JP3268986B2 - Power circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a power supply circuit, and more particularly, to an improvement of a power supply circuit having an overvoltage protection circuit.

[0002]

2. Description of the Related Art FIG.
This will be described with reference to FIG. As shown in FIG. 2, this circuit is a power supply circuit having a bridge circuit 1, a transformer 2, and an overvoltage protection circuit 3, and is a circuit that generates a DC output voltage Vout from an AC input voltage Vin. This power supply circuit is provided with an overvoltage protection circuit 3 for protecting the circuit when an overvoltage occurs for some reason.

According to the above circuit, when the power is first turned on, the AC input voltage Vin is smoothed by the bridge circuit 1 and output to the primary coil Np1 of the transformer 2. The electromotive force induced in the primary coil Np1 is transmitted to the secondary coil Np2, and the diode D1 and the capacitor C1
After being rectified by the rectifier, it is supplied to a load (not shown) as an output voltage Vout.

In this circuit, as shown in FIG. 2, an auxiliary coil Np3 is wound coaxially with the primary coil Np1.
An electromotive force that varies in proportion to the voltage generated in the secondary coil Np2 is also induced in the auxiliary coil Np3, and this voltage Vcc1 is output to the overvoltage protection circuit 3. This voltage Vcc
In accordance with 1, the overvoltage protection circuit 3 determines the state of the output voltage. That is, the output voltage Vout is within the normal operation range, the voltage Vcc1 is not so high, and the Zener voltage 2 of the Zener diode ZD1 provided in the overvoltage protection circuit 3
When the voltage does not exceed 7 V, this is OFF, and the switching transistor Q provided in the overvoltage protection circuit 3 is turned off.
2 and Q3 are also turned off, and the overvoltage protection circuit 3 has no effect on the entire power supply circuit.

[0005] However, if the output voltage Vout becomes abnormally high for some reason, the voltage Vcc1 generated by the auxiliary coil Np3 also becomes high. As a result, when the Zener voltage of the Zener diode ZD1 exceeds 27V, this becomes O
N, and the switching transistor Q in FIG.
Since the base potential of 2 rises, Q2 turns on. Then, the base potential of the switching transistor Q3 drops, so that the transistor Q3 is also turned on, and accordingly, the switching transistor Q4 is also turned on.

When the switching transistor Q4 is turned on, a current flows from the reference voltage circuit 4 connected to the input terminal of Vcc1 to the current sense comparator 5 via the switching transistor Q4, and flows to the ground potential. Since the switching transistors Q2 and Q3 form a thyristor in the circuit configuration as shown in FIG. 2, once turned on, the state is maintained. As a result, Vcc1 keeps decreasing and eventually reaches the ground voltage.

In this manner, the overvoltage protection circuit 3
1 is forcibly lowered to the ground potential, a signal from the control IC 3 is not supplied to the control electrode of the drive switching element (not shown) connected in series to the primary coil Np1, and the power Since the operation of the entire circuit is stopped, it is possible to prevent the circuit itself or loads (not shown) from being damaged by overvoltage, even if an excessive voltage occurs due to abnormal operation. Had become.

[0008]

However, as the switching power supply circuit becomes more sophisticated, a function to detect the low input voltage protection / overvoltage protection circuit from the auxiliary voltage of the control IC and to stop the power supply safely is provided. Have been. In addition, the input voltage range tends to be wider, and the output power range tends to be larger. In addition, the output voltage may be varied.

If a switching power supply system adapted to such a use condition is to be constructed while maintaining the protection function, the following problems have occurred. When the input voltage is minimum and the output power is maximum, the auxiliary voltage rises to reach the overvoltage protection detection level (the condition where Vcc1 rises the most). When the input voltage is at the maximum and the output power is at the minimum, the auxiliary voltage falls and becomes a low input voltage detection level (the condition that Vcc1 falls the lowest).

Conventionally, a resistor has been inserted in series in the auxiliary voltage winding as shown in FIG. 2 to cope with the problem. More specifically, in the overvoltage protection circuit, in the above power supply circuit, the output voltage Vout is supplied to a load (not shown) and the output current Io is supplied to the load. The phenomenon that Vcc1 in FIG. 2 fluctuates in a logarithmic function has occurred due to the magnitude of.

FIG. 3 shows the state of the fluctuation. The graph of FIG. 3 is a graph for explaining the relationship between the output current Io and the voltage Vcc1. That is, when the resistance value of the resistor RD is changed, it can be made substantially constant depending on the value of the detection current Io. For example, in FIG.
When the resistance RD is 15Ω, the voltage Vcc1 fluctuates in the range of 22V to 25V, and when the resistance RD is 30Ω, the voltage Vcc1 is 20V to 23V.
Therefore, the above problem has been dealt with by selecting the resistor RD in accordance with the standard of the overvoltage protection circuit to be used. However, it was practically impossible to prepare two types of resistors and select a resistance value according to the load current.

If the voltage Vcc1 fluctuates due to the fluctuation of the output current Io, the following problem occurs. First, since the overvoltage protection circuit has an operating voltage range, if a voltage exceeding this range is supplied, the overvoltage protection circuit operates even if the voltage is not an overvoltage, and a malfunction occurs.

Second, when the damping resistance is large and the load power is small, the voltage becomes lower than the operating range voltage, and the low input voltage protection circuit operates to cause a malfunction. Third,
The overvoltage protection circuit 3 determines whether or not the output voltage is an overvoltage output voltage due to a malfunction such as the protection being activated even though the voltage Vcc1 fluctuates due to a voltage fluctuation such as noise. Can not exactly do. It is considered that such a situation may occur, and sufficient overvoltage protection measures may not be taken. Therefore, it is desired that the voltage Vcc1 be maintained at a constant voltage regardless of the output current Io. But
It was very difficult.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks. First, a current inflow portion is connected to one end of the damping resistor, and a current outflow portion is connected to the other end. If the overvoltage is lower than the predetermined voltage, the semiconductor device is selected, and if the voltage is lower than a predetermined voltage, the semiconductor device is driven in a mode for selecting the semiconductor device. Is turned off) and the semiconductor element is selected and turned on (no current flows through the damping resistor at all)
During the mode, the semiconductor element is operated linearly, and Vcc can be stabilized without largely fluctuating.

Second, the semiconductor element is a first NPN transistor, the collector of which is connected so as to extract the base current of the base of the first NPN transistor.
NPN transistor, a Zener diode connected between the emitter of the second transistor and GND, and a connection between the intersection of the attenuation resistor and the first NPN transistor and the base of the second NPN transistor. A first resistor, and a second resistor connected between the base of the second NPN transistor and the GND, and a voltage generated at R13 serving as a bleeder resistor is:
It is constantly compared with the voltage of the second transistor VBE + VZD. This signal changes the base potential of Q11, so that the Ic current of Q11 changes and the bleeder resistance R13
Is controlled so as to be the same potential as VBE + VZD of the second transistor. Therefore, the attenuation rate of the detection voltage due to the attenuation resistance can be selectively reduced to Vcc1.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A switching power supply circuit according to an embodiment of the present invention will be described below with reference to FIG. This circuit, as shown in FIG.
1, transformer 12, control IC 13, selection switching circuit 1
4, a circuit having a damping resistor RD and an auxiliary coil Np13, which is a power supply circuit for which overvoltage protection is taken.

The configuration of each part of this circuit will be described below. The bridge circuit 11 is a circuit that rectifies the AC input voltage Vin and supplies the rectified AC input voltage Vin to the primary coil Np11 of the transformer 12. The transformer 12 includes primary coils Np11 and Np11,
It has a secondary coil Np12 and transmits the rectified input voltage Vin to the secondary side as Vout.

The control IC 13 is an IC for controlling the operation of a circuit having a built-in overvoltage protection circuit. The configuration and operation of the built-in overvoltage protection circuit are the same as those of the circuit shown in FIG. It is almost the same. The auxiliary coil Np13 is a coil wound coaxially with the primary coil Np11, and generates a detection voltage Vk that changes in proportion to the voltage induced in the secondary coil Np12.

One end of the damping resistor RD has an auxiliary coil N.
This element is connected to one end of p13 via a diode Dst and attenuates the detection voltage Vk to generate an attenuation detection voltage Vg. Here, Dst regulates the direction of the flow of the current supplied from the coil Np13. The selection switching circuit 14 surrounded by a dotted line is a circuit including a transistor Q12 and the like connected in parallel with the resistor RD, and detects the detection voltage Vk and the attenuation detection voltage Vg according to the magnitude of the detection voltage Vk.
Is a circuit for selecting and supplying the selected signal to the control IC 13.

This selection switching circuit has an NPN structure in which an emitter and a collector are connected to both ends of an attenuation resistor RD, a first resistor R11 is connected between the collector and the base, and a diode D13 is connected between the base and the emitter. A first transistor Q11, a collector connected to the base of the first transistor Q11 via a second resistor R12, and an emitter connected to one end of the auxiliary coil on the side not connected to the attenuation resistor RD. A second transistor Q12 of NPN type connected via ZD, and a first transistor connected between one end of the attenuation resistor and the emitter of the first transistor Q11 and the base of the second transistor Q12. Of the second transistor Q12
And a third resistor R15 connected between the emitter of the second transistor Q12 and the diode Dst.

Between the first resistor R14 and the control IC 13 (corresponding to the overvoltage protection circuit 3 in FIG. 2), there is provided a diode D12 having a cathode on the control IC 13 side to regulate the current flow. And both ends of the diode D12 and G
ND and a capacitor Cst connected between the ND. Hereinafter, the operation of the above circuit will be described.

According to the above circuit, when the power is turned on, first, the AC input voltage Vin is smoothed by the bridge circuit 11 and output to the primary coil Np11 of the transformer 12. The electromotive force induced in the primary coil Np11 is 2
The output voltage Vout is transmitted to the secondary coil Np12 and rectified by the diode D11 and the capacitor C11.
Is supplied to a load (not shown).

In this circuit, as shown in FIG. 1, an auxiliary coil Np13 is wound coaxially with the primary coil Np11, and the secondary coil Np1 is also wound on the auxiliary coil Np13.
2, a detection voltage Vk, which is an electromotive force that varies in proportion to the magnitude of the electromotive force, is generated. The circuit described above detects the detection voltage V
Control I with overvoltage protection circuit mounted according to the magnitude of k
C13 operates and operates in the same manner as in the prior art in that the circuit operation is forcibly stopped when an abnormal voltage is generated. However, in the present invention, unlike the conventional in that a selection switching circuit 14 is added, Perform a characteristic operation. The operation will be described below while touching that point.

First, it is assumed that the ON voltage between the collector and the emitter of the transistor Q11 in FIG. 1 is 0V. That is,
This is a case where the output voltage Vout increases and the detection voltage Vk increases accordingly. If this is (I), (I). When Vcc1 + i1 × RD + 2Vf <Vk The voltage of the bleeder resistor R13 is V2 of the second transistor.
BE + VZD voltage or more, the second transistor
Q12 is completely on. When Q12 is completely turned on, the base potential of the first transistor Q11 is reduced, so that Q11 is also completely turned off. Therefore, the voltage from the attenuation resistor RD is supplied. D13 is a reverse bias voltage protection diode at the base and emitter of Q11.

Therefore, the current supplied from the auxiliary coil Np13 flows through the resistor RD as shown by the solid line arrow in FIG. 1, and the voltage Vcc1 supplied to the control IC 13 is the attenuation detection attenuated by the resistor RD. The voltage Vg is supplied. On the other hand, if the following case where the detection voltage Vk becomes small is defined as (III), (III). When Vcc1 + 2Vf> Vk The voltage of the bleeder resistor R13 is equal to the V of the second transistor.
Be + VZD or less, and the second transistor Q12 is completely off. When Q12 is completely turned off, the base potential of the first transistor Q11 rises, so that Q11 turns on.

Therefore, the current supplied from the auxiliary coil Np13 flows through the transistor Q11 without passing through the resistor RD as shown by the dotted line in FIG. 1, and as the voltage Vcc1 supplied to the control IC 13, , The detection voltage Vk generated by the auxiliary coil Np13 is supplied as it is. The following mode (II) is an intermediate mode between the above (I) and (III). (II). Vcc1 + 2Vf ≦ Vk ≦ Vcc1 + i1 + 2
In the case of Vf In this region, the voltage of the bleeder resistor R13 is constantly compared with the voltage VBE + VZD of the second transistor. Since this signal changes the base potential of Q11, Ic of Q11
The current changes, and the potential of the bleeder resistor R13 is controlled so as to be the same as VBE + VZD of the second transistor.

To summarize the above operation, when the detection voltage Vk is high, the attenuation resistor RD is selected, and as the voltage Vcc1, the attenuation detection voltage Vg is selected. When the detection voltage Vk is low, the attenuation resistor RD is not selected. Is only the resistance inherent in the first transistor Q11, and the detection voltage Vk which is hardly attenuated is supplied as it is as the voltage Vcc1. In an intermediate region where both the damping resistor and the transistor Q11 are selected, a value between the damping voltage Vg and the voltage Vk as it is is selected.

For example, in FIG.
When the resistance RD is 15Ω, the voltage Vcc1 fluctuates in a range of about 22V to 25V. When the resistance RD is 30Ω, the voltage Vcc1 is about 20V to 2V.
Since the voltage fluctuates in the range of 3 V, the fluctuation width is about 3 V regardless of which resistor is used.
In the range of A to 2A, 15Ω is selected as the resistance RD.
If 30Ω is selected as RD in the range of A to 6A, the voltage Vcc1 falls within the fluctuation range of about 22V to about 23V, so that the voltage Vcc1 is smaller than when a single resistor is used.
The variation width of cc1 can be reduced.

As can be seen from FIG. 3 and the above discussion, when the output current Io is small, RD is reduced and the output current Io
Is larger, the voltage Vcc is increased by increasing RD.
It can be seen that the fluctuation range of 1 can be reduced.
Further, it is already known that there is a correlation that when the output current Io increases, the detection voltage Vk also increases.
From this, it can be seen that the fluctuation range of the voltage Vcc1 can be reduced by decreasing RD when the detection voltage Vk is small and increasing RD when the detection voltage Vk is large.

In the circuit according to this embodiment, the auxiliary coil N
The resistance RD connected to the output of p13 is switched according to the increase or decrease of the detection voltage Vk, that is, the increase or decrease of the output current Io. When the detection voltage Vk is small as shown in FIG.
RD is reduced by bypassing the current through the transistor Q11. When the detection voltage Vk is large, the current flows through the attenuation resistor RD to attenuate the resistance, and the resistance is selected by the selection switching circuit 14 according to the magnitude of the detection voltage Vk. Since the switching operation is performed, the fluctuation range of the voltage Vcc1 caused by the fluctuation of the output current Io can be reduced.

Therefore, at this time, it is possible to prevent a situation in which it is not possible to accurately determine whether or not the control IC 13 is overvoltage, or to supply a voltage necessary for driving the control IC 13. become. Therefore, the circuit operation can be stabilized. Next, FIG. 4 will be described. The vertical axis represents Vcc1 and the horizontal axis represents Vk. Further, a graph in which 270Ω is virtually set as the load of the control IC 13, and a graph in which 0Ω is selected as RD in FIG. 2 and a graph in which 220Ω is selected as RD are shown in the bottom.

Here, as an overvoltage, Vcc = 20 volts,
A range A of 10 volts is set as the low input voltage, and when the voltage is 20 volts or more or 10 volts or less, the protection operation of the control IC operates. Here, a range B of 13 volts to 16 volts is set as the normal operation range. First, in the conventional circuit of FIG. 2, the range B (RD = 0
Ω), Vk ranges from 13 volts to 16 volts C, and in range B (RD = 220Ω), Vk is 2
Take a range of D from 4.4 volts to 30 volts.

Returning to FIG. 1, if the transistor Q11 is turned on and the on-resistance is set to 0Ω and the transistor Q11 is turned off and RD = 220Ω can be selected, the range of VK in which Vcc1 of the range B can be obtained is obtained. Is 16 volts ~
A wide range E of 32 volts can be taken. Therefore,
In the range of I where Vk is 30 volts or more, the damping resistance is selected. From 18 volts to 30.0 volts, the damping resistance and Q
Eleven is the II range where both are selected, and ~ 18 volts or less is the III range where Q11 is selected.

Therefore, in the normal operating range B of Vcc1, 1
A wide Vk can be obtained from 6.4 volts to 32 volts. As described above, the control IC 13 equipped with the overvoltage protection circuit judges the operation by the substantially constant voltage Vcc1 irrespective of the fluctuation of the output current Io. If the output voltage Vout becomes abnormally high for some reason, the control IC 13 The IC operates to keep Vcc1 falling, and eventually reaches the ground potential.

By forcibly lowering Vcc1 to the ground potential in this way, the operation of the entire power supply circuit is practically stopped. Therefore, even if an excessive voltage is generated due to abnormal operation or the like. Accordingly, it is possible to prevent the circuit from being broken. In this embodiment, the selection switching circuit as shown in FIG. 1 is used. However, the present invention is not limited to this, and a circuit that performs an operation of switching the resistance value of the resistor RD according to the magnitude of the detection voltage Vk. The same effect can be obtained regardless of the type.

[0036]

As described above, according to the power supply circuit of the present invention, the selection switching circuit operates in three modes based on the magnitude of the detection voltage. That is, the transistor Q11
Between the selection and non-selection of the detection voltage generated by the attenuation resistor and the transistor Q11, and the detection voltage generated as it is without the current flowing through the attenuation resistor. Since the voltage between the attenuation detection voltage and the detection voltage as it is is supplied to the voltage protection circuit, the voltage level input to the overvoltage protection circuit fluctuates according to the increase or decrease of the output current, It is possible to solve the conventional problem of insufficient protection.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a power supply circuit according to an embodiment of the present invention.

FIG. 2 is a circuit diagram of a power supply circuit according to a conventional example.

FIG. 3 is a graph illustrating a conventional problem.

FIG. 4 is a diagram showing circuit characteristics in FIG.

[Explanation of symbols]

 Reference Signs List 11 bridge circuit 12 transformer 13 control IC (overvoltage protection circuit) 14 selection switching circuit Np13 auxiliary coil Vin input voltage Vout output voltage Io output current RD attenuation resistance Vk detection voltage Vg attenuation detection voltage

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/28 H02H 7/12 H02J 1/00 309

Claims (2)

(57) [Claims] 1. A bridge circuit for rectifying an AC input voltage, comprising: a primary coil and a secondary coil, wherein a voltage induced in the primary coil by the rectified input voltage is converted to a secondary coil. And an auxiliary coil that generates a detection voltage that is wound coaxially with the primary coil and fluctuates in proportion to a voltage induced in the secondary coil. And an over-voltage protection circuit that detects the decay detection voltage in a power supply circuit having an attenuation resistor that generates an decay detection voltage obtained by attenuating the detection voltage. A first semiconductor having an inflow portion connected thereto and a current outflow portion connected to the other end.
An inflow portion is connected to the body element and a control electrode of the first semiconductor element;
A Zener diode is connected between the outlet and GND.
Bleeder resistance is connected between the control electrode and GND.
A second semiconductor element, and when the attenuation detection voltage exceeds the predetermined voltage range,
The overvoltage protection circuit forcibly grounds and stops the attenuation detection voltage attenuated by the attenuation resistor, and when the attenuation detection voltage is within the predetermined voltage range, the semiconductor element is controlled by the control electrode of the semiconductor element. Is not operated, and the voltage between the control electrode and the outflow portion of the second semiconductor element is
A voltage obtained by adding the breakdown voltage of the Zener diode;
By comparing with the voltage of the bleeder resistor,
The magnitude of the detection voltage is determined, and when the detection voltage is large,
Mode for inputting the detection voltage to the overvoltage protection circuit
Operating the semiconductor element when the detection voltage is low.
The detection through the damping resistor and the semiconductor element.
Inputting the attenuated voltage to the overvoltage protection circuit
A power supply circuit characterized in that it is driven in different modes. 2. The first semiconductor device and the second semiconductor device,
The semiconductor element is an NPN transistor, and the first N
A second NPN transistor having a collector connected to extract a base current of the base of the PN transistor;
A Zener diode connected between the emitter of the second NPN transistor and GND, an intersection of the attenuation resistor and the first NPN transistor, and the second NP
2. The power supply circuit according to claim 1, further comprising a resistor connected between the base of the N transistor and a bleeder resistor connected between the base of the second NPN transistor and the GND.