JP2009165323A - Switching power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply device which prevents wasteful power consumption and prevents an overvoltage of the output voltage. <P>SOLUTION: The switching power supply device 1 includes: a dummy load circuit 3 which is connected between a second secondary-side output B which is not feedback-controlled and a first secondary-side output A which is feedback-controlled and supplies a dummy current I<SB>B1</SB>from the second secondary-side output B toward the first secondary-side output A; and a dummy current control circuit 4 which detects a difference between the secondary-side output voltage (15[V]) which is not feedback-controlled and the secondary-side output voltage (7[V]) which is feedback-controlled using a Zener diode ZD and energizes the Zener diode ZD when the detected voltage difference exceeds a predetermined value and causes the dummy current I<SB>B1</SB>to flow into the dummy load circuit 3. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a switching power supply device, and more particularly, to a switching power supply device including a dummy load circuit for preventing an output voltage from becoming an overvoltage state.

As a conventional switching power supply, for example, a switching power supply according to Patent Document 1 is shown in FIG.
In the switching power supply unit 1 ', transformer T has one primary winding _{T 1} and two secondary windings _{T 2a} and _{T 2b,} the switching element _{Q 1} made of MOSFET on the primary winding _{T 1} A DC power supply E is connected via The gate of the switching element Q _1, a switching control circuit 2 for controlling the duty ratio of the ON / OFF time of the switching element Q ₁ is connected.

The secondary winding T _2a of transformer T, a rectifying diode D _a and a smoothing capacitor C _a are connected. The secondary output voltage V _A is output from the first secondary output A provided at these connection points. Further, in the secondary winding T _2b of transformer T, a rectifying diode D _b and the smoothing capacitor C _b is connected. The secondary output voltage V _B is output from the second secondary output B provided at these connection points.

The secondary output voltages V _A and V _B are determined by the turn ratio of each winding of the transformer T. As an example, the secondary output voltages V _A and V _B are 7 [V] and 15 [V], respectively. It is. Further, a load circuit (not shown) driven by 7 [V] is connected to the first secondary output A, and a load driven by 15 [V] is connected to the second secondary output B. A circuit (not shown) is connected.

The smoothing capacitor C _a voltage dividing resistors R ₁ and R ₂ are connected in parallel, voltage dividing point of the voltage dividing resistors R ₁ and R ₂ are connected to the switching control circuit 2. Thus, the secondary-side output voltage V _A is fed back to the switching control circuit 2, the switching control circuit 2 controls the switching element Q ₁ so that the secondary side output voltage V _A becomes constant.

In this switching power supply device 1 ′, the secondary output voltage on the side not subjected to feedback control, that is, the consumption current of the load circuit driven by the secondary output voltage V _B decreases, and the second secondary output B When the current supplied to the load circuit decreases excessively, the secondary output voltage V _B becomes in an overvoltage state without being proportional to the turns ratio of the transformer T, as shown by the broken line in FIG. .
Therefore, the switching power supply device 1 ′ includes a dummy load circuit 3 ′ and a dummy current control circuit 4 ′, and when the secondary output voltage V _B satisfies a predetermined condition, the dummy current circuit 3 ′ is used as a dummy current. I _B1 flows, so that an overvoltage state of the secondary output voltage V _B is prevented.

Specifically, the dummy load circuit 3 'is made of the switching element Q ₂ and the dummy resistor R ₃ connected in series, connected between the second secondary output B of the first secondary output A Is done. The dummy current control circuit 4 'includes a resistance R ₄ connected between the base and emitter of the switching element Q _2, consists of a MOSFET is connected to the base of the switching element Q ₂ via a resistor R ₅ switching element Q ₃ is provided. Further, the dummy current control circuit 4 ′ is connected to the voltage dividing resistors R ₆ and R ₇ connected in series between the second secondary output B and GND, and to the connection point between them, and the switching element Q _The microcomputer 5 which controls ₃ is provided.

The microcomputer 5 monitors the secondary output voltage V _B through the voltage at the voltage dividing point of the voltage dividing resistors R ₆ and R ₇ . If this voltage value is equal to or less than the predetermined value, i.e., when the steady state (see FIG. 4), the switching element Q ₃ are being OFF by the microcomputer 5. Also, since the switching element _{Q 2} is turned OFF, the dummy current _{I B1} is not flowed to the dummy load circuit 3 '.

On the other hand, when the consumption current of the load circuit driven by the secondary-side output voltage V _B becomes excessively decreased to an overvoltage condition (see FIG. 4), the microcomputer 5 detects this makes ON the switching element Q _3. Accordingly, the current flows _{I B3} to the resistor _{R 4} and _{R 5,} the base current to the switching element _{Q 2} of the dummy load circuit 3 '(drive current _{I B2)} flows, the switching element _{Q 2} is turned ON. Then, the dummy current I _B1 flows from the second secondary output B to the first secondary output A via the dummy load circuit 3 ′, and the secondary output voltage V _B enters an overvoltage state. Is prevented.

The secondary output voltage V _A tends to increase due to the dummy current I _B1 flowing through the voltage dividing resistors R ₁ and R ₂ , but the secondary output voltage V _A is feedback-controlled by the switching control circuit 2. Therefore, an overvoltage condition will not occur.
JP 2005-341695 A

However, it was flowed to GND through the (1) in the conventional switching power supply device 1 'shown in FIG. 3, the drive current I _B2 of the switching element Q ₂ during overvoltage condition suppression, resistor R ₅ and the switching device Q ₃ . (2) In the switching power supply 1 ′, the current I _B4 flowing through the voltage _dividing resistors R ₆ and R ₇ connected between the second secondary output B and GND is in an overvoltage state or a steady state. It was always flowing into GND. (3) Furthermore, since the switching power supply device 1 ′ uses the microcomputer 5 to detect the overvoltage state of the secondary output voltage V _B , it also requires power for driving the microcomputer 5 itself. It was.

  That is, the conventional switching power supply device 1 'shown in FIG. 3 consumes power wastefully in the above three points, and further reduction in power consumption has been demanded.

  Therefore, an object of the present invention is to provide a switching power supply device that can prevent an output voltage from being in an overvoltage state without wasting power.

In order to solve the above problems, a switching power supply device according to the present invention provides:
A DC voltage is converted into a plurality of secondary output voltages using a first switching element and a transformer having a plurality of secondary windings, and one of the secondary output voltages is kept constant by feedback control. A switching power supply apparatus that is configured to output a secondary side output voltage that is subjected to the feedback control and a secondary side output voltage that is not subjected to the feedback control. A dummy having a second switching element and a dummy load connected in series between the secondary side output and a dummy current flowing through the dummy load when the second switching element is turned on. A load circuit; and a dummy current control circuit that switches between an ON state and an OFF state of the second switching element. A detection element that detects a voltage difference between the output voltage of the side output and the output voltage of the second secondary output, and when the voltage difference exceeds a predetermined value, the detection element changes from a non-conduction state to a conduction state. By switching, the second switching element is switched from an OFF state to an ON state.

  Preferably, the current flowing out of the dummy current control circuit flows toward the first secondary output together with the dummy current.

  Preferably, the detection element is a Zener diode.

  Preferably, the output voltage of the first secondary output is lower than the output voltage of the second secondary output.

  ADVANTAGE OF THE INVENTION According to this invention, the switching power supply device which can prevent that an output voltage will be in an overvoltage state can be provided, without consuming electric power wastefully.

  Hereinafter, preferred embodiments of a switching power supply device according to the present invention will be described with reference to the drawings.

A switching power supply apparatus according to the present invention is shown in FIG.
In the switching power supply unit 1, the transformer T has one primary winding _{T 1} and two secondary windings _{T 2a} and _{T 2b,} the switching element _Q 1 made of MOSFET on the primary winding _{T 1} ( A DC power source E is connected via a “first switching element” of the present invention. The gate of the switching element Q _1, a switching control circuit 2 for controlling the duty ratio of the ON / OFF time of the switching element Q ₁ is connected.

The secondary winding T _2a of transformer T, a rectifying diode D _a and a smoothing capacitor C _a are connected. The secondary output voltage V _A is output from the first secondary output A provided at these connection points. Further, in the secondary winding T _2b of transformer T, a rectifying diode D _b and the smoothing capacitor C _b is connected. The secondary output voltage V _B is output from the second secondary output B provided at these connection points.

The secondary output voltages V _A and V _B are determined by the turn ratio of each winding of the transformer T. As an example, the secondary output voltages V _A and V _B are 7 [V] and 15 [V], respectively. It is. Further, a load circuit (not shown) driven by 7 [V] is connected to the first secondary output A, and a load driven by 15 [V] is connected to the second secondary output B. A circuit (not shown) is connected.

The smoothing capacitor C _a voltage dividing resistors R ₁ and R ₂ are connected in parallel, voltage dividing point of the voltage dividing resistors R ₁ and R ₂ are connected to the switching control circuit 2. Thus, the secondary-side output voltage V _A is fed back to the switching control circuit 2, the switching control circuit 2 controls the switching element Q ₁ so that the secondary side output voltage V _A becomes constant.

A dummy load circuit 3 for preventing an overvoltage state of the secondary output voltage V _B includes a switching element Q ₂ (corresponding to a “second switching element” of the present invention) connected in series and a dummy resistor R ₃ (this Equivalent to the “dummy load” of the invention, and is connected between the second secondary output B and the first secondary output A. The dummy current control circuit 4 includes resistors R ₄ and R ₅ and a Zener diode ZD (corresponding to the “detection element” of the present invention). Resistor R ₄ is connected between the base and emitter of the switching element Q _2, resistors R ₅ and a Zener diode ZD are connected in series between the base and the first secondary output A of the switching element Q _2.

In the switching power supply device 1 shown in FIG. 1, the Zener diode ZD plays a role of detecting a voltage difference between the secondary output voltages V _A and V _B. That is, whether or not the secondary output voltage V _B is in an overvoltage state by detecting a voltage difference from the reference voltage using the secondary output voltage _VA maintained constant by feedback control as a reference voltage. Is determined.

Specifically, when the secondary output voltage V _A is 7 [V] and the secondary output voltage V _B is 15 [V], the Zener voltage of the Zener diode ZD is 8 (= 15−7) [V ] Is set. Then, it is determined if a voltage exceeding this to both ends of the Zener diode ZD is applied and the secondary-side output voltage V _B is overvoltage, switched zener diode ZD is from a non-conductive state to the conductive state, current flow .
Note that the Zener voltage may be set to a value slightly higher than 8 [V] in consideration of minute fluctuations in the secondary output voltages V _A and V _B.

When the secondary-side output voltage V _B is in a steady state (see FIG. 4), the voltage exceeding the 8 [V] across the Zener diode ZD will not be applied, no current flows to the Zener diode ZD (non Conduction state). Therefore, the driving current _{I B2} of the switching element _{Q 2} is not flow, the switching element _{Q 2} is remains OFF, the dummy current _{I B1} to the dummy load circuit 3 does not flow.

On the other hand, when the current consumption of the load circuit driven by the secondary side output voltage V _B decreases and the secondary side output voltage V _B enters an overvoltage state (see FIG. 4), 8 [V] across the Zener diode ZD. Is applied, the Zener diode ZD becomes conductive and current begins to flow. Accordingly, the driving current _{I B2} of the switching element _{Q 2} flows through the resistor _{R 5} and a Zener diode ZD, the switching element _{Q 2} is turned ON. Then, the dummy current I _B1 flows from the second secondary output B to the first secondary output A via the dummy load circuit 3, and an overvoltage state of the secondary output voltage V _B is prevented. The

In the switching power supply apparatus 1 shown in FIG. 1, in a steady state, the switching element Q ₂ and the Zener diode ZD is turned OFF none, no current flows at all to the dummy load circuit 3 and the dummy current control circuit 4.
When the overvoltage state is suppressed, the current I _B3 flowing through the dummy current control circuit 4 and the drive current I _B2 of the switching element Q ₂ are all changed from the second secondary output B to the first secondary output A. It flows toward. The dummy current I _B1 and the currents I _B2 and I _B3 are required by a voltage dividing circuit including resistors R ₁ and R ₂ and a load circuit (not shown) connected to the first secondary output A. Since it is effectively utilized as a current, no wasteful current flows through the switching power supply device 1 as a whole.
Furthermore, since the switching power supply device 1 shown in FIG. 1 does not require a microcomputer (see FIG. 3), power consumption can be reduced accordingly.

  As mentioned above, although preferable embodiment of the switching power supply device which concerns on this invention was described, this invention is not limited to the said structure.

For example, the switching power supply device 1 may have three or more secondary outputs A, B... N as shown in FIG. Even in this case, the dummy currents I _{B1 to} I _N1 flowing out of the dummy load circuits 3 _{b to} 3 _n and all the currents flowing out of the dummy current control circuits 4 _b to 4 _n are the first two whose output voltages are feedback controlled. It is sent to the secondary output A and used effectively.
The secondary output voltages V _{B to} V _N output from the second secondary outputs B... N need to be higher than the secondary output voltage V _A. Even when a dummy current flows from any one of the plurality of second secondary outputs B... N to the first secondary output A, the secondary output voltage V _A is obtained by feedback control. Since it is kept constant, it does not affect other secondary output voltages.

1 is a circuit diagram showing an embodiment of a switching power supply device according to the present invention. It is a circuit diagram of the switching power supply concerning a modification. It is a circuit diagram of the conventional switching power supply device. It is a graph which shows the relationship between the output voltage of a switching power supply device, and the consumption current of the load circuit driven with the said voltage.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Switching power supply device 2 Switching control circuit 3 Dummy load circuit 4 Dummy current control circuit 5 Microcomputer Q ₁ 1st switching element Q ₂ 2nd switching element R ₃ Dummy resistance (dummy load)
ZD Zener diode (detection element)

Claims (4)

A DC voltage is converted into a plurality of secondary output voltages using a first switching element and a transformer having a plurality of secondary windings, and one of the secondary output voltages is kept constant by feedback control. A switching power supply device
Between the first secondary output that outputs the secondary output voltage for which the feedback control is performed and the second secondary output that outputs the secondary output voltage for which the feedback control is not performed. A dummy load circuit having a second switching element and a dummy load connected to each other, and a dummy current flowing through the dummy load when the second switching element is turned on,
A dummy current control circuit for switching between the ON state and the OFF state of the second switching element,
The dummy current control circuit includes a detection element that detects a voltage difference between an output voltage of the first secondary output and an output voltage of the second secondary output, and the voltage difference has a predetermined value. The switching power supply device according to claim 1, wherein the second switching element is switched from an OFF state to an ON state when the detection element is switched from a non-conducting state to a conducting state.   2. The switching power supply device according to claim 1, wherein a current flowing out of the dummy current control circuit flows toward the first secondary output together with the dummy current.   The switching power supply device according to claim 1, wherein the detection element is a Zener diode.   4. The switching power supply device according to claim 1, wherein an output voltage of the first secondary output is lower than an output voltage of the second secondary output. 5.

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