JPH0797902B2 - Push-pull converter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a push-pull converter including an overcurrent protection circuit whose operating point is constant regardless of an input voltage.

(Prior Art) FIG. 7 shows a circuit of a conventional push-pull converter, in which 1 is a filter, 2 is a rectifying / smoothing circuit, and 3 is a filter.
Is a converter control oscillator, 4a and 4b are switching elements (MOSFETs are used here), 5 is a power conversion transformer, 5a is a primary winding, 5b and 5c are secondary windings, and 6a and 6b are available. Saturation reactor, 7a, 7b, 8a, 8b, 9 are diodes, 10 is a smoothing coil, 11 is a smoothing capacitor, 12 is a load, 13
a, 13b are two current transformers, 14 is a transistor for controlling the conduction angle of the saturable reactor, 15 is an error amplifier, 16a, 1
6b is a secondary winding of a current transformer, 17a and 17b are voltage conversion resistors, 18a and 18b are diodes, 19 is a smoothing capacitor, 20, 23 and 25 are resistors, 21 is a zener diode, and 22 and 24 are Capacitor 26 is a transistor and block 27 is an overcurrent protection circuit for load 12.

In operation, however, the voltage rectified and smoothed by the rectifying and smoothing circuit 2 is converted into a voltage by the power conversion transformer 5, the output of the secondary winding thereof is rectified, and a DC voltage is supplied to the load 12.

As an overcurrent protection function, when the secondary side outputs of the current transformers 13a and 13b reach a specified voltage, an overcurrent protection circuit
The transistor 26 of 27 is turned on, and the conduction angle control is performed by controlling the transistor 14 to protect the load from overcurrent.

(Problems to be Solved by the Invention) In this conventional overcurrent protection circuit, two current transformers 13a and 13b are used and saturable reactors 6a and 6b are used.
Since the conduction angle is controlled by, the area of the output voltage of the current transformers 13a and 13b becomes S ₁ = S _{2 in} the case of AC85V input as shown in FIG. Ground level x ₁
Then, T _off (x ₀ −x ₁ ) = T _on · x ₁ . Where x ₁ = (T _off /
T) x ₀ .

Where x ₀ is the primary: secondary winding ratio of the current transformer is 1: u,
When a load connected to the secondary winding is R _CT and a current flowing in the primary winding is I ₀ , a voltage represented by x ₀ = I ₀ / u · R _CT .

That is, in the circuit of FIG. 7, the conduction angle changes according to the input voltage as shown in FIG. As a result, the output voltages of the two current transformers 13a and 13b change. In this case, as shown in FIG. 9, the relationship between the output current to the load and the output voltage differs depending on whether the AC input voltage is high (solid line), medium (broken line), or low (thin broken line). Therefore, the starting point of overcurrent protection depends on the input voltage, and the output voltage of the overcurrent protection circuit 27 has a drawback that it changes depending on whether the input voltage is high, medium, or low.

The present invention has been proposed in order to improve the above-mentioned drawbacks, and an object thereof is to make the output voltage of a current transformer constant regardless of whether the input voltage is high or low, and thus the operation of the overcurrent protection circuit regardless of the input voltage. An object of the present invention is to provide an inexpensive push-pull converter that maintains stable points and operates stably.

(Means for Solving the Problem) In order to achieve the above object, the present invention provides a DC voltage obtained through an AC input voltage through a rectifying and smoothing circuit and a current through an oscillation circuit including a switching element. The directions are alternately changed and applied to the primary winding of the power conversion transformer, and one ends of the first and second secondary windings of the power conversion transformer are connected to one of the current transformers having a center tap. While connecting to both ends of the secondary winding, connect the center tap of the primary winding of the current transformer to one end of the load,
The other end of the load is connected to the positive side of the rectifying / smoothing circuit on the secondary side of the power conversion transformer, and the output of the secondary winding of the current transformer is applied to the overcurrent protection circuit of the load to output the output overload. To control the output current,
It has achieved the above objectives.

In addition, the direct current voltage obtained from the AC input voltage through the rectifying and smoothing circuit is applied to the primary winding of the power conversion transformer while alternately changing the current direction through the oscillation circuit including the switching element. , One end of the first and second secondary windings of this power conversion transformer is connected to form a center tap,
Connect this center tap to one end of the load and
A saturable reactor connected in series to the other end of the secondary winding, connected to one end of the primary winding of one current transformer having a center tap via a rectifying diode, and connected to the second end of the power conversion transformer. The secondary winding is connected to the other end of the primary winding of the current transformer through a saturable reactor connected in series with the other end of the secondary winding, and the center tap of the primary winding of the current transformer is connected. The output of the secondary winding of the current transformer is connected to the line on the positive side of the load to the overcurrent protection circuit of the load to control the output current against the output overload, thereby achieving the above object. is doing.

Furthermore, one terminal on the output side of the rectifying and smoothing circuit obtained by rectifying the AC input voltage is connected to the center tap of the primary winding of one current transformer, and both ends of the primary winding are
The other terminal of the rectifying and smoothing circuit is connected to one end of each of the first and second primary windings of the power conversion transformer, and the other end of the power conversion transformer is alternately turned on and off by an oscillation circuit. And rectify the voltage generated in the first and second secondary windings of the power conversion transformer and supply it to the load, and protect the output of the secondary winding of the current transformer from overcurrent of the load. The output current is applied to the circuit to control the output current against the output overload, thereby achieving the above object.

(Function) Conventionally, two current transformers are used to individually detect the current to operate the overcurrent protection circuit, whereas according to the present invention, one is provided with a center tap in the primary winding. By detecting the load current using the current transformer of No. 2, the operating point of the overcurrent protection circuit is made constant regardless of the level of the AC input voltage, and stable overcurrent protection can be performed.

Moreover, since the number of current transformers is one, the number of parts can be reduced by that amount, the circuit configuration can be simplified, and an inexpensive push-pull converter can be obtained.

Example 1 Next, an example of the present invention will be described.

FIG. 1A shows a first embodiment of a push-pull converter provided with the overcurrent protection circuit of the present invention. In the figure, 1
Is a filter, 2 is a rectifying / smoothing circuit, 3 is a converter control oscillator, 4a and 4b are switching elements, 5 is a power conversion transformer, 5a is its primary winding, 5b and 5c are secondary windings, and 6a and 6b are Saturable reactor, 7a and 7b are rectifying diodes, 8a and 8b are diodes, 9 is a commutation diode, 10 is a smoothing coil,
11 is a smoothing capacitor, 12 is a load, 13 is a current transformer, 14 is a transistor for controlling the conduction angle of the saturable reactor,
15 is an error amplifier, 16 is a secondary winding of a current transformer, 17
Is a resistor for voltage conversion, 18 is a diode, 19 is a smoothing capacitor, 20, 23, 25 are resistors, 21 is a zener diode, 2
Reference numerals 2 and 24 are capacitors, 26 is a transistor, and a block 27 surrounded by a broken line is an overcurrent protection circuit.

As shown in FIG. 1B, the current transformer 13 has a center tap 29 on its primary winding 28, and both ends a and b of the primary winding of the current transformer 13 and the center tap ha. Both ends of the secondary winding 30 are shown in Fig. 1 (a).
Are connected as shown in.

Therefore, in the present invention, the one using the two current transformers 13a and 13b is replaced with the center tap in the primary winding in the present invention.
The current is detected by one current transformer 13 provided with 29.

The saturable reactors 6a and 6b, the rectifying diodes 7a and 7b, the smoothing coil 1 are provided in the secondary circuit of the power conversion transformer 5.
0, the smoothing capacitor 11, the diodes 8a, 8b, 9 and the transistor 14 are connected to obtain a DC voltage Vo and a DC current Io as outputs, which are applied to the load 12.

In this case, for the conduction angle control during normal operation, the transistor 14 is controlled by the output of the error amplifier 15, and the diodes 8a and 8b are controlled.
Via a controlled reset current to the saturable reactors 6a, 6b.

In other words, the overcurrent protection circuit 27 has two
The secondary winding 16 detects the current on the secondary winding side of the power conversion transformer 5, converts the voltage via the resistor 17 connected in parallel with the secondary winding 16, and rectifies this to form the base of the transistor 26. In addition, if the secondary output of the power conversion transformer 5 is high, the transistor 26 of the overcurrent protection circuit 27 is turned on, and this output controls the transistor 14 for controlling the conduction angle,
By controlling the reset currents of the saturable reactors 6a and 6b and controlling the conduction angle, the drooping characteristic is obtained.

As described above, in the related art, the current is detected individually by the two current transformers, whereas in the present invention, the primary winding is used.
One current transformer 13 with center tap 29 on 28
Is configured to detect the current. As apparent from a comparison between FIG. 2 and FIG. 8 described in the section of the above-mentioned conventional example, in the present invention, the current transformer 13
The output voltage of the current transformer (shown as CT in FIG. 2) does not constantly change even when the input voltage changes, for example, AC85V, AC100V, AC132V.

Therefore, as shown in FIG. 3, a voltage / current characteristic in which the operating point of the overcurrent protection circuit 27 does not change depending on the input voltage is obtained, and the push-pull converter can be operated stably.

(Embodiment 2) FIG. 4 shows a second embodiment of the present invention. In this example, the current transformer 13 provided on the secondary circuit side of the power conversion transformer 5
The insertion position of is different.

That is, the first and second secondary windings of the power conversion transformer 5
One end of the primary winding of the current transformer 13 is connected to the other end of the first secondary winding 5b through the saturable reactor 6a and the rectifying diode 7a which are connected to one ends of the 5b and 5c. Connected.

Further, the other end of the primary winding of the current transformer 13 is connected to the other end of the second secondary winding 5c of the power conversion transformer 5 via a saturable reactor 6b and a rectifying diode 7b connected in series. ing.

The center tap of the current transformer 13 is connected to the line on the positive electrode side of the load 12.

The secondary winding of the current transformer 13, that is, the current detector is provided on the side of the overcurrent protection circuit 27 as in the first embodiment, and the transistor 26 is provided when the detected current value reaches a specified value. The transistor 14 for controlling the conduction angle of the saturable reactors 6a and 6b is controlled via the above so as to prevent an overcurrent to the load 12.

(Embodiment 3) FIG. 5 shows a third embodiment of the present invention. This example is different from the first example in that one current transformer 13 is provided on the primary circuit side of the power conversion transformer 5 to detect the current on the primary circuit side.

That is, one terminal on the output side of the rectifying / smoothing circuit 2 is connected to the center tap of the current transformer 13.

Further, both ends of the primary winding of the current transformer 13 are respectively connected to one ends of the first and second primary windings of the power conversion transformer 5, and the other ends of the primary windings of the power conversion transformer 5 are alternately arranged. It is configured to be connected to the other end of the rectifying / smoothing circuit 2 via switching elements 4a and 4b that are turned on and off.

The secondary circuit side of the power conversion transformer 5 is the first circuit shown in FIG.
This embodiment has the same circuit configuration as that in which the current transformer 13 is removed, and the operation in this embodiment is basically the same as that of the first embodiment except that the current transformer 13 detects the current in the primary circuit of the power conversion transformer 5. Same as the first embodiment, the current is detected by the current transformer 13, the overcurrent protection circuit 27 is operated accordingly, and the reset current of the saturable reactors 6a, 6b is controlled via the transistors 26, 14. ing.

(Embodiment 4) FIG. 6 shows a fourth embodiment of the present invention. This example is common with the third embodiment in that the current transformer 13 is brought to the primary circuit side of the power conversion transformer 5.

Other circuits are the same as those of a normal push-pull converter.

That is, in the primary circuit of the power conversion transformer 5, the current is detected via the current transformer 13, and the output is sent from the overcurrent protection circuit 27 in response to this, and this output is added to the pulse width control circuit 28. There is. The pulse width control circuit 28 outputs the output of the error amplifier 15 according to the DC output on the load 12 side, and the output of the error amplifier 15 is a circuit-isolating photocoupler 29.
A predetermined pulse width control output is sent to the driver 30 (driving circuit), the switching elements 4a and 4b are alternately turned on and off by the output of the driver 30, and thus the switching element 4a, The on / off time of 4b is controlled so that the output induced in the secondary winding of the power conversion transformer 5 becomes constant so that a stable DC output can be obtained on the secondary circuit side of the power conversion transformer 5. There is.

It goes without saying that the specific configuration of the overcurrent protection circuit 27 in this embodiment is the same as that shown in FIGS. 1, 4, and 5.

Further, the present invention is not limited to the push-pull converter shown in the figure, and it goes without saying that the present invention can be applied to other push-pull converters in general.

(Effects of the Invention) As described above, the current detection is individually performed by the two current transformers in the related art.
Stable overcurrent protection function because the current transformer output voltage does not change depending on the input voltage and the operating point of the overcurrent protection circuit does not change. It is possible to obtain a push-pull converter having

Further, since only one current transformer is required, the circuit components can be reduced, the circuit configuration can be simplified, and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a push-pull converter in a first embodiment of the present invention, FIG. 2 is an explanatory diagram of an output waveform of a current transformer in the present invention, and FIG. 3 is a load in an overcurrent protection circuit in the present invention. Output current / output voltage characteristic chart, No. 4
FIG. 6 is a circuit diagram of the second embodiment of the present invention, FIG. 5 is a circuit diagram of the third embodiment of the present invention, and FIG. 6 is a fourth diagram of the present invention.
FIG. 7 is a circuit diagram in the embodiment, FIG. 7 is a circuit diagram in the conventional example, FIG. 8 is an explanatory diagram of an output waveform of a current transformer in the conventional example, and FIG. 9 is an output current to a load in an overcurrent protection circuit in the conventional example. / Shows output voltage characteristics. 1 ... Filter 2 ... Rectifier smoothing circuit 3 ... Converter control oscillator 4a, 4b ... Switching element 5 ... Power conversion transformer 5a ... Primary winding 5b, 5c ... Secondary winding 6a, 6b ... … Saturable reactor 7a, 7b …… Diode 8a, 8b …… Diode 9 …… Diode 10 …… Smoothing coil 11 …… Smoothing capacitor 12 …… Load 13 …… Current transformer 14 …… Saturation reactor conduction angle Control transistor 15 …… Error amplifier 16 …… Current transformer secondary winding 17 …… Voltage conversion resistor 18 …… Diode 19 …… Smoothing capacitor 20,23,25 …… Resistance 21 …… Zener diode 22… … Capacitor 24 …… Capacitor 26 …… Transistor 27 …… Overcurrent protection circuit 28 …… Primary winding 29 …… Center tap 30 …… Secondary winding

Claims (3)

[Claims] 1. A primary winding of a power conversion transformer in which a DC voltage obtained from an AC input voltage through a rectifying / smoothing circuit is alternately changed in a current direction through an oscillation circuit including a switching element. While applying the voltage, one ends of the first and second secondary windings of the power conversion transformer are connected to both ends of the primary winding of one current transformer having a center tap, and the primary of the current transformer is connected. The center tap of the winding is connected to one end of the load, and the other end of the load is connected to the positive side of the rectifying / smoothing circuit on the secondary side of the power conversion transformer.
Further, the push-pull converter is characterized in that the output of the secondary winding of the current transformer is given to an overcurrent protection circuit of the load to control the output current against the output overload. 2. A primary winding of a power conversion transformer, wherein a direct current voltage obtained from an AC input voltage through a rectifying / smoothing circuit is alternately changed in a current direction through an oscillation circuit including a switching element. In addition to applying the voltage, one end of the first and second secondary windings of the power conversion transformer is connected to form a center tap, the center tap is connected to one end of the load, and the other of the first secondary winding is connected. A saturable reactor connected in series to an end, connected to one end of a primary winding of one current transformer having a center tap via a rectifying diode, and a second secondary winding of the power conversion transformer. While being connected to the other end of the primary winding of the current transformer through a saturable reactor connected in series with the other end and a rectifying diode,
The center tap of the primary winding of the current transformer is connected to the line on the positive side of the load, and the output of the secondary winding of the current transformer is applied to the overcurrent protection circuit of the load to prevent output overload. Push-pull converter characterized by controlling output current. 3. One terminal on the output side of a rectifying / smoothing circuit obtained by rectifying an AC input voltage is connected to a center tap of a primary winding of one current transformer, and both ends of the primary winding are connected. Are connected to one ends of the first and second primary windings of the power conversion transformer, respectively, and the other end is connected to the other of the rectifying and smoothing circuit via a switching element which is alternately turned on and off by an oscillation circuit. Connected to a terminal, rectifies the voltage generated in the first and second secondary windings of the power conversion transformer and supplies the rectified voltage to the load, and outputs the output of the secondary winding of the current transformer to the overcurrent of the load. A push-pull converter characterized by being provided to a protection circuit and controlling output current against output overload.