JPH0357710B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a DC-DC converter (DC-DC converter) used in a stabilized DC power supply.

[Conventional technology and its problems]

A DC- DC converters are already known. By the way, the conventional typical on-off type DC-DC converter (FCC) is
It is configured as a separately excited type that includes an oscillator to create a pulse width modulated wave (PWM wave), which controls the on/off of the conversion switching transistor.
Therefore, the circuit has become complex and expensive. As a solution to this kind of drawback, Japanese Patent Publication No. 18364/1983 discloses a self-excited on/off type DC-DC converter in which a control winding is provided in the transformer and the voltage is adjusted by controlling the control winding. has been done. However, in this method, a control winding must be provided, making it difficult to significantly downsize and simplify the device.

As a DC-DC converter with a simple circuit configuration,
There is an on-off type self-excited DC-DC converter, but it has the disadvantage that the oscillation frequency increases at light loads, resulting in a large number of switching operations and a large switching loss. Another disadvantage is that it is difficult to use field effect transistors as switching elements.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a DC-DC converter that can self-oscillate with a relatively simple circuit and easily achieve voltage control.

[Means for solving problems]

In order to solve the above problems and achieve the above objects, the present invention turns on and off the voltage supplied from a DC power supply, and turns on when the voltage of the control element reaches a threshold voltage. A conversion switch configured as shown in FIG. A transformer having a switch driving winding connected to the control terminal to turn on the voltage converting switch; a rectifying and smoothing circuit connected to the output side of the voltage converting winding; a first capacitor connected to the conversion switch so as to drive the conversion switch; a first charging circuit for charging the first capacitor; and a first charging circuit for determining when the conversion switch starts to turn off. 2 capacitor, and the second capacitor in response to the start of turning on the conversion switch.
a second charging circuit that starts charging the capacitor; and a second charging circuit that is connected to the conversion switch and the first and second capacitors, and that starts charging the second capacitor in response to the second capacitor being charged to a predetermined value. a control circuit that cuts off the on-drive of the conversion switch by the voltage of the winding, turns the conversion switch into an OFF state, and puts the first and second capacitors in a discharge state; and the first capacitor. a DC-DC voltage control circuit that controls the output voltage of the rectifying and smoothing circuit by changing the time width until the voltage of the first capacitor reaches the threshold voltage by changing the charging current of the first capacitor. It is related to converters.

The conversion switch in the above invention is, for example, an element such as the FET 5 in the embodiment, and the voltage conversion winding is a device for voltage conversion, such as the primary winding 3 and the secondary winding 6 in the embodiment. The related windings, the switch drive winding being, for example, the tertiary winding 14 of the embodiment, the control circuit being, for example, a circuit including the control transistor 19 of the embodiment, and the voltage control circuit being, for example, the phototransistor 25 This is a circuit that includes

[Effect]

In the above invention, when the first capacitor is charged to a predetermined value, the off period of the conversion switch ends and the on period begins. Also, when the second capacitor is charged to a predetermined value, the on period ends;
Entering an off period. Therefore, the conversion switch operates on and off based on the charging and discharging of the first and second capacitors. Changing the charging current of the first capacitor changes the off time width of the conversion switch.

〔Example〕

Next, a DC-DC converter according to an embodiment of the present invention will be explained with reference to FIGS. 1 and 2. In Figure 1, one DC power supply terminal 1 has a transformer 2.
One end of the primary winding 3 of is connected. Between the other end of the primary winding 3 and the other DC power supply terminal 4, an N-channel insulated gate type (MOS type) field effect transistor is connected as a conversion switching element.
FET5 is connected. FET5 has a drain as the first terminal, a source as the second terminal, and a gate as the control terminal. When a voltage higher than the threshold voltage V _TH is applied to the gate, the drain and source are in a conductive state (on state). ).

The secondary winding 6 electromagnetically coupled to the primary winding 3 has
A rectifying and smoothing circuit 11 consisting of two diodes 7 and 8, a reactor 9, and a capacitor 10 is connected. Output terminals 12 and 13 connected to the rectifying and smoothing circuit 11 are parts that supply a stabilized output voltage to a load.

14 is the tertiary winding of the transformer, and the primary winding 3
and is electromagnetically coupled to the secondary winding 6. One end of the tertiary winding 14 is connected to the gate of the FET 5 via the first capacitor 15, and the other end is connected to the source. Two resistors 16a are connected between one power supply terminal 1 and the gate as a first charging circuit.
16b is connected. These two resistors 16
a and 16b are provided to obtain the charging time constant of the first capacitor 15.

The on-time end control circuit according to the present invention has a second
capacitor 17 and a second capacitor 17 as a second charging circuit.
and a resistor 18. A second capacitor 17 is connected in parallel to the tertiary winding 14 via a second resistor 18 for determining this charging time constant. Note that a diode 17a is connected in parallel to the capacitor 17 to keep this reverse charging voltage constant.
is connected.

The control transistor 19 is connected between the gate and source of the FET 5 via a reverse blocking diode _D1 , and is connected to the voltage V _C2 of the second capacitor 17.
controlled by. That is, the second capacitor 17 is connected between the base and emitter of the control transistor 19, and when the voltage of the second capacitor 17 exceeds a predetermined value, the control transistor 1
9 is configured to be turned on.

As a constant voltage control circuit, voltage detection resistors 20 and 21 are connected between output terminals 12 and 13, one input terminal is connected to a voltage dividing point of these resistors 20 and 21, and the other input terminal is connected to a reference voltage. An error amplifier 23 connected to the source 22, a light emitting diode 24 connected to the output terminal of the error amplifier 23, and a phototransistor 25 optically coupled to the light emitting diode 24 are provided. The phototransistor 25 is provided as a variable impedance element for changing the charging time constant of the first capacitor 15, and is connected in parallel to the resistor 16b and the first capacitor 15.

In order to reset the residual magnetism of the transformer 2 by the flyback voltage, a resistor 27 is connected in parallel to the primary winding 3 via a diode 26. Further, a capacitor 28 is connected in parallel to the resistor 27 in order to suppress flyback voltage.

(Operation) When DC voltage is supplied from the pair of power supply terminals 1 and 4, the resistors 16a and 16b and the first capacitor 1
5. The circuit consisting of the tertiary winding 14 starts charging the first capacitor 15. The first capacitor 15 is charged according to a charging time constant determined by the capacitance _C1 , the values of the resistors 16a and 16b, and the impedance of the phototransistor 25,
This voltage V _C1 between both ends gradually increases as shown from t ₁ to _{t 4} in FIG. 2E. FET5 is the threshold voltage
Since it has V _TH (rise voltage), it does not turn on immediately in synchronization with power-on, but turns on at time t ₃ when the first capacitor 15 is charged to the threshold voltage V _TH or higher. When FET5 turns on,
As shown in FIG. 2C, the drain-source voltage V _DS of the FET 5 becomes low, and conversely, since the power supply voltage is applied to the primary winding 3, the voltage increases across the tertiary winding 14 as shown in FIG. 2D. V ₃ is generated, and this voltage V ₃ of the tertiary winding 14 becomes a positive feedback voltage and is applied to the gate of the FET 5. The voltage at the power supply terminal 1 is the voltage at the tertiary winding 14.
Since the voltage of V is set higher than ₃ , the FET
5 is on during the period t ₃ to _{t 4} as well, and the sum of the capacitor voltage V _C1 and the tertiary winding voltage V ₃ becomes the gate voltage V _G. Note that a current flows through the source-gate capacitance of the FET 5 based on the positive voltage obtained in the tertiary winding 14. As a result, the positive voltage on the tertiary winding 14 allows the capacitor 15 to discharge and then charge the capacitor 15 to the opposite polarity. When the FET 5 is turned on, a drain current _ID flows as shown in FIG. 2B, and a voltage that turns on the diode 7 is generated in the secondary winding 6 of the transformer, and this is smoothed to become an output voltage.

From the time t ₃ when FET 5 turns on and a positive voltage is generated across the tertiary winding 14, the second capacitor 1
7 starts charging in the positive direction, and this charging voltage V _C2 gradually increases as shown in FIG. 2F. When the capacitor voltage V _C2 becomes equal to or higher than the rising voltage of the control transistor 19, the transistor 19 is turned on, the gate and source are short-circuited, the gate voltage V _G becomes zero, and the FET 5 is turned off.
While the transistor 19 is kept on due to the storage time, the charge in the first capacitor 15 is discharged by the circuit of the diode D ₁ , the transistor 19, and the tertiary winding 14, and
7 charges are also released.

Next, the operation after startup will be explained. At t ₁ in Figure 2
When FET5 turns off, the flyback voltage
V _F is generated, and the tertiary winding voltage is as shown in Figure 2D.
The direction of V ₃ is reversed. The reverse voltage of the tertiary winding 14 generated between t ₁ and _{t 2} is blocked by the transistor 19 and the diode D ₁ , but slightly charges the first capacitor 15 through the source-gate capacitance. The first capacitor 15 is a resistor 16a, 16b
is charged with the power supply voltage through the capacitor voltage
V _C1 gradually increases as shown in Figure 2 E, as at the time of startup, but the gate voltage V _G is the capacitor voltage
Since it is the sum of V _C1 and _the tertiary winding voltage V ₃ , _it changes as shown _{in FIG .} The voltage becomes the gate voltage V _G. When the reset of the transformer 2 is completed at t ₂ , the flyback voltage V _F also disappears, so that the capacitor voltage V _C1 becomes the gate voltage V _G. The charging of the first capacitor 15 further progresses,
When the threshold voltage _VTH of FET5 is reached at _t3 , FET5 is turned on. The on period from _t3 to _t4 is
It is determined by the R ₂ C ₂ time constant circuit consisting of the value R ₂ of the resistor 18 and the value C ₂ of the second capacitor 17, as in the case of startup.

The output voltage is controlled by controlling the resistance value of the phototransistor 25. Now, if the output voltage becomes lower than a certain value, the output of the error amplifier 23 becomes low, the amount of light from the light emitting diode 24 decreases, and the resistance value of the phototransistor 25 increases.
Therefore, the current in the bypass circuit formed by the phototransistor 25 connected in parallel to the resistor 16b and the capacitor 15 decreases, and conversely, the charging current of the first capacitor 15 increases. For this reason, Figure 2 E
As shown by the dotted line, the voltage of the first capacitor 15 is
The time it takes for V _C1 to reach the threshold voltage V _TH of FET5 becomes shorter. As a result, the off time T _OFF
becomes shorter than t ₁ to _{t 3} . Since the on-time T _ON is the same as t ₃ to _{t 4} and is constant regardless of the output voltage, the on-off period is shortened and the duty ratio is increased, so that the output voltage is increased and returned to a predetermined value. When the output voltage is higher than the reference value, the operation is opposite to the above.

As is clear from the above, since this circuit is not a self-oscillating circuit that utilizes the saturation of the switching transistor, the switching period does not become shorter when the load is light, but rather becomes longer. Therefore, the number of times of switching during light loads is reduced, and switching loss is reduced. Furthermore, although it is an on-off type, self-sustained oscillation is possible. Furthermore, the number of times configuration can be simplified and the size can be reduced.

[Another example]

Next, another embodiment of the present invention shown in FIGS. 3 and 4 will be described. However, parts common to FIGS. 1 and 2 are designated by the same reference numerals and their explanations will be omitted. In this embodiment, a resistor 29 is connected in parallel to the second capacitor 17 instead of the reverse bias diode _D2 .

Furthermore, a transistor 30 is connected in parallel to the first capacitor 15 via a diode D ₃ , and the base of this transistor 30 is connected via a resistor 31 to the lower end of the tertiary winding 14 . Therefore,
A flyback voltage occurs during the period t ₁ to t ₂ in Fig. 4, and the reverse voltage is applied to the tertiary winding 1 as shown in Fig. 4D.
4, the transistor 30 is turned on during this period from _t1 to _t2 , and the charge in the first capacitor 15 is discharged. Therefore, as shown in Figure 4E,
Charging of the first capacitor 15 begins at time _t2 after the reset of the transformer 2. This results in
It is possible to completely prevent the gate voltage V _G from reaching the threshold voltage V _TH before the transformer 2 is reset. Furthermore, when the reset of the transformer 2 is completed, vibration (not shown) occurs in the primary winding 3 of the transformer 2, but since the first capacitor 15 is in an uncharged state, there is a risk that the FET 5 may be accidentally turned on. There is no.

At time _t2 , the transistor 30 is turned off, so charging of the first capacitor 15 starts from this time, and at time _t3 , the FET 5 is turned on.

32 is an overcurrent detection resistor, which is connected in series to the source of FET5. transistor 19
The emitter of is not directly connected to the source but is connected to the resistor 3.
It is connected to the left end of 2. Therefore, the voltage V _C2 of the second capacitor 17 and the voltage V _O across the resistor 32
is applied between the base and emitter of transistor 19. Therefore, when an overcurrent flows, the transistor 19 is turned on while the charging voltage V _C2 of the second capacitor 17 is low, and the FET 5
is turned off, and the on period t ₃ to t ₄ of the FET 5 becomes shorter.

[Modified example]

The present invention is not limited to the embodiments described above, and the following modifications are possible, for example.

(a) The circuits shown in FIGS. 1 and 3 may be modified into the circuit shown in FIG. 5. That is, the first capacitor 1
5 may be connected to the lower end of the tertiary winding 14. Also, the backflow blocking diode _D1 may be moved to the gate line. Further, a phototransistor 25 may be connected in parallel to the resistor 16a. Alternatively, a quaternary winding 14a may be provided, and the resistor 18 and capacitor 17 may be connected thereto.

(b) FET5 can be replaced with a bipolar transistor. In addition, phototransistor 2
Connect a bipolar transistor in place of 5,
This may be controlled by the output of the error amplifier 23. Also, transistor 19 is FET
It can be done.

(c) The conversion switching element may be composed of a plurality of switching elements in which FETs or bipolar transistors are connected in series or in parallel.

(d) The upper end of the resistor 16a may be connected to the lower end of the primary winding 3.

(e) The resistors 16a and 16b may be omitted and a phototransistor 25 may be connected thereto, and this phototransistor 25 may be used as the first charging circuit of the present invention.

〔Effect of the invention〕

According to the above invention, a self-excited DC-DC converter in which the on-width of the switch is substantially fixed and the off-width is variable can be obtained with a simple circuit configuration.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing a DC-DC converter according to an embodiment of the present invention, Fig. 2 is a waveform diagram of each part of Fig. 1, and Fig. 3 shows a DC-DC converter of another embodiment. Circuit diagram, Figure 4 is a waveform diagram of each part of Figure 3,
FIG. 5 is a circuit diagram showing a modified DC-DC converter. 1...Power terminal, 3...Primary winding, 5...
FET, 6... Secondary winding, 11... Rectifier smoothing circuit,
14... Tertiary winding, 15... First capacitor,
16a, 16b...Resistor, 17...Second capacitor, 18...Resistor, 19...Control transistor, 25...Phototransistor.

Claims (1)

[Scope of Claims] 1. A conversion switch that turns on and off a voltage supplied from a DC power source and is configured to be turned on when the voltage at a control terminal reaches a threshold voltage; A voltage conversion winding for obtaining an output voltage corresponding to on/off of the conversion switch; and a voltage conversion winding that is electromagnetically coupled to the voltage conversion winding and connected to the control terminal to turn on the conversion switch. a rectifying and smoothing circuit connected to the output side of the voltage converting winding; and a rectifying and smoothing circuit connected in series with the driving winding so as to turn on the converting switch by a charging voltage. a connected first capacitor; a first charging circuit for charging the first capacitor; a second capacitor for determining when the conversion switch starts to turn off; a second charging circuit that starts charging the second capacitor in response to turning on; a second charging circuit that is connected to the conversion switch and the first and second capacitors, and that charges the second capacitor to a predetermined value; In response to this, the conversion switch is turned off by cutting off the on-drive of the conversion switch by the voltage of the drive winding,
and a control circuit for bringing the first and second capacitors into a discharge state; and a time period for the voltage of the first capacitor to reach the threshold voltage by changing the charging current of the first capacitor. and a voltage control circuit that controls the output voltage of the rectifying and smoothing circuit by changing the width.