JP3490049B2 - Switching power supply - Google Patents

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 device, and more particularly to a step-down chopper type switching power supply device capable of reducing power consumption under light load.

[0002]

2. Description of the Related Art FIG. 6 shows a circuit configuration of a conventional switching power supply device described in Japanese Patent Laid-Open No. 10-191625. The conventional switching power supply device shown in FIG. 6 drops the positive DC voltage applied to the main input terminal 101 to a predetermined voltage value by the switching element 102 and the voltage conversion circuit 103, which are N-type MOSFETs, and outputs the main output. It is a step-down chopper type switching power supply device that outputs to a terminal 104.

The switching power supply device includes a control circuit power supply capacitor 10 connected in parallel between the source of the switching element 102 and the output side of the output voltage detection circuit 105.
The control circuit 107 has a control circuit 107 driven by the power supply voltage Vc generated by the control circuit 6, and the switching element 102 is controlled by a control signal Vg output from the control circuit 107. The power supply voltage Vc is the output voltage detection circuit 105.
Fluctuates according to the control current Ic output from.

An outline of the operation of the switching power supply device configured as described above will be described below. FIG. 7 shows current-voltage waveforms at various parts of the switching power supply device shown in FIG.

First, until the control circuit 107 is activated, the power source switching block 108 is activated by the activation power source block 10.
9 is closed so as to connect the control circuit power supply capacitor 106.

Next, when the input voltage Vin is applied to the main input terminal 101, the control circuit power supply capacitor 10 is passed from the starting power supply block 109 through the power supply switching block 108.
A current flows through 6, and the power supply voltage Vc of the control circuit 107 rises. When the value of the power supply voltage Vc becomes equal to or higher than the starting voltage value of the control circuit 107, the control circuit 107 operates. At this time, the output voltage Vo applied to the main output terminal 104 is 0V.
Is.

When the control circuit 107 starts its operation, the comparator 111 generates a triangular wave carrier signal voltage generated by the triangular wave generation circuit 110 constituting the control circuit 107 and a resistance-divided voltage of the power supply voltage Vc of the control circuit 107. Be compared.

The comparison signal output from the comparator 111 is P
It is input to the WM (pulse width modulation) pulse generation circuit 112, and as a result, the control signal Vg shown in FIG. 7 is applied to the control terminal of the switching element 102. This control signal Vg
Is turned on in a predetermined time width, and the power supply voltage Vc
It is variable depending on. While the control signal Vg is on, the switching element 102 is in the on state, and the drain current Ip flowing through the switching element 102 becomes the voltage conversion circuit 103.
Flows into the coil.

Next, when the switching element 102 is turned off by the control signal Vg of the control circuit 107,
The electric energy stored in the coil is supplied to the main output terminal 104 through the diode of the voltage conversion circuit 103. Here, the output voltage of the main output terminal 104 rises,
Power supply voltage Vc of control circuit 107, forward voltage Vf of diode of voltage conversion circuit 103, output voltage detection circuit 105
Forward voltage Vf of diode and output voltage detection circuit 1
When the breakdown voltage Vz of the No. 05 Zener diode becomes larger than the sum of the respective voltage values (Vc + Vf−Vf + Vz = Vc + Vz), the output voltage is detected from the high-level side terminal of the main output terminal 104 while the switching element 102 is in the off state. The control current I is supplied to the control circuit power supply capacitor 106 through the diode and the Zener diode of the circuit 105.
c flows in, and the value of the output voltage Vo is fed back to the control circuit 107. Here, when the power supply voltage Vc of the control circuit 107 becomes sufficiently high, the power supply switching block 108 switches the main output terminal 104 to supply the power supply voltage Vc to the control circuit 107.

Next, the comparator 111 compares the triangular wave carrier signal voltage generated by the triangular wave generation circuit 110 with the voltage obtained by resistance-dividing the power supply voltage Vc, and the switching element 102 in one triangular wave, that is, one carrier. Of the on-duty of the PWM pulse generation circuit 112
And the pulse width applied to the switching element 102 is determined.

As described above, in the conventional switching power supply device, the duty ratio of the switching element 102 is variably controlled by feeding back the output voltage Vo, and the main output terminal 1 is controlled.
By improving the voltage accuracy of 04, the main output terminal 1
The output voltage Vo of 04 is controlled to be a predetermined value.

[0012]

However, in the conventional switching power supply device, although the drain current Ip flowing through the switching element 102 becomes small at the time of light load or no load such as standby, this drain current Ip becomes zero.
Therefore, a certain amount of current flows even when the load is light. Therefore, even when the load is light, a loss due to switching occurs in the switching element 102, and the lighter the load, the greater the proportion of the loss in the switching element 102. As a result, the efficiency of the power source is lowered, and there is a problem that power saving cannot be achieved during standby of the power source.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems, reduce switching loss at light load to reduce power consumption, and improve power efficiency at light load in a chopper type switching power supply. To do.

[0014]

In order to achieve the above object, the present invention provides a switching power supply device with a power supply voltage of the control means which is detected by the output voltage detection means and is fed back to the control means. Based on this, the output of the switching signal to the switching means is stopped.

Specifically, a first switching power supply device according to the present invention receives a first DC voltage and a switching means for receiving a first DC voltage and an output signal from the switching means. Second less than absolute voltage
Voltage conversion means for converting and outputting to the DC voltage, control means for controlling the operation of the switching means, output voltage detection for detecting the voltage value of the second DC voltage, and feeding back the detected detection signal to the control means. Means and one electrode connected to the output side of the output voltage detection means, the other electrode connected to the output side of the switching means, the power supply voltage generation means for generating the power supply voltage of the control means, the control means , An oscillator for generating and outputting a switching signal to be applied to the switching means, and when the value of the power supply voltage of the control means is larger than the upper limit value, the output of the switching signal to the oscillator is stopped and the power supply voltage When the value is smaller than the lower limit value, it has a light load detection unit that starts outputting a switching signal to the transmission unit.

According to the first switching power supply device, when the value of the power supply voltage of the control means is larger than the upper limit value, the output of the switching signal to the oscillating unit is stopped, and the value of the power supply voltage of the control means is the lower limit. When it is smaller than the value, it has a light load detection unit that starts outputting a switching signal to the transmission unit, so that the current consumed at a light load is reduced and the output voltage of the device is the second voltage. When the DC voltage rises, the amount of current fed back from the output voltage detection means for detecting the voltage value of the second DC voltage to the control means increases. As a result, the power supply voltage of the control means rises, and the light load detection section of the control means stops the switching operation of the switching element at the time of light load, the loss in the switching means is reduced, and the power consumption at light load can be reduced. The power efficiency of the chopper type switching power supply device can be improved.

A second switching power supply device according to the present invention receives a first DC voltage at an input terminal and an output signal from the switching element,
Voltage conversion circuit for converting and outputting the DC voltage of the second DC voltage smaller than the absolute value of the first DC voltage, a control circuit for controlling the operation of the switching element, and a voltage of the second DC voltage. The output voltage detection circuit that detects the value and feeds back the detected detection signal to the control circuit, the anode is connected to the output side of the output voltage detection circuit, the cathode is connected to the output side of the switching element, and the power supply for the control circuit A control circuit power supply capacitor that generates a voltage is provided, and the control circuit generates an error voltage signal that is a difference between the power supply voltage and the reference voltage and an oscillator that generates and outputs a switching signal to be applied to the switching element. The error amplifier that outputs the current, the current detection circuit that detects the current flowing through the switching element and outputs the detected detection signal, the error voltage signal and the detection signal are compared, and the compared comparison signal is output. Output comparator, a switching signal control circuit that controls the current amount and output of the switching signal based on the comparison signal, and if the error voltage signal is smaller than the lower limit voltage value, switch the switching signal control circuit to the switching element. Output of the switching signal of
If the error voltage signal is greater than the upper limit voltage value has a light load detection circuit starts outputting the switching signal to the switching signal control circuit, a light load detection times
The path is a reference voltage source and one input terminal is
Output voltage, and the other input terminal receives the error voltage signal.
It has a light load detection comparator, and the output voltage of the reference voltage source.
The pressure value is lowered by the output signal from the light load detection comparator.
The limit voltage value or the upper limit voltage value is set .

According to the second switching power supply device, when the current consumed at a light load decreases and the second DC voltage which is the output voltage of the device rises, the voltage value of the second DC voltage is detected. The amount of current fed back from the output voltage detection circuit to the control circuit increases. At this time, the power supply voltage of the control circuit rises, and 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, the light load detection circuit stops the output of the switching signal to the switching signal control circuit when the error voltage signal is smaller than the lower limit voltage value. Since the power consumption under load can be reduced, the power efficiency of the chopper type switching power supply device can be improved.

In the second switching power supply device, it is preferable that the light load detection circuit has a hysteresis characteristic when transitioning from the output stop state of the switching signal to the output state. In this case, for example, when the output of the switching signal to the switching element is stopped, the value of the second DC voltage decreases and, conversely, the voltage value of the error voltage signal from the error amplifier increases. Here, when the error voltage signal exceeds the upper limit voltage value, the light load detection circuit immediately starts outputting the switching signal to the switching signal control circuit, so that the output stop period of the switching signal can hardly be set. However, by providing a hysteresis characteristic until the error voltage signal exceeds the upper limit voltage value,
It is possible to reliably set the output suspension period of the switching signal.

In the second switching power supply device, it is preferable that the light load detection circuit has detection voltage varying means for variably setting the value of the lower limit voltage or the upper limit voltage. In this way, the load current value during standby can be optimized, and the number of options for a system incorporating this device increases.

In the second switching power supply device, the reference potential of the control circuit is the same as the output terminal of the switching element, and the control circuit can detect the second DC voltage when the switching signal is in the off state. preferable. By doing so, control by the high-speed switching frequency becomes easy, and the second DC voltage which is the output voltage can be controlled with high accuracy. Further, since the reference potential of the control circuit is the same as that of the output terminal of the switching element, the control circuit and the switching element can be easily integrated into one chip.

In the second switching power supply device, it is preferable that the output voltage detection circuit includes a series connection circuit of the output voltage setting element and the diode. This way,
For example, the second DC voltage value can be easily set or changed simply by exchanging the output voltage setting element such as a Zener diode. Therefore, it is possible to realize a general-purpose power supply device that is as easy to use as a linear regulator.

In the second switching power supply device, the polarity of the second DC voltage is preferably negative. By doing so, it becomes possible to cope with a system that requires a negative control voltage source.

In the second switching power supply device, it is preferable that the value of the first DC voltage is approximately 100V or more and the value of the second DC voltage is approximately 25V or less. With this configuration, when the first AC voltage, which is the input voltage, is converted and input from the commercial AC power source, the cost is reduced,
Miniaturization and higher performance will be more remarkable.

In the second switching power supply device, the switching element and the control circuit are arranged so that the input terminal and the output terminal of the switching element and the input terminal on the anode side of the control circuit power supply capacitor in the control circuit serve as external connection terminals. It is preferably integrated and formed on one semiconductor substrate. With this configuration, the switching element and the control circuit can be integrated into one chip, so that the number of parts can be significantly reduced and the size of the switching power supply device can be reduced.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows a schematic circuit configuration of a switching power supply device according to an embodiment of the present invention. As shown in FIG. 1, in the switching power supply device according to the present embodiment, an input voltage Vin, which is a first positive DC voltage applied to a main input terminal 11, is supplied to a switching element 12 and an N-type power MOSFET. This is a step-down chopper type switching power supply device that drops to an output voltage Vo which is a second DC voltage having a predetermined voltage value by a voltage conversion circuit 13 and outputs it to the main output terminal 14.

A smoothing capacitor 10 for smoothing the input voltage Vin is connected between the high level side and the low level side of the main input terminal 11. A predetermined load 15 is connected between the high level side and the low level side of the main output terminal 14, and a load current Io flows through the load 15.

In this switching power supply device, the power supply voltage Vc generated by the control circuit power supply capacitor 17 connected in parallel between the source, which is the output terminal of the switching element 12, and the output side of the output voltage detection circuit 16. The switching element 12 is controlled by a control signal Vg output from the control circuit 18. That is, the reference potential of the source of the switching element 12 and the reference potential of the control circuit 18 are the same, so that the switching element 12 is substantially controlled by the power supply voltage Vc. Further, the power supply voltage Vc is changed by the control current output from the output voltage detection circuit 16.

In the voltage conversion circuit 13, the anode is the main output terminal 1
A first diode 131 which is connected to the low level side of 4 and whose cathode is connected to the high level side of the main output terminal 14;
A coil 132 connected between the cathode of the first diode 131 and the high level side of the main output terminal 14, a cathode connected to the low level side of the main output terminal 14, and an anode connected to the output side of the coil 132. It is composed of a connected capacitor 133.

The output voltage detection circuit 16 comprises a second diode 161 having anodes connected in series with each other and a Zener diode 162 as an output voltage setting element.
The cathode of the second diode 161 is connected to the anode of the power supply capacitor 17 for the control circuit, and the Zener diode 162 is connected.
The cathode of is connected to the high level side of the main output terminal 14.

The control circuit 18 determines the difference between the oscillator 21 which is applied to the switching element 12 and which generates and outputs a switching signal having an oscillation frequency of about 100 kHz, and the difference between the power supply voltage Vc dropped through the resistor and the reference voltage. Error amplifier 22 for generating and outputting the error voltage signal VEAO
And the current detection circuit 23 that detects the drain current ID flowing through the switching element 12 and outputs the detected detection signal VCL, and the error voltage signal VEAO and the detection signal VCL are compared, and the drain current that outputs the compared comparison signal is compared. The detection comparator 24, the switching signal control circuit 25 that controls the current amount and output of the switching signal based on the comparison signal, and the switching signal control circuit 25 when the error voltage signal VEAO is smaller than the lower limit voltage value. And the output of the switching signal to the switching element 12 is stopped, and when the error voltage signal VEAO is larger than the upper limit voltage value, the switching signal control circuit 25 starts outputting the switching signal and the light load detection circuit 40. have. Here, the negative phase input terminal of the error amplifier 22 is also connected to the source of the switching element 12 via a resistor.

Further, the control circuit 18 is connected between the drain of the switching element 12 and the negative phase input terminal of the error amplifier 22 and supplies an internal circuit current supply circuit 29 for supplying a current for starting to the control circuit 18. And a start / stop circuit 30 that is connected to the output side of the internal circuit current supply circuit 29 via a switch and controls the operation of the switching signal control circuit 25 when the control circuit 18 is started or stopped.

The switching signal control circuit 25 receives the output signal of the light load detection circuit 40 at the set terminal and the output signal of the drain current detection comparator 24 at the reset terminal, and the first input terminal. The terminal receives the output signal of the start / stop circuit 30, and the second input terminal receives the maximum duty cycle signal MDC from the oscillator 21.
Receiving the RS flip-flop circuit 2 at the third input terminal.
NAND circuit 27 for receiving the output signal from 6 and NAN
The gate driver 28 includes an inverter that receives the output signal of the D circuit 27 and outputs a control signal Vg that is obtained by inverting and amplifying the received output signal.

The light load detection circuit 40 includes a reference voltage source 41.
And a light load detection comparator 42 that receives the error voltage signal VEAO from the error amplifier 22 at its positive phase input terminal and a reference voltage VR from the reference voltage source 41 at its negative phase input terminal, and a load at one input terminal. The AND comparator 43 receives the output signal of the detection comparator 42 and receives the clock signal CLK from the oscillator 21 at the other input terminal. The reference voltage source 41 is set so that the value of the reference voltage VR can be changed by receiving 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,
It outputs a high level signal to the AND circuit 43.
On the other hand, 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 becomes low level. The output of the control signal Vg from 28 is stopped.

On the output side of the error amplifier 22, a PNP type bipolar transistor which short-circuits an overcurrent to the source of the switching element 12 when the error amplification signal VEAO output from the error amplifier 22 becomes excessive. An overcurrent protection circuit 31 is provided.

The switching power supply device according to this embodiment is also characterized in that the switching element 12 and the control circuit 18 are formed monolithically on the semiconductor substrate. The substrate formation region formed on the substrate at this time is represented by reference numeral 19 in FIG. This substrate forming region 19
A drain terminal TD connected to the drain of the switching element 12, a source terminal Ts connected to the source of the switching element 12, and a control terminal Tc connected to the anode of the control circuit power supply capacitor 17. Of at least three input / output terminals are provided.

Further, although the switching power supply device does not limit the voltage values of the input voltage Vin and the output voltage Vo, as an example, the value of the input voltage Vin is 100V to 200V and the value of the output voltage Vo is 25V. For example, since the number of components of the switching power supply device is reduced by this one-chip construction, the size of the switching power supply device can also be reduced, so that further downsizing and cost reduction can be realized.

Further, an N-type MOSFE is used as the switching element.
Although T is used, an NPN type bipolar transistor may be used.

The operation of the switching power supply device configured as described above at a light load will be described below with reference to the drawings.

FIG. 2 shows the operation timing of the switching power supply device according to this embodiment. First, until the control circuit 18 is activated, the activation / stop circuit 30 operates the internal circuit current supply circuit 29 and the control circuit power supply capacitor 16
Close to connect with the anode of.

Next, when the input voltage Vin is applied to the main input terminal 11, a current flows from the internal circuit current supply circuit 29 to the anode of the control circuit power supply capacitor 16 and the control circuit 18
The power supply voltage Vc of is increased. When the power supply voltage Vc becomes equal to or higher than the starting voltage of the control circuit 18, the control circuit 18 operates, so that the starting / stopping circuit 30 disconnects the internal circuit current supply circuit 29 from the control circuit power supply capacitor 16.

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.

After that, for example, if a load fluctuation occurs such that the load current Io decreases so that the load current Io decreases, the power supply to the load 15 becomes excessive and the voltage value of the output voltage Vo rises slightly. When the output voltage Vo rises, a current is supplied from the output voltage detection circuit 16 to the anode of the control circuit power supply capacitor 17, and the control voltage Vc also rises.

When the control voltage Vc rises, the control circuit 18
At, the voltage applied to the negative phase terminal of the error amplifier 22 rises, so that the voltage value of the error voltage signal VEAO output from the error amplifier 22 falls. At this time, the value of the detection signal VCL output from the drain current detection circuit 23 also decreases, so the switching power supply device according to the present embodiment is in a so-called current mode in which the pulse width of the switching signal is controlled by the load current Io. It can be said to be a PWM control method.

The light load detection comparator 42 which receives the error voltage signal VEAO at its positive phase terminal receives the error voltage signal V
When the value of EAO becomes smaller than the lower limit voltage value VR1, A
Since the low-level signal is output to the ND circuit 43, the gate driver 2 of the switching signal control circuit 25
8 outputs only the low level control signal Vg, and the switching operation of the switching element 12 is stopped. Almost at the same time, receiving the low-level output signal of the light load detection comparator 42, the output voltage VR of the reference voltage source 41 is changed from the lower limit voltage value VR1 to the upper limit voltage value VR2.

In a light load or no load state such as during standby, power is not supplied to the voltage conversion circuit 132, so that power is supplied to the load 15 by the capacitor 1.
Since it is started only from 33, the output voltage Vo
Gradually decreases. As a result, the error voltage signal VEAO from the error amplifier 22 gradually rises, but 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. Therefore, as shown in FIG. To
The switching operation by the switching element 12 is not immediately restarted.

Further, when the error voltage signal VEAO exceeds the upper limit voltage value VR2 due to the decrease of the output voltage Vo, the output signal from the light load detecting comparator 42 becomes the high level again. The receiving AND circuit 43 can output a high-level output signal, so that the switching operation of the switching element 12 is restarted.
Almost at the same time, the output voltage VR of the reference voltage source 41 is received in response to the high level output signal of the light load detection comparator 42.
Is reset from the upper limit voltage value VR2 to the lower limit voltage value VR1.

Next, when the switching operation by the switching element 12 is restarted in the standby state, the drain current ID flowing through the switching element 12 becomes larger than the current value at the time of detecting the light load. Power supply becomes excessive, the output voltage Vo rises again,
The error voltage signal VEAO from the error amplifier 22 drops. Here, the error voltage signal VEAO is the lower limit voltage value VR1.
When it becomes smaller than this, the output of the switching signal to the switching element 12 is stopped again.

In the present embodiment, the reference voltage VR output from the reference voltage source 41 stops the switching operation by detecting a light load, and the reference voltage VR is changed from the lower limit voltage value VR1 to the upper limit voltage value. Even if the error voltage signal VEAO rises by changing to VR2,
The hysteresis characteristic is given to the reference voltage VR so that the switching operation is not started immediately. Thereby, while the light load or the no load is detected, the switching control for the switching element 12 is in the intermittent oscillation state in which the stop and the restart of the switching operation are repeated.

The output voltage Vo drops during the switching stop period in the intermittent oscillation state, and the degree of this drop depends on the load current Io. That is, as the load current Io decreases, the output voltage Vo decreases more slowly.

The switching stop period in the intermittent oscillation state becomes longer as the load current Io becomes smaller. That is, as the load becomes lighter, the switching operation of the switching element 12 decreases.

Thus, taking a switching power supply device having an output power of 1 W as an example, in the present embodiment, compared with a conventional switching power supply device having a power consumption of 2.2 W and a power supply efficiency of 45%. The power consumption is 1.2 W and the power supply efficiency is 83%, so that low power consumption and high efficiency can be realized.

(First Modification) A first modification of the 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 device according to a first modification of the embodiment of the present invention. In FIG. 4, the same components as those shown in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

As shown in FIG. 4, the switching power supply device according to the first modification has a light load detection voltage adjustment terminal TR provided at one end at the end portion of the substrate forming region 19, and is provided with a light load. It has a light load detection voltage adjusting resistor 51 as a detection voltage varying means which is connected to the negative phase input terminal of the detection comparator 42 and has the other end connected to the low level side of the main input terminal 11.

As described above, even when the switching element 12 and the control circuit 18 are integrated into one chip by the resistor 51 provided outside the substrate formation region 19, the lower limit of the light load detection circuit 40 is obtained. The voltage value VR1 or the upper limit voltage value VR2 can be changed according to the application of the power supply device.

(Second Modification) A second modification of the embodiment of the present invention will be described below with reference to the drawings.

FIG. 5 shows a schematic circuit configuration of a switching power supply device according to a second modification of the embodiment of the present invention. In FIG. 5, the same components as those shown in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.

As shown in FIG. 5, in the switching power supply device according to the second modification, the configuration of the voltage conversion circuit 13A is as shown in FIG.
And the configuration of the voltage conversion circuit 13 in the switching power supply device of FIG. 4 is different.

That is, in the voltage conversion circuit 13A, the first diode 131 is connected in series between the source of the switching element 12 and the output terminal 14A and its cathode is connected to the source, and the coil 132 is connected to the first terminal. The first diode 13 in parallel with the first capacitor 133
1 is connected to the cathode side.

By adopting such a configuration of the voltage conversion circuit 13A, since the polarity of the main output terminal 14A can be made negative without changing the polarity of the main input terminal 11, the control voltage source of negative polarity can be obtained. In a system requiring the above, the negative voltage source can be realized without changing the configuration of each circuit on the substrate formation region 19 having the switching element 12 and the control circuit 18.

In the second modification as well, the light load detection voltage adjusting resistor 51 according to the first modification may be provided.

In the embodiment of the present invention and each modification, the input voltage Vin is assumed to be a DC voltage. Therefore, for example, when inputting an AC voltage, the input AC voltage may be rectified into a DC voltage and then input.

[0066]

According to the switching power supply device of the present invention, when the value of the power supply voltage of the control means is larger than the upper limit value, the output of the switching signal to the oscillating unit is stopped and the power supply voltage of the control means is changed. When the value is smaller than the lower limit value, it has a light load detection unit that starts outputting a switching signal to the transmission unit. Loss is reduced and power consumption at light load can be reduced, so that power efficiency can be improved.

[Brief description of drawings]

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

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

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

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

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

FIG. 6 is a schematic circuit diagram showing a conventional switching power supply device.

FIG. 7 is a timing chart showing an operation of a conventional switching power supply device.

[Explanation of symbols]

Reference Signs List 10 smoothing capacitor 11 main input terminal 12 switching element (switching means) 13 voltage conversion circuit (voltage conversion means) 131 first diode 132 coil 133 capacitor 14 main output terminal 15 load 16 output voltage detection circuit (output voltage detection means) 161 Second diode 162 Zener diode (output voltage setting element) 17 Power supply capacitor for control circuit (power supply voltage generation means) 18 Control circuit (control means) 19 Substrate formation region 21 Oscillator (oscillation part) 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 (Light Load Detection Section) 41 Criteria Voltage source 42 Light load detection comparator 43 AND circuit 51 Light load detection voltage adjusting resistor (detection voltage varying means) Ts source terminal TD drain terminal Tc control terminal TR light load detection voltage adjusting terminal 13A voltage conversion circuit (voltage Conversion means) 14A Main output terminal

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuharu Hayashi 1-1 Sachimachi, Takatsuki City, Osaka Prefecture Matsushita Electronic Industrial Co., Ltd. (72) Yuji Yamanishi 1-1 Sachimachi, Takatsuki City, Osaka Matsushita Electronic Industry Co., Ltd. (56) Reference JP-A-11-353038 (JP, A) JP-A-10-191625 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H02M 3/155

Claims (8)

(57) [Claims] 1. A switching element that receives a first DC voltage at an input terminal, and a first switching element that receives an output signal from the switching element.
Voltage conversion circuit that converts the DC voltage of the first DC voltage into a second DC voltage that is smaller than the absolute value of the first DC voltage and outputs the second DC voltage, a control circuit that controls the operation of the switching element, and the second DC voltage An output voltage detection circuit for detecting the voltage value of, and feeding back the detected detection signal to the control circuit, an anode is connected to the output side of the output voltage detection circuit, and a cathode is connected to the output side of the switching element, A control circuit power supply capacitor that generates a power supply voltage for the control circuit; and the control circuit, an oscillator that generates and outputs a switching signal applied to the switching element, and a difference between the power supply voltage and a reference voltage. An error amplifier that generates and outputs an error voltage signal consisting of: a current detection circuit that detects a current flowing through the switching element and outputs a detected detection signal; A comparator that compares a voltage signal with the detection signal and outputs a compared comparison signal; a switching signal control circuit that controls the current amount and the output of the switching signal based on the comparison signal; and the error voltage signal When it is smaller than the lower limit voltage value, the switching signal control circuit stops outputting the switching signal to the switching element, and when the error voltage signal is larger than the upper limit voltage value, the switching signal control circuit A light load detection circuit that starts outputting the switching signal with respect to the light load detection circuit , the light load detection circuit includes a reference voltage source and one input terminal.
Receives the output voltage from the reference voltage source, and the other input terminal
A light load detecting comparator whose child receives the error voltage signal.
The output voltage value of the reference voltage source is the light load detection value.
Output signal from the comparator for
A switching power supply device, wherein the switching power supply device is set to have the upper limit voltage value . Wherein said light-load detection circuit, according to claim 1, in which a transition in the output state from the output stop state of the switching signal, characterized in that it has a hysteresis characteristic
The switching power supply device according to. 3. The switching power supply device according to claim 1 , wherein the light load detection circuit includes detection voltage varying means for variably setting the value of the lower limit voltage or the upper limit voltage. 4. The reference potential of the control circuit is the same potential as the output terminal of the switching element, and the control circuit detects the second DC voltage when the switching signal is in an off state. Characterized by
The switching power supply device according to claim 1 . 5. The switching power supply device according to claim 1 , wherein the output voltage detection circuit includes a series connection circuit of an output voltage setting element and a diode. 6. The switching power supply device according to claim 1 , wherein the polarity of the second DC voltage is negative. 7. The value of the first DC voltage is approximately 100V.
The switching power supply device according to claim 1 , wherein the value of the second DC voltage is approximately 25 V or less. 8. The switching element and the control circuit are arranged such that an input terminal and an output terminal of the switching element and an input terminal on the anode side of the control circuit power supply capacitor in the control circuit are external connection terminals. The switching power supply device according to claim 1 , wherein the switching power supply device is integrated and formed on one semiconductor substrate.