JP2001224166A - Switching power supply unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce electric power consumption for positively high power efficiency by reducing switching loss under light load, using a simple structure. SOLUTION: A control circuit 15 includes an error amplifier 22 of generating an error voltage signal VEAO, formed out of a difference between a auxiliary power supply voltage Vcc and a reference voltage, a current-detecting circuit 23 of detecting drain current ID flowing through a switching element 14 and outputting an element current detecting signal VCL, and a drain current detecting comparator 24 for comparing the error voltage signal VEAO with the element current detecting signal VCL and outputting a comparison signal. The control circuit 15 involves a light load detecting circuit 40 for stopping the output of a switching signal to a switching element 14 for a switching signal control circuit 25, if the error voltage signal VEAO is lower than a lower limit voltage value, and starting the output of the switching signal for the switching signal control circuit 25, if the error voltage signal VEAO is higher than an upper voltage value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply, and more particularly, to a step-down chopper type switching power supply capable of reducing power consumption at a light load.

[0002]

2. Description of the Related Art (First Conventional Example) A switching power supply according to a first conventional example will be described with reference to the drawings.

FIG. 7 shows a circuit configuration of a conventional switching power supply in which an input side and an output side are electrically insulated. In the switching power supply device shown in FIG. 7, for example, the alternating current from the commercial power supply input to the input terminal is:
It is rectified by a rectifier 101 composed of a diode bridge or the like. Subsequently, the DC voltage is smoothed by the input capacitor 102 and becomes the DC voltage Vin, and the transformer 103 for power conversion is used.
Is input to The transformer 103 includes a first primary winding 10
3a, a second primary winding 103b and a secondary winding 103c, and the generated DC voltage Vin is input to the first primary winding 103a.

The first primary winding 103a of the transformer 103
Is controlled by a switching element 104 composed of a power MOSFET or the like. By the switching operation of the switching element 104 at this time, an electromotive force is generated in the secondary winding 103c of the transformer 103 by magnetic induction.

The current caused by the electromotive force generated in the secondary winding 103c is equal to the current of the diode 11 connected to the secondary winding 103c.
0 and rectified and smoothed by the output capacitor 111 and supplied to the load 112 as DC power of the output voltage Vo.

The second primary winding 103b of the transformer 103
Also, a DC electromotive force is generated by the first primary winding 103a, and the DC current output from the second primary winding 103b is rectified and smoothed by the auxiliary power supply unit 120 including the diode 121 and the capacitor 122. And an auxiliary power supply voltage Vcc is generated.

[0007] The control circuit 130 driven by the auxiliary power supply voltage Vcc outputs a control signal to the gate of the switching element 104. Here, the auxiliary power supply voltage Vcc is proportional to the output voltage Vo supplied from the secondary winding 103c of the transformer 103 to the load 112, and is also used as a feedback signal for stabilizing the output voltage Vo.

[0008] The control circuit 130 includes the switching element 10
Oscillator 131 which outputs a switching signal to be applied to the oscillator 4
An error amplifier 132 that outputs an error voltage signal VEAO consisting of a difference between the auxiliary power supply voltage Vcc and the reference voltage; and a drain current detection circuit that detects a drain current ID flowing through the switching element 104 and outputs an element current detection signal VCL. 133, a comparator 134 that compares the error voltage signal VEAO with the element current detection signal VCL, and outputs a comparison result.
And a switching signal control circuit 135 for controlling the output of the switching signal based on the comparison signal.

The switching signal control circuit 135 receives a clock signal CLK from the oscillator 131 at a set terminal and an output signal of the comparator 134 at a reset terminal.
The S flip-flop circuit 136 and the oscillator 1
31 receives the maximum duty cycle signal MDC from
It comprises a NAND circuit 137 that receives an output signal from the RS flip-flop circuit 136 at another input terminal, and a gate driver 138 that receives an output signal of the NAND circuit 137, inverts and amplifies the output signal, and outputs a control signal. .

Hereinafter, the operation of the switching power supply device configured as described above will be described.

In FIG. 7, first, immediately after the apparatus is started, when an AC current from a commercial power supply is input to the rectifier 101, the input AC current is rectified by the rectifier 101 and the input capacitor 102. The DC voltage Vin is smoothed and converted to a DC voltage Vin, and the converted DC voltage Vin is applied to the first primary winding 103 a of the transformer 103. At this time, the DC voltage Vin is also supplied to the second primary winding 10 by the internal circuit current supply circuit 139 included in the control circuit 130.
3b, the capacitor 122 of the auxiliary power supply unit 120
Is charged.

Thereafter, in the auxiliary power supply section 120, when the auxiliary power supply voltage Vcc reaches the starting voltage of the control circuit 130, the control circuit 130 starts operating. Thereby, the control of the switching operation to the switching element 104 is started, and the start / stop circuit 140 stops the internal circuit current supply circuit 139.

The control circuit 130 controls the switching operation of the switching element 104 based on the auxiliary power supply voltage Vcc so that the output voltage Vo to the load 112 is stabilized at a predetermined voltage value. Specifically, the output voltage Vo to the load 112 and the auxiliary power supply voltage Vcc are
3 and a voltage proportional to the turns ratio between the second primary winding 103b and the secondary winding 103c, and an error voltage signal VEAO from the error amplifier 132 and a voltage from the drain current detection circuit 133 to the comparator 134. A comparison is made with the element current detection signal VCL, and when both the signals VEAO and VCL become equal to each other, a high-level output signal is output to the reset terminal of the RS flip-flop circuit 136.

Next, as shown in the timing chart of FIG. 8, when the current supply to the load 112 decreases and the load supply current Io decreases during a load change, the output voltage Vo slightly increases. In response, the auxiliary power supply voltage Vcc of the auxiliary power supply section 120 on the feedback side also increases, and the error amplifier 132
, The error voltage signal VEAO decreases.

When the error voltage signal VEAO and the element current detection signal VCL become equal in a state where the error voltage signal VEAO is lowered, such as when no load such as a load change or a standby time and when the load is light, a comparator 134 is used. , A reset signal is output to the reset terminal of the RS flip-flop circuit 136, so that a signal for turning off the switching element 104 is output from the NAND circuit 137 at a timing earlier than during a steady load. As a result, switching element 1
In No. 04, the on-state time during the switching operation is shortened, so that the drain current ID flowing through the switching element 104 decreases.

As described above, the control circuit 130 of the conventional switching power supply device controls the magnitude of the drain current ID flowing through the switching element 104 in accordance with the load supply current Io supplied to the load 112, thereby reducing the lightness. A current mode control method capable of reducing power consumption under load is employed.

(Second Conventional Example) Japanese Patent Application Laid-Open No. 10-304658, which is a second conventional example, discloses an switching power supply device that stabilizes an output voltage and has small power loss. The switching power supply according to this conventional example temporarily suppresses the on / off operation of the switching element when the value of the output voltage rises above the upper limit voltage, and turns off the on / off operation of the switching element when the output voltage falls below the lower limit voltage. The light-load switching control unit to be restarted has a configuration in which the switching unit is connected in parallel with the output-side capacitor.

[0018]

However, in the switching power supply according to the first conventional example, the drain current ID flowing through the switching element 104 is reduced at the time of a light load such as a standby time, but the drain current ID is completely reduced. Therefore, a certain amount of current flows even when there is no load. Therefore, even when there is no load, the current is lost due to the switching operation of the switching element 104.
04, the ratio of the current loss increases. as a result,
There is a problem in that the efficiency of the power supply is reduced and power saving during standby of the power supply cannot be achieved.

Further, as shown in Japanese Patent Application Laid-Open No. 10-304658, if a configuration is adopted in which an open / close control section for controlling the on / off operation of the switching element is provided on the output side, the control circuit on the input side can be used. There is a problem that the switching control unit at the time of light load on the output side must be completely electrically insulated, and cannot be integrated on one semiconductor chip.

An object of the present invention is to solve the above-mentioned conventional problems. An object of the present invention is to reduce the power consumption by reducing the switching loss at a light load with a simple configuration, thereby reducing the power consumption in a chopper type switching power supply. Is to be surely improved.

[0021]

A first switching power supply according to the present invention has a transformer, an input terminal connected to a first primary winding of the transformer, and a first switching power supply via the transformer. A switching element that receives the DC voltage, is connected to a secondary winding of the transformer, and rectifies and smoothes the secondary output voltage of the transformer, thereby converting the first DC voltage to the first DC voltage. An output voltage generation circuit for generating and outputting a second DC voltage smaller than the absolute value, a control circuit for controlling operation of the switching element, and a second primary winding of the transformer,
A power supply voltage generating circuit that generates a power supply voltage of a control circuit by generating a primary side output voltage proportional to the secondary side output voltage, and rectifying and smoothing the generated primary side output voltage; The control circuit generates and outputs a switching signal to be applied to the switching element, a current detection circuit detects a current flowing through the switching element and outputs the current as an element current detection signal, and a difference between a power supply voltage and a reference voltage. An error amplifier that generates and outputs an error voltage signal, a comparator that compares the element current detection signal and the error voltage signal, and outputs a comparison signal, and a current amount and output of a switching signal based on the comparison signal. A switching signal control circuit for controlling the switching signal control circuit when the error voltage signal is smaller than the lower limit voltage value. It stops outputting the switching signal,
A light load detection circuit that starts outputting a switching signal to the switching signal control circuit when the error voltage signal is larger than the upper limit voltage value.

According to the first switching power supply, when the current consumed at the time of light load decreases and the second DC voltage which is the output voltage of the device increases, the secondary output voltage, that is, the second DC voltage The value of the power supply voltage generated by the power supply voltage generation circuit that generates the primary-side output voltage is increased. As a result, the voltage value of the error voltage signal from the error amplifier that generates the error voltage signal composed of the difference between the power supply voltage for the control circuit and the reference voltage decreases. At this time, when the error voltage signal is smaller than the lower limit voltage value, the light load detection circuit stops outputting the switching signal to the switching element with respect to the switching signal control circuit. Since the power consumption under load can be reduced, the power efficiency of the chopper type switching power supply device can be improved.

A second switching power supply according to the present invention is a switching element having a transformer, an input terminal connected to a first primary winding of the transformer, and receiving a first DC voltage via the transformer. Connected to the secondary winding of the transformer, and rectifying and smoothing the secondary output voltage of the transformer, so that the first DC voltage is smaller than the absolute value of the first DC voltage from the first DC voltage. An output voltage generating circuit for generating and outputting the DC voltage of the second DC voltage, a control circuit for controlling the operation of the switching element, detecting a voltage value of the second DC voltage, and insulating the detected signal as a feedback signal to the control circuit. An output voltage detection circuit that feeds back in a state, is connected to a second primary winding of the transformer, generates a primary output voltage proportional to the secondary output voltage, and generates the generated primary output voltage. By rectifying and smoothing, the power supply of the control circuit A power supply voltage generation circuit for generating a voltage, wherein the control circuit generates and outputs a switching signal to be applied to the switching element, and a current detection for detecting a current flowing through the switching element and outputting it as an element current detection signal A circuit, a feedback voltage conversion circuit that detects a feedback signal included in the power supply voltage, converts the detected feedback signal into a feedback voltage signal that changes in a direction opposite to the increase or decrease, and outputs the feedback voltage signal, and an element current detection A comparator that compares the signal with the feedback voltage signal and outputs the compared signal; a switching signal control circuit that controls the current amount and output of the switching signal based on the comparison signal; Is smaller, the output of the switching signal to the switching element to the switching signal control circuit is stopped, and the feedback voltage signal is It is larger than limit voltage value and a light-load detection circuit starts outputting the switching signal to the switching signal control circuit.

According to the second switching power supply, when the current consumed at the time of light load decreases and the second DC voltage which is the output voltage of the device increases, the primary output which is proportional to the second DC voltage is increased. The value of the power supply voltage generated by the power supply voltage generation circuit that generates the voltage increases. Further, the amount of current of the feedback signal that is fed back from the output voltage detection circuit that detects the voltage value of the second DC voltage to the control circuit in an insulated state also increases. As a result, the power supply voltage of the control circuit rises, and the feedback voltage signal output from the feedback voltage transformation circuit and converted by the feedback signal changes in the direction opposite to the increase or decrease in the detected feedback signal. The voltage value decreases. At this time, when the feedback voltage signal is smaller than the lower limit voltage value, the light load detection circuit stops outputting the switching signal to the switching element with respect to the switching signal control circuit.
Since the loss in the switching element is reduced and the power consumption under a light load can be reduced, the power efficiency of the chopper type switching power supply device can be improved.

In the first or second switching power supply, it is preferable that the upper limit voltage is set to be larger than the lower limit voltage. With this configuration, for example, when the output of the switching signal to the switching element is stopped, the value of the second DC voltage decreases, and in the case of the first switching power supply, the voltage of the error voltage signal from the error amplifier is reduced. The value increases, and in the case of the second switching power supply, the voltage value of the feedback voltage signal from the feedback voltage conversion circuit increases. Here, if the error voltage signal or the feedback voltage signal exceeds the upper limit voltage value and becomes large, the light load detection circuit immediately starts outputting the switching signal to the switching signal control circuit, so that the output of the switching signal is stopped. Although the period can hardly be set, if the upper limit voltage value is made larger than the lower limit voltage value, a time margin (hysteresis characteristic) occurs until the error voltage signal or the feedback voltage signal exceeds the upper limit voltage value. The output suspension period of the switching signal can be reliably set.

In this case, the value of the upper limit voltage is about 20% of the maximum value of the amplitude in the element current detection signal, and the value of the lower limit voltage is about 15% of the maximum value of the amplitude in the element current detection signal.
%.

In the first or second switching power supply device, it is preferable that the light load detection circuit has a detection voltage varying means capable of variably setting a lower limit voltage or an upper limit voltage. By doing so, the load current value at the time of standby can be optimized, so that the number of options for a system incorporating this device can be increased.

[0028]

(First Embodiment) A first embodiment of the present invention.
An embodiment will be described with reference to the drawings.

FIG. 1 shows a schematic circuit configuration of a switching power supply according to a first embodiment of the present invention. As shown in FIG. 1, the switching power supply according to the first embodiment converts a smoothed first DC voltage applied to a main input terminal 10 from, for example, an AC current from a commercial power supply to a transformer ( While the voltage is applied to the primary side of the transformer 13, the switching operation by the switching element 14
Output voltage generation circuit 1 provided on the secondary side of transformer 13
6 is an insulated chopper type switching power supply which drops to an output voltage Vo which is a second DC voltage and outputs it to a main output terminal 17.

Hereinafter, the switching power supply device will be described in detail.

The transformer 13 has a first primary winding 13a,
It has a second primary winding 13b and a secondary winding 13c.

A rectifier 11 composed of a diode bridge or the like for rectifying an AC current and an input capacitor 12 for smoothing a rectified signal to generate a DC voltage Vin are connected in parallel to the main input terminal 10. . The generated DC voltage Vin is supplied to the first primary winding 13 of the transformer 13.
After being input to a, it is input to the drain terminal TD of the switching element 14 composed of, for example, an N-type power MOSFET. Here, the source terminal Ts of the switching element 14
Is connected to a low-level terminal of the main input terminal 10, and a control signal output from a control circuit 15 that controls the operation of the switching element 14 is input to a gate of the main input terminal 10.

An output voltage generating circuit 16 is connected to the secondary winding 13c of the transformer 13. The output voltage generation circuit 16 generates a first diode 16 that is generated by magnetic induction of the switched DC voltage Vin applied to the first primary winding 13a and rectifies a current due to the generated electromotive force.
1 and an output capacitor 16 for smoothing the rectified signal
And 2.

The main output terminal 17 connected to the output voltage generating circuit 16 has a load 18 connected between its high-level terminal and its low-level terminal, and the load 18 has a load supply current Io. Flows.

A power supply circuit 19 as a power supply voltage generating circuit for generating an auxiliary power supply voltage Vcc for the control circuit 15 is connected to the second primary winding 13b of the transformer 13. The power supply circuit 19 generates a second diode 191 that is generated by the switched DC voltage Vin applied to the first primary winding 13a and rectifies a current due to the generated electromotive force.
And a power supply capacitor 192 for smoothing the rectified signal.
It is composed of Here, the second primary winding 13
b is provided so that the auxiliary power supply voltage Vcc is proportional to the output voltage Vo. The auxiliary power supply voltage Vcc generated by the power supply circuit 19 is connected to the control terminal Tc of the control circuit 15.
Is applied to

In this embodiment, the area surrounded by the broken line 20, that is, the switching element 14 and the control circuit 15 are included, and at least three terminals of the drain terminal TD, the source terminal Ts, and the control terminal Tc are connected to the outside. An area where output is possible is called an on-substrate formation area 20, which indicates that the on-substrate formation area 20 can be formed on one semiconductor chip.

The control circuit 15 receives an oscillator 21 for generating and outputting a switching signal having an oscillation frequency of about 100 kHz, which is applied to the switching element 14, and an auxiliary power supply voltage Vcc dropped via a resistor at an opposite phase terminal. , An error voltage signal VEA comprising a difference from a reference voltage received at the positive phase terminal
An error amplifier 22 that generates and outputs O and a drain current ID flowing through the switching element 14 are detected, the detected drain current ID is converted into a voltage, and an element current detection signal V
A drain current detection circuit 23 that outputs the signal as CL; a drain current detection comparator 24 that compares the error voltage signal VEAO with the element current detection signal VCL and outputs a comparison signal; and a switching signal based on the comparison signal. Switching signal control circuit 25 for controlling current amount and output
When the error voltage signal VEAO is smaller than the lower limit voltage value, the output of the switching signal to the switching element 14 to the switching signal control circuit 25 is stopped, and when the error voltage signal VEAO is larger than the upper limit voltage value. Has a light load detection circuit 40 that starts outputting a switching signal to the switching signal control circuit 25. Here, the negative-phase input terminal of the error amplifier 22 is also connected to the source terminal Ts of the switching element 14 via a resistor.

Further, the control circuit 15 includes a drain terminal TD of the switching element 14 and a control terminal T
c, and an internal circuit current supply circuit 29 for supplying a start-up current to the control circuit 15, and an output side of the internal circuit current supply circuit 29 connected to the control circuit 15 via a switch. And a start / stop circuit 30 for controlling the operation of the switching signal control circuit 25 at the time of starting or stopping.

The switching signal control circuit 25 includes an RS flip-flop circuit 26 which receives the output signal of the light load detection circuit 40 at the set terminal S and receives the output signal of the drain current detection comparator 24 at the reset terminal R; Receives an output signal of the start / stop circuit 30 at an input terminal thereof and a maximum duty cycle signal M from the oscillator 21 at a second input terminal thereof.
A NAND circuit 27 that receives DC and receives an output signal from an RS flip-flop circuit 26 at a third input terminal;
A gate driver 28 comprising an inverter that receives an output signal of the AND circuit 27 and outputs a control signal obtained by inverting and amplifying the received output signal to the gate of the switching element 14.

The light load detection circuit 40 includes a reference voltage source 41
A light-load detecting comparator 42 which receives an error voltage signal VEAO from the error amplifier 22 at a positive-phase input terminal and receives a reference voltage VR from a reference voltage source 41 at a negative-phase input terminal, and a load at one input terminal. An AND circuit 43 receives an output signal of the detection comparator 42 and receives a clock signal CLK from the oscillator 21 at another input terminal. The reference voltage source 41 is configured to be able to change the value of the reference voltage VR in response to the output of the light load detection comparator 42.

The light load detection comparator 42 compares the input error voltage signal VEAO with the reference voltage VR, and when the error voltage signal VEAO is larger than the reference voltage VR,
A high-level signal is output to the AND circuit 43.
Conversely, when the error voltage signal VEAO is smaller than the reference voltage VR, a low-level signal is output to the AND circuit 43, so that the output signal of the RS flip-flop circuit 26 is at a low level. The output of the control signal from 28 can be stopped.

On the output side of the error amplifier 22, there is provided an overcurrent protection circuit 31 composed of a PNP-type bipolar transistor for clamping the maximum value of the error voltage signal VEAO, and the error voltage signal VEAO exceeds the clamp value. In this case, the overcurrent is short-circuited to the source terminal Ts of the switching element 14 so that the switching element 1
4 can be protected.

In the switching power supply according to the present embodiment, the DC voltage Vin and the output voltage Vo are not limited. However, as an example, the DC voltage Vin is 100 V to 100 V.
Assuming that the output voltage Vo is 200 V and the value of the output voltage Vo is 25 V, the number of components of the switching power supply device can be reduced by this one-chip configuration. realizable.

The switching element 14 has an N-type MOS
Although an FET is used, an NPN-type bipolar transistor may be used instead.

Hereinafter, the operation of the switching power supply device configured as described above will be described with reference to the drawings.

FIG. 2 shows the operation timing of the switching power supply according to the present embodiment. First, until the control circuit 15 is started, the start / stop circuit 30 is closed so as to connect the internal circuit current supply circuit 29 and the anode of the power supply capacitor 192 in the power supply circuit 19.

Next, when the apparatus is started up and an alternating current starts to be input to the main input terminal 10, the internal circuit current supply circuit 2
Current flows from 9 to the anode of the power supply capacitor 192, and the value of the auxiliary power supply voltage Vcc of the control circuit 15 increases. When the value of the auxiliary power supply voltage Vcc reaches the start-up voltage of the control circuit 15, the control circuit 15 can operate. Therefore, the start / stop circuit 30 connects the internal circuit current supply circuit 29 and the power supply capacitor 19 with each other. Disconnect. This allows the internal circuit current supply circuit 29 to operate only at the time of startup, so that power consumption of the control circuit 15 during normal operation can be suppressed.

Next, as shown in FIG. 2, during a steady load, the value of the reference voltage VR of the reference voltage source 41 is set to the lower limit voltage value VR1.

Thereafter, for example, when a load change that results in a light load such that the load supply current Io decreases, the load 18
Becomes excessive, the voltage value of the output voltage Vo slightly increases. As the value of the output voltage Vo increases, the auxiliary power supply voltage Vcc of the power supply circuit 19 on the feedback side also increases.

When the auxiliary power supply voltage Vcc increases, the voltage applied to the opposite phase terminal of the error amplifier 22 increases in the control circuit 15, so that the voltage value of the error voltage signal VEAO output from the error amplifier 22 decreases. . At this time, the voltage value of the element current detection signal VCL output from the drain current detection circuit 23 also decreases.

As described above, the switching power supply according to the present embodiment has a so-called current mode PW in which the pulse width of the switching signal is changed by the load supply current Io.
This is the M control method.

The light load detection comparator 42 which receives the error voltage signal VEAO at the positive phase terminal receives the error voltage signal VEAO.
When the value of O becomes smaller than the lower limit voltage value VR1, AND
Since a low-level signal is output to the circuit 43, the gate driver 28 of the switching signal control circuit 25 outputs only a low-level control signal, and the switching operation of the switching element 14 stops. At about the same time, the output voltage VR of the reference voltage source 41 receives the low level output signal of the light load detection comparator 42, and the lower limit voltage value V
The voltage is changed from R1 to the upper limit voltage value VR2.

When a light load or no load occurs as in a standby state, power is not supplied to the output voltage generating circuit 16, so that power is supplied to the load 18 only from the output capacitor 162. Therefore, the output voltage Vo gradually decreases. Thereby, the error amplifier 22
, The output voltage VR of the reference voltage source 41 is set to the upper limit voltage VR2 which is higher than the lower limit voltage VR1, and therefore, as shown in FIG. The switching operation is not immediately restarted.

Further, when the output voltage Vo decreases and the error voltage signal VEAO exceeds the upper limit voltage value VR2, the output signal from the light load detection comparator 42 becomes high level again and receives this. Since the AND circuit 43 can output a high-level output signal, the switching operation of the switching element 14 is restarted. At about the same time, the output voltage VR of the reference voltage source 41 is reset from the upper limit voltage value VR2 to the lower limit voltage value VR1 upon receiving the high level output signal of the light load detection comparator 42.

Next, when the switching operation by the switching element 14 is restarted in the standby state, the drain current ID flowing through the switching element 14 is larger than the current value when the light load is detected. Becomes excessive, the output voltage Vo rises again,
The error voltage signal VEAO from the error amplifier 22 decreases. Therefore, as described above, when the error voltage signal VEAO becomes smaller than the lower limit voltage value VR1, the output of the switching signal to the switching element 14 is stopped again.

In the first embodiment, the reference voltage source 4
The switching operation is stopped by detecting the light load state of the reference voltage VR output from the reference voltage VR1. Further, by changing the reference voltage VR from the lower limit voltage value VR1 to the upper limit voltage value VR2, the error voltage signal VEAO is changed. Even if the voltage rises, a hysteresis characteristic is given to the reference voltage VR so that the switching operation is not immediately started. As a result, while light load or no load is detected, the switching control of the switching element 14 is in an intermittent oscillation state in which the switching operation is repeatedly stopped and restarted.

The output voltage Vo drops during the switching stop period in the intermittent oscillation state, and the degree of the drop depends on the load supply current Io. In other words, as the load supply current Io decreases, the output voltage Vo decreases more gradually.

The switching suspension period in the intermittent oscillation state becomes longer as the load supply current Io becomes smaller. That is, the switching operation of the switching element 14 decreases as the load becomes lighter.

Note that if the light load detection voltage value for stopping or restarting the operation of the switching element 14 is set too high, the transformer 13 generates noise. On the other hand, if the light load detection voltage value is set too low, the operation will be intermittent (intermittent mode).
It is difficult to make the transition. Therefore, the optimum light load detection voltage value is determined by these trade-offs.
Therefore, the lower limit voltage value VR1 which is one light load detection voltage value
Is set to about 15% of the overcurrent protection voltage value that regulates the drain current ID flowing through the switching element 14, and the upper limit voltage value VR2, which is another light load detection voltage value, is set to about 20% of the overcurrent protection voltage value. Is preferred.

For example, in the case of a switching power supply having an output of 0.3 W, the power consumption of the conventional power supply is 1 W and the power supply efficiency is about 30%. Power is 0.45W and power efficiency is 6
7%, which confirms that low power consumption and high efficiency are achieved.

In addition, the switching power supply device according to the present embodiment includes only the primary-side, that is, the input-side control circuit 15 and the switching element 14 in the on-substrate formation region 20, so that the semiconductor integrated circuit is formed into one chip. Can be easily performed, and the number of parts can be reduced, so that the cost can be easily reduced.

(Modification of First Embodiment) A modification of the first embodiment of the present invention will be described below with reference to the drawings.

FIG. 4 shows a schematic circuit configuration of a switching power supply according to a modification of the first embodiment of the present invention. In FIG. 4, the same components as those shown in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.

As shown in FIG. 4, in the switching power supply according to the present modification, one end of the switching power supply is connected to a light load detection voltage adjusting terminal TR provided at the end of the on-substrate formation region 20 so as to detect a light load. A light load detection voltage adjusting resistor 51 is connected to the negative phase input terminal of the comparator 42 and the other end is connected to the low level side of the main input terminal 10 as detection voltage variable means.

As described above, the provision of the light load detection voltage adjusting resistor 51 allows the light load detection voltages VR1 and VR2 to be appropriately adjusted. The load supply current I when the switching operation of the switching element 14 is stopped or restarted.
o can be optimized. Therefore, even when the switching element 14 and the control circuit 15 are integrated into one chip, the lower limit voltage value VR1 or the upper limit voltage value VR2 of the light load detection circuit 40 can be changed according to the use of the power supply device. .

In this modification, the light-load detection voltage adjusting resistor 51 is provided outside the region 20 on the substrate. However, the present invention is not limited to this. You may make it incorporate in the formation area 20 on a board | substrate.

(Second Embodiment) Hereinafter, a second embodiment of the present invention will be described with reference to the drawings.

FIG. 5 shows a schematic circuit configuration of a switching power supply according to a second embodiment of the present invention. In FIG. 5, the same components as those of the switching power supply device shown in FIG.

As shown in FIG. 5, the switching power supply according to the second embodiment includes a secondary winding 13 of a transformer 13.
c in parallel with the output voltage V, which is the second DC voltage.
An output voltage detection circuit 61 for detecting o, and a photocoupler capable of feeding back the detected signal to the primary side control circuit 15 as a feedback signal Icc in an insulated state are provided. In the present embodiment, the photocoupler is connected to the output voltage detection circuit 61.
It is assumed that it is included in a part of

The photocoupler comprises a light emitting diode section 62a connected to the secondary side output voltage detecting circuit 61 and a light receiving transistor section 62b connected between the primary side power supply circuit 19 and the control terminal Tc. Have been.

Between the control terminal Tc and the comparator 24 for detecting the drain current and the light load detection circuit 40, the feedback fed back from the output voltage detection circuit 61 through the resistor and the first shunt regulator 63. A feedback voltage conversion circuit 64 that converts the signal Icc into a feedback voltage signal VCO that changes in the direction opposite to the increase and decrease and outputs the signal is connected.

The output voltage detecting circuit 61 is connected to the main output terminal 17
, A first resistor 611 and a second resistor 612 for dividing the output voltage Vo, and a control terminal connected to the first resistor 611 and the second resistor 612 at a connection portion 613.
And the anode is connected to the connection portion 61 of the second resistor 612.
3 and a second shunt regulator 614 composed of a thyristor whose cathode is connected to the cathode of the light emitting diode unit 62a. The anode of the light emitting diode unit 62a is connected to the cathode (output terminal) of the first diode 161 of the output voltage generation circuit 16 via a resistor.

A gain adjusting resistor 65 for the feedback voltage conversion circuit 64 is connected between the input terminal of the feedback voltage conversion circuit 64 and the source terminal Ts of the switching element 14.

The first shunt regulator 63 receives a voltage obtained by stepping down the feedback signal Icc at the source, and has a drain connected to the gain adjusting resistor 65.
Receiving the source potential of the P-type MOSFET at the opposite phase terminal,
The reference voltage is received at the positive phase terminal, and the comparison result is P-type MOSFET.
And a comparator for outputting to the gate of T. Thus, the feedback voltage conversion circuit 64 does not start until the auxiliary power supply voltage Vcc reaches a predetermined voltage.

The feedback voltage conversion circuit 64 receives a voltage generated in the gain adjusting resistor 65 by the feedback signal Icc at the negative phase terminal, receives a reference voltage at the positive phase terminal, and outputs the output of the comparator 641 to the gate. P-type MOSFE for receiving signals
T642, P-type MOSFET 6 for gate and drain
A first N-type MOSFET 643 receiving the output voltage of C.42
And a second N-type MOSFET 644 sharing a gate with the first N-type MOSFET 643. Second N-type MOSF receiving power supply voltage via resistor 645
The drain of the ET 644 is an output terminal of the feedback voltage signal VCO, and includes a first N-type MOSFET 643 and a second N-type MOSFET 643.
The source of the MOSFET 644 is connected to the source terminal Ts.

In the drain current detection comparator 24 according to the second embodiment, the element current detection signal VCL is not directly input to the opposite phase terminal. Instead, the opposite phase terminal is connected to the current source 66 and the PNP bipolar. It is connected between the transistor 67 and the emitter. On the other hand, the element current detection signal VCL
Is input to the base of the PNP bipolar transistor 67.

Hereinafter, the features of the configuration of the switching power supply according to the second embodiment will be described. In the first embodiment, since the secondary-side load state is detected only by the winding feedback to the primary side of the transformer 13, the feedback current flowing from the control terminal Tc is only about 50 μA. Therefore, it is necessary to increase the gain of the error amplifier 22.

On the other hand, in the second embodiment, an output voltage detection circuit 61 is provided on the secondary side to detect the load state on the secondary side, and the detection result is fed back to the primary side by a photocoupler. ing. As a result, the current amount of the feedback signal Icc, which is the feedback current flowing from the control terminal Tc into the control circuit 15, reaches about 10 mA, and the error amplifier 22 becomes unnecessary. Instead, the voltage V65 generated in the gain adjusting resistor 65 is configured to detect the secondary-side load state through the feedback voltage conversion circuit 64.

Hereinafter, the operation of the switching power supply device configured as described above will be described with reference to the drawings.

FIG. 6 shows the operation timing of the switching power supply according to this embodiment. First, until the control circuit 15 is started, the start / stop circuit 30 is closed so as to connect the internal circuit current supply circuit 29 and the anode of the power supply capacitor 192 in the power supply circuit 19.

Next, the apparatus is started up and the main input terminal 10 is activated.
When an alternating current is input to the internal circuit current supply circuit 29
, A current flows to the anode of the power supply capacitor 192, and the auxiliary power supply voltage Vcc of the control circuit 15 rises. When the auxiliary power supply voltage Vcc reaches the start-up voltage of the control circuit 15, the control circuit 15 can operate. Therefore, the start / stop circuit 30 includes the internal circuit current supply circuit 29 and the power supply capacitor 1
The connection with the terminal 92 is disconnected. This allows the internal circuit current supply circuit 29 to operate only at the time of startup, so that power consumption of the control circuit 15 during normal operation can be suppressed.

Next, as shown in FIG. 6, during a steady load, the value of the reference voltage VR of the reference voltage source 41 is set to the lower limit voltage value VR1.

Thereafter, for example, when a load change that results in a light load such that the load supply current Io decreases, the load 18
Becomes excessive, the voltage value of the output voltage Vo slightly increases. As the value of the output voltage Vo increases, the auxiliary power supply voltage Vcc of the power supply circuit 19 on the feedback side increases, and the current amount of the feedback signal Icc increases. Specifically, in FIG. 5, when the value of the output voltage Vo increases, the potential of the connection portion 613 for generating the resistance voltage division in the output voltage detection circuit 61 on the secondary side increases, and the second shunt regulator 614 increases. Becomes conductive. Thereby, a forward current flows through the light emitting diode portion 62a of the photocoupler,
From the light receiving transistor section 62b on the primary side to the control terminal Tc
Then, a feedback signal Icc proportional to the amount of light emitted from the light emitting diode unit 62a on the secondary side is injected.

When the amount of current of the feedback signal Icc injected into the control terminal Tc increases, the feedback voltage conversion circuit 64 in the control circuit 15 outputs the feedback signal Icc having the increased amount of current from the comparator 641 to the opposite-phase terminal. P-type MOSFET
The output value for the 642 gate drops. as a result,
Since the P-type MOSFET 642 has low impedance and the drain potential of the P-type MOSFET 642 rises, the second N-type MOSFET receiving the drain potential at its gate
The FET 644 also has a low impedance, and the second N
The voltage value of feedback voltage signal VCO output from the drain of type MOSFET 644 decreases. At this time, the element current detection signal VC output from the drain current detection circuit 23
The voltage value of L also decreases.

The light load detection comparator 42 receiving the feedback voltage signal VCO at the positive phase terminal receives the feedback voltage signal VCO.
When the value of O becomes smaller than the lower limit voltage value VR1, AND
Since a low-level signal is output to the circuit 43, the gate driver 28 of the switching signal control circuit 25 outputs only a low-level control signal, and the switching operation of the switching element 14 stops. At about the same time, the output voltage VR of the reference voltage source 41 receives the low level output signal of the light load detection comparator 42, and the lower limit voltage value V
The voltage is changed from R1 to the upper limit voltage value VR2.

In a light load or no load state such as during standby, power supply to the output voltage generating circuit 16 is stopped, so that power supply to the load 18 is performed only from the output capacitor 162. Therefore, the output voltage Vo gradually decreases. As a result, the feedback voltage signal VCO from the feedback voltage conversion circuit 64 gradually increases.
Since the output voltage VR of the reference voltage source 41 is set to the upper limit voltage VR2 higher than the lower limit voltage VR1, the switching operation by the switching element 14 is not immediately restarted.

Further, when the output voltage Vo decreases and the feedback voltage signal VCO exceeds the upper limit voltage value VR2, the output signal from the light load detection comparator 42 becomes high level again and receives this. Since the AND circuit 43 can output a high-level output signal, the switching operation of the switching element 14 is restarted. At about the same time, the output voltage VR of the reference voltage source 41 is reset from the upper limit voltage value VR2 to the lower limit voltage value VR1 upon receiving the high level output signal of the light load detection comparator 42.

Next, when the switching operation by the switching element 14 is restarted in the standby state, the drain current ID flowing through the switching element 14 is larger than the current value when the light load is detected. Becomes excessive, the output voltage Vo rises again,
The feedback voltage signal VCO from the feedback voltage conversion circuit 64 decreases. Therefore, as described above, when the feedback voltage signal VCO becomes smaller than the lower limit voltage value VR1, the output of the switching signal to the switching element 14 is stopped again.

Also in the second embodiment, the reference voltage source 4
The switching operation is stopped by detecting the light load state of the reference voltage VR output from 1 and the feedback voltage signal VCO is changed by changing the reference voltage VR from the lower limit voltage value VR1 to the upper limit voltage value VR2. Even if it rises,
The reference voltage VR is provided with a hysteresis characteristic so that the switching operation is not immediately started. As a result, while light load or no load is detected, the switching control of the switching element 14 is in an intermittent oscillation state in which the switching operation is repeatedly stopped and restarted.

The output voltage Vo decreases during the switching stop period in the intermittent oscillation state, and the degree of the decrease depends on the load supply current Io. In other words, as the load supply current Io decreases, the output voltage Vo decreases more gradually.

The switching suspension period in the intermittent oscillation state becomes longer as the load supply current Io becomes smaller. That is, the switching operation of the switching element 14 decreases as the load becomes lighter.

Also, in the second embodiment, if the light load detection voltage value at which the operation of the switching element 14 is stopped or restarted is set too high, the transformer 13 generates noise. On the other hand, if the light load detection voltage value is set too low, the transition to the intermittent operation state (intermittent mode) becomes difficult. Therefore, the optimum light load detection voltage value is determined by these trade-offs. Therefore, the lower limit voltage value VR1 that is one light load detection voltage is set to approximately 15% of the overcurrent protection voltage that regulates the drain current ID flowing through the switching element 14, and the upper limit voltage value V that is another light load detection voltage value is set.
Preferably, R2 is approximately 20% of the overcurrent protection voltage.

The output voltage detection circuit 61 shown in FIG.
The circuit configuration of the feedback voltage conversion circuit 64 is not limited to these, and may be any circuit configuration having equivalent functions.

[0094]

According to the first switching power supply of the present invention, there is provided an error amplifier for outputting an error voltage signal comprising a difference between a power supply voltage for a control circuit, which is generated by feedback from an output side, and a reference voltage. A light load detection circuit that stops outputting the switching signal to the switching element for the switching signal control circuit when the error voltage signal is smaller than the lower limit voltage value, so that the loss in the switching element is reduced, Since the power consumption under a light load can be reduced, the power efficiency of the chopper type switching power supply semiconductor device can be improved.

According to the second switching power supply of the present invention, the effect of the first switching power supply can be obtained, and the output voltage detection circuit for detecting the voltage value of the second DC voltage is provided. Since the amount of current of the feedback signal fed back from the output voltage detection circuit to the control circuit in an insulated state can be increased, it is not necessary to provide an error amplifier.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram showing a switching power supply device according to a first embodiment of the present invention.

FIG. 2 is a timing chart showing the operation of the switching power supply according to the first embodiment of the present invention.

FIG. 3 is a timing chart showing a reference voltage used for a light load detection comparator in the switching power supply according to the first embodiment of the present invention.

FIG. 4 is a schematic circuit diagram showing a switching power supply according to a modification of the first embodiment of the present invention.

FIG. 5 is a schematic circuit diagram showing a switching power supply according to a second embodiment of the present invention.

FIG. 6 is a timing chart showing an operation of the switching power supply according to the second embodiment of the present invention.

FIG. 7 is a schematic circuit diagram showing a conventional switching power supply device.

FIG. 8 is a timing chart showing an operation of a conventional switching power supply device.

[Explanation of symbols]

Reference Signs List 10 main input terminal 11 rectifier 12 input capacitor 13 transformer (transformer) 13a first primary winding 13b second primary winding 13c secondary winding 14 switching element 15 control circuit 16 output voltage generation circuit 161 first 162 Output capacitor 17 Main output terminal 18 Load 19 Power supply circuit (power supply voltage generation circuit) 191 Second diode 192 Power supply capacitor 20 Substrate formed area 21 Oscillator 22 Error amplifier 23 Current detection circuit 24 Drain current detection comparator 25 Switching signal control circuit 26 RS flip-flop circuit 27 NAND circuit 28 Gate driver 29 Internal circuit current supply circuit 30 Start / stop circuit 31 Overcurrent protection circuit 40 Light load detection circuit 41 Reference voltage source 42 Light load detection comparator 43 AND circuit 51 Light load Detection voltage adjusting resistor (detection voltage varying means) 61 Output voltage detection circuit 611 First resistor 612 Second resistor 613 Connection section 614 Second shunt regulator 62a Light emitting diode section 62b Light receiving transistor section 63 First Shunt regulator 64 Feedback voltage conversion circuit 641 Comparator 642 P-type MOSFET 643 First N-type MOSFET 644 Second N-type MOSFET 645 Resistor 65 Gain adjustment resistor 66 Current source 67 PNP bipolar transistor Ts Source terminal TD Drain Terminal Tc control terminal TR Light load detection voltage adjustment terminal

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Yoshiaki Hachiya, 1006 Kazuma Kadoma, Osaka Prefecture, Matsushita Electric Industrial Co., Ltd. 72) Inventor Yo Shiomi 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. CC01 DD04 EE02 EE07 FD01 FD24 FD41 FF19 FG02 VV01 XX15

Claims (5)

[Claims] 1. A transformer, a switching element having an input terminal connected to a first primary winding of the transformer, receiving a first DC voltage via the transformer, and a secondary of the transformer. Connected to a winding to rectify and smooth the secondary output voltage of the transformer,
An output voltage generating circuit that generates and outputs a second DC voltage smaller than the absolute value of the first DC voltage from the DC voltage of the first DC voltage; a control circuit that controls the operation of the switching element; 2 to generate a primary output voltage proportional to the secondary output voltage, and rectify and smooth the generated primary output voltage to control the control circuit. A power supply voltage generation circuit that generates a power supply voltage, wherein the control circuit generates and outputs a switching signal to be applied to the switching element, and detects a current flowing through the switching element, and detects the current flowing through the switching element as an element current detection signal. A current detection circuit for outputting, an error amplifier for generating and outputting an error voltage signal composed of a difference between the power supply voltage and the reference voltage, and an element current detection signal and the error voltage signal. Compare,
A comparator that outputs a compared comparison signal; a switching signal control circuit that controls a current amount and an output of the switching signal based on the comparison signal; and the switching when the error voltage signal is smaller than a lower limit voltage value. The output of the switching signal to the switching element is stopped for the signal control circuit, and the output of the switching signal to the switching signal control circuit is started when the error voltage signal is larger than the upper limit voltage value. A switching power supply device comprising a light load detection circuit. 2. A transformer, a switching element having an input terminal connected to a first primary winding of the transformer, receiving a first DC voltage via the transformer, and a secondary of the transformer. Connected to a winding to rectify and smooth the secondary output voltage of the transformer,
An output voltage generation circuit that generates and outputs a second DC voltage smaller than the absolute value of the first DC voltage from the DC voltage of the first DC voltage; a control circuit that controls the operation of the switching element; An output voltage detection circuit that detects a voltage value of a voltage and returns the detected signal to the control circuit as a feedback signal in an insulated state; and an output voltage detection circuit that is connected to a second primary winding of the transformer, A power supply voltage generating circuit that generates a power supply voltage of the control circuit by generating a primary side output voltage proportional to a voltage and rectifying and smoothing the generated primary side output voltage; A circuit configured to generate and output a switching signal to be applied to the switching element; a current detection circuit configured to detect a current flowing through the switching element and output the detected current as an element current detection signal; A feedback voltage conversion circuit that detects the feedback signal included in the source voltage, converts the detected feedback signal into a feedback voltage signal that changes in a direction opposite to the increase or decrease, and outputs the feedback voltage signal; and the element current detection signal and the feedback voltage. Compare with the signal
A comparator that outputs a compared comparison signal; a switching signal control circuit that controls a current amount and an output of the switching signal based on the comparison signal; and the switching when the feedback voltage signal is smaller than a lower limit voltage value. The output of the switching signal to the switching element is stopped for the signal control circuit, and the output of the switching signal to the switching signal control circuit is started when the feedback voltage signal is larger than the upper limit voltage value. A switching power supply device comprising a light load detection circuit. 3. The switching power supply according to claim 1, wherein the value of the upper limit voltage is set to be larger than the value of the lower limit voltage. 4. The value of the upper limit voltage is about 20% of the maximum value of the amplitude in the element current detection signal, and the value of the lower limit voltage is about 15% of the maximum value of the amplitude in the element current detection signal. The switching power supply device according to claim 3, wherein 5. The switching power supply according to claim 1, wherein the light load detection circuit includes a detection voltage varying unit that can variably set a value of the lower limit voltage or the upper limit voltage. apparatus.