JPH043594Y2 - - Google Patents

Description

[Detailed description of the invention] "Industrial Application Field" This invention relates to a DC-DC converter suitable for a power supply circuit whose load is a capacitor charged to a high voltage.

"Prior Art" Automatic oscillation circuits using semiconductor elements such as transistors and transformers are widely used as means for outputting high DC voltage from a DC power source such as a dry cell battery.

Among these types of oscillation circuits, there is a blocking oscillation circuit that uses one oscillation transistor, but oscillation circuits that rectify only half a cycle of AC output to create DC output are shown in Figures 3 and 4. The circuit configuration shown is known.

The oscillation circuit shown in FIG. 3 is composed of a step-up transformer 1, an oscillation transistor 2, a resistor 3 forming a time constant circuit, and a capacitor 4. An AC output voltage is rectified by a diode 5 and a DC voltage is output.
In addition, reference numeral 6 in this figure is a battery power source;
7 is a power switch, and 8 is a capacitor connected as a load.

In this oscillation circuit, the oscillation transistor 2 is
During the ON period, load current flows and charges the capacitor 8.

The output voltage Eo at this time is Eo=E・(Ns/
Np). Note that E is the voltage of the battery power source 6, Np is the number of turns of the primary coil P of the step-up transformer 1, and Ns is the number of turns of the secondary coil S of the step-up transformer 1.

The oscillation circuit shown in FIG. 4 is different from the oscillation circuit shown in FIG. 3 described above in the connection polarity of the secondary coil of the step-up transformer 1, as shown in the figure.

In this oscillation circuit, no load current flows during the cycle in which the oscillation transistor 2 is ON, and from the time when the oscillation transistor 2 is OFF and the current in the primary coil P of the step-up transformer 1 is cut off, the load current flows to the capacitor 8. flows.

That is, the magnetic energy accumulated around the primary coil P during the OFF period of the oscillation transistor 2 is taken out from the secondary coil by flyback.

Therefore, the output voltage of this oscillation circuit is not determined by the turns ratio of the step-up transformer, but depends on the magnitude of the current flowing through the primary coil P immediately before the oscillation transistor 2 turns OFF, and whether the oscillation transistor is ON or OFF.
is proportional to the speed of movement between the two states.

As a result, this flyback type oscillation circuit outputs a high voltage.

``Problems to be solved by the invention'' As can be seen from the above, the flyback type oscillation circuit shown in FIG. When charging and discharging the capacitor 8 are repeated in a short period of time, for example, when charging and discharging is repeated at a frequency of once every few seconds or several tens of times per second, the magnet of the step-up transformer 1 There is a drawback that the power conversion efficiency is lowered due to insufficient reset.

The reason for this is mainly related to the following points.

(1) The oscillation period of the oscillation transistor 2 is designed to be short.

(2) The configuration is such that the current flowing through the primary coil P increases immediately before the oscillation transistor 2 turns off.

(3) Since the permissible limit of the voltage that can be applied to the oscillation transistor 2 is relatively low, the turns ratio n of the step-up transformer 1 has a relatively large value.

As an example, if the capacitor 8 is charged to 500 volts and the voltage applied between the collector and emitter of the oscillation transistor 2 is 50 volts as the limit, the voltage appearing in the primary coil P of the step-up transformer 1 is (50-E). Since it is necessary to keep it below volts, if Ns/Np=n is chosen as 20, then 500/20=25
Volt is applied between the collector and emitter of the oscillation transistor 2.

On the other hand, since the inductance of the secondary coil S has a value of n ² Lp with respect to the inductance Lp of the primary coil P, this inductance n ² Lp increases considerably due to the relationship in which the turns ratio n is determined as described above.
Therefore, when the charge on the capacitor 8 is zero or close to zero, a load current flows through the secondary coil S for an extremely long time.

As a result, together with the short oscillation period of the oscillation transistor 2, the transistor 2 enters the next ON period before the current in the secondary coil S has sufficiently decreased.

Such a phenomenon has an adverse effect on the above-described oscillation circuit including the step-up transformer 1 having a magnetic core, and causes damage to the oscillation transistor 2.

The above phenomenon will now be explained in more detail with reference to FIG. Note that FIG. 5 is a B-H curve of the magnetic core included in the step-up transformer 1.

The magnetic iron core is the first part of the oscillation transistor 2.
When it is ON, it is magnetized toward a to b, and when it is turned off, if there is enough time (when the charge of the capacitor 8 is zero or close to zero), it returns to b to c and the magnetic reset occurs. If this transistor 2 continues to be turned on, it will be magnetized toward c to b, and thereafter, when the loop from c to b is repeated, stable operation will be achieved.

Oscillation transistor 2 turns ON while returning from b to c
Then, it becomes magnetized toward d to e, and similarly, e
If it turns from OFF to ON before returning to ~d, it will be magnetized towards f~g.

Correctly, the loops c to b, d to e, and f to g have the same length when viewed from the H axis, so when the magnets are not reset, they gradually enter a region where the magnetic flux is saturated.

Therefore, before the power pole of the secondary coil S of the step-up transformer 1 approaches zero, the oscillation transistor 2
When the transformer enters the ON period, the magnetic core is driven in the direction of increasing magnetizing force without being sufficiently magnetically reset, and is gradually forced into a state of magnetic flux saturation, and in extreme cases, the transformer function may be lost.

In reality, the charging voltage of the capacitor 8 gradually increases, and the time it takes for the current in the secondary coil S to reach zero gradually decreases, so the OFF period of the oscillation transistor 2 is adjusted so that magnetic reset is performed. There are few extreme adverse effects such as loss of transformer function.

However, as long as the operation in which the oscillation transistor 2 enters the next ON period with insufficient magnetic reset is repeated, the power conversion efficiency between the input and the output decreases. This is step-up transformer 1
leaving behind the electrical energy supplied by the
This is because the motion does not use up this energy.

Furthermore, the above operation increases the amount of input power and causes the oscillation transistor 2 to generate heat.

"Means for Solving the Problems" In view of the above-mentioned problems, the present invention aims to develop a DC-DC converter that has a high oscillation frequency and is suitable for capacitor loads.

Therefore, in the present invention, the input coil current of the step-up transformer supplied from the battery power source is intermittent by the oscillating transistor, and the output coil voltage of the step-up transformer, which is boosted by the flyback voltage of the coil, is connected via the rectifying diode. In a DC-DC converter configured to charge a capacitor,
A resistive member provided in the output current path of the output coil is applied with the voltage generated in this resistive member in each oscillation operation added to the battery power supply voltage, and conduction is established until the voltage of the resistive member decreases below a predetermined level. and a switching member connected between the base and emitter of the oscillation transistor so as to turn on in response to conduction of the constant voltage element and turn off in response to non-conduction, and When the voltage of the resistive member decreases to a predetermined level of zero or close to zero for each oscillation operation, the switching member
We propose a DC-DC converter configured to turn off the oscillation transistor and turn on the oscillation transistor.

"Example" Next, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing a DC-DC converter according to the present invention, in which the same circuit components are given the same reference numerals as in the conventional example described above.

As shown in the figure, in this embodiment, a resistor 10 is connected between the secondary coil S of the step-up transformer 1 and the ground line 9, and a voltage corresponding to the current flowing through the secondary coil S is generated in the resistor 10.

Further, resistors 11 and 1 are connected between the connection between the secondary coil S and the resistor 10 and the positive electrode side of the battery power source 6.
2 and a constant voltage element 13, and a transistor 14 that is biased and turned on by the voltage generated in the resistor 11;
The base is inputted by this transistor 14.
Transistor 15 for switching operation to turn on
is provided.

The transistor 15 is connected between the base and emitter of the oscillation transistor 2, and when the impedance between the collector and emitter of the transistor 15 is sufficiently reduced, the oscillation transistor 2 receives the bias voltage and the positive feedback voltage at the same time. It drops and stops the oscillation operation.

Resistors 16 and 17 connected in parallel to capacitor 8
A constant voltage element 18 that receives the voltage divided by these resistors 16 and 17, and a transistor 19 that receives base input from this constant voltage element 18 form a constant voltage control circuit for the capacitor 8. When the voltage reaches the same voltage, the constant voltage element 18 becomes conductive, turning on the transistor, which increases the collector current of the transistor 14, lowers the impedance of the transistor 15, and stops the oscillation operation of the oscillation transistor 2. to work.

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

The current Ic flowing to the collector of the oscillation transistor 2 during oscillation and the voltage Vd generated in the output current detection resistor 10 have a relationship approximately shown in FIG. 2, and as the capacitor 8 is charged, the duration of the output current changes. To gradually becomes shorter.

Furthermore, the voltage Vd generated across the resistor 10 becomes a negative voltage when the ground line 9 is referenced, and this voltage Vd
Considering the absolute value of , the following formula holds true.

E+｜Vd｜−Vz>V _BE E: Voltage of battery power supply 6 Vz: Zener voltage of constant voltage element 13 V _BE : Forward voltage drop between base and emitter of transistor 14 As long as the above formula holds, the transistor Since transistor 14 is forward biased, this collector current increases and transistor 15 is forward biased, so that oscillation transistor 2 remains OFF.
That is, while the voltage Vd is above the expected level,
In other words, the oscillation transistor 2 is reliably activated until the load current approaches zero and the voltage Vd sufficiently decreases.
Turn off.

During this time, the time constant circuit consisting of the resistor 3 and capacitor 4 is in a state where it applies a forward bias voltage to the oscillation transistor 2, so the voltage Vd falls below the expected level, the collector current of the transistor 14 decreases, and the collector current of the transistor 15 decreases. When the impedance between the collector and emitter increases, the oscillation transistor 2 is immediately turned on and the oscillation operation continues.

In this way, the oscillation transistor 2 remains OFF until the load current reaches zero or approaches zero, so
The magnetic core of the step-up transformer 1 is reliably reset, and a configuration for high-speed oscillation operation (high-frequency oscillation) becomes possible.

To increase the speed, the product of the resistance value of the resistor 3 and the capacitance value of the capacitor 4 of the time constant circuit is made small.

That is, the value of the above product is determined so as to correspond to the duration To when the capacitor 8 is charged and the duration To of the output current becomes appropriately shortened.

However, the zener voltage Vz of the constant voltage element 13 is
It is necessary to satisfy the condition of Vz>E and set so that the transistor 14 is not forward biased even when the voltage Vd is zero.

In this type of DC-DC converter, it is most effective to increase the speed by setting the product of the circuit factors of the time constant circuits 3 and 4 to a small value. The explanation will be given below.

The oscillation frequency is f, 1/2 of its period 1/f is the ON period of oscillation transistor 2, and the other 1/2 is its OFF.
The period, Icm is the peak value of its collector current, and the inductance Lp of the primary coil P of step-up transformer 1 is
Then, the output power W becomes W=η・(1/2)・Lp・I ² cm・f. However, η is the transmission efficiency, and η<1. Moreover, the relationship between the voltage E of the battery power source 6 and the above-mentioned inductance Lp is as follows: E=2·Lp·(Icm/f).

When the step-up transformer 1 is made small and handles a large amount of power, it is not possible to increase the inductance Lp, so the collector current Icm of the oscillation transistor 2 is designed to be large. but,
Since the collector current Icm is limited by the capacitance of the oscillation transistor 2, the oscillation frequency f is increased.

Further, setting the product of the circuit factors of the time constant circuits 3 and 4 to a large value causes an invalid time in view of the operating principle of the blocking oscillation circuit, resulting in a significant deterioration of the power exchange efficiency.

Therefore, it is preferable that the product of the circuit factors of the time constant circuits 3 and 4 is a relatively small value determined by the oscillation frequency in the intermediate period of the charging process of the capacitor 8.

On the other hand, in FIG. 1, when capacitor 8 is charged to a predetermined voltage, transistor 19 is turned on due to conduction of constant voltage element 18. As a result, the collector current of the transistor 14 increases,
The impedance between the collector and emitter of transistor 15 decreases and oscillation transistor 2 turns off.
will be maintained.

That is, when the capacitor 8 is charged to a predetermined charging voltage, the oscillation of the DC-DC converter stops, and if the charging voltage of the capacitor 8 becomes somewhat lower, the constant voltage element 18 becomes non-conductive. Since the DC-DC converter starts oscillating, the constant voltage element 18 and the transistor 19 function as an output voltage control circuit that controls the charging voltage of the capacitor 8 to be substantially constant.

``Effect of the invention'' As mentioned above, in this invention, the oscillation transistor is operated until the load current reaches zero or close to zero.
Since a switching member is provided to turn off the transformer, the magnetic core of the step-up transformer is reliably reset magnetically, resulting in a DC-DC converter with an oscillation circuit that uses less input power and has high power conversion efficiency.

In addition, since the magnetic reset is performed reliably, the power conversion efficiency does not decrease even if the oscillation frequency is set high, making the DC
-Becomes a DC converter.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a DC-DC converter showing an embodiment of the present invention, Fig. 2 is a waveform diagram showing the relationship between the collector current of the oscillation transistor and the voltage generated by the output current detection resistor, and Fig. 3 and 4 are circuit diagrams of a DC-DC converter shown as a conventional example, and FIG. 5 is a B-H curve diagram showing the magnetization state of a magnetic core included in a step-up transformer. 1... Step-up transformer, 2... Oscillation transistor, 3, 4... Resistor and capacitor forming a time constant circuit, 10... Resistor for output current detection, 11...
...Resistance for bias voltage, 13... Constant voltage element,
14, 15...Transistors for preventing magnetic saturation.

Claims (1)

[Scope of utility model registration request] A DC system that uses an oscillation transistor to intermittent the input coil current of a step-up transformer supplied from a battery power source, and charges a capacitor via a rectifier diode with the output coil voltage of the step-up transformer, which is boosted by the flyback voltage of the coil. −
In a DC converter, a resistive member is provided in the output current path of the output coil, and the voltage generated in this resistive member during each oscillation operation is added to the battery power supply voltage and applied, and the voltage of the resistive member is reduced below a predetermined level. A constant voltage element that is made to conduct until the constant voltage element becomes conductive, and a switching member that is connected between the base and emitter of the oscillation transistor so that it turns on in response to the conduction of the constant voltage element and turns off when it becomes non-conductive. The DC-DC converter is configured to include: the switching member is turned off and the oscillation transistor is turned on when the voltage of the resistance member decreases to a predetermined level of zero or close to zero for each oscillation operation.