JPH06189545A - Switching power supply - Google Patents

Abstract

PURPOSE:To provide a switching power supply which can control drop of oscillation frequency under a heavy load condition. CONSTITUTION:A sub-coil 2d is provided in a transformer 2. Moreover, a resistor 34 connected in series to one end of the sub-coil 2d, a capacitor 36 connected between the resistor 34 and the other end of the sub-coil 2d, a diode 38 for selectively extracting a forward voltage V6 during the OFF period of a MOSFET 4 which is the voltage across the capacitor 36, and a switch circuit 40 which turns ON when a voltage extracted from the diode 38 exceeds a predetermined value and thereby forcedly turns on the MOSFET 4 are also provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited and flyback type (that is, RCC type) switching power supply device, and more particularly to a means for suppressing a decrease in oscillation frequency under heavy load.

[0002]

2. Description of the Related Art A conventional example of this type of switching power supply device is shown in FIG. This switching power supply device is of the RCC system, in which a primary winding 2a of the transformer 2 is connected in series with a MOSFET 4 which is an example of a switching element, and the output of a bias winding 2c of the transformer 2 is a self-excited oscillation control circuit. The MOSFET 4 is oscillated by being fed back to the gate of the MOSFET 4 via 6, and the energy stored in the transformer 2 from the secondary winding 2b to the rectifying diode 2 when the MOSFET 4 is turned off.
It is configured to be taken out as a DC output via 6.

As the oscillation control circuit 6, various known circuits can be adopted. In this example, one end of the bias winding 2c and M are used.
It comprises a capacitor 14 inserted in series between the gates of the OSFET 4, a resistor 16 and a control transistor 18 connected to bypass the gate of the MOSFET 4 to ground. Also, a resistor 1 connected in series with each other
A time constant circuit 8 composed of 0 and a capacitor 12 is connected across the bias winding 2c so that the voltage of the capacitor 12 is applied to the base of the control transistor 18. Furthermore, for constant voltage control of the output voltage V ₂ on the secondary side, a diode 20 and a photocoupler 22 (more specifically, the phototransistor side thereof) are connected between one end of the bias winding 2c and the capacitor 12. is doing. 24
Is the starting resistance.

A rectifier diode 26 and an output voltage detection circuit 28 are connected to the secondary winding 2b of the transformer. The output voltage detection circuit 28 uses the shunt regulator 3
0, an output voltage V ₂ is divided, and a voltage dividing resistor 32 for supplying the divided voltage to the reference voltage terminal of the shunt regulator 30 and a photocoupler 22 are provided. The shunt regulator 30 compares the output voltage V ₂ with a reference voltage. The photodiode of the photo coupler 22 is caused to emit light according to the difference,
Thereby, feedback is applied to the oscillation control circuit 6 on the primary side.

Explaining the oscillation operation, the input voltage V
_{When 1} is applied, it passes through the starting resistor 24
Applied to the gate of FET4, MOSFET4 becomes conductive. As a result, a voltage is applied to the primary winding 2a of the transformer 2 and at the same time, a voltage V ₃ is generated in the bias winding 2c. This is MO through the capacitor 14 and the resistor 16.
Applied to the gate of SFET4, MOSFET4 turns on rapidly. At this time, the voltage of the secondary winding 2b of the transformer 2 is applied in the opposite direction to the rectifier diode 26,
No current flows in the secondary winding 2b, and energy is stored in the transformer 2. At the same time, a charging current flows through the capacitor 12 forming the time constant circuit 8 through the resistor 10 and the base potential of the control transistor 18 gradually rises.

When the voltage on the capacitor 12 begins to conduct for the control transistor 18 for a given value, this causes M
The gate of OSFET4 is bypassed to ground and M
The OSFET4 cannot be maintained in the ON state, and the primary winding 2a
Voltage decreases, and the voltage V ₃ of the bias winding 2c also decreases. Since this is positive feedback, MOSFET 4 turns off rapidly. When the MOSFET 4 is turned off, the energy stored in the transformer 2 is supplied from the secondary winding 2b to the output side through the rectifying diode 26.

After that, when the stored energy is completely discharged, a voltage V ₃ is generated in the bias winding 2c due to the residual energy slightly left in the secondary winding 2b, and M is again generated.
The OSFET 4 is turned on, and the above operation is repeated.

[0008] Referring to the constant voltage control of the output voltage V _2, the output voltage V ₂ rises above the specified value, the greater the current flowing through the photodiode of the shunt regulator 30 and the photocoupler 22, the photocoupler accordingly The current flowing in the phototransistor 22 also becomes large, which causes the capacitor 12 in the oscillation control circuit 6 to be charged quickly, so that the control transistor 18 is turned on quickly and the gate of the MOSFET 4 is bypassed to the ground. Becomes shorter and the output voltage V ₂ drops. In this way, the output voltage V ₂
Constant voltage control is performed.

[0009]

The RCC type switching power supply device as described above generally has a characteristic that the oscillation frequency decreases as the load increases.

More specifically, the oscillation frequency is f, the input power at that frequency is P, the input voltage is V ₁ , the output voltage is V ₂ , and the peak value of the drain current I _D flowing through the MOSFET 4 is I _DP . Further, the inductance of the primary winding 2a of the transformer 2 is L _P , the winding ratio of the winding number N _P of the primary winding 2a and the winding number N _S of the secondary winding 2b is N, and A of the core of the transformer 2 is A.
If the L value is AL,

## EQU1 ## f = 2P / (L _P I _DP ² ) = V ₁ V ₂ / (N V ₁ + V ₂ ) (N _P ² ALI _DP ) where the peak of the drain current I _D changes depending on the load. the value I _DP is only the peak value I _DP that increase the oscillation frequency f as the load becomes heavier drops inevitably.

As described above, in the conventional RCC type switching power supply device, the oscillation frequency decreases as the load becomes heavier. Therefore, the transformer 2 must be upsized in order to efficiently output an output even at a low frequency. There is a problem that the cost increases and the required space increases.

Therefore, a main object of the present invention is to provide a switching power supply device capable of suppressing a decrease in the oscillation frequency under heavy load.

[0013]

In order to achieve the above object, the switching power supply device of the present invention is provided with an auxiliary winding in the transformer, and a resistor and a resistor connected in series to one end of the auxiliary winding. A capacitor connected between this resistor and the other end of the auxiliary winding; and a diode for selectively extracting the voltage across the capacitor, which is the voltage in the OFF direction of the switching element. A switch circuit is provided, which is turned on when the voltage taken out by the diode exceeds a predetermined value, thereby forcibly turning on the switching element.

[0014]

According to the above construction, during the switching operation, the output voltage of the auxiliary winding repeatedly charges the capacitor in both the forward and reverse directions through the resistor connected to the auxiliary winding. The voltage across the capacitor, which is in the direction in which the switching element is off, is selectively taken out by the diode. Therefore, after a certain period of time elapses after the switching element is turned off, the voltage taken out by this diode becomes a predetermined value or more, whereby the switch circuit is turned on and the switching element is forcibly turned on. By forcibly turning on the switching element, the off period of the switching element is shortened.

When the switching element is forcibly turned on, the voltage direction of the secondary winding of the transformer reverses, so that the stored energy in the transformer stops flowing out from the secondary winding. Therefore, the current flowing in the primary winding rises largely in a trapezoidal shape, and in such a case, the required energy can be accumulated in the transformer in a shorter time compared to the conventional triangular waveform, so the switching element The on period will also be short.

With the above operation, it is possible to prevent the oscillation frequency from falling below a certain level even if the load becomes heavy.

[0017]

1 is a circuit diagram showing a switching power supply device according to an embodiment of the present invention. The same or corresponding portions as those of the conventional example in FIG. 6 are denoted by the same reference numerals, and the differences from the conventional example will be mainly described below.

In this embodiment, an auxiliary winding 2d having a winding direction opposite to that of the primary winding 2a, that is, the same winding direction as that of the secondary winding 2b is further provided on the primary side of the transformer 2. Therefore, the auxiliary winding 2d has a downward voltage V as shown by a solid line in FIG.
₄ occurs, and an upward voltage V ₅ as indicated by a broken line in FIG. 1 is generated during the off period.

A resistor 34 is serially connected to one end side (winding end side) of the auxiliary winding 2d, and a capacitor 36 is connected between the resistor 34 and the other end side of the auxiliary winding 2d. Further, the anode side of the diode 38 is connected to the connection portion of the resistor 34 and the capacitor 36. MOSF
During the switching operation of ET4, the capacitor 36 is repeatedly charged in both the forward and reverse directions, but the diode 38
Is the voltage across this capacitor 36 and is the MOSF
ET4 selectively takes out the voltage in the OFF direction, that is, the upward voltage V ₆ in FIG.

A switch circuit 40 is provided between the cathode side of the diode 38 and the other end side of the auxiliary winding 2d. In this example, the switch circuit 40 has bias resistors 42 and 44 connected in series between the cathode side of the diode 38 and the other end side of the auxiliary winding 2d and a connection portion of the resistors 42 and 44. Base is diode 38
Has a switching transistor 46 having an emitter connected to the cathode of the transistor 46, and when the voltage on the cathode side of the diode 38 exceeds a predetermined value, the transistor 46 is turned on.
The base of the transistor is biased above a predetermined level
Turns on and outputs the voltage on the cathode side of the diode 38. In this example, the voltage output from the switch circuit 40 is MOSFET through the diode 48 for reverse blocking.
4 is applied to the gate of MOSFET 4, thereby forcing MOSFET 4, which was previously turned off, to turn on.

When the MOSFET 4 is turned on as described above, the voltage V ₃ is generated in the bias winding 2c of the transformer 2 as described above, and thereafter the operation of the oscillation control circuit 6 maintains the on state of the MOSFET 4 for a predetermined period. It

With the circuit configuration as described above, the MOSFE
The timing for forcibly turning on T4 is
The time T (see FIG. 2) from when the ET4 is turned off until it is forcibly turned on is determined by the values of the resistor 34 and the capacitor 36 which determine the time constant of the rise of the voltage V ₆ across the capacitor 36, and the resistor 42. It can be adjusted by the ratio of 44 or the like.

According to the above construction, since the oscillation frequency becomes high as a characteristic of the RCC method when the load is light, the capacitor 36 is not charged with a voltage sufficient to turn on the switch circuit 40. Without auxiliary winding 2
It does not happen that the MOSFET 4 is forcibly turned on by the above circuit on the d side. That is, the oscillation control circuit 6
Thus, the same oscillation control as in the conventional example is performed.

When the load becomes heavy (that is, the output current I ₂ becomes large), the oscillation frequency decreases as a characteristic of the RCC method only by the control by the oscillation control circuit 6. And MO
When the off period of the SFET4 is about to be longer than the time T, M is generated by the above-mentioned circuit on the auxiliary winding 2d side.
OSFET4 is forced on. Therefore MOS
The off period of the FET 4 is shortened.

When the MOSFET 4 is forcibly turned on, the direction of the voltage of the secondary winding 2b of the transformer 2 is reversed, so that the stored energy in the transformer 2 stops flowing out from the secondary winding 2b. Therefore, the drain current I _D flowing in the MOSFET 4 via the primary winding 2a of the transformer 2 rises largely in a trapezoidal shape. In such a case, the transformer 2 can be formed in a shorter time than the conventional triangular waveform.
Since the required energy can be stored in the
The on period of the SFET4 can be shortened.

With the above-described operation, it is possible to prevent the oscillation frequency from dropping below a certain level even if the load becomes heavy.

An example of the experimental results will be described. The output current I
_When both ₂ are 3 A, the waveforms of the drain current I _D and the drain-source voltage V _DS of the MOSFET 4 are as shown in FIG. 3 in the conventional switching power supply device shown in FIG. 6, and the oscillation frequency is It fell to about 109 KHz. On the other hand, in the switching power supply device of this embodiment, the same waveform is as shown in FIG. 2, the oscillation frequency is about 181 KHz, and it can be seen that the decrease is greatly suppressed.

Further, as can be seen by comparing FIG. 2 and FIG. 3, the peak value I _DP of the drain current I _D becomes smaller in this embodiment when the same output current I _{2 of} 3 A is supplied. There is. It is considered that this is because the drain current I _D rises in a trapezoidal shape and sufficient energy can be accumulated in the transformer 2 even if the peak value I _DP is not so large.

2 and 3, the output current I ₂ is 3 A.
However, the change in the oscillation frequency f and the peak value I _{DP of the} drain current I _D when the output current I ₂ is changed
Graphs of changes in the are shown in FIGS. 4 and 5, respectively.

As can be seen from FIG. 4, in the conventional example, the oscillating frequency is greatly reduced as the output current I ₂ is increased, whereas in this example, the oscillating frequency is obtained when the output current I ₂ is about 2 A or more. Is kept almost constant.

Further, as can be seen from FIG. 5, in the conventional example, the peak value I _DP of the drain current I _D increases almost linearly with the increase of the output current I _2. In the example, the increase of the peak value I _DP is gradual.

The winding direction of the auxiliary winding 2d newly provided in the transformer 2 may be the same as that of the primary winding 2a instead of the above-mentioned embodiment, and in this case, the winding end of the auxiliary winding 2d is finished. The resistor 34, the diode 38, etc. may be connected to the side.

Further, as the switching element, a bipolar transistor may be used instead of the MOSFET 4 as described above.

[0034]

As described above, according to the present invention, since the auxiliary winding and the circuit connected thereto are provided, it is possible to prevent the oscillation frequency from dropping below a certain level even if the load becomes heavy. You can As a result, efficiency is improved even with a small transformer, so that the transformer can be downsized and the cost and space can be reduced accordingly.

Further, even when the same output is produced, the peak value of the current flowing through the switching element can be made smaller than that of the conventional example, so that the capacity of the switching element can be made smaller and the cost can be reduced. Can be planned.

Further, since the oscillation frequency does not drop below a certain value, the efficiency is not significantly reduced even if the oscillation frequency is designed to be low as a whole, and as a result, it is advantageous in terms of measures against EMI noise (electromagnetic interference).

[Brief description of drawings]

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

FIG. 2 is a diagram showing an example of waveforms of drain current and drain-source voltage in the switching power supply device of FIG.

FIG. 3 is a diagram showing an example of waveforms of drain current and drain-source voltage in the conventional switching power supply device of FIG.

FIG. 4 is a diagram showing an example of a relationship between an output current and an oscillation frequency.

FIG. 5 is a diagram showing an example of a relationship between an output current and a peak value of a drain current.

FIG. 6 is a circuit diagram showing an example of a conventional switching power supply device.

[Explanation of symbols]

 2 transformer 2a primary winding 2b secondary winding 2c bias winding 2d auxiliary winding 4 MOSFET (switching element) 6 oscillation control circuit 34 resistance 36 capacitor 38 diode 40 switch circuit

Claims (1)

[Claims] 1. A transformer having a primary winding, a secondary winding and a bias winding, a switching element connected in series to the primary winding of the transformer, and an output from the bias winding of the transformer. RC provided with a self-excited oscillation control circuit for controlling switching of a switching element
In the C type switching power supply device, an auxiliary winding is provided on the transformer, and a resistor connected in series to one end of the auxiliary winding and a resistor connected between the resistor and the other end of the auxiliary winding. And a diode that selectively takes out the voltage across the capacitor, which is the voltage across the capacitor and is in the direction in which the switching element is in the off period, and turns on when the voltage taken out by this diode exceeds a predetermined value, A switching power supply device is provided with a switch circuit for forcibly turning on the switching element thereby.