JPS6336231B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a self-excited switching regulator using an isolation transformer.

A self-excited switching regulator usually uses a choke coil and is constructed as a non-isolated type (input and output). If an attempt is made to separate the input and output using an isolation transformer, it is difficult to realize a self-excited type. This is because the input side and the output side are coupled by the mutual inductance of the transformer, so oscillation cannot be sustained unless the input side always oscillates in some form. Therefore, conventionally, as shown in FIG. 1, it has to be configured as a separately excited type with a variable frequency oscillator 4. In FIG. 1, 1 and 5 are rectifier and smoothing circuits, 2 is an isolation transformer, 3 is a switching transistor, 6 is a voltage detection circuit, and 7 is a photocoupler. The output voltage is detected by a voltage detection circuit 6, and the frequency of the oscillator 4 is changed via a photocoupler 7 to keep the output voltage constant. However, such a configuration requires an oscillator, and has the disadvantage that the circuit configuration is complicated and cannot be miniaturized.

In view of the above, an object of the present invention is to provide a self-excited switching regulator configured as a self-excited type while separating input and output using an isolation transformer.

An embodiment of the present invention will be described below with reference to FIG. In FIG. 2, 10 is a rectifying and smoothing circuit consisting of a diode bridge 11 and a capacitor 12, 20 is an isolation transformer, and 30 is a switching transistor. 50 is a rectifying and smoothing circuit consisting of a diode 51 and a capacitor 52, and 60 is a resistor 61 and a constant voltage diode 62.
This is a voltage detection circuit consisting of an LED (light emitting diode) 63, and the LED 63 is optically coupled to a PHT (phototransistor) 47 to form a photocoupler. 80 is a protection circuit for absorbing the spike voltage generated in the primary winding of the isolation transformer 20 when the switching transistor 30 switches, and is composed of a diode 81, a capacitor 82, and a resistor 83. The resistor 41 is for current detection, and the voltage generated across the resistor 41 is integrated by an integrating circuit consisting of a resistor 42 and a capacitor 46, and the integrated voltage is transmitted through the resistor 43.
Applied to the base of PHT47. In addition, the collector of the switching transistor 30 and the PHT47
A positive feedback resistor 44 is connected between the base of the positive feedback resistor 44 and the base of the positive feedback resistor 44 . The switching transistor 30 is a resistor 4
The base current supply circuit 5 is supplied with a base current and turned off, but this base current is cut off when the PHT 47 is turned on.

The voltage waveforms at points a, b, c and e, f in Fig. 2 are shown in Fig. 3, and
The light output d of the LED 63 is also shown in FIG.
The operation shown in FIG. 2 will be explained with reference to FIG. In the following description, V1 to V7 are as follows: V1, V2; Voltage at point a (V1<V2) V3; Voltage at point C where LED 63 lights up V4; Voltage at point C where LED 63 goes off (V3>V4) V5 ; Voltage V6 at point e at which the PHT 47 is turned on when there is no optical input; Voltage V7 at point e at which the PHT 47 is turned off when there is no optical input; Base voltage at which the PHT 47 is turned on when there is no optical input. First, when the power is turned on and the voltage at point a changes from 0 to V1, the transistor 30 immediately turns on.
The voltage at point e gradually increases. This voltage
When V5 is reached, the PHT 47 is turned on, and the current that was flowing to the base of the transistor 30 through the resistor 45 begins to flow through the PHT 47, so the base current is cut off and the transistor 30 is turned off. Then, capacitor 46 starts discharging, so the voltage at point e gradually decreases, and when it drops to V6,
PHT47 is turned off, base current is again supplied to transistor 30, and this transistor 30
is turned on. At first, this operation is repeated, and the transistor 30 is turned on at regular intervals.
The time it is on is _t1 . During this process, the capacitor 52 is gradually charged, and the voltage at point c gradually increases. Therefore, when the power is turned on, an excessive current flows into the capacitor 52 and rapid charging is avoided, and the life and reliability of the capacitor 52 are improved. When the transistor 30 is turned on several times, the voltage at point c reaches V3 one after another before the voltage at point e reaches V5. Therefore, PHT47 turns on the LED before the voltage at point e reaches V5.
The transistor 30 is turned on by the optical output of the transistor 63, and the base current is cut off, thereby turning off the transistor 30. Then, when the voltage at point c decreases and reaches V4, the LED 63 turns off. Since the capacitor 46 is discharging while the transistor 30 is off, the voltage at point e has further decreased and is now below V6.
Therefore, when there is no optical input, PHT 47 is immediately turned off, and as a result, transistor 30 is turned on. Therefore, the voltage at point c rises again. When the voltage reaches V3, the LED 63 lights up, the PHT 47 turns on, and the transistor 30 turns off. This operation is repeated and the transistor 30 is turned on.
Although it is repeatedly turned off, the voltage at point e is always lower than V6 during this process, and the on/off period of the transistor 30 (particularly the time t2 when the transistor 30 is on) includes the optical coupling between the LED 63 and the PHT 47. Determined by the closed loop constant, t2 is t1
become smaller.

The positive feedback resistor 44 improves the switching speed of the transistor 30 and reduces the rise and fall of the waveform in both cases of oscillation in the loop via the above-mentioned integrating circuit and oscillation in the loop via optical coupling. It functions to suppress power loss in the transistor 30. Furthermore, the ratio of the resistor 44 and the resistor 43 becomes a large factor that determines the hysteresis voltage V5-V6 in the case of oscillation by a loop via an integrating circuit and the hysteresis voltage V3-V4 in the case of oscillation by a loop via optical coupling. .

When the voltage at point a increases from V1 to V2, the voltage at point c will reach V3 sooner, so
The phase in which LED63 lights up becomes faster, and PHT47
Since the transistor 30 is turned on at an earlier phase, the time t3 during which the transistor 30 is turned on becomes even shorter.

If the load is short-circuited and excessive output current flows,
Since a large voltage is generated across the resistor 41 and the voltage at point e increases, the PHT 47 is turned on and the transistor 30 is turned off regardless of the optical input. Therefore, for example, if the load is short-circuited and the voltage at point c becomes 0V, the LED
Even in the case where the switch 63 does not light up, it is possible to prevent excessive current from flowing and prevent the insulation transformer 20 and the transistor 30 from burning out. In this way, when an excessive current flows, a closed loop via the integrating circuit operates, so the output characteristics of this switching regulator are as shown in Figure 4, and when the rated maximum output current is exceeded, the overcurrent protection function is activated. works.

Although the illustrated voltage detection circuit 60 is used, if a differential amplifier is used to perform more accurate voltage detection, the constant voltage characteristics of the DC output will be further improved.

As described above with respect to the embodiments, according to the present invention, a self-excited switching regulator that isolates the input and output using an isolation transformer can be realized with an extremely simple configuration. Moreover, even if the load is short-circuited and an excessive output current flows, a large voltage will be generated across the voltage detection resistor and the base voltage of the phototransistor will rise, so the phototransistor will turn on regardless of the optical input and will not switch. Since the switching transistor is turned off, this self-excited switching regulator performs an overcurrent protection function.
Therefore, even if the load is short-circuited and the light-emitting diode does not light up, it is possible to prevent an excessive current from flowing and to prevent the insulation transformer or the switching transistor from burning out.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional example, FIG. 2 is a block diagram of an embodiment of the present invention, FIG. 3 is a waveform diagram of each part of FIG. 2, and FIG. 4 is a graph showing output characteristics. 1, 10, 5, 50... Rectifier smoothing circuit, 2, 2
0...Isolation transformer, 3,30...Switching transistor, 4...Variable frequency oscillator, 6,
60... Voltage detection circuit, 7... Photocoupler, 8
0...Spike voltage protection circuit.

Claims (1)

[Claims] 1. An isolation transformer, a switching transistor connected in series to the primary winding of this isolation transformer and connected to a DC input source, a base current supply circuit that supplies a base current to this switching transistor, and a base current supply circuit that supplies a base current to this switching transistor. A phototransistor connected to cut off the current, a current detection resistor connected in series to the primary winding of the isolation transformer and the series circuit of the switching transistor, and integrating the voltage generated across this resistor. an integrating circuit that applies this integrated voltage to the base of the phototransistor; a positive feedback resistor connected between the connection point of the primary winding of the isolation transformer and the switching transistor and the base of the phototransistor; A self-excited switching device comprising a rectifying and smoothing circuit connected to the secondary winding of the isolation transformer, and a voltage detection circuit that detects the voltage of the rectified and smoothed output and lights up a light emitting diode optically coupled to the phototransistor. Regulator.