JP2001078447A - Switching power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent excessive stress from exerting on a switching device. SOLUTION: The voltage of an input voltage source 100 is detected by voltage dividing resistors 28, 29, the detected voltage is supplied to a comparator 30 and compared with voltage from a reference voltage source 31, and the compared output turns on a switch 32. By turning on the switch 32, the resistor 33 is connected in parallel with a resistor 27. The charging time of a capacitor 20 is charged by the same current that passes through resistors 18, 27, 33 in an oscillator 19, and a saw-tooth frequency formed becomes high, so that the switching frequencies of the switching devices 2, 3 become high. That is to say, the higher the input voltage is, the more current passes through the switching devices 2, 3. By setting the switching frequency through overcurrent protection operation so as to be higher when the input voltage is high than when the input voltage is low, increase in switching loss is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a switching power supply circuit suitable for use in, for example, a power supply circuit of an electronic device. Specifically, in a switching power supply circuit driven by an upper-side composite resonance, protection against overload operation can be favorably performed.

[0002]

2. Description of the Related Art For example, an upper side composite current resonance converter (switching power supply circuit) is often used as a power supply circuit used in electronic equipment. That is, FIG. 8 shows a basic circuit configuration of such a composite current resonance converter.

In the circuit configuration shown in FIG. 8, the input voltage source 70 is alternately switched by switching elements 72 and 73 driven by a drive circuit 71, thereby connecting one of the connected resonance inductance 74 and the main transformer 75. The secondary winding 75 a, the resonance capacitor 76, the resonance current detection resistor 77, the charge / discharge capacitors 78 and 79 provided in parallel with the switching elements 72 and 73, and the commutation diode 8
The current flowing between 0 and 81 is changed. Thus, the secondary windings 75b, 75c, 75 of the main transformer 75
It supplies desired energy to the load connected to d ....

In this circuit, as the switching elements 72 and 73, for example, a MOS (Metal) is used.
Oxide (Semiconductor) type FET
(Field Effect Transistor)
When an element is used, the above-described charge / discharge capacity 78, 7
9, and the commutation diodes 80 and 81 can also be formed as parasitic devices built in the switching elements 72 and 73, respectively. The resonance inductance 74 includes
It is also possible to use the leakage inductance of the main transformer 75.

In order to explain the operation of this circuit, the switching elements 72 and 73 repeat exactly the same operation in which the phases are inverted. Therefore, the operation of the switching element 73 will be described below. That is, in FIG. 9, in a period a immediately after the switching element 72 is turned off by the drive signal from the drive circuit 71, the charge to the capacitor 78 and the discharge current from the capacitor 79 change from the resonance inductance 74 to the main transformer 7.
5, the primary winding 75a → the resonance capacitor 76 → the detection resistor 77 → the input voltage source 70. This current flows until the charging and discharging of the capacitors 78 and 79 are completed.

Next, in the period b after the charging and discharging of the capacitors 78 and 79 are completed, the resonance capacitor 76 is charged in the commutation mode via the commutation diode 81. In the period c after the completion of the charging, the driving circuit 71
From the resonance capacitor 76 via the switching element 73 which is turned on by the drive signal from the main transformer 75 → the primary winding 75a of the main transformer 75 → the resonance inductance 74 → the switching element 73 → the detection resistor 77. Is performed. This discharge is continued until the switching element 73 is turned off by the drive signal from the drive circuit 71.

When the switching element 73 is turned off, the charging and discharging currents of the capacitors 78 and 79 flow again.
At this time, the same operation as described above is performed on the switching element 72. The zero-cross operation of the switching elements 72 and 73 is almost ideally performed by the combined current waveform of the three modes of the resonance system. Thus, one cycle of operation is completed, and this operation is repeatedly performed in the above-described circuit. Note that FIG. 9 shows the waveforms of two different types of voltage at the time of the low input voltage and at the time of the high input voltage, respectively, on the left and right sides of the drawing.

On the other hand, the relationship between the input impedance and the switching frequency in this circuit is as shown by a curve a in FIG. 10 in a steady operation, for example, and the operation is performed in a range C of the curve a. That is, in this circuit, the oscillation frequency is set to the resonance capacitor 76 and the resonance inductance 74.
By changing at a frequency higher than the resonance frequency of the primary winding 75a of the main transformer 75,
A desired impedance can be obtained in the range of -B. Performing the operation while maintaining the state in which the operating frequency is higher than the resonance frequency is called upper side operation.

Therefore, in order to keep the output voltage constant in the above-described circuit, the switching frequency is changed according to the input voltage. Here, when the input voltage is low, the switching frequency is lowered because the output voltage is low, and conversely, when the input voltage is high, the switching voltage is raised because the output voltage is high. Therefore, in order to perform such control, the above-described circuit detects an output on the secondary side of the main transformer 75 and feeds back the detected output to the primary side to change the oscillation frequency of the drive circuit 71, Control for adjusting the switching frequency to keep the output voltage constant is performed.

That is, in the above-described circuit, for example, the output of the secondary winding 75b of the main transformer 75 is supplied to the smoothing capacitor 83b through the diode bridge circuit 82b. The output voltage formed by the smoothing capacitor 83b is supplied to the error amplifier 86 through the control resistor 84 and the light emitting diode 85a of the photocoupler 85. The output of the secondary winding 75c of the main transformer 75 is supplied to the error amplifier 86 by the diode bridge circuit 82, for example.
c, supplied through the smoothing capacitor 83c,
The voltage of the reference voltage source 87 is supplied.

Thus, the error amplifier 86 compares the voltage of the reference voltage source 87 with the output of the secondary winding 75b, and changes the amount of light emitted from the light emitting diode 85a according to the comparison result. Further, the light emitted from the light emitting diode 85a is received by the phototransistor 85b of the photocoupler 85, and the impedance of the phototransistor 85b is controlled. And this phototransistor 8
5b is connected through a resistor 88 to an oscillator 89, in which the connected capacitor 90 is charged with the same current as flowing through the resistor 88.

Further, the capacitor 90 is discharged when its voltage reaches a predetermined upper limit, and is charged again when the voltage becomes lower limit by this discharge. As a result, a sawtooth wave having an arbitrary frequency is formed in the capacitor 90, and the oscillation frequency of the oscillator 89 is determined based on the sawtooth wave. In accordance with the oscillation frequency, the drive signal is formed in the drive circuit 71 so as to alternately turn on / off at a duty of about 50%, and the switching operation of the switching elements 72 and 73 is performed.

That is, when the output voltage on the secondary side of the main transformer 75 becomes higher than the set value, the error is detected by the error amplifier 86 and the current flowing through the photocoupler 85 increases. As a result, the current flowing through the resistor 88 of the oscillator 89 increases, and the frequency of the sawtooth wave formed by shortening the charging time of the capacitor 90 increases. Here, FIG.
As shown by the curve a of 0, in the current resonance converter of the upper side operation, the higher the switching frequency of the switching elements 72 and 73 with respect to the resonance frequency, the higher the impedance becomes, and the power transmission to the secondary side is limited. You.

For this reason, in the above-mentioned circuit, when the output voltage increases, the switching frequency of the switching elements 72 and 73 increases, which limits the power transmission to the secondary side, and as a result, the output voltage decreases. Become.
Conversely, when the output voltage decreases, the current flowing through the photocoupler 85 decreases, the frequency of the sawtooth wave decreases, the power transmitted to the secondary side of the main transformer 75 increases, and the output voltage increases. You. Thus, in the circuit described above,
Control is performed so that the output voltage fluctuation is inversely corrected and made constant.

[0015]

At the time of the low input voltage shown on the left side of FIG. 9, the periods b and c in FIG.
In the example, the current flows through the primary winding 75a of the main transformer 75, but the current flows through the secondary side only in the middle of the periods b and c (period d). That is, in this circuit, there is a power transmission period in which power is supplied to the output via the main transformer 75, and a power non-transmission period in which current flows only through the primary winding 75a of the main transformer 75. Also, during the power transmission period, an exciting current component that does not contribute to the current transmitted to the secondary side flows through the primary side.

On the other hand, the conduction period of the current transmitted to the secondary side of the main transformer 75 is substantially fixed by the resonance inductance 74 and the resonance capacitor 76. Therefore, when the switching frequency is low and the conduction angle on the secondary side is small, In order to obtain the same output current, the peak value of the current increases. Also, as for the exciting current component flowing only on the primary side of the main transformer 75, the lower the frequency and the longer the on-time, the greater. As described above, the peak value of the current flowing through the switching element is different between when the input voltage is low and when it is high, even when the same output power is supplied.

Therefore, in the above-mentioned circuit, in a steady operation, for example, the higher the input voltage, the smaller the peak value of the current flowing through the switching element. The situation at this time is shown in the above-mentioned operation waveform at the time of a high input voltage on the right side of FIG. On the other hand, considering an operation in an overload state, when an overload such as a load short circuit occurs, the above-described relationship between the input impedance and the switching frequency is represented by, for example, a curve b
It becomes as shown in. In the curve b, as shown in the range D in the figure, the impedance inversion phenomenon occurs in the same operating frequency range as described above.

That is, at the time of overload operation, the resonance frequency may be higher than the frequency at the time of operation, and there is a possibility that a so-called lower-side operation may be performed. Once in such a state, the impedance of the system increases, the output voltage decreases, and the operation is in the direction of decreasing the switching frequency as in the control direction described above. This means that at the time of overload, the switching frequency is fixed to the lowest frequency determined by the external time constant, and the lower side operation different from the original upper side operation is continued on the switching frequency.

Further, when the converter shown in the principle diagram of FIG. 8 enters the lower side operation,
A sharp surge current appears due to the effects of the reverse recovery time of the commutation diodes 80 and 81 and the capacitance components at both ends of the switching elements 72 and 73. Thereby, the switching elements 72, 7
For example, a switching waveform (current) as shown in FIG. The flow of the switching waveform current in which such a sharp surge current appears causes a great stress on the switching elements 72 and 73.

Therefore, in order to prevent such an abnormal operation, for example, the resonance current is converted into a voltage by the detection resistor 77, and
When this voltage becomes equal to or higher than a set threshold value, the switching frequency is forcibly raised to a set high frequency (frequency control method), and the switching frequency is controlled so that the peak value of the resonance current becomes the threshold value ( Peak current limiting method) When the resonance current exceeds the threshold value in each cycle, the switching element is instantaneously pulse-by-pulsed.
It is necessary to provide an overcurrent protection circuit such as turning off with a pulse (peak current limiting method).

That is, in the circuit shown in FIG. 8, the voltage detected by the detection resistor 77 is supplied to the integration circuit of the resistor 92 and the capacitor 93 through the diode 91. Further, the integrated value is supplied to a comparator 94 and compared with a voltage from a reference voltage source 95. A comparison output when the integrated value exceeds the reference voltage is supplied to an oscillator 89.
When the comparison output is supplied, for example, an overcurrent protection operation as described in (1) to (5) is performed. FIG. 12 shows operation waveforms when such an overcurrent protection circuit operates.

However, in such an overcurrent mode, the current flowing through the switching element has a sawtooth shape, and the off-state current is large, so that the switching loss increases as compared with the steady state. When the input voltage further increases, the switching frequency is constant in the frequency control method, but the current peak value and the applied voltage both increase.In the peak current limiting method, the applied voltage increases even if the peak current is constant. , The loss may increase and the switching element may be subjected to more stress.

That is, the operation waveforms in these cases are shown in FIG.
3, as shown in FIG. Note that the waveform at the time of low input voltage and the waveform at the time of high input voltage are shown on the left side of the drawing.
Here, in the case of the frequency control method shown in FIG. 13, the switching frequency is constant, but the current peak value and the applied voltage are both large, thereby greatly increasing the loss of the switching element. In the case of the peak current limiting method shown in FIG. 14, the peak current is constant but the applied voltage is large, and the switching frequency is also high, so that the loss of the switching element is increasing.

When the loss of the switching element is increased in this way, excessive stress is applied to the switching element, which may lead to thermal destruction or the like. Conventionally, measures such as using a large-capacity switching element or providing a heat sink have been taken. However, for example, a switching element having a large capacity is expensive and may increase the manufacturing cost.
In addition, when a heat radiating plate is provided, a space for the heat radiating plate is required, and the installation thereof causes problems such as an increase in manufacturing cost.

The present application has been made in view of the above points, and a problem to be solved is that, for example, in a switching power supply circuit driven by an upper-side type composite resonance, the switching power supply circuit is required to operate within the operating frequency range. Although it is necessary to provide an overcurrent protection circuit to prevent the impedance reversal phenomenon in the above, in the conventional circuit, excessive stress may be applied to the switching element, leading to thermal destruction and the like. Increasing the capacity or providing a heat sink causes problems such as an increase in manufacturing cost and a need for a space for installation.

[0026]

Therefore, in the present invention, the switching frequency is controlled in accordance with the input voltage, or the set value for determining the overload operation is made variable in accordance with the input voltage. However, according to this, it is possible to prevent a situation in which excessive stress is applied to the switching element to cause thermal destruction or the like, particularly by suppressing an increase in the loss of the switching element when the input voltage is high. Special measures such as an increase in manufacturing cost caused for protection of the element and installation of special equipment can be eliminated.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the first means of the present invention is as follows.
A switching power supply circuit driven by an upper-side composite resonance, comprising overcurrent protection means for detecting a resonance current or a switching current and setting a switching frequency higher than a resonance frequency determined by a resonance impedance during an overload operation. And a control means for controlling the switching frequency according to the input voltage.

According to a second aspect of the present invention, there is provided a switching power supply circuit driven by an upper side composite resonance, wherein a resonance frequency determined by a resonance impedance during overload operation by detecting a resonance current or a switching current. Overcurrent protection means for setting a switching frequency higher than that of the first embodiment, and a set value for determining an overload operation by detecting a resonance current or a switching current in the overcurrent protection means is made variable in accordance with an input voltage. .

FIG. 1 is a block diagram showing the configuration of a first embodiment of an upper side composite current resonance converter to which a switching power supply circuit according to the present invention is applied. .

In FIG. 1, the input voltage source 100 is alternately switched by switching elements 2 and 3 driven by a drive circuit 1 to connect a resonance inductance 4 and a primary winding 5 a of a main transformer 5. The current flowing between the charge and discharge capacitors 8, 9 and the commutation diodes 10, 11 provided in parallel with the resonance capacitor 6, the resonance current detection resistor 7, and the switching elements 2, 3 is changed.
Thereby, the secondary windings 5b, 5c,
5d is to supply desired energy to the load connected thereto.

In this circuit, as the switching elements 2 and 3, for example, a MOS (Metal) is used. Ox
ide Semiconductor (FET) type FET (F
field Effect When a Transistor element is used, the above-described charge / discharge capacitances 8 and 9 and commutation diodes 10 and 11 can be formed as parasitic devices built in the switching elements 2 and 3, respectively. The resonance inductance 4 can use the leakage inductance of the main transformer 5.

Further, for example, the secondary winding 5 of the main transformer 5
The output of b is supplied to the smoothing capacitor 13b through the diode bridge circuit 12b. The output voltage formed by the smoothing capacitor 13b is supplied to the error amplifier 16 through the control resistor 14 and the light emitting diode 15a of the photocoupler 15. The output of the secondary winding 5c of the main transformer 5, for example, is supplied to the error amplifier 16 through the diode bridge circuit 12c and the smoothing capacitor 13c, and the voltage of the reference voltage source 17 is supplied.

Thus, the error amplifier 16 compares the voltage of the reference voltage source 17 with the output of the secondary winding 5b, and changes the amount of light emitted from the light emitting diode 15a according to the result of the comparison. Further, the light emitted from the light emitting diode 15a is received by the phototransistor 15b of the photocoupler 15, and the impedance of the phototransistor 15b is controlled. And this phototransistor 15
b is connected through a resistor 18 to an oscillator 19, which charges a connected capacitor 20 with the same current as flowing through the resistor 18.

The capacitor 20 is discharged when its voltage reaches a predetermined upper limit, and is charged again when the voltage reaches the lower limit. As a result, a sawtooth wave having an arbitrary frequency is formed in the capacitor 20, and the oscillation frequency of the oscillator 19 is determined based on the sawtooth wave. According to the oscillation frequency, the drive circuit 1 alternately turns on / off with a duty of about 50%.
A drive signal that is repeatedly turned off is formed, and the switching operation of the switching elements 2 and 3 is performed.

As a result, the main transformer 15
When the output voltage on the secondary side becomes higher than the set value, the error amplifier 1
At 6, it is detected and the current flowing through the photocoupler 15 increases. As a result, the current flowing through the resistor 18 of the oscillator 19 increases, and the frequency of the sawtooth wave formed by shortening the charging time of the capacitor 20 increases. Here, as described in the above description of FIG. 10, in the current resonance converter of the upper side operation, as the switching frequency of the switching elements 2 and 3 becomes higher with respect to the resonance frequency, the impedance becomes higher, and the power to the secondary side becomes higher. Communication is limited.

For this reason, in this circuit, when the output voltage increases, the switching frequency of the switching elements 2 and 3 increases, thereby restricting the power transmission to the secondary side, and as a result, the output voltage decreases. . In addition,
Conversely, when the output voltage decreases, the current flowing through the photocoupler 15 decreases, the frequency of the sawtooth wave decreases, the power transmitted to the secondary side of the main transformer 15 increases, and the output voltage increases. In this manner, in this circuit, control is performed to reversely correct the output voltage fluctuation and make it constant.

In this circuit, the resonance current is converted into a voltage by the detection resistor 7, and only the positive portion of the converted voltage is passed through the diode 21 through the resistor 22 and the capacitor 2.
3 and a voltage corresponding to the averaged resonance current is formed. Further, the integrated value is supplied to the comparator 24 and compared with the voltage from the reference voltage source 25, and the switch 26 is turned on by the comparison output when the integrated value exceeds the reference voltage. And this switch 2
By turning on 6, the resistor 27 is connected in parallel with the series circuit of the phototransistor 15b and the resistor 18.

As a result, when the switch 26 is turned on,
When the current flows through the resistors 18 and 27, the current flowing through the resistor 18 becomes equivalently large, and the charging time of the capacitor 20 charged with the same current is shortened. Wave frequency increases. That is, in this case, when the switch 26 is turned on, the control for increasing the switching frequency is performed more than the control for the steady operation through the photocoupler 15, and the frequency control method described in the overcurrent protection operation is performed. A protection operation is performed.

That is, at the time of overload or output short circuit, the resonance current increases because the impedance of the primary winding 15a of the main transformer 15 decreases. Therefore, the voltage generated in the detection resistor 7 increases, and when the integrated value exceeds the threshold voltage from the reference voltage source 25, the comparator 24 outputs an "H" signal. Then, the switch 26 is turned on by the "H" signal, and the charging current of the capacitor 20 of the oscillator 19 is increased by the resistor 27. Forcibly increasing the switching frequency prevents the lower side operation.

In the above configuration, since only the positive portion of the resonance current is extracted by the diode 21, the resonance current flowing through the switching element 2 is observed. There is also a method of observing the positive and negative voltages using two diodes. In that case, a resonance current flowing through the switching element 3 is also observed. There is also a method in which a detection resistor is inserted between the switching element 3 and the ground, and a switching current flowing through the switching element 3 is observed. These methods can also detect abnormalities such as overload and output short circuit.

Further, in this circuit, the voltage of the input voltage source 100 is detected by the voltage dividing resistors 28 and 29. This detection voltage is supplied to the comparator 30 and compared with the voltage from the reference voltage source 31, and the switch 32 is turned on by the comparison output when the detection voltage exceeds the reference voltage. When the switch 32 is turned on, the resistor 33 is connected in parallel with the resistor 27.
As a result, the charging current of the capacitor 20 of the oscillator 19 is further increased by the resistor 33, and the switching frequency is increased.

That is, as described above, in the operation at the same switching frequency, the higher the input voltage, the larger the current flowing through the switching elements 2 and 3. Therefore, in the above-described circuit, the switching frequency due to the overcurrent protection operation is set higher when the input voltage is high than when the input voltage is low, thereby preventing an increase in switching loss. FIG. 2 shows operation waveforms at this time. Here, the waveform at the time of the low input voltage on the left is the same as that in FIGS. 13 and 14 described above, and the switching loss is small in the waveform at the time of the high input voltage on the right as compared with this.

Therefore, in this circuit, by controlling the switching frequency in accordance with the input voltage, it is possible to suppress an increase in the loss of the switching element especially when the input voltage is high, and excessive stress is applied to the switching element. It can prevent situations such as thermal destruction,
Special measures such as an increase in manufacturing cost caused for protection of the switching element and installation of special equipment can be eliminated.

Thus, for example, in a switching power supply circuit driven by the upper side composite resonance,
It is necessary to provide an overcurrent protection circuit to prevent the impedance reversal phenomenon within the operating frequency range.However, in the conventional circuit, excessive stress may be applied to the switching element, leading to thermal destruction, etc. As
According to the present invention, these problems can be easily solved by increasing the capacity of the switching element or providing a heat sink, which causes problems such as an increase in manufacturing cost and a space for installation. It can be eliminated.

In the above-described circuit, the switching frequency at the time of the overcurrent protection operation is switched in two stages. For example, as shown in FIG. Depending on the resistor 2
The current value of the variable current source 35 provided in place of 7, 33 may be controlled. Thus, for example, by configuring the current value of the variable current source 35 to increase as the output of the operational amplifier 34 increases, it is possible to minimize an increase in switching loss over the entire input voltage range. More accurate overcurrent protection can be performed.

FIG. 4 is a block diagram showing a configuration of a second embodiment of an upper side composite current resonance converter to which the switching power supply circuit according to the present invention is applied. In FIG. 4, the same reference numerals are given to portions corresponding to those in FIG.

In FIG. 4, the input voltage source 100 is alternately switched by switching elements 2 and 3 driven by the drive circuit 1 to connect the resonance inductance 4 and the primary winding 5 a of the main transformer 5. The current flowing between the charge and discharge capacitors 8, 9 and the commutation diodes 10, 11 provided in parallel with the resonance capacitor 6, the resonance current detection resistor 7, and the switching elements 2, 3 is changed.
Thereby, the secondary windings 5b, 5c,
5d is to supply desired energy to the load connected thereto.

Further, for example, the secondary winding 5 of the main transformer 5
The output of b is supplied to the smoothing capacitor 13b through the diode bridge circuit 12b. The output voltage formed by the smoothing capacitor 13b is supplied to the error amplifier 16 through the control resistor 14 and the light emitting diode 15a of the photocoupler 15. The output of the secondary winding 5c of the main transformer 5, for example, is supplied to the error amplifier 16 through the diode bridge circuit 12c and the smoothing capacitor 13c, and the voltage of the reference voltage source 17 is supplied.

Thus, the error amplifier 16 compares the voltage of the reference voltage source 17 with the output of the secondary winding 5b, and changes the amount of light emitted from the light emitting diode 15a according to the comparison result. Further, the light emitted from the light emitting diode 15a is received by the phototransistor 15b of the photocoupler 15, and the impedance of the phototransistor 15b is controlled. And this phototransistor 15
b is connected through a resistor 18 to an oscillator 19, which charges a connected capacitor 20 with the same current as flowing through the resistor 18.

Further, the capacitor 20 is discharged when its voltage reaches a predetermined upper limit, and is charged again when the voltage becomes lower limit by this discharge. As a result, a sawtooth wave having an arbitrary frequency is formed in the capacitor 20, and the oscillation frequency of the oscillator 19 is determined based on the sawtooth wave. According to the oscillation frequency, the drive circuit 1 alternately turns on / off with a duty of about 50%.
A drive signal that is repeatedly turned off is formed, and the switching operation of the switching elements 2 and 3 is performed.

As a result, the main transformer 15
When the output voltage on the secondary side becomes higher than the set value, the error amplifier 1
At 6, it is detected and the current flowing through the photocoupler 15 increases. As a result, the current flowing through the resistor 18 of the oscillator 19 increases, and the frequency of the sawtooth wave formed by shortening the charging time of the capacitor 20 increases. Here, as described in the description of FIG. 10 above, in the current resonance converter of the upper side operation, as the switching frequency of the switching elements 2 and 3 becomes higher with respect to the resonance frequency, the impedance becomes higher and the power to the secondary side becomes higher. Communication is limited.

For this reason, in this circuit, when the output voltage increases, the switching frequency of the switching elements 2 and 3 increases, thereby restricting power transmission to the secondary side, and as a result, the output voltage decreases. . In addition,
Conversely, when the output voltage decreases, the current flowing through the photocoupler 15 decreases, the frequency of the sawtooth wave decreases, the power transmitted to the secondary side of the main transformer 15 increases, and the output voltage increases. In this manner, in this circuit, control is performed to reversely correct the output voltage fluctuation and make it constant.

Further, in this circuit, the resonance current is converted into a voltage by the detection resistor 7, and the positive part of the converted voltage is supplied to the integrating circuit of the resistor 22 and the capacitor 23 through the diode 21, and the averaged resonance voltage is converted. A voltage corresponding to the current is formed. Further, this integrated value is calculated by the comparator 24.
And compared with the voltage from the reference voltage source 25. When the integrated value exceeds the reference voltage, the comparison output is output to the oscillator 1.
9. When this comparison output is supplied, a protection operation such as the peak current limiting method described in the above-mentioned overcurrent protection operation is performed.

That is, at the time of overload or output short-circuit, the resonance current increases because the impedance of the primary winding 15a of the main transformer 15 becomes small. Therefore, the voltage generated in the detection resistor 7 increases, and when the integrated value exceeds the threshold voltage from the reference voltage source 25, the comparator 24 outputs an "H" signal. This "H" signal is supplied to the oscillator 19, for example, to discharge the capacitor 20 instantaneously and to turn off the switching element 2 or to control the charging current so that the current detection value is equal to the set value. 2 is limited.

In the above-described configuration, only the positive portion of the resonance current is extracted by the diode 21, so that the resonance current flowing through the switching element 2 is observed. There is also a method of observing the positive and negative voltages using two diodes. In that case, a resonance current flowing through the switching element 3 is also observed. There is also a method in which a detection resistor is inserted between the switching element 3 and the ground, and a switching current flowing through the switching element 3 is observed. These methods can also detect abnormalities such as overload and output short circuit.

Further, in this circuit, the comparator 24 is provided with a reference voltage source 25 having a lower voltage than the reference voltage source 25. Further, a switch 37 for selecting these voltage sources 25 and 36 is provided. Further, the voltage of the input voltage source 100 is
The detected voltage is supplied to a comparator 30 and compared with a voltage from a reference voltage source 31. The switch 37 is switched by the comparison output when the detected voltage exceeds the reference voltage.

That is, in the operation at the same switching frequency as described above, the higher the input voltage, the larger the current flowing through the switching elements 2 and 3. Therefore, in the above-described circuit, by setting the threshold value of the resonance current at which the overcurrent protection operation is started at the time of the high input voltage to be lower than that at the time of the low input voltage,
The overcurrent protection operation is started earlier to prevent an increase in switching loss. The operation waveform at this time is shown in FIG.
Shown in Here, the waveform at the time of low input voltage on the left is shown in FIG.
3, which is the same as that of FIG. 14, and has a smaller switching loss in the waveform at the time of the high input voltage on the right side.

As described above, even in the steady operation, the amplitude of the resonance current changes with the input voltage, and the higher the input voltage, the smaller the amplitude. Therefore, the threshold voltage for overcurrent detection is reduced when the input voltage is high. Even if it is changed, there is no influence on the steady operation. Thereby, in the above-described circuit, even if the threshold value of the resonance current at which the overcurrent protection operation is started at the time of the high input voltage is lower than that at the time of the low input voltage,
This has no effect on the steady-state operation, and can quickly start the overcurrent protection operation at the time of a high input voltage, thereby preventing an increase in switching loss.

Therefore, in this circuit, by setting the set value for judging the overload operation in accordance with the input voltage, it is possible to suppress an increase in the loss of the switching element particularly when the input voltage is high. Can prevent thermal stress, etc. due to excessive stress applied to the device, and increase the production cost for protecting the switching element and eliminate the need for special measures such as installation of special equipment. it can.

Thus, for example, in a switching power supply circuit driven by the upper side composite resonance,
It is necessary to provide an overcurrent protection circuit to prevent the impedance reversal phenomenon within the operating frequency range.However, in the conventional circuit, excessive stress may be applied to the switching element, leading to thermal destruction, etc. As
According to the present invention, these problems can be easily solved by increasing the capacity of the switching element or providing a heat sink, which causes problems such as an increase in manufacturing cost and a space for installation. It can be eliminated.

In the above-described circuit, the set value for the determination during the overcurrent protection operation is switched in two stages. For example, an operational amplifier 34 is provided instead of the comparator 30 as shown in FIG. The voltage value of the variable voltage source 38 provided in place of the voltage sources 25 and 36 may be controlled according to the output. Thus, for example, by configuring the voltage value of the variable voltage source 38 to decrease as the output of the operational amplifier 34 increases, it is possible to minimize an increase in switching loss over the entire input voltage range.
More accurate overcurrent protection can be performed.

Further, the present invention can be implemented by combining the above-described first embodiment and second embodiment. That is, in FIG. 7, for example, the switch 32 for connecting the resistor 33 is controlled by the output of the comparator 30, and the switch 37 for selecting the voltage sources 25 and 36 is controlled by the output. With such a configuration, it is possible to perform control that combines the above two overcurrent protection operations. Note that this combination
The above-described operational amplifier 34, variable current source 35, and variable voltage source 3
8 is also possible.

Thus, according to the switching power supply circuit described above, the switching power supply circuit is driven by the upper side composite resonance, detects the resonance current or the switching current, and sets the switching frequency higher than the resonance frequency determined by the resonance impedance during the overload operation. By providing overcurrent protection means for controlling the switching frequency in accordance with the input voltage and providing control means for controlling the switching frequency in accordance with the input voltage, it is possible to prevent a situation in which excessive stress is applied to the switching element and thermal destruction or the like is caused. It is possible to eliminate the need for a special treatment such as an increase in manufacturing cost caused for protection of the device and installation of a special device.

Further, according to the switching power supply circuit described above, the switching power supply circuit is driven by the upper side composite resonance, detects the resonance current or the switching current, and sets the switching frequency higher than the resonance frequency determined by the resonance impedance during the overload operation. In addition to providing an overcurrent protection unit that performs overload protection, the setting value for determining overload operation by detecting a resonance current or a switching current in the overcurrent protection unit is made variable according to the input voltage, so that excessive stress is applied to the switching element. In addition, it is possible to prevent a situation such as thermal destruction and the like, and it is possible to increase a manufacturing cost generated for protection of the switching element and to eliminate a special measure such as installation of a special device.

The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention.

[0066]

According to the first aspect of the present invention, by controlling the switching frequency in accordance with the input voltage, it is possible to suppress an increase in the loss of the switching element, especially when the input voltage is high. It is possible to prevent a situation in which excessive stress is applied to cause thermal destruction or the like, and it is possible to increase a manufacturing cost generated for protection of the switching element and to eliminate a special measure such as installation of a special device. Things.

According to the second aspect of the present invention, the set value for determining the overload operation is made variable in accordance with the input voltage, thereby suppressing an increase in switching element loss particularly when the input voltage is high. It is possible to prevent a situation in which excessive stress is applied to the switching element, which leads to thermal destruction, etc., and increase special costs such as an increase in manufacturing cost for protection of the switching element and installation of special equipment. It can be made unnecessary.

Further, according to the third aspect of the present invention, by combining the first embodiment and the second embodiment,
Control that combines two overcurrent protection operations can be performed.

Thus, for example, in a switching power supply circuit driven by the upper side composite resonance,
It is necessary to provide an overcurrent protection circuit to prevent the impedance reversal phenomenon within the operating frequency range.However, in the conventional circuit, excessive stress may be applied to the switching element, leading to thermal destruction, etc. As
According to the present invention, these problems can be easily solved by increasing the capacity of the switching element or providing a heat sink, which causes problems such as an increase in manufacturing cost and a space for installation. It can be eliminated.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of an upper side composite current resonance converter to which a switching power supply circuit according to the present invention is applied.

FIG. 2 is a diagram for explaining the operation.

FIG. 3 is a block diagram showing a configuration of a modification of the first embodiment of the upper-side composite current resonance converter to which the switching power supply circuit according to the present invention is applied;

FIG. 4 is a block diagram showing a configuration of a modification of the second embodiment of the upper-side composite current resonance converter to which the switching power supply circuit according to the present invention is applied;

FIG. 5 is a diagram for explaining the operation.

FIG. 6 is a block diagram showing a configuration of a modification of the second embodiment of the upper-side composite current resonance converter to which the switching power supply circuit according to the present invention is applied.

FIG. 7 is a block diagram showing a configuration of still another modified example of the upper side composite current resonance converter to which the switching power supply circuit according to the present invention is applied.

FIG. 8 is a block diagram showing a configuration of a conventional upper side composite current resonance converter.

FIG. 9 is a diagram for explaining the operation.

FIG. 10 is a diagram for explaining the operation.

FIG. 11 is a diagram for explaining the operation.

FIG. 12 is a diagram for explaining the operation.

FIG. 13 is a diagram for explaining the operation.

FIG. 14 is a diagram for explaining the operation.

[Description of Signs] 100: input voltage source, 1: drive circuit, 2, 3: switching element, 4: resonance inductance, 5: main transformer,
5a: primary winding, 5b, 5c, 5d: secondary winding, 6
... Resonant capacitor, 7 ... Detector resistor, 8, 9 ... Charge / discharge capacity, 10,11 ... Commutating diode, 12b, 12c, 1
2d: diode bridge circuit, 13b, 13c, 1
3d: smoothing capacitor, 14: control resistor, 15: photocoupler, 15a: light emitting diode, 15b: phototransistor, 16: error amplifier, 17, 25, 31: reference voltage source, 18, 22, 27, 33: resistor Vessel, 19 ... oscillator, 20, 23 ... capacitor, 21 ... diode, 2
4, 30 comparator, 26, 32 switch, 2
8,29… Voltage divider

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H730 AA20 AS00 BB26 BB57 BB70 BB76 DD04 DD26 EE04 EE07 EE73 FD41 FF19 FG09 XX04 XX15 XX16 XX22 XX32 XX41

Claims (3)

[Claims] 1. A switching power supply circuit driven by an upper-side composite resonance, wherein a switching frequency is set higher than a resonance frequency determined by a resonance impedance during an overload operation by detecting a resonance current or a switching current. A switching power supply circuit comprising: overcurrent protection means; and control means for controlling the switching frequency according to an input voltage. 2. A switching power supply circuit driven by upper side composite resonance, wherein a switching frequency is set higher than a resonance frequency determined by a resonance impedance during an overload operation by detecting a resonance current or a switching current. A switching power supply, further comprising: an overcurrent protection unit, wherein a set value for determining the overload operation by detecting the resonance current or the switching current in the overcurrent protection unit is made variable in accordance with an input voltage. circuit. 3. The switching power supply circuit according to claim 2, further comprising control means for controlling said switching frequency by said input voltage.