JP2598259Y2 - Switching power supply - Google Patents

Description

[Detailed description of the invention] <Industrial application field> The present invention relates to a switching power supply, and conducts a circuit for changing a maximum pulse width provided in a control circuit for performing pulse width control according to a DC input voltage. A switching power supply that can set the maximum pulse width to a predetermined value without lowering the efficiency even when the input voltage changes in a wide voltage range by including Is provided.

<Conventional Technology> Switching power supplies have been widely known as DC power supplies for computer terminal equipment and the like. A general configuration of a switching power supply has a circuit configuration in which a pulse width control circuit is built in and a switching element that switches a DC input is driven by a pulse output from the pulse width control circuit. FIG. 5 is a block diagram of this type of conventional switching power supply, 1 is a commercial AC power supply, 2 is a rectifying and smoothing circuit, 3 is a transformer, 4 is a switching element, 5 is an output circuit, 6 is a control circuit, and 7 is Output terminal.

The rectifying and smoothing circuit 2 rectifies and smoothes AC power supplied from the AC power supply 1 and supplies a DC input voltage Vin.

The transformer 3 has at least an input winding 31 and an output winding 32. The switching element 4 includes a bipolar transistor or a field-effect transistor, and has a main electrode connected in series to an input winding 31 of the transformer 3.
Switching Vin.

The output circuit 5 converts the switching output generated in the output winding 32 of the transformer 3 into DC and outputs a DC output voltage V0. As the circuit configuration of the output circuit 5, two types of a forward type and a flyback type are well known.

The control circuit 6 integrates circuit elements and circuit components necessary for pulse width control into a single control IC 61.
And an insulating coupler 62 such as a photocoupler. The control IC 61 has an oscillation circuit for generating a saw-tooth shape and the like, a pulse width modulation circuit, and the like inside thereof. 4 is pulse width controlled.
The control IC 61 has an operating power supply VCC through a resistor R1.
Is given. The control IC 61 has a resistor R4 and a capacitor C1 that determine the oscillation frequency of the internal oscillation circuit.
Are externally connected to the terminals RT and CT.

Furthermore, resistors R2 and R3 are connected between the terminal Vref and the terminal G, and a connection point P1 between the resistors R2 and R3 is connected to the terminal DT. The resistors R2 and R3 divide the voltage applied between the terminal Vref and the terminal G (ground), and the potential between the connection point P1 and the ground, that is, the terminal voltage VD of the resistor R3 causes the maximum pulse of the output pulse to be divided. Set the width to a constant value.

The reason why the maximum pulse width is set to a constant value is to prevent the duty ratio of the switching element 4 from becoming larger than specified by the maximum pulse width even when the load becomes heavy or a pulse load. This is for avoiding magnetic saturation and protecting circuits and circuit components. The transformer 3 has a product Em · T of the maximum value Em of the input DC voltage Vin and the maximum pulse width Tm.
In consideration of m, the design is made so as not to cause magnetic saturation.

However, when the DC input voltage Vin increases, the duty ratio of the switching element 4 decreases, and the pulse width is controlled to decrease. Therefore, when the DC input voltage Vin increases, the maximum pulse width Tm decreases. However, there is no problem in performance. However, in the case of the conventional example shown in FIG. 5, the maximum pulse width Tm is fixed, so that the capacity of the transformer 3 has to be larger than actually required. This increases the shape and weight, requires additional mounting space, and raises costs.

As conventional technology aimed at solving the above-mentioned problems,
No. 57-90386 is known. As shown in FIG. 6, this prior art uses a resistor between a connection point P1 of a resistor R2 and a resistor R3 and a connection point P2 of a startup resistor R11 and a resistor R12 connected to a power supply line (a). R5 is connected, and the divided voltage VD by the resistors R2 and R3 is changed by the DC input voltage Vin.

FIGS. 7A and 7B are waveform diagrams showing the operation of the circuit shown in FIG. First, as shown in FIG.
When the DC input voltage Vin is at a certain level, the divided voltage at the terminal DT is set to VD1, and the oscillation waveform of the internal oscillator of the control IC 61 is set to the sawtooth waveform F, the sawtooth waveform F is lower than the divided voltage VD1. In the high region, an output pulse Fm having a maximum pulse width Tm1 as shown in FIG. 7 (b) is obtained, and the switching element 4 is driven by this output pulse Fm.

When the DC input voltage Vin increases, FIG. 7 (a)
As shown in (5), since the divided voltage VD1 seen at the terminal DT rises from VD2, the maximum pulse width of the output pulse Fm is reduced from Tm1 to Tm2. When the DC input voltage Vin changes in the lower direction, the control is performed in the opposite direction to increase the maximum pulse width of the output pulse Fm.

As described above, when the DC input voltage Vin increases, the maximum pulse width Tm decreases from Tm1 to Tm2. Therefore, when designing the transformer 3, the ET product is set to a small value, and the transformer 3 is reduced in size. , Measurement, small capacity and low cost,
The magnetic saturation can be prevented.

<Problem to be solved by the invention> However, in Japanese Utility Model Laid-Open No. 57-90386, the resistance R2 and the resistance R
3 connection point P1 and the starting resistor R connected to the power line
11 and the connection point P2 of the resistor R12, the resistor R5 is connected, and the divided voltage VD at the terminal DT is changed through the resistor R5. become worse.

Further, the switching power supply is often designed so that it can be used in a wide input voltage range, such as operating at 100 V AC and 200 V AC. In such a type of switching power supply, it is desirable from the viewpoint of efficiency to detect the difference between AC 100V and AC 200V and to set the divided voltage suitable for each of them in a stepwise manner. However, Shokai Sho57
In -90386, such a stepped voltage setting cannot be performed because the divided voltage that determines the maximum pulse width is adjusted by the resistor R5.

Therefore, an object of the present invention is to solve the above-mentioned conventional problems, and to set a maximum pulse width to a predetermined value with high efficiency even when the input voltage changes in a wide voltage range. It is to provide.

<Means for Solving the Problems> In order to solve the problems described above, a switching power supply according to the present invention includes a transformer, a switching element for switching a DC input provided through an input winding of the transformer, and the transformer. And a control circuit for controlling the switching element. The control circuit includes a resistor circuit that determines a maximum pulse width of an output pulse given to the switching element, and a circuit that changes the maximum pulse width.

The resistor circuit determines the maximum pulse width by a divided voltage determined by a resistance division ratio.

The circuit that changes the maximum pulse width includes a voltage detection element that is turned on or off according to the voltage value of the DC input, and when the voltage detection element is turned on, according to the DC input voltage. It is connected to the resistor circuit so as to change the divided voltage of the resistor circuit.

A circuit configured by connecting the circuit for changing the maximum pulse width and the resistor circuit is connected to a power supply line for supplying the DC input.

The transformer has a shield conductor, and the shield conductor is connected to the power supply line on the positive electrode side or the power supply line on the negative electrode side that supplies the DC input, or to a position that maintains the same potential as the power supply line.

The transformer has a coil bobbin and a core according to a normal structure. The coil bobbin has a body, and the core is attached to the coil bobbin through a hole provided inside the body of the coil bobbin.

The shield conductor is wound around a body of a coil bobbin of a transformer, and the input winding and the output winding are wound on the shield conductor.

<Operation> The control circuit has a resistor circuit that determines the maximum pulse width of the output pulse given to the switching element and a circuit that changes the maximum pulse width. Following the voltage change, the maximum pulse width is variably set, and when designing the transformer, the ET product is set to a small value to reduce the size, measurement, capacity, and cost of the transformer. Meanwhile, the magnetic saturation can be prevented.

Moreover, the resistance circuit is a circuit that determines the maximum pulse width by the divided voltage determined by the resistance division ratio, and the circuit that changes the maximum pulse width is a voltage detection circuit that is turned on or off according to the DC input voltage value. Elements are connected to the resistance circuit so as to change the divided voltage of the resistance circuit by conduction and non-conduction of the voltage detection element, and these circuits are connected to a power supply line supplying a DC input Therefore, until the input voltage reaches a certain level, the maximum pulse width is set to a value determined by the divided resistance ratio of the resistance circuit, and during this time, the power consumption of the circuit including the voltage detection element is not involved.

When the input voltage reaches a certain level and the voltage detection element is turned on, the divided voltage set by the resistor circuit is increased, and control is performed to reduce the maximum pulse width. Therefore, the divided voltage for setting the maximum pulse width changes stepwise. Thereby, for example, a switching power supply suitable for common use as AC 100 V and AC 200 V is obtained.

As described above, since the resistor circuit determines the maximum pulse width by the divided voltage determined by the resistance division ratio, the divided voltage must be accurately controlled in order to accurately set the maximum pulse width. . However, the value of the divided voltage is quite small. Therefore, the divided voltage may fluctuate due to radiation noise generated from the transformer.

As a countermeasure, in the present invention, the transformer has a shield conductor. In a circuit configuration in which a circuit configured by connecting a circuit for changing the maximum pulse width and a resistance circuit is connected to a power supply line for supplying a DC input, the shield conductor is connected to the power supply line on the positive electrode side for supplying the DC input. Alternatively, it is connected to the power supply line on the negative electrode side or a position for maintaining the same potential as the power supply line. According to this circuit configuration, suppressing the radiation noise of the transformer prevents the divided voltage of the resistor circuit that determines the maximum pulse width from fluctuating due to the radiation noise of the transformer, and allows the maximum pulse width to be set accurately. . This means that it is not necessary to design a transformer, a choke coil, and the like with an extra margin, and it is possible to reduce the size of these components.

The shield conductor is wound around a body of a coil bobbin of a transformer, and the input / output winding is wound on the shield conductor. According to this structure, noise generated from the core passing through the trunk can be blocked by the shield conductor. If there is no shield conductor,
A stray capacitance is generated between the core passing through the body and the winding wound around the body. Due to the stray capacitance, radiation noise is generated in the core, and the radiation noise jumps into peripheral circuits, causing large noise in a portion having high impedance and causing malfunction of the circuit. In the present invention, the shield conductor is wound around the body of the coil bobbin of the transformer, and the input and output windings are wound on the shield conductor. Between,
Does not generate stray capacitance that causes radiation noise.

<Embodiment> FIG. 1 is an electric circuit diagram of a switching power supply according to the present invention. In the figure, the same reference numerals as those in FIG. 5 indicate the same components. In the embodiment, the circuit for changing the maximum pulse width includes a connection point P1 and resistors R11 and R12.
And a connection point P2 is connected to a series circuit of a voltage detection element D1 composed of a Zener diode and a resistor R5. One end of the resistor R11 and one end of the resistor R3 are connected to power supply lines (a) and (b), respectively.

When the AC power supply 1 is selected in the range of 100V AC or 200V AC, the Zener voltage of the voltage detection element D1 is changed to 100V AC.
Is set to a value that is non-conductive at the DC input voltage Vin corresponding to, and is conductive at the DC input corresponding to 200 V AC.
Therefore, at a DC input voltage Vin corresponding to AC 100V, the maximum pulse width is mainly determined by the resistance division ratio of the resistors R2 and R3, and when supporting AC 200V, the resistors R11, R2, R
3, depends on R5 and zener voltage. Therefore, even when the input voltage is selected in a wide range, it can be sufficiently applied. The setting operation of the maximum pulse width in this case is as described with reference to FIG.

In addition, a voltage detection element D1 composed of a Zener diode, etc.
In this configuration, the voltage is detected, so that the loss is small and the efficiency is high.

In the embodiment, the transformer 3 has a shield conductor 8 for suppressing radiation noise. 9 is a shield conductor 8
Are connected to the lead wire. The shield conductor 8 is connected to a ground potential which is a stable potential. The ground potential is equal to the power supply line (b) on the negative electrode side. In addition, as shown by dotted lines (a) and (b) in FIG. 1, the power supply line may be connected to a positive power line (a) or a negative power line (b). The positive-side power supply line (a) or the negative-side power supply line (b) that supplies a DC input is immediately after the rectifying and smoothing circuit 2 that is often used in this type of switching power supply, so that the potential is stable. doing.

The divided voltage VD at the point P1 that determines the maximum pulse width is, for example, 1.
This is a low voltage value having a control range of 5 to 2.5 V, and is easily affected by noise. In the present invention, the shield conductor 8 has the above-described shield conductor 8, and the shield conductor 8 is a positive power supply line (a) or a negative power supply line (b) for supplying a DC input, or a ground that is at a position that maintains the same potential as the power line. , The radiation noise of the transformer 3 can be suppressed. Therefore, the divided voltage VD at point P1 that determines the maximum pulse width
Is prevented from fluctuating due to the radiation noise of the transformer 3,
The maximum pulse width can be set accurately. Therefore, the transformer 3
This eliminates the need to design an excessively large margin for the coil and the choke coil and the like, and makes it possible to reduce the size of these components.

FIG. 2 is a diagram schematically showing the structure of the transformer 4 having the shield conductor 8. The shield conductor 8 is wound around the body 331 of the coil bobbin 33, and The input winding 31 and the output winding 32 are wound. 341 and 342 are cores. The shield conductor 8 electrically insulates both ends 81 and 82 as shown in FIG. 3 so as not to form a short-circuit ring. Alternatively, as shown in FIG.
The shield conductor 8 may be formed by winding over the wire 1, and one end of the wire 9 may be connected to the stable potential described above.

According to the above embodiment, radiation noise generated in the transformer 3 is suppressed by the shield conductor 8. That is, the shield conductor 8 is wound around the body 331 of the coil bobbin 33 of the transformer 3, and the input / output windings 31 and 32 are wound on the shield conductor 8. Of which, torso 33
No stray capacitance that causes radiation noise is generated between the core portion (middle leg) passing through 1 and the windings 31 and 32.
Therefore, if the shield conductor 8 is not provided, the radiation noise generated in the cores 341 and 342 does not occur due to the stray capacitance between the core portion passing through the body 331 and the available windings 31 and 32.

In addition, since the shield conductor 8 is provided on the body 331 of the bobbin 33 and the windings 31 and 32 are wound thereon, the electric insulation between the windings 31 and 32 can be reduced. No assembly is easy.

<Effects of the Invention> As described above, according to the present invention, even when the input voltage changes in a wide voltage range, the maximum pulse width can be set to a predetermined value with high efficiency. For this reason, it is not necessary to design a transformer or a choke coil with an excessive margin, and it is possible to reduce the size of these components.

[Brief description of the drawings]

FIG. 4 is an electric circuit diagram of the switching power supply according to the present invention,
FIG. 2 is a model diagram showing a structure of a transformer used in the switching power supply according to the present invention, FIG. 3 is a perspective view showing a shield conductor, FIG. 4 is a perspective view showing another embodiment of the same, FIG. 5 is an electric circuit diagram of a conventional switching power supply, FIG. 6 is an electric circuit diagram of another conventional switching power supply, and FIG.
FIG. 6 is a waveform chart showing the operation of the switching power supply shown in FIG. 2: Rectifying smoothing circuit, 3: Transformer 31: Input winding, 32: Output winding 4: Switching element 5: Output circuit, 6: Control circuit 61: Control IC R2, R3: Resistance D1: Voltage detection element

Claims (1)

(57) [Scope of request for utility model registration] 1. A transformer, a switching element for switching a DC input supplied through an input winding of the transformer, and an output circuit for converting a switching output generated in an output winding of the transformer into DC and outputting the DC. A control circuit that controls the switching element, wherein the control circuit includes a resistor circuit that determines a maximum pulse width of an output pulse given to the switching element, and a circuit that changes the maximum pulse width. The resistance circuit is a circuit that determines the maximum pulse width by a divided voltage determined by a resistance division ratio, and the circuit that changes the maximum pulse width is conductive or non-conductive according to a voltage value of a DC input. Including a voltage detection element that becomes conductive, when the voltage detection element is conductive, the resistance circuit of the resistance circuit according to the DC input voltage A circuit configured to connect the resistance circuit and a circuit that changes the maximum pulse width is connected to the power supply line that supplies the DC input so as to change the voltage and voltage. Wherein the transformer includes a coil bobbin, a core, and a shield conductor, the coil bobbin has a body, and the core is provided inside the body of the coil bobbin. The shield conductor is attached to the coil bobbin through a hole, and the shield conductor is wound around the body of the coil bobbin, and the power supply line on the positive electrode side or the power supply line on the negative electrode side for supplying the DC input or the like. The input winding and the output winding are wound on the shield conductor. Ing power.