JP2990742B2 - Switching power supply overcurrent protection circuit - Google Patents

Description

The present invention relates to an overcurrent protection circuit for a switching power supply, more specifically, a switching power supply provided with a constant voltage circuit using a saturable reactor having a constant current drooping characteristic. An overcurrent protection circuit is provided.

[Conventional technology]

AC / DC or DC / DC converters are used as DC power supplies for computer-related equipment including communication transmission equipment and OA equipment.However, redundant operation is performed by connecting multiple units in parallel, and power supply reliability is reduced. Is increasing.

FIG. 4 shows a redundant operation by three converters 201, 202, and 203.
208, 209 and reverse current blocking diodes 210, 211, 212 are provided, and overcurrent detectors 204, 205, 206 for opening a circuit breaker that has detected an overcurrent are provided.

[Problems to be solved by the invention]

Now, when the load balance to each converter during the overload operation is lost, the unit to which the maximum load is applied first detects the overload and stops. As a result, the load of the stopped unit is distributed to the remaining units, so that the overload stop spreads one after another, resulting in a total stoppage of the system.

If the load distribution to each unit during overload operation is not equal, the output current of the unit to which overload is applied is large, so that an excessive current also flows to the capacitor for the smoothing circuit, which is a component of this unit. As a result, there is a disadvantage that the life of the capacitor is shortened.

FIG. 3 shows a switching power supply having a constant current drooping characteristic using a saturable reactor, which is provided to eliminate the above-mentioned disadvantage.
A switching power supply including a rectifying circuit including diodes 104 and 105 and a smoothing circuit including a reactor 102 and a capacitor 108 is provided with a circuit having the following constant current drooping characteristics.

That is, an operational amplifier 107 having a non-inverting input terminal connected to a reference voltage circuit via a voltage dividing circuit composed of resistors 112 and 113 and an inverting input terminal connected to a G line.
A base terminal connected to the output terminal of 07 via a resistor 111,
An emitter terminal connected to the base terminal via a resistor 110 and connected to a control power supply, and a P-type transistor 106 connected to the output side of the saturable reactor 101 via a resistor 109 and a diode 103. .

The output of the operational amplifier 107, which compares the voltage loss in the current detection resistor 114 with the reference voltage so as to keep the voltage constant and amplifies the difference voltage, is applied to the saturable alicator via the P-type transistor 106, but the output current is large. When the output current is small, the reset current is increased, and when the output current is small, the reverse is performed to keep the output current constant.

However, the constant current drooping characteristic of this circuit shows a constant current drooping or fold-back characteristic (character-shaped characteristic). However, each characteristic depends on the characteristics of the resistor, the operational amplifier, the transistor, the diode, and the saturable reactor. Since it is difficult to make the characteristics of the units uniform, it has been difficult to appropriately adjust the load balance of each unit during overload operation.

The present invention has been made in order to eliminate the above-described disadvantages of the related art, and shows a constant voltage output characteristic up to a rated current, and shows a linear drooping characteristic when reaching an overload detection point, and It is an object of the present invention to provide an overcurrent protection circuit that can easily adjust a drooping slope by changing a resistance constant.

[Means for solving the problem]

In order to achieve the above-described object, an overcurrent protection circuit for a switching power supply according to the present invention includes a saturable reactor 5,
Rectifier circuit consisting of two diodes 7 and 8, reactor 6
And a smoothing circuit comprising a capacitor 9 and a constant voltage circuit which is constituted by a first P-type transistor which is operated by the AVR voltage and negatively feeds back to the output side of the saturable reactor 5 via the resistor 19 and the backflow preventing diode 1. In the switching power supply provided, the resistors 11 and 12 provided in the DC output circuit
A voltage dividing circuit comprising a non-inverting input terminal for inputting a divided voltage and a non-inverting input terminal connected to a G line via a resistor 13 while receiving a reference voltage via a resistor 13, and a negative feedback by a resistor 15. An operational amplifier 10 having a circuit, a base terminal connected to an output terminal of the operational amplifier 10 via a resistor 20, a control power supply and an emitter terminal via a resistor 17, and a resistor 18 and a diode 2 And a second P-type transistor having a collector terminal connected to the output side of the saturable reactor 5 and a current detection resistor 16 inserted in the G line.

[Action]

Assuming that the DC output voltage is V ₀ and the DC output current is I ₀ , the first
The characteristics of the droop circuit constituted by the operational amplifier circuit in the figure are represented by the following equations.

Here, V _ref is a reference voltage, V _OP is the output voltage of the operational amplifier 10, also, K is equal to:.

K = R ₁ R ₂ + R ₂ R ₅ + R ₁ R ₅ (2) In the equation (1), R ₁ / R ₅ is the reciprocal of the voltage amplification factor of the operational amplifier 10 and is negligibly small.

Since the first term KR ₆ / R ₂ R ₅ and the second term on the right side are constants, equation (1) can be regarded as a linear equation relating to I ₀ .

The inclination of the drooping characteristic determined by -KR ₆ / R ₂ R _5, but it is possible to adjust the inclination by adjusting the R _2.

〔Example〕

Hereinafter, an embodiment according to the present invention will be described with reference to FIG.

FIG. 1 is a block diagram of an overcurrent protection circuit of an insulated switching power supply having a saturable reactor.
On the output side of said saturable reactor 5 of a switching power supply comprising a saturable reactor 5 on the secondary side of 22, a rectifier circuit comprising two diodes 7 and 8, a smoothing circuit comprising a reactor 6 and a capacitor 9, It is provided with a constant voltage circuit and an overload protection circuit according to the present invention.

The first P-type transistor that inputs the AVR voltage of the constant voltage circuit to the base terminal has its collector terminal connected to the output side of the saturable reactor 5 via the resistor 19 and the backflow prevention diode 1.

The overcurrent protection circuit according to the present invention has an inverting input terminal for inputting a voltage of a voltage dividing circuit composed of two resistors 11 and 12 connected to a DC output circuit, a reference voltage being input via a resistor 14, and a G A non-inverting input terminal connected to the line via a resistor 13, an operational amplifier 10 having a negative feedback circuit with a resistor 15, an output terminal of the operational amplifier 10 and a resistor 20
And a base terminal connected to a control power supply and its own emitter terminal via a resistor 17, and a collector terminal connected to the output side of the saturable reactor 5 via a resistor 18 and a backflow prevention diode 2. It is constituted by a second P-type transistor, and a current detecting resistor 16 is inserted between two resistors 12 and 13 connected to the G line.

A saturable reactor in which a winding is wound around a toroidal core (saturable core) having square magnetic hysteresis characteristics is connected to the secondary side of the output transformer of a switching power supply, and a saturable reactor corresponding to the error voltage detected at the output terminal Although the PWM control is performed by the reset current to the inverter, the constant voltage control circuit according to the prior art and the overcurrent protection circuit according to the present invention are both connected via the first and second P-type transistors and the backflow prevention diodes 1 and 2. The reset current is supplied to the saturable reactor 5.

As is clear from FIG. 1, the two reverse current blocking diodes 1 and 2 constitute an OR circuit, so that the one where the anode side first goes to "H" level is turned on, and the others remain off.

Therefore, if the load current is smaller than the rated current,
The first P-type transistor connected to the prior art constant voltage circuit becomes active earlier than the second P-type transistor of the overcurrent protection circuit according to the present invention, and conversely when the load current becomes larger than the overcurrent set value. Adjust the output characteristics of the operational amplifier so that As a result, when the load current is smaller than the rated current, the constant voltage circuit according to the prior art operates preferentially to perform constant voltage control, and when the load current is larger than the overcurrent set value, the overcurrent protection circuit according to the present invention has priority. In operation, the output voltage-output current characteristic of this circuit shows a linear drooping characteristic at an overcurrent set value exceeding the rated current as shown in FIG.

As is apparent from the above equation (1), the slope of the drooping characteristic is determined by −K · R ₆ / R ₂ R _5. Therefore, the slope of the drooping characteristic is adjusted by changing the resistance value R ₂ of the resistor 12. can do.

Therefore, the drooping characteristic of the overload protection circuit of each converter operated in parallel can be adjusted by the resistor 12, and the load distribution can be equalized.
Down can be prevented.

〔The invention's effect〕

As described above, the overcurrent protection circuit of the switching power supply according to the present invention is a switching power supply provided with a constant voltage circuit using a saturable reactor, in which the voltage of the voltage dividing circuit provided in the DC output circuit and the voltage loss of the current detection resistor are reduced. An operational amplifier for comparing and amplifying the corrected reference voltage division,
A P-type transistor whose ON / OFF is controlled by the output of the operational amplifier, and the output of the P-type transistor is negatively fed back to the output side of the saturable reactor via a resistor and a diode.

In other words, up to the rated current, a constant voltage circuit using a saturable reactor according to the prior art exhibits a constant voltage output characteristic preferentially, but when an overload occurs and the overcurrent set value is reached, the overcurrent protection circuit according to the present invention has a priority. It operates in such a manner as to exhibit a linear drooping characteristic, and the inclination of this drooping characteristic is adjustable.

Therefore, even during overload operation, the load balance of each unit that is operating redundantly can be appropriately adjusted, so that a system down due to load concentration can be prevented, and a smoothing capacitor constituting a unit due to overload concentration. There is also an advantage of avoiding shortening of the service life.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an overload protection circuit for a switching power supply according to the present invention, FIG. 2 is a droop characteristic of the overload protection circuit, and FIG. 3 is a block diagram of a control circuit having droop characteristics according to the prior art. FIG. 4 is a block diagram of the converter in redundant operation. 1,2,7,8 ... Diode, 3,4 ... Transistor, 5 ... Saturable reactor, 6 ... Reactor, 9 ... Capacitor, 10 ... Op amp, 11-20 ... Resistance.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H02M 1/00 H02M 3/28 G05F 1/573

Claims (1)

(57) [Claims] 1. A saturable reactor provided on the secondary side of a transformer, a rectifier circuit comprising a rectifier diode and a freewheeling diode, a smoothing circuit comprising a reactor and a capacitor, and an AVR circuit provided in a DC output circuit. Equipped with a base terminal for inputting voltage, an emitter terminal connected to the base terminal via a resistor and inputting the control power supply V _CC , and a negative feedback of the collector current to the output side of the saturable reactor via a reverse current blocking diode and a resistor A switching power supply provided with a constant voltage circuit constituted by a first P-type transistor, wherein a reference voltage is applied to a voltage via an inverting input terminal for inputting a divided voltage from an output voltage detecting voltage dividing circuit connected in parallel to a DC output circuit and a resistor. It has a non-inverting input terminal connected to the G line via a resistor while receiving the voltage _Vref , and a negative feedback circuit using a resistor An operational amplifier, a collector terminal connected to the output side of the saturable reactor via a reverse current blocking diode and a resistor, and a control power supply V _CC and its own emitter connected to the output terminal of the operational amplifier via a resistor and a resistor. A second P-type transistor having a base terminal connected to the terminal; a resistor connected to the G line in the output voltage detecting voltage dividing circuit; and a resistor connected to the G line at the non-inverting input terminal of the operational amplifier. An overcurrent protection circuit for a switching power supply, comprising: a current detection resistor inserted into the switching power supply;