JPH047669Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a so-called self-excited DC-DC converter that inputs a DC voltage and obtains a desired DC voltage.

[Technical background of the invention and its problems] The circuit configuration of a conventional flyback type constant voltage output DC-DC converter is shown in FIG. In the same figure
Vin is the DC input voltage, Vo is the DC output voltage, T is a transformer having a first winding N ₁ and a second winding N ₂ , and Q ₁ is an oscillation transistor (hereinafter simply " CNT is an oscillation/control circuit, A is an error amplifier, E is a reference voltage, D ₁ is a rectifying diode, and C _{1 is} a smoothing capacitor.

The DC-DC converter having the above configuration is a so-called separately excited DC-DC converter, and its collector voltage waveform is as shown in FIG. 8a. In Figure 8a, t ₁ is when the oscillation transistor Q ₁ is ON and diode D ₁ is OFF, and at t ₂ , Q ₁ is OFF, flyback occurs, and diode D ₁ is ON.
, and t ₃ is both Q ₁ and diode D ₁
This is when it is OFF.

Further, the relationship between t ₁ and t ₂ is t ₂ /t ₁ =K ₁ Vin/Vo (1). (However, K ₁ is a proportional constant determined by the winding ratio of N ₁ and N _2. ) Also, the output voltage Po (=Vo・Iout) is Po=K ₂ t ₁ ² /t ₁ +t ₂ +t ₁ …(2) becomes. (However, _K2 is a proportionality constant determined by Vin, the inductance of _N1 , etc.) Next, the self-excited DC-DC converter will be explained with reference to FIG. Components in FIG. 7 having the same functions as those in FIG. 6 are designated by the same reference numerals, and detailed explanation thereof will be omitted. The self-excited oscillation circuit consists of a third winding _N3 provided on the transformer T, a capacitor _C2 , and a resistor _R2 . The control circuit includes a control circuit transistor Q ₂ (hereinafter also simply referred to as "Q ₂ "), a Zener diode Z, and a resistor R ₃
It consists of. Furthermore, R ₁ is a starting resistance for Q ₁ .

Next, the operation of the above configuration will be explained. When the input voltage Vin is applied and current flows to the base of the oscillation transistor Q ₁ via the starting resistor R ₁ , the oscillation transistor Q ₁ is turned on and current begins to flow through the first winding N ₁ . This makes the third winding N ₃
A voltage is induced in , current flows into the base via resistor R ₂ and capacitor C ₂ , Q ₁ turns on further, and the collector current continues to increase. However, the current supplied to the base of Q ₁ via capacitor C ₂ decreases as capacitor C ₁ is charged, and eventually it becomes impossible to secure enough base current to keep Q ₁ turned on. . This results in _Q1
Since the collector current of Q1 stops increasing and positive feedback is applied, the voltage induced in the third winding rapidly decreases and is even reversed, bringing _Q1 into the OFF state.

Immediately after that, the energy stored in the transformer so far becomes a flyback voltage and is supplied to the secondary side, and at the same time, _Q1 is turned OFF to the third winding.
Generate the voltage to do this. When the energy stored in the transformer is completely released, the above _Q1
There is no longer any voltage generated to turn OFF, and due to the restart by _R1 and the positive feedback action of the base winding,
_Q1 suddenly turns ON.

In this way, the ON-OFF operation of _Q1 is performed continuously. Then, the voltage induced in the second winding N ₂ is rectified and smoothed by the rectification and smoothing circuit 2, thereby obtaining the output voltage Vo.

Now, as the output voltage Vo increases, the current flowing to the base of Q ₂ increases. At this time, the current flowing from the power supply voltage to the base of Q ₁ via resistor R ₁ and the current flowing from the third winding N ₃ to the base of Q ₁ via resistor R ₂ and capacitor C ₂ are controlled by the control transistor. The current flowing into the base of Q ₁ is reduced because it is shunted to Q ₂ . Therefore, when _Q1 turns ON, the current flowing through the first winding _N1 stops increasing in a short period of time, quickly transitions to the OFF state, and transformer T
As a result, the energy stored in
The output voltage Vo obtained by the second winding decreases.

By controlling _Q1 in a feedback manner in accordance with changes in the output voltage Vo in this way, a desired output voltage Vo can be obtained (constant voltage control).

However, the conventional self-excited DC-
In a DC converter, the capacitor C ₂ is fully charged and the base current of Q ₁ stops flowing.
_Q1 is turned off and the voltage induced in the third winding _N3 is reversed. Next, as mentioned above, when the voltage induced in the third winding is reversed again by the linking operation, _Q1 immediately turns on, so the time _t3 shown in Figure 8a becomes zero, and the self-excited type with the above configuration The collector voltage waveform in the DC-DC converter becomes the waveform shown in FIG. 8b. In Fig. 8, t ₁ is when Q ₁ is ON and D ₁ is OFF, and t ₂ is when Q ₁ is OFF.
This is when _D1 is in the ON state.

In addition, in this case, the relationship between t ₁ and t ₂ is the same as the above equation (1), but the output voltage Po is set to t ₃ = 0 of the waveform shown in Figure 8a in the separately excited DC-DC converter. Equivalent to the case (i.e. there is no period of t ₃ )
Therefore, Po=K ₂ t ₁ ² /t ₁ +t ₂ =K′ ₂ t ₁ …(3). (However, K′ ₂ is a proportionality constant.) This equation (3) shows that when the output voltage (since it is a constant voltage output, in other words, the output current) decreases by 1/10, t ₁ It also shows that the amount decreases to 1/10 proportionally. This also increases the oscillation frequency by 10 times. Therefore, the conventional self-excited DC-DC converter shown in FIG. 7 has the following drawbacks when the output power is small, that is, when the load is light.

In other words, the conventional self-excited DC-DC shown in Figure 7
Since the converter does not have the period t ₃ in the separately excited converter shown in Fig. 6, it depends on the length of t _1. At light loads, the ON time of Q ₁ becomes shorter and the frequency becomes higher. It was very difficult to obtain stability in the operation during time. To achieve stable operation under light loads, the frequency characteristics of the entire circuit, including the switching speed of _Q1 , must be able to handle high frequencies. However, creating a circuit that can handle such high frequencies not only increases costs, but even if such a circuit could be obtained, it would be difficult to create a circuit that can handle such high frequencies.
EMI noise problem arises.

[Purpose of the invention] The present invention was developed in view of the above circumstances, and is a self-excited DC-DC converter that can suppress frequency fluctuations and obtain stable operation even under light loads, and is inexpensive. The purpose is to provide DC-DC converters.

[Summary of the invention] In order to achieve the above object, the outline of the invention is as follows: a switching circuit that turns on and off the oscillation transistor by a voltage generated in the second winding;
A self-excited DC that has a control circuit that obtains a flyback output voltage from the third winding during OFF operation and keeps the output voltage constant by controlling the oscillation transistor according to this output voltage.
- In the DC converter, a transistor is provided between the base of the oscillation transistor and the ground terminal, and the transistor is connected so that the bias voltage of the transistor is obtained from the winding of the transformer via a diode. .

[Embodiments of the invention] Hereinafter, embodiments of the invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing a first embodiment of the DC-DC converter of the present invention. In FIG. 1, components having the same functions as those of the conventional DC-DC converter are designated by the same reference numerals, and detailed explanation thereof will be omitted.

The DC-DC converter shown in Figure 1 is a conventional DC converter.
-The difference from a DC converter is that a circuit is provided at the base of the oscillating transistor to delay the turn-on of the oscillating transistor. This turn-on delay circuit is connected to the fourth winding of the transformer T.
_N4 , a diode Da with one end connected to this fourth winding, a transistor Qa with its collector connected to the base of _Q1 , and its emitter connected to the emitter of _Q1 ,
Consists of voltage dividing resistors Ra and Rb. The polarity of the fourth winding _N4 is changed to the diode at _t2 when flyback occurs (that is, when the oscillation transistor _Q1 turns off).
Da and transistor Qa are turned on, and the number of turns of the fourth winding _N4 and the voltage dividing resistor are
The values of Ra and Rb are set so that when the transistor Qa is turned on, the transistor Qa is saturated. Note that, as explained in the conventional example, the transistor _Q2 is a control transistor essential to the self-commutated converter, and is set to operate in a linear region (not in a saturation region) in order to perform quick control operations.

Next, the operation of the DC-DC converter configured as described above will be explained.

Now, consider the point at which _Q1 starts oscillating, flyback occurs when it turns off, and the flyback ends. At this point, as described above, the other circuits except the second control circuit are trying to move in the direction of turning on _Q1 immediately. However, the transistor Qa, which was in the saturated state and in the ON state when the flyback occurred, does not immediately turn OFF because there is a surplus carrier even after the flyback ends and the base current is no longer supplied. The ON state continues for a while until it disappears. Until this surplus carrier disappears, the transistor Qa remains in the ON state, so the base and emitter of _Q1 are short-circuited. Therefore, when the flyback ends, _Q1 cannot immediately turn on, but remains off for a while. This state is exactly as shown in Figure 8a above.
This is the state of _t3 . Also, the time it takes for this surplus carrier to disappear is determined by the transistor Qa and the voltage dividing resistor.
It can be adjusted by changing Ra and Rb.
The surplus carrier of the diode Da is the voltage dividing resistor Ra,
This is to prevent the wire from being pulled out faster than necessary via the fourth winding _N4 . Then, the surplus carrier disappears and the transistor Qa
When Q 1 becomes OFF, Q ₁ rapidly becomes ON due to positive feedback from the starting resistor R ₁ and the third winding N ₃ . From now on, the above operation will be repeated.

As described above, according to the above configuration, the turn-on delay circuit causes the collector voltage waveform of _Q1 to
Since the state of t ₃ shown in Figure 8a can be maintained for a while, the frequency will not increase even under light loads, and therefore the DC-DC converter will operate stably even under light loads. . In addition, the frequency characteristics of the entire circuit, including the switching speed of _Q1 , are cheaper than those that support high frequencies, and they do not generate EMI noise.

FIG. 2 is a circuit diagram showing a second embodiment of the DC-DC converter of the present invention. The second embodiment differs from the first embodiment in that a second winding N ₂ is used instead of the fourth winding N ₄ provided in the transformer T of the first embodiment, The point is that the output is used for bias via the diode Da. This makes it possible to provide a DC-DC converter that is cheaper and has a simpler structure than the first embodiment. Other effects are similar to those of the first embodiment.

FIG. 3 is a circuit diagram showing a third embodiment of the DC-DC converter of the present invention. The third embodiment differs from the first embodiment in that a buffer is provided in the bias circuit section of the transistor Qa. This buffer consists of _N1a added to the first winding _N1 of transformer T, diode Db, and voltage dividing resistor.
Rc, Rd and starting resistance of transistors Qb, Qa
It consists of Ra. This prevents the connection of the turn-on delay circuit from affecting other circuits.
Stable oscillation can be performed. Other functions and effects are similar to those of the first embodiment.

FIG. 4 is a circuit diagram showing a fourth embodiment of the DC-DC converter according to the present invention. The fourth embodiment differs from the first embodiment in that the transistor Q ₂ used in the control circuit is used in place of the transistor Qa in the turn-on delay circuit, and the output point of the second winding N ₂ , Zener diode Z and transistor
A diode Da and a resistor between the connection point of the base of Q ₂
This is the point where Ra is connected in series. As a result, the number of transistors used can be reduced, resulting in a DC-DC converter that is cheaper than the first embodiment. Other functions and effects are similar to those of the first embodiment.

It should be noted that the present invention is not limited to the above-described embodiments; for example, as shown in FIG. 5, Qa can be not only an NPN type transistor but also a PNP type transistor.

[Effects of the invention] As described above, according to the invention, it is possible to provide a self-excited DC-DC converter that has high operational stability even under light loads and is inexpensive.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a self-excited DC-DC converter according to the present invention, FIG. 2 is a circuit diagram similarly showing a second embodiment, and FIG. 3 is a circuit diagram similarly showing a second embodiment. 3 is a circuit diagram showing the third embodiment, and the fourth embodiment is a circuit diagram showing the third embodiment.
The figure is a circuit diagram showing the fourth embodiment, FIG. 5 is a circuit diagram showing the fifth embodiment, and FIG. 6 is a circuit diagram showing the configuration of a conventional separately excited DC-DC converter. FIG. 7 is a circuit diagram showing the configuration of a conventional self-excited DC-DC converter, and FIG. 8a,
FIG. 1B is a diagram showing a collector voltage waveform of an oscillation transistor in a conventional DC-DC converter. T...Transformer, _N1 ...First winding, _N2 ...Second winding, _N3 ...Third winding, _N4 ...Fourth winding, Vin...DC Input voltage, Vo...DC output voltage, _Q1 ...Oscillation transistor, _Q2 ...Control circuit transistor, Qa...Turn-on delay transistor, Qb...Buffer transistor,
R ₁ , R ₂ , R ₃ , R ₄ , Ra, Rb, Rc, Rd...Resistance,
C ₁ , C ₂ , C ₃ ... Capacitor, D ₁ , Da, Db ... Diode, Z ... Zener diode.

Claims (1)

[Scope of utility model registration request] A transformer having first, second, and third windings; an oscillation transistor connected to the input via the first winding of the transformer; and a voltage generated in the second winding to generate the oscillation. A switching circuit turns on and off the oscillation transistor, and obtains a flyback output voltage from the third winding when the oscillation transistor turns OFF, and controls the oscillation transistor according to this output voltage to control the output voltage. Self-excited type with a control circuit that keeps the
A self-excited DC-DC converter, characterized in that a transistor is provided between the base of the oscillation transistor and the ground terminal, and the transistor is connected so that the bias voltage of the transistor is obtained from the winding of the transformer via a diode. DC-DC converter.