JP5126967B2 - Switching power supply - Google Patents

Description

  The present invention relates to a switching power supply, and more particularly to a switching power supply capable of detecting an overvoltage state and an overcurrent state of a secondary output.

Various switching power supply devices are used depending on the application. For example, the switching power supply device 1 ′ shown in FIG. 4 converts an AC voltage V _in (for example, 100 V to 240 V) into a predetermined DC voltage V _out, and converts the overvoltage state and the overcurrent state of the secondary output. When detected, the switching operation of the switching element is stopped to prevent damage to the switching power supply device 1 ′ itself and damage to the load 3 connected to the secondary side output (see, for example, Non-Patent Document 1).

The switching power supply device 1 ′ includes a control circuit 2. As shown in FIG. 2, the control circuit 2 includes a switching element Q ₁ , a switching control unit 21 that controls the switching operation, a current monitoring unit 22 that monitors a current flowing through the switching element Q ₁ , and a control circuit 2. And a voltage monitoring unit 23 for monitoring the supplied drive voltage. As the control circuit 2, for example, an IC “STR-A6251” manufactured by Sanken Electric Co., Ltd. can be used.

Referring to FIGS. 2 and 4, the switching power supply unit 1 'includes a transformer T, transformer T has a primary winding T _1, the secondary winding T _2, and an auxiliary winding T _S. The primary winding T ₁ is connected to the drain of the switching element Q ₁ via the terminal D of the control circuit 2. The source of the switching element _{Q 1} is connected to the resistor _{R 1} via a terminal OCP.

The secondary winding T ₂ of the transformer T, a rectifier diode D ₂ and the smoothing capacitor C ₂ is connected, the induced AC voltage is rectified and smoothed in the secondary winding T ₂ and the secondary-side output voltage V _out Is output as The amount of the secondary output voltage V _out is fed back to the primary side by voltage dividing resistors R ₂ and R ₃ and photocouplers PC and PC ′. Then, switching control unit 21 of the control circuit 2, based on the voltage of the terminal FB which changes in accordance with the secondary output voltage V _out, controls the duty ratio of the switching element Q _1. As a result, the secondary output voltage _Vout is kept constant.

In addition, a rectifier diode D _S and a smoothing capacitor C _S are connected to the auxiliary winding T _S. Switching power supply unit 1 'generates a DC voltage VCC AC voltage induced in the auxiliary winding T _S rectifies and smoothes. The voltage VCC is used as a drive voltage for the control circuit 2.

As described above, the switching power supply device 1 ′ can detect an overcurrent state of the secondary side output, that is, a state where the current _IO flowing from the secondary side output to the load 3 is excessively large.
Specifically, when the secondary-side output is in an overcurrent state, even overcurrent flowing through the switching element to Q ₁ control circuit 2. This current flows through the resistor _{R 1} through terminal OCP, the voltage at the terminal OCP is increased. The current monitoring unit 22 determines that an overcurrent state occurs when the voltage at the terminal OCP exceeds a predetermined value. Then, switching control unit 21 changes the duty ratio of the switching element Q _1, by reducing the secondary-side output voltage V _out, to alleviate the over-current condition.

When the secondary output voltage _Vout decreases, the drive voltage VCC of the control circuit 2 also decreases in proportion to this. When a driving voltage VCC reaches the minimum operating voltage of the control circuit 2, the switching operation of the switching element Q ₁ is stopped, an overcurrent condition is fully resolved.
If an overcurrent state occurs, components (secondary winding T ₂ , rectifier diode D ₂ ) on the path through which the overcurrent flows may generate heat and be damaged.

Further, the switching power supply device 1 ′ can detect an overvoltage state of the secondary side output, that is, a state where the secondary side output voltage _Vout greatly exceeds a predetermined value.
For example, if some abnormality occurs in the photocouplers PC and PC ′ and the secondary output voltage V _out is not fed back to the primary side, the voltage at the terminal FB decreases. At this time, the control circuit 2 determines that the secondary-side output voltage V _out is lowered than the predetermined value, continues to increase the secondary side output voltage V _out, the overvoltage condition.

When the secondary output voltage _Vout increases, the drive voltage VCC of the control circuit 2 also increases in proportion to this. Then, the driving voltage VCC is a predetermined value (hereinafter, "the overvoltage protection setting voltage") exceeds, determines that the voltage monitoring unit 23 of the control circuit 2 is in an overvoltage state, stops the switching operation of the switching element Q ₁ , Eliminate the overvoltage condition.
Note that the load 3 may be damaged if an overvoltage state occurs.
Product Catalog “STR-A6200 Series”, Sanken Electric Co., Ltd., Internet, <http://www.sanken-ele.co.jp/news/contents/str-a6200j.pdf>, as of February 4, 2008

  As described above, in the switching power supply device 1 ′ shown in FIG. 4, both the overcurrent state and the overvoltage state are determined based on the drive voltage VCC. FIG. 5 shows the relationship among the steady-state drive voltage VCC, the operation lower limit voltage of the control circuit 2, and the overvoltage protection setting voltage when an IC “STR-A6251” manufactured by Sanken Electric Co., Ltd. is used as the control circuit 2. .

In FIG. 5A, the driving voltage VCC (= 15 [V]) in the steady state is set to a value close to the operation lower limit voltage (= 10 [V]), so that the overcurrent state is eliminated early. . In this case, since the difference voltage between the drive voltage VCC during normal operation and the overvoltage protection setting voltage (= 32 [V]) is large, the overvoltage state continues for a long time and the overvoltage applied to the load 3 increases.
On the other hand, in FIG. 5B, the driving voltage VCC (= 25 [V]) in the steady state is set to a value close to the overvoltage protection setting voltage (= 32 [V]), so that the overvoltage state is eliminated early. ing. In this case, since the difference voltage between the driving voltage VCC in the steady state and the operation lower limit voltage (= 10 [V]) is large, the overcurrent state continues for a long time.
That is, in the conventional switching power supply device 1 ′, it is impossible to achieve both the early cancellation of the overcurrent state and the early cancellation of the overvoltage state.

  Therefore, an object of the present invention is to provide a switching power supply device that can quickly resolve both an overcurrent state and an overvoltage state.

In order to solve the above problems, a switching power supply device according to the present invention provides:
A control circuit having a first switching element connected to a primary winding of the transformer and a switching control unit that controls the first switching element; While rectifying and smoothing the AC voltage induced in the secondary winding to generate the secondary output voltage, the secondary output voltage is fed back to the switching control unit to keep the secondary output voltage constant. And generating a first DC voltage for driving the control circuit by rectifying and smoothing an AC voltage induced in a first auxiliary winding provided on the primary side of the transformer, If the DC voltage has reached the overvoltage protection setting voltage, wherein a switching power supply device that reduces the secondary side output voltage, the dynamic of the first DC voltage and the control circuit Voltage difference between the lower limit voltage, the to be smaller than the difference between the voltage of the overvoltage protection setting voltage and said first DC voltage, said first DC voltage is set, further first the primary side of the transformer Two auxiliary windings are provided to rectify and smooth the sum voltage of the AC voltage induced in the first auxiliary winding and the AC voltage induced in the second auxiliary winding to generate a second DC When the detection element that generates a voltage and detects the amount of the second DC voltage detects that the second DC voltage exceeds a predetermined voltage, the first DC voltage is changed to the second DC voltage. The overvoltage protection set voltage is reached by forcibly pulling up toward the DC voltage.

  Preferably, in the switching power supply device, the detection element is a constant voltage diode.

  Preferably, the switching power supply device includes a second switching device including an emitter connected to a high potential side of the second auxiliary winding and a collector connected to a high potential side of the first auxiliary winding. Further comprising an element, the cathode side of the constant voltage diode and the base of the second switching element are connected, the anode side of the constant voltage diode and the low potential side of the first auxiliary winding are connected, When the second DC voltage rises and the constant voltage diode is switched from a non-conductive state to a conductive state, the second switching element is turned on, and the first switching element is turned on via the second switching element. The DC voltage is raised to the overvoltage protection set voltage.

  Preferably, in the switching power supply device, the predetermined voltage is set to be equal to or higher than the overvoltage protection set voltage.

  Preferably, in the switching power supply device, the control circuit is an IC packaged in one package.

  According to the present invention, even when the drive voltage VCC at the time of steady state is set to a value close to the operation lower limit voltage of the control circuit, the overvoltage state of the secondary output is eliminated early, and the overvoltage applied to the load is reduced. A possible switching power supply device can be provided.

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

FIG. 1 shows an embodiment of a switching power supply device according to the present invention.
The switching power supply device 1 includes a control circuit 2. As shown in FIG. 2, the control circuit 2 flows through the switching element Q ₁ (corresponding to the “first switching element” of the present invention), the switching control unit 21 for controlling the switching operation, and the switching element Q ₁ . A current monitoring unit 22 that monitors the current and a voltage monitoring unit 23 that monitors the drive voltage supplied to the control circuit 2 are provided. In the present embodiment, an IC “STR-A6251” manufactured by Sanken Electric Co., Ltd. is used as the control circuit 2.

The switching power supply device 1 includes a transformer T. The transformer T includes a primary winding T ₁ , a secondary winding T ₂ , a first auxiliary winding T _S1 provided on the primary side, and a first auxiliary winding T _S1 provided on the primary side. It has two auxiliary windings _TS2 . The primary winding T ₁ is connected to the drain of the switching element Q ₁ via the terminal D of the control circuit 2. The source of the switching element _{Q 1} is connected to the resistor _{R 1} via a terminal OCP.
The switching power supply device 1 includes a rectifier smoothing circuit consisting of rectifier diode D ₁ and a smoothing capacitor C ₁ Tokyo, AC voltage V _in input is converted into a DC voltage by rectifying and smoothing circuit, the switching element It is converted to an AC voltage again by the switching operation of the Q _1.

The secondary winding T ₂ of the transformer T, a rectifier diode D ₂ and the smoothing capacitor C ₂ is connected, the induced AC voltage is rectified and smoothed in the secondary winding T ₂ and the secondary-side output voltage V _out (10 [V] in this embodiment) is output. The amount of the secondary output voltage _Vout is fed back to the primary side by voltage dividing resistors R ₂ and R ₃ and photocouplers PC and PC ′.
Specifically, when the secondary output voltage V _out changes, the light emission amount of the light-emitting photocoupler PC increases or decreases. As a result, the collector-emitter voltage of the photocoupler PC ′ on the light receiving side varies, and the voltage at the terminal FB changes. Then, switching control unit 21 of the control circuit 2, based on the voltage of the terminal FB which changes in accordance with the secondary output voltage V _out, controls the duty ratio of the switching element Q _1. As a result, the secondary output voltage _Vout is kept constant.

A rectifier diode D _S1 and a smoothing capacitor C _S1 are connected to the first auxiliary winding T _S1 . The switching power supply device 1 rectifies and smoothes the AC voltage induced in the first auxiliary winding TS ₁ to generate a DC voltage VCC (corresponding to “first DC voltage”). The voltage VCC is used as a drive voltage for the control circuit 2.
Furthermore, the switching power supply device 1 mainly includes a second auxiliary winding T _S2 , a rectifier diode D _S2 , a smoothing capacitor C _S2 , a constant voltage diode ZD (corresponding to the “detection element” of the present invention), and a switching element Q. ₂ (corresponding to the “second switching element” of the present invention). The breakdown voltage of the constant voltage diode ZD is 33 [V] (corresponding to the “predetermined voltage” in the present invention). The collector of the switching element _{Q 2} are connected to the VCC terminal of the control circuit 2 via the diode _{D 3.} The operation of the voltage raising circuit 4 will be described in detail later.

Here, the diode D ₃ is the reverse voltage is prevented from being applied to the switching element Q ₂ during startup. The resistor R ₄ is a resistor for preventing malfunction due to the leakage current of the switching element Q ₂ , and the resistor R ₅ is a resistor for limiting the base current of the switching element Q ₂ , and the resistors R ₆ , R ₇ is a resistor for setting a constant current drooping characteristic.

In this embodiment, the secondary winding T ₂ of the number of turns: number of turns of the first auxiliary winding T _S1: number of turns of the second auxiliary winding T _S2 = 6: 9: is set to 7. As above, the secondary-side output voltage _{V out} is because 10 [V] at the time of steady generated in the secondary winding _{T 2,} the driving voltage VCC generated by the first auxiliary winding _{T S1} is 15 [V] DC voltage generated by the second auxiliary winding _{T S2} is 11.7 [V].

  The operation lower limit voltage of the control circuit 2 (STR-A6251) is 10 [V], and the overvoltage protection set voltage is 32 [V]. Further, as described above, the drive voltage VCC is 15 [V]. That is, in the present embodiment, the drive voltage VCC is set so that the difference voltage between the drive voltage VCC and the operation lower limit voltage of the control circuit 2 is smaller than the difference voltage between the overvoltage protection setting voltage and the drive voltage VCC. Yes.

In the switching power supply device 1 shown in FIG. 1, when the secondary side output is in an overcurrent state, even overcurrent flowing through the switching element to Q ₁ control circuit 2. This current flows through the resistor _{R 1} through terminal OCP, the voltage at the terminal OCP is increased. The current monitoring unit 22 determines that an overcurrent state occurs when the voltage at the terminal OCP exceeds a predetermined value. Then, switching control unit 21 changes the duty ratio of the switching element Q _1, to execute the overcurrent protection operation exhibiting a constant current drooping characteristic. As a result, the secondary output voltage _Vout decreases.

When the secondary output voltage _Vout decreases, the drive voltage VCC of the control circuit 2 also decreases. When a driving voltage VCC reaches the minimum operating voltage of the control circuit 2 (= 10 [V]) , the switching operation of the switching element Q ₁ is stopped, an overcurrent condition is removed. In the present embodiment, the steady-state drive voltage VCC (= 15 [V]) is set to a value close to the operation lower limit voltage of the control circuit 2, so that the constant current drooping region (secondary-side output voltage V in steady state) is set. _The voltage range sandwiched between “ _out” and the operation lower limit voltage of the control circuit 2) can be reduced, and the overcurrent state can be eliminated early (see FIG. 5A).

Next, the operation in the overvoltage state will be described with reference to FIG. Reference numeral _{V CS2} in Figure 3 is the voltage across the smoothing capacitor _{C S2.} It is generated at both ends of the smoothing capacitor C _S2 by rectifying and smoothing the sum voltage of the AC voltage induced in the first auxiliary winding T _S1 and the AC voltage induced in the second auxiliary winding T _S2. Voltage (= 11.7 + 15 = 26.7 [V], corresponding to “second DC voltage”) is applied.

At time T ₁ , an abnormality occurs in the photocouplers PC and PC ′, and a feedback signal for keeping the secondary output voltage V _out by the photocouplers PC and PC ′ constant is not transmitted from the secondary side to the primary side. Then, the secondary side output voltage _Vout starts to rise, and the drive voltage VCC and the voltage _VCS2 also rise in proportion to this.
When the voltage _{V CS2} at time _{T 2} reaches 33 [V] (the breakdown voltage of the constant voltage diode ZD), a constant voltage diode ZD is switched from the nonconductive state to the conductive state. Thus, since through the resistor R ₅ could flow to the base current of the switching element Q _2, the switching element Q ₂ is turned ON. The current through the switching element Q ₂ and the diode D ₃ charges the smoothing capacitor C _S2, rapidly pulling drive voltage VCC (first DC voltage) towards the second DC voltage.
When a driving voltage VCC reaches a overvoltage protection setting voltage of the control circuit 2 32 [V] at time T _3, determines that the voltage monitoring unit 23 is over-voltage condition, over-voltage protection operation is performed. That is, the switching operation of the switching element Q ₁ is stopped, an overvoltage state of the secondary side output is eliminated.

Incidentally, the secondary winding _{T 2} of the number of turns: the first turns = 6 turns + second auxiliary winding _{T S2} of the auxiliary winding _{T S1:} 9 + 7 = 3: since it is 8, the overvoltage protection operation starts The secondary-side output voltage V _out at this time is calculated as 12.4 (= 33 ÷ 8 × 3) [V]. However, since there is a delay time from when the overvoltage protection operation is performed to when the switching operation is stopped, the secondary output voltage _Vout actually rises to 14 [V]. On the other hand, the voltage V _CS2 rises to 37 (= 14 ÷ 3 × 8) [V] because the secondary output voltage V _out rises to 14 [V].

  As a result, according to the switching power supply device 1 shown in FIG. 1, the voltage boosting circuit 4 rapidly raises the first DC voltage, so that the overvoltage protection function can be operated early and the overvoltage state can be eliminated.

On the other hand, in the voltage pull-up circuit 4 conventional switching power supply device not equipped with a 1 ', a secondary winding T ₂ of the number of turns: turns = 6 of the auxiliary winding T _S: set to 9, the secondary of the steady state The drive voltage VCC when the side output voltage _Vout is 10 [V] is 15 (= 10 ÷ 6 × 9) [V]. Since the overvoltage protection set voltage is 32 [V], the overvoltage protection operation is not executed until the drive voltage VCC rises from 15 [V] to 32 [V] (2.13 times the voltage ratio).
Therefore, in the conventional switching power supply device 1 ′ shown in FIG. 4, the secondary side output voltage V _out rises to about 21 [V], which is 2.13 times that at the normal time (10 [V]), and is overvoltage only. A protection operation is performed.

On the other hand, in the switching power supply device 1 according to the present embodiment, as described above, the secondary side output voltage _Vout rises only to about 14 [V]. That is, in the switching power supply device according to the present invention, the overvoltage applied to the load 3 can be reduced by eliminating the overvoltage state at an early stage.

The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the spirit of the present invention.
For example, although IC “STR-A6251” manufactured by Sanken Electric Co., Ltd. is used as the control circuit 2, the present invention is not limited to this, and STR-Y6400 series manufactured by the same company or equivalents manufactured by other companies can also be used. That is, as the control circuit of the present invention, (i) the first switching element and the switching control unit are accommodated in one packaged IC, and (ii) the overvoltage protection operation is the first DC voltage (VCC). Depending on the size, an IC that does not have a function to stop latching other than the overvoltage protection operation according to the first DC voltage is preferably used.

1 is a circuit diagram showing an embodiment of a switching power supply device according to the present invention. It is a block diagram of the control circuit used for a switching power supply device. It is a graph which shows operation | movement of the switching power supply device which concerns on this invention in an overvoltage state. It is a circuit diagram which shows the conventional switching power supply device. It is a figure explaining the relationship between the drive voltage VCC of constant time, the operation | movement minimum voltage of a control circuit, and an overvoltage protection setting voltage, Comprising: (A) is when the drive voltage VCC is set to the value close | similar to an operation | movement minimum voltage, (B) is a case where the drive voltage VCC is set to a value close to the overvoltage protection set voltage.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Switching power supply device 2 Control circuit 21 Switching control part 22 Current monitoring part 23 Voltage monitoring part 3 Load 4 Voltage raising circuit Q ₁ 1st switching element Q ₂ 2nd switching element T Transformer T ₁ Primary winding T ₂ 2 Secondary winding T _S1 First auxiliary winding T _S2 Second auxiliary winding VCC Drive voltage (first DC voltage)
Voltage generated by rectifying and smoothing V _CS2 sum voltage (second DC voltage)
V _out Secondary output voltage ZD Constant voltage diode (detection element)

Claims (5)

A control circuit having a first switching element connected to a primary winding of the transformer and a switching control unit that controls the first switching element; While rectifying and smoothing the AC voltage induced in the secondary winding to generate the secondary output voltage, the secondary output voltage is fed back to the switching control unit to keep the secondary output voltage constant. And generating a first DC voltage for driving the control circuit by rectifying and smoothing an AC voltage induced in a first auxiliary winding provided on the primary side of the transformer, When the DC voltage reaches an overvoltage protection setting voltage, the switching power supply device reduces the secondary output voltage,
The first DC voltage is set such that a difference voltage between the first DC voltage and an operation lower limit voltage of the control circuit is smaller than a difference voltage between the overvoltage protection setting voltage and the first DC voltage. Set,
A second auxiliary winding is further provided on the primary side of the transformer,
Rectifying and smoothing the sum voltage of the AC voltage induced in the first auxiliary winding and the AC voltage induced in the second auxiliary winding to generate a second DC voltage;
When it is detected by the detection element that detects the amount of the second DC voltage that the second DC voltage exceeds a predetermined voltage, the first DC voltage is directed toward the second DC voltage. A switching power supply device that is forcibly pulled up to reach the overvoltage protection set voltage. The switching power supply device according to claim 1, wherein the detection element is a constant voltage diode. A second switching element having an emitter connected to the high potential side of the second auxiliary winding and a collector connected to the high potential side of the first auxiliary winding;
The cathode side of the constant voltage diode and the base of the second switching element are connected, the anode side of the constant voltage diode and the low potential side of the first auxiliary winding are connected, and the second DC When the voltage rises and the constant voltage diode is switched from the non-conductive state to the conductive state, the second switching element is turned on, and the first DC voltage is supplied to the overvoltage via the second switching element. The switching power supply device according to claim 2, wherein the switching power supply device is pulled up to a protection set voltage. 4. The switching power supply device according to claim 1, wherein the predetermined voltage is set to be equal to or higher than the overvoltage protection set voltage. 5. The switching power supply device according to claim 1, wherein the control circuit is an IC packaged in one package.

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