JP2002010633A - Switching power supply source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent occurrence of magnetic saturation of a transformer which has a small dimension without increasing switching loss. SOLUTION: In a composition having a PWM circuit 5 which stabilizes an output voltage by making on-off operation of a FET 15 with a period of an output of an oscillator circuit 6 and controlling the ratio of the on-off operation according to a voltage error of a DC secondary output 32, a frequency controller 7 which sets the output frequency of the oscillator circuit 6 to a first frequency when the switching of the FET 15 is started and sets the output frequency of the oscillator circuit 6 to a second frequency when a specified time is elapsed after the switching has been started is provided, and the first frequency is set higher frequency than the second frequency.

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 increasing a switching frequency in PWM control at the start of switching and lowering it after a predetermined period.

[0002]

2. Description of the Related Art In a switching power supply, when a switching operation is started, the voltage of a secondary side DC output is reduced to 0V.
, So that the FET, which is a switching element, is controlled such that the ON period is maximized. Therefore, the maximum value of the current flowing through the FET becomes an extremely large value. For this reason, magnetic saturation tends to occur in the transformer, and it is necessary to use a transformer having a large saturation magnetic capacity. However, a transformer having a large saturation magnetic capacity has a large shape and is expensive. For this reason, a resistor is inserted between the source of the FET and the ground level, and when the voltage between the terminals of the resistor rises to a voltage at which the base current flows through the control transistor, the FET is turned off, thereby turning off the FET. Has been used (referred to as a first related art).

A conventional technique for preventing the occurrence of magnetic saturation has been proposed as Japanese Utility Model Publication No. 62-104587. That is, the maximum value of the output voltage of the error amplifier is limited to a predetermined range by using a Zener diode. As a result, the width of the pulse for turning on the FET is prevented from being narrowed beyond a predetermined range. Therefore, the two FETs whose on / off are complementary shift to the on state when the on pulse is given even when the characteristics vary. For this reason, since no magnetic bias occurs in the transformer, the occurrence of magnetic saturation is prevented, and an excessive current is prevented from flowing through the FET (the second conventional technique).

[0004]

However, when the first prior art is used, the following problems have occurred. That is, when applied to an in-vehicle television, the voltage of the primary side DC source is a voltage of 12 to 13V. Also,
The power of the secondary side DC output is about 50W. Therefore,
The average value of the current of the primary DC source is about 4 to 5A.
Therefore, when the ON ratio of the FET is, for example, 30%, the current flowing through the FET is 10% in the steady state.
It becomes the number A. On the other hand, when the switching operation of the FET is started, it is necessary to raise the voltage of the secondary side DC output from 0 V, so the maximum value of the current flowing through the FET is:
The value is larger than the steady state. Therefore, when the switching operation is started in the switching power supply having the above-described configuration, the maximum value of the current of the FET is a value of several tens of amps. The value of the resistor connected between the source of the FET and the ground level is set so that the voltage between the terminals is about 0.6 V when the current value to be limited is reached. Therefore,
In the case of the above switching power supply, the resistance value is 20 mΩ
Degree and extremely small. And a resistor having such a small value is expensive. In addition, there is also a problem that the pattern design becomes difficult because the influence of the DC impedance of the pattern constituting the wiring path increases. Further, since the voltage of the primary side DC source is low, the ratio of the voltage between the terminals of the resistor to the voltage of the primary side DC source becomes large, which causes a reduction in switching efficiency.

Further, in the second prior art, in a configuration including two FETs whose on and off are complementary to each other, the minimum value of the width of the on-pulse is prevented from becoming smaller than a predetermined value, so that the transformer is biased. It is a technique for preventing the occurrence of magnetism. Therefore, it has been difficult to apply this technique to prevent magnetic saturation caused by a long on-time.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and an object of the present invention is to increase the switching frequency at the start of a switching operation and to lower the switching frequency after a predetermined period has elapsed, thereby reducing the switching loss. It is an object of the present invention to provide a switching power supply that can prevent the occurrence of magnetic saturation of a transformer even when a small-sized transformer is used without increasing the number of transformers.

In addition to the above object, the switching frequency is switched by using a control signal for instructing the degaussing circuit to degauss, so that it is not necessary to add a microcomputer control program, and the terminals of the microcomputer are connected. An object of the present invention is to provide a switching power supply capable of preventing an increase in the number of occupied devices.

In addition to the above object, by changing the element constant of the oscillation circuit by turning on and off the control transistor for controlling the operation of the degaussing circuit, the number of elements added for changing the switching frequency can be reduced. It is an object of the present invention to provide a switching power supply.

[0009]

In order to solve the above-mentioned problems, a switching power supply according to the present invention comprises: a switching element for switching a current flowing through a primary coil; and a switching element which is turned on / off by an output cycle of an oscillation circuit. By controlling the on / off ratio based on the voltage error of the secondary DC output, the present invention is applied to a switching power supply including a PWM circuit for stabilizing the voltage of the secondary DC output, and the switching of the switching element is performed. When the operation is started, the output frequency of the oscillation circuit is set to a first frequency, and when a predetermined period has elapsed after switching of the switching element has started, the output frequency of the oscillation circuit is set to a second frequency. A frequency control unit is provided, wherein the first frequency is higher than the second frequency.

That is, when the rate at which the switching elements are turned on is constant, the higher the switching frequency, the shorter the on-period of the switching elements. Therefore,
When the switching frequency becomes the first frequency, even when the ratio of turning on increases, the switching element switches from on to off before magnetic saturation occurs in the transformer. After a predetermined period has elapsed, the switching frequency changes from the first frequency to the second frequency, so that an increase in switching loss is avoided.

In addition to the above configuration, a microcomputer for transmitting a degaussing control signal for a predetermined period when switching of the switching element is started, and a degaussing circuit for erasing the magnetization of the CRT when the degaussing control signal is transmitted. The frequency control unit sets the output frequency of the oscillation circuit to the first frequency when the degaussing control signal is transmitted, and stops the transmission of the degaussing control signal. Then, the output frequency of the oscillation circuit is set to the second frequency.

That is, the degaussing control signal is transmitted at the start of the operation of the television receiver. Therefore, the switching frequency is set to the first frequency at the start of the switching operation. The transmission of the degaussing control signal is stopped when a period of about several seconds has elapsed. Therefore,
After a few seconds, the switching frequency becomes the second
Frequency. In addition, the above operation occurs only when the microcomputer sends out a degaussing control signal to a terminal set for degaussing control.

[0013] In addition to the above configuration, a control transistor for closing the connection to the ground level when the degaussing control signal is transmitted is provided, and the degaussing circuit performs degaussing when the control transistor closes the connection to the ground level. When applied to a switching power supply of a television receiver, the oscillation circuit increases the oscillation frequency as the value of the resistor connected to the frequency setting terminal decreases, and the frequency control unit has one terminal connected to the frequency setting terminal. And a setting resistor having the other terminal grounded, and an auxiliary resistor having one terminal connected to the frequency setting terminal and the other terminal connected to the control transistor.

That is, the frequency control section for changing the output frequency of the oscillation circuit is constituted by the set resistor and the auxiliary resistor.

[0015]

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 block diagram showing an electrical configuration when an embodiment of the switching power supply according to the present invention is applied to an in-vehicle television receiver. Although a switching power supply having a commercial power supply as a primary-side input is also provided, the illustration of the switching power supply is omitted.

In FIG. 1, a battery 1 has an output voltage of 1
It is 2 V (other voltages may be 24 V), and constitutes a primary-side DC source. For this reason, the plus terminal of the battery 1 is led to one terminal of the primary coil L1. The negative terminal of the battery 1 is grounded. The other terminal of the primary coil L1 is connected to the drain of the FET 15, which is a switching element. The source of the FET 15 is grounded.

The block including the diode D1 and the capacitor C1 rectifies and smoothes the output of the secondary coil L2, and generates a secondary DC output 32. The resistance R1 and the resistance R
2 is to divide the voltage of the secondary-side DC output 32 and send the divided voltage to the PWM circuit 5.

The oscillation circuit 6 is a circuit whose oscillation frequency is determined by the value of a resistor connected between the frequency setting terminal 61 and the ground level, and sends out an oscillation output to the PWM circuit 5. The oscillation frequency is configured to increase when the value of the resistor connected between the frequency setting terminal 61 and the ground level decreases.

The PWM circuit 5 is a block for turning on and off the switching element in accordance with the cycle of the output of the oscillation circuit 6, and based on the divided voltage of the secondary DC output 32,
A voltage error of the secondary DC output 32 is detected. Then, the voltage of the secondary DC output 32 is stabilized at a predetermined voltage (for example, 110 V) by changing the on / off ratio of the FET 15 based on the detected voltage error. The drive circuit 4 is a block that drives the FET 15 by amplifying the output of the PWM circuit 5.

The television unit 3 is a block including a tuner, an intermediate frequency amplifier circuit, a detection circuit, a video signal processing circuit, an audio signal processing circuit, a deflection circuit, and a CRT (not shown). Perform the main operation of the television receiver. The degaussing circuit 8 is a block for erasing the magnetization of the CRT. For this purpose, a degauss coil 23 and a posistor 22 are connected in series.
The degauss coil 23 and the posistor 22 connected in series are connected to a commercial power supply via a relay contact 21.

The microcomputer 9 is a block for controlling main operations as a television receiver. Therefore, by controlling the PWM circuit 5 and the drive circuit 4, the start of switching of the FET 15 and the stop of switching of the FET 15 are controlled. Further, the operation of the television unit 3 is controlled. Further, in order to control the operation of the degaussing circuit 8, a degaussing control signal 33 is transmitted to the base of the control transistor Q2 via the resistor R5.

The control transistor Q2 outputs a degaussing control signal 3
When 3 (H level) is transmitted, it is turned on,
The element closes the connection to the ground level. Therefore, when the control transistor Q2 is turned on, the relay coil 11 is driven and the connection of the relay contact 21 is closed. (When the demagnetization control signal 33 is not sent, the resistance R6 sets the base of the control transistor Q2 to high impedance. The DC source supplied to the relay coil 11 is a DC source in which the output of the battery 1 is stabilized at 12V.

The frequency control section 7 is a block for switching the output frequency of the oscillation circuit 6 between a first frequency and a second frequency (the first frequency is higher than the second frequency). . Therefore, one terminal is connected to the frequency setting terminal 61 and the other terminal is connected to the grounded setting resistor R.
4. An auxiliary resistor R3 whose one terminal is connected to the frequency setting terminal 61, and a diode D2 whose anode is connected to the other terminal of the auxiliary resistor R3. And
The diode D2 has its cathode guided to the collector of the control transistor Q2 (the diode D2 prevents the current through the relay coil 11 from flowing to the auxiliary resistor R3 when the control transistor Q2 is turned off). .

The frequency control section 7 has the above-described configuration. Therefore, when the control transistor Q2 is turned off, only the setting resistor R4 is connected between the frequency setting terminal 61 and the ground level. On the other hand, when the control transistor Q2 is turned on, the frequency setting terminal 6
1 and the ground level, a setting resistor R4 and an auxiliary resistor R
3 will be connected in parallel. Therefore, the oscillation frequency (output frequency) of the oscillation circuit 6 is higher (first frequency) than the frequency (second frequency) when the control transistor Q2 is off when the control transistor Q2 is on. (The first frequency is 100KH
z, the second frequency is 50 KHz.

Although only L2 is shown for the secondary coil wound around the transformer 2, a secondary coil for obtaining a DC source of 25V is wound in an actual machine. In addition, a rectifying and smoothing circuit for outputting the secondary coil is provided. The 25 V DC source is supplied to the television unit 3. The electric power supplied to the television unit 3 is about 50W.

In this embodiment, the FET 15 is turned on before the energy stored in the transformer 2 is completely discharged. For this reason, as shown in FIG. 2, the FET 15 is turned on (at time T).
A considerable amount of current flows from 1), and thereafter, the current increases as time elapses. After turning on, 50μ
When S (time period t1) has elapsed (time T2), magnetic saturation occurs in the transformer 2. Therefore, after the time T2, the rate of increase of the current sharply increases.

Further, the PWM circuit 5 in the present embodiment
Is to change the time ratio of turning on the FET 15 within a range of 50% or less, so that the secondary side DC output 32
Voltage is stabilized. For this reason, even when switching is started in a state where the voltage of the secondary-side DC output 32 is 0 V, the time ratio at which the FET 15 is turned on does not exceed 50%.

FIG. 3 shows the FET 1 immediately after the start of switching.
5 is an explanatory diagram showing a current change of the FET 15 and a current change of the FET 15 after a predetermined period has elapsed. FIG. The operation of the embodiment will be described with reference to FIG.

When starting operation of the television section 3, the microcomputer 9 controls to start the operation of the PWM circuit 5 and the operation of the drive circuit 4. Further, the degaussing control signal 33 is transmitted for only 5 seconds (predetermined period). Therefore, the control transistor Q2 is turned on for 5 seconds.

For this reason, during the above-mentioned 5 seconds, the auxiliary resistor R3 of the frequency control section 7 is grounded via the diode D2 and the control transistor Q2. Therefore, between the frequency setting terminal 61 and the ground level, the setting resistor R4 and the auxiliary resistor R3 are connected in parallel. For this reason, the output frequency transmitted from the oscillation circuit 6 to the PWM circuit 5 is 100 KHz (first frequency).

On the other hand, when the FET 15 starts switching, the voltage of the secondary DC output 32 is 0V.
For this reason, the PWM circuit 5 generates a signal whose ON ratio is 50% and sends it to the drive circuit 4. As a result, the FET 15 performs a switching operation of repeatedly turning on and off every 50 μS. On the other hand, transformer 2
In, magnetic saturation does not occur when the ON period of the FET 15 is within 50 μS. Therefore, during the ON period, F
The current flowing through the ET 15 shows a moderate increase as shown by 31a, and the maximum value of the current is suppressed to a range that does not cause the destruction of the FET 15 (period t3 = 50 μS, period t3).
2 = 100 μS). Then, in response to the increase in the voltage of the secondary DC output 32, the rate at which the FET 15 is turned on gradually decreases.

When five seconds have elapsed, the transmission of the degaussing control signal 33 is stopped (the path 33 is at L level). Therefore, the control transistor Q2 turns off from on. Therefore, it is equivalent to a state where only the setting resistor R4 is connected between the frequency setting terminal 61 and the ground level. As a result, the output frequency of the oscillation circuit 6 becomes 50
KHz (second frequency). However, at this time, since the voltage of the secondary DC output 32 has risen to 110 V, the rate at which the FET 15 is turned on is 25%.
%. Therefore, the period during which the FET 15 is turned on is about 50 μS (period t5). For this reason, no magnetic saturation occurs in the transformer 2, so that the FET 1
The maximum value of the current of No. 5 is suppressed to a range that does not cause the destruction of the FET 15 (period t4 = 200 μS).

When the switching frequency of the FET 15 becomes 50 KHz, the switching loss
15 is smaller than the switching loss when the switching frequency of No. 15 is 100 KHz.

Note that the present invention is not limited to the above embodiment, and the PWM circuit 5 has been described as being configured such that the time ratio for turning on the switching element changes within a range of 50% or less. The same can be applied to the case of a configuration in which the value is changed within an arbitrary range, such as a range of not more than% or a range of not more than 70%.

The first frequency is 100K
Although the case where the frequency is set to Hz has been described, any frequency can be set as long as the FET 15 (switching element) can be turned off after the FET 15 (switching element) is turned on and before the transformer 2 does not cause magnetic saturation. it can.

The case where the output frequency is the first frequency is set to 5 seconds.
The other period can be, for example, 10 seconds, 30 seconds, or 1 minute.

[0037]

As described above, in the switching power supply according to the present invention, the switching element is turned on / off by the cycle of the output of the oscillation circuit, and the on / off ratio is controlled based on the voltage error of the secondary DC output. When the switching of the switching element is started in the configuration including the PWM circuit for stabilizing the voltage of the secondary-side DC output, the output frequency of the oscillation circuit is set to the first frequency. When a predetermined period elapses after the switching of the elements is started, a frequency control unit that sets the output frequency of the oscillation circuit to the second frequency is provided, and the first frequency is higher than the second frequency. Therefore, since the voltage of the secondary-side DC output rises from 0 V, even when the ratio at which the switching element is turned on increases, the switching element turns from on to off before magnetic saturation occurs in the transformer. After a predetermined period has elapsed, the switching frequency changes from the first frequency to the second frequency, so that an increase in switching loss is avoided. Therefore, even when a small-sized transformer is used, it is possible to prevent the occurrence of magnetic saturation of the transformer without increasing the switching loss.

Furthermore, a microcomputer for sending out a degaussing control signal for a predetermined period when switching of the switching element is started, and a degaussing circuit for erasing the magnetization of the CRT when the degaussing control signal is sent out. When applied to a switching power supply of a television receiver, the frequency control unit sets an output frequency of the oscillation circuit to a first frequency when a degaussing control signal is transmitted, and when the transmission of the degaussing control signal is stopped. The output frequency of the oscillation circuit is set to a second frequency.
Therefore, the microcomputer only sends the degaussing control signal to the terminal set for degaussing control, and the switching frequency becomes the first frequency at the start of switching, and the switching frequency becomes Becomes the second frequency. This eliminates the need to add a microcomputer control program,
An increase in the number of occupied terminals of the microcomputer can be prevented.

Further, the apparatus further comprises a control transistor for closing the connection to the ground level when the degaussing control signal is sent out, and the degaussing circuit comprises a television receiver for degaussing when the control transistor closes the connection to the ground level. When applied to a switching power supply of a device, the oscillation circuit increases the oscillation frequency as the value of the resistor connected to the frequency setting terminal decreases, and the frequency control unit has one terminal connected to the frequency setting terminal and the other Are connected to a frequency setting terminal and the other terminal is connected to a control transistor. Therefore, since the frequency control unit that changes the output frequency of the oscillation circuit includes the setting resistor and the auxiliary resistor, the number of elements added to change the switching frequency can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an electrical configuration when an embodiment of a switching power supply according to the present invention is applied to a television receiver.

FIG. 2 is an explanatory diagram illustrating a relationship between an on-time of a switching element and a current.

FIG. 3 is an explanatory diagram showing a change in current of the switching element immediately after switching starts and a change in current of the switching element after a predetermined period has elapsed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Battery 5 PWM circuit 6 Oscillation circuit 7 Frequency control unit 8 Degaussing circuit 9 Microcomputer 22 Posistar 23 Degauss coil 32 Secondary DC output 33 Degaussing control signal 61 Frequency setting terminal L1 Primary coil Q2 Control transistor R3 Auxiliary resistance R4 Setting resistance

Claims (3)

[Claims] 1. A switching element for switching a current flowing through a primary coil, a switching element being turned on / off by an output cycle of an oscillation circuit, and a ratio of the on / off being controlled based on a voltage error of a secondary DC output. Thus, in a switching power supply including a PWM circuit for stabilizing the voltage of the secondary DC output, when switching of a switching element is started, an output frequency of the oscillation circuit is set to a first frequency, When a predetermined period elapses after the switching of the switching element is started, a frequency control unit that sets the output frequency of the oscillation circuit to a second frequency is provided, and the first frequency is set to a frequency higher than the second frequency. A switching power supply characterized in that: 2. A television comprising: a microcomputer for transmitting a degaussing control signal for a predetermined period when switching of a switching element is started; and a degaussing circuit for erasing the CRT magnetism when the degaussing control signal is transmitted. In the switching power supply of the receiving device, the frequency control unit sets an output frequency of the oscillation circuit to a first frequency when a degaussing control signal is transmitted,
2. The switching power supply according to claim 1, wherein the output frequency of the oscillation circuit is set to a second frequency when the transmission of the degaussing control signal is stopped. 3. A television receiver comprising: a control transistor for closing a connection to a ground level when the degaussing control signal is transmitted; and a degaussing circuit for degaussing when the control transistor closes a connection to a ground level. In the switching power supply, the oscillation circuit increases the oscillation frequency as the value of the resistor connected to the frequency setting terminal decreases, and the frequency control unit includes one terminal connected to the frequency setting terminal, and the other terminal grounded. 3. The switching power supply according to claim 2, wherein the switching power supply includes a set resistor and an auxiliary resistor having one terminal connected to the frequency setting terminal and the other terminal connected to the control transistor.