JPH0641386U - Switching power supply - Google Patents

Abstract

(57) [Abstract] [Purpose] Limiting the output current due to changes in the input power supply voltage, while eliminating the need to design the current rating of the secondary side rectifier diode and the size of the pulse transformer to be larger than the rated load. Eliminate the fluctuation of points. [Configuration] to the primary winding n ₁ of the pulse transformer T the current through the series switching transistor Q ₁ is detected by the current detecting circuit 2, is compared with the reference voltage Vref and the detection voltages V ₁ in the comparator 7. "H" of the output of the comparator 7, "L" output of the PWM control circuit 8 based on "L", "H" to be changed in OFF the switching transistors Q _1, turned ON. Utilizing the fact that the DC power supply voltage Vcc generated by the DC power supply circuit 10 connected to the auxiliary winding n ₃ and used as the power supply input of the PWM control circuit 8 changes according to the input power supply voltage V _D , the auxiliary winding voltage detection circuit 11 And a reference voltage correction circuit 12 are provided to correct the reference voltage Vref according to the detection voltage _VDET .

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to an IC-type PWM (pulse width modulation) control switching power supply in a switching power supply that performs switching control at high frequencies to stabilize the output.

[0002]

[Prior art]

FIG. 5 shows a circuit diagram of a conventional switching power supply of this type. In the figure, 1 is a direct current input power source, T is a pulse transformer, n ₁ is its primary winding, n ₂ is a secondary winding, n ₃ is an auxiliary winding, and Q ₁ is an N-channel power MOS-FET. (Hereinafter referred to as a switching transistor), 2 is a drain current detection circuit, 3 is a DC input power source 1, a primary winding n ₁ of the pulse transformer T and a primary side circuit formed by connecting a switching transistor Q ₁ in series, Reference numeral 4 is a rectifying / smoothing circuit including a rectifying diode D ₂ and a smoothing capacitor C _2, and 5 is a secondary side circuit in which the rectifying / smoothing circuit 4 is inserted between the secondary winding n ₂ and the output terminal 6. R ₁ is a bypass resistor, 7 is a comparator, 8 is a PWM control circuit, 9 is a semiconductor device including the comparator 7 and the PWM control circuit, 10 is a rectifying diode D ₁ and a smoothing capacitor C on the auxiliary winding n ₃ side. ₁ is a DC power supply circuit that generates a DC power supply voltage Vcc and supplies power to the power supply input terminal of the PWM control circuit 8.

An input power supply voltage V _D is supplied from the DC input power supply 1. Now, assuming that the ON control signal of “H” level is supplied to the gate of the switching transistor Q ₁ from the PWM control circuit 8 in the semiconductor element 9 and the switching transistor Q ₁ is conducting, the DC input power source 1 Current flows through the primary winding n ₁ of the pulse transformer T. The voltage induced in the auxiliary winding n ₃ is rectified by the rectifying diode D ₁ in the DC power supply circuit 10 and smoothed to DC by the smoothing capacitor C ₁ , and is supplied to the power supply input terminal of the PWM control circuit 8 as the DC power supply voltage Vcc. Is applied.

The drain current of the switching transistor Q ₁ is converted into a detection voltage V ₁ in the drain current detection circuit 2 and supplied to the non-inverting input terminal (+) of the comparator 7. When the detected voltage V ₁ is equal to or lower than the reference voltage Vref, the output of the comparator 7 does not invert and remains at “L” level, so the output state from the PWM control circuit 8 to the switching transistor Q ₁ is also at “H” level. Keeps the ON control signal of, and the switching transistor Q ₁ maintains the ON state.

When the drain current flowing through the switching transistor Q ₁ increases and the detected voltage V ₁ by the drain current detection circuit 2 exceeds the reference voltage V ref, the output of the comparator 7 is inverted to the “H” level, and therefore PWM control is performed. circuit 8 button switches the mode to the state of outputting an OFF control signal of "L" level to the switching tiger Njisuta Q _1, a switching transistor Q ₁ is inverted to the OFF state. Then, no current flows in the primary winding n ₁ of the pulse transformer T, so that the power supply to the secondary winding n ₂ is momentarily cut off. Then, the output current I _O is prevented from exceeding a predetermined value and increasing. At this time, the current from the DC input power source 1 for charging the smoothing capacitor C ₁ of the DC power supply circuit 10 through the bypass resistor R _1, the power supply to the PWM control circuit 8 is maintained.

Through the ON / OFF control of the switching transistor Q ₁ by the PWM control circuit 8 as described above, the output current I _O is limited to fall within a predetermined threshold range.

[0007]

[Problems to be solved by the device]

In the case of the conventional switching power supply described above, when the input power supply voltage V _{D from the} DC input power supply 1 changes, the limit point of the output current I _O changes accordingly. That is, FIG. 6 is a characteristic diagram showing the relationship between the output voltage V _O and the limiting point of the output current I _{O with} the input power supply voltage V _D as a parameter. _{When D} increases, the limiting point of the output current I _O changes in the increasing direction, and when the input power supply voltage V _D decreases, the limiting point of the output current I _O changes in the decreasing direction.

As described above, since the limiting point of the output current I _O changes with the change of the input power supply voltage V _D , in the design of the rectifier diode D ₂ on the secondary side and the design of the pulse transformer T, the output current I _It must be designed according to the maximum limit of _O. Then, the current rating of the rectifying diode D ₂ and the size of the pulse transformer T naturally become larger than necessary with respect to the rated load, which causes a problem that the cost increases and the size of the power source increases.

The present invention was devised in view of such circumstances, and it is necessary to design the current rating of the secondary side rectifying diode and the size of the pulse transformer to be larger than the rated load. The purpose is to eliminate the fluctuation of the output current limit point due to the change of the input power supply voltage, even though it is eliminated.

[0010]

[Means for Solving the Problems]

 In the switching power supply according to the present invention, a rectifying and smoothing circuit is inserted between a primary side circuit in which a primary winding of a pulse transformer and a switching element are connected in series, and a secondary winding of the pulse transformer and an output terminal. A secondary side circuit, a current detection circuit that detects a current flowing through the switching element and outputs it as a detection voltage, a comparator that compares the detection voltage with a reference voltage, and a detection circuit that detects when the detection voltage exceeds the reference voltage. A PWM control circuit that outputs an OFF control signal to the switching element when a comparator output is input and an ON control signal when a comparator output is input when the detected voltage is less than a reference voltage, and an auxiliary winding of the pulse transformer. PWM control in a switching power supply equipped with a DC power supply circuit that generates a DC power supply voltage from the An auxiliary winding voltage detection circuit for detecting a change in the DC power supply voltage output from the DC power supply circuit in the IC, and the reference voltage of the comparator is corrected based on the detected voltage by the auxiliary winding voltage detection circuit. It is characterized by the addition of a reference voltage correction circuit.

[0011]

[Action]

 The present invention utilizes the phenomenon that the DC power supply voltage generated by the DC power supply circuit connected to the auxiliary winding changes according to the change of the input power supply voltage applied to the primary winding of the pulse transformer. Yes, the change in the DC power supply voltage is detected by the auxiliary winding voltage detection circuit, and the reference voltage correction circuit is operated based on the detected voltage to correct the reference voltage of the comparator. This is to eliminate the change in the output current limit point.

[0012]

【Example】

 Hereinafter, an embodiment of a switching power supply according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing a switching power supply according to an embodiment.

One end of the primary winding n ₁ of the pulse transformer T is connected to the positive electrode of the DC input power supply 1, and the other end of the primary winding n ₁ is an N-channel type power MOS-FET: Q ₁ (in the following, The drain of a switching transistor Q ₁ ) is connected. Control button quenching transistor source for Q ₁ is connected to the negative electrode and the ground GND of the DC input power source 1 via the drain current detection circuit 2, or more 1 of the pulse transformer T primary winding n ₁ and the switching transistor Q _1, the drain current The series circuit with the detection circuit 2 constitutes the primary side circuit 3. The secondary winding n ₂ of the pulse transformer T is connected to the rectifying / smoothing circuit 4 composed of the rectifying diode D ₂ and the smoothing capacitor C _2, and both ends of the smoothing capacitor C ₂ are connected to the output terminal 6 and the secondary winding n ₂ is connected. The line n ₂ , the rectifying / smoothing circuit 4, and the output terminal 6 form a secondary side circuit 5.

The drain current detection circuit 2 detects the drain current flowing through the switching transistor Q ₁ and inputs it as a detection voltage V ₁ to the non-inverting input terminal (+) of the comparator 7. The reference voltage Vref is applied to the inverting input terminal (−) of the comparator 7. Comparator 7, with increasing drain current outputs "H" level when the detection voltage V ₁ is greater than the reference voltage Vref, the detection voltages V ₁ with the decrease in the drain current is the reference voltage Vref or less When it becomes, it outputs "L" level. The PWM control circuit 8 has its input terminal connected to the output terminal of the comparator 7, and its output terminal connected to the gate of the switching transistor Q ₁ . This PWM control circuit 8 outputs an “L” level, which is an OFF control signal, to the gate of the switching transistor Q ₁ when the “H” level is input from the comparator 7, and conversely, the comparator 7 outputs an “L” level. The "H" level, which is an ON control signal, is output to the gate of the switching transistor Q _{1 when} the "level" is input. The comparator 7 and the PWM control circuit 8 are collectively configured as a semiconductor element 9. The auxiliary winding n ₃ of the pulse transformer T is connected to the DC power supply circuit 10 which is composed of a rectifying diode D ₁ and a smoothing capacitor C ₁ and generates a DC power supply voltage Vcc. The positive electrode of the smoothing capacitor C ₁ in the DC power supply circuit 10 is connected to the positive electrode of the DC input power supply 1 via the bypass resistor R _1, and the negative electrode of the smoothing capacitor C ₁ is connected to the ground GND.

The circuit configuration described above is the same as that of the conventional example (FIG. 5). In this embodiment, the following circuit is added to the above circuit configuration.

Between the DC power supply circuit 10 which is connected to the auxiliary winding n ₃ and generates the DC power supply voltage Vcc and the PWM control circuit 8 which receives the DC power supply voltage Vcc at its power supply input terminal, the DC power supply voltage Vcc An auxiliary winding voltage detection circuit 11 that detects a change is provided. A reference voltage correction circuit 12 for correcting the reference voltage Vref of the comparator 7 based on the detection voltage V _DET detected by the auxiliary winding voltage detection circuit 11 is provided. The reference voltage correction circuit 12 lowers the reference voltage Vref when the DC power supply voltage Vcc rises and the detection voltage V _DET rises, and conversely, the DC power supply voltage Vcc falls and the detection voltage V _DET falls. When it does, the reference voltage Vref is increased. That is, the auxiliary winding voltage detection circuit 11 and the reference voltage correction circuit 12 prevent the fluctuation of the limit point of the output current I _O from the output terminal 6 regardless of the change of the input power supply voltage V _D. It is configured to do. The auxiliary winding voltage detection circuit 11 and the reference voltage correction circuit 12 are also collectively configured as the constituent elements of the semiconductor element 9 like the comparator 7 and the PWM control circuit 8.

FIG. 2 is a circuit diagram showing the entire switching power supply by exemplifying the configurations of the auxiliary winding voltage detection circuit 11 and the reference voltage correction circuit 12 at specific levels. That is, the auxiliary winding voltage detection circuit 11 for detecting the change in the DC power supply voltage Vcc is composed of voltage dividing resistors R ₂ and R ₃ connected in series between the positive electrode of the smoothing capacitor C ₁ and the ground GND. ing. The output from the resistance division point of the voltage dividing resistors R ₂ and R ₃ becomes the detection voltage V _{DE T.} The reference voltage correction circuit 12 is composed of an NPN transistor Q ₂ and a resistor R ₄ . The transistor Q ₂ has its base connected to the resistance dividing point of the voltage dividing resistors R ₂ and R ₃ , its collector connected to the inverting input terminal (−) of the comparator 7, and its emitter connected to the ground GND via the resistor R _4. Has been done. Since the collector current of the transistor Q ₂ increases as the detection voltage V _DET increases, the reference voltage Vref applied to the inverting input terminal (−) of the comparator 7 drops. Therefore, even if the detection voltage V ₁ by the drain current detection circuit 2 to be compared with the reference voltage Vref is lower than the standard level, the output of the comparator 7 is inverted from the “L” level to the “H” level. On the contrary, since the collector current of the transistor Q ₂ decreases as the detection voltage V _DET decreases, the reference voltage Vref applied to the inverting input terminal (−) of the comparator 7 increases. Therefore, even if the detected voltage V ₁ by the drain current detection circuit 2 to be compared with the reference voltage Vref is higher than the standard level, the output of the comparator 7 is inverted from the “H” level to the “L” level.

Next, the operation of the switching power supply configured as above will be described. The input power supply voltage V _D is supplied from the DC input power supply 1. Now, assuming that the "H" level ON control signal is supplied to the gate of the switching transistor Q ₁ from the PWM control circuit 8 in the semiconductor element 9 to make the switching transistor Q ₁ conductive, the direct current input power supply 1 The current flows through the primary winding n ₁ of the pulse transformer T. An induced voltage is generated in the secondary winding n ₂ of the pulse transformer T, which is rectified by the rectifying diode D ₂ in the rectifying and smoothing circuit 4 and smoothed by the smoothing capacitor C ₂ to obtain a constant output voltage V _O. It is output from the output terminal 6. The voltage induced in the auxiliary winding n ₃ is rectified by the rectifying diode D ₁ in the DC power supply circuit 10, smoothed to DC by the smoothing capacitor C ₁ , and is supplied as the DC power supply voltage Vcc to the power input terminal of the PWM control circuit 8. Applied to.

The drain current of the switching transistor Q ₁ is converted into a detection voltage V ₁ in the drain current detection circuit 2 and supplied to the non-inverting input terminal (+) of the comparator 7. When the detected voltage V ₁ is equal to or lower than the reference voltage Vref, the output of the comparator 7 does not invert and remains at “L” level, and therefore the output state from the PWM control circuit 8 to the gate of the switching transistor Q ₁ is also “H”. The "ON" level control signal is maintained, and the switching transistor Q ₁ maintains the ON state.

When the drain current flowing through the switching transistor Q ₁ increases and the detected voltage V ₁ by the drain current detection circuit 2 exceeds the reference voltage Vref, the output of the comparator 7 is inverted to the “H” level, so that the PWM control is performed. circuit 8 is switched to a state of outputting the "L" level OFF control signal to the gate of the switching tiger Njisuta Q _1, the switching transistor Q ₁ is inverted to the OFF state. Then, no current flows in the primary winding n ₁ of the pulse transformer T, so that the power supply to the secondary winding n ₂ is momentarily cut off. Then, the output current I _O is prevented from increasing above a predetermined value. At this time, the current from the DC input power source 1 for charging the smoothing capacitor C ₁ of the DC power supply circuit 10 through the bypass resistor R _1, the power supply to the PWM control circuit 8 is maintained.

Through the ON / OFF control of the switching transistor Q ₁ by the PWM control circuit 8 as described above, the output current I _O is limited to fall within a predetermined threshold range.

When the input power supply voltage V _{D from} the DC input power supply 1 changes, the DC power supply voltage Vcc from the DC power supply circuit 10 changes accordingly. The situation is shown in FIG.

That is, as the input power supply voltage V _D rises, the DC power supply voltage Vcc also rises, and conversely, as the input power supply voltage V _D drops, the DC power supply voltage Vcc also falls.

In the conventional example, when the input power supply voltage V _D rises, the limiting point of the output current I _O from the output terminal 6 changes in the increasing direction, and conversely, when the input power supply voltage V _D drops, the output current I _O decreases. The limit point of _O was changing in the decreasing direction (see Fig. 6). The present invention prevents such a variation of the limiting point of the output current I _O by adding the auxiliary winding voltage detection circuit 11 and the reference voltage correction circuit 12.

That is, when the input power supply voltage V _D rises, the DC power supply voltage Vcc by the DC power supply circuit 10 also rises proportionally, but this increase in the DC power supply voltage Vcc is caused by the auxiliary winding voltage detection circuit 11. It is detected as an increase in the detection voltage V _DET . That is, the detection voltage V _DET at the resistance division points of the voltage dividing resistors R ₂ and R ₃ increases. Then, the collector current of the transistor Q ₂ increases, and the reference voltage Vref drops below the standard. As a result, the detected voltage V ₁ by the drain current detection circuit 2 becomes relatively higher than the reference voltage Vref at a level lower than the standard, and the output of the comparator 7 is inverted to the “H” level when it is earlier than the standard. Then, the output from the PWM control circuit 8 is inverted to the “L” level which is the OFF control signal, so that the switching transistor Q ₁ is turned off. That is, since the current flowing through the primary winding n ₁ is cut off at a lower level of the drain current flowing through the switching transistor Q ₁ , the limit point of the output current I _O does not rise even if the input power supply voltage V _D rises. It will be kept constant.

Conversely, when the input power supply voltage V _D drops, the DC power supply voltage Vcc also drops proportionally, and this is detected as a drop in the detection voltage V _DET by the auxiliary winding voltage detection circuit 11. It Then, the collector current of the transistor Q ₂ decreases and the reference voltage V ref rises above the standard. As a result, the detected voltage V ₁ by the drain current detection circuit 2 becomes relatively lower than the reference voltage Vref at a level higher than the standard, and the output of the comparator 7 is inverted to the “L” level to output from the PWM control circuit 8. Is inverted to the "H" level which is an ON control signal, so that the switching transistor Q ₁ is turned ON. That is, since the current flowing through the primary winding n ₁ is once interrupted, the current is allowed to flow again through the primary winding n ₁ at an earlier time than the standard, so that the input power supply voltage V 1 _{Even if D} drops, the limit point of the output current I _O does not fall and is maintained at a constant value. FIG. 4 is a characteristic diagram showing the relationship between the output voltage V _O and the limiting point of the output current I _O with respect to changes in the input power supply voltage V _D. The limit point of the output current I _O does not change regardless of the change of the input power supply voltage V _D.

As described above, even if the input power supply voltage V _D changes, the limit point of the output current I _O is not changed by correcting the reference voltage V ref, so that the rectifying diode D ₂ on the secondary side is There is no need to increase the current rating or the size of the pulse transformer T with respect to the rated load, and it is possible to design according to the rated load.

[0028]

[Effect of device]

 As described above, according to the present invention, by adding the auxiliary winding voltage detection circuit and the reference voltage correction circuit, the output current limit point does not change even if the input power supply voltage changes. There is no need to increase the current rating of the rectifier diode on the secondary side and the size of the pulse transformer more than necessary as in the conventional example, which can reduce the cost and the size of the power supply.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switching power supply using a PWM control semiconductor device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing the entire switching power supply by exemplifying the configurations of an auxiliary winding voltage detection circuit and a reference voltage correction circuit in a specific level in the embodiment.

FIG. 3 is a characteristic diagram showing changes in the DC power supply voltage by the DC power supply circuit with respect to changes in the input power supply voltage in the example.

FIG. 4 is a characteristic diagram showing a relationship between an output voltage and an output current limit point with respect to a change in an input power supply voltage in an example.

FIG. 5 is a circuit diagram of a switching power supply using a semiconductor device for PWM control according to a conventional example.

FIG. 6 is a characteristic diagram showing a relationship between an output voltage and an output current limit point with respect to a change in an input power supply voltage in a conventional example.

[Explanation of symbols]

1 DC input power supply 2 Drain current detection circuit 3 Primary side circuit 4 Rectification smoothing circuit 5 Secondary side circuit 6 Output terminal 7 Comparator 8 PWM control circuit 9 Semiconductor element 10 DC power supply circuit 11 Auxiliary winding voltage detection circuit 12 Reference voltage correction Circuit T Pulse transformer n ₁ Primary winding n ₂ Secondary winding n ₃ Auxiliary winding Q ₁ Switching transistor (Power MOS-F
ET) Q ₂ Reference voltage correction transistor D ₁ Rectifier diode D ₂ Rectifier diode C ₁ Smoothing capacitor C ₂ Smoothing capacitor R ₁ Bypass resistor R ₂ Voltage dividing resistor R ₃ Voltage dividing resistor I _O Output current V _O Output voltage V _D Input power supply voltage Vcc DC power supply voltage Vref Reference voltage V ₁ Detection voltage V _DET Detection voltage by auxiliary winding voltage detection circuit

Claims (1)

[Scope of utility model registration request] 1. A rectifying / smoothing circuit is interposed between a primary side circuit in which a primary winding of a pulse transformer and a switching element are connected in series, and a secondary winding of the pulse transformer and an output terminal. A secondary circuit, a current detection circuit that detects a current flowing through the switching element and outputs the detected current as a detection voltage,
A comparator for comparing the detection voltage and a reference voltage,
A PWM control circuit that outputs an OFF control signal to the switching element when the comparator output is input when the detection voltage exceeds the reference voltage and outputs an ON control signal when the comparator output is input when the detection voltage is equal to or lower than the reference voltage; A switching power supply comprising: a DC power supply circuit for generating a DC power supply voltage from an auxiliary winding of a pulse transformer to supply power to the PWM control circuit; an auxiliary device for detecting a change in the DC power supply voltage output from the DC power supply circuit. A switching power supply comprising: a winding voltage detection circuit; and a reference voltage correction circuit for correcting the reference voltage of the comparator based on the voltage detected by the auxiliary winding voltage detection circuit.