JP7624334B2 - Switching Power Supply - Google Patents

Description

The present invention relates to a multiple-output switching power supply with multiple outputs, and more specifically to a switching power supply equipped with a voltage-controlled output circuit that controls the output voltage and a non-voltage-controlled output circuit that does not control the output voltage.

Patent Document 1 describes an invention for a multi-output switching power supply in which one output is configured as a voltage-controlled output circuit and the other outputs are configured as non-voltage-controlled output circuits, and in order to stabilize the output voltage of the non-voltage-controlled output circuit, power is supplied from the voltage-controlled output circuit to the non-voltage-controlled output circuit.

6 is a circuit diagram of a conventional switching power supply described in Patent Document 1. This switching power supply includes a voltage-controlled output circuit having a control circuit that rectifies and smoothes the voltage output from a winding P2 on the secondary side of a transformer and controls the voltage V1, and a non-voltage-controlled output circuit that rectifies and smoothes the voltage output from another winding P3 on the secondary side and uses a constant voltage circuit IC1 to produce an output voltage V3.
In this switching power supply, a constant voltage adjustment circuit is connected between the voltage controlled output circuit and the non-voltage controlled output circuit, and when the output voltage of the non-voltage controlled output circuit falls due to a heavy load on the output of the non-voltage controlled output circuit, current is caused to flow from the voltage controlled output circuit to the non-voltage controlled output circuit via transistor Q2, increasing the voltage V2 of the non-voltage controlled output circuit and stabilizing the input voltage of the constant voltage circuit IC1.

In other words, when the load connected to the voltage-controlled output circuit is unloaded or lightly loaded and does not fluctuate, the on period of the switching element Q1 controlled by the control circuit does not change. Therefore, when the output of the non-voltage-controlled output circuit is heavily loaded, the voltage of the secondary winding drops and the output voltage V3 of the non-voltage-controlled circuit drops.
Therefore, when the voltage V2 of the non-voltage controlled output circuit falls below a value satisfying a predetermined relationship, the constant voltage adjustment circuit operates to cause current to flow from the voltage controlled output circuit to the non-voltage controlled output circuit, thereby suppressing a decrease in the output voltage of the non-voltage controlled output circuit.

JP 2002-272108 A

However, when power is supplied from the voltage-controlled output circuit to the non-voltage-controlled output circuit by the constant voltage adjustment circuit shown in FIG. 6, there is a problem in that the greater the difference between the voltage V1 of the voltage-controlled output circuit and the voltage V2 of the non-voltage-controlled output circuit, the greater the loss.
For example, when this switching power supply is used as the power supply for a printer, the voltage V1 of the voltage-controlled output circuit is about 65 V, the voltage at which the printer operates, and the output voltage V3 of the non-voltage-controlled output circuit is about 5 V. If the current supplied from the voltage-controlled output circuit is 0.6 A, the loss in the constant voltage adjustment circuit becomes the product of the voltage difference between voltages V1 and V2 and the supplied current, which is approximately 36 W, and is very large.

The present invention solves the above problems by providing a switching power supply that does not supply power from a voltage-controlled output circuit to a non-voltage-controlled output circuit, but rather provides a non-voltage-controlled output circuit with multiple winding output terminals on the secondary winding of a transformer and inputs the multiple winding output terminals, and when the load on the non-voltage-controlled output circuit is small (light load), the non-voltage-controlled output circuit supplies power to the load using the winding output terminal with a relatively low induced voltage as its input, among the multiple winding output terminals, and when the load is large (heavy load), supplies power to the load using the winding output terminal with a relatively high induced voltage as its input.

That is, a multi-output switching power supply is provided that includes a transformer having a primary winding and multiple secondary windings, a switching element connected to the primary winding of the transformer, a voltage-controlled output circuit connected to at least one of the multiple secondary windings, and a non-voltage-controlled output circuit connected to the other secondary windings, in which a switching voltage obtained by turning a DC voltage on and off with the switching element is applied to the primary winding of the transformer, and a voltage induced in the multiple secondary windings is output to a first load connected to the voltage-controlled output circuit and a second load connected to the non-voltage-controlled output circuit. In this switching power supply, the secondary winding of the transformer connected to the non-voltage controlled output circuit has multiple winding output terminals that generate different voltages, the voltage controlled output circuit converts the voltage induced in the secondary winding of the transformer into DC voltage, stabilizes it using a predetermined control circuit, and supplies it to a first load, and the non-voltage controlled output circuit is capable of inputting the voltage induced in the multiple winding output terminals, and selectively supplies the DC voltage to the second load based on one of the voltages induced in the multiple winding output terminals depending on the size of the second load.

With this configuration, even if the load on the non-voltage-controlled output circuit increases, the voltage induced in the winding output terminal is selected without receiving power from the voltage-controlled output circuit, suppressing voltage drops and reducing circuit losses.

In addition, the non-voltage controlled output circuit is characterized in that, when the output voltage output to the second load is higher than a predetermined value, it connects to the terminal among the multiple winding output terminals with the lower induced voltage to supply it to the second load, and when the voltage of the second load drops below the predetermined value, it connects to the terminal with the higher induced voltage to suppress the voltage drop of the second load.
With this configuration, even if the voltage of the non-voltage controlled output circuit drops, the voltage at the winding output terminal of the secondary winding can be switched to suppress a drop in the output voltage.

Furthermore, the non-voltage controlled output circuit is a circuit in which the output voltage for the second load is divided by a resistor and input to a voltage detection element, one end of the voltage detection element is connected to the control terminal of a voltage switching element, one terminal of a current path of the voltage switching element is connected to a winding output terminal having a higher induced voltage among the multiple winding output terminals, and the other terminal of the current path of the voltage switching element is connected to a terminal having a lower induced voltage among the multiple winding output terminals, wherein when the output voltage for the second load is higher than a predetermined value, a current flows through the voltage detection element and the voltage switching element is turned off, and a voltage is output to the second load based on the voltage from the winding output terminal having the lower induced voltage, and when the output voltage for the second load falls below the predetermined value, the current flowing through the voltage detection element is cut off and the voltage switching element is turned on, and a voltage is output to the second load based on the voltage from the winding output terminal having the higher induced voltage.
In this way, by switching the voltages of the multiple winding output terminals using the voltage switching element, the non-voltage controlled output circuit can be configured as a circuit with little loss.

The voltage detection element is a shunt regulator, and the voltage switching element is an NPN transistor.
This makes it possible to switch the induced voltage in the secondary winding in response to a voltage drop in the load.

By configuring as described above, the switching power supply of the present invention can automatically switch the voltage at the winding output terminal of the secondary winding when the load on the non-voltage controlled output circuit increases and the output voltage drops, thereby suppressing the drop in the output voltage and reducing power loss.

1 is a circuit diagram of a switching power supply according to an embodiment of the present invention. FIG. 2 is a circuit diagram of a non-voltage controlled output circuit. 1A and 1B are diagrams illustrating the operation of a non-voltage controlled output circuit. FIG. 13 is a diagram for explaining, for comparison, the output voltage switching operation of the prior art. FIG. 11 is a circuit diagram of another embodiment of a non-voltage controlled output circuit. FIG. 1 is a circuit diagram of a conventional switching power supply described in Patent Document 1.

A switching power supply according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a circuit diagram of a switching power supply according to an embodiment of the present invention.
This switching power supply 1 includes a switching element Q1 that switches a DC voltage Vin to convert the voltage, a transformer T having a primary winding N1 to which the converted switching voltage is supplied and two secondary windings, a first secondary winding N2 and a second secondary winding N3, a voltage-controlled output circuit 11 that converts the switching voltage induced in the first secondary winding N2 to a DC voltage using a DC conversion circuit (rectifier/smoothing circuit) consisting of a diode D1 and a smoothing capacitor C1, a control circuit 12 such as a control IC that controls the switching element Q1 so that the output voltage of the voltage-controlled output circuit 11 becomes a predetermined voltage, and a non-voltage-controlled output circuit 10 to which a plurality of voltages induced in the second secondary winding N3 are input.

The second secondary winding N3 has a plurality of winding output terminals which generate different voltages, and the plurality of winding output terminals are connected to the input of a non-voltage controlled output circuit 10. A first load is connected to the output of the voltage controlled output circuit 11, and a second load is connected to the output of the non-voltage controlled output circuit 10.
The control circuit 12 is a well-known circuit that controls the on/off cycle of the switching element Q1 so that the output voltage V1 of the voltage-controlled output circuit 11 becomes a predetermined voltage value.
The DC voltage Vin may be a DC voltage obtained by rectifying and smoothing a commercial AC power supply voltage, or may be a DC voltage based on power generated by a power generating means such as a solar cell.

FIG. 2 is a circuit diagram of the non-voltage controlled output circuit 10. As shown in FIG.
The winding output terminal at the end of the secondary winding N3 of the transformer T (hereinafter referred to as the "winding end terminal") is connected to the anode of the diode D2, the winding output terminal at the middle of the secondary winding N3 (hereinafter referred to as the "center tap") is connected to the anode of the diode D3, and the start of the secondary winding is connected to GND. In other words, the voltage V2 at the winding end terminal is higher than the voltage V3 at the center tap.

The cathode of diode D2 is connected to the collector (one terminal of the current path) of transistor Q1 (corresponding to the "voltage switching element" of this application) and one end of resistor R1. The voltage on the cathode side of diode D2 (collector voltage of transistor Q1) is V2. The cathode of diode D3 is connected to the emitter (the other terminal of the current path) of transistor Q1 and is output to a second load. In this case, the voltage on the cathode side of diode D3 (emitter voltage of transistor Q1) V3 is the same as the output voltage of non-voltage controlled output circuit 10 (output voltage output to the second load) Vo.

The base (control terminal) of the transistor Q1 is connected to the other end of the resistor R1 and one end of the resistor R2. The other end of the resistor R2 is connected to the cathode of a shunt regulator U1 (corresponding to the "voltage detection element" of the present application). The reference terminal of the shunt regulator U1 is connected to the connection point of resistors R3 and R4 that are connected in series between the output terminal of the non-voltage controlled output circuit 10 and GND. The anode of the shunt regulator U1 is connected to GND.
The voltage V2 on the cathode side of the diode D2 and the voltage V3 on the cathode side of the diode D3 are respectively the induced voltages output to the winding end terminal and the center tap minus the voltage drops of the diodes D2 and D3. A capacitor C2 is connected between the cathode side of the diode D3 and GND.

The operation of the non-voltage controlled output circuit 10 configured as above will be described.
In the shunt regulator U1, the voltage at the connection point between resistors R3 and R4 (the voltage obtained by dividing the output voltage Vo by resistors R3 and R4) is input to the reference terminal of the shunt regulator U1, the input voltage to the reference terminal is compared with a reference voltage inside the shunt regulator U1, and when the input voltage to the reference terminal is high, current flows through the shunt regulator U1 and its function as a switch is turned on (the shunt regulator U1 is conductive).

When the output voltage Vo output to the second load is higher than a predetermined value, the shunt regulator U1 turns on, and current flows from the winding output terminal where the induced voltage of the secondary winding N3 of the transformer T is relatively high, through the diode D2, resistor R1, and resistor R2. When current flows, the base voltage of the transistor Q1 falls below the emitter voltage V3 due to the voltage drop caused by resistor R1, and the transistor Q1 turns off. At this time, no current flows from the cathode (voltage V2) side of the diode D2 to the cathode (voltage V3) side of the diode D3, so the voltage V3 becomes the output voltage Vo as it is and is output to the load (second load).

On the other hand, when the second load becomes large and the output voltage Vo falls below a predetermined value, the voltage divided by the resistors R3 and R4 also falls, and the input voltage of the reference terminal of the shunt regulator U1 falls below the reference voltage. Then, the switch function of the shunt regulator U1 is turned off (the shunt regulator U1 is non-conductive). As a result, no voltage drop occurs due to the shunt regulator U1 of the resistor R1, so the base voltage becomes higher than the emitter voltage V3 of the transistor Q1, and the transistor Q1 turns on. A current flows from the cathode (voltage V2) side of the diode D2 to the cathode (voltage V3) side of the diode D3 via the transistor Q1. As a result, the drop in the voltage V3 is suppressed, and the voltage V2 is output to the load as the output voltage Vo.
In this case, the output voltage Vo is not exactly the voltage V2, but is the voltage obtained by subtracting the collector-emitter voltage of the transistor Q1 from the voltage V2.

In other words, when the load on the non-voltage controlled output circuit 10 is small (light load), the voltage of the winding output terminal where the induced voltage of the secondary winding N3 is relatively low is output to the load, and when the load on the non-voltage controlled output circuit is large (heavy load), the voltage of the winding output terminal where the induced voltage of the secondary winding N3 is relatively high is output to the load.
Here, the predetermined value of the output voltage Vo is a voltage value determined by the reference voltage value inside the shunt regulator U1 and the resistance values of the resistors R3 and R4, i.e., the predetermined value = (R3 + R4) / R4 x reference voltage value.

FIG. 3 is a diagram for explaining the operation of the non-voltage controlled output circuit 10.
The horizontal axis is the output current of the second load, and the vertical axis is the voltage. The voltage V1 shown by the dashed line is the output voltage V1 of the voltage-controlled output circuit 11 shown in Fig. 1. The continuous curve in which the voltage gradually decreases as the output current increases (shown by a dotted line, but shown by a solid line in the region where the output current is large) is the voltage V2, and the continuous curve in which the voltage gradually decreases as the output current increases and the voltage value is smaller than V2 (shown by a dotted line, but shown by a solid line in the region where the output current is small) is the voltage V3. The curve in which the voltage becomes constant at point A from the solid line portion of voltage V3 and then switches to the solid line portion of voltage V2, i.e., the curve consisting only of solid lines with a straight line inserted in the middle, shows the output voltage Vo to the second load.

At point A, the output voltage Vo does not immediately switch to voltage V2 but remains constant (a predetermined value) because the shunt regulator U1 operates to keep the output voltage Vo constant. When the output current increases further and voltage V2 drops, the shunt regulator U1 turns off completely, which turns on the transistor Q1 and outputs voltage V2 as is to the second load.
In both curves, the voltage decreases as the output current increases. The turns ratio of the secondary winding N3 is set so that the voltage V2 is about 10 to 15% higher than the voltage V3.

Point A in Fig. 3 is the point in time when the output current when the output voltage Vo becomes a constant value (e.g., 0.1 A). Here, the output voltage Vo is maintained at a constant value from voltage V3 and then switches to voltage V2. Since transistor Q1 is not operating at the output current up to point A, there is no power loss regardless of the output current value. When the output current increases and exceeds point A, transistor Q1 operates, maintains a constant value, switches to voltage V2, and the output voltage Vo also decreases.
For example, when the output current is 0.6 A (corresponding to point B of voltage Vo in FIG. 3), the loss of non-voltage controlled output circuit 10 is 0.30 W if the difference between voltages V2 and V3, which are the voltages applied between the collector and emitter of transistor Q1, is 0.5 V.

The losses in the non-voltage controlled output circuit 10 of this embodiment are compared with the losses in the conventional circuit shown in FIG.
Fig. 4 is a diagram for explaining, for comparison, the output voltage switching operation of the prior art shown in Fig. 6. The horizontal axis is the output current, and the vertical axis is the voltage, which is the same as in Fig. 3. The dashed line and the dashed dotted line in Fig. 4 represent voltages V1 and V3 shown in Fig. 6, and the solid line, which decreases with an increase in the output current and then becomes a constant value at point A, represents voltage V2.
For example, if the output currents at points A and B shown in FIG. 4 are 0.1 A and 0.6 A, the same as in FIG. 3, and if voltage V1 is 65 V and voltage V3 is 5 V, then there is no loss in the constant voltage adjustment circuit at point A, but at point B, the loss is the product of the difference between voltage V1 and voltage V3 (60 V) and the output current of 0.6 A, which is 36 W.
Compared to the losses in the constant voltage regulator circuits of the prior art, the losses in the non-voltage controlled output circuit 10 of the present invention are significantly reduced.

Although the non-voltage controlled output circuit 10 shown in FIG. 2 does not include the constant voltage circuit IC1 shown in FIG. 6, it may include a constant voltage circuit.
Moreover, although the voltage switching element is configured with an NPN transistor, a MOSFET may also be used.

Fig. 5 is a circuit diagram of another embodiment of a non-voltage controlled output circuit. Compared to Fig. 2, the element U2 (voltage detection element) that detects a drop in the output voltage and outputs it to the transistor Q1 is different, but since the other configurations are the same, the specifications of the elements such as resistors of the non-voltage controlled output circuit 20 are the same.
This element U2 can use a comparator that compares a terminal that detects a drop in the output voltage Vo with a reference voltage, and is configured so that when the voltage drops, the output of the comparator operates the transistor Q1.

In addition, in this embodiment, there are two secondary windings, the first secondary winding N2 and the second secondary winding N3, but there may be three or more secondary windings. In this case, a voltage-controlled output circuit is connected to one of the secondary windings, and a non-voltage-controlled output circuit is connected to the other secondary winding, and the present invention can be applied to at least one of the non-voltage-controlled output circuits connected to the other secondary windings.

1... switching power supply, 10... non-voltage controlled output circuit, 11... voltage controlled output circuit, 12... control circuit, 20... non-voltage controlled output circuit.

Claims (2)

A multi-output switching power supply comprising a transformer having a primary winding and a plurality of secondary windings, a switching element connected to the primary winding of the transformer, a voltage controlled output circuit connected to at least one of the plurality of secondary windings, and a non-voltage controlled output circuit connected to another secondary winding, wherein a switching voltage obtained by turning a DC voltage on/off with the switching element is applied to the primary winding of the transformer, and a voltage induced in the plurality of secondary windings is output to a first load connected to the voltage controlled output circuit and a second load connected to the non-voltage controlled output circuit,
a secondary winding of the transformer connected to the non-voltage controlled output circuit has a plurality of winding output terminals each having a different generated voltage;
the voltage controlled output circuit converts a voltage induced in the secondary winding of the transformer into a direct current, stabilizes the direct current by a predetermined control circuit, and supplies the direct current to the first load;
the non-voltage controlled output circuit is capable of inputting voltages induced in the plurality of winding output terminals, and selectively supplies to the second load a voltage that has been converted to DC based on one of the voltages induced in the plurality of winding output terminals depending on the magnitude of the second load; detects an output voltage to be output to the second load, and when the output voltage is higher than a predetermined value, connects to the winding output terminal having the lower induced voltage among the plurality of winding output terminals, and when the output voltage falls below the predetermined value, connects to the winding output terminal having the higher induced voltage ;
The non-voltage controlled output circuit is a circuit in which an output voltage to be output to the second load is divided by a resistor and input to a voltage detection element, one end of the voltage detection element is connected to a control terminal of a voltage switching element, one terminal of a current path of the voltage switching element is connected to a winding output terminal having a high induced voltage among the plurality of winding output terminals, and the other terminal of the current path of the voltage switching element is connected to a winding output terminal having a low induced voltage among the plurality of winding output terminals,
a switching power supply comprising: when an output voltage output to the second load is higher than a predetermined value, a current flows through the voltage detection element, the voltage detection element is turned on and the voltage switching element is turned off, and a voltage is output to the second load based on the voltage from the winding output terminal having the low induced voltage; and when the output voltage output to the second load falls below a predetermined value, the current flowing through the voltage detection element is interrupted, the voltage detection element is turned off and the voltage switching element is turned on, and a voltage is output to the second load based on the voltage from the winding output terminal having the high induced voltage . the voltage detection element is a shunt regulator,
2. The switching power supply according to claim 1 , wherein the voltage switching element is an NPN transistor.

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