JPH028551Y2 - - Google Patents

Description

[Detailed explanation of the invention] This invention uses a DC-DC transistor that self-oscillates by utilizing the saturation of a current transformer connected to the base circuit.
This relates to miniaturization of converters.

The conventional self-excited transistor DC-DC converter shown in Figure 1, which self-oscillates using the saturation of a current transformer connected between the emitter and base of the transistor, turns on transistors Q ₁ and Q ₂ alternately. Then, a current of opposite polarity is alternately passed from the DC power supply to the primary winding _N1 of the output transformer T, and the output from the secondary winding is rectified and smoothed by the diodes _D1 , _D2 and the capacitor C. This provides a DC output with any desired voltage value that is isolated from the DC power source. Also, at this time, self-oscillation is performed by controlling the transistors Q ₁ and Q ₂ on and off as follows. In other words, a starting circuit (not shown) first turns on the transistor.
With Q ₂ in the on state, the primary winding of output transformer T
The current flowing through N ₁ is passed through the primary winding n ₁ of the saturable current transformer CT to saturate the saturable current transformer CT, thereby setting the base input of transistor Q ₂ to zero and turning it on. OFF state and also apply the voltage obtained in the tertiary winding N ₂ of the output transformer T to the base of the transistor Q ₁ via the windings n ₃ , n ₄ of the saturable current transformer CT; Turn on. and the winding of the current transformer CT through the primary winding N ₁ of the output transformer T
Saturation caused by the current flowing through n ₁ changes Q ₁ from the on state to the off state. Then, in the same manner as above, the transistors Q ₁ and Q ₂ are alternately turned on and off to perform self-oscillation and obtain an output voltage. In the figure, C ₁ and C ₂ are voltage dividing capacitors, and R ₁ and R ₂ are balancing resistors.

By the way, it is no exaggeration to say that what determines the size of such a DC-DC converter is the size of the output transformer T. Therefore, in order to meet the demand for smaller and lighter converters,
There is a demand for output transformers to be smaller and lighter, and as a means of achieving this, conventional methods have been to increase the on-off period, that is, the self-oscillation frequency, of transistors Q ₁ and Q ₂ . In addition, in this case, the self-oscillation frequency of the converter is determined by the time from when the input is applied to the current transformer CT until it becomes saturated, as described above, so as a method to increase the oscillation frequency, conventionally A method is being used to speed up saturation by minimizing the cross-sectional area of . Also, the output voltage of the secondary winding is V
(volt), the number of turns is n (turns), the cross-sectional area of the iron core is A (cm ² ), and the maximum magnetic flux density is Bm (kilogauss).
Then, the oscillation frequency f (Hz) is given by the following equation.

f=V/4BmAn×10 ⁸ As is clear from the above equation, the oscillation frequency f is inversely proportional to the number of turns n when everything other than the number of turns n is constant. Therefore, a method is used to increase the oscillation frequency by making the turns ratio of the secondary windings n ₁ , n ₂ (n ₄ ) of the saturable current transformer CT as close to 1:1 as possible.

However, it is difficult to reduce the cross-sectional area of the iron core below the current level, and it is also difficult to reduce the primary/secondary winding ratio below the current level due to the relationship with the amplification factors of the transistors Q ₁ and Q ₂ . Therefore, with these methods, it is impossible to increase the oscillation frequency below the current level.
Also, from the above equation, if the output voltage V of the secondary winding of the saturable current transformer can be increased, the oscillation frequency f will also increase. It is only necessary to make the resistance large, and for this purpose, it is possible to make the balancing resistors R ₁ and R ₂ high resistance. However, in this case, the base current of the transistor changes depending on the collector current,
Adoption of this method is not preferable because it causes fluctuations in the voltage drop, which in turn causes fluctuations in the oscillation frequency, making it impossible to obtain a stable output voltage. Therefore, in the conventional circuit shown in Figure 1, the balancing resistor
Since the voltage drop of R ₁ and R ₂ is extremely small, 2
The voltage drop of the next winding, that is, the voltage V in the previous equation, is determined by the voltage drop of the transistor Q ₁ or Q ₂ (for example, 0.7 V compared to 0.1 V of the resistors R ₁ and R ₂ ),
The oscillation frequency f cannot be increased below the value determined by the voltage V.

This invention aims to increase the oscillation frequency by adding a simple circuit to the emitter-base circuit of the transistor, and at the same time, shortens the storage time of the transistor to speed up the turn-off and to reduce the variation in storage time between the two transistors. This helps increase the oscillation frequency, thereby making it possible to further downsize the output transformer and hence the DC-DC converter. Next, the present invention will be explained in detail using the drawings.

FIG. 2 is a circuit diagram (first embodiment) showing one embodiment of the present invention.
The same reference numerals as in the figures indicate equivalent parts), and the features of the present invention are as follows. In other words, in series with the respective base circuits of transistors Q ₁ and Q ₂ , unidirectional non-linear resistance elements, such as diodes D ₃ and D ₄ , are connected in series to obtain a positive voltage drop with a polarity that allows the base current to flow. ,diode
Reverse bias capacitor C ₃ in parallel with D ₃ and D ₄
and connect C ₄ . The unidirectionality of the diode is used to create a reverse bias voltage for the transistor in the capacitor, and at the same time, even if the base current changes due to the collector current due to the nonlinear resistance of the diode, a nearly constant voltage drop is generated. In this way, it is possible to increase the oscillation frequency.

Since the circuit operations of transistors Q ₁ and Q ₂ are the same, the operation of the present invention will be described first with respect to a state in which transistor Q ₂ is turned on by a starting circuit (not shown). When _the transistor Q ₂ is turned on _, as explained in FIG.
Primary winding N ₁ → Primary winding n ₁ of saturable current transformer CT →
Current flows in the path from the collector/emitter of transistor Q ₂ to the DC power supply. Then, a base current determined by the ratio of the windings n ₁ and n ₂ of the current transformer CT is supplied to the transistor Q ₂ , and Q ₂ is turned on, and this state continues until the saturable current transformer CT is saturated. Ru. Next, when the current transformer CT is saturated and the base current no longer flows, the voltage drop across the diode D ₄ during the on period causes the charge in the capacitor C ₄ charged under the polarity shown in the figure to n ₂ →transistor
It is discharged in the path of the emitter base of Q ₂ ,
A reverse bias current flows through transistor _Q2 . Therefore, the storage time of the transistor is shortened, and the current transformer CT is turned off more quickly when it is saturated, which helps increase the oscillation frequency.

On the other hand, when the base current flows through transistor Q ₂ , the diode connected in series with the base circuit
A voltage drop occurs across D ₄ , and the voltage across the secondary winding n ₂ of current transformer CT is determined by the sum of the emitter-base voltage drop of transistor Q ₂ and the voltage drop across diode D ₄ . Therefore, the voltage of the secondary winding _n2 can be made higher than in the conventional circuit, which is determined only by the voltage drop between the emitter and the base of the transistor _Q2 . As a result, the output voltage V in the previous equation is increased, and the oscillation frequency can be increased even if it is not possible to reduce the cross-sectional area of the iron core or the number of turns. Moreover, in the present invention, in order to obtain the voltage drop, a non-linear element is used instead of a fixed resistor in which the voltage drop changes linearly with the current flowing there. Therefore, even if the base current changes due to changes in the collector current of the transistor due to changes in the load size, the voltage drop does not change.
The oscillation frequency can be kept constant. In addition, the diode
It goes without saying that the resistance values of the balance resistors R _{1 and R 2 placed} in parallel with D 3 and D 4 are selected so as to obtain the required voltage drop across the diodes D ₃ and D ₄ .

According to experiments, the cross-sectional area of saturable current transformer CT is 0.036
cm ² , its maximum magnetic flux density is 4 kg, and the number of turns is n ₁ = 1 turn, n ₃ = 15 turns, and n ₂ = n ₄ = 5 turns, then the voltage of winding n ₂ in the conventional circuit shown in Figure 1 is
0.7 volts, whereas in this invention it is about 2 volts.
I was able to double the voltage to 1.7 volts. In addition, the oscillation frequency of the conventional model is approximately 25KHz,
In the present invention, we were able to increase the frequency to about 60KHz, which is more than twice that.

As is clear from the above explanation, according to the present invention, the self-excited oscillation frequency of the self-excited transistor DC-DC converter can be increased and the output transformer can be made smaller and lighter, which has great practical effects. .

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional device, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. Q ₁ , Q ₂ ... transistor, R ₁ , R ₂ ... balance resistor, T ₁ ... output transformer, N ₁ , N ₂ ... its winding, D ₁ , D ₂ ... rectifier diode, C...Smoothing capacitor, CT...Saturable current transformer, n ₁ , n ₂ ,
n ₃ , n ₄ ... winding, C ₁ , C ₂ ... voltage dividing capacitor, D ₃ , D ₄ ... positive direction voltage drop diode,
C ₃ , C ₄ ... Reverse bias capacitor.

Claims (1)

[Scope of utility model registration request] A first and a second connected in series and connected to a DC power supply.
An output transformer having a transistor, a primary winding, a tertiary winding, and an output winding through which current flows alternately in forward and reverse directions by alternately turning on the first and second transistors, and a load by rectifying and smoothing the output. a saturable current transformer having a circuit for supplying the output transformer, a winding through which the primary winding current of the output transformer flows, a winding through which the tertiary winding current flows, and first and second output windings. and its first and second output winding voltages utilize the saturation of a saturable current transformer connected between the emitters and bases of the first and second transistors via first and second balancing resistors, respectively. Transistor that automatically oscillates
In the DC-DC converter, first and second diodes for obtaining positive direction voltage drop and first and second capacitors for obtaining reverse bias voltage of the transistor are connected in parallel with the first and second balance resistors, respectively. and when the first and second transistors are turned on,
By utilizing the nonlinear resistance of the first and second diodes, a constant voltage drop is obtained despite changes in the load current, and the output winding voltage of the saturable current transformer is increased. A transistor DC- that self-oscillates using the saturation of a saturable current transformer, which is characterized by applying reverse bias when the transistor is off using the charging voltage of two capacitors to shorten the storage time and increase the oscillation frequency. DC converter.