JP3616028B2 - Rise control circuit used in switching converter - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a rise control circuit used for a switching converter.
[0002]
[Prior art]
A conventional switching converter including an overcurrent control circuit will be described with reference to FIG.
DC power input to the input terminals 11 and 12 from the outside is applied to the primary winding of the transformer 13 via the switching element SW. Since the switching element SW is periodically turned on and off by a signal output from the control circuit 2, the excitation current i1 flowing through the primary winding of the transformer 13 is also periodically turned on and off. For this reason, AC power is induced in the secondary winding of the transformer 13.
[0003]
The AC power that appears in the secondary winding of the transformer 13 is rectified by a rectifier circuit including diodes 24 and 25, smoothed by a smoothing circuit including a coil 26 and a capacitor 19, and converted into DC power. . This DC power is supplied to the load 18. Further, the control circuit 2 detects the output voltage appearing at the output terminal 16, reflects the difference between the output voltage and the target voltage or the change in the output voltage in the on / off duty of the switching element SW, and stabilizes the output voltage.
[0004]
In this type of switching converter, an overcurrent protection circuit is provided in order to prevent an excessive current from flowing when a component failure occurs.
In the switching converter of FIG. 4, the exciting current i1 flowing through the primary winding of the transformer 13 can be detected by the resistor 27 connected in series with the switching element SW. The overcurrent detection circuit 1 detects the magnitude of the excitation current i1 as a voltage appearing at the resistor 27.
[0005]
When the overcurrent detection circuit 1 detects an excessive current, a control signal for suppressing output power is input from the overcurrent detection circuit 1 to the control circuit 2. In that case, the control circuit 2 reduces the on / off duty of the switching element SW and suppresses the input power of the transformer 13. As a result, the output power appearing at the output terminals 16 and 17 is also limited.
[0006]
[Problems to be solved by the invention]
Incidentally, in order to stabilize the output voltage in the switching converter as shown in FIG. 4, it is necessary to increase the capacitance of the capacitor 19. However, when the capacitance of the capacitor 19 is large, the following problem occurs.
Assume that the power supply to the input terminals 11 and 12 is started while the power supply to the input terminals 11 and 12 is cut off and the capacitor 19 is discharged. At this time, since the capacitor 19 is rapidly charged, the impedance of the circuit connected to the secondary side winding of the transformer 13 becomes small, and the secondary side winding of the transformer 13 temporarily has a current higher than the steady current. A large inrush current (i2) flows. Therefore, at this time, an excessive current (i1) also flows through the primary side winding of the transformer 13.
[0007]
When an excessive current (i1) flows also in the primary side winding of the transformer 13, the overcurrent detection circuit 1 detects that the current is excessive, and the control circuit 2 suppresses the on-duty of the switching element SW.
When the on-duty of the switching element SW is reduced, the output power appearing in the secondary winding of the transformer 13 is suppressed, and the current flowing through the capacitor 19 is suppressed, so that the time required for charging the capacitor 19 is increased. For this reason, the rise of the output voltage from when the power is turned on until the steady voltage appears at the output terminals 16 and 17 is delayed. In order to speed up the rise of the output voltage, the capacity of the capacitor 19 must be reduced.
[0008]
The present invention provides a rise control circuit used in a switching converter capable of raising an output voltage in a short time even when a capacitor having a large capacity is connected to a smoothing circuit on the secondary side of a transformer. Objective.
[0009]
[Means for Solving the Problems]
Rise control circuit used in the switching converter of claim 1, a transformer, a switching element arranged on the primary side of the transformer, a switching control unit for controlling the switching element, generated in the secondary side of the transformer A smoothing unit for rectifying and smoothing electric power; an input current detecting unit for detecting a current flowing on a primary side of the transformer as a first voltage; and a first voltage output from the input current detecting unit and a predetermined second A current detection unit that determines that the current flowing on the primary side of the transformer is an overcurrent when the first voltage is higher than the second voltage, and the current detection unit use the current flowing through the primary side when it is determined that the overcurrent, the switching converter and a suppressing overcurrent suppressing portion controlling said switching control unit In the rise control circuit that is, the rising control circuit, the DC power to the primary side of the transformer is synchronized with the timing of stopping supply start / supply receives the binary signal changes state in occurs at the start of the DC power supply wherein Based on the state change of the binary signal, transistor control means for generating a base voltage for turning on the transistor for a predetermined time, and when the transistor is turned on, the first voltage output from the input current detection unit is obtained. And a first voltage control means for lowering the voltage than the second voltage .
[0010]
According to the first aspect, at least immediately after the power is turned on, the control of the overcurrent suppressing unit is automatically switched to a state in which the suppression of the overcurrent suppressing unit is relaxed or inhibited by the control of the rise control circuit . For this reason, when the power is turned on, the capacitor provided in the smoothing section is quickly charged, so even if an excessively large current flows, the current is not suppressed, so the capacitor is charged in a short time and the output voltage is reduced in a short time. Can be launched.
[0011]
Power at the timing other than the introduction of, the function of suppressing the overcurrent suppressing portion acts, such as by component failure to suppress the immediate current if excessive current flows through, Ru can protect the circuit and the like.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
For one embodiment of the rise control circuit used in the switching converter of the present invention, described with reference to FIGS.
[0017]
FIG. 1 is an electric circuit diagram showing the main part of the switching converter of this embodiment. FIG. 2 is a time chart showing an operation example of the rise control circuit 56 .
In this embodiment, the input current detection unit , the overcurrent suppression unit, and the rise control circuit according to claim 1 correspond to the resistor 27, the overcurrent detection circuit 55 , and the rise control circuit 56, respectively. The first voltage in claim 1 corresponds to the voltage V1, and the second voltage in claim 2 corresponds to the voltage V2.
[0018]
Referring to FIG. 1, the switching converter includes a transformer 13, a switching element (FET) 14, a control circuit 15, resistors 21, 27, capacitors 19, 22, and 28, diodes 23 to 25, a coil 26, an overcurrent detection circuit. 55 and a rise control circuit 56 are provided.
[0019]
DC power input to the input terminals 11 and 12 from the outside is applied to the primary winding of the transformer 13 via the switching element 14. Since the switching element 14 is periodically turned on and off by a signal output from the control circuit 15, the excitation current i1 flowing through the primary winding of the transformer 13 is also periodically turned on and off. For this reason, AC power is induced in the secondary winding of the transformer 13.
[0020]
The AC power that appears in the secondary winding of the transformer 13 is rectified by a rectifier circuit including diodes 24 and 25, smoothed by a smoothing circuit including a coil 26 and a capacitor 19, and converted into DC power. . This DC power is supplied to the load 18. Further, the control circuit 15 detects the output voltage appearing at the output terminal 16, reflects the difference between the output voltage and the target voltage and the change in the output voltage in the on / off duty of the switching element 14, and stabilizes the output voltage.
[0021]
This switching converter includes an overcurrent detection circuit 55 as a protection circuit for preventing an excessive current from flowing when a component failure occurs. In this switching converter, the resistor 27 connected in series with the switching element 14 detects the exciting current i1 flowing through the primary winding of the transformer 13. The overcurrent detection circuit 55 detects the magnitude of the excitation current i1 as the voltage Vd appearing in the resistor 27.
[0022]
When the overcurrent detection circuit 55 detects an excessive current, the control circuit 15 reduces the on-duty of the switching element 14 in accordance with the control signal SG1 output from the overcurrent detection circuit 55.
When the on-duty of the switching element 14 is reduced, the input power of the primary side winding (N1) of the transformer 13 is suppressed, and the output power of the secondary side winding (N2) is also limited. Therefore, it is possible to prevent an excessive current from flowing.
[0023]
However, since a large current flows to quickly charge the capacitor 19 having a large capacity when the power is turned on, the overcurrent detection circuit 55 may detect an excessive current in this case as well. When the overcurrent detection circuit 55 detects an excessive current, the output power is suppressed, so that the rise of the output voltage appearing at the output terminals 16 and 17 is delayed.
In order to speed up the rise of the output voltage, a rise control circuit 56 is provided in the switching converter of FIG. The signal SG2 applied to the control input terminal 60 of the rise control circuit 56 is a binary signal that changes in synchronization with the on / off of the input voltage Vin at the input terminals 11 and 12, as shown in FIG.
[0024]
That is, when a main power supply (not shown) is turned on and a predetermined DC power appears at the input terminals 11 and 12 as the voltage Vin, the signal SG2 changes from the low level L to the high level H, the main power supply is cut off, and the input terminals 11 and 12 When the power supply to 12 is stopped, the signal SG2 changes from the high level H to the low level L.
The rise control circuit 56 includes resistors 36, 38, 42 and 43, a transistor 37, a diode 39 and a capacitor 41.
[0025]
When the main power is turned on and the signal SG2 changes from the low level L to the high level H, a pulse-like waveform as shown in FIG. 2 appears in the signal SG3. That is, since the impedance of the capacitor 41 is small until the capacitor 41 is charged, a large voltage temporarily appears in the signal SG3.
While the voltage of the signal SG3 exceeds the threshold value Vth determined by the characteristics of the transistor 37, the transistor 37 is in a conductive state. In this example, the circuit characteristics are determined so that the transistor 37 is conductive only for 10 msec after the main power is turned on.
[0026]
The operational amplifier 29 and the resistor 31 in the overcurrent detection circuit 55 constitute a differential amplifier. The voltage V1 is applied to one input terminal of the differential amplifier, and the voltage V2 is applied to the other input terminal.
The voltage V2 is constant because it is obtained by dividing the fixed reference voltage Vref by the resistors 32 and 33.
[0027]
Since the voltage V1 is obtained by dividing the voltage Vd proportional to the current i1 detected by the resistor 27 by the voltage dividing circuit formed by the resistors 34 and 35, the voltage V1 changes according to the current i1. Further, when the transistor 37 is conductive, the resistor 36 is connected in parallel with the resistor 35 and the voltage dividing ratio of the voltage dividing circuit changes, so that the voltage V1 changes according to the on / off state of the transistor 37.
[0028]
When the transistor 37 is non-conductive, when the current i 1 flowing through the resistor 27 exceeds a predetermined upper limit (overcurrent) (V1> V2), the voltage of the signal SG1 output from the operational amplifier 29 is Become negative. In this case, the control circuit 15 makes the on-duty of the control signal SG4 given to the switching element 14 smaller than usual. As a result, the output power appearing at the secondary winding of the transformer 13 is suppressed.
[0029]
On the other hand, when the transistor 37 becomes conductive, the voltage V1 decreases, and even if the current i1 temporarily exceeds the upper limit value (V1 <V2), the signal SG1 output from the operational amplifier 29 The voltage becomes positive. In this case, the control circuit 15 maximizes the on-duty of the control signal SG4 given to the switching element 14 within a controllable range.
[0030]
That is, immediately after the main power is turned on, even when a large current flows, the on-duty of the switching element 14 is increased by the control of the rise control circuit 56, so that a large amount of power is output to the secondary winding of the transformer 13. The capacitor 19 is charged in a short time. Therefore, the voltage rise at the output terminals 16 and 17 can be accelerated.
[0031]
In steady operation other than when the main power supply is turned on, the transistor 37 of the rise control circuit 56 is non-conductive. Therefore, when some abnormality occurs and the current i 1 exceeds the upper limit value, the control circuit 15 controls the transformer. The output power appearing in the 13 secondary windings is suppressed, and the current is suppressed.
( Switching converter related to the present invention )
FIG. 3 is an electric circuit diagram showing a main part of the switching converter related to the present invention .
[0032]
In FIG. 3, the elements corresponding to those of the first embodiment are indicated by the same reference numerals as those in FIG. The following description is omitted for the same parts as those of the first embodiment.
The switching converter of FIG. 3 is configured to control the voltage V2 in order to accelerate the rise of the voltage at the output terminals 16 and 17 when the main power is turned on.
[0033]
That is, a series circuit composed of the transistor 52 and the resistor 53 is connected in parallel with the resistor 32. When the transistor 52 becomes conductive, the resistor 53 is connected in parallel with the resistor 32, so that the voltage V2 becomes higher than usual.
The time constant circuit 51 temporarily (for example, 10 msec) controls the signal SG5 to the low level L at a timing (when the main power is turned on) synchronized with the rising of the input signal SG2. When the signal SG5 is at the low level L, the transistor 52 is turned on and the voltage V2 increases.
[0034]
When the transistor 52 is non-conductive, when the current i 1 flowing through the resistor 27 exceeds a predetermined upper limit (overcurrent) (V1> V2), the voltage of the signal SG1 output from the operational amplifier 29 is Become negative. In this case, the control circuit 15 makes the on-duty of the control signal SG4 given to the switching element 14 smaller than usual. As a result, the output power appearing at the secondary winding of the transformer 13 is suppressed.
[0035]
On the other hand, when the transistor 52 becomes conductive, the voltage V2 increases, and even when the current i1 temporarily exceeds the upper limit value (V1 <V2), the signal SG1 output from the operational amplifier 29 The voltage becomes positive. In this case, the control circuit 15 maximizes the on-duty of the control signal SG4 given to the switching element 14 within a controllable range.
[0036]
That is, immediately after the main power is turned on, even when a large current flows, the on-duty of the switching element 14 is increased by the control of the transistor 52, so that a large amount of power is output to the secondary side winding of the transformer 13, The capacitor 19 is charged in a short time. Therefore, the voltage rise at the output terminals 16 and 17 can be accelerated.
[0037]
In steady operation other than when the main power is turned on, the transistor 52 is non-conductive. Therefore, if some abnormality occurs and the current i 1 exceeds the upper limit value, the control circuit 15 controls the secondary side of the transformer 13. The output power appearing in the winding is suppressed, and the current is suppressed.
[0038]
【The invention's effect】
As described above, according to the rise control circuit of the present invention, even when a capacitor with a large capacity is connected to the smoothing circuit on the secondary side of the transformer, the output voltage can be raised in a short time when the power is turned on.
[Brief description of the drawings]
FIG. 1 is an electric circuit diagram showing a main part of a switching converter according to a first embodiment.
FIG. 2 is a time chart showing an operation example of a rise control circuit.
FIG. 3 is an electric circuit diagram showing a main part of a switching converter related to the present invention .
FIG. 4 is a block diagram showing a conventional switching converter.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Overcurrent detection circuit 2 Control circuit 11, 12 Input terminal 13 Transformer 14 Switching element 15 Control circuit 16, 17 Output terminal 18 Load 19, 22, 28 Capacitor 21, 27 Resistor 23, 24, 25 Diode 26 Coil 29 Operational amplifier 31-36 Resistors 37, 52 Transistors 38, 42, 43, 53 Resistor 39 Diode 41 Capacitor 51 Time constant circuit 55 Overcurrent detection circuit 56 Rise control circuit 60 Control input terminal SW Switching element

Claims (1)

A transformer, a switching element arranged on the primary side of the transformer, a switching control unit for controlling the switching element, a smoothing section for rectifying and smoothing the power generated in the secondary side of the transformer, the primary of the transformer Comparing the first voltage output from the input current detection unit with a predetermined second voltage, and detecting the first voltage as the first voltage. A current detector that determines that the current flowing on the primary side of the transformer is an overcurrent when higher than two voltages; and the current detector that determines that the current that flows on the primary side of the transformer is an overcurrent, In a rise control circuit used in a switching converter including an overcurrent suppression unit that suppresses control of the switching control unit ,
The rise control circuit includes:
A binary signal whose state changes in synchronization with the timing of starting / stopping the supply of DC power to the primary side of the transformer is received, and a predetermined time is determined based on the state change of the binary signal generated at the start of the DC power supply Transistor control means for generating a base voltage to turn on the transistor over
First voltage control means for lowering the first voltage output from the input current detection unit to be lower than the second voltage by turning on the transistor;
A rise control circuit for use in a switching converter .

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