Abstract:
In one embodiment, a method for soft-start in a power converter includes the following: providing a feedback signal indicative of the output voltage of the power system at a first input terminal of an error amplifier in a negative feedback loop of the power converter; providing a reference voltage at a second input terminal of the error amplifier; comparing the feedback signal against the reference voltage to generate a control signal for regulating an output voltage of the power converter; charging a soft-start capacitor coupled to the second input terminal of the error amplifier with a current for establishing the reference voltage; and adjusting the current in response to the control signal so that the error amplifier is prevented from saturation.

Description:
BACKGROUND 
     1. Field of Invention 
     The present invention relates to power conversion, and more particularly, to a soft start system and method for a power converter. 
     2. Description of Related Art 
     Power converters are essential for many modern electronic devices. Among other capabilities, a power converter can adjust voltage level downward (buck converter and its derivatives) or adjust voltage level upward (boost converter and its derivatives). A power converter may also convert from alternating current (AC) power to direct current (DC) power, or vice versa. A power converter may also function to provide an output at a regulated level (e.g., 5.0V). Power converters are typically implemented using one or more switching devices, such as transistors, which are turned on and off to deliver power to the output of the converter. Control circuitry is provided to regulate the turning on and off of the switching devices, and thus, these converters are known as “switching regulators” or “switching converters.” Such a power converter may be incorporated into or used to implement a power supply—i.e., a switching mode power supply (SMPS). The power converters may also include one or more capacitors or inductors for alternately storing and outputting energy 
     Some power converters may employ a soft-start circuit in order to begin operation after power on. One kind of soft-start circuit can be a closed-loop soft-start which maintains an error amplifier of the power converter in its linear operating mode to actively control the output voltage of the power converter to follow a reference voltage V REF  at the non-inverting input of the error amplifier. The power converter is able to follow the reference voltage V REF  until the error amplifier output is saturated—i.e. its output is not limited by the supply rails of the error amplifier. 
     Typically, during start up, the reference voltage V REF  at the non-inverting input of the error amplifier rises with a predetermined speed, dV REF /dt. That speed is calculated during initial circuit design and based on the following factors: (1) the value of the soft start capacitor (C SS ) coupled to the non-inverting input of the error amplifier; and (2) the value of a soft start resistor (R SS ) or the amplitude of an I SS  dc current source coupled to the soft start capacitor for charging the same. 
     The rise time of the reference voltage dV REF /dt at the non-inverting input of the error amplifier determines how quickly the output capacitor C OUT  is charged from an initial condition of 0V to its final value, where the output voltage V OUT  can be regulated by the power converter. The rise time of the output voltage dV OUT /dt is proportional to the rise time of the reference voltage dV REF /dt. 
     To support this rate of rise dV OUT /dt at the output during soft start, the power converter must deliver the sum of two current components. The first current component charges the output capacitor and is a function of the value of the output capacitor and rate of rise at the output (C OUT *dV OUT /dt). The second current component provides power to the load (I LOAD ). The total output current (I OUT ) delivered by the power converter then is: 
     
       
         
           
             
               I 
               OUT 
             
             = 
             
               
                 
                   C 
                   OUT 
                 
                 · 
                 
                   
                     ⅆ 
                     
                       V 
                       OUT 
                     
                   
                   
                     ⅆ 
                     t 
                   
                 
               
               + 
               
                 I 
                 LOAD 
               
             
           
         
       
     
     The equation above shows that during soft start, the output current I OUT  depends not only on the dV REF /dt value calculated by the designer but also the converter&#39;s output capacitance C OUT  and the actual load during soft start. Furthermore, the output capacitance C OUT  might have a significant tolerance and its value can be easily multiplied by additional capacitance added by the end user of the power supply. In addition, a power supply is typically required to reliably start up with any load. All these effects will greatly influence the required output current I OUT  during start up. 
     A problem arises if the above-calculated output current I OUT  exceeds the maximum output current of the power converter. When this happens, the power converter can no longer operate in a closed-loop operating mode. In particular, the maximum output current is usually established by a current limit circuit and is slightly higher than the specified maximum load current I LOAD . When the current of the power converter exceeds the maximum output current limit, the error amplifier goes into saturation and looses control over the output voltage V OUT , such that the output voltage V OUT  does not follow the reference voltage V REF  as it should. Ultimately, when the output voltage V OUT  reaches its final value, the error amplifier will need to recover from its saturated state. During this recovery, the output voltage V OUT  overshoots, which is an undesired phenomenon in power supplies. 
     SUMMARY 
     According to an embodiment of the present invention, a system is provided for soft-start in a power converter. The system includes an error amplifier having a first input terminal and a second input terminal. The error amplifier receives a feedback signal indicative of the output voltage of the power system at the first input terminal, and receives a reference voltage at the second input terminal. The error amplifier is operable to compare the feedback signal against the reference voltage to generate a control signal for regulating an output voltage of the power converter. A soft-start capacitor, coupled to the second input terminal of the error amplifier, is operable to be charged for establishing the reference voltage. An adjustable current source is operable to provide a current to charge the soft-start capacitor for establishing the reference voltage. The adjustable current source receives the control signal from the error amplifier, and adjusts the current in response to the control signal so that the error amplifier is prevented from saturation. 
     According to another embodiment of the present invention, a method for soft-start in a power converter includes the following: providing a feedback signal indicative of the output voltage of the power system at a first input terminal of an error amplifier in a negative feedback loop of the power converter; providing a reference voltage at a second input terminal of the error amplifier; comparing the feedback signal against the reference voltage to generate a control signal for regulating an output voltage of the power converter; charging a soft-start capacitor coupled to the second input terminal of the error amplifier with a current for establishing the reference voltage; and adjusting the current in response to the control signal so that the error amplifier is prevented from saturation. 
     According to yet another embodiment of the present invention, a system is provided for soft-start in a power converter. The system includes an error amplifier in a negative feedback loop of the power converter, the error amplifier having a first input terminal and a second input terminal. The error amplifier receives a feedback signal indicative of the output voltage of the power system at the first input terminal, and receives a reference voltage at the second input terminal. The error amplifier is operable to compare the feedback signal against the reference voltage to generate a control signal for regulating an output voltage of the power converter. A reference voltage adjustment circuitry, coupled to the second input terminal of the error amplifier, is operable to adjust the reference voltage in response to the control signal so that the error amplifier is prevented from saturation. 
     Important technical advantages of the present invention are readily apparent to one skilled in the art from the following figures, descriptions, and claims 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a block diagram of an exemplary implementation of a closed loop start-up system, according to an embodiment of the invention. 
         FIG. 2  is an exemplary diagram illustrating the value of a soft-start current as a function of the output of an error amplifier, according to an embodiment of the invention. 
         FIG. 3  is an exemplary diagram illustrating minimum soft-start time as a function of the total output current of a power converter, according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention and their advantages are best understood by referring to  FIGS. 1 through 3  of the drawings. Like numerals are used for like and corresponding parts of the various drawings. 
     In various embodiments, the present invention prevents overshoot by maintaining or keeping the control loop closed during soft start. In some embodiments, the present invention prevents saturation of the error amplifier in the control loop of a power converter during soft start (i.e., the error amplifier stays in its linear operating range). This technique can be implemented using analog or digital circuit design techniques. 
       FIG. 1  is a block diagram of an exemplary implementation of a closed-loop start-up system  10 , according to an embodiment of the invention. Such start-up system  10  can be used with or incorporated in a power converter, such as a switching regulator which provides a regulated output voltage V OUT  (e.g., 5V). Start-up system  10  provides a relatively fast, load-independent soft-start for the power converter. As shown, start-up system  10  includes resistor network  12 , an error amplifier  14 , a soft-start capacitor (Css)  16 , and an adjustable current source (Iss)  18 . 
     The resistor network  12  comprises resistor  20  and  22  coupled in series, for example, between output voltage V OUT  for the power converter and ground (GND). As used herein, the terms “coupled” or “connected,” or any variant thereof, covers any coupling or connection, either direct or indirect, between two or more elements. Resistor network  12  may function as a output voltage feedback divider. That is, resistor network  12  divides the value of the output voltage V OUT  and provides it as feedback at a terminal or pin FB. 
     The error amplifier  14  has an inverting (−) terminal and a non-inverting (+) terminal. The inverting (−) terminal of error amplifier  14  is coupled to the resistor network  12  through an impedance  24  (Z 3 ) to receive the feedback signal. The non-inverting (+) terminal receives a reference voltage V REF (t), the value of which can vary with time. The error amplifier  14  compares the feedback signal against the reference voltage V REF (t), and in response, outputs a control signal V CTRL . The control signal V CTRL  may be provided for controlling switching for regulating output voltage of the power converter. The output of the error amplifier is coupled to its inverting (−) terminal through an impedance  26  (Z 4 ). 
     Soft-start capacitor  16  and adjustable current source  18  are coupled to the non-inverting (+) terminal of error amplifier  14  and function to generate or develop the reference voltage V REF (t). Reference voltage V REF (t), which varies with time, corresponds to the voltage of the soft-start capacitor  16 . Adjustable current source  18  provides a current Iss for charging the soft-start capacitor  16 . As soft-start capacitor  16  is charged, the value of the reference voltage V REF (t) increases. Adjustable current source  18  is coupled to and receives the control signal V CTRL  output from the error amplifier  14 . The magnitude of current I SS  output from adjustable current source  18  is controlled by the control signal V CTRL  and can be adjusted between a minimum and maximum value. 
     The output current I OUT  of the power converter, in which start-up system  10  is incorporated, is proportional to the control voltage signal V CTRL  output from the error amplifier  14 . As the output current I OUT  approaches its maximum value where current limit operation would commence, control voltage V CTRL  also approaches its maximum voltage (saturation). If the current demand of the power converter&#39;s load exceeds the current capability of the converter, the error amplifier  14  would saturate. With start-up system  10 , however, the error amplifier  14  controls the soft-start current source  18 . This arrangement can reduce the current needed to charge the output capacitor. As a result, the saturation of the error amplifier  14  can be prevented and the control loop can stay in its linear operating range. Thus, closed loop soft-start is maintained. 
     To accomplish this, in one embodiment, the soft-start current I SS  (output from adjustable current source  18  for charging the soft-start capacitor  16 ) is reduced when the output of the error amplifier  14  (control signal V CTRL ) rises and nears the saturation voltage. Start-up system  10  moves to equilibrium as the reduced Iss current slows down the rising rate of the output voltage (dV OUT /dt), thus reducing the total output current (I OUT ) demand of the power converter. 
     In an alternative embodiment, an adjustable voltage source can be used instead of the adjustable current source  18 . Such adjustable voltage source would also be responsive to the control signal V CTRL , output from the error amplifier  14 . 
     In various embodiments, all or a portion of the start-up system  10  shown in  FIG. 1  can be implemented on a single or multiple semiconductor dies (commonly referred to as a “chip”) or discrete components. Each die is a monolithic structure formed from, for example, silicon or other suitable material. For implementations using multiple dies or components, the dies and components can be assembled on a printed circuit board (PCB) having various traces for conveying signals therebetween. In one embodiment, for example, error amplifier  14 , soft-start capacitor  16 , and adjustable current source  18  can be provided on a single chip or die, or on one or more separate die, and resistors  20  and  22  of resistor network  12  are implemented as discrete components. 
       FIG. 2  is an exemplary diagram  100  illustrating the value of soft-start current Iss as a function of the output of error amplifier  14 , according to an embodiment of the invention. Soft-start current I SS  is output from adjustable current source  18 , which is controlled by the control voltage V CTRL  from error amplifier  14 . As such, the value of soft-start current I SS  varies with control voltage V CTRL  The magnitude of soft-start current I SS  is at its highest (e.g., I SS MAX ) when the control voltage V CTRL  from error amplifier  14  is low. As the value of the control voltage V CTRL  rises, the magnitude of soft-start current I SS  decreases. 
       FIG. 3  is an exemplary diagram  200  illustrating soft-start time (t SS ) as a function of the total output current I OUT  of a power converter with the start-up system  10 , according to an embodiment of the invention. The soft-start time t SS  varies with the output current I OUT . The soft-start time t SS  is at its lowest (e.g., t SS MIN ) when the output current I OUT  is low. As the magnitude of the output current I OUT  rises, the soft-start time t SS  increases. The soft-start time t SS  is at its highest when the output current I OUT  is at its maximum. 
     Thus, in various embodiments, the present invention provides an advantage in that it prevents or keeps the error amplifier  14  from saturation. 
     Another advantage of the present invention, in some embodiments, is that it allows faster start-up with light load. In particular, with previously developed techniques, the soft start time is designed for worst case conditions, which results in extremely long soft-start time to ensure closed-loop operation when the circuit starts up under full load conditions. 
     Yet another advantage is that the present invention facilitates design for soft start circuitry. The start-up system  10  is transparent for the designer, and the control of the soft start current Iss is automatically adjusted by the output of the error amplifier  14 . 
     Still yet another advantage is that the present invention, in various embodiments, is insensitivity to tolerances for the output capacitor and load current variations of a power converter. 
     Another advantage, in some embodiments, is that the present invention can significantly reduce or substantially eliminate overshoot of the output voltage V OUT  of the power converter during start up. This is especially important in power factor correction (PFC) applications but can also relevant in low voltage applications where there is little tolerance for overshoot. 
     Embodiments of the present invention, such as start-up system  10 , can be used in any closed-loop start-up system. Furthermore, embodiments of the present invention can be used in a wide variety of power converter topologies including, for example, isolated or non-isolated applications, buck converters, boost converters, buck/boost converters, flyback converters, SEPIC converters, etc. Embodiments of the present invention may be used with a variety of control methods including, for example, analog control, digital control, voltage-mode control, current-mode control, average current-mode control, etc. 
     For example, in alternative embodiment, the exemplary analog circuit of  FIG. 1  can be replaced by its digital equivalent circuit. In such a digital implementation, the voltage signals (such as the voltage across the soft-start capacitor  16 , output voltage of the error amplifier  14 , and the scaled output voltage provided by the resistive network having resistors  20  and  22 ) are represented by numerical quantities. In one embodiment for a digital implementation, the reference (Vref(t)) to the error amplifier  14  is a numerical value between zero and the final value of said reference representing the desired output voltage of the converter. In a digital implementation, the error amplifier  14  can be implemented with a digital compensator. Furthermore, the output voltage of the converter or system can also represented by its numerical equivalent after an analog-to-digital conversion, which can be obtained by directly converting the output voltage to a digital quantity or by converting only the error between the actual and desired output voltage of the converter to a digital quantity. In such an implementation, soft-start can be provided by incrementing the numerical value of the reference from zero to its final value. Increments to the numerical reference number can be made in predetermined time intervals to achieve the desired output voltage ramp up time. For a digital implementation, the time interval between the increments to the numerical value of the reference may also be responsive to the control signal from output of the digital compensator (the equivalent of error amplifier  14 ). 
     Furthermore, in addition to a pure analog or pure digital implementations, analog and digital circuits can be combined in various manners to achieve the desired functionality covered by the spirit of the invention. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims. That is, the discussion included in this application is intended to serve as a basic description. It should be understood that the specific discussion may not explicitly describe all embodiments possible; many alternatives are implicit. It also may not fully explain the generic nature of the invention and may not explicitly show how each feature or element can actually be representative of a broader function or of a great variety of alternative or equivalent elements. Again, these are implicitly included in this disclosure. Where the invention is described in device-oriented terminology, each element of the device implicitly performs a function. Neither the description nor the terminology is intended to limit the scope of the claims.