JPS6223542B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a power supply circuit, and particularly to, for example, an input/output isolated switching type power supply circuit.

In recent years, many switching type power supply circuits (so-called switching regulators) have been put into practical use, especially as power supplies for IC circuits and the like. This switching type power supply circuit has advantages such as being smaller in size and having less power loss than conventional power supply circuits. By the way, some switching type power supply circuits have a common ground for input and output, and others have input and output insulated by a high-frequency transformer. The former common ground type is suitable as a power supply circuit for a DC power supply, but when used for an AC power supply such as a commercial power supply, an AC transformer must be inserted between the commercial power supply and the input. There must be insulation between the two. However, since this AC transformer is large, it has the disadvantage that the circuit becomes large. On the other hand, in an input/output insulation type power supply circuit, the input side and the output side are insulated by a high frequency transformer, so there is no need to insulate between the input of the power supply circuit and the AC power supply. Furthermore, high frequency transformers are approximately 1/4 to 1/4 the size of AC transformers.
Because it is about 1/5 the size, the entire circuit can be significantly smaller than conventional power supply circuits. In this way, an input/output isolated type power supply circuit has great merits when used in an AC power supply.

FIG. 1 is a circuit diagram showing a conventional input/output isolated switching type power supply circuit which is the background of the present invention. In the configuration, for example, an AC power supply of 100V is applied to the rectifier circuit 1. This rectifier circuit 1 includes a diode bridge 11, a smoothing capacitor 12, and the like. The rectified output of the rectifier circuit 1 is applied to the collector of a transistor 24 via a primary winding 21 of a high frequency transformer 20. The emitter of this transistor 24 is grounded. Further, the high frequency transformer 20 is provided with a winding 22 for generating a base current of the transistor 24, and the current generated in the winding 22 is applied to the base of the transistor 24 via a capacitor 25. Further, the rectified output of the rectifier circuit 1 is connected to the transistor 2 through a resistor 26.
4 and the capacitor 25, and also to the phototransistor 27. Note that this phototransistor 27 constitutes a photocoupler in cooperation with a light emitting diode 30, which will be described later.

On the other hand, at both ends of the secondary winding 23 of the high frequency transformer 20, there is a series circuit consisting of a capacitor 29 charged by the current induced in the secondary winding 23 and a diode 28 for charging this capacitor 29 to one polarity. Connected. Furthermore, a series circuit of the phototransistor 27, a light emitting diode 30, a resistor 31, and a constant voltage diode 32, which constitute a photocoupler, is connected to both ends of the capacitor 29. Furthermore, a load 33 is connected to both ends of the capacitor 29.

FIG. 2 is a waveform diagram of each part of the circuit of FIG. 1.
The operation of the circuit shown in FIG. 1 will be explained below with reference to FIG.

First, the transistor 24 begins to turn on due to the base current flowing from the rectifier circuit 1 through the resistor 26. At this time, the voltage change of the primary winding 21 of the high frequency transformer 20, that is, the change of the collector voltage E2 of the transistor 24, is caused by the change in the base current winding 21.
is positively fed back to the base of the transistor 24. Therefore, as shown in FIG. 2, the current i1 increases rapidly. Here, when the transistor 24 is on, that is, when the voltage E2 is at a low level, a voltage is applied to the output rectifying diode 28 in the opposite direction, so that no current flows to the secondary side. However, when the current i1 increases to a certain point, the base current of the transistor 24 becomes insufficient or h _FE becomes insufficient, and the transistor 24 becomes unable to maintain the on state.
The impedance between the collector and emitter begins to rise. When the impedance begins to rise, the voltage across the primary winding 21 of the high frequency transformer 20 begins to decrease, and the voltage across the base current winding 22 also decreases. Therefore, the base current of the transistor 24 decreases, and the transistor 24 is rapidly turned off in a positive feedback manner. Therefore, as shown in FIG. 2, the current i1 and the voltage E1 induced in the base current winding 22 decrease rapidly.

At this time, the peak current of the transistor 24 is Ip
If the inductance of the transformer 20 is L, the energy E stored in the transformer 20 is
E=1/ ^2LIp2 . This energy E turns off the transistor 24 and the high frequency transformer 2
The voltage is discharged to the secondary side of 0, is smoothed by a capacitor 29 via a diode 28, and is then supplied to a load 33. Here, the current i2 flowing to the secondary side decreases with time as shown in FIG. 2, and finally stops, but when this current i2 stops, a surge voltage is generated. This surge voltage also appears in the base current winding 22 of the high frequency transformer 20, thereby turning on the transistor 24 again. After this, the same operation as described above is repeated.

Note that when the voltage across the capacitor 29 becomes too large during an overload and exceeds the threshold voltage of the high voltage diode 32, current is supplied to the light emitting diode 30 and it emits light. In response, the phototransistor 27 forming a photocoupler with the light emitting diode 30 becomes conductive, thereby bypassing the base current of the transistor 24. When the voltage across the capacitor 29 is large, the bypass current also becomes large, so that the on period of the transistor 24 becomes shorter and the off period of the transistor 24 becomes longer, and as a result, the capacitor 29
Acts in the direction of lowering the voltage across the terminal.

By the way, the circuit of FIG. 1 has a base current winding 22.
This has the disadvantage that the high frequency transformer 20 becomes large.

Therefore, the main object of the present invention is to overcome the above-mentioned drawbacks and to provide a miniaturized isolated switching type power supply circuit that does not require a base current winding.

In summary, this invention applies to the primary coil in an isolated power supply circuit that applies the output of a rectifier circuit to a primary coil such as a high frequency transformer, rectifies and smoothes the voltage induced in a secondary coil, and applies the resulting voltage to a load. A first switching means for controlling the current flowing through the coil, a second switching means for controlling the first control voltage applied to the first switching means, and a second switching means for controlling the current flowing through the coil. The device includes a charging circuit that generates a second control voltage to be applied to the switching means, and a diode that bypasses the discharge current of the charging circuit to the first switching means when the first switching means is turned on.

Hereinafter, this invention will be described in more detail with reference to embodiments shown in the drawings.

FIG. 3 is a circuit diagram showing an embodiment of the present invention. In terms of construction, this embodiment is similar to the circuit shown in FIG. 1 except for the following points, and corresponding parts are given the same reference numerals and their explanation will be omitted. The output of the rectifier circuit 1 is connected to the primary winding 4 of the high frequency transformer 40.
1 to the collector of a transistor 43 as a first switching means. The emitter of this transistor 43 is grounded. Further, the output of the rectifier circuit 1 is applied via a resistor 44 to the emitter of a transistor 45 serving as a second switching means, and is also applied to a capacitor 46 for charging the base current of this transistor 45. The collector output of the transistor 45 is given to the base of the transistor 43, and
It is applied to phototransistor 27. Further, the base of the transistor 45 is connected to its own emitter via a resistor 47, and is grounded via a constant voltage diode 48. Furthermore, the base of transistor 45 is connected to resistor 49 and diode 5.
0 to the collector of the transistor 43.

On the other hand, on the secondary side, a series circuit of a resistor 51 and a resistor 52 is connected to both ends of the capacitor 29, that is, to both ends of the load 33. Further, a transistor 53 is inserted between the resistor 31 and the constant voltage diode 32, and the base of this transistor 53 is connected to the resistor 5.
1 and the resistor 52.

FIG. 4 is a waveform diagram of each part of the circuit of FIG. 3.
The operation of the circuit shown in FIG. 3 will be described below with reference to FIG. 4.

First, when the power is turned on in the initial state, transistors 43 and 45 are off;
Therefore, capacitor 46 begins to charge through resistor 44. Therefore, the charging voltage Ec of the capacitor 46 increases as shown in FIG. When this charging voltage Ec reaches the threshold voltage of the constant voltage diode 48, the transistor 45 is turned on, and the discharge current of the capacitor 46 flows through the emitter-collector path of this transistor 45 to the base of the transistor 43. is turned on. Then, since the collector voltage Ect becomes low level, the diode 50 is turned on, the base current of the transistor 45 increases, and the capacitor 46
The discharge current increases at once in a positive feedback manner and flows to the base of the transistor 43. Therefore, the transistor 43 becomes saturated. Here, while the transistor 43 is on, a voltage is applied to the diode 28 in the opposite direction, so no current flows to the secondary side. Therefore, the load on the transistor 43 becomes only the inductance L of the high frequency transformer 40, and the coil current iL and the collector current ic of the transistor 43 increase linearly as shown in FIG. In this way, the transistor 43
After reaching the saturated state, the base current decreases and exits from the saturated state, the collector voltage Ect increases, the diode 50 becomes non-conductive, the base current of the transistor 45 becomes 0, and the transistor 4
5 is off. In this way, transistor 43 is rapidly turned off in a positive feedback manner.

When the transistor 43 is turned off, the diode 28 on the secondary side becomes forward biased, and the energy stored in the high frequency transformer 40 is released to the secondary side. This released energy is rectified by the diode 28 and the smoothing capacitor 29, and is applied to the load 33. At this time, transistor 4
Since capacitors 3 and 45 are off, charging of the capacitor 46 starts again, and the voltage Ec rises again. In addition,
The voltage regulator diode 48 etc. is selected so that this voltage Ec is still about 1/2 to 1/3 of the threshold voltage of the voltage regulator diode 48 when the energy release to the secondary side is finished, so high frequency transformer 40
The transistors 43 and 45 remain off until the energy is completely released to the secondary side.

When the energy is completely released to the secondary side, an oscillating waveform is generated at the collector of the transistor 43, but at a portion of this oscillating waveform that touches the low level side, the diode 50 is turned on, and the transistor 43
As the base current flows, the transistor 45 is turned on, and similarly to the initial operation, the transistors 43 and 45 are turned on in a positive feedback manner. Thereafter, this operation is repeated and the output voltage E0 increases. When this output voltage E0 exceeds the threshold voltage of the constant voltage diode 32 and the transistor 53 is turned on, a light emitting diode 30 is generated. In response, the phototransistor 27 forming a photocoupler with the light emitting diode 30 becomes conductive, thereby bypassing the base current of the transistor 43. When the output voltage E0 is large, the bypass current also becomes large, so that the on period of the transistor 43 becomes shorter and the off period becomes longer, which works in the direction of lowering the output voltage E0. Thus, the output voltage E achieved by bypassing the base current of transistor 43 by phototransistor 27
Since the control function of E0 is performed in a direct current manner, by increasing the value of the output voltage smoothing capacitor 29, it is possible to bring the ripple value of the output voltage E0 close to zero. Further, when the transistor 43 is turned on or off, positive feedback is applied by the transistor 45, and the collector voltage of the transistor 43 is
The waveform of Ect becomes extremely rapid as shown in Figure 4. Therefore, the power loss consumed by the transistor 43 is reduced. In addition, the transistor 43
When conducting, no current flows to the secondary side, so
There is no power loss (heat generation) in the transistor 43 when it is conductive. Furthermore, since the current flowing through the resistor 44 when the transistor 43 is off is charged to the capacitor 46 without being wasted, power loss is reduced.

FIG. 5 is a circuit diagram showing another embodiment of the invention. In the embodiment shown in FIG. 6, instead of detecting the output voltage E0, the output current flowing through the load 33 is detected and the base current of the transistor 43 is controlled so that the output current is constant.

Note that the phototransistor 27 is connected to the capacitor 4.
Transistor 4 is connected to both ends of transistor 6.
The base current of the transistor 43 may be controlled by bypassing a portion of the discharge current of the capacitor 46 that is about to flow to the base of the transistor 43 via the phototransistor 27.

As described above, according to the present invention, a base current winding unlike the conventional one is not required, so the transformer can be downsized, and a power supply circuit that is downsized as a whole can be obtained. Furthermore, since it is an isolated power supply circuit, it is possible to obtain a power supply circuit that has less noise and can be used even when the input and output ground levels are different.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional power supply circuit which is the background of this invention. FIG. 2 is a waveform diagram of each part of the circuit of FIG. 1. FIG. 3 is a circuit diagram showing an embodiment of the present invention. FIG. 4 is a waveform diagram of each part of the circuit of FIG. 3. FIG. 5 is a circuit diagram showing another embodiment of the invention. In the figure, 1 is a rectifier circuit, 28 is a secondary rectifier diode, 29 is a smoothing capacitor, 30 is a light emitting diode, 33 is a load, 41 is a primary winding, 4
2 is a secondary winding, 43 and 45 are transistors,
46 is a charging capacitor, 48 is a constant voltage diode, and 50 is a bypass diode.

Claims (1)

[Claims] 1. A rectifier circuit that rectifies alternating current, a primary coil to which the output of the rectifier circuit is applied, a secondary coil that is magnetically coupled to the primary coil, and a secondary coil that is connected in parallel with the secondary coil, and a capacitor charged by a current induced in a secondary coil; a diode inserted between the capacitor and the secondary coil and charging the capacitor to one polarity; a load to which voltage across the capacitor is applied; a first switching means for intermittently controlling the current flowing through the primary coil in response to a first control voltage; a second switching means for controlling the first control voltage in response to a second control voltage; means, a charging circuit that is charged by the output of the rectifier circuit and generates the second control voltage; and a charging circuit that supplies a discharge current of the charging circuit to the first switching means when the first switching means is turned on. Power supply circuit with bypassing diodes. 2. The power supply circuit according to claim 1, further comprising a photocoupler that changes the first control voltage according to the voltage across the capacitor.