JPS5935566A - Switching power source - Google Patents

Abstract

PURPOSE:To stabilize the reverse bias of a main transistor by inputting a pulse which alternately turns two transistors connected in series with a powe source ON and OFF, and employing a parallel circuit of a constant-voltage element and a condenser. CONSTITUTION:When the base of a transistor 19 is at a high level, a transistor 19 is conducted, while a transistor 20 is interrupted, a forward bias is applied to a main transistor 2, and a condenser 21 is simultaneously charged to a voltage which is determined by a constant-voltage element 22. When the base of the transistor 19 becomes low level, the transistor 19 is interrupted, while the transistor 20 is conducted, and a reverse bias can be applied to the transistor 2 at the voltage which is charged in the condenser 21. Since the reverse bias voltage is determined only by the element 22 when the capacity of the condenser 21 is larger than the prescribed value, no change occurs in the reverse bias voltage to the variation in the duty cycle of the input pulse.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a switching power supply device that transmits power by switching transistors at a repetition frequency higher than an audible frequency.

Conventional configuration and its problems Figure 1 shows an example of the power stage and drive stage circuits of a conventional switching power supply. 1 is a transformer, W is the primary winding of the transformer 1, and W2 is the secondary winding of the transformer 1. It is the next 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 transistor 3 whose emitter is connected to the base of the main transistor 2 is connected to a positive 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 transistors 3 and 6 are both connected to the primary winding VV1 (!) through a resistor 8 to the input power supply terminal 9, and the emitters are connected to the bias power supply 7.
It is connected to the collector of transistor 0 of q. The base and emitter of the transistor 100 are connected by a series circuit of a diode 11 and a resistor 12 for protection of the transistor 0. Further, the base of the transistor 0 is connected to the power supply terminal 9 via a series circuit of a capacitor 13 and a resistor 14. A connection point between the capacitor 13 and the resistor 14 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 the Jonahals pulse (a) is applied to the base of the transistor 15, the main transistor 2Vi 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 storage 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 significantly improves the performance of the switching power supply. This is generally known. Therefore, as shown in FIG. 1, a negative polarity bias power supply 7 is installed in addition to the positive polarity bias power supply 5 as a bias power supply for driving the main transistor 2. An NPN type transistor 3 and a PNP type transistor 6 are arranged in a totem pole shape between the two transistors, and when the voltage applied to the base of the transistor 16 is low, a negative voltage is applied to the base of the transistor 2. Therefore, a reverse voltage can be applied to the main transistor 2. The disadvantage of this circuit is that it requires a bias power supply 7 with a negative polarity, and since the input is a power supply with a positive polarity, in order to obtain a power supply with a negative polarity, a negative polarity power supply winding must be added to the transformer 1. An extra line is required, and a rectification path is also required.

Another drawback is the drive circuit of main transistor 2 (
Transistors 3, 6, 10. (consists of resistors 4 and 8)
Since the potential of the common line and the potential of the ground differ by the voltage of the negative polarity bias power supply 7, the capacitor 13. resistance 1
2. A level shift circuit composed of a diode 11 is required. The diode 11 in this case is the capacitor 1
By using 3, there is no generation and filtration due to large reverse voltage.
This prevents the I transistor 10 from being destroyed.

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 W1 or W2 of the transformer 1 becomes complicated, resulting in increased costs and decreased reliability. There was a problem in that it invited

In FIG. 2, the negative bias power supply 7 is removed from the circuit in FIG. 1, and a parallel circuit consisting 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 for driving the main transistor 2, and the capacitor 17 is generally called a speed-up capacitor. 3(E), (C1 is the voltage applied to the base of the main transistor 2 when the time constant of the capacitor 17 and resistor 18 in the circuit of FIG. 2 is set larger than the pulse repetition period. This figure shows the waveform of the base-emitter voltage ■BE.

Figure 3 (input pulse Gl in Figure 2) shows the voltage vBE waveform when the duty cycle is small;
) is the voltage vBE waveform when the input pulse (a) has a large duty cycle. The reverse bias applied to the base of the main transistor 2 changes in proportion to the duty cycle of the input pulse (a), as shown in FIGS. 3(A) and 3(B). This is because the voltage charged in the capacitor 17 is proportional to the integral value of the pulse current supplied by the drive circuit of the main transistor 2 (the circuit consisting of the transistors 3 and 6 and the resistor 4). be. Furthermore, even if the input pulse duty cycle is the same, if the voltage of the positive bias power supply 6 changes, the output current of the drive circuit of the main transistor 2 will also change proportionally, and the reverse bias voltage will change. 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 the transformer 1 is equipped with a bias winding (not shown). Since the rectified voltage is used as a positive bias power supply, if the voltage of the input power supply 9 decreases, the reverse bias voltage will decrease proportionally as shown in Figure 3. I end up.

Figure 4 (4, β) shows the capacitor 17 and resistor 18 in Figure 2.
Voltage v between the base and emitter of main transistor 2 when the time constant is set smaller than the pulse repeating period
It is a diagram illustrating the waveform of BE. 'In this case, the duty cycle of the input pulse (a) has almost no effect on the peak value of the reverse bias.

However, since the peak value of the reverse bias is expressed by the following equation, it can be compared to the voltage of the positive polarity bias power supply.

In other words, if the input voltage decreases in the same way as in Figure 3 (B), (Q), the peak value of the reverse bias voltage will be (8) in Figure 4.
It changes from to (B).

In this way, the fluctuation of the voltage at the input terminal 9, the input/< Lus(a
) The reverse bias voltage fluctuates due to variations in the duty cycle. In general, the emitter-base breakdown voltage vEBO when the collector of the main transistor 2 is open is relatively low (s
V~7V), so the reverse bias voltage changes greatly.
The circuit shown in the figure had the disadvantage that it was not possible to apply an inverse force to the light component.

Further, as a requirement of the switching power supply device, when the duty cycle of the input pulse is small, it is necessary to shorten the accumulation time, and for this purpose, it is necessary to apply a larger reverse bias. On the other hand, 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 is not applied over the entire range in which the main transistor 2 is off, but only during the transition period from on to off.

Therefore, the collector-emitter breakdown voltage of main transistor 2 is the collector-emitter breakdown voltage vcEo when the base is open.
need to be evaluated. Also, since the voltage between the collector and emitter of transistor 6 is about 0.7 V or less,
Since the collector and emitter of the transistor 6 are almost in an open state, even if a resistor is inserted between the collector and emitter of the transistor 6, it is necessary to evaluate it using vCER. That is, when considering the breakdown voltage between the collector and emitter, the operation shown in FIG. 4 is the same as if there was no reverse bias.

OBJECTS 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 significantly smaller number of elements than in the past, and in which many of the constituent elements can be easily made into monolithic ICs. It is something.

Structure of the Invention In order to achieve the above object, the present invention connects two transistors in series to a power supply by their collectors or emitters, and connects the bases of the two transistors to the bases of the two transistors, which are turned on alternately.

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, if the capacitance of the capacitor is large to some extent, the capacitor is charged by the voltage determined by the constant voltage element, and a reverse bias is applied to the main transistor 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 it is possible to apply a reverse bias approximately equal to the emitter-base breakdown voltage of the main transistor. is possible,
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 the main transistor can be switched at a repetition frequency higher than the audio frequency. can.

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 the figure, 1 is the primary winding @'
A transformer has VV1 and a secondary winding aW2, 2 is a main transistor, and 5 is a positive bias power supply. 19 is an NPN type transistor whose collector is open and connected to the insulator source 5; 2o is an NPN type transistor whose collector is connected to the emitter of the transistor 19, and whose emitter is connected to the input voltage power supply terminal 16 together with the emitter of the main transistor 2; A capacitor 21 and a constant voltage element 2 are connected between the emitter of the transistor 19, the collector of the transistor 2o, and the base of the main transistor 2.
A parallel circuit with 2 is connected.

A pulse such as NO (a) is applied to the base of the transistor 19, and a pulse (a) is applied to the base of the transistor 200, and when the pulse (a) is 7. As a result, when the base of transistor 19 is at high level, transistor 19 is on, and transistor 2 is on.
0 is off, a forward bias is applied to the main transistor 2, and at the same time, the capacitor 21 is charged 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. Figure 6 shows the base voltage waveform of main transistor 2 ((c)
, 倶). In Figure 6) is the input curve ζrus (a).

If the duty cycle of (1)) is small, Fig. 6 CB
) shows waveforms when the input pulses (a) and (b) have large duty cycles. In the circuit of this embodiment, if the capacitance of the capacitor 21 is larger than a certain value, the input pulse ← is determined only by the constant voltage element, so the input pulse ←) 6
As shown in the figure, there is no change in the reverse bias voltage.

In addition, since there is no effect on voltage fluctuations of the input power source, it is possible to always apply a negative voltage up to the emitter-base breakdown voltage vEBo 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 breakdown voltage between the collector and emitter when an inverse voltage (Ias) 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 region 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 much smaller number of parts than 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 a low cost. can.

FIG. 7 is an electrical circuit diagram of a switching power supply 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 the pulses are current. Transistors 19 and 20 connected in series with the limiting 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.

The output voltage is then output from output terminals 36 and 36 via a secondary rectifier diode 30 and a filter capacitor 31. The resistors 32 and 33 are output voltage dividing resistors, and the output voltage divided by the resistors 32 and 33 is compared with the voltage of the reference power supply 26 by the error amplifier 25. The error amplifier 25 also corrects the pulses sent to the pulse width modulator 23 so that a constant output voltage is always output from the output terminals 36 and 36.

The circuit shown in FIG. 7 is a general flyback type switching power supply circuit, and in this case, the input power supply is conveniently used as a positive polarity bias power supply for driving the main transistor 2. Main transistor 2 for the same reason as
The reverse bias voltage 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. Unlike the switching power supply shown in FIG. It is used as a positive bias power supply for driving the main transistor 2. As a result, the seventh
The power consumption of the resistor 4 can be reduced compared to the switching power supply circuit shown in the figure. 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 passing 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 transferred to the diode 37. The power is rectified by the power supply and used as a positive bias power supply 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, reducing switching loss and increasing switching speed. Needless to say, even when the current transformer 43 is provided at the emitter of the main transistor 2, similar effects can be obtained.

FIG. 1o shows an example of application to a chopper type switching power supply, in which 40 is a choke inductor, 41 is a flywheel diode, and 42 is a filter capacitor. In this case main transistor 2. Both transistors 19 and 20 are PINP type, but transistors 19 and 20 are N
It can also be configured as a PIN type.

Note that in the conventional switching power supply shown in FIG. 1, when stages other than the first power stage are implemented as monolithic ICs, the potential of the common line is lower in the drive stage and the stages preceding it, so only the stage before the part indicated by the broken line 14 is implemented as an IC. It is possible. However, in the present invention, the portion shown by gland 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 applies pulses to the bases of the two transistors to turn the two transistors on and off alternately. By connecting a parallel circuit consisting of a constant voltage element and a capacitor between the connection point of the two transistors and the base of the main transistor, it is possible to convert the drive stage into an IC, which was previously impossible. On the other hand, it is possible to form a single-chip IC including the drive stage, and in combination with a simple reverse bias circuit with a small number of parts, it is possible to supply a small, lightweight, and highly efficient switching power supply at low cost.

[Brief explanation of the drawing]

Fig. 1 is an electrical circuit diagram of a conventional switching power supply device, Fig. 2 is an electrical circuit diagram of the main parts of the device, Figs. 3 and 4 are waveform diagrams explaining the operation of the device, and Fig. 6 is FIG. 6 is an electric circuit diagram of the main parts of the switching power supply device according to the first embodiment of the present invention, and FIG.
/i, an electric circuit diagram of the same device; FIG. 8 is an electric circuit diagram of a switching power supply device in a second embodiment of the present invention;
The figure is an electric circuit diagram of a switching power supply device according to a third embodiment of the present invention, and FIG. 10 is an electric circuit diagram of a switching power supply device according to a fourth embodiment of the present invention. 2... Main transistor, 19.20... Group transistor, 21... Conde/su, 22...
- Constant voltage element. Name of agent: Patent attorney Toshio Nakao (1 person wJ)
Z Figure 1 Figure 3 Figure 4 Figure 6

Claims (1)

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