JP2002374671A - Overcurrent protective circuit for separately excited converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a separately excited converter of a transistor oscillator which does not use IC is insufficient to protect against the overcurrent and one which uses IC causes its cost to be raised by that amount. SOLUTION: When a switching element Q1 is ON, a voltage proportional to a DC input voltage is induced in a tertiary winding n3, a capacitor C1 is charged by a voltage, obtained by combining a voltage through a Zener diode D2 and a resistor R5 and a voltage drop in a resistor R3 to serve the bias voltage of a transistor Q2. When the element Q1 is OFF, the voltage of the winding n3 is inverted from a positive voltage into a negative voltage, and a bias voltage charged in the capacitor C1 is discharged through the diode D1, the diode D2 and a resistor R4 and the resistor R5. Accordingly, the time constant of charging the capacitor C1 is changed according to increase or decrease of the input voltage. This causes deviation of an overcurrent start point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit for a separately excited converter.

[0002]

2. Description of the Related Art FIG. 4 is a circuit diagram of an overcurrent protection circuit of a separately-excited converter according to a first conventional example. In the figure, a switching element Q1 switches a DC input voltage, and an oscillator 1 generates a switching frequency of the switching element Q1. The transformer T1 includes first to third primary windings n1 to n3,
Secondary side output is diode (rectifier) D3, capacitor C
The power is converted to direct current by 2 and supplied to the load. The tertiary output serves as a power supply to the oscillator 1. The resistor R1 is the oscillator 1
Is the starting resistance.

The overcurrent protection circuit 5 comprises resistors R2 and R3 and a transistor Q2. When the load current on the secondary side of the transformer T1 increases, the switching current flowing through the resistor R3 also increases. When the switching current due to the increase exceeds a specified value, the transistor Q2 starts operating. When a current flows through the transistor Q2, the bias voltage of the switching element Q1 is reduced, and the switching voltage is reduced.

FIG. 5 is an output voltage / output current characteristic diagram of the overcurrent protection circuit of the separately-excited converter according to the first conventional example. In the figure, the vertical axis is the switching output voltage V on the primary side of the transformer T1, and the horizontal axis is the switching output current I flowing on the primary side of the transformer T1. Normally, the output voltage V is independent of the current value of the output current I.
Is kept constant. However, when the load current on the secondary side of the transformer T1 abnormally increases due to a short circuit or the like, the output current I decreases despite the decrease in the output voltage V.
Is increasing.

A separately-excited converter using a transistor oscillator that does not use an IC as shown in FIG. 4 has an advantage that the cost can be reduced as compared with the use of an IC. But,
The protection circuit against the overcurrent is only the constant power control by the series resistor R3 of the switching element Q1. for that reason,
At the time of short circuit, an excessive current has caused damage to circuit components such as the diode D3 and the switching element Q1.

FIG. 6 is an overcurrent protection circuit diagram of a separately-excited converter according to Conventional Example 2. It is to be noted that components denoted by the same reference numerals as those in the already described drawings indicate the same elements, and the description thereof will be omitted (hereinafter the same). The difference between the overcurrent protection circuit 5 of the conventional example 1 and the overcurrent protection circuit 6 is that the photocouplers PHC-1 and PHC-2 stabilize the secondary load voltage of the transformer T1.

FIG. 7 is an overcurrent protection circuit diagram of a separately-excited converter according to Conventional Example 3. The separately-excited power supply IC 7 includes a main switch, overcurrent protection, an OR circuit, an oscillator, a comparison circuit, and the like. If the voltage drop of the overcurrent protection is the output voltage of the Hor / and photocoupler is H, H is output from the OR circuit and compared with the output from the oscillator by the comparison circuit. Controlled. That is, the voltage drop of the overcurrent protection is Hor / and
If the output voltage of the photocoupler is H, the main switch operates to reduce the primary voltage of the transformer T1.

The externally-excited power supply IC 7 usually has the I
C has an independent oscillator (many triangular waves) inside, and the main switch (transistor orF)
ET). In addition, a secondary feedback signal and an overcurrent detection signal are added to a comparator to arbitrarily change the width of a pulse signal from an oscillator, thereby stabilizing an output voltage and performing overcurrent protection. The overcurrent protection circuit stops the IC itself when the output voltage decreases due to the overcurrent and reaches a certain voltage (see FIG. 8). The voltage drop of the emitter resistance of the switching transistor is used for detecting the overcurrent. The waveform here is a triangular wave, which is coupled to the base of the overcurrent detection transistor by a capacitor.

[0009]

However, the conventional separately-excited converter using a transistor oscillator without using an IC has insufficient protection against overcurrent. Also, IC
However, there is a problem that the cost of the conventional separately-excited converter using the transistor oscillator using the device becomes high.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides an overcurrent protection circuit for a separately-excited converter which has an enhanced overcurrent protection function in a separately-excited converter using a transistor oscillator without using an IC. The purpose is to do.

[0011]

According to the present invention, there is provided an overcurrent protection circuit for a separately-excited converter, comprising: a switching element for switching a DC input voltage;
In an overcurrent protection circuit for a separately-excited converter provided in a separately-excited converter including a transformer T1 including primary to third windings and an oscillator 1 for generating a switching frequency of the switching element Q1, the switching element is connected in series to the switching element Q1. A transformer T1 is provided in which the polarity of the primary winding n1 is the same as the polarity of the tertiary winding n3 serving as a power supply to the oscillator 1, and the switching flowing through a resistor R3 connected in series with the switching element Q1. Voltage drop due to current,
The combined voltage of the voltage corresponding to the increase of the induced voltage of the tertiary winding n3 and the voltage corresponding to the decrease of the induced voltage of the tertiary winding n3 is set as the bias voltage of the transistor Q2, and the voltage due to the switching current flowing through the resistor R3. It is characterized in that the bias voltage of the switching element Q1 is controlled by the bias voltage of the transistor Q2 in accordance with the increase of the drop, so that the switching element Q1 has a square-shaped characteristic to reduce the primary voltage and current of the transformer T1.

The overcurrent protection circuit of the separately-excited converter according to the present invention includes a voltage corresponding to an increase in the induced voltage of the tertiary winding n3 and a voltage corresponding to a decrease in the induced voltage on the base of the transistor Q2. Two sets of diodes D1,
The resistor R4, the Zener diode D2, and the resistor R5
It is characterized in that it is connected to the same polarity side of the tertiary winding n3.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The overcurrent protection circuit of a separately excited type converter according to the present invention is different from a self-excited type in which a switching element performs both an oscillating action and a switching operation. This is a separately-excited type in which a switching element is driven by a signal synchronized with a voltage. Further, instead of using a separately-excited power supply IC which has a complicated circuit configuration and high cost, a diode and a resistor are used to give the overcurrent protection circuit a square-shaped characteristic.

[0014]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram illustrating an overcurrent protection circuit of a separately-excited converter according to an embodiment of the present invention. In the figure, T1 is a primary winding n1, a secondary winding n2, and a tertiary winding n3.
, Q1 is a switching element for switching a DC input voltage, 1 is an oscillator for generating the switching frequency of the switching element Q1, 2 is a transistor Q2, a diode D1, a zener diode D2, resistors R2 to 5, and a capacitor C1. The DC output circuit includes a secondary winding n2 and a diode D
3. It is composed of a capacitor C2.

When a voltage is input to the oscillator 1 through the starting resistor R1, a pulse-like oscillation waveform is output, and the switching element Q1 is turned on. Switching element Q1 is ON
In this case, a voltage proportional to the DC input voltage is induced in the tertiary winding n3. And a Zener diode D2 and a resistor R5
The combined voltage of the voltage passed through and the voltage drop of the resistor R3 is charged in the capacitor C1, and becomes the bias voltage of the transistor Q2.

When the switching element Q1 is OFF (when a secondary voltage is generated), the voltage of the tertiary winding n3 is inverted between plus and minus, and passes through the diode D1, the Zener diode D2, the resistors R4 and R5. The bias voltage charged in the capacitor C1 is discharged.

Therefore, when the input voltage increases or decreases, the capacitor C
The time constant for charging 1 changes, which causes a shift in the overcurrent start point. Input voltage → large Overcurrent point is small (enters overcurrent with small current) Input voltage → small Overcurrent point is large (enters overcurrent with large current) However, when input voltage is large, input current decreases The reverse is true when the voltage drop across the resistor R3 is small and the input voltage is small. Therefore, they are canceled out, and the deviation of the overcurrent point is reduced.

By this adjustment, it is possible to prevent the deviation of the overcurrent point, to make it faster when the voltage is low, to make it slower when the voltage is high, and to set the reverse pattern arbitrarily.
In a normal overcurrent protection circuit 2, a diode D1 and a resistor R4
Is not required, but it is easier to adjust. Further, the temperature fluctuation of the transistor Q2 can be canceled by the diode D1.

FIG. 2 is an output voltage / output current characteristic diagram of the overcurrent protection circuit of the separately excited converter according to the present invention.
In the figure, the vertical axis is the switching output voltage V on the primary side of the transformer T1, and the horizontal axis is the transformer T1.
1 is a switching output current I flowing to the primary side of the I.
Normally, the output voltage V is kept constant regardless of the current value of the output current I. Due to short circuit etc., the transformer T1
If the load current on the secondary side increases abnormally, the output voltage V
And the output current I both decrease.

FIG. 3 is a circuit diagram of an AC / DC converter using an overcurrent protection circuit of a separately excited converter according to an embodiment of the present invention. Oscillation circuit 4 through starting resistor R5
When a voltage is input to (configured by Q6, Q7, C7, C8, and R9 to 13), a pulsed oscillation waveform is output to the diode D6. These are diodes D6, D5 and resistor R8
To turn on the switching element Q1. When the switching element Q1 is ON, a voltage proportional to the DC input voltage is induced in the tertiary winding n3. Then, the combined voltage of the voltage passing through the Zener diode D9 and the resistor R16 and the voltage drop of the resistor R6 is charged in the capacitor C6, which becomes the bias voltage of the transistor Q5.

When the switching element Q1 is OFF (when a secondary voltage is generated), the voltage of the tertiary winding n3 is inverted between plus and minus, and passes through the diode D8, the Zener diode D9, and the resistors R15 and R16. Capacitor C6
To discharge the charged bias voltage.

Therefore, when the input voltage increases or decreases, the capacitor C
The time constant for charging 6 changes, which causes a shift in the overcurrent start point. Input voltage → large Overcurrent point is small (enters overcurrent with small current) Input voltage → small Overcurrent point is large (enters overcurrent with large current) However, when input voltage is large, input current decreases The reverse is true when the voltage drop across the resistor R3 is small and the input voltage is small. Therefore, they are canceled out, and the deviation of the overcurrent point is reduced.

By this adjustment, the deviation of the overcurrent point can be prevented, the speed can be reduced when the voltage is low, the speed can be reduced when the voltage is high, and the reverse pattern can be arbitrarily set.

[0024]

As described above, the overcurrent protection circuit of the separately-excited converter according to the present invention can obtain the fold-back characteristic of the separately-excited converter without using an IC, and can prevent the circuit from being damaged during overcurrent. It can be prevented and a small cost increase is possible.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an overcurrent protection circuit of a separately-excited converter according to an embodiment of the present invention.

FIG. 2 is an output voltage / output current characteristic diagram of an overcurrent protection circuit of a separately-excited converter according to the present invention.

FIG. 3 is a circuit diagram of an AC / DC converter using an overcurrent protection circuit of a separately-excited converter according to an embodiment of the present invention.

FIG. 4 is an overcurrent protection circuit diagram of a separately-excited converter according to Conventional Example 1.

FIG. 5 is an output voltage / output current characteristic diagram of an overcurrent protection circuit of a separately-excited converter according to Conventional Example 1.

FIG. 6 is an overcurrent protection circuit diagram of a separately-excited converter according to Conventional Example 2.

FIG. 7 is an overcurrent protection circuit diagram of a separately-excited converter according to Conventional Example 3.

FIG. 8 is an output voltage / output current characteristic diagram of an overcurrent protection circuit of a separately-excited converter according to Conventional Example 3.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Oscillator 2, 3 Overcurrent protection circuit 4 Oscillation circuit 5, 6 Overcurrent protection circuit 7 Separately-excited power supply IC C1-11 Capacitor D1, 3-8, 10-12 Diode D2, 9 Zener diode F1 Fuse L1 Reactor n1 3 1st to 3rd winding Q1 Switching element Q2, 4 to 8 Transistor R1 to 22 Resistance PHC-1, Photocoupler T1 Transformer

Continued on the front page F-term (reference)

Claims (2)

[Claims] 1. A switching element (Q1) for switching a DC input voltage, a transformer (T1) including first to third primary windings, and an oscillator (1) for generating a switching frequency of the switching element (Q1). In the overcurrent protection circuit of the separately-excited converter provided in the separately-excited converter, the polarity of the primary winding (n1) connected in series to the switching element (Q1) and the power supply to the oscillator (1) A transformer (T) having the same polarity as the next winding (n3)
1), and a resistor (R) connected in series to the switching element (Q1).
3) the combined voltage of the voltage drop due to the switching current flowing through the switch, the voltage corresponding to the increase of the induced voltage of the tertiary winding (n3), and the voltage corresponding to the decrease of the induced voltage of the tertiary winding (n3) The bias voltage of the transistor (Q2) is controlled by the bias voltage of the transistor (Q2) as the voltage drop due to the switching current flowing through the resistor (R3). An overcurrent protection circuit for a separately-excited converter, characterized by having a square-shaped characteristic for reducing the primary voltage and current of the converter. 2. A pair of diodes (D1) for applying a voltage corresponding to an increase in the induced voltage of the tertiary winding (n3) and a voltage corresponding to a decrease in the induced voltage to the base of the transistor (Q2). And a resistor (R4), a Zener diode (D2), and a resistor (R5) are connected to the same polarity side of the tertiary winding (n3). Current protection circuit.