JP5565909B2 - Self-excited flyback converter - Google Patents

Description

  The present invention relates to a self-excited flyback converter including a circuit for preventing a decrease in oscillation frequency under heavy load.

2. Description of the Related Art Currently, switching power supply units that are highly efficient and can meet the needs for low power consumption are commonly used in power supply units of electronic appliances such as household appliances and office equipment.
Among them, the self-excited flyback converter (RCC) is an isolated converter and is self-excited and has a simple basic operation. Therefore, the self-excited flyback converter (RCC) is employed in various power supply units.

  In a switching power supply device based on such a self-excited flyback converter, the self-excited flyback converter usually has an oscillation frequency that decreases as the output load increases, and decreases as the input voltage decreases (Patent Document 2). [0009] to [0011] paragraph). Therefore, the transformer design needs to take into consideration that the oscillation frequency is lowered, and the transformer is larger than a flyback converter that uses a control IC and operates at a fixed oscillation frequency.

  In the case of a flyback converter with a fixed oscillation frequency, the current waveform flowing in the main switching element is a trapezoidal wave, whereas in the case of a self-excited flyback converter, the current waveform is a triangular wave (Patent Documents 1 to 3). (Refer to each waveform diagram of 3). For this reason, in the self-excited flyback converter, as the oscillation frequency decreases under heavy load, the peak value of the current flowing through the main switching element increases, and loss due to current (conduction loss due to the main switching element having on-resistance and There is a disadvantage that switching loss generated during switching) is large.

  Therefore, circuits as shown in FIGS. 2 to 4 have been devised so far to prevent the oscillation frequency from being lowered under heavy load (see Patent Documents 1 to 3).

FIG. 2 is a circuit diagram of the self-excited flyback converter disclosed in Patent Document 1 (see FIG. 1 of Patent Document 1).
In the self-excited flyback converter 100 ′ shown in FIG. 2, in order to realize a self-excited operation, a starting resistor 24 and a transformer primary side auxiliary winding (hereinafter referred to as “gate drive winding”) wound around the transformer 2 as an auxiliary winding. Or 2) (referred to as “bias winding”). In the example of FIG. 2, the bias winding 2c and the primary winding 2a have the same polarity (for the transformer 2, the side marked with ● is the one end side and the opposite side is the other end side, and so on). . The number of turns of the bias winding 2c is set in advance within a range in which the voltage output to one end of the bias winding 2c can output a voltage higher than the threshold voltage for driving the gate of the main switching element 4. .

The basic operation of the self-excited flyback converter 100 ′ shown in FIG. 2 when the oscillation frequency drop prevention circuit 7 ′ is not operating is outlined. As is known, first, the main switching element 4 is temporarily turned on by the starting resistor 24 for starting. As a result, the input voltage V1 is applied to the primary winding 2a of the transformer 2, and a proportional voltage is induced in the bias winding 2c according to the turn ratio with the primary winding 2a. The main switching element 4 is completely turned on by the voltage. The turn-on period after the main switching element 4 is completely turned on is determined by the oscillation control circuit 6 and the output voltage detection circuit 28 shown in FIG. Note that the oscillation control circuit 6 and the output voltage detection circuit 28 are known per se, and a detailed description of the configuration and operation thereof will be omitted.
When the main switching element 4 is in a turn-on state, a reverse bias is applied to the secondary rectifier diode 26 by the voltage induced in the secondary winding 2b, and no current flows through the secondary winding 2b. While the main switching element 4 is in the turn-on state, the secondary winding 2b is excited by the input voltage V1, and energy is stored in the transformer 2.
On the other hand, when the main switching element 4 is in the turn-off state, the secondary winding 2b is reset with the output voltage V2, and the stored energy is released to the load connected to the output terminal on the secondary side.

Next, the operation of the self-excited flyback converter 100 ′ of FIG. 2 including the case where the oscillation frequency reduction preventing circuit 7 ′ operates will be described.
In the bias winding 2c, an upward voltage V3 as indicated by an arrow indicated by a solid line in FIG. 2 is generated during the ON period of the main switching element 4, and during the OFF period of the main switching element 4, A downward voltage in the direction opposite to the voltage V3 is generated, and the capacitor 13 is repeatedly charged in both forward and reverse directions via the resistor 9 during the switching period of the main switching element 4.
As described above, the self-excited flyback converter has a characteristic that the length of the on-period and the off-period of the main switching element 4 increases as the light load changes to the heavy load state. The voltage V7 charged to the downward polarity as indicated by the solid line gradually increases.
Therefore, when the load of the switching power supply becomes larger than a predetermined value, the voltage V7 becomes larger than the forward voltage between the base and the emitter of the PNP transistor 5 during the OFF period of the main switching element 4, and the PNP transistor 5 Turns on.

When the PNP transistor 5 is turned on, a downward voltage in the direction opposite to the voltage V3 indicated by the solid line in FIG. 2 is generated in the bias winding 2c during the off period of the main switching element 4 as described above. A current flows from the other end of the winding 2 c via a path that returns to one end of the bias winding via the PNP transistor 5, the diode 3, and the current limiting resistor 16.
Further, during the OFF period of the main switching element 4, the charging voltage Vc of the DC blocking capacitor 14 is charged to the polarity indicated by the solid line in FIG. 5 is turned on, the DC blocking capacitor 14 causes the charging voltage Vc to pass through the DC blocking capacitor 14 between the gate and the source of the main switching element 4, the PNP transistor 5 and the diode 3 to be connected to the DC blocking capacitor. Discharge occurs through the path returning to 14, and the gate voltage of the main switching element 4 rises to a value equal to or higher than the on-threshold level. Therefore, the main switching element 4 is forcibly turned on.

  When the main switching element 4 is turned on, the voltage V3 is generated in the bias winding 2c of the transformer 2 as described above, and the voltage V3 is applied to the gate of the main switching element 4 via the current limiting resistor 16 and the DC blocking capacitor 14. The ON state of the main switching element 4 is maintained for a predetermined period determined by the constant voltage operation of the oscillation control circuit 6. The timing for forcibly turning on the main switching element 4 by the circuit configuration as described above, that is, the predetermined time T until the forcible turning on after the main switching element 4 is turned off (see FIG. 5 of Patent Document 1). It can be adjusted by changing the time constant of the resistor 9 and the capacitor 13.

According to the above configuration, when the load is light, the oscillation frequency is high as a characteristic of the self-excited flyback converter as described above, so that a voltage sufficient to turn on the PNP transistor 5 in the capacitor 13 is charged. No, forcibly turning on the main switching element 4 does not occur. That is, the oscillation control circuit 6 performs the same oscillation operation as when the oscillation frequency reduction preventing circuit 7 ′ is not operating.
On the other hand, when the load is heavy (that is, the output current I2 is large), the oscillation frequency decreases as a characteristic of the self-excited flyback converter only by the control by the oscillation control circuit 6. When the off period of the main switching element 4 is going to be longer than the period T, the main switching element 4 is forcibly turned on by the oscillation frequency reduction preventing circuit 7 ′. When the oscillation frequency reduction preventing circuit 7 'operates, the off period of the main switching element 4 is shortened in this way.

FIG. 3 is a circuit diagram of the self-excited flyback converter (100 ″) disclosed in Patent Document 2 (see FIG. 1 of Patent Document 2).
The outline of the oscillation frequency reduction preventing circuit 1 disclosed in Patent Document 2 will be described in sequence. The primary side of the transformer 2 has a winding direction opposite to that of the primary winding 2a, that is, the same winding direction as that of the secondary winding 2b. An auxiliary winding 2d is further provided. Therefore, a downward voltage V4 as indicated by a solid line in FIG. 1 is generated in the auxiliary winding 2d during the ON period of the main switching element 4, and upward as indicated by a broken line in FIG. 3 during the OFF period. The voltage V5 is generated.

  A resistor 34 is connected in series 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. During the switching operation of the main switching element 4, the capacitor 36 is repeatedly charged in both forward and reverse directions. However, the diode 38 is a voltage across the capacitor 36 and has a direction in which the main switching element 4 is in the off period. The voltage, that is, the upward voltage V6 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 includes a bias resistor 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 between the resistors 42 and 44. The base has a switching transistor 46 whose emitter is connected to the cathode of the diode 38. When the voltage on the cathode side of the diode 38 exceeds a predetermined value, the base of the transistor 46 is biased to a predetermined value or more. 46 is turned on, and the voltage on the cathode side of the diode 38 is output. In this example, the voltage output from the switch circuit 40 is applied to the gate of the main switching element 4 via the diode 48 for preventing backflow, thereby forcing the main switching element 4 that has been turned off until then. Turned on.

  When the main switching element 4 is turned on as described above, the voltage V3 is generated in the bias winding 2c of the transformer 2 as described above, and thereafter, the on state of the main switching element 4 is maintained for a predetermined period by the action of the oscillation control circuit 6. Maintained.

  The timing at which the main switching element 4 is forcibly turned on by the circuit configuration as described above, that is, the time T until the main switching element 4 is forcibly turned on after the main switching element 4 is turned off (see FIG. 2 of Patent Document 2). It can be adjusted by the values of the resistor 34 and the capacitor 36 that determine the time constant of the rise of the voltage V6 across the capacitor 36, the ratio of the resistors 42 and 44, and the like.

  According to the above configuration, when the load is light, the oscillation frequency is high as a characteristic of the self-excited flyback converter as described above. Therefore, a voltage sufficient to turn on the switch circuit 40 in the capacitor 36 is charged. In other words, the main switching element 4 is not forcibly turned on by the circuit as described above on the auxiliary winding 2d side. That is, the oscillation control circuit 6 performs the same oscillation control as when the oscillation frequency lowering prevention circuit 1 is not operating.

  On the other hand, when the load is heavy (that is, the output current I2 is large), the oscillation frequency decreases as a characteristic of the self-excited flyback converter only by the control by the oscillation control circuit 6. When the off period of the main switching element 4 is going to be longer than the time T, the main switching element 4 is forcibly turned on by the circuit as described above on the auxiliary winding 2d side. Therefore, the off period of the main switching element 4 is shortened.

FIG. 4 is a circuit diagram of the self-excited flyback converter (100 ′ ″) disclosed in Patent Document 3 (see FIG. 1 of Patent Document 3).
In the example disclosed in Patent Document 3, as shown in FIG. 4, a rectifying diode D3, a backflow prevention diode D4, a charging time constant connected to a second auxiliary winding Np3 provided on the primary side of the transformer 320, as shown in FIG. An oscillation frequency drop prevention circuit 340 is formed from the resistor R2 and the capacitor C3 that determine the above. R3 is a discharge resistance.

  The second auxiliary winding Np3 is provided so as to have a winding direction that is apparently opposite to the winding direction of the auxiliary winding Np2 when the minus side of the smoothing capacitor C1 is used as a reference. Accordingly, a voltage having the same polarity as that of the primary winding Np1 is induced in the auxiliary winding Np2, but a voltage having a reverse polarity is induced in the second auxiliary winding Np3. The voltage value is a value corresponding to the winding ratio with the primary winding Np1.

The general operation of the oscillation frequency reduction preventing circuit 340 having the above configuration will be described. First, when the control element Q2 is turned off by the on / off control signal CNT from the control signal generating circuit 332 based on the feedback signal Vf, the main switching element Q1 is turned off. Then, a current flows from the second auxiliary winding Np3 through the rectifier diode D3 and the charging resistor R2, and flows into the charging capacitor C3.
When the voltage of the charging capacitor C3 exceeds the ON voltage of the main switching element Q1, the switching element Q1 is forcibly turned on.

  That is, the time from when the main switching element Q1 is turned off by the on / off control signal CNT from the control signal generating circuit 332 until it is forcibly turned on by the oscillation frequency reduction preventing circuit 340 (FIG. 2 of Patent Document 3). The time shown in (B) (see tb to t0) is determined by the charging time constant of the resistor R2 and the capacitor C3. Therefore, in the example disclosed in Patent Document 3, the oscillation frequency at the maximum load can be regulated to the oscillation frequency determined by the set time constant.

  On the other hand, in the case of the minimum load, the oscillation frequency becomes high. That is, the on / off cycle time of the main switching element Q1 is minimized. In this case, the voltage of the charging capacitor C3 does not exceed the ON voltage of the main switching element Q1, and the oscillation frequency lowering prevention circuit 340 does not operate, so that the voltage control circuit 313 is operated at a normal oscillation frequency.

  As described above, each of the circuits in FIGS. 2 to 4 uses the voltage generated in the bias winding when the main switching element is turned off, sets the time constant using a resistor and a capacitor, and performs main switching for a certain period of time. After the element is turned off, the main switching element is forcibly turned on to prevent the oscillation frequency from being lowered. 2) The resistors and capacitors that determine the time constant in FIG. 2 are the resistor 9 and the capacitor 13, and ii) The resistors and capacitors that determine the time constant in FIG. Yes, iii) The resistors and capacitors that determine the time constant of FIG. 4 are the resistor R2 and the capacitor C3.

  By the way, the voltage of the bias winding used in the oscillation frequency reduction prevention circuit shown in FIGS. 2 to 4 is a voltage when the main switching element is off, and the voltage of the secondary winding is rectified and smoothed. Occurs during the period when power is being supplied to the load.

  Here, with respect to the secondary side output, feedback control for making the secondary side output voltage constant is performed in any of the circuits of FIGS. 2 to 4, and as the output load increases, The voltage increases. Accordingly, the voltage of the bias winding also increases as the output load increases, similar to the voltage of the secondary winding.

  Then, in the circuits of FIGS. 2 to 4 that have been devised so far, the voltage generated in the bias winding when the output load further increases from the point where the output load increases and the oscillation frequency reduction prevention circuit operates. As a result, the timing to forcibly turn on the main switching element (after setting the time constant using resistors and capacitors and turning off the main switching element for a certain period of time) becomes early, resulting in an oscillation frequency reduction prevention circuit The oscillation frequency rises from the oscillation frequency at the time when is operated.

  Therefore, in the circuits of FIGS. 2 to 4 that have been devised so far, if the output load continues to increase, the oscillation frequency will increase from the oscillation frequency at the time when the oscillation frequency lowering prevention circuit operates. An increase in switching loss during loading has been a problem.

  In addition, if an overcurrent protection circuit provided separately in the self-excited flyback converter is activated as a result of the output load being overloaded, the on-period of the main switching element is reduced and the off-period is lengthened. As a result, it is necessary to lower the oscillation frequency. If the oscillation frequency reduction prevention circuit is operating at this time, the opposite controls will be performed, and the secondary output can be sufficiently reduced. It was difficult.

JP 2006-115593 A Japanese Patent Laid-Open No. 6-189545 JP-A-6-133543

  Therefore, the present invention prevents the oscillation frequency from increasing from the oscillation frequency when the decrease in the oscillation frequency stops even if the output load further increases after the output load increases and the oscillation frequency decrease prevention circuit operates. It is an object of the present invention to provide a self-excited flyback converter capable of suppressing an increase in switching loss due to an increase in oscillation frequency at the time.

  Further, according to the present invention, when an overcurrent protection circuit provided separately in the self-excited flyback converter is activated as a result of the output load being overloaded, the oscillation frequency reduction prevention circuit is stopped and the overcurrent protection circuit is stopped. It is an object of the present invention to provide a self-excited flyback converter that can prevent the operation of the protection circuit from being affected.

  In addition, the present invention can prevent a decrease in the oscillation frequency when the output load increases without separately adding a winding for preventing a decrease in the oscillation frequency to the transformer, and solve the above problems. It is an object of the present invention to provide a self-excited flyback converter having a configuration capable of achieving the above.

A self-excited flyback converter according to the present invention capable of solving the above-described problems includes: (1) a transformer having a primary winding, a secondary winding, and a bias winding;
A main switching element connected in series with the primary winding and driven by a voltage induced in the bias winding;
An oscillation control circuit for controlling the switching of the main switching element by feeding back the output from the secondary winding;
A switch circuit connected to the main switching element and turning on the main switching element from an off state;
A time constant circuit including a first capacitor and a first resistor, wherein the first capacitor is set to a threshold voltage or higher by using an output voltage from the bias winding when the main switching element is in an off state; An oscillation frequency lowering prevention circuit that forcibly turns on the switch circuit by charging, and
The oscillation frequency reduction preventing circuit further includes a constant current circuit that charges the first capacitor with a constant current when the main switching element is turned off.

In the present invention, it is preferable that (2) one end of the bias winding be connected to a control terminal of the main switching element via a current limiting resistor and a DC blocking capacitor. Because
A drive auxiliary power source comprising a first diode and a second capacitor connected in series between one end and the other end of the bias winding;
The switch circuit is connected between a connection point between the DC blocking capacitor and the control terminal of the main switching element and a connection point between the first diode and the second capacitor of the drive auxiliary power supply,
When the switch circuit is turned on, the electric charge charged in the second capacitor is sent to the switch circuit, and a drive current for turning on the main switching element is supplied to the control terminal of the main switching element. It is a feature.

  In the self-excited flyback converter of (2), (3) a constant current for stopping the charging operation of the first capacitor by the constant current circuit between the constant current circuit and the other end of the bias winding. It is preferable to further include a circuit operation stop means.

In the self-excited flyback converter of (3) above, (4) a first Zener diode having a cathode connected to the other end of the bias winding and an anode connected to the constant current circuit is used as the constant current circuit operation stop means. Function,
It is preferable that the charging operation of the first capacitor by the constant current circuit is stopped by the conduction of the first Zener diode.

Further, in the self-excited flyback converter of (4), (5) the constant current circuit has three terminals, a first end is connected to one end of the bias winding, and the first Zener A second end connected to the anode of the diode, and a third end connected to the control terminal of the switch circuit;
The time constant circuit includes a parallel circuit in which the first capacitor and the first resistor are connected in parallel. One end of the parallel circuit is a control terminal of the switch circuit and a third of the constant current circuit. It is preferable that the other end is connected to a connection point between the first diode and the second capacitor of the auxiliary power source for driving.

In the self-excited flyback converter of (3), (6) the switch circuit includes a PNP transistor,
The constant current circuit includes a constant current control transistor composed of an NPN transistor, a second Zener diode, a second diode, and a second resistor and a third resistor.
The cathode of the second diode of the constant current circuit serving as the first end is at a connection point between one end of the bias winding and the current limiting resistor, and the anode of the second diode is the second resistor. Via the third resistor, the emitter of the constant current control transistor, the collector of the constant current control transistor to the base of the PNP transistor of the switch circuit, and the base of the constant current control transistor to the second end. To the anode of the first Zener diode,
The anode of the second Zener diode is connected to the connection point between the anode of the second diode and the second resistor of the constant current circuit, and the cathode of the second Zener diode is connected to the base of the constant current control transistor. It is preferable that each connection point is connected to the connection point with the third resistor.

  In the present specification, the expression “gate”, “drain” or “source” in relation to the main switching element 4 means that a field effect transistor is assumed as the main switching element 4. It is a manifestation and is simply expressed as such for convenience. Therefore, the main switching element 4 is not limited to the field effect transistor on the basis of the expression. The main switching element 4 is not particularly limited as long as it performs an equivalent function. In that case, the corresponding expression is naturally applicable to a case where a three-terminal expression different from the field effect transistor is used. It will be obvious.

  i) According to the present invention, even if the output load further increases after the oscillation frequency decrease prevention circuit operates (the output load increases), the oscillation frequency does not increase from the oscillation frequency when the decrease in the oscillation frequency stops. Thus, it is possible to provide a self-excited flyback converter capable of suppressing an increase in switching loss due to an increase in oscillation frequency during heavy load.

  ii) According to the present invention, when an overcurrent protection circuit provided separately in the self-excited flyback converter is activated as a result of the output load being overloaded, the oscillation frequency reduction prevention circuit is stopped. Thus, it is possible to provide a self-excited flyback converter capable of sufficiently restricting the secondary side output.

  iii) Further, according to the present invention, it is possible to prevent a decrease in the oscillation frequency when the output load is increased without separately adding a winding for preventing a decrease in the oscillation frequency to the transformer. A self-excited flyback converter capable of realizing each function of ii) can be provided.

1 is a circuit diagram showing one embodiment of a self-excited flyback converter according to the present invention. FIG. It is a circuit diagram which shows an example of the self-excitation flyback converter which concerns on a prior art example. It is a circuit diagram which shows another example of the self-excitation flyback converter which concerns on a prior art example. It is a circuit diagram which shows another example of the self-excitation flyback converter which concerns on a prior art example.

  Hereinafter, the self-excited flyback converter according to the present invention will be described in detail with reference to one embodiment.

[Basic configuration]
FIG. 1 shows a circuit diagram of a self-excited flyback converter 100 according to the present embodiment. In addition, regarding the description of the configuration of FIG. 1, the same reference numerals are used for the same portions used in the description of FIGS.

First, the basic configuration and operation of the self-excited flyback converter 100 according to the present embodiment will be described. In the self-excited flyback converter 100 according to the present embodiment, the main switching element 4 is connected in series to the primary winding 2a of the transformer 2. The output of the bias winding 2c of the transformer 2 is fed back to the gate of the main switching element 4 via the self-excited oscillation control circuit 6 to oscillate, and the stored energy in the transformer 2 is turned off when the main switching element 4 is turned off. Is taken out from the secondary winding 2b through the rectifier diode 26 as a direct current output. In this embodiment, an N-channel MOSFET is used as an example of the main switching element 4.
In the self-excited flyback converter 100 according to the present embodiment, the starting resistor 24 is provided between one end of the primary winding 2a and the gate of the main switching element 4 as in the conventional example. .

The oscillation control circuit 6 can take various configurations. The self-excited flyback converter 100 according to this embodiment includes a current limiting resistor 16, a DC blocking capacitor 14, and a gate of the main switching element 4 inserted in series between one end of the bias winding 2 c and the gate of the main switching element 4. Is connected to the ground to control transistor 18. Further, a time constant circuit 8 comprising a resistor 10 and a capacitor 12 connected in series with each other is connected to both ends of the bias winding 2 c so that the voltage of the capacitor 12 is applied to the base of the control transistor 18.
Further, a diode 20 and a photocoupler 22 (more specifically, the phototransistor side) are connected between one end side of the bias winding 2c and the capacitor 12 for constant voltage control of the output voltage V2 on the secondary side. ing.

  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 includes a shunt regulator 30, a voltage dividing resistor 32 that divides the output voltage V 2 and supplies it to the reference voltage terminal of the shunt regulator 30, and the photocoupler 22. V2 is compared with the reference voltage, and the photodiode of the photocoupler 22 is caused to emit light according to the difference, thereby providing feedback to the oscillation control circuit 6 on the primary side.

Further, in this embodiment, an overcurrent protection circuit 74 having the following configuration is provided in the self-excited flyback converter 100 separately.
That is, the overcurrent protection circuit 74 of this embodiment includes the control transistor 18, the current detection resistor 31 having one end connected to the source of the main switching element 4 and the other end connected to the ground, It comprises a base current limiting resistor 33 (of the control transistor 18) connected between the connection point of the switching element 4 and the base of the control transistor 18. The detailed operation of the overcurrent protection circuit 74 will be separately described later.

Here, the basic oscillation operation of the self-excited flyback converter 100 according to the present embodiment, that is, the operation of the self-excited flyback converter 100 when the oscillation frequency lowering prevention circuit 7 described later is not operating will be described. When the input voltage V1 is applied, it is applied to the gate of the main switching element 4 through the starting resistor 24, and the main switching element 4 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 V3 is generated in the bias winding 2c. This is applied to the gate of the main switching element 4 through the current limiting resistor 16 and the DC blocking capacitor 14, and the main switching element 4 is rapidly turned on.
At this time, since 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 through the secondary winding 2b, and energy is accumulated in the transformer 2. At the same time, a charging current flows through the resistor 10 in the capacitor 12 constituting the time constant circuit 8, and the base potential of the control transistor 18 gradually increases.

  When the voltage of the capacitor 12 reaches a predetermined value and the control transistor 18 starts to conduct, the gate of the main switching element 4 is thereby bypassed to the ground and the main switching element 4 cannot be kept on, and the primary winding 2a The voltage decreases, and the voltage V3 of the bias winding 2c also decreases. Since this is a positive feedback, the main switching element 4 is rapidly turned off. When the main switching element 4 is turned off, the energy stored in the transformer 2 is supplied from the secondary winding 2b to the output side through the rectifier diode 26.

  Thereafter, when the stored energy has been released, the residual energy slightly left in the secondary winding 2b generates a voltage V3 in the bias winding 2c, and the main switching element 4 is turned on again, as described above. The operation is repeated.

  Further, the constant voltage control of the output voltage V2 will be described. When the output voltage V2 increases to a specified value or more, the current flowing through the photodiodes of the shunt regulator 30 and the photocoupler 22 increases, and the photocoupler 22 of the photocoupler 22 is correspondingly increased. The current flowing through the transistor also increases, and as a result, the capacitor 12 in the oscillation control circuit 6 is quickly charged, so that the control transistor 18 conducts quickly and bypasses the gate of the main switching element 4 to the ground. As a result, the main switching element 4 is shortened and the output voltage V2 is lowered, and thus the constant voltage control of the output voltage V2 is performed.

The operation of the overcurrent protection circuit 74 of this embodiment will be described. First, the overcurrent protection circuit 74 according to this embodiment is proportional to the increase in the secondary output current I2 of the self-excited flyback converter 100. The current Id flowing through the main switching element 4 is also increased. That is, the amount of the current Id flowing through the main switching element 4 proportional to the amount of the secondary output current I2 is converted into a voltage by the current detection resistor 31, and the obtained voltage signal is transmitted to the control transistor 18 through the base current limiting resistor 33. Applied to the base.
In this embodiment, when the secondary output current I2 is in an overload state, the voltage across the current detection resistor 31 rises to a voltage that turns on the control transistor 18, and the current detection resistor 31 is actually set. Is increased to a voltage that turns on the control transistor 18, the control transistor 18 is turned on, and the gate voltage of the main switching element 4 is extracted. Thereby, the main switching element 4 is turned off, and the self-excited flyback converter 100 is protected against overcurrent (overcurrent protection operation).
In addition, after the secondary output current I2 is out of the overload state, the self-excited flyback converter 100 is not affected by the overcurrent protection circuit 74 and operates according to the basic operation described above. Accordingly, the oscillation frequency lowering prevention circuit 7 described below operates.

[Oscillation frequency drop prevention circuit]
Next, the oscillation frequency reduction preventing circuit 7 will be described. In this embodiment, the oscillation frequency drop prevention circuit 7 includes a diode 37 (corresponding to a “first diode” of the present invention) and a capacitor 39 (“ A drive auxiliary power source 72 comprising a diode 3 and a PNP transistor 5, a connection point between the DC blocking capacitor 14 and the gate of the main switching element 4, and the drive auxiliary power source 72. The switch circuit 17 connected between the connection points of the diode 37 and the capacitor 39, the first end and the second end between one end and the other end of the bias winding 2c, and the base of the PNP transistor 5 of the switch circuit 17 The constant current circuit 71 connected to the third end, the control terminal of the switch circuit 17 (PNP transistor 5) at one end and the constant current circuit 71 The connection point between the third end and the other end consists constant circuit 73 when connected to a connection point between the diode 37 and the capacitor 39 of the drive for the auxiliary power source 72,.
The detailed configuration of each component of the oscillation frequency decrease prevention circuit 7 and the detailed operation of the oscillation frequency decrease prevention circuit 7 will be separately described later.

  In this embodiment, a Zener diode 25 (corresponding to the “first Zener diode” of the present invention) is provided as a constant current circuit operation stop means between the other end of the bias winding 2 c and the constant current circuit 71. It is connected. The Zener diode 25 has an anode connected to the constant current circuit 71 and a cathode connected to the other end of the bias winding 2c. Details of the function of the Zener diode 25 as the constant current circuit operation stopping means will be described later in detail.

  In the drive auxiliary power source 72, the anode of the diode 37 is connected to the connection point between one end of the bias winding 2c and the current limiting resistor 16, and the cathode of the diode 37 is connected to the other end of the bias winding 2c via the capacitor 39. Is configured.

  The switch circuit 17 includes a PNP transistor 5, a diode 3, and a resistor 29. The cathode of the diode 3 is a connection point between the DC blocking capacitor 14 and the gate of the main switching element 4, and the anode of the diode 3 is a resistor 29. The emitter of the PNP transistor 5 is connected to the collector of the PNP transistor 5 between the connection points of the diode 37 and the capacitor 39 of the auxiliary power source 72 for driving.

  The constant current circuit 71 includes a constant current control transistor 19 formed of an NPN transistor, a Zener diode 23 (corresponding to the “second Zener diode” of the present invention), a diode 21 (corresponding to the “second diode” of the present invention), and It comprises a resistor 9 (corresponding to the “second resistor” of the present invention) and a resistor 27 (corresponding to the “third resistor” of the present invention), and the cathode of the diode 21 corresponding to the first end is biased At the connection point between one end of the winding 2c and the current limiting resistor 16, the anode of the diode 21 is connected to the emitter of the constant current control transistor 19 via the resistor 9, and the collector of the constant current control transistor 19 corresponding to the third end. Is connected to the base of the PNP transistor 5 corresponding to the control terminal of the switch circuit 17 and to one end of the base of the constant current control transistor 19. The other end of the resistor 27 (corresponding to the second end) is the anode of the Zener diode 25, the anode of the Zener diode 23 is the connection point between the anode of the diode 21 and the resistor 9, and the cathode of the Zener diode 23 is the constant current. It is configured by being connected to a connection point between the base of the control transistor 19 and the resistor 27.

  In the constant current circuit 71 of the present embodiment in which the constant current transistor 19, the Zener diode 23, the diode 21, and the resistors 9 and 27 are combined as described above, the voltage applied to both ends of the resistor 9 is Since the constant voltage is obtained by subtracting the base-emitter voltage VBE of the constant current transistor 19 from the Zener voltage, assuming that the resistance value of the resistor 9 is constant, the current flowing through the resistor 9 is a constant current. In the constant current circuit 71 of the present embodiment, the constant current operation is performed in this way.

One end of the time constant circuit 73 is a connection point between the base of the PNP transistor 5 of the switch circuit 17 and the collector of the constant current control transistor 19 of the constant current circuit 71, and the other end is a diode 37 and a capacitor 39 of the auxiliary power source 72 for drive. And a capacitor 13 (corresponding to the “first capacitor” of the present invention) connected to the connection point (emitter of the PNP transistor 5) and charged with a constant current by the constant current circuit 71 and charged to the capacitor 13 It is constituted by a parallel connection circuit of a discharge resistor 35 (corresponding to the “first resistor” of the present invention) for discharging the remaining charge.
The time constant circuit 73 controls the switching operation of the switch circuit 17 by sending a signal from one end thereof to the base of the PNP transistor 5 of the switch circuit 17.

That is, in this embodiment, in the oscillation frequency reduction preventing circuit 7, the voltage of the bias winding 2c when the main switching element 4 is off is not directly used, but the transistor 19, the Zener diode 23, the diode 21 and the The constant current circuit 71 is composed of the resistors 9 and 27, and the capacitor 13 of the time constant circuit 73 is charged with a constant current so that it is not affected even if the voltage of the bias winding 2c changes.
However, in the self-excited flyback converter 100 of this embodiment, a constant current circuit operation stop means is separately provided, and is provided separately in the self-excited flyback converter 100 as a result of the output load being overloaded. When the overcurrent protection circuit 74 operates, the constant current circuit operation stop means operates to stop the operation of the constant current circuit 71, and the oscillation frequency lowering prevention circuit 7 is stopped.

  In this embodiment, in order to sufficiently drive the main switching element 4, a drive auxiliary power source 72 is formed by a diode 37 and a capacitor 39 and connected to the emitter of the transistor 5, and the main switching element 4 is prevented from lowering the oscillation frequency. The circuit configuration is such that the drive current for forcibly turning on is directly applied from the transistor 5 to the gate of the main switching element 4 through the resistor 29 and the diode 3.

Further, in this embodiment, a Zener diode 25 is added as a constant current circuit operation stop means between the other end of the bias winding 2 c and the resistor 27 corresponding to the second end of the constant current circuit 71.
As a result, the overload protection circuit 74 provided separately in the self-excited flyback converter 100 operates as a result of the output load being overloaded, and the main switching is performed in proportion to the decrease in the secondary output voltage V2. When the voltage of the bias winding 2c when the element 4 is off falls below the zener voltage of the zener diode 25, the oscillation frequency reduction preventing circuit 7 is configured to stop operating. That is, according to the configuration of the present embodiment, the overcurrent protection circuit 74 and the oscillation frequency reduction prevention circuit 7 separately provided in the self-excited flyback converter 100 do not perform the opposite controls, and the overcurrent The protection circuit 74 can sufficiently reduce the secondary output.

In addition, in the present embodiment, the base threshold voltage of the constant current control transistor 19 is generally equal to the magnitude relationship among the base threshold voltage of the constant current control transistor 19, the Zener voltage of the Zener diode 23, and the Zener voltage of the Zener diode 25. A typical value is 0.5 to 0.6V.
Next, with respect to the Zener voltage of the Zener diodes 23 and 25, a voltage generated when one end side of the bias winding 2c is inverted to minus (the other end side becomes positive), Zener voltage of the Zener diode 23 + Zener diode 25 Zener voltage + voltage dropped at diode 21 + sum of voltage across resistor 27 must be equal to each other.
Therefore, the Zener voltage of the Zener diode 23 + the Zener voltage of the Zener diode 25 is a value smaller than the voltage generated when one end of the bias winding 2c is inverted to minus.
Here, the Zener voltage of the Zener diode 25 is such that the overload protection circuit 74 of the self-excited flyback converter 100 according to this embodiment operates as a result of the output load being overloaded, and one end side of the bias winding 2c is When the voltage generated when the voltage is inverted to minus starts to decrease, the voltage at which the constant current transistor 19 is turned off and the constant current circuit 71 is determined to be turned off when the voltage starts decreasing. Can be designed according to the case.
On the other hand, the Zener voltage of the Zener diode 23 is a voltage for determining the constant current value in the constant current circuit 71, and is a value considerably lower than the Zener voltage of the Zener diode 25.

[Operation]
Hereinafter, a detailed operation of the self-excited flyback converter 100 when the oscillation frequency reduction preventing circuit 7 having the above configuration is in an operating state will be described.
First, when the main switching element 4 is turned on, a current flows through the primary winding 2a of the transformer 2, and a positive voltage is generated on one end side of the bias winding 2c. As a result, the bias winding 2 c drives the main switching element 4 from one end side thereof so that the main switching element 4 is maintained in the ON state through the current limiting resistor 16 and the DC blocking capacitor 14.
At this time, the capacitor 39 is charged through the diode 37 from one end of the bias winding 2c.

Thereafter, when the secondary output voltage V2 detected by the shunt regulator 30 of the output voltage detection circuit 28 reaches a predetermined voltage, a current flows through the light emitting diode of the photocoupler 22.
Then, since the phototransistor of the photocoupler 22 is turned on, a voltage is applied to the base of the transistor 18 from one end side of the bias winding 2c through the diode 20 and the phototransistor of the photocoupler 22, and finally the transistor 18 is turned on. Become.
When the transistor 18 is turned on, the gate voltage of the main switching element 4 is extracted by the transistor 18, and thereby the main switching element 4 is turned off.

When the main switching element 4 is turned off, one end of the bias winding 2c is inverted to minus (the other end becomes plus).
Here, when the voltage of the bias winding 2c exceeds the Zener voltage of the Zener diode 25, a current flows through the resistor 27 to the base of the Zener diode 23 and the constant current control transistor 19, and the constant current control transistor 19 is turned on. When the constant current control transistor 19 is turned on, the charge charged in the capacitor 39 flows to the constant current transistor 19 at a constant current through the discharge resistor 35 and the capacitor 13 constituting the time constant circuit 73. At this time, the capacitor 13 is charged with a constant current, and when the voltage across the capacitor 13 reaches the base-emitter voltage necessary for turning on the PNP transistor 5 of the switch circuit 17, the PNP transistor 5 is turned on. Thus, the electric charge charged in the capacitor 39 is applied to the gate of the main switching element 4 through the transistor 5, the resistor 29 and the diode 3, and the main switching element 4 is forcibly turned on. Thereafter, the on / off operation of the main switching element 4 is repeatedly performed.

  Next, the operation of the constant current circuit operation stopping means will be described in order. In this embodiment, as a result of the output load being overloaded, the overcurrent protection circuit 74 provided separately in the self-excited flyback converter 100 is operated, and the main output is proportional to the decrease of the secondary output voltage V2. When the voltage of the bias winding 2c when the switching element 4 is OFF falls below the Zener voltage of the Zener diode 25, the oscillation frequency drop prevention circuit 7 is configured to stop operating. That is, according to the configuration of the present embodiment, the overcurrent protection circuit 74 and the oscillation frequency reduction prevention circuit 7 separately provided in the self-excited flyback converter 100 do not perform the opposite controls, and the overcurrent The protection circuit 74 can sufficiently reduce the secondary output.

As described above, in this embodiment, the oscillation frequency reduction preventing circuit 7 is provided with the constant current circuit 71. Even if the voltage of the bias winding 2c changes by charging the capacitor 13 with a constant current. The operation of the oscillation frequency lowering prevention circuit 7 is configured so as not to affect the operation.
Therefore, according to this embodiment, even if the output load further increases after the oscillation frequency decrease prevention circuit 7 operates (the output load increases), the oscillation frequency is lower than the oscillation frequency when the decrease in the oscillation frequency stops. Therefore, it is possible to provide a self-excited flyback converter in which increase in main switching loss due to increase in oscillation frequency during heavy load is suppressed.

In this embodiment, a Zener diode 25 is connected between the other end of the bias winding 2 c and the resistor 27 corresponding to the second end of the constant current circuit 71 as a constant current circuit operation stop means.
Therefore, according to the present embodiment, when the overload protection circuit 74 separately provided in the self-excited flyback converter 100 is operated as a result of the output load being overloaded, the oscillation frequency reduction preventing circuit 7 is operated. It is possible to provide a self-excited flyback converter capable of stopping the operation and sufficiently narrowing the secondary side output.

  Furthermore, in the present embodiment, it is possible to prevent a decrease in the oscillation frequency when the output load is increased without additionally adding a winding for preventing a decrease in the oscillation frequency to the transformer 2, and at the time of the heavy load. It is possible to provide a self-excited flyback converter capable of realizing the functions of suppressing an increase in switching loss and stopping the operation of the oscillation frequency reduction preventing circuit 7 when the overcurrent protection circuit 74 is operated.

[Modification]
The self-excited flyback converter according to the present invention has been described based on one embodiment. However, the present invention is not limited to the configuration described in the above-described embodiment, and various modifications can be made.

  For example, in this embodiment, the overcurrent protection circuit 74 is configured by using the control transistor 18 for extracting the gate voltage of the main switching element 4, and the configuration of the overcurrent protection circuit is the configuration described in this embodiment. For example, although the number of parts is increased, an overcurrent protection transistor is additionally provided in parallel with the control transistor 18 and the base current limiting resistor 33 is connected to the base of the overcurrent protection transistor. I do not care. Even with such a configuration, an operation similar to that of the overcurrent protection circuit of the present embodiment using the control transistor 18 can be obtained.

  As described above, the present invention can suppress i) an increase in switching loss due to an increase in oscillation frequency under heavy load, and ii) operate an oscillation frequency decrease prevention circuit when an overcurrent protection circuit operates. It is clear that it is new and useful to provide a self-excited flyback converter that can be turned off and not affect the operation of the overcurrent protection circuit.

2 Transformer 2a Primary Winding 2b Secondary Winding 2c Bias Winding 3 Diode 4 Main Switching Element 5 Transistor 6 Oscillation Control Circuit 7 Oscillation Frequency Reduction Prevention Circuit 8 Time Constant Circuit 9 Resistance (Second Resistance)
10 resistor 12 capacitor 13 capacitor (first capacitor)
14 DC blocking capacitor 16 Current limiting resistor 17 Switch circuit 18 Control transistor 19 Constant current control transistor 20 Diode 21 Diode (second diode)
22 Photocoupler 23 Zener diode (second Zener diode)
24 Starting resistor 25 Zener diode (first Zener diode)
26 Rectifier diode 27 Resistance (third resistance)
28 Output Voltage Detection Circuit 29 Resistor 30 Shunt Regulator 31 Current Detection Resistor 32 Voltage Dividing Resistor 33 Base Current Limiting Resistor 35 Discharge Resistor (First Resistor)
37 diode (first diode)
39 Capacitor (second capacitor)
71 Constant current circuit 72 Auxiliary power supply for drive 73 Time constant circuit 74 Overcurrent protection circuit 100, 100 ′, 100 ″, 100 ′ ″ Self-excited flyback converter

Claims (6)

A transformer having a primary winding, a secondary winding and a bias winding;
A main switching element connected in series with the primary winding and driven by a voltage induced in the bias winding;
An oscillation control circuit for controlling the switching of the main switching element by feeding back the output from the secondary winding;
A switch circuit connected to the main switching element and turning on the main switching element from an off state;
A time constant circuit including a first capacitor and a first resistor, wherein the first capacitor is set to a threshold voltage or higher by using an output voltage from the bias winding when the main switching element is in an off state; An oscillation frequency lowering prevention circuit that forcibly turns on the switch circuit by charging, and
The self-excited flyback converter, wherein the oscillation frequency reduction preventing circuit further includes a constant current circuit that charges the first capacitor with a constant current when the main switching element is turned off. The self-excited flyback converter according to claim 1, wherein one end of the bias winding is connected to a control terminal of the main switching element via a current limiting resistor and a DC blocking capacitor.
A drive auxiliary power source comprising a first diode and a second capacitor connected in series between one end and the other end of the bias winding;
The switch circuit is connected between a connection point between the DC blocking capacitor and the control terminal of the main switching element and a connection point between the first diode and the second capacitor of the drive auxiliary power supply,
When the switch circuit is turned on, the electric charge charged in the second capacitor is sent to the switch circuit, and a drive current for turning on the main switching element is supplied to the control terminal of the main switching element. Features a self-excited flyback converter.   The constant current circuit operation stop means for stopping the charging operation of the first capacitor by the constant current circuit is further provided between the constant current circuit and the other end of the bias winding. Self-excited flyback converter. A first Zener diode having a cathode connected to the other end of the bias winding and an anode connected to the constant current circuit functions as the constant current circuit operation stop means;
The self-excited flyback converter according to claim 3, wherein the first Zener diode is turned on to stop the charging operation of the first capacitor by the constant current circuit. The constant current circuit has three terminals, a first end connected to one end of the bias winding, a second end connected to an anode of the first Zener diode, and a control terminal of the switch circuit. 3 ends connected,
The time constant circuit includes a parallel circuit in which the first capacitor and the first resistor are connected in parallel. One end of the parallel circuit is a control terminal of the switch circuit and a third of the constant current circuit. 5. The self-excited flyback according to claim 4, wherein the other end is connected to a connection point between the first diode and the second capacitor of the auxiliary power source for driving, at a connection point with the end. converter. The switch circuit includes a PNP transistor,
The constant current circuit includes a constant current control transistor composed of an NPN transistor, a second Zener diode, a second diode, and a second resistor and a third resistor.
The cathode of the second diode of the constant current circuit serving as the first end is at a connection point between one end of the bias winding and the current limiting resistor, and the anode of the second diode is the second resistor. Via the third resistor, the emitter of the constant current control transistor, the collector of the constant current control transistor to the base of the PNP transistor of the switch circuit, and the base of the constant current control transistor to the second end. To the anode of the first Zener diode,
The anode of the second Zener diode is connected to the connection point between the anode of the second diode and the second resistor of the constant current circuit, and the cathode of the second Zener diode is connected to the base of the constant current control transistor. 4. The self-excited flyback converter according to claim 3, wherein the self-excited flyback converter is connected to a connection point with the third resistor.

Images