JPS6232705B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse width control circuit for use in converters or DC-DC converters.

In an isolated converter, one of the output transformers
When controlling the next-side switching element based on the secondary-side DC output voltage, an error signal between the output detection voltage and the reference voltage was transmitted via a photocoupler as an insulating element. However, control circuits using photocouplers have the following drawbacks.

(a) Since the frequency response speed of the photocoupler is slow, there is a risk of hunting phenomenon occurring when the conversion frequency is high.

(b) Since semiconductor elements have a withstand voltage, they are vulnerable to high surge voltages.

(c) Photocouplers have lower reliability than other semiconductor devices.

(d) Since the photocoupler's transmission efficiency deteriorates significantly,
Settings must be made with this deterioration in mind.

SUMMARY OF THE INVENTION An object of the present invention is to provide a pulse width control circuit that can isolate the control input side and the control output side without using a photocoupler.

To achieve the above object, the present invention includes a primary winding of a transformer connected to a controlled DC power source, and a transformer connected to the primary winding for selectively connecting the controlled DC power source to the primary winding. a first switching element connected in series; and a resistor and a capacitor connected in parallel or series to the primary winding so that the capacitor charging voltage changes in response to changes in the voltage of the primary winding. a series circuit, a second switching element for selectively forming a discharge circuit for the capacitor, and an oscillator for generating pulses at a predetermined duty factor, and controlling the first switching element on and off. , a switching control circuit that controls on/off the second switching element in an antiphase relationship with the first switching element, and a secondary winding of the transformer that is electrically insulated from the primary winding. , a variable resistance element connected in parallel to the secondary winding, a control circuit for changing the resistance value of the variable resistance element, and a comparison for comparing the voltage of the capacitor with a reference voltage and sending out a rectangular wave pulse. This relates to a pulse width control circuit consisting of a

According to the above invention, since a photocoupler is not used, a highly reliable control circuit can be provided. Furthermore, it is possible to provide a device that does not have the various drawbacks caused by the use of photocouplers. Furthermore, since an insulating transformer is provided in the circuit that controls charging of the capacitor, the configuration can be simplified.

Next, embodiments of the present invention will be described with reference to the drawings.

A first diagram showing a converter according to an embodiment of the present invention.
In the figure, the primary winding 3 of the output transformer 2 is connected to the main DC power supply 1 by the conversion switching transistor 4.
A smoothing circuit 7 is connected to the secondary winding 5 of the transformer 2 via a rectifier diode 6. The smoothing circuit 7 includes a diode 8, a reactor 9, and a capacitor 10.

In order to control the voltage of the DC output terminal 11 to a constant value, a voltage detection line 12 is connected to the output terminal 11, and this voltage detection line 12 is connected to the error amplifier 1.
is coupled to one input terminal of 3. The other input terminal of this error amplifier 13 is connected to a reference voltage source 14.
is connected, an output voltage corresponding to the difference between the reference voltage supplied from the reference voltage source 14 and the output detection voltage is obtained from the error amplifier 13.

15 is an insulation and control transformer, which has a primary winding 16 and a secondary winding 17. A first transistor 18 serves as a first switching element, and is connected in series to the primary winding 16. 1
9 is a charging current adjusting resistor, 20 is a triangular wave generating capacitor, and these series circuits are connected in parallel to the primary winding 16 via a transistor 18. 21 is a second switching element;
This transistor is connected in parallel to the series circuit of the resistor 19 and the capacitor 20 in order to selectively form a discharge circuit for the capacitor 20. First transistor 18 and second transistor 21
A square wave oscillator 22 and an inverter 23 are provided as a switching control circuit for turning on and off the first transistor 1 in opposite phase.
An oscillator 22 is coupled directly to the base of the second
The output of an oscillator 22 is coupled to the base of a transistor 21 via an inverter 23. 24
is a controlled DC power supply, which is connected to the primary winding 16 via a resistor 32 and also to the capacitor 20 via resistors 32 and 19.

A third transistor 26 is connected in parallel to the secondary winding 17 via a rectifier diode 25 .
This transistor 26 functions as a variable resistance element that operates in class A, and has a resistor 27 at its base.
The output terminal of the error amplifier 13 is coupled thereto. The error amplifier 13 functions as a control circuit for the transistor 26, and the resistance value of the transistor 26 changes based on the error output. A resistor 31 connected between the detection line 12 and the collector of the transistor 26 allows a dummy current to flow therethrough to stabilize the class A operation of the transistor 11.

A voltage comparator 28 sends a comparison output between the reference voltage V _R of the reference voltage source 29 and the voltage V _C of the capacitor 20 to the drive amplifier circuit 30.

Next, the operation of the circuit shown in FIG. 1 will be explained with reference to the waveform diagram shown in FIG. Now, a voltage is generated in the secondary winding of the output transformer 2 by turning on and off the conversion transistor 4, and this is rectified and smoothed to obtain a DC output voltage at a predetermined level at the output terminal 11, and this output voltage When a voltage corresponding to the difference between and the reference voltage source 14 is applied to the base of the transistor 26,
The collector-emitter resistance value of transistor 26 changes based on the error output, and a voltage V _S related to the error output is obtained across transistor 26 as shown in FIG. 2B.

On the other hand, as shown in FIG. 2A, when a pulse is generated from the oscillator 22 during the period t ₁ to t ₃ and the transistor 18 is turned on, the voltage of the controlled DC power supply 24 is supplied to the primary winding 16 . At this time, since the voltage of the secondary winding 17 is clamped by the voltage V _S of the transistor 26, the voltage V _P of the primary winding 16 becomes a value determined by the voltage V _S on the secondary side and the turns ratio. . That is, the voltage V _P of the primary winding 16 has a value controlled by the output of the error amplifier 13. Since this voltage V _P is applied to the capacitor 20 via the resistor 19,
As shown in the figure, the capacitor 20 is charged at a rate dependent on the voltage V _P . And capacitor 20
When voltage V _C crosses reference voltage V _R at time t ₂ , the output of comparator 28 switches to a high level as shown in FIG. 2D. At time t ₂ , the output of the oscillator 22 becomes low level, so the transistor 18 is turned off and the transistor 21 is turned on, forming a discharge circuit for the capacitor 20 and reducing the voltage V of the capacitor 20.
_C gradually decreases and crosses the reference voltage V _R at time t ₄ and the output of comparator 28 switches to a low level. As a result, a control pulse having a pulse width corresponding to the voltage of the capacitor 20, that is, the peak value of the triangular wave, is generated as shown in FIG. 2D.

After that, at time _t9 , if the output voltage of the converter decreases, the output voltage of the error amplifier 13 also decreases, and the resistance value of the transistor 26 increases,
The voltage V _S increases as shown in FIG. 2B. As a result, the voltage V _P of the primary winding 16 during the ON period of the transistor 18 also becomes high, and the capacitor 20 is charged to a high voltage corresponding to V _P . That is,
It is charged with a steep slope as shown in the period from _t9 to _t11 , and at the end of the on-period of the transistor 18, _t11
, resulting in a high peak voltage. Note that if the value of the resistor 19 is R, the capacitance of the capacitor 20 is C, and the charging current is i, then the capacitor voltage V _C is expressed as V _C =1/C∫idt, and the step response of the voltage V _P is The output is V _C =
It is expressed as V _P (1-ε ^-t/RC ).

When transistor 18 is turned off and transistor 21 is turned on at time _t11 , a discharge circuit is formed,
The voltage across capacitor 20 decreases to below V _R at time t ₁₂ , and a pulse with a time width of t ₁₀ to t ₁₂ is obtained from comparator 28 . Thereafter, at time _t13 , transistor 18 is turned on again, and charging of capacitor 20 is started. By the way, since the discharge rate is always constant, the capacitor 20 is not completely discharged during the period t ₁₁ to _{t 13} and the capacitor voltage V _C at t ₁₃ does not become zero. Therefore, the next charge is performed in a state where the voltage is added to the voltage at time _t13 . In this way, even if the voltage of the capacitor 20 does not return to zero, no problem will occur if the relationship between the triangular wave and the reference voltage V _R is appropriately set.

When the comparator 28 generates an output whose pulse width is controlled as shown in FIG. It will be done.

As is clear from the above, according to this example,
It becomes possible to obtain an output having a pulse width corresponding to the error output without using a photocoupler, and the drawbacks caused by using a photocoupler can be solved.

Furthermore, since the transformer 15 is placed in the charging control circuit for the capacitor 20 for insulation, the configuration can be simplified.

Next, FIG. 3 showing another embodiment of the present invention will be described. However, in Fig. 3, the numbers 1 to 30 and 32
Components indicated by are substantially the same as those indicated by the same reference numerals in FIG. 1, so a description thereof will be omitted. In this embodiment, a resistor 31 in FIG. 1 is not provided. Therefore, the voltage V _S of the transistor 26 is equal to the voltage V S of the transistor 18 on the primary side.
Occurs only while is on. And the primary winding 1
The same operation occurs when a variable resistor is connected at point 6, and the voltage determined by the voltage division ratio between resistor 25 and the equivalent resistance in primary winding 16 is applied across the circuit consisting of capacitor 20 and resistor 19. is applied to 1
Since the equivalent resistance in the next winding 16 changes in response to the output of the error amplifier 13, it is possible to control the pulse width in the same way as in FIG.

Next, FIG. 4 showing still another embodiment of the present invention will be described. However, since the parts indicated by reference numerals 1 to 31 in FIG. 4 are substantially the same as those shown by the same reference numerals in FIG. 1, the explanation thereof will be omitted. In this embodiment, a circuit consisting of a resistor 19 and a capacitor 20 is connected in series with the primary winding 16.
Even when connected in this way, the primary winding 16
The voltage V _P changes in response to the output of the error amplifier 13, changing the rate of charging of the capacitor 20 and allowing pulse width control.

Although the embodiments of the present invention have been described above, the present invention is not limited thereto and can be further modified. For example, transistors 18,2
1, 26 and FET may be used. Further, the time constant for discharging the capacitor 20 may also be changed by the output of the error amplifier 13. Also, capacitor 20
The voltage may be made into a sawtooth waveform. It is also applicable to pulse width control other than converters. Further, the oscillation frequency of the output of the oscillator 22 and the duty factor may be changed.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a converter according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing the states of points A to D in FIG. 1, and FIG. 3 is a circuit diagram showing a converter according to another embodiment of the present invention. FIG. 4 is a circuit diagram showing a converter according to still another embodiment of the present invention. In the symbols used in the drawings, 13 is an error amplifier, 15 is a transformer, 16 is a primary winding,
17 is a secondary winding, 18 is a first transistor, 1
9 is a resistor, 20 is a capacitor, 21 is a second transistor, 22 is an oscillator, 23 is an inverter, 2
4 is a controlled DC power supply, 25 is a diode, 26 is a third transistor, 27 is a resistor, and 28 is a comparator.

Claims (1)

[Claims] 1. A primary winding of a transformer connected to a controlled DC power source; and a transformer connected in series with the primary winding for selectively connecting the controlled DC power source to the primary winding. a first switching element, and a series circuit of a resistor and a capacitor connected in parallel or in series to the primary winding so that the capacitor charging voltage changes in response to a change in the voltage of the primary winding. , a second switching element for selectively forming a discharge circuit for the capacitor, and an oscillator that generates a pulse at a predetermined duty factor, controlling the first switching element on and off, and controlling the second switching element. a switching control circuit that controls on/off a switching element of the transformer in an antiphase relationship with the first switching element; a secondary winding of the transformer that is electrically insulated from the primary winding; a variable resistance element connected in parallel to a winding; a control circuit for changing the resistance value of the variable resistance element; and a comparator for comparing the voltage of the capacitor with a reference voltage and sending out a rectangular wave pulse. Pulse width control circuit consisting of. 2. The pulse width control circuit according to claim 1, wherein the first and second switching elements and the variable resistance element are each transistors.