JPS6377376A - Overcurrent protective system - Google Patents

Abstract

PURPOSE:To inhibit the value of overcurrents, and to contract the maximum current standards of each device by providing a control means focussing the operating point of overcurrent protection at a point without being subject to the effect of input voltage. CONSTITUTION:Voltage proportional to input voltage is generated in a capacitor C1, and voltage proportional to output voltage through a coil 33 is generated in a capacitor C2. These voltage is applied to resistors R2, R3 and a control means for a Zener diode DZ1, thus adjusting base voltage where a transistor Q2 is brought to an ON state. Accordingly, a current value under the state of overcurrents is reduced to a drooping type without being subject to the effect of input voltage, thus focussing the operating point of overcurrent protection at a point.

Description

[Detailed Description of the Invention] [Summary] The present invention relates to a ringing choke, which is a type of DC-DC converter that converts a DC input on the primary side into a DC voltage on the secondary side isolated by a transformer by switching transistors. The overcurrent current applied to the power supply circuit of the converter (RCC) is increased from the normal output voltage value to the low voltage at the overcurrent operating point without affecting input voltage fluctuations, especially when the output current is overcurrent. This invention relates to an overcurrent protection method that reduces the value without increasing it.

In an RCC type converter, generally when a switching transistor on the primary side is turned on, a voltage is generated in a coil on the secondary side, and this is smoothed to obtain an output voltage. At this time, the current flowing between the collector and emitter of the transistor flows through a resistor connected to the emitter and is converted into a voltage. An overcurrent protection method is used in which the base current of the transistor is controlled by this voltage and the transistor is turned off when the collector current increases above a certain value.

In the present invention, a voltage proportional to the input DC voltage is generated in the first capacitor, and a voltage proportional to the output voltage is generated in the second capacitor. A resistor and a Zener diode are used as control means for increasing or decreasing the voltage of the first capacitor when the input voltage increases or decreases, and using the fluctuating voltage to reduce fluctuations in the collector current flowing through the transistor at the time of overcurrent. It is formed using

This control means makes it possible to narrow down the operating point at the time of overcurrent to one point, regardless of fluctuations in input voltage.

By confining the overcurrent operating point to one point regardless of input voltage fluctuations, the present invention can suppress the overcurrent value, reduce the maximum current specification for each device, and make each device smaller. can be used. In addition, since the overall current can be reduced, the amount of heat generated is also small, and therefore a small heat sink etc. can be used, and there is an effect that a compact power supply can be constructed in terms of packaging.

[Industrial application field]

The present invention relates to a circuit configuration of a ringing choke (RCC'), which is a type of DC-DC converter that converts a DC input into an isolated DC voltage by switching transistors. In particular, when the output is short-circuited and the output current becomes overcurrent, it is possible to suppress the overcurrent value and keep the overcurrent protection operating point at a single point without being affected by input voltage fluctuations. This invention relates to an overcurrent protection method that makes it possible.

[Traditional technique]

When using a DC power source as an input power source, or when supplying DC voltage to integrated circuit elements in personal computers, microcomputers, minicomputers, etc., the 100V AC voltage is first converted to a high DC voltage, and then the transistor A DC-DC converter is used that converts the DC voltage into another DC voltage via a converter by switching. This DC-DC converter instantaneously converts a high direct current voltage into an alternating current voltage by switching transistors, then transforms the voltage to the secondary side using a transformer, and performs rectification and smoothing. Since this converter can use a transformer, it has the advantage that not only the primary side and the secondary side are DC isolated, but also that it can convert to any voltage by appropriately selecting the number of turns of the transformer. Furthermore, because the transistor is used with the transistor on or off, there is little loss and the efficiency is generally high. Advances in semiconductor technology have made switching transistors faster and with higher voltage resistance.
High-current power transistors also have high frequencies, large capacity, and miniaturized transistors that have been put into practical use. There are various basic circuits of DC-DC converters, such as push-pull type, half-bridge type, and full-bridge type, but the ringing choke (RCC) type has the advantage of being an extremely simple circuit. However, in general, this RCC method has a drawback that low voltage accuracy is somewhat poor when load fluctuations are large, since voltage changes occur due to output current fluctuations.

FIG. 9 shows a circuit according to the conventional RCC type overcurrent protection system.

Terminal 2 is at zero potential, and a DC voltage of, for example, 130 V is supplied to terminal 1 from an AC voltage via an AC-DC converter. Transistor Q1 is a switching transistor. When the transistor Q is on, the NPN
) Current flows from the collector to the emitter of transistor Q. When the transistor Q is on, magnetic energy is accumulated in the iron core 32 of the transformer 3, and the voltage half-wave rectified by the diode 4 on the secondary side is converted to a constant DC voltage by the smoothing capacitor 5, and the voltage is applied to the output load resistor 6. Supplied. When a current flows between the collector and emitter of the transistor Q, a voltage is generated at the other end, point A, which is not the ground terminal of the resistor RflET connected to the emitter side. Since point A is connected to the base of NPN transistor Q2 via resistor 20, and the emitter of transistor Q2 is connected to ground point 2, the voltage generated at point A causes the base voltage of transistor Q2 to become the base-emitter voltage. As VBE drops (0.6 volts to 0.8 volts), transistor Q2 becomes active or saturated. If the output load resistance 6 has a certain size and a sufficient normal DC voltage (for example, 5V) is supplied to the load resistance 6, the voltage generated at point A is not so large, and the transistor Qz It is in the off state. In this case, the transistor Q,
The coil 33 connected to the primary side of the transformer 3 is turned on.
The base current of the transistor Q is controlled via the capacitor C by the voltage detected from the capacitor C. When the collector current of the transistor Q increases beyond a certain value, the transistor Q is turned off and the magnetic energy stored in the transformer 3 is released via the secondary coil 31.

In this manner, when operating normally, the transistor Q1 is turned on and off at regular intervals.

However, if the value of the load resistor 6 is very small and the output is short-circuited, an overcurrent will flow through the load resistor 6 in the secondary circuit. This overcurrent is caused by the secondary coil 31
The magnetic energy is transmitted to the coil 30 on the primary side, and a current corresponding to the overcurrent on the secondary side is generated, and this current flows between the collector and emitter of the transistor Q.

Due to this current, the voltage at point A becomes a much higher value than the normal voltage. This voltage is supplied to the base of transistor Q2 through resistor 20, so transistor Q
2 is turned on. When the transistor Q2 is turned on, a current flows to the collector of the transistor Q2 from the terminal 1 through the resistor 21, and the collector potential becomes low. Since this collector is connected to the base of the transistor Q1, the base voltage of the transistor Q is lowered, and the transistor Q is forcibly turned off. When the transistor Ql is turned off, no current flows between the collector and emitter, and the potential at point A drops to 0. When the potential at point A drops to O■, the base voltage of transistor Q2 becomes VB! , and the transistor Q2 is turned off. When transistor Q2 turns off, transistor Q.

Since the base voltage of Q increases, the transistor Q turns on. When Q is turned on, the potential at point A increases and Qt is turned on. Therefore, the output load resistance 6
An overcurrent flows until the output is 0, that is, the output is short-circuited, and during that time, the transistors Q1 and Qt are repeatedly turned on and off so that when one is on, the other is off. By repeating this turning on and off, the output voltage supplied to the load resistor 6 gradually decreases. That is, in the conventional overcurrent method, in an overcurrent state, transistor Q1 is turned off by turning on transistor Q2, and the collector current flowing through transistor Q is suppressed. However, it is not possible to keep Ql in an off state all the time. It protects against overcurrent by repeatedly turning on and off. Therefore, the characteristics of the output voltage and output current on the secondary side of the circuit according to the conventional overcurrent protection method are as shown in FIG. If the load resistance 6 has a certain value, the output voltage will be normal 5.
However, when the load resistance 6 becomes smaller, an overcurrent state occurs, and the output voltage gradually decreases while the overcurrent gradually increases until the output becomes short-circuited. Furthermore, in the circuit of FIG. 9 according to this conventional overcurrent protection method, there is some variation in the alternating current voltage (A
For example, when the voltage drops from 100V to 90V or rises to 110V, as shown in FIG. 10, the voltage-current characteristics of the output vary depending on the fluctuation of the input. In particular, at the ultimate point where the load resistance 6 becomes 0 ohm and the output becomes short-circuited, this difference is large, and the operating point of the overcurrent protection, that is, the ultimate operating point where the output becomes short-circuited, is due to input fluctuations. to be influenced.

[Problem that the invention seeks to solve]

In this way, in a circuit that follows the conventional overcurrent protection method, by turning on transistor Q2, transistor Q is turned off and overcurrent is suppressed. To provide overcurrent protection, the characteristics of the output voltage and output current on the secondary side in an overcurrent state are such that the output voltage drops while the overcurrent increases, and at the ultimate overcurrent protection operating point, the input voltage Its characteristics differ depending on the fluctuation. As a result, the overcurrent value is large and is affected by input voltage fluctuations, so each device needs to have a large current rating. This means that the size of each device cannot be reduced, and that the overall current is large and that a large heat sink or the like is required due to the generation of heat. Therefore, as a mounting technology, it has the disadvantage of requiring a large power supply. The present invention eliminates such conventional drawbacks, narrows down the operating point of overcurrent protection to one point regardless of input voltage fluctuation, and further makes the characteristics of output voltage and output current drooping at the time of overcurrent. Provides an overcurrent protection method that enables

[Means for solving problems]

A circuit according to the overcurrent protection method of the present invention is shown in FIG. In the circuit of FIG. 1, transistor QI.

Qt corresponds to transistors Q, , Q2 of the circuit according to the conventional overcurrent protection scheme shown in FIG. 9, respectively. Similarly, the coil 33 and capacitor C are also connected to the coil 33 in FIG.
and corresponds to capacitor C.

In addition to such a conventional circuit, the present invention includes a first capacitor CI that generates a voltage proportional to the DC voltage, and a second capacitor C2 that generates a voltage proportional to the output voltage via the coil 33. , two diodes D, , D, which turn off when transistor Q2 turns on,
A resistor R2 and a zener are inserted to increase or decrease the voltage of the first capacitor C1 when the voltage of the human power increases or decreases, and to reduce the fluctuation of the collector current of Qt at the time of overcurrent due to the fluctuating voltage. There is a diode DZ. Furthermore, when there is no overcurrent state, one end α of the capacitor C2 that holds the output voltage at a certain value
The divided potential of the negative potential of point A and the positive potential of point A is divided by the transistor Q
The resistor R3 is added in order to operate the circuit so that the divided potential becomes smaller when the output voltage decreases during overcurrent. That is, in the present invention, the voltage at point A is adjusted by resistor R2 + Zener diode DZ, and resistor R1 to reach the base-emitter voltage VIIE that should turn on transistor Q2, and the voltage at point A is adjusted to reach the voltage VIIE between the base and emitter that should turn on transistor Q2. A control means is formed to narrow down the operating point of current protection to one point.

[For production]

In the circuit shown in FIG. 1, the present invention generates a voltage proportional to the input voltage in the capacitor C2, generates a voltage proportional to the output voltage in the capacitor C2, and connects these voltages to the resistor R2, the Zener diode DZ, and the control means of resistor R3 to adjust the base voltage that turns on transistor Q2, and as shown in the output characteristics in Figure 2, the current value in the overcurrent state is adjusted without being affected by the input voltage. To reduce the overcurrent protection to a drooping type, and narrow down the operating point P of overcurrent protection to one point.

〔Example〕

Next, a circuit according to the overcurrent protection method of the present invention will be explained with reference to the drawings.

FIG. 1 is a configuration diagram of a control circuit according to the overcurrent protection method of the present invention.

First, with reference to FIG. 3, the functions of the resistor R1 and Zener diode D Z + inserted to narrow down the operating point of overcurrent protection to one point P regardless of input voltage fluctuations will be explained. Figure 3 shows transistors Q and Q2 of the circuit in Figure 1.
This is the circuit around the . The emitter terminal of transistor Q is point A, the other end of capacitor C2 that holds a voltage proportional to the output voltage, which is not ground point 2, is α, and the other end of capacitor C1, which holds a voltage proportional to the input voltage, is not ground point 2, etc. The end is shown in FIG. 3 as β. And between point A and ground point 2, there is a resistor RDI? A resistor R2 is connected between the point A and the base of the transistor Q2.

A series circuit of a resistor RX and a Zener diode DZ is connected between the base terminal and point β. Further, a resistor R1 is connected between the base of the transistor Q2 and the point α.

In the conventional circuit, as shown in FIG. 9, if there are no resistors R2 and R3, there is a Zener diode DZ.

Nor. There is a resistor R1, but if the resistor R2 and Zener diode DZ are not present, the output load resistance will be small, and if an overcurrent condition occurs, the output characteristics during overcurrent will be as shown in Figure 4. , shift to the right. This will be explained first.

In such a circuit, resistor R2 and Zener diode D
Since Z is absent, in the circuit of FIG. 3, a voltage proportional to the input voltage is not fed back to the base voltage of transistor Q2. Now, if the output power is constant, the power on the primary and secondary sides of the transformer 3 is always equal, so if the input voltage increases, in order to keep the power constant, the power on the primary side of the transformer 3 must be equal. The current () flowing through the side coil 30 becomes smaller. That is,
When the input voltage increases from, for example, ioo v to 110 s, the current s becomes small. Since this current (2) is applied to the resistor RDtT connected to the emitter side of the transistor Q1, the voltage at point A becomes small. Then, the base voltage of transistor Q2 becomes smaller, so transistor Q
2 progresses to the on state at a slower rate. Transistor Q2
The fact that it is still in the off state means that the transistor Q is in the on state, so when the input voltage rises, the collector current I flowing through the transistor Q is suppressed by the slow speed at which Q goes to the off state. I can't. Therefore, the current on the output side cannot be suppressed, and in the event of overcurrent, as shown in Figure 4, when the input voltage increases,
・The overcurrent point will shift to the right. In other words, if the operating point of overcurrent protection when the input voltage is 100V is P, then
When the input voltage increases to 110V, the operating point of the overcurrent protection is P', and this P' is shifted to the right of P.

The output characteristic curve in Figure 4 already has a shape in which the curve at the overcurrent value wraps inward due to the presence of the resistor R1, but in the conventional circuit without R2 and the Zener diodes DZI and R3, the input voltage increases. In this case, not only does the overcurrent shift to the right, but the output characteristic becomes a curve that gradually descends diagonally downward to the right, as shown in FIG.

In the present invention, as shown in FIG. 3, a resistor R2, a Zener diode DZ, and a resistor R3 are inserted.

By inserting these elements, the operating point of overcurrent protection can be narrowed down to one point P, regardless of input voltage fluctuations, as shown in the output characteristics of FIG. 2 or FIG. 5. That is, in the present invention, a voltage proportional to the input voltage is generated at the point β at one end of the first capacitor CI, a voltage proportional to the output voltage is generated at the α point in the capacitor C2, and these voltages are connected to the resistor Rz. By feeding back the voltage to the base voltage of the transistor Q2 using the Zener diode DZ+ and the resistor R3 and adjusting the amount, the overcurrent characteristics can be adjusted toward the operating point P of the overcurrent protection without being affected by the input voltage. It becomes possible to narrow it down to one point. In this case, the function of the resistor R2 and the Zener diode DZ is mainly controlled so that the ultimate point at which the load becomes short-circuited, that is, the operating point P of overcurrent protection, is narrowed down to one point regardless of input voltage fluctuations. Resistance R1
Compared to the current value before entering the overcurrent, the value of the overcurrent until it reaches the ultimate overcurrent protection operating point P is smaller, and the output characteristics are such that it curls inward in the overcurrent state. to control.

First, the reason why the operating point P of the overcurrent protection is narrowed down to one point even if the input voltage fluctuates due to the function of the resistor R2 and the Zener diode DZ+ will be explained below.

In the circuit of FIG. 3, the voltage V3 at point β is a voltage proportional to the input voltage and is the voltage generated at one end β of the first capacitor C1. Now, consider the case where the input voltage increases from 100V to 110V. If the input voltage increases, V3, which is a voltage proportional to the input voltage, naturally increases. Voltage V
3, a current flows through the resistor R2 and the Zener diode DZ.

In this case, both ends of the Zener diode D Z + are at a constant voltage due to the action of the Zener diode.

Now, let vIIE be the base voltage when transistor Q2 is turned on. When the input voltage increases and (3) increases, the base voltage of the transistor Q2 also increases. Therefore, when the input voltage increases, the speed at which transistor Q2 turns on becomes faster. When transistor Q2 turns on, transistor Q1 turns off, and the current I flowing through Q becomes smaller. Then, the voltage V at point A decreases, and the base voltage of transistor Q2 decreases.In other words, if the input voltage increases, the voltage V at point A decreases.
The base voltage of Q2 increases, but as Q is turned off, the base voltage of Q2 decreases. However, the decrease in the base voltage of transistor Q2 is actually compensated for by the increase in input voltage. As a result, even if there is a fluctuating voltage at the input, fluctuations in the collector current I flowing through the transistor Q1 at the time of overcurrent are reduced.

In the present invention, even if the input terminal voltage increases from 100■ to 110■, the base voltage VB! to turn on the transistor Q2! The speed at which the input voltage is applied is the same as when the input voltage is 100V. This is the same even when the input voltage drops from 100V to 90V. When the input voltage drops, v3 drops, the base potential of transistor Q2 drops, and transistor Ql turns on, and the potential at point A goes up. This increased potential causes the base potential of transistor Q2 to rise, but this is canceled by the decrease in input voltage. Therefore, even if the input voltage drops from 100V to 90V, the speed at which the transistor Q2 reaches the base potential 1 for turning on is as follows: It will be the same.

If we look at this in terms of output characteristics, as shown in Figure 2 or Figure 5, even when the input voltage is 90V or 110V,
This means that the operating points P of overcurrent protection intersect at one point, as in the case of 100V. As explained above, the function of the resistor R2 and the Zener diode DZ is to suppress fluctuations in the input voltage.

As shown in Figure 2 or Figure 5, the input voltage is 90V,
At 100 V or 110 V (7) respectively, the overcurrent value corresponding to the overcurrent protection operating point P is smaller than the maximum current value in the overcurrent state, and the reason why the output characteristics are narrowed inward is due to the resistance R5. By works. This will be explained next using FIGS. 6, 7, and 8.

6 and 7 have the same circuit configuration, but in FIG. 6, the output load resistance 6 has a certain value and is in an overcurrent state before entering the operating point P of overcurrent protection. This is a state before the output voltage reaches a low output voltage corresponding to the operating point P of overcurrent protection. In this state, it is assumed that the output voltage has a value other than zero.

One end α of the capacitor C2, which is not the ground point 2, stores the output voltage generated in the coil 33 of the circuit of FIG. This is because when the diode D2 is on, a current flows into the black spot of the coil 33 due to the closed loop composed of the capacitor C2, the diode Dt, and the coil 33, and a negative voltage is generated in the capacitor C2. That is, it is assumed that the potential at point α is negative and is -■2.

If the base voltage of the transistor Q2 reaches the base-emitter forward voltage (2), the transistor Q2 is turned on and the transistor Ql is turned off.

In other words, the potential at point A when transistor Q2 is turned on is ■
8 and the potential of -V2, which is the potential at point α, becomes a problem.

Assume now that the transistor Q1 is in the on state and that a current with the maximum overcurrent is flowing, and that the maximum voltage value of the output characteristic corresponds to (2). Since this voltage (1) is a voltage generated across the resistor RDEr, it is a positive voltage. On the other hand, the voltage at one end α of the resistor R3 is a negative voltage of −v2 proportional to the output voltage. Since the resistors R3 and R3 are commonly connected to the base terminal of the transistor Q2, the base voltage VIE that should turn on the transistor Q2 is the positive voltage V, and the negative voltage -v2 is connected to the resistor R+.
+ Rs...(1) In formula (1),
■It is assumed that 2 is not O. In other words, even in an overcurrent state, the ultimate overcurrent operating point P is not reached, the output voltage is close to the normal value of 5, and the collector current I corresponding to the overcurrent at that time is It is flowing to Q1. In such a state, if the output negative resistor 6 becomes 0Ω, that is, becomes short-circuited and reaches the overcurrent protection operating point P, the output voltage becomes almost O. At this time, the voltage stored in the capacitor C2 changes from -V2 to -v2. This state, ie, the state when the load is short-circuited, is the circuit shown in FIG.

At this time, the voltage generated at point A by the current I flowing through the transistor Q1 is Vl. In this circuit state, the base voltage of the transistor Q2 becomes VIE. The base voltage at which the transistor Q2 should be turned on is given by the equation (1) in which vl is changed to V l and V Z is changed to v2. Since the output is shorted, v2 is almost zero. Equation (1) when v2 is O is given by the following equation (2).

■ in equation (1) and ■1 in equation (2) are both transistor Q
This is the base voltage for turning on the circuit 2, and this is a constant value of about 0.6V to 0.8V. When formula (2) is transformed, it becomes the following formula (3).

On the other hand, when formula (1) is transformed, the following formula (4) is obtained.

・ ・ ・(4) From equations (3) and (4), the magnitude relationship between ■1 and V is Vl
>v, ...(5).

As shown in the output characteristics of Fig. 8, this means that the voltage of R DEr at the operating point P of overcurrent protection when the output is short-circuited is higher than the voltage when the overcurrent becomes the largest.
Small means good. That is, since the base voltage for turning on the transistor Q2 and the Rtill voltage to reach the on-voltage of ■1 is smaller than ■1, V is smaller.
At the operating point P, the current value I is small. In other words, if vl is small, it means that vl is small.
Since RIIET×I, it operates when ■ is small.

This is the reason why the output characteristics are narrowed inward.

In this way, in the present invention, the overcurrent operating point P is set to one point by the resistor R2 and the Zener diode DZ+ without being affected by the input voltage, and the overcurrent output characteristic is narrowed inward to the P point by the resistor R3. It will be complicated.

〔Effect of the invention〕

The present invention makes it possible to narrow down the overcurrent operating point to a single point regardless of input voltage fluctuations, thereby suppressing the overcurrent value and reducing the maximum current specification for each device. You can use a small one as a. In addition, since the overall current can be reduced, the amount of heat generated is also small, and therefore a small heat sink etc. can be used, and there is an effect that a compact power supply can be constructed in terms of packaging.

[Brief explanation of the drawing]

Figure 1 is a circuit configuration diagram according to the overcurrent protection method of the present invention, Figure 2 is an output according to the overcurrent protection method of the present invention, and Figure 3 is at the operating point P of the overcurrent protection of the overcurrent protection method of the present invention. A circuit diagram explaining the operation. Figure 4 shows the output characteristics when the resistor R2 and Zener diode DZ are not present in Figure 3. Figure 5 shows the output characteristics when the resistor R2 and Zener diode DZ of the overcurrent protection method of the present invention are present. Output characteristics of Figures 6 and 7
The figure is a circuit diagram explaining the function of the resistor R3 in the overcurrent protection system of the present invention, and FIG. 8 shows the output characteristics due to the function of the resistor R3 in the overcurrent protection system of the present invention. Fig. 9 is a circuit configuration diagram according to the conventional overcurrent protection method, and Fig. 10 is a % formula at output according to the conventional overcurrent protection method. ...Resistance, 30.31.33...Coil, Q,,QZ...NPN l-transistor, RDE
T, R1, R2, R:l...Resistance, D+, Dz
・ ・ ・Diode, DZI... Zener diode, CI, C2... Capacitor, A, β, α... Terminal. Patent Applicant: Panafacom Co., Ltd. Book 4S U-Me ■Electricity 7 holes = 5 methods to prompt the circuit, t-bottom diagram Figure 1 Figure 2 Circuit diagram for familiarization Figure 3 Output turtle tt (A) Figure 4 ■ Output turtle bi (A) Park Ming The energizing current, L, resistance R2, and the output of the tzedegion OZ+S'CTN, 5, 6, 7, The power distribution of the valve September is 4°'F''b according to the low weight of the light 4 and the low weight of the tR3. Arrival H student figure 10 procedure amendment document December lJ, 1986

Claims (1)

[Claims] 1) In a converter circuit that converts a DC input on a primary side into a DC voltage on a secondary side using a transformer by switching a transistor, a switching circuit connected to a coil on the primary side of the transformer is provided. a first transistor, a second transistor that controls the base voltage of the first transistor, a first capacitor that holds a voltage proportional to the DC voltage on the primary side, and a voltage proportional to the DC voltage on the secondary side. a second capacitor that holds the voltage of the first capacitor; a first control means that controls the voltage of the base terminal of the second transistor based on the voltage of the first capacitor; and a second control means that applies a divided voltage to the base terminal of the second transistor with a voltage proportional to the current flowing through the transistor, and when an overcurrent occurs at an operating point of overcurrent protection independent of fluctuations in input voltage. An overcurrent protection method characterized by narrowing down the output characteristics. 2) The collector terminal of the first transistor is connected to one end of the primary coil of the transformer, the emitter terminal is connected to one end of the detection resistor, and the base terminal of the second transistor is connected to one end of the primary coil of the transformer. The first transistor is connected to the emitter terminal of the first transistor via a first resistor, the collector terminal is connected to the base terminal of the first transistor, the emitter terminal is connected to the other end of the detection resistor, and the first transistor is connected to the emitter terminal of the first transistor via a first resistor. A capacitor No. 1 generates at one end a voltage proportional to the DC input voltage applied between the other end of the primary coil of the transformer and the other end of the detection resistor, and the other end generates a voltage proportional to the DC input voltage applied between the other end of the primary coil of the transformer and the other end of the detection resistor. the second capacitor, one end of which is connected to the other end of the detection resistor, one end of which generates a voltage proportional to the output voltage generated via the transformer, and the other end of which is connected to the other end of the detection resistor The first control means is connected between the one end of the first capacitor and the base terminal of the second transistor, and includes a series circuit comprising a second resistor and a Zener diode. The set second control means has a third resistor connected to the one end of the second capacitor and the base terminal of the transistor Q_2, and when the input voltage increases or decreases, the set second control means The voltage of the second capacitor is increased or decreased, and the fluctuating voltage is applied to the base terminal of the second transistor via the series circuit, and the voltage of the second capacitor is also increased or decreased.
2. The overcurrent protection system according to claim 1, wherein the overcurrent protection method is applied to the base terminal of the second transistor through a resistor.