JPH0254029B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a self-excited DC-DC converter, particularly a forward type self-excited DC-DC converter that can obtain a predetermined DC output by controlling the off period of a switching element.
Related to DC-DC converters.

Prior Art A conventional DC-DC converter will be explained with reference to FIG. This DC-DC converter has an oscillator 19
This is a so-called separately excited switching regulator. The oscillation output of the oscillator 19 is applied to a pulse width modulator 20, and the output of this pulse width modulator 20 is applied to one of the drive transformers 15 through an amplifier 21.
It is sent to the next winding 22. Therefore, drive transformer 1
When an excitation current is supplied to the primary winding 22 of the transistor 5, the secondary winding 16 receives a current that forward biases the transistor 5 as a switching element through a capacitor 17 and a diode 18 that are connected in parallel with each other. flows. Therefore, the transistor 5 is turned on, and current flows from the positive terminal 1 of the DC power source to the negative terminal 2 through the primary winding 4 of the output transformer 3 and the transistor 5. When current flows through the primary winding 4, a voltage is induced in the secondary winding 6, and the current is supplied to the load 11 through the rectifying diode 7, the smoothing choke 8, and the smoothing capacitor 10.

When pulse width modulator 20 produces no output,
Since no current is generated in the secondary winding 16, the transistor 5 is turned off. The excitation energy stored in the output transformer 3 during the on period of the transistor 5 is:
The flyback voltage absorption circuit 12 absorbs the voltage during the off period of the transistor 5. The flyback voltage absorption circuit 12 has a diode 13 connected in series and an impedance circuit 14 connected in parallel with the primary winding 4. The impedance circuit 14 is composed of, for example, a resistor and a capacitor connected in parallel.
The flyback voltage absorption circuit 12 absorbs the excitation energy as a current flowing from the primary winding 4 through the diode 13 and the impedance circuit 14 to the primary winding 4.

The output voltage applied to the load 11 is determined by a voltage divider 23 consisting of resistors 24 and 25 connected in series.
and is applied to one input terminal of the error amplifier 27. The reference voltage of the power supply 26 is applied to the other input terminal of the error amplifier 27. The output of the error amplifier 27 is sent to the pulse width modulator 20, which generates an output with a narrow pulse width when the voltage applied to the load 11 is high; When the voltage is low,
Generates output with wide pulse width. Conventional DC-DC
In the converter, the voltage applied to the load 11 is adjusted to be constant as described above.

FIG _. 6 shows the _operating waveforms of each part of the circuit shown in FIG. The emitter voltage V _CE ,d is the voltage V ₆ applied to the secondary winding 6 of the output transformer 3. In the above waveform c,
The flyback voltage generated in the primary winding 4 of the output transformer 3 during the off period of the transistor 5 is denoted by Vf.

Problems to be Solved by the Invention The conventional converter described above has a built-in oscillator 19, and outputs the output of the oscillator 19 to a pulse width modulator 20.
is modulated by the drive transformer 1 through the amplifier 21.
This is a separately excited converter having a circuit that sends power to the primary winding 22 of No. 5. As described above, separately excited converters have disadvantages in that the number of components increases and that an auxiliary power source is required to drive the switching elements. Therefore, in order to eliminate these drawbacks, a control winding is added to the output transformer as described in Japanese Patent Publication No. 52-18364, and while the on-time of the switching element is kept constant, the control winding is A converter has been proposed in which the off-time of the switching element is varied by controlling the current flowing through the switching element, thereby obtaining a constant output voltage. However, this converter required an additional control winding and therefore took a long time to manufacture.

SUMMARY OF THE INVENTION An object of the present invention is to provide a self-excited DC-DC converter that eliminates the above drawbacks, has a simple circuit configuration, and can be easily manufactured.

Another object of the present invention is to provide a self-excited DC-DC converter that has a simple circuit configuration, can be easily manufactured, and can protect switching elements from overcurrent.

Means for Solving the Problems In the self-excited DC-DC converter of the present invention, a switching element is connected in series with the primary winding of an output transformer, and a pulse width is controlled by a drive circuit that supplies a drive signal to the switching element. In addition to providing a setting circuit,
A flyback voltage absorption circuit is connected in parallel with the primary winding, and the impedance of the flyback voltage absorption circuit is changed in response to the DC output of the secondary winding of the output transformer.

Further, in another self-commutated DC-DC converter of the present invention, a flyback voltage absorption circuit is connected in parallel with the primary winding of the output transformer, and an overcurrent detection means is connected in series with the primary winding. , characterized in that the impedance of the flyback voltage absorption circuit is changed in accordance with the magnitude of the DC output obtained from the secondary winding of the output transformer and the magnitude of the signal obtained from the overcurrent detection means. .

Function: In a self-excited DC-DC converter, the impedance of the flyback voltage absorption circuit is changed in response to the DC output obtained from the secondary winding of the output transformer. This changes the flyback voltage, changes the off period of the switching element, and stabilizes the DC output. Also, when overcurrent occurs,
The signal detected by the overcurrent detection means is also input to the flyback voltage absorption circuit. For this reason,
The flyback voltage is reduced, the off-time of the switching element is increased, and the magnitude of the output current is limited.

Embodiments Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 4. Among the parts shown in these figures, the same parts as in FIG. 5 are given the same reference numerals, and the explanation thereof will be omitted.

As shown in FIG. 1, the forward type self-excited DC-DC converter of the present invention includes a drive circuit 30 that drives a transistor 5 and a pulse width setting circuit 3.
1 is provided. The drive circuit 30 includes an output transformer 3
It is composed of a tertiary winding 29, a resistor 33, a capacitor 17 and a diode 18 connected in parallel. Further, the pulse width setting circuit 31 connects the tertiary winding 29
a resistor 34 connected in parallel and in series with
A CR timer circuit 36 has a base connected between the resistor 34 and the converter 35, a collector connected to the base of the transistor 5, and an emitter connected to the emitter of the transistor 5, that is, the tertiary winding 29. It consists of a transistor 37 connected to the other end and the negative terminal 2 of the DC power supply. In addition, in the converter of FIG. 1, the error amplifier 2
An LED 38 is connected to the output terminal of 7 as a light emitting element, and the LED 38 constitutes a photocoupler with a light receiving transistor 39 as a light receiving element provided in the flyback voltage absorption circuit 12. The flyback voltage absorption circuit 12 includes a reverse current blocking diode 1.
3 and a variable impedance circuit 40,
It is connected in parallel with the primary winding 4 of the output transformer 3.
The variable impedance circuit 40, as shown as an example in FIG. It has an emitter connected to one end of the line 4. Further, in the converter shown in FIG. 1, a starting resistor 28 is connected between the base of the transistor 5 and the positive terminal 1 of the DC power supply.

In the converter shown in FIG. 1, when the power is turned on, current flows through the starting resistor 28 to the base of the transistor 5, and the transistor 5 is turned on. Therefore, current flows from the positive terminal 1 to the negative terminal 2 through the primary winding 4 and the collector-emitter of the transistor 5. Therefore, output transformer 3 of 2
A voltage that forward biases the diode 7 is induced in the next winding 6, and this voltage supplies a current through the diode 7, the smoothing circuit 8, and the capacitor 10. Further, at this time, a voltage that biases the transistor 5 in the forward direction is also induced in the tertiary winding 29, and the transistor 5 is maintained in a forward biased state. The primary winding 4 works as an inductance coil, so the transistor 5
The collector current of increases with time.

The current in the tertiary winding 29 is also supplied to the CR timer circuit 36 and the capacitor 35 is charged through the resistor 34. When capacitor 35 is charged to a level above the threshold voltage of the base of transistor 5, transistor 5 is turned on and the base of transistor 5 goes low. Transistor 5 is therefore turned off and no current flows through primary winding 4 and transistor 5. When transistor 5 is turned off, the excitation energy stored in output transformer 3 creates a flyback voltage in primary winding 4. This flyback voltage is absorbed by the current flowing through the flyback voltage absorption circuit 12, and at this time, the capacitor 35
The electric charges charged in the tertiary winding 29 are discharged through the resistor 34 and the tertiary winding 29 by the flyback voltage.

When excitation energy is released by the current flowing through the flyback voltage absorption circuit 12, the tertiary winding 2
A small kick voltage is generated at transistor 9, and transistor 5
is turned on again and the above operation is repeated.

Hereinafter, it will be explained that the output voltage can be adjusted to a constant voltage by varying the flyback voltage. The amount of light emitted by the light emitting diode 38 is determined by the output of the error amplifier 27, and the error amplifier 27 generates a high level output when the output voltage to the load 11 is high, and generates a high level output when the output voltage to the load 11 is low. , generates a low level output. Therefore, when the output voltage to the load 11 is high, the amount of light emitted from the light emitting diode 38 increases and the light receiving transistor 3
9's emitter current increases. As a result, the base current of transistor 42 increases, the emitter-collector impedance of transistor 42 decreases, and the flyback voltage decreases. Therefore, the period during which the excitation energy is held in the output transformer 3 becomes longer, and the off-period of the transistor 5 becomes longer. Conversely, when the output voltage to the load 11 is low, the emitter of the transistor 42
The impedance between the collectors increases and the flyback voltage increases. Therefore, the period during which the excitation energy is retained within the output transformer 3 becomes shorter,
The off period of transistor 5 becomes shorter. In this way, the impedance of the flyback voltage absorption circuit 40 is determined by the output of the error amplifier 27.
It will be appreciated that the emitter-collector flyback voltage level of transistor 42 can be varied in response to the magnitude of the DC output to load 11.

FIG. 2 shows the operating waveforms of each part shown in FIG. 1, where a is the base-emitter voltage V _BE of the transistor 5, b is the collector current l _C of the transistor 5,
c is the collector-emitter voltage of transistor 5
V _CE , d is the voltage V ₆ applied to the secondary winding 6 of the output transformer 3. As is clear from FIG. 2a, the on time of the transistor 5 is determined by the pulse width setting circuit 3.
1, it is almost constant. That is, transistor 5
The off point is determined by the time constant of the CR timer circuit 36, and the capacitor 35 is connected to the tertiary winding 2
It is charged by the voltage induced in 9.

FIG. 3 shows a second embodiment of the invention. In this embodiment, a voltage across a resistor 44 serving as overcurrent detection means is inputted to a variable impedance circuit 40 between the positive side terminal 1 of the DC power supply and one end of the primary winding 4. A specific example of this is shown in Figure 4b.
-C. In FIG. 4b, transistor 4
The base of the resistor 2 is connected to one end of the resistor 44 via the resistor 45, and the emitter thereof is connected to the other end of the resistor 44. Further, a capacitor 46 is connected between the collector and emitter of the transistor 42.

The operation of this circuit is similar to that shown in FIG. 4a in that transistors 39 and 42 are controlled in accordance with the output voltage, and the output voltage is controlled to a constant voltage.

By the way, when an overcurrent flows through the resistor 44,
When the voltage applied to the resistor 44 exceeds a predetermined voltage, the transistor 42 is turned on. transistor 42
When turned on, the charge in the capacitor 46 is discharged,
The terminal voltage of capacitor 46 decreases. Therefore, when transistor 5 is turned off, the excitation energy stored in output transformer 3 is used to charge capacitor 46 and the flyback voltage is lower than in the previous situation. Therefore, the off period of the transistor 5 becomes longer, and the current flowing through the transistor 5 is limited.

FIG. 4c shows a circuit in which an overcurrent detection transistor 43 is provided separately from the transistor 42. This circuit operation is basically the same as that in FIG. 4b, so its explanation will be omitted.

The embodiments of the present invention described above can be modified in various ways within the scope of the same technical idea. For example, a pnp transistor can be used in place of npn transistor 42 in FIG. 4a. Further, an impedance circuit including a capacitor and a resistor may be connected between the emitter and collector of the transistor 42. Furthermore, in FIGS. 4b and 4c, the capacitor 46 can be omitted by providing a circuit for holding the detected voltage across the resistor 44.

Further, the impedance of the flyback voltage absorption circuit may be changed by controlling the transistor 42 on and off in accordance with the DC output to the load 11.

Effects of the Invention As mentioned above, the pulse width of the drive signal supplied to the switching element is kept almost constant by the pulse width setting circuit, and the impedance of the flyback voltage absorption circuit is changed in accordance with the magnitude of the DC output to the load, thereby controlling the switching element. The DC output is controlled to a predetermined value by controlling the off period of the DC-DC, and the circuit configuration is simple and easy to manufacture.
You can get a converter.

Furthermore, since the impedance of the flyback voltage absorption circuit is changed by the signal obtained from the overcurrent detection means connected to the primary winding, overcurrent can be limited.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of the self-excited DC-DC converter of the present invention, Figure 2 is the operating waveform of each part of the circuit shown in Figure 1, and Figure 3 is another example of the self-excited DC-DC converter of the present invention. A circuit diagram showing an embodiment, FIG. 4 shows various embodiments of a flyback voltage absorption circuit, and a
is a basic circuit, b and c are a circuit combined with overcurrent detection means, Fig. 5 is a circuit diagram of a conventional separately excited DC-DC converter, and Fig. 6 shows each part of the circuit shown in Fig. 5. The operating waveforms are shown. 3... Output transformer, 4... Primary winding, 5... Transistor (switching element), 6... Secondary winding, 12... Flyback voltage absorption circuit, 30...
...Drive circuit, 44...Resistor (overcurrent detection means).

Claims (1)

[Claims] 1. A self-excited DC-type device that turns on and off a switching element connected in series to the primary winding of an output transformer to extract DC output from the secondary winding of the output transformer.
In the DC converter, a pulse width setting circuit for setting the pulse width of the drive signal is provided in a drive circuit that supplies a drive signal to the switching element, a flyback voltage absorption circuit is connected in parallel with the primary winding, and the drive circuit supplies the drive signal to the switching element. A self-excited DC-DC converter characterized in that the impedance of the flyback voltage absorption circuit is changed in accordance with the magnitude of the output. 2. The flyback voltage absorption circuit according to claim 1, wherein the flyback voltage absorption circuit has a variable impedance circuit, and the variable impedance circuit changes the impedance of the flyback voltage absorption circuit in accordance with the magnitude of the DC output. Self-excited DC-DC
converter. 3. The above-mentioned pulse width setting circuit has a CR timer circuit connected to the tertiary winding of the above-mentioned output transformer, and a switching element that is turned on by the output of the CR timer circuit and determines the pulse width of the drive signal. A self-excited DC-DC converter according to claim 1. 4. The automatic switching regulator according to claim 1, wherein the DC-DC converter is a forward type switching regulator that generates a DC output from the secondary winding of the output transformer when the switching element is turned on. Excited DC-DC converter. 5 The flyback voltage absorption circuit has a light receiving element that applies a signal that changes the impedance of the variable impedance circuit, and the light receiving element is a light emitting element that generates light of a magnitude corresponding to the magnitude of the DC output. The self-excited DC-DC converter according to claim 2, which is arranged opposite to the self-excited DC-DC converter. 6. A self-excited DC-DC that turns on and off a switching element connected in series to the primary winding of the output transformer to extract DC output from the secondary winding of the output transformer.
In the converter, a pulse width setting circuit for setting a pulse width of the drive signal is provided in a drive circuit that supplies a drive signal to the switching element, and
A flyback voltage absorption circuit is connected in parallel with the secondary winding, and an overcurrent detection means is connected in series with the primary winding, and the circuit corresponds to the magnitude of the DC output and the signal obtained from the overcurrent detection means. A self-excited DC-DC converter, characterized in that the impedance of the flyback voltage absorption circuit is changed by changing the impedance of the flyback voltage absorption circuit. 7 The flyback voltage absorption circuit has a variable impedance circuit, and the variable impedance circuit controls the flyback voltage absorption circuit in accordance with the magnitude of the DC output and the magnitude of the signal obtained from the overcurrent detection means. A self-excited DC-type according to claim 6 that changes impedance.
DC converter.