JPH11332237A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress increase in the number of elements by employing a plus voltage generated from an auxiliary power supply circuit as the operating power supply for the output element of an insulating element when a switching transistor is turned off. SOLUTION: When a switching transistor Q1 is turned off, an auxiliary power supply circuit 2 filters a voltage induced in the primary coil L1 wound around a high frequency transformer 6 to produce a plus voltage, which is employed as the operating power supply for the output element of an insulating element, i.e., a phototransistor Q3. Since the plus voltage being generated from the auxiliary power supply circuit 2 satisfies a specified level so long as the secondary DC output voltage does not drop below a set level, such a voltage level as can supply a current to the base of a control transistor Q2 can be sustained. Consequently, increase in the number of elements can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply for providing a positive feedback to a switching transistor using a feedback coil, and more particularly to a switching power supply obtained by rectifying and smoothing the output of a coil wound around a high-frequency transformer. The present invention relates to a switching power supply that uses the obtained positive voltage as an operation power supply for an output element of an insulating element that feeds back a voltage error of an output.

[0002]

2. Description of the Related Art One of the prior arts for simplifying an RCC type switching power supply, which can constitute a switching power supply with a simple circuit, is disclosed in Japanese Utility Model Application Laid-Open Publication No. 63-6 / 1988.
There is a technique proposed as No. 92. In this technique, paying attention to the fact that the forward voltage of the light emitting diode of the photocoupler becomes constant, the light emitting diode is connected between the emitter of the transistor for performing error detection and the ground level. Therefore, the light emitting diode performs an operation of transmitting an error of the output voltage to the phototransistor as a light emitting element of the photocoupler, and also operates as a reference voltage source.
As a result, a dedicated element for generating the reference voltage is not required.

Even when the above technique is used, when the load on the primary side is reduced in a state where the load is extremely light, the voltage of the secondary side output is temporarily reduced immediately before the switching operation is stopped. A situation of rising occurs. The reason for this temporary voltage increase will be described with reference to FIGS. In addition, P1 shown in FIG. 2 indicates a voltage change of the primary side input, and P3 indicates a voltage change of the secondary side output. P4 indicates the envelope of the signal waveform at the terminal of the feedback coil L3. In the following description, the voltage drop of the resistor R5 is ignored, and the resistor R21
Is not connected.

When the switching transistor Q1 in FIG. 4 is turned on, the voltage generated in the feedback coil L3 is applied to the collector of the phototransistor Q3 via the resistor R5. At this time, the voltage generated in the feedback coil L3 is proportional to the voltage of the primary side input (see P1 and P4). On the other hand, the emitter of the phototransistor Q3 is guided to the base of the control transistor Q2 via the diode D4. Therefore, the collector voltage of the phototransistor Q3 is changed to the base-emitter voltage (approximately 0.6 V) of the control transistor Q2, the forward voltage (approximately 0.6V) of the diode D4, and the collector-emitter voltage (approximately) of the phototransistor Q3. Only when the sum is higher than 0.3 V), a base current can flow through the control transistor Q2. When the collector voltage of the phototransistor Q3 is lower than the above-mentioned added value, current cannot flow through the base of the control transistor Q2.
Therefore, when the voltage generated in the feedback coil L3 becomes lower than the above-mentioned added value (the voltage V1 at P4) (at time T
1) Even when the phototransistor Q3 is turned on, no current flows through the base of the control transistor Q2. Therefore, after the time T1, the voltage of the secondary output is not stabilized. As a result, as shown by a broken line 51, the voltage of the secondary output temporarily increases, causing a failure in a circuit serving as a load.

[0005] The resistor R21 is an element provided for preventing occurrence of the above-mentioned problem. That is,
When the resistor R21 is connected, the phototransistor Q
To the collector of No. 3, a voltage via a resistor R21 is applied in addition to the voltage generated in the feedback coil L3. Therefore, even when the voltage of the feedback coil L3 becomes lower than the voltage V1, the voltage of the collector of the phototransistor Q3 becomes
It is maintained at a voltage higher than voltage V1. Therefore, at time T1
Thereafter, the base of the control transistor Q2
A current corresponding to the output of the phototransistor Q3 flows,
The voltage of the secondary output is stabilized. As a result, the voltage of the primary side input becomes V2 or less, and the voltage of the feedback coil L3 becomes V2.
Even when the value becomes 1 or less, the voltage of the secondary output is maintained at the set voltage V3 as indicated by the solid line of P3.

[0006]

However, the resistance R2
In the case where 1 is connected, the following problem has occurred.
That is, when the voltage rise indicated by the broken line 51 occurs (time T1), the primary-side input voltage V2 is about 20V. On the other hand, the minimum value (voltage V1) allowed as the collector voltage of the phototransistor Q3 is about 1.5V. Therefore, the value of the resistor R21 cannot be increased. As a result, when the voltage of the primary-side input becomes the voltage during normal operation (approximately 140 V), the value of the current flowing through the resistor R21 increases, and the loss on the primary side increases. The increase in the loss greatly increases the power consumption in the standby mode in which the load is light.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a positive coil obtained by rectifying and smoothing the output of a coil wound around a high-frequency transformer. By supplying the voltage to the output element of the insulating element that feeds back the voltage error of the output, it is possible to prevent an abnormal rise in the secondary output voltage when the voltage of the primary input decreases without causing an increase in loss. And to provide a switching power supply that can perform the switching.

According to a second aspect of the present invention, in addition to the above object, a plus voltage obtained by rectifying and smoothing the output of an auxiliary coil which is a dedicated coil is used as an operating power supply for an output element of an insulating element. Accordingly, an object of the present invention is to provide a switching power supply capable of suppressing an increase in the number of elements.

[0009]

According to a first aspect of the present invention, there is provided a switching power supply for switching a current of a primary coil wound around a high-frequency transformer, and a base current of the switching transistor. A control transistor for controlling the
The present invention is applied to a switching power supply including an insulating element that feeds back a current corresponding to a voltage error of a secondary side DC output to a base of a control transistor, and when the switching transistor is turned off, a coil wound around the high-frequency transformer is used. An auxiliary power supply circuit that generates a positive voltage by extracting and smoothing the generated voltage is provided, and the positive voltage generated by the auxiliary power supply circuit is used as an operation power supply of the output element of the insulating element.

That is, when the switching transistor is turned off, the auxiliary power supply circuit extracts a voltage generated in a coil wound around the high-frequency transformer and smoothes it, thereby generating a positive voltage. On the other hand, when the switching transistor is turned off, the voltage generated in the coil corresponds to the voltage of the secondary DC output. Therefore, the plus voltage generated by the auxiliary power supply circuit is a voltage that satisfies the predetermined value unless the voltage of the secondary-side DC output drops below the set value. For this reason, the voltage supplied to the output element (for example, a phototransistor) of the insulating element is maintained at a voltage that allows a current to flow through the base of the control transistor even when the voltage of the primary input decreases. As a result, even when the voltage of the primary-side input decreases, the voltage of the secondary-side DC output is stabilized at the set value. Further, the plus voltage generated by the auxiliary power supply circuit is constant regardless of the voltage of the primary side input. Therefore, even when the voltage of the primary-side input increases, the power consumption of the positive voltage does not increase.

In a switching power supply according to a second aspect of the present invention, the coil is an auxiliary coil wound separately from a feedback coil for providing positive feedback to the switching transistor.

That is, to extract the voltage generated in the auxiliary coil when the switching transistor is turned off, a diode may be connected to the terminal of the auxiliary coil.
Then, a positive voltage can be generated only by smoothing the rectified output of the diode using a capacitor.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below.
This will be described with reference to the drawings. FIG. 1 is a circuit diagram showing an electrical connection of an embodiment of a switching power supply according to the present invention. Elements having the same functions as those of the prior art shown in FIG. 4 are denoted by the same reference numerals as those shown in FIG. Has been granted.

In FIG. 1, a smoothing capacitor C8 is connected between the plus IN + output from the diode bridge 8 to which the commercial power is led and the minus IN- output from the diode bridge 8. Also,
The switching transistor Q1 is an element for switching a current flowing through the primary coil L1.
Therefore, one terminal of the primary coil L1 has a positive IN
+ Is connected, and the other terminal is connected to the collector of the switching transistor Q1. Further, the emitter of the switching transistor Q1 is connected to the negative IN through a resistor R4 for detecting the emitter current as a voltage.
Connected to-.

A resistor R1 having one terminal connected to the plus IN + is an element for supplying a starting current to the switching transistor Q1. Therefore, the other terminal of the resistor R1 is connected to the base of the switching transistor Q1 via the resistor R2 for limiting the base current.
A small-capacity capacitor C3 is connected between the base and the emitter of the switching transistor Q1 to prevent parasitic oscillation. Further, a noise snubber 5 including a capacitor, a resistor, and a diode is connected in parallel to the primary coil L1.

The control transistor Q2 is an element for controlling the base current of the switching transistor Q1, thereby stabilizing the voltage of the secondary DC output (hereinafter referred to as DC output) 21. Therefore, the collector of the control transistor Q2 is connected to the base of the switching transistor Q1. The emitter of a phototransistor (output element of an insulating element) Q3 for transmitting a voltage error of the DC output 21 is connected to the base of the control transistor Q2 via a diode D4. The base of the control transistor Q2 is connected to minus IN- via a noise absorbing capacitor C2. The capacitor C2 is connected in parallel with a charge discharging resistor R3. Then, in order to limit the current flowing through the switching transistor Q1 when the power is turned on, the voltage detected by the resistor R4 is applied to the base of the control transistor Q2 via the diode D2. The emitter of the control transistor Q2 is connected to minus IN-.

The feedback coil L3, one terminal of which is connected to minus IN-, is a coil for performing self-excited oscillation by giving positive feedback to the switching transistor Q1. Therefore, the other terminal of the feedback coil L3 is connected to the resistor R1 and the resistor R1 via the diode D1.
2 to the connection point. Further, a capacitor C1 is connected in parallel with the diode D1 in order to accelerate the turn-off of the switching transistor Q1. The other terminal of the feedback coil L3 is connected to the collector of the phototransistor Q3 via the resistor R5. A series circuit including a resistor R7 and a capacitor C4 is connected between the collector of the phototransistor Q3 and minus IN-.

The auxiliary coil L4 is a coil wound separately from the feedback coil L3 (one terminal is common to one terminal of the feedback coil L3), and the switching transistor Q1 is turned off. At this time, a positive voltage is generated at the other terminal. For this reason, the anode of the diode D3 for rectifying the output of the auxiliary coil L4 is connected to the other terminal of the auxiliary coil L4. A smoothing capacitor C5 is connected between the cathode of the diode D3 and minus IN-. The diode D
The cathode of No. 3 is connected to the collector of the phototransistor Q3 via a resistor R6 for limiting the current. That is, the block 2 including the diode D3, the capacitor C5, and the resistor R6 is an auxiliary power supply circuit described in the claims, and extracts and smoothes a voltage generated in the auxiliary coil L4 when the switching transistor Q1 is turned off. , Generate a positive voltage. Then, the generated plus voltage is supplied to the collector of the phototransistor Q3.

The anode of a rectifying diode D6 is connected to one terminal of the secondary coil L2 of the high frequency transformer 6. A π-type smoothing circuit composed of two capacitors C6 and C7 and a choke coil L5 is connected to the cathode of the rectifying diode D6. The other terminal of the secondary coil L2 is grounded. The voltage of the DC output 21 sent from the π-type smoothing circuit is 5 V because it is an operating power supply for a microcomputer.

The error detecting circuit 4 is a block including a voltage dividing circuit for dividing the DC output 21, a reference voltage source for generating a reference voltage, a circuit for calculating a difference between the output of the voltage dividing circuit and the reference voltage, and the like. Thus, a voltage error of the DC output 21 is detected. Then, the detected voltage error is fed back to the control transistor Q2 via the photocoupler 3, which is an insulating element, so that the voltage of the DC output 21 is stabilized.

The present embodiment is used as a power source for, for example, a television, a video cassette deck, or a CD player. Therefore, the high-frequency transformer 6
In addition, a secondary coil for direct current output is wound, and a rectifying and smoothing circuit is connected to each of the secondary coils. Omitted.

The operation of the embodiment having the above configuration will be described with reference to FIGS. In addition, P2 of FIG.
The envelope of the signal waveform of the collector of the phototransistor Q3 is shown.

During the period when the switching transistor Q1 is turned on, the following occurs. That is, the voltage generated in the feedback coil L3 corresponds to the plus IN + voltage. Therefore, when the voltage of the plus IN + decreases due to a power failure, the voltage generated in the feedback coil L3 decreases. For this reason, the collector voltage of the phototransistor Q3 also decreases. On the other hand, during the period when the switching transistor Q1 is off, the voltage generated in the feedback coil L3 corresponds to the voltage change of the DC output 21. Therefore, the voltage at the connection point between the feedback coil L3 and the resistor R5 is
This is the same as the voltage change indicated by the envelope of P4. That is, in the plus region, the voltage drops in response to the plus IN + voltage drop, and in the minus region, it becomes substantially constant in accordance with the voltage of the DC output 21.

The voltage generated in the auxiliary coil L4 also shows the same change as the voltage change generated in the feedback coil L3, but the polarity is reversed. Therefore, the diode D3
Is when the switching transistor Q1 is turned off,
Only the voltage generated in the auxiliary coil L4 is extracted. Accordingly, the voltage between the terminals of the capacitor C5 is constant regardless of the plus IN + voltage when the voltage of the DC output 21 is constant. Further, when the voltage of the DC output 21 decreases, the voltage between the terminals of the capacitor C5 decreases later than the voltage decrease of the DC output 21 (indicating a change similar to P3 indicating the voltage change of the DC output 21). .

The two types of voltages are a resistor R5 and a resistor R5.
And is supplied to the collector of the phototransistor Q3. Therefore, the envelope of the signal waveform applied to the collector of the phototransistor Q3 has a shape in which the width of the level change of the envelope indicated by P4 is compressed in accordance with the ratio of each value of the resistors R5 and R6. . And the resistance R
The 0 level is shifted by a voltage determined by the ratio of each value of 5 to the resistor R6 and the voltage between the terminals of the capacitor C5. Therefore, the envelope shown by P2 is obtained.

From the above, before the time T1, the voltage supplied to the collector of the phototransistor Q3 is a voltage at which a current can flow through the base of the control transistor Q2, so that the DC output 21 is stable at 5V. Will be done. Then, even at time T1, the voltage supplied to the collector of the phototransistor Q3 is the voltage V
1 (voltage at which base current does not flow through control transistor Q2 even when phototransistor Q3 is on)
It is maintained at a higher voltage. Therefore, even after the time T1, a current flows through the base of the control transistor Q2, so that the voltage of the DC output 21 is maintained at 5 V, and an abnormal voltage rise does not occur. Then, at the time T2, the voltage of the plus IN + becomes a voltage at which the voltage of the DC output 21 cannot be maintained at 5 V. Therefore, after the time T2, the voltage of the DC output 21 starts to gradually decrease. . Then, at time T3, the switching operation stops.

The plus voltage generated by the auxiliary power supply circuit 2 is constant regardless of the plus IN + voltage. For this reason, even in the normal standby mode in which the plus IN + voltage is approximately 140 V, the plus voltage output from the auxiliary power supply circuit 2 is maintained at a voltage similar to the voltage at the time T1. Therefore, an increase in power consumption (an increase in loss) due to the resistor R6 is prevented.

FIG. 4 shows a coil for an auxiliary power supply circuit,
FIG. 2 is a circuit diagram showing an electrical connection of an embodiment shared with a feedback coil L3, showing an electrical connection of a portion different from the embodiment shown in FIG. 1 in connection.

The terminal 32 of the feedback coil L3 (the terminal which becomes negative when the switching transistor Q1 is turned on) is connected to the negative IN- in the embodiment shown in FIG. Connected to cathode. The anode of the diode D9 is connected to minus IN-. Further, the terminal 31 of the feedback coil L3 (the terminal which becomes positive when the switching transistor Q1 is turned on) is connected to the anode of the diode D1, as in the embodiment shown in FIG.

The anode of the diode D3 is
Is connected to the auxiliary coil L4 in the embodiment shown in FIG. 5, but is connected to the terminal 32 of the feedback coil L3 in this embodiment. Further, the cathode of a diode D8 is connected to the terminal 31 of the feedback coil L3.
The anode 8 is connected to minus IN-.

That is, the present embodiment has a configuration in which the auxiliary coil L4 is omitted from the embodiment shown in FIG. 1 and two diodes D8 and D9 are added. In addition,
During the period in which the energy stored in the high-frequency transformer 6 is released, the switching transistor Q1 can be maintained in the off state, so that the capacitance of the capacitor C1 is smaller than that of the embodiment shown in FIG. Changed to a large value.

The operation of the above configuration will be described. When the switching transistor Q1 is turned on, the terminal 31 becomes positive and the terminal 32 becomes negative. Accordingly, the positive voltage generated at the terminal 31 of the feedback coil L3 flows through the diode D1 to the base of the switching transistor Q1. The current to be fed back to the feedback coil L3 flows to the terminal 32 via the diode D9.

On the other hand, when the switching transistor Q1 is turned off, the terminal 31 becomes negative and the terminal 32 becomes positive. Therefore, the positive voltage generated at the terminal 32 flows through the diode D3. Then, the current to be fed back to the feedback coil L3 passes through the diode D8,
It will flow to the terminal 31. Therefore, the diode D
8, an auxiliary power circuit 33 including a capacitor C5, a resistor R6, and a diode D3.
Turns off, the voltage generated in the feedback coil L3 is taken out, and the circuit operates as a circuit for generating a positive voltage.

[0034]

According to the first aspect of the present invention, there is provided a switching power supply comprising: a switching transistor for switching a current of a primary coil; a control transistor for controlling a base current of the switching transistor; By applying to a switching power supply with an insulation element that feeds back a current corresponding to the output voltage error, when the switching transistor is turned off, the voltage generated in the coil wound around the high-frequency transformer is taken out and smoothed. And an auxiliary power supply circuit for generating a positive voltage, wherein the positive voltage generated by the auxiliary power supply circuit is used as an operating power supply for the output element of the insulating element.
That is, when the switching transistor is turned off, the auxiliary power supply circuit generates a positive voltage from the voltage generated in the coil. Therefore, the plus voltage generated by the auxiliary power supply circuit is a voltage that satisfies the predetermined value unless the voltage of the secondary-side DC output drops below the set value. For this reason, the voltage supplied to the output element of the insulating element is maintained at a voltage that allows a current to flow through the base of the control transistor even when the voltage of the primary input decreases. Therefore, even when the voltage of the primary side input decreases, the voltage of the secondary side DC output is stabilized at the set value. Further, since the plus voltage generated by the auxiliary power supply circuit is constant irrespective of the voltage of the primary input, even when the voltage of the primary input becomes high, the power consumption of the plus voltage does not increase. Therefore, it is possible to prevent an abnormal increase in the secondary output voltage when the voltage of the primary input decreases, without increasing the loss.

Further, in the switching power supply according to the present invention, the coil is an auxiliary coil wound separately from a feedback coil for providing a positive feedback to the switching transistor. That is, when the switching transistor is turned off, to extract the voltage generated in the auxiliary coil, a diode may be connected to the terminal of the auxiliary coil. Then, a positive voltage can be generated only by smoothing the rectified output of the diode using a capacitor. For this reason, it is possible to suppress an increase in the number of elements.

[Brief description of the drawings]

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

FIG. 2 is a timing chart showing waveforms of main signals according to the embodiment.

FIG. 3 is a circuit diagram showing an electrical connection of an embodiment in which a coil for an auxiliary power supply circuit is shared with a feedback coil.

FIG. 4 is a circuit diagram showing a conventional electrical connection.

[Explanation of symbols]

 Reference Signs List 2 auxiliary power circuit 3 photocoupler (insulating element) 6 high frequency transformer 21 secondary side DC output 33 auxiliary power circuit L1 primary coil L3 feedback coil L4 auxiliary coil Q1 switching transistor Q2 control transistor Q3 phototransistor (output element of insulating element)

Claims (2)

[Claims] 1. A switching transistor for switching a current of a primary coil wound around a high-frequency transformer, a control transistor for controlling a base current of the switching transistor, and a voltage error of a secondary DC output at a base of the control transistor. In a switching power supply including an insulating element that feeds back a corresponding current, when a switching transistor is turned off, a voltage generated in a coil wound around the high-frequency transformer is extracted and smoothed to generate a positive voltage. A switching power supply, comprising: an auxiliary power supply circuit, wherein a positive voltage generated by the auxiliary power supply circuit is used as an operation power supply of an output element of the insulating element. 2. The switching power supply according to claim 1, wherein the coil is an auxiliary coil wound separately from a feedback coil that provides positive feedback to the switching transistor.