Patent Application
20120182000
1. Field of the Invention
The present invention relates to a soft start circuit, and more particularly, to a soft start circuit capable of preventing power converter semiconductor devices from being burned out and preventing overshooting of an output voltage.
2. Description of the Related Art
In general, a power converter is a device for raising or lowering the voltage level of input power to a predetermined level to output power having a predetermined voltage. A power converter is indispensable in various devices for SMPS (Switching Mode Power Supply), motor driving, or the like.
This power converter utilizes a voltage control method, by which an output voltage is compared with a reference voltage to control an on-off function of a switching element contained in the power converter, in order to control the output voltage. According to this voltage control method, since a reference voltage of an error amplifier may be high at the time of early driving of the power converter, the switching element of the power converter is operated at the maximum duty ratio, and thus, an output capacitor installed at an output terminal of the power converter is charged to a predetermined output voltage level. In this case, a section in which the current of an inductor or a transformer of the power converter is built up may be enlarged. This causes an initial inrush current to be generated.
In order to prevent this initial inrush current, a soft start method of gradually increasing the duty ratio is being studied. However, the soft start method has a defect in that overshooting may be generated in an output voltage while a transient state is changed to a normal state.
Further, the output voltage is gradually decreased at the time of the turning off of the power converter. Therefore, when the power converter is again turned on, while the output voltage remains high, a voltage difference between a reference voltage and the output voltage is large and thus, current flows inversely through an inductor of the power converter. This may cause semiconductor devices inside the power converter to be burned out.
An aspect of the present invention provides a soft start circuit capable of preventing power converter semiconductor devices from being burned out and preventing the overshooting of an output voltage.
According to an aspect of the present invention, there is provided a soft start circuit, including: a resonator generating a resonant voltage; a voltage detector detecting an output voltage of a power converter; a reference voltage selector selecting one of a voltage detected by the voltage detector and the resonant voltage as a reference voltage, based on a driving signal for turning the power converter on or off; and an error amplifier generating a control signal for controlling the output voltage of the power converter from the reference voltage selected by the reference voltage selector and the voltage detected by the voltage detector.
The reference voltage selector may select the resonant voltage as the reference voltage when the driving signal is a first driving signal for turning the power converter on, and select the voltage detected by the voltage detector as the reference voltage when the driving signal is a second driving signal for turning the power converter off.
The reference voltage selector may include: a first switching element having a first terminal connected to an output terminal of the voltage detector and a second terminal connected to an output terminal of the resonator, the first switching element opening or closing the connection between the first terminal and the second terminal by a voltage difference between a first control terminal to which a switching signal is inputted and the second terminal; and a second switching element having a third terminal connected to the first control terminal of the first switching element and a fourth terminal connected to the ground, the second switching element opening or closing the connection between the third terminal and the fourth terminal by a voltage difference between a second control terminal to which a switching signal is inputted and the fourth terminal.
The reference voltage selector may further include a voltage follower between the first terminal of the first switching element and the output terminal of the voltage detector.
The resonator may include: a driving power; a resistor having one end connected to the driving power; and a resonant capacitor connected between the other terminal of the resistor and the ground. Herein, an output terminal which is a contact point of the other end of the resistor and the resonant capacitor may be connected to the second terminal of the first switching element.
The reference voltage selector may further include a resistor having one end connected to the fourth terminal of the second switching element and the other end connected to the ground.
The reference voltage selector may include: a comparator having a negative (−) terminal connected to the output terminal of the voltage detector and a positive (+) terminal connected to the output terminal of the resonator; a third switching element having a third control terminal connected to an output of the comparator, a fifth terminal connected to the output terminal of the resonator, a sixth terminal connected to the ground; and a diode having an anode connected to the third control terminal and a cathode to which a driving signal for turning the power converter on or off is applied.
The resonator may include: a driving power; a resistor having one end connected to the driving power; and a resonant capacitor connected between the other terminal of the resistor and the ground. Herein, an output terminal which is a contact point of the other end of the resistor and the resonant capacitor may be connected to the fifth terminal of the third switching element.
The reference voltage selector may further include a resistor having one end connected to the sixth terminal of the third switching element and the other end connected to the ground.
The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings so that they could be easily practiced by those skilled in the art to which the present invention pertains. However, in describing the exemplary embodiments of the present invention, detailed descriptions of well-known functions or constructions are omitted so as not to obscure the description of the present invention with unnecessary detail.
In addition, like reference numerals denote parts performing similar functions and actions throughout the drawings.
In addition, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising,” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements.
More specially, the power converter 10 may include a DC/DC converter, such as, a Buck converter, a boost converter, a Buck-boost converter, a flyback converter, a half bridge converter, or the like. The power converter 10 converts the input voltage Vin into a constant output voltage Vo to output the output voltage Vo.
The soft start circuit 100 receives the feedback on the output voltage Vo of the power converter 10 to generate the control signal Verr, and transmits the control signal Verr to the PWM signal generator 20. Constitutions and operations of this soft start circuit 100 will be described in detail later.
The PWM signal generator 20 controls a plurality of switching elements included in the power converter 10 according to the control signal Verr transmitted from the soft start circuit 100.
When the external signal for controlling the driving of the power converter 10 is inputted, the on-off signal generator 30 generates the driving signal
Hereinafter, a circuit of a soft start circuit 100 according to an exemplary embodiment of the present invention will be described in detail with reference to
In the exemplary embodiment of the present invention, a first switching element Q1 and a second switching element Q2 may include a BJT (Bipolar Junction Transistor) or an FET (Field Effect Transistor), and the BJT is figuratively shown in
The soft start circuit 100 according to the exemplary embodiment of the present invention may include an RC resonator 120 generating an RC resonant voltage, a voltage detector 110 detecting an output voltage Vo of a power converter 10, a reference voltage selector 140 selecting one of the voltage Vo′ detected by the voltage detector 110 and the RC resonant voltage Vr as a reference voltage Vref, based on the driving signal for turning the power converter 10 on or off, and an error amplifier 130 generating a control signal Verr for controlling the output voltage Vo of the power converter 10 from the reference voltage Vref selected by the reference voltage selector 140 and the voltage Vo′ detected by the voltage detector 110.
The voltage detector 110, which may include resistors R1 and R2 connected in series, detects the output voltage Vo of the power converter 10, and outputs the voltage Vo′ divided according to a law of resistance division.
The RC resonator 120 may include a resistor R having one end connected to a driving power Vdd and the other end connected to a resonant capacitor C, and the resonant capacitor C connected between the other end of the resistor R and the ground. An output terminal of the RC resonator which is a contact point of the other end of the resistor R and the resonant capacitor C may be connected to the second terminal of the first switching element Q1. This RC resonator 120 generates the RC resonant voltage Vr by charging the resonant capacitor C by the driving power Vdd.
In the error amplifier 130, the voltage Vo′ detected by the voltage detector 110 is applied to a negative (−) terminal of an OP amp 131 and the reference voltage Vref is applied to a positive (+) terminal of the OP amp 131. In addition, the error amplifier 130 may be constituted as a negative feedback circuit in which a first compensator 132 and a second compensator 133 for appropriate gain control are added. The first compensator 132 and the second compensator 133 may be constituted of elements such as resistors, capacitors, inductors, and the like. This error amplifier 130 receives the reference voltage Vref and the voltage Vo′ detected by the voltage detector 110, generates the control signal Verr based on a difference between both of the inputted voltages Vref and Vo′, and outputs the generated control signal Verr to the PWM signal generator 20. As such, the first compensator 132 and the second compensator 133 are constituted of the appropriate elements, and thus, the above-described control signal Verr can be generated to have various signal types, such as P (Proportional) control, PI (Proportional Integral) control, PID (Proportional Integral Differential) control, and the like. This matter is obvious to those skilled in the art, and thus a detailed description thereof will be omitted.
The reference voltage selector 140 selects the RC resonant voltage Vr as the reference voltage Vref when the driving signal
More specially, the reference voltage selector 140 may include a first switching element Q1 and a second element Q2. The first switching element Q1 has a first terminal connected to an output terminal of the voltage detector 110 and a second terminal connected to an output terminal of the RC resonator 120. The first switching element Q1 opens or closes the connection between the first terminal and the second terminal by a voltage difference between a first control terminal to which a switching signal is inputted and the second terminal. The second switching element Q2 has a third terminal connected to the first control terminal of the first switching element Q1 and a fourth terminal connected to the ground. The second switching element Q2 opens or closes the connection between the third terminal and the fourth terminal by a voltage difference between a second control terminal to which a driving signal DC_ON inputted and the fourth terminal.
In addition, a voltage follower 141 may be disposed between the first terminal of the first switching element Q1 and the output terminal of the voltage detector 110.
Hereinafter, an operational principle of the soft start circuit 100 according to the exemplary embodiment of the present invention will be described in detail with reference to
Referring to
Meanwhile, the PWM signal generator 20 receives the control signal Verr generated by the error amplifier 130 to generate a PWM signal SW for tracking the reference voltage Vref, and transmits the generated PWM signal SW to the power converter 10. The power converter 10 controls inside switching elements (not shown) based on the PWM signal SW transmitted from the PWM signal generator 20, thereby controlling the output voltage Vo to track the reference voltage Vref. Herein, the reference voltage Vref is a smoothly rising RC resonant voltage, and thus, this can prevent overshooting of the output voltage Vo in the power converter 10.
Then, in the case that at t1, a signal DC OFF for turning the power converter 10 off, a second driving signal (
More specially, as the power converter 10 is turned off, the voltage Vo′ detected by the voltage detector 110 is reduced to become smaller than the RC resonant voltage Vr. Herein, an amount of voltage corresponding to a voltage difference between the RC resonant voltage Vr and the voltage Vo′ detected by the voltage detector 110 is discharged to the output terminal of the voltage follower 141 as a current. In other words, the voltage follower 141 functions as a current sink. Therefore, as shown in
Then, in the case that at t2, a signal DC ON for restarting the power converter 10, the first driving signal (
As for an explanation of this exemplary embodiment of the present invention, a third switching element Q3 may include a BJT (Bipolar Junction Transistor) or an FET(Field Effect Transistor), and the FET(Field Effect Transistor) is figuratively shown in
Referring to
In addition, the RC resonator 120 may include a resistor R having one end connected to a driving power Vdd, and a resonant capacitor C connected between the other end of the resistor R and the ground. An output terminal of the RC resonator which is a contact point of the other end of the resistor R and the resonant capacitor C may be connected to the fifth terminal of the third switching element Q3.
Hereinafter, an operational principle of the soft start circuit 100 according to the present exemplary embodiment of the present invention will be described in detail.
Referring to
Meanwhile, the PWM signal generator 20 may receive the control signal Verr generated by the error amplifier 130 to generate a PWM signal SW for tracking the reference voltage Vref, and transmit the generated PWM signal SW to the power converter 10. The power converter 10 controls inside switching elements (not shown) based on the PWM signal SW transmitted from the PWM signal generator 20, thereby controlling the output voltage Vo to track the reference voltage Vref. Herein, the reference voltage Vref is a smoothly rising RC resonant voltage, and thus, this can prevent overshooting of the output voltage Vo in the power converter 10.
Then, at t1, in the case that a signal DC OFF for turning the power converter 10 off, the second driving signal (
More specially, at t1, the RC resonant voltage Vr is greater than the voltage Vo′ detected by the voltage detector 110. In other words, it is because the voltage Vo′ detected by the voltage detector 110 is not greater than the reference voltage Vref by tracking the RC resonant voltage Vr which is the reference voltage at t1. Therefore, a high (H)-level voltage signal is outputted from the comparator 300, and thus the third switching element Q3 is turned on. As the third switching element Q3 is turned on, the RC resonant voltage Vr charged in the resonant capacitor C is discharged through the third switching element Q3. However, when the voltage Vr charged in the resonant capacitor C is discharged such that the voltage Vr is smaller than the voltage Vo′ detected by the voltage detector 110, a low (L)-level voltage signal is outputted from the comparator 300 and the third switching element Q3 is turned off. As the third switching element Q3 is turned off, the resonant capacitor C starts to be recharged. When the voltage charged in the resonant capacitor C is equal to or greater than the voltage Vo′ detected by the voltage detector 110, the third switching element Q3 is again turned on. Therefore, the voltage charged in the resonant capacitor C starts to be discharged through the third switching element Q3. Through repetitive performance of the above-described procedure, the voltage Vo′ detected by the voltage detector 110 is transmitted to the error amplifier 130 as the reference voltage Vref, while the driving signal
Then, at t2, when a signal DC ON for turning the power converter 10 on, a first driving signal (
As such, according to the exemplary embodiment of the present invention, since the reference voltage Vref and the voltage Vo′ detected by the voltage detector 110 have the same voltage level, even at the time of restarting of the power converter 10, current may be prevented from flowing inversely through the inductor (not shown) at the output terminal of the power converter 10. Therefore, the exemplary embodiment of the present invention is capable of preventing semiconductor devices inside the power converter 10 such as switching elements from being burned out.
As set forth above, according to an embodiment of the present invention, semiconductor devices of the power converter may be prevented from being burned out and may prevent overshooting of an output voltage by selecting one of the resonant voltage and the output voltage of the power converter as the reference voltage inputted to the error amplifier according to the driving signal for turning the power converter on or off.
While the present invention has been shown and described in connection with the exemplary embodiements, it will be apparent to those skilled in the art that modification and variation can be made withough departing from the spirit and scope of the invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2011-0005577 | Jan 2011 | KR | national |
This application claims the priority of Korean Patent Application No. 10-2011-0005577 filed on Jan. 19, 2011, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
