a voltage signal. In the load state, the output voltage is switched by detecting the decrease in the voltage signal due to intermittent oscillation.

  In the switching circuit 2, a diode D4 and a resistor R7, and a parallel circuit composed of a capacitor C6 and a resistor R8 are connected in series between terminals of the secondary winding S, and the secondary winding S generated during the OFF period of the switching element Q1. Rectifying smoothing and integration of the pulse voltage generated in A series circuit of a Zener diode ZD1, a resistor R9, and a resistor R10 is connected to both terminals of the parallel circuit including the capacitor C6 and the resistor R8, and a base / emitter of the NPN transistor Q2 is connected to both terminals of the resistor R10. The collector terminal of the NPN transistor Q2 is connected to the connection point between the output voltage detection terminal R of the error amplifier IC2 and the resistors R4 and R5 via the resistor R6.

FIG. 2 is a timing chart showing signals at various parts in the first embodiment. The load current IL shown in FIG. 2 gradually decreases from time t0 to time t2, and the light load state continues from time t2 to t5. Further, the voltage Vc6 of the capacitor C6 of the switching circuit 2 also decreases in proportion to the load current. Here, after time t1 when the load current becomes light load current, the drain-source voltage Vds of the switching element Q1 becomes in an intermittent oscillation state, and a ripple occurs in the voltage Vc6 of the capacitor C6 of the switching circuit 2, and the voltage Vc6 at time t3. Falls below the voltage Vz obtained by adding the Zener voltage of the Zener diode ZD1 and the base emitter voltage of the NPN transistor Q2. Therefore, at time t3, the NPN transistor Q2 is turned off, the resistor R6 connected between the output voltage detection terminal of the error amplifier IC2 and the collector of the NPN transistor Q2 is released, and the output voltage detection terminal of the error amplifier IC2 rises. .
That is, the NPN transistor Q2 is turned on in the load current state up to time t3 when the rated load and the light load are not detected. The collector terminal of the NPN transistor Q2 is connected to the output voltage detection terminal of the secondary side error amplifier IC2 via the resistor R6, and the resistors R5 and R6 are connected in parallel to detect the rated output voltage. Yes.

  Therefore, when the light load current is reached at time t3 shown in FIG. 2, the NPN transistor Q2 is turned off. When the NPN transistor Q2 is turned off, the resistor R6 is released and the detection terminal voltage of the secondary side error amplifier IC2 rises. Therefore, the reference voltage Vr (not shown) is compared with the divided voltage of the resistors R4 and R5, and the difference is calculated. Based on the voltage, feedback control is performed to the control circuit IC1 on the primary side. That is, the standby voltage is switched to a standby voltage lower than the rated voltage before time t3 based on the resistors R4 and R5.

Here, the rated voltage and standby voltage of the output voltage are set as follows.
Vr: Reference voltage rated voltage of the secondary side error amplifier IC2: Vr × (R4 + R5 // R6) / (R5 // R6) Equation 1
Standby voltage: Vr × (R4 + R5) / (R5) Equation 2
In order to return from the low output voltage after switching the output voltage to the rated voltage, the pulse voltage width of the secondary winding S increases by increasing the load current IL. The load current increases from time t4, the voltage Vc6 of the capacitor C6 of the switching circuit 2 increases, and exceeds the voltage Vz at time t5. For this reason, the NPN transistor Q2 is switched from the OFF state to the ON state at time t5, and returns to the rated voltage shown in Equation 1 at time t6.

  As described above, without detecting the load current IL with the current detection resistor, the switching circuit 2 determines whether there is no load or light load, and has a function of switching the setting of the output voltage from the rated voltage to the standby voltage. And the efficiency when the load current flows can be improved.

FIG. 3 is a diagram showing a circuit configuration of a switching power supply device 1a that is Embodiment 2 of the present invention.
Note that parts substantially the same as those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted.
In the second embodiment, the second output voltage switching circuit 3 is added in order to reliably set the load current value to shift to the standby voltage at the time of no load / light load.

  The second output voltage switching circuit 3 has a diode D5 and a capacitor C7 connected in series between the terminals of the secondary winding S, and rectifies the pulse voltage generated in the secondary winding S generated during the OFF period of the switching element Q1. Smooth. Further, a series circuit of resistors R12 and R13 is connected in parallel to the capacitor C7, and a base / emitter terminal of an NPN transistor Q3 is connected to both ends of the resistor R13. The collector terminal of the NPN transistor Q3 is connected to the output voltage detection terminal R of the secondary side error amplifier IC2 via the resistor R11, and the resistors R5, R6, and R11 are connected in parallel between the output voltage detection terminal R and GND, The output rated voltage is detected.

  FIG. 4 is a timing chart showing signals at various parts in the second embodiment. The load current IL shown in FIG. 4 gradually decreases from time t10 to time t12, and the light load state continues from time t12 to t14. Further, the voltage Vc6 of the capacitor C6 of the switching circuit 2 also decreases in proportion to the load current. Here, after time t11 when the load current becomes light load current, the drain-source voltage Vds of the switching element Q1 is in an intermittent oscillation state, and a ripple occurs in the voltage Vc6 of the capacitor C6 of the switching circuit 2, and the voltage Vc6 at time t12. Falls below the voltage Vz obtained by adding the Zener voltage of the Zener diode ZD1 and the base-emitter voltage of the NPN transistor Q2. Therefore, at time t3, the NPN transistor Q2 is turned off, the resistor R6 connected between the output voltage detection terminal of the error amplifier IC2 and the collector of the NPN transistor Q2 is released, and the output voltage detection terminal of the error amplifier IC2 rises. .

  Therefore, the output voltage switching by the on / off operation of the NPN transistor Q2 is the same as that in the first embodiment, but the details up to the output voltage switching are different and will be described below.

  FIG. 5 is a timing chart showing signals at various parts in the second embodiment in which the vicinity of the times t11 to t13 shown in FIG. 4 is enlarged. Here, in FIG. 5, the resistance value of the output voltage detection terminal R of the secondary side error amplifier IC2 shown in FIG. 4 is replaced with the output voltage detection terminal voltage of the secondary side error amplifier IC2.

The output voltage detection terminal voltage of the secondary side error amplifier IC2 is equal to or higher than the reference voltage Vr of the secondary side error amplifier IC2 (not shown), and the ripple voltage is superimposed as the switching element Q1 is turned on / off.
Immediately after the switching element Q1 is turned off, the capacitors C5 to C7 and the load side capacitor C10 are charged, and the output voltage detection terminal voltage of the secondary side error amplifier IC2 slightly increases. Next, the period of the time constant time td2 of the second output voltage switching circuit 3 becomes a discharge period of the load current and the capacitor C5 and the capacitor C10 on the load side, and slightly decreases.
The time constant time td2 of the second output voltage switching circuit 2 indicates the discharge time constant by the capacitor C7 and the resistors R12 and R13, and the NPN transistor Q3 is turned on after charging the capacitors C5 to C7 and the capacitor C10 on the load side. Indicates the time to transition off.
When the time constant time td2 elapses, the NPN transistor Q3 is turned off, so that the resistor R11 is released from the GND terminal, and the resistance value between the output voltage detection terminal and the GND of the secondary side error amplifier IC2 is the parallel resistance value of the resistors R5 and R6. As shown in the figure, the voltage rises by V2. For this reason, when the output voltage detection terminal of the secondary side error amplifier IC2 rises to the voltage V2, the time until it reaches the reference voltage Vr is extended, and as a result, the timing for switching on the switching element Q1 is extended.
Here, when the switching cycle Ts of the switching element Q1 exceeds the time constant time td1 of the first output voltage switching circuit 2, the NPN transistor Q2 of the first output voltage switching circuit 2 is turned off, and the secondary side error amplifier. The resistance value between the output voltage detection terminal of IC2 and GND is switched to the resistance value of the resistor R5 alone. At this time, the output voltage detection terminal voltage of the secondary side error amplifier IC2 rises by the voltage V1 shown in the figure as a voltage change. That is, the setting of the output voltage is switched from the rated voltage to the standby voltage.

Here, the output voltage of Example 2 is set as follows.
The state in which the rated voltage is being output is expressed by Equation 3 because the NPN transistors Q2 and Q3 of the first output voltage switching circuit 2 and the second output voltage switching circuit 3 are in the on state.
Rated voltage: Vref x (R4 + (R5 // R6) // R11) / ((R5 // R6) // R11)
Next, when the load of the switching element Q1 is intermittently oscillated at a light load (no load) and the second output voltage switching circuit 3 is turned off after the time constant td2, the output voltage to start the next switching is as follows. It becomes the following formula.
Rated voltage (after td1): Vref × (R4 + (R5 // R6)) / (R5 // R6) Equation 4

  Therefore, the rated voltage of the second embodiment has the hysteresis voltage determined by the resistor R11, and the voltages of the expressions 3 to 4 are output. The switching off period of the intermittent oscillation period of the second embodiment corresponds to a period from when the voltage of Expression 3 is decreased to the voltage of Expression 4, but the parameter that is decreased by the hysteresis voltage is the circuit current of the secondary error amplifier IC2 + It depends on the time constant between the total value of the load current IL and the total value of the capacitances of the output smoothing capacitor C5 and the load capacitor C10. Here, since the circuit current of the secondary side error amplifier IC2 and the capacitances of the output smoothing capacitor C5 and the load capacitor C10 are fixed, the OFF period of the intermittent oscillation period is inversely proportional to the load current.

Here, the conditions for the output voltage to switch from the rated voltage to the standby voltage are as follows.
The capacitances of the output smoothing capacitor C5 and the load capacitor C10 are c5 and c10, respectively, and the circuit current of the secondary side error amplifier IC2 is IE. From the relational expression of CV = it,
V2> (IL + IE) × td1 / (c5 + c10) Equation 5
If you sort this out and replace the display with a load current that switches to standby voltage,
IL <{V2 × (c5 + c10) / td1} −IE Expression 6
Therefore, it is possible to set the load current IL to be subjected to output voltage switching from the hysteresis voltage V2 determined by the time constant td1 of the first output voltage switching circuit 2 and the resistor R11 of the second output voltage switching circuit 3. become.
Note that the return from the standby voltage to the rated voltage is performed when the charging voltage of the capacitor C6 exceeds the threshold value Vz, as in the first embodiment.
Further, the second output voltage switching circuit 3 sets the output voltage at the time of no load / light load within a range not exceeding the output voltage accuracy in order to obtain a minute change corresponding to the hysteresis voltage of V2.

  Although the present invention has been specifically described above with reference to the embodiments, it goes without saying that the present invention is not limited to the above-described embodiments, and can be modified without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1, 1a, 1b ... Switching power supply device 2 ... 1st output voltage switching circuit 3 ... 2nd output voltage switching circuit DB1 ... Rectifier C1-C7, C10 ... Capacitor Q1 ... Switching elements Q2, Q3... NPN transistors D1 to D6... Diode IC1... Primary side control circuit IC2... Secondary side error amplifiers R1 to 13. ... Primary winding S of transformer T ... Secondary winding ZD1 of transformer T ... Zener diode PC1-1, -2 ... Photocoupler

Claims (5)

A switching power supply device that rectifies and smoothes an AC voltage and converts it to a predetermined DC voltage,
A transformer provided in the switching power supply device, and the transformer includes at least a primary winding and a secondary winding;
The secondary winding includes a secondary side rectifying and smoothing circuit including a diode and a capacitor;
An error amplifier that sends a signal for detecting the voltage of the secondary side rectifying and smoothing circuit through a resistor and controlling the voltage to the predetermined DC voltage;
The secondary winding includes a voltage switching circuit provided with means for rectifying and integrating the pulse voltage generated in the secondary winding to detect the voltage,
The voltage switching circuit is configured to change a detection resistance value of the error amplifier to a standby voltage lower than the predetermined DC voltage when the integrated voltage becomes less than a predetermined voltage. Switching power supply. The switching power supply device includes an on / off type DC-DC converter,
2. The switching power supply device according to claim 1, wherein the detection circuit rectifies and integrates a pulse voltage generated in the secondary winding when switching off, and detects the voltage. A switching power supply device that rectifies and smoothes an AC voltage and converts it to a predetermined DC voltage,
A transformer provided in the switching power supply device, and the transformer includes at least a primary winding and a secondary winding;
The secondary winding includes a secondary side rectifying and smoothing circuit including a diode and a capacitor;
Supply the voltage of the secondary side rectifying and smoothing circuit as an output voltage to the load,
An error amplifier that sends a signal for detecting the voltage of the secondary side rectifying and smoothing circuit through a resistor and controlling the voltage to the predetermined DC voltage;
The secondary winding includes a first voltage switching circuit having a function of detecting a time constant by rectifying a pulse voltage generated in the secondary winding and integrating the voltage with a resistor and a capacitor;
A second voltage switching circuit having a time constant set shorter than that of the first voltage switching circuit;
The first voltage switching circuit changes a detection resistance value of the error amplifier when the pulse voltage generated in the secondary winding becomes less than a predetermined first time constant, and Comprising means for switching to a standby voltage lower than the DC voltage;
The second voltage switching circuit changes a detection resistance value of the error amplifier when the pulse voltage generated in the secondary winding becomes less than a predetermined second time constant, and A switching power supply comprising a means for switching to a voltage lower than a direct current voltage and higher than the standby voltage. The switching power supply device includes an on / off type DC-DC converter,
4. The voltage detection circuit according to claim 3, wherein the first voltage switching circuit and the second voltage switching circuit detect a voltage by rectifying and integrating a pulse voltage generated in the secondary winding when switching off. 5. Switching power supply. The switching of the output voltage by the first voltage switching circuit is as follows:
In the time when the output voltage reduced by the sum of the capacitance of the secondary side rectifying and smoothing circuit and the load capacitance connected to the output terminal and the load current reaches the voltage switched by the second voltage switching circuit,
5. The switching power supply device according to claim 3, wherein the switching power supply device is performed when the time constant is longer than a time constant of the first voltage switching circuit.

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