JP2003052172A - Self-excited switching power supply circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent breakdown of a switching element by impeding input of overcurrent under the transitional condition, such as input of a current, in order to prevent saturation of an inductance element. SOLUTION: A current-detecting element is allocated in a current path, connected with a main switching element and operation of an impedance conversion circuit connected between an input terminal, and control terminal of the main switching element is controlled by the output of the current-detecting element. When the output of the detecting element exceeds the predetermined value, an input signal of the impedance conversion circuit is bypassed to turn off the main switching element. With the state becoming a steady condition, a multivibrator is made to transit to oscillating condition.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited switching power supply circuit, and more particularly to a self-excited switching power supply circuit having a circuit for preventing magnetic saturation of an inductance element (transformer) used therein.

[0002]

2. Description of the Related Art There are several types of self-excited switching power supply circuits, but in general, a multi-vibrator is used to oscillate and a main switching element and an auxiliary switching element are alternately turned on and off to operate. is there. Energy is stored in an inductance element such as a transformer when the main switching element is turned on and off, and energy is output from the inductance element. Figure 2
Has been proposed as such a self-excited switching power supply circuit having a relatively large output power.

The circuit shown in FIG. 2 is basically an application of a monostable multivibrator, which stores energy in a transformer T when the main switching element Q ₁ is on.
Main switching element Q ₁ is rectified its energy by the diode D ₁ in the off, performs smoothing to operation to output a DC voltage to the output terminal V ₀ by the capacitor C _1. Of course, it is also possible to obtain an AC output from the output terminals 3a to 3c.

The input terminal 1 and the control terminal of the main switching element Q ₁ are connected via the resistor R ₁ and the impedance conversion circuit 5, and the primary winding of the transformer T and the auxiliary switching element Q _{2 are connected.}
The control terminal of is connected via a resistor R ₃ and a capacitor C ₂ to form a monostable multivibrator. In addition,
In this example, the voltage at the output terminal is detected, and the output compared with the reference voltage is fed back to the control terminal of the auxiliary switching element Q ₂ via the resistor R ₇ to control the voltage.

In the above impedance conversion circuit, the connection point between the bases is connected to one end of the main current path of the auxiliary switching element Q ₂ , and the connection point between the emitters is connected to the control terminal of the main switching element Q _1. Different polarity
It is composed of a complementary connection circuit of two transistors Q ₄ and Q ₅ . The collector of the NPN transistor Q ₄ of the impedance conversion circuit is connected to the input terminal 1, and the collector of the PNP transistor Q ₅ is connected to GND. As a result, the input capacitance of the main switching element and the auxiliary switching element can be separated, and the auxiliary switching element Q ₂ is not affected even when a transistor with a large input capacitance is used, and stable self-excited oscillation operation is possible. It becomes possible to do.

A bipolar type transistor may be used as the main switching element Q ₁ (transistor).
When the MOSFET is used as in this example, the following problems are likely to occur. That is, it is the resistance from the input terminal 1 to GND that limits the current that flows into the transformer when the main switching element Q ₁ is on. The resistance due to the wiring, the internal resistance of the transformer T, the main switching element Q
There is an internal resistance of ₁ , and it is necessary to make them as small as possible in order to improve power conversion efficiency.

Due to the above demands of the circuit configuration, in a transient state in which the input power is supplied from the input terminal 1 and the circuit performs a steady operation, a considerable amount of current flows into the transformer T (depending on the specification, However, there is a problem that the transformer causes magnetic saturation, and in some cases, the switching element is destroyed. The same applies when the transformer is a single-winding inductance element.

[0008]

SUMMARY OF THE INVENTION The present invention limits the excessive transient current flowing at the start of operation of a switching power supply to prevent magnetic saturation of an inductance element (transformer),
As a result, a highly reliable self-excited switching power supply that does not damage the switching element (transistor) is provided.

[0009]

SUMMARY OF THE INVENTION The present invention solves the above problems by adding means for detecting an excessive transient current and thereby shutting off the main switching element.

That is, the main switching element and the auxiliary switching element constitute a multivibrator, and the main switching element connected to the inductance element is turned on,
In a self-excited switching power supply circuit that is turned off to rectify the voltage induced in the inductance element and smoothes it to obtain a DC output, the impedance between the main current path of the auxiliary switching element and the control terminal of the main switching element. A conversion circuit is connected, and means for detecting an input current in a current path from the input terminal through the main switching element is provided, and when the input current exceeds a predetermined value, the output of the means causes the impedance circuit to pass through the impedance circuit. It is characterized in that the main switching element is turned off, the current flowing into the inductance element is temporarily blocked, and the current exceeding the predetermined input current is prevented.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION As means for detecting an input current, a current detection resistor or a current transformer is used, and these are arranged between an input terminal, an inductance element and a switching element, and reach GND, and the detection signal is impedance. It is input to the input side of the conversion circuit so that the impedance conversion circuit has a switching signal ON / OFF function. As a result, the ON / OFF control of the main switching element can be performed temporarily.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the present invention.
Common components are given the same reference numerals as in FIG.
The input terminal 1 and the control terminal of the main switching element Q ₁ are resistors R ₁
Is connected via the impedance conversion circuit 5, and the primary winding of the transformer T and the control terminal of the auxiliary switching element Q ₂ are connected via the resistor R ₃ and the capacitor C ₂ to form a monostable multivibrator. 2 is the same as the conventional circuit shown in FIG. The connection and operation of the impedance conversion circuit 5 are also the same.

In the example shown in FIG. 1, the conventional one shown in FIG.
The following elements are added to the circuit. Main switching
Element Q₁Current detection resistor R between GND and GND_SConnect the main switch
Element Q₁Source and current sense resistor R_SConnection point and NPN
Langista Q₆The base is resistance R ₉Connected through
It Transistor Q₆The collector of the impedance conversion times
Transistor Q in path 5_Four, Q_FiveConnected to the control terminal of
The emitter is connected to GND. The transistor Q₆And G
Capacitor C connected between ND₃Is resistance R₉With time constant
Is a capacitor for setting.

Next, the operation of this circuit will be described. When the input voltage V _IN is applied to the input terminal 1, the small resistance R _S , GN passes through the primary winding of the transformer T and the main switching element Q _1.
The current flows through the D route. Micro resistance (current detection resistance) R _S
Generates a voltage drop of R _S × I _IN ≡V _S.

[0015] The voltage drop V _S is the forward base of the switching signal controlling the transistor Q ₆ - exceeds the emitter voltage drop V _BE6, the transistor Q ₆ is turned on, so bypassing the switching signal input to the impedance conversion circuit 5 , The main switching element Q ₁ is turned off, and the current flowing into the transformer T is cut off.

The voltage that drops on the detection resistor R _S over time
When V _S becomes V _S <V _BE6 , the transistor Q ₆ is turned off, the switching signal turns on the main switching element Q ₁ , a current flows into the transformer T, and normal operation starts. By such an operation, the operation when the excessive transient current flows can be controlled.

The present invention can be applied to circuits other than the above circuit configuration. For example, the transistors Q ₁ to Q ₆ are may be each other types of devices (bipolar or MOSFET). It is also possible to control the on / off level of the transistor Q ₆ by applying a minute voltage between the base and the emitter of the transistor Q ₆ that does not turn on unless V _S is applied, as a bias. Other current detection means such as a current transformer may be used instead of the detection resistor R _S. In this case, it may be arranged at any position where the primary current can be detected. The transformer may be a single roll. If a complementary type is used for all the elements, a negative polarity output can be obtained, so that a negative output circuit can be configured.

[0018]

According to the present invention, since the input current detecting circuit and the input current limiting circuit can be configured and the switching signal can be controlled at the input side of the impedance conversion circuit, the transient at the time of input of the input voltage is achieved. It is possible to limit excessive input current during operation. It could be reduced to about half that of the conventional one.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional self-excited switching power supply circuit.

[Explanation of symbols]

Q ₁ : Main switching transistor Q ₂ : Auxiliary switching transistor T: Transformer R _S : Detection resistor

Claims (3)

[Claims] 1. A multivibrator comprising a main switching element and an auxiliary switching element, wherein a main switching element connected to an inductance element is turned on and off to rectify and smooth a voltage induced in the inductance element. In a self-excited switching power supply circuit that obtains a DC output, an impedance conversion circuit is connected between the main current path of the auxiliary switching element and the control terminal of the main switching element, and means for detecting the input current is provided. When the value exceeds a predetermined value, the main switching element is turned off via the impedance circuit by the output of the means, the current inflow to the inductance element is temporarily cut off, and the current inflow exceeding the predetermined input current flows. A self-excited switching power supply circuit characterized by preventing 2. A main switching element and an auxiliary switching element constitute a multivibrator, and a main switching element connected to a primary winding of a transformer is turned on / off to be induced in a secondary winding of the transformer. In a self-excited switching power supply circuit that rectifies and smoothes a voltage to obtain a DC output, an impedance conversion circuit is connected between the main current path of the auxiliary switching element and the control terminal of the main switching element to detect the input current. When the input current exceeds a predetermined value, the output of the means turns off the main switching element via the impedance circuit, temporarily interrupts the inflow of current to the transformer, and outputs the predetermined input. A self-excited switching power supply circuit characterized by preventing the inflow of current exceeding the current. 3. The polarity of the impedance conversion circuit, wherein a connection point between bases is connected to one end of a main current path of the auxiliary switching element, and a connection point between emitters is connected to a control terminal of the main switching element. 3. The self-excited switching power supply device according to claim 1 or 2, which is a complementary connection circuit of two transistors different from each other.