JPS6361870B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention provides the following advantages: by operating a transistor at a repetition frequency higher than an audible frequency,
The present invention relates to a switching power supply device that transmits electric power.

Conventional configuration and its problems Figure 1 shows an example of the power stage and drive stage circuits of a conventional switching power supply, where 1 is the transformer, _W1 is the primary winding of the transformer 1, and _W2 is the It is the secondary winding and is connected to the output rectifier circuit. 2 is a main transistor whose collector is connected to the primary winding _W1 and whose emitter is grounded. The collector of an NPN type transistor 3 whose emitter is connected to the base of the main transistor 2 is connected to a positive polarity bias power supply 5 via a resistor 4. The collector of a PNP type transistor 6 whose emitter is connected to the base of the main transistor 2 is connected to a bias power supply 7 of negative polarity.
The bases of the transistors 3 and 6 are both connected to the input power supply terminal 9 via a resistor 8 together with the primary winding _W1 , and the emitters are connected to the collector of an NPN type transistor 10 connected to the bias power supply 7. There is. The base and emitter of the transistor 10 are connected by a parallel circuit including a diode 11 and a resistor 12 for protection of the transistor 10. Further, the base of the transistor 10 is connected to the power supply terminal 9 via a series circuit of a capacitor 13 and a resistor 14. capacitor 13 and resistor 1
The connection point with 4 is connected to the collector of an NPN type transistor 15 whose emitter is grounded.
Note that 16 is an input power supply terminal.

When a pulse such as pulse a is applied to the base of the transistor 15, the main transistor 2 is turned on when the base of the transistor 15 is at a high level, and the main transistor 2 is turned off when the base of the transistor 15 is at a low level. By applying a reverse bias to the base of the main transistor 2, the accumulation time and turn-off time can be shortened, and the withstand voltage between the collector and emitter of the main transistor 2 can be improved, which generally improves the performance of the switching power supply significantly. is known for. Therefore, the first
As shown in the figure, in addition to the positive polarity bias power supply 5, a negative polarity bias power supply 7 is installed as a bias power supply for driving the main transistor 2, and between the positive polarity bias power supply 5 and the negative polarity bias power supply 7.
NPN type transistor 3 and PNP type transistor 6 are arranged in a totem pole type, and when the pulse applied to the base of transistor 15 is low, by applying a negative voltage to the base of transistor 2, the main transistor 2 is inverted. A bias can be applied. The disadvantage of this circuit is that it requires a negative polarity bias power supply 7, and since the input is a positive polarity power supply, in order to obtain a negative polarity power supply, a negative polarity power supply winding must be added to the transformer 1. Extra wires are required and a rectifier circuit is also required. Another drawback is that the common line potential of the drive circuit of the main transistor 2 (consisting of transistors 3, 6, 10, and resistors 4, 8) and the ground potential differ by the voltage of the negative bias power supply 7. A level shift circuit consisting of a capacitor 13, a resistor 12, and a diode 11 is required. The diode 11 in this case prevents the transistor 10 from being destroyed by an excessive reverse voltage generated by using the capacitor 13. As described above, in order to apply a reverse bias to the main transistor 2, the conventional switching power supply device requires a large number of parts, and the winding W ₁ of the transformer 1
Another problem was that the complication of _W2 led to increased costs and decreased reliability.

In Figure 2, the negative bias power supply 7 is removed from the circuit in Figure 1, and a parallel circuit of a capacitor 17 and a resistor 18 is connected between the emitters of transistors 3 and 6 and the base of main transistor 2. be. This circuit is an example of a conventional reverse bias application method that does not require a negative bias current to drive the main transistor 2, and is an example of a conventional reverse bias application method for driving the main transistor 2.
7 is generally called a speed-up capacitor. 3A, B, and C show the base applied to the base of the main transistor 2 when the time constant of the capacitor 17 and the resistor 18 in the circuit of FIG. 2 is set larger than the pulse repetition period. This figure shows the waveform of V _BE between emitters.

Figure 3A shows the voltage V _BE waveform when the duty cycle of input pulse a in Figure 2 is small;
Figure B shows the voltage V _BE waveform when the duty cycle of input pulse a is large. The reverse bias applied to the base of the main transistor 2 is
It changes in proportion to the duty cycle of input pulse a, as shown in FIG. This means that the voltage charged in the capacitor 17 is the drive circuit of the main transistor 2 (a circuit consisting of transistors 3, 6 and resistor 4).
This is because it is proportional to the integral value of the pulse current supplied by the pulse current. Furthermore, even if the duty cycle of the input pulse a is the same, if the voltage of the positive polarity bias power supply 5 changes, the output current of the drive circuit of the main transistor 2 will also change proportionally, and the reverse bias voltage will change proportionally. and it fluctuates.
In reality, the positive polarity bias power supply 5 is not provided alone, but the input power supply 9 is used as a positive polarity bias power supply, or a bias winding (not shown) is provided in the transformer 1, and it is rectified. Since it is used as a positive polarity bias power supply,
If the voltage of the input power source 9 decreases, the reverse bias voltage also decreases proportionally as shown in FIG. 3c.

Figures 4A and 4B illustrate the waveform of the voltage V _BE between the base and emitter of the main transistor 2 when the time constant of the capacitor 17 and resistor 18 in Figure 2 is set smaller than the pulse repetition period. be. In this case, the duty cycle of the input pulse a has almost no influence on the peak value of the reverse bias. However, since the peak value of the reverse bias is expressed by the following equation, it is proportional to the voltage of the positive polarity bias power supply.

(Reverse bias voltage peak value) = (Voltage of positive polarity bias power supply 5) x (Resistance value of resistor 18) / (Resistance 4
) + (resistance value of the resistor 18) In other words, similarly to FIGS. 3B and 3C, when the input voltage decreases, the peak value of the reverse bias voltage changes from A to B in FIG. 4.

In this way, the reverse bias voltage fluctuates due to fluctuations in the voltage at the input terminal 9 and fluctuations in the duty cycle of the input pulse a. Generally main transistor 2
Since the emitter-to-base breakdown voltage V _EBO when the collector is open is relatively low (5V to 7V), the circuit shown in FIG. 2, in which the reverse bias voltage changes greatly, cannot apply a sufficient reverse bias.

Further, as a requirement of the switching power supply device, the smaller the duty cycle of the input pulse, the shorter the storage time is required, and for this purpose, it is necessary to apply a larger reverse bias. In contrast, in the operation shown in FIG. 3, when the duty cycle of the input pulse is small, almost no reverse bias is applied, which can be said to be a fatal defect.

In addition, in the operation shown in FIG. 4, the reverse bias remains unchanged throughout the entire range in which the main transistor 2 is off, but only during the transition period from on to off.

Therefore, the breakdown voltage between the collector and emitter of the main transistor 2 needs to be evaluated in detail by the breakdown voltage V _CEO between the collector and emitter when the base is open. Furthermore, when the voltage between the collector and emitter of transistor 6 becomes about 0.7V or less, the collector and emitter of transistor 6 becomes almost open, so even if we consider the case where a resistor is inserted between the collector and emitter of transistor 6, V _CER must be used for detailed pricing. That is, when considering the withstand voltage between the collector and the emitter, the operation shown in FIG. 4 is the same as if there was no reverse bias.

Purpose of the Invention In view of the above-mentioned drawbacks, the present invention provides a switching power supply device that can apply a reverse bias to the base of the main transistor with a much smaller number of elements than conventional ones, and in which many of the constituent elements can be easily fabricated into monolithic ICs. It is.

Structure of the Invention In order to achieve the above object, the present invention connects two transistors in series to a power supply through their collectors or emitters, and connects the bases of the two transistors to the bases of the two transistors. ,
A pulse to turn it off is input, and a parallel circuit of a constant voltage element and a capacitor is connected between the connection point of the two transistors and the base of the main transistor.

With this configuration, when the capacitor has a certain large capacity, the capacitor is charged by the voltage determined by the constant voltage element, and the reverse bias of the main transistor is applied by the capacitor. As a result, the reverse bias of the main transistor is determined only by the constant voltage element and is not affected by variations in the duty cycle of the input pulse. For the same reason, the reverse bias of the main transistor is not affected by voltage fluctuations of the input power supply, so by selecting a constant voltage element, a reverse bias approximately equal to the emitter-base breakdown voltage of the main transistor can be applied. It is possible to always apply the best reverse bias voltage to the base of the main transistor with a simpler circuit than conventional switching power supplies, and under the best conditions it is possible to switch the main transistor at a repetition rate above the audio frequency. It can be made to work.

DESCRIPTION OF EMBODIMENTS A switching power supply device according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 5 is an electrical circuit diagram of the main parts of a switching power supply device in an embodiment of the present invention, in which 1 is a transformer having a primary winding _W1 and a secondary winding W2, 2 is a main transistor, and 5 is a transformer having a primary winding _W1 and a secondary winding W2. is a positive polarity bias power supply. 19 is an NPN type transistor whose collector is connected to the bias power supply 5; 20 is a collector whose collector is connected to the emitter of the transistor 19;
It is an NPN type transistor whose emitter is connected to the input voltage power supply terminal 16 together with the emitter of the main transistor 2. A capacitor 2 is connected between the emitter of the transistor 19 and the collector of the transistor 20 and the base of the main transistor 2.
1 and a constant voltage element 22 are connected in parallel. A pulse such as pulse a is applied to the base of transistor 19, and a reverse pulse such as pulse b is applied to the base of transistor 20, such that when pulse a is high, it becomes low and when pulse a is low, it becomes high. . As a result, when the base of the transistor 19 is at a high level, the transistor 19 is on, the transistor 20 is off, and a forward bias is applied to the main transistor 2, and at the same time, the capacitor 2
1, charging is performed to a voltage determined by the constant voltage element 22. When the base of the transistor 19 becomes low level, the transistor 19 is turned off and the transistor 20 is turned on, so that a reverse bias equal to the voltage charged in the capacitor 21 can be applied to the main transistor 2. The base voltage waveform of the main transistor 2 is shown in FIGS. 6A and 6B. FIG. 6A shows the waveform when the duty cycle of input pulses a and b is small, and FIG. 6B shows the waveform when the duty cycle of input pulses a and b is large. In the circuit of this embodiment, if the capacitance of the capacitor 21 is larger than a certain level, the reverse bias voltage is determined only by the constant voltage element. There is no change in reverse bias voltage. Further, since there is no effect on voltage fluctuations of the input power supply, a reverse bias can always be applied up to the emitter-base withstand voltage V _EBO when the collector of the main transistor 2 is open. Further, the breakdown voltage between the collector and emitter of the main transistor 2 is the collector-emitter breakdown voltage V _CEX when a reverse bias is applied to the base, and at the same time, the safe operation area is widened.
For some transistors, if the reverse bias is applied too much, the safe operation area becomes narrower, but in the case of this embodiment, the best conditions can be achieved by selecting the voltage of the constant voltage element.

In this way, this embodiment uses a very small number of parts compared to conventional circuits, and can bring out near the best performance that a transistor can exhibit in terms of turn-off characteristics, collector-emitter breakdown voltage, and safe operation area at low cost. I can do it.

FIG. 7 is an electrical circuit diagram of a switching power supply device according to an embodiment of the present invention, in which the pulses sent from the oscillator 24 have pulse on/off intervals determined by the pulse width modulator 23, and are used for current limiting. Transistors 19 and 20 connected in series with the resistor 4 and the power supply terminal 9 of the input power source are alternately turned on and off. 27 is a reverse voltage protection diode between the collector and emitter of the main transistor 2, 28 is a snubber capacitor, and 29 is a snubber resistor. And the output voltage is the secondary side rectifier diode 30
and output from output terminals 35 and 36 via filter capacitor 31. Resistors 32 and 33
is a resistor for output voltage division, and resistors 32 and 3
The output voltage divided by the reference power supply 2 and
6 by an error amplifier 25.
The error amplifier 25 is configured to correct the pulse sent from the pulse width modulator 23 so that a constant output voltage is always output from the output terminals 35 and 36.

The circuit shown in FIG. 7 is a general flyback type switching power supply circuit, in which the input power supply is used as a positive polarity bias power supply for driving the main transistor 2, but the circuit shown in FIG. For this reason, the reverse bias voltage to the main transistor 2 is not affected by variations in the input power supply.

FIG. 8 is a circuit diagram of a switching power supply according to another embodiment of the present invention, and unlike the switching power supply shown in FIG.
W ₃ is wound, and its output is rectified by a diode 37 to serve as a positive bias power source for driving the main transistor 2. As a result, the power consumption of the resistor 4 can be reduced compared to the switching power supply circuit shown in FIG. Note that the resistor 38 is a resistor for flowing a starting current.

FIG. 9 shows an example in which the collector current feedback base drive method is applied. By flowing the collector current of the main transistor 2 through the current transformer 43, a current source proportional to the collector current is obtained, and the current is transmitted by the diode 37. It is rectified and serves as a positive bias power source for driving the main transistor 2. With this circuit, it is possible to prevent more base current than necessary from flowing through the main transistor 2, and the switching speed can be increased and switching loss can be reduced. Needless to say, the same effect can be obtained even when the current transformer 43 is provided at the emitter of the main transistor 2.

Figure 10 shows an example of application to a chip type switching power supply, where 40 is a chiyo inductor, 4
1 is a flywheel diode, and 42 is a filter capacitor. In this case the main transistor 2,
Both transistors 19 and 20 are of PNP type, but transistors 19 and 20 can also be configured of NPN type.

In addition, in the conventional switching power supply shown in Figure 1, when converting parts other than the power stage into monolithic ICs, the potential of the common line is different between the drive stage and the stages preceding it, so it is only possible to use ICs in the stage before the part indicated by the broken line 14. It is. However, in the case of the present invention, the portion indicated by the broken line 34 in FIGS. 7 to 10 can be made into a monolithic IC.

Effects of the Invention As described above, the present invention connects two transistors in series to a power supply through their collectors or emitters, and connects the bases of the two transistors to alternately turn on and off. By inputting pulses and connecting a parallel circuit of a constant voltage element and a capacitor between the connection point of the two transistors and the base of the main transistor, it was impossible to convert the drive stage into an IC. In contrast, it is possible to use a single-chip IC, including the drive stage, and in combination with a simple reverse bias circuit with a small number of components, it is possible to supply a small, lightweight, and highly efficient switching power supply at low cost. .

[Brief explanation of drawings]

Fig. 1 is an electric circuit diagram of a conventional switching power supply device, Fig. 2 is an electric circuit diagram of main parts of a switching power supply device in a second conventional example, and Figs. 3 and 4 explain the operation of the same device. Waveform diagram, Figure 5 is an electrical circuit diagram of the main part of the switching power supply device which is the basis of the present invention, Figure 6 is a waveform diagram explaining the operation of the same device, and Figure 7 is a switching power supply according to an embodiment of the present invention. An electric circuit diagram of the device, FIG. 8 is an electric circuit diagram of a switching power supply device in a second embodiment of the present invention,
FIG. 9 is an electrical circuit diagram of a switching power supply device according to a third embodiment of the present invention, and FIG. 10 is an electrical circuit diagram of a switching power supply device according to a fourth embodiment of the present invention. 2... Main transistor, 19, 20... Transistor, 21... Capacitor, 22... Constant voltage element.

Claims (1)

[Claims] 1 The collector and emitter of the first transistor and the collector and emitter of the second transistor are connected in series to a power supply, and the first transistor and the second transistor are connected in series to a power supply.
A pulse is input to the base of the transistor so that the first transistor and the second transistor are turned on and off alternately, and the connection point between the first transistor and the second transistor is connected to the base of the main transistor. A switching power supply device with a parallel circuit of a constant voltage element and a capacitor connected between the base and the base.