JP2003304682A - Semiconductor device for controlling switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor device for controlling switching power supply, wherein soft start is accomplished and current loss due to a switching element can be reduced under lightload. <P>SOLUTION: Under lightload, two reference voltages are switched and compared with an error voltage VEAO through a light load detection circuit 18, and one of the reference voltages is set a light load detection lower limit voltage for stopping the oscillations of the switching element 1. The other is a light load detection upper limit voltage for starting oscillation of the switching element 1. Thus, the switching element 1 is caused to oscillate intermittently. The actuation start voltage V c<SB>-</SB>on of a start/stop circuit 9 is set to a value higher than a supply voltage V c<SB>-</SB>cnt1 when the error voltage VEAO matches with the light-load detection lower limit voltage. Thus, the switching element 1 is prevented from starting oscillation, until the error voltage VEAO exceeds the light load detection upper limit voltage, after the supply voltage VCC reaches the actuation start voltage V c<SB>-</SB>on at start time. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device for controlling a switching power supply, which can realize soft start at the time of starting and reduction of power consumption at the time of light load or no load.

[0002]

2. Description of the Related Art A conventional semiconductor device for controlling a switching power supply will be described below. FIG. 10 is a circuit diagram showing an example of a conventional semiconductor device for controlling a switching power supply.
In this semiconductor device 29, a switching element 1 such as a power MOSFET and a control circuit for performing switching control of the switching element 1 are integrated in one chip, and a high voltage terminal (DRAIN terminal) and a GND terminal of the switching element 1 are integrated. (SOURCE terminal) and the control terminal (CONTROL terminal) of the control circuit for inputting the control signal of the switching element 1.

In FIG. 10, the negative input terminal of the error amplifier 2 is supplied with the power supply voltage Vcc of the semiconductor device 29, and the positive input terminal thereof is supplied with a predetermined reference voltage. The error amplifier 2 compares the power supply voltage Vcc with the reference voltage, and when the power supply voltage Vcc is below the reference voltage, outputs the error voltage VEAO to the plus input terminal of the drain current detection comparator 4. That is, the power supply voltage Vcc and the error voltage VEAO have a relationship that the error voltage VEAO decreases as the power supply voltage Vcc increases. The negative input terminal of the drain current detection comparator 4 is supplied with the detection voltage VCL output from the drain current detection circuit 3 connected to the high voltage terminal of the switching element 1. The drain current detection circuit 3 includes a switching element 1
The switching element current ID flowing in is detected, converted into a voltage, and output as a detection voltage VCL.

The drain current detection comparator 4 detects an error voltage VEA between the detection voltage VCL corresponding to the switching element current ID and the power supply voltage Vcc of the semiconductor device 29 and the reference voltage.
O is compared, and when both are equal, a signal is output to the reset terminal of the RS flip-flop circuit 13.

The overcurrent protection circuit 5 fixes the maximum value of the error voltage VEAO output from the error amplifier 2 to prevent an overcurrent from flowing through the switching element 1. The oscillator 6 outputs a clock signal 7 for determining the switching frequency of the switching element 1 and a maximum duty cycle signal 8 for determining the maximum duty cycle of the switching element 1, respectively. The clock signal 7 output from the oscillator 6 is applied to the set terminal of the RS flip-flop circuit 13, and the output of the RS flip-flop circuit 13 is output to the NAND circuit 16. The maximum duty cycle signal 8 output from the oscillator 6 is directly input to the NAND circuit 16.

An internal circuit current supply circuit 14 for supplying a power supply current of the semiconductor device 29 is connected to the high voltage terminal of the switching element 1. The internal circuit current supply circuit 14 controls the start / stop circuit 27 for controlling the start and stop of the semiconductor device 29 so that the power supply voltage V
It operates only when cc is lower than the starting voltage. The output of the start / stop circuit 27 is NAND
It is input to the circuit 16.

The overheat protection circuit 15 is a circuit for stopping the oscillation of the switching element 1 when the chip temperature of the semiconductor device 29 rises above a set value. The output of the overheat protection circuit 15 is the NAND circuit 16 Has been entered in.

In the NAND circuit 16, the clock signal 7 of the switching element 1 output from the oscillator 6 via the RS flip-flop circuit 13, the maximum duty cycle signal 8 of the switching element 1 output from the oscillator 6, and the activation. / Start signal Von output from the stop circuit 27
_off and four signals output from the overheat protection circuit 15 are input, and the output is given to the drive circuit 17 of the switching element 1 as a switching control signal of the switching element 1.

The drive circuit 17 controls the switching of the switching element 1 based on the switching control signal given from the NAND circuit 16. FIG. 11 is a circuit diagram showing an example of a switching power supply device configured using a conventional semiconductor device for switching power supply control. In this switching power supply device, a commercial AC voltage is rectified by a rectifier 19 such as a diode bridge, smoothed by a capacitor 20, converted into a DC voltage VIN, and applied to a power conversion transformer 21.

The transformer 21 for power conversion includes a first primary winding 21a, a second primary winding 21b, and a secondary winding 2
1c, and the DC voltage VIN is applied to the first primary winding 21a.

The DC voltage VIN applied to the first primary winding 21a of the transformer 21 is switched by the switching element 1 provided in the semiconductor device 29. Then, due to the switching operation of the switching element 1, an electromotive force due to magnetic induction is generated in the secondary winding 21c of the transformer 21, and a current is taken out to the secondary winding 21c of the transformer 21. The current extracted to the secondary winding 21c is
The output voltage Vo is rectified and smoothed by the diode 24 and the capacitor 25 connected to the secondary winding 21c.
Is supplied to the load 26.

In the second primary winding 21b of the transformer 21, electromotive force due to magnetic induction is also generated by the switching operation of the switching element 1, and a current is taken out. The current output from the second primary winding 21b is rectified and smoothed by the diode 22 and the capacitor 23, which are auxiliary power supply units, and output as the power supply voltage Vcc.
The power supply voltage Vcc output from the auxiliary power supply unit is
The semiconductor device 29 is input to the control terminal of the semiconductor device 29 and
It is used as the power supply voltage of. This power supply voltage Vcc
Is a voltage proportional to the output voltage Vo supplied from the secondary winding 21c of the transformer 21 to the load 26.
It is also used as a feedback signal for stabilizing o.

The operation of the thus configured switching power supply device will be described below. When the AC voltage from the commercial power source is input to the rectifier 19, the rectifier 19 and the capacitor 2
It is rectified and smoothed by 0 and converted into a DC voltage VIN. This DC voltage VIN is applied to the first primary winding 21a of the transformer 21. In addition, DC voltage VIN
Is applied to the second primary winding 21b via the internal circuit current supply circuit 14 in the semiconductor device 29, and the power supply voltage Vcc
The capacitor 23 for charging is charged.

Thereafter, the power supply voltage Vcc is changed to the semiconductor device 29.
When the start-up voltage set by the start-up / stop circuit 27 in the internal circuit reaches the start-up voltage, the start-up signal Von_off output from the start-up / stop circuit 27 to the NAND circuit 16 becomes a high-level signal,
The control circuit is activated and control of the switching operation by the switching element 1 is started. At the same time, the start / stop circuit 27 outputs a signal Son_off for stopping the current supply from the internal circuit current supply circuit 14. By such an operation, the power consumption of the semiconductor device 29 during the normal operation is kept low.

The semiconductor device 29 controls the switching operation of the switching element 1 based on the power supply voltage Vcc so that the output voltage Vo to the load 26 is stabilized at a predetermined voltage. Output voltage Vo and power supply voltage Vc
The voltage c is a voltage proportional to the turn ratio between the second primary winding 21b and the secondary winding 21c of the transformer 21.

That is, as shown in the time chart of FIG. 12, the DC voltage VIN is applied to the second primary winding 21b via the internal circuit current supply circuit 14 in the semiconductor device 29 to supply the power supply voltage Vcc. The power supply voltage Vcc of the semiconductor device 29 rises by charging the capacitor 23 (FIG. 12 (c)). Then, when the power supply voltage Vcc reaches the activation start voltage Vc_on, the activation signal Von_off output from the activation / stop circuit 27 changes from low level to high level (FIG. 12 (e)), and is input to the NAND circuit 16 to switch the switching element. Oscillation of 1 is started. At the same time, the charging current from the internal circuit current supply circuit 14 to the capacitor 23 for the power supply voltage Vcc is turned off. Switching element 1
Is started, the current Io supplied to the load 26 is increased.
The output voltage Vo rises until the output voltage reaches the steady state, and the power supply voltage Vcc becomes a value according to the output voltage Vo.

However, in the conventional semiconductor device 29 for switching power supply control, the starting voltage Vc_on is set low, and the power supply voltage Vcc is set to the starting voltage Vc_on.
The oscillation of the switching element 1 is started at the same time. Therefore, the error voltage VEA from the error amplifier 2 at the time of starting the oscillation of the switching element 1 (at the time of starting)
The value of O becomes a high value (FIG. 12 (d)), and the error amplifier 2
During the period until the error voltage VEAO from the voltage drop to a constant voltage corresponding to the load 26, the switching element current ID oscillates in a current state equal to or higher than a constant value corresponding to the load 26, and thereafter, a constant voltage corresponding to the load. Value (Fig. 12
(F)). Therefore, at the time of starting, the current Io is rapidly supplied to the load 26 (FIG. 12 (b)), and the rise of the output voltage Vo becomes steep (FIG. 12 (a)).

Further, during a light load such as a standby time when the current Io supplied to the load 26 connected to the output of the switching power supply becomes small, the current Io supplied to the load 26 decreases (see FIG. 12 (b)). )), The output voltage Vo to the load 26 rises (FIG. 12A), and in proportion to this, the power supply voltage Vcc
Rises (FIG. 12 (c)), and the error voltage VEAO from the error amplifier 2 falls (FIG. 12 (d)). Then, when the error voltage VEAO becomes equal to the detection voltage VCL, the drain current detection comparator 4 outputs a reset signal to the reset terminal of the RS flip-flop circuit 13. As a result, the NAND circuit 16 causes the switching element 1
A signal to turn off is output. As a result, the switching element 1 has a shorter on-time, and the switching element current ID flowing through the switching element 1 is reduced (FIG. 12).
(e)).

Further, even when there is no load at which the current Io supplied to the load 26 becomes zero, the switching element current ID flowing through the switching element 1 can be reduced as in the case of a light load.

As described above, in this conventional semiconductor device for controlling a switching power supply, since it is necessary to supply the internal circuit current of the semiconductor device through the transformer, the switching element can be used even under light load or no load. The current that flows cannot be zero, and a certain amount of current will flow. Therefore, by controlling the switching element current according to the current supplied to the load connected to the output of the switching power supply device, as a current mode control method that can reduce the power consumption during light load or no load There is. In other words, at light load and no load,
By shortening the ON time of the switching element 1 to reduce the switching element current flowing through the switching element 1, the power consumption of the switching element during light load or no load is reduced.

However, since the switching element current is passed in synchronism with the clock signal from the oscillator, there is a limit to the reduction of power consumption by this conventional method, which is called power saving at light load or no load. There is a problem that the request cannot be realized.

[0022]

In order to solve the above problems, the present invention provides a light load detection lower limit voltage and a light load detection upper limit voltage, and when the error voltage VEAO falls below the light load detection lower limit voltage, a switching element is provided. The switching operation of the switching element is stopped and the switching operation of the switching element is started when the error voltage VEAO exceeds the light load detection upper limit voltage, so that the switching operation of the switching element is repeatedly stopped and restarted under light load or no load. Intermittent oscillation is used to reduce the current loss in the switching element and to activate the control circuit (semiconductor device) to a voltage higher than the power supply voltage Vcc when the error voltage VEAO matches the light load detection lower limit voltage. Set the starting voltage and start the power supply (when the switching element starts oscillating). After the voltage Vcc reaches the starting voltage, the switching operation of the switching element is started when the error voltage VEAO exceeds the light load detection upper limit voltage, so that the switching operation of the switching element is performed in a state where the error voltage VEAO is low. It is an object of the present invention to provide a semiconductor device for controlling a switching power supply that realizes a soft start and supplies a current to a load and a rise of an output voltage is gentle.

[0023]

A semiconductor device for controlling a switching power supply according to claim 1 of the present invention comprises a switching element for a switching power supply and a control circuit for controlling a switching operation of the switching element, A switching power supply control for comparing an error voltage between a power supply voltage of a circuit and a preset reference voltage with a voltage according to a current flowing through the switching element, and controlling a switching operation of the switching element based on the comparison result. In the semiconductor device for use, the control circuit stops the switching operation of the switching element when the power supply voltage of the control circuit rises and the error voltage falls below the light load detection lower limit voltage, and the power supply voltage of the control circuit decreases. When the error voltage exceeds the light load detection upper limit voltage, the switch of the switching element is switched. The light load detection circuit that starts the power supply operation and the start voltage for starting the control circuit have a voltage higher than the power supply voltage of the control circuit when the error voltage matches the light load detection lower limit voltage. And a start / stop circuit for switching the start / stop state of the control circuit according to the power supply voltage of the control circuit,
At the time of starting, when the power supply voltage of the control circuit reaches the start-up start voltage, the start-up / stop circuit sets the control circuit in the start-up state, and thereafter, the power-supply voltage of the control circuit decreases and the error voltage is lightly loaded. When the voltage exceeds the detection upper limit voltage, the switching operation of the switching element is started by the light load detection circuit, and during light load or no load, the switching operation of the switching element is repeatedly stopped and restarted by the light load detection circuit. Is characterized by.

A semiconductor device for controlling a switching power supply according to a second aspect of the present invention is the semiconductor device for controlling a switching power supply according to the first aspect, wherein the light load detection circuit includes the light load detection lower limit voltage. When the light load reference voltage source that switches and outputs the light load detection upper limit voltage is compared with the error voltage and the output voltage of the light load reference voltage source, and the error voltage falls below the light load detection lower limit voltage. A light load detection comparator that outputs a signal for stopping the switching operation of the switching element, and outputs a signal for starting the switching operation of the switching element when the error voltage exceeds the light load detection upper limit voltage. And is provided.

A semiconductor device for controlling a switching power supply according to claim 3 of the present invention is the semiconductor device for controlling a switching power supply according to claim 1 or 2, wherein the start / stop circuit is A start / stop reference voltage source for switching and outputting a start start voltage and a stop voltage for stopping the control circuit, and a power supply voltage of the control circuit and an output voltage of the start / stop reference voltage source are compared, When the power supply voltage of the control circuit exceeds the start start voltage, a signal for putting the control circuit into the start state is output, and when the power supply voltage of the control circuit falls below the stop voltage, the control circuit is put into the stop state. A first start / stop comparator that outputs a signal for

A semiconductor device for controlling a switching power supply according to claim 4 of the present invention is the semiconductor device for controlling a switching power supply according to claim 1 or 2, wherein the start / stop circuit is a resistor. A start-up for comparing the power supply voltage of one of the divided control circuits with an internal reference voltage, and outputting a signal for putting the control circuit into an activated state when the power supply voltage of the control circuit exceeds the activation start voltage. A signal for comparing the power supply voltage of the other resistance-divided control circuit and the internal reference voltage with each other and bringing the control circuit into a stopped state when the power supply voltage of the control circuit falls below the stop voltage. And a stop comparator for outputting.

A semiconductor device for controlling a switching power supply according to a fifth aspect of the present invention is the semiconductor device for controlling a switching power supply according to the first or second aspect, wherein the start / stop circuit is a resistor. When comparing the power supply voltage of one of the divided control circuits and the internal reference voltage,
When the power supply voltage of the control circuit exceeds the startup start voltage, a signal for putting the control circuit into a startup state is output, and the power supply voltage of the other resistance-divided control circuit is compared with the internal reference voltage. In this case, a second start / stop comparator is provided, which outputs a signal for bringing the control circuit into a stopped state when the power supply voltage of the control circuit falls below the stop voltage.

A semiconductor device for controlling a switching power supply according to a sixth aspect of the present invention is the semiconductor device according to any of the third to fifth aspects, in which the start / stop circuit is a power supply voltage of the control circuit. Is provided with a clamp circuit for clamping to a clamp voltage.

A semiconductor device for controlling a switching power supply according to a seventh aspect of the present invention is the semiconductor device for controlling a switching power supply according to the sixth aspect, wherein the clamp voltage has hysteresis.

As described above, according to the semiconductor device for controlling a switching power supply of the present invention, the switching operation of the switching element is intermittently oscillated by repeating stop and restart under light load or no load, and current loss in the switching element is caused. At the same time, the switching operation of the switching element is started with a low error voltage at the time of starting, and a soft start is realized to make the current supply to the load and the rise of the output voltage gentle. You can

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an example of a semiconductor device for controlling a switching power supply according to the present embodiment. The members corresponding to those described with reference to FIG. 10 are designated by the same reference numerals, and the description thereof will be omitted.

The semiconductor device 28 shown in FIG. 1 is provided with a light load detection circuit 18 to which the error voltage VEAO output from the error amplifier 2 is applied. This light load detection circuit 1
8 is provided with a light load detecting comparator 11. The error voltage VEAO output from the error amplifier 2 is applied to the positive input terminal of the light load detection comparator 11.
A reference voltage VR output from the light load reference voltage source 10 is applied to the negative input terminal. The light load detection comparator 11 receives the input error voltage VEAO and the reference voltage VR.
And a predetermined output signal VO1 is output to the AND circuit 12.
Is output. As a result, the light load detection circuit 18 can detect the light load state and the no load state.

The output signal VO1 of the light load detection comparator 11 is also supplied to the light load reference voltage source 10,
The light load reference voltage source 10 receives the output signal VO1 of the light load detection comparator 11 and changes the reference voltage VR.

The operation of the light load detection circuit 18 is shown in FIG.
, The output signal VO of the light load detection comparator 11
According to 1, the reference voltage VR from the light load reference voltage source 10
Is light load detection lower limit voltage VR1 and light load detection upper limit voltage VR
Switch to 2.

That is, when the error voltage VEAO falls below the light load detection lower limit voltage VR1, the light load detection comparator 11 operates as AN.
Since the low level signal is output to the D circuit 12, the oscillation of the switching element 1 is stopped. At this time, the light load reference voltage source 10 receives this signal and changes the output voltage to the light load detection upper limit voltage VR2. Then, the error voltage VE
When AO exceeds the light load detection upper limit voltage VR2, the light load detection comparator 11 outputs a high level signal to the AND circuit 12, and the oscillation of the switching element 1 restarts.
At this time, the light load reference voltage source 10 receives this signal and changes the output voltage to the light load detection lower limit voltage VR1.
By operating the light load detection circuit 18 in this manner, it is possible to create an intermittent oscillation state in which the switching operation of the switching element is repeatedly stopped and restarted under light load or no load. Further, the power supply voltage Vcc when the error voltage VEAO matches the light load detection lower limit voltage VR1 is set to Vc.
_cnt1, error voltage VEAO is light load detection upper limit voltage VR2
If the power supply voltage Vcc when Vcc_cnt2 is equal to Vc_cnt2, the relationship between them is Vc_cnt1> Vc_cnt2.

The AND circuit 12 is supplied with the clock signal 7 output from the oscillator 6 as another input,
The output of the AND circuit 12 is the RS flip-flop circuit 1
3 is given to the set terminal. In other words, during normal operation, the AND circuit 12 is supplied with the high-level signal from the light load detection comparator 11 and the clock signal 7 from the oscillator 6, and the switching element 1 operates at the switching frequency according to the clock signal 7. Switching operation is performed. The output of the RS flip-flop circuit 13 is NAND
It is output to the circuit 16. Therefore, the NAND circuit 16
Include the output signal of the RS flip-flop circuit 13, the maximum duty cycle signal 8 of the switching element 1 output from the oscillator 6, the output signal from the start / stop circuit 9, and the output signal from the overheat protection circuit 15. Have been entered respectively. Then, the output of the NAND circuit 16 is
It is given to the drive circuit 17 of the switching element 1 as a switching control signal of the switching element 1.

The drive circuit 17 controls the switching of the switching element 1 based on the switching control signal provided from the NAND circuit 16. FIG. 3 is a circuit diagram showing an example of a switching power supply device configured using the switching power supply control semiconductor device according to the present embodiment. The members corresponding to the members described with reference to FIG. 11 are designated by the same reference numerals and the description thereof will be omitted.

Next, examples of the configuration of the start / stop circuit 9 in the present embodiment are shown in FIGS. 4, 5 and 6. In the conventional semiconductor device, in the start / stop circuit, the reference voltage for setting the start state and the stop state of the control circuit (semiconductor device) is set low, and the oscillation of the switching element is started in the state where the power supply voltage Vcc is low. Was. Therefore, the error voltage VEAO at the time of oscillation of the switching element becomes high, and as a result, the current is rapidly supplied to the load and the output voltage Vo rises sharply.

Therefore, in the semiconductor device, in the start / stop circuit, the reference voltage (start start voltage Vc_on) for setting the control circuit (semiconductor device) in the start state and the reference voltage (stop voltage Vc_off) for setting the stop state. ) Are separately set, and the starting voltage Vc_on is set higher than the voltage Vc_cnt1 for stopping the oscillation of the switching element 1, so that the oscillation of the switching element is started in the state where the power supply voltage Vcc is high. . By doing so, the error voltage VEAO during oscillation of the switching element can be lowered, and soft start can be realized.

First, the start / stop circuit shown in FIG. 4 will be described. The start / stop circuit shown in FIG. 4 is provided with a start / stop comparator (first start / stop comparator) 40 for setting the start start voltage and the stop voltage of the control circuit. A reference voltage from the start / stop reference voltage source 41 is input to the negative input terminal of the start / stop comparator 40, and a signal obtained by resistance-dividing the power supply voltage Vcc for detecting the power supply voltage Vcc is input to the positive input terminal. Is entered. Then, the start / stop reference voltage source 41 is the start / stop comparator 4
When it receives an output of 0, it changes to two reference voltages. That is,
When the power supply voltage Vcc exceeds the start voltage Vc_on, the power supply voltage Vcc is compared with the stop voltage Vc_off.
Further, when the power supply voltage Vcc falls below the stop voltage Vc_off, the reference voltage is switched so that the power supply voltage Vcc is compared with the activation start voltage Vc_on. Then, when the power supply voltage Vcc exceeds the startup start voltage Vc_on, the startup signal Von_off
And the signal Son_off for stopping the current supply from the internal circuit current supply circuit 14 becomes a high level signal, and when the power supply voltage Vcc is below the stop voltage Vc_off, the start signal Von_off and the signal Son_off are low level signals. So that

Further, the start / stop circuit shown in FIG.
A clamp circuit including a clamp comparator 42 and a clamp switching element 43 is provided. A reference voltage (internal reference voltage) Vb1 is input to the positive input terminal of the clamp comparator 42, and a signal obtained by resistance-dividing the power supply voltage Vcc for detecting the power supply voltage Vcc is input to the negative input terminal. And the clamp comparator 4
The power supply voltage Vcc is clamped by inputting the output voltage of 2 to the switching element 43 for clamping. The power supply voltage Vcc when the switching element 43 for clamping is turned on becomes the clamp voltage Vc_clp, and the clamp voltage Vc_clp and the activation start voltage /
The resistance for detecting the power supply voltage Vcc and the reference voltage Vb1 are set so that the relationship of the stop voltage is Vc_clp>Vc_on> Vc_off (1).

Further, the relationship between the power supply voltage Vc_cnt1 when the error voltage VEAO matches the light load detection lower limit voltage VR1 and the power supply voltage Vc_cnt2 when the error voltage VEAO matches the light load detection upper limit voltage VR2 is Vc_clp. >Vc_on>Vc_cnt1>Vc_cnt2> Vc_off (2) The resistance for detecting the power supply voltage Vcc and the reference voltage Vb1 are set. Thus, the power supply voltage Vc
When c exceeds the clamp voltage Vc_clp, the power supply voltage V
By clamping cc with the clamp voltage Vc_clp, the recovery time from the oscillation stopped state can be shortened.

Next, the start / stop circuit shown in FIG. 5 will be described. The start / stop circuit shown in FIG. 5 is provided with a comparator for starting start voltage (comparator for start) 50 and a comparator for detecting stop voltage (stop comparator) 51 separately. A reference voltage (internal reference voltage) Vb1 is input to the negative input terminal of the comparator 50 for detecting the start-up voltage, and a signal obtained by resistance-dividing the power supply voltage Vcc for detecting the power supply voltage Vcc is input to the positive input terminal. Is entered. Further, a reference voltage (internal reference voltage) Vb1 is input to the minus input terminal of the comparator 51 for detecting the stop voltage, and a signal obtained by resistance-dividing the power supply voltage Vcc for detecting the power supply voltage Vcc is input to the plus input terminal. Is entered. Then, the output of the comparator 50 for detecting the start voltage is input to the set terminal of the RS flip-flop circuit 52, and the output of the comparator 51 for detecting the stop voltage is input to the reset terminal of the RS flip-flop circuit 52. Start voltage Vc_on
And the stop voltage Vc_off are set. That is, when the power supply voltage Vcc becomes the activation start voltage Vc_on, a signal is output to the set terminal to activate the activation signal Von_off and the signal S for stopping the current supply from the internal circuit current supply circuit 14.
A signal is output to the reset terminal so that on_off becomes a high level signal, and when the power supply voltage Vcc becomes the stop voltage Vc_off, the start signal Von_off and the signal Son
The power supply voltage Vcc is set so that _off becomes a low level signal.
Is divided by resistors and the reference voltage (internal reference voltage) V
Set b1.

Further, similarly to the start / stop circuit shown in FIG. 4, a clamp circuit having a clamp comparator 42 and a clamp switching element 43 is provided.
The power supply voltage Vcc when the switching element 43 for clamping is turned on becomes the clamp voltage Vc_clp,
A resistance for detecting the power supply voltage Vcc and a reference voltage (internal reference voltage) Vb1 are set.

In the start / stop circuit shown in FIG. 5, the reference voltages of the comparator 50 for detecting the start voltage, the comparator 51 for detecting the stop voltage, and the clamp comparator 42 are all the same value Vb1 (internal reference). Voltage), the start voltage Vc_on, the stop voltage Vc_off, and the clamp voltage Vc
It is characterized in that the relationship of c_clp can be more stably set to the state of Vc_clp>Vc_on> Vc_off (3).

Further, similar to the start / stop circuit shown in FIG. 4, the power supply voltage Vcc is detected so that the relationship between Vc_cnt1 and Vc_cnt2 is Vc_clp>Vc_on>Vc_cnt1>Vc_cnt2> Vc_off (4). A resistance and a reference voltage (internal reference voltage) Vb1 are set.

Next, the start / stop circuit shown in FIG. 6 will be described. The start / stop circuit shown in FIG. 6 is provided with a start / stop comparator (second start / stop comparator) 60 for setting the start start voltage and the stop voltage of the control circuit. A reference voltage (internal reference voltage) Vb1 is input to the positive input terminal of the start / stop comparator 60. Also,
The power source voltage Vcc is divided into two resistors by the negative input terminal.
Two voltage signals are input via the switching elements 62 and 63. By thus switching the signal input to the negative input terminal of the start / stop comparator 60, the start voltage Vc_on and the stop voltage Vc_off are set.

The operation of the start / stop circuit will be described below. First, before the control circuit is activated, NAN
The signal PU input to the D circuit 61 is at a low level, and as a result, before the control circuit is activated, the signal PU is NAN.
The output of the D circuit 61 is at a high level, and the switching element 62 is turned on. In addition, the inverter circuit 6
Since the output of 4 becomes low level, the start signal Von_off and the signal Son_off for stopping the current supply from the internal circuit current supply circuit 14 also become low level, the switching element 63 is turned off, and the signal is connected to the switching element 62. The voltage signal from the resistor is the start / stop comparator 60.
Is input to.

When this voltage signal exceeds the reference voltage Vb1, that is, the power supply voltage Vcc changes to the start-up voltage Vc.
When it exceeds _on, a high level signal is input from the start / stop comparator 60 to the NAND circuit 61 and the signal PU also becomes high level, so the output of the NAND circuit 61 becomes low level and the switching element 62 is turned off. It becomes a state. Further, since the output of the inverter circuit 64 becomes high level, the activation signal Von_off and the signal Son_off also become high level, the switching element 63 is turned on, and the voltage signal from the resistor connected to the switching element 63 is activated / deactivated. Input to the container 60.

That is, the power supply voltage Vcc is the start voltage V
When it exceeds c_on, the power supply voltage Vcc is the stop voltage Vc_off.
And the power supply voltage Vcc is equal to the stop voltage V
When it falls below c_off, the power supply voltage Vcc is the start-up voltage Vc.
A resistor for detecting the power supply voltage Vcc and a reference voltage (internal reference voltage) Vb1 are set so as to be compared with _on.

Further, similar to the start / stop circuit shown in FIG. 4, a clamp circuit having a clamp comparator 42 and a clamp switching element 43 is provided.
The power supply voltage Vcc when the switching element 43 for clamping is turned on becomes the clamp voltage Vc_clp,
A resistance for detecting the power supply voltage Vcc and a reference voltage (internal reference voltage) Vb1 are set.

In the start / stop circuit shown in FIG. 6, all the reference voltages of the start / stop comparator 60 and the clamp comparator 42 are set to the same value Vb1 (internal reference voltage), so that the start start voltage Vc_on is obtained. The relationship between the stop voltage Vc_off and the clamp voltage Vc_clp can be more stably set to a state of Vc_clp>Vc_on> Vc_off (5).
Further, since the start voltage and the stop voltage are detected by the single comparator 60, the number of constituent elements of the circuit can be reduced.

Further, similar to the start / stop circuit shown in FIG. 4, the power supply voltage Vcc is detected so that the relationship between Vc_cnt1 and Vc_cnt2 is Vc_clp>Vc_on>Vc_cnt1>Vc_cnt2> Vc_off (6). The resistance and the reference voltage Vb1 are set.

The operation of the switching power supply device using the semiconductor device for switching power supply control configured as described above will be described with reference to the time chart shown in FIG. First, the DC voltage VIN is applied to the second primary winding 21b via the internal circuit current supply circuit 14 in the semiconductor device 29, and the capacitor 23 for the power supply voltage Vcc is charged to supply the power supply voltage of the semiconductor device 29. Vcc rises (Fig. 7
(C)).

Then, the power supply voltage Vcc is equal to the starting voltage V
When it reaches c_on, the start signal Von_off output from the start / stop circuit 9 changes from low level to high level (see FIG. 7).
(E)) It is input to the NAND circuit 16 and the control circuit is activated, but at this point the power supply voltage Vcc is already higher than the voltage Vc_cnt1 (FIG. 7).
(C)), the output signal VO1 of the light load detection comparator 11 is at a low level (FIG. 7 (g)), and the switching operation of the switching element 1 does not start (FIG. 7).
(H)).

When the power supply voltage Vcc reaches the activation start voltage Vc_on, the activation signal Von_off changes from the low level to the high level, and the current supply from the high voltage terminal to the control terminal via the internal circuit current supply circuit 14 disappears. Power supply voltage V
cc decreases. The power supply voltage Vcc is the voltage Vc_cnt
When it decreases to 2, the output signal VO1 of the light load detection comparator 11 changes from low level to high level (see FIG.
(G)), the switching operation of the switching element 1 is started at this point for the first time (FIG. 7 (h)).

At this time, the power supply voltage Vcc is higher than the voltage value when the current Io supplied to the load 26 is in a steady state, so that the error voltage VEAO is low and the switching element current ID is small. Oscillation of No. 1 is started (FIG. 7 (h)). After that, the power supply voltage V
As the cc decreases, the switching element current ID increases and settles to a constant value according to the load state. By this operation, the switching element current ID at the time of starting the switching element is prevented from increasing, and the switching element current ID gradually changes from a small value to a large value, so that a soft start can be realized (FIG. 7).
(H)).

Next, the operation during a light load such as a standby time when the current Io supplied to the load 26 becomes small will be described with reference to the time chart of FIG. First, the transition operation from the steady load state to the light load state will be described. The light load detection comparator 11 compares the error voltage VEAO from the error amplifier 2 with the reference voltage VR from the light load reference voltage source 10. The reference voltage VR from the light load reference voltage source 10 is the light load detection lower limit voltage VR1 in the steady load state (FIG. 7 (f)). When the supply current Io to the load 26 connected to the output of the switching power supply device decreases (see FIG. 7).
(B)), the output voltage VO rises (FIG. 7 (a)), the power supply voltage Vcc rises in proportion to this (FIG. 7 (c)),
The output voltage VEAO of the error amplifier 2 decreases (see FIG. 7).
(D)).

When the error voltage VEAO falls below the light load detection lower limit voltage VR1, that is, when the power supply voltage Vcc rises to the voltage Vc_cnt1, the output signal VO1 of the light load detection comparator 11 becomes low level (FIG. 7). (G)). As a result, the output of the AND circuit 12 becomes low level,
The switching operation of the switching element 1 stops. At the same time, the output signal VO1 of the light load detection comparator 11
In response to the reference voltage VR from the light load reference voltage source 10.
Switches from the light load detection lower limit voltage VR1 to the light load detection upper limit voltage VR2 (FIG. 7 (f)).

When the switching operation by the switching element 1 is stopped and the switching element 1 is turned off, no current flows in the switching element 1 (FIG. 7 (h)). Therefore, the power supply to the load 26 is lost, and the output voltage VO to the load 26 gradually decreases. As a result, the error voltage VEAO gradually rises, but the reference voltage from the light load reference voltage source 10 becomes the light load detection upper limit voltage VR2 which is higher than the light load detection lower limit voltage VR1, and as shown in FIG. Moreover, the switching operation by the switching element 1 is not immediately restarted. When the output voltage VO to the load 26 further decreases and the error voltage VEAO exceeds the light load detection upper limit voltage VR2, that is, when the power supply voltage Vcc drops to the voltage Vc_cnt2, the output of the light load detection comparator 11 is output. The signal VO1 becomes high level, and the switching operation of the switching element 1 is restarted. At the same time, the light load reference voltage source 10 receives the output signal VO1 of the light load detection comparator 11.
Reference voltage VR is switched from the light load detection lower limit voltage VR2 to the light load detection upper limit voltage VR1 (FIG. 7).
(F)).

When the switching operation by the switching element 1 is restarted, a current is supplied to the load, the output voltage VO rises, and the output voltage VEAO of the error amplifier 2 falls. Then, when the error voltage VEAO falls below the light load detection lower limit voltage VR1 again, that is, when the power supply voltage Vcc rises to the voltage Vc_cnt1, the output signal VO1 of the light load detection comparator 11 becomes low level (see FIG. g)). As a result, the output of the AND circuit 12 becomes low level,
The switching operation of the switching element 1 stops.

Thus, the reference voltage VR from the light load reference voltage source 10 is the output signal V of the light load detection comparator 11.
Since the light load detection lower limit voltage VR1 and the light load detection upper limit voltage VR2 are switched according to O1, the switching operation of the switching element 1 at the time of a light load is in an intermittent oscillation state in which stop and restart are repeated. The output voltage VO to the load 26 decreases during the period during which the intermittent oscillation is stopped, and the degree of this decrease depends on the supply current to the load 26. That is, the smaller the current supplied to the load 26, the smaller the load 26.
The output voltage VO to the output decreases gradually, and the intermittent oscillation stop period becomes longer. That is, the lighter the load, the less the switching operation of the switching element 1.

Even when there is no load when the current Io supplied to the load 26 is zero, the switching operation of the switching element 1 can be brought into an intermittent oscillation state in which stop and restart are repeated as in the case of a light load. .

In the switching power supply control semiconductor device, the clamp voltage of the power supply voltage Vcc is set to Vc_clp by the clamp comparator 42.
When the power supply voltage Vcc exceeds the voltage Vc_clp, the switching element 43 for clamping is turned on, and when the power supply voltage Vcc falls below the voltage Vc_clp, the switching element 4 for clamping.
3 is turned off.

Here, it is better to set the clamp voltage to a value having hysteresis. Specifically, after reaching the clamp voltage Vc_clp once, the clamp release voltage Vc_clpR for recovering from the clamped state is set to a value lower than the clamp voltage Vc_clp. Also,
The relationship between the start voltage Vc_on and the stop voltage Vc_off is set to Vc_clp>Vc_on>Vc_clpR> Vc_off (7). If the clamp release voltage Vc_clpR is set in this way, after the power supply voltage Vcc reaches the clamp voltage Vc_clp, the power supply voltage Vcc is released from the clamp release voltage Vc_clp.
Since the switching element 43 for clamping is turned on until reaching R, the voltage drop of the power supply voltage Vcc can be accelerated. That is, the power supply voltage Vcc instantaneously rises due to an abnormal state on the secondary side of the switching power supply device or a sudden output voltage fluctuation, the power supply voltage Vcc becomes equal to or higher than the voltage Vc_cnt1, the oscillation of the switching element 1 stops, and further, Even when the clamp voltage Vc_clp is reached, the recovery time from the oscillation stopped state can be shortened.

An example in which a circuit for providing hysteresis in the clamp voltage is added to the start / start circuit as described above is shown in FIGS. The start / stop circuit shown in FIG.
In the start / stop circuit shown in (1), a reference voltage source 80 for clamping is added in order to provide hysteresis in the clamp voltage. The reference voltage source 80 is the clamp comparator 4
It is connected to the positive input terminal of 2 and receives the output of the clamp comparator 42 and outputs different reference voltages. That is, when the power supply voltage Vcc exceeds the clamp voltage Vc_clp, the switching element 43 for clamping is turned on to clamp the power supply voltage Vcc, and at the same time, the reference voltage output from the reference voltage source 80 is changed to clamp the power supply voltage Vcc. Hysteresis is provided in the clamp voltage by making it compared with the release voltage Vc_clpR.

The start / stop circuit shown in FIG. 9 is different from the start / stop circuit shown in FIG.
1, 92 and an inverter circuit 93 are added so that the power supply voltage Vcc when the switching element 43 for clamping turns on becomes the clamp voltage Vc_clp, and the switching element 43 for clamping changes from on to off. Power supply voltage Vcc at this time is the clamp release voltage Vc_clp
The power supply voltage Vcc is resistance-divided so that it becomes R.

The operation of the start / stop circuit will be described below. First, the power supply voltage Vcc is the clamp voltage Vc_c.
Before it exceeds lp, the signal P input to the NAND circuit 90
U is at a low level, and as a result, the output of the NAND circuit 90 is at a high level, and the switching element 91
Is turned on. Further, since the output of the inverter circuit 93 becomes low level, the switching element 92 is also turned off, and the voltage signal from the resistor connected to the switching element 91 is input to the clamp comparator 42.

When the power supply voltage Vcc rises and this voltage signal exceeds the reference voltage (internal reference voltage) Vb1,
That is, when the power supply voltage Vcc exceeds the clamp voltage Vc_clp, the clamping switching element 43 is turned on to clamp the power supply voltage Vcc, and at the same time, the signal PU also becomes high level, and the high-level signal from the clamp comparator 42 is NAND. Since it is input to the circuit 90, the output of the NAND circuit 90 becomes low level and the switching element 91
Is turned off. Further, since the output of the inverter circuit 93 becomes high level, the switching element 92 is turned on, the voltage signal from the resistor connected to the switching element 92 is input to the clamp comparator 42, and the power supply voltage Vcc is released from the clamp release voltage. Compare with Vc_clpR.

Then, when the power supply voltage Vcc drops and this voltage signal falls below the reference voltage b1, that is, the power supply voltage Vc.
When c falls below the clamp release voltage Vc_clpR, the switching element 43 for clamping is turned off.

In this start / stop circuit, the clamp voltage Vc_clp and the clamp release voltage Vc_clpR are set in this way, and the clamp voltage has hysteresis. Also,
In this start / stop circuit, the clamp voltage Vc_clp
And clamp release voltage Vc_clpR and start voltage Vc_on,
In order to stabilize the relationship of the stop voltage Vc_off, the reference voltages of the start / stop comparator 60 and the clamp comparator 42 are set to the same Vb1 (internal reference voltage).

[0072]

As described above, according to the semiconductor device for controlling a switching power supply of the present invention, at the time of starting, the switching operation of the switching element is started in a state where the error voltage is low, and the soft start is realized. The current supply to the IC and the rise of the output voltage can be made gradual.In addition, the switching operation of the switching element is intermittently oscillated by repeatedly stopping and restarting at light load or no load to reduce the switching operation period. Thus, the current loss in the switching element can be reduced.

Further, by providing the clamp circuit, even if the power supply voltage transiently rises due to an abnormal state of the secondary side of the switching power supply device or a sudden output voltage fluctuation, the oscillation of the switching element is stopped, The recovery time from this abnormal oscillation stop state can be minimized.

[Brief description of drawings]

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

FIG. 2 is a time chart diagram for explaining the operation of the light load detection circuit according to the embodiment of the present invention.

FIG. 3 is a circuit diagram showing an example of a switching power supply device configured using the switching power supply control semiconductor device according to the embodiment of the present invention.

FIG. 4 is a circuit diagram showing an example of a start / stop circuit according to an embodiment of the present invention.

FIG. 5 is a circuit diagram showing an example of a start / stop circuit according to an embodiment of the present invention.

FIG. 6 is a circuit diagram showing an example of a start / stop circuit according to an embodiment of the present invention.

FIG. 7 is a time chart diagram for explaining the operation of the switching power supply device configured using the semiconductor device for controlling the switching power supply according to the embodiment of the present invention.

FIG. 8 is a circuit diagram showing an example of a start / stop circuit according to an embodiment of the present invention.

FIG. 9 is a circuit diagram showing an example of a start / stop circuit according to an embodiment of the present invention.

FIG. 10 is a circuit diagram showing an example of a conventional semiconductor device for controlling a switching power supply.

FIG. 11 is a circuit diagram showing an example of a switching power supply device configured by using a conventional semiconductor device for controlling a switching power supply.

FIG. 12 is a time chart diagram for explaining the operation of a switching power supply device configured using a conventional semiconductor device for controlling a switching power supply.

[Explanation of symbols]

1 switching element 2 Error amplifier 3 Drain current detection circuit 4 Drain current detection comparator 5 Overcurrent protection circuit 6 oscillators 7 clock signals 8 Maximum duty cycle signal 9, 27 Start / stop circuit 10 Light load reference voltage source 11 Light load detection comparator 12 AND circuit 13 RS flip-flop circuit 14 Internal circuit Current supply circuit 15 Overheat protection circuit 16 NAND circuit 17 Drive circuit 18 Light load detection circuit 19, 22, 24 Rectifier 20, 23, 25 capacitors 21 transformer 21a First primary winding 21b Second primary winding 21c Secondary winding 26 load 28, 29 Semiconductor device for controlling switching power supply 40 Start / Stop Comparator (First Start / Stop Comparator) 41 Start / stop reference voltage source 42 Clamp comparator 43 Switching element for clamp 50 Comparator for detecting start-up voltage (start-up comparator) 51 Comparator for detecting stop voltage (comparator for stop) 52 RS flip-flop circuit 60 Start / Stop Comparator (Second Start / Stop Comparator) 61, 90 NAND circuit 62, 63, 91, 92 switching elements 64, 93 Inverter circuit Reference voltage source for 80 clamps

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 5H730 AA14 AS01 BB43 BB57 CC01                       DD04 EE02 EE07 EE56 FD25                       FD41 FF01 FF05 FG01 FG21                       VV03 VV06 XC04 XC14

Claims (7)

[Claims] 1. A switching element for a switching power supply, and a control circuit for controlling the switching operation of the switching element, wherein an error voltage between a power supply voltage of the control circuit and a preset reference voltage, and the switching element. In the semiconductor device for switching power supply control, which compares the voltage according to the current flowing through the device, and controls the switching operation of the switching element based on the comparison result,
The control circuit stops the switching operation of the switching element when the power supply voltage of the control circuit rises and the error voltage falls below the light load detection lower limit voltage, and the power supply voltage of the control circuit decreases to cause the error voltage. Is above the light load detection upper limit voltage, the light load detection circuit that starts the switching operation of the switching element, and the start voltage for starting the control circuit, the error voltage is equal to the light load detection lower limit voltage Is set to a voltage higher than the power supply voltage of the control circuit when
Start-up of the control circuit according to the power supply voltage of the control circuit
A start / stop circuit for switching the stop state, and at the time of start, when the power supply voltage of the control circuit reaches the start start voltage, the start / stop circuit sets the control circuit in the start state, and then the control circuit When the power supply voltage decreases and the error voltage exceeds the light load detection upper limit voltage, the light load detection circuit starts the switching operation of the switching element, and at the time of light load or no load,
A semiconductor device for controlling a switching power supply, wherein stop and restart of switching operation of a switching element are repeated by the light load detection circuit. 2. A light load reference voltage source for switching and outputting the light load detection lower limit voltage and the light load detection upper limit voltage, the light load detection circuit, and the output of the error voltage and the light load reference voltage source. A voltage, and outputs a signal for stopping the switching operation of the switching element when the error voltage is below the light load detection lower limit voltage, and the switching when the error voltage is above the light load detection upper limit voltage. The semiconductor device for controlling a switching power supply according to claim 1, further comprising a light load detection comparator that outputs a signal for starting a switching operation of the element. 3. A start / stop reference voltage source for the start / stop circuit to switch and output the start start voltage and a stop voltage for stopping the control circuit, a power supply voltage of the control circuit, and the start. / Comparing with the output voltage of the stop reference voltage source, when the power supply voltage of the control circuit exceeds the start start voltage, a signal for putting the control circuit into a start state is output, and the power supply voltage of the control circuit is 3. The switching power supply control according to claim 1, further comprising a first start / stop comparator which outputs a signal for bringing the control circuit into a stopped state when the voltage falls below a stop voltage. Semiconductor device. 4. The start / stop circuit compares a power supply voltage of one of the resistance-divided control circuits with an internal reference voltage, and when the power supply voltage of the control circuit exceeds the start start voltage, the control circuit The comparator for start-up that outputs a signal for setting the start-up state and the power-supply voltage of the other resistance-divided control circuit and the internal reference voltage are compared, and the power-supply voltage of the control circuit determines the stop voltage. 3. The switching power supply control semiconductor device according to claim 1, further comprising a stop comparator which outputs a signal for stopping the control circuit when the voltage falls below the limit. 5. When the start / stop circuit compares the power supply voltage of one of the resistance-divided control circuits with an internal reference voltage, the control is performed when the power supply voltage of the control circuit exceeds the start start voltage. When the power supply voltage of the control circuit is output below the stop voltage when outputting the signal for activating the circuit and comparing the power supply voltage of the other resistance-divided control circuit with the internal reference voltage. 3. The semiconductor device for controlling a switching power supply according to claim 1, further comprising a second start / stop comparator that outputs a signal for turning off the control circuit. 6. The semiconductor device according to claim 3, wherein the start / stop circuit includes a clamp circuit for clamping a power supply voltage of the control circuit to a clamp voltage. A characteristic semiconductor device for switching power supply control. 7. The semiconductor device for controlling a switching power supply according to claim 6, wherein the clamp voltage has hysteresis.