Patent Publication Number: US-2009231886-A1

Title: Power supply and bootstrap circuit thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 97108696, filed on Mar. 12, 2008. The entirety the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a power supply and a bootstrap circuit thereof, and more particularly, to a power supply with less power consumption and a bootstrap circuit consuming power only during a starting course. 
     2. Description of Related Art 
     Along with the approaching energy crisis, the environmental awareness is gradually increasing among the public opinions. In recent years, US government has proposed some of energy efficiency standards, such as 80 Plus program, Energy Star program and the like so as to expectably govern the energy consumptions of electronic products. The above-mentioned energy efficiency standards have been applied to computer systems already, which restrict the power consumption of a computer in standby state to be lower than 3 W. Considering a power supply in a computer system usually has a conversion efficiency of 60%-70% only, much electrical energy is transformed to thermal energy and dissipated into the atmospheric air. Therefore, a power supply becomes one of principal energy-consuming parts in a computer system. Moreover, the above-mentioned power consumption restriction seriously challenges the existing relevant designs of a conventional power supply. 
     Although a current power supply has adopted quasi-resonance technique and synchronized rectification technique to promote the efficiency thereof, but many auxiliary circuits thereof still have unsolved power loss issues.  FIG. 1  is a schematic circuit drawing of a power supply with a conventional bootstrap circuit. In  FIG. 1 , a resistor  102  and a capacitor  104  form a conventional bootstrap circuit. The capacitance of the capacitor  104  in the bootstrap circuit must be large enough to feed a pulse width modulation control circuit (PWM control circuit)  106  with a sufficient electric current. The resistance of the resistor  102  must be large enough too, so that any inrush current passing through the resistor  102  is avoided to damage the components. The problem for the above-mentioned power supply rests in that although the PWM control circuit  106  only consumes less than one watt during driving an metal oxide semiconductor transistor (MOS transistor)  108 ; however, the resistor  102  still consumes a quite large power after starting the power supply, which leads the computer system not to meet the above-mentioned energy efficiency standards. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a bootstrap circuit, which is suitable for a power supply and consumes power only during the start of the power supply. 
     The present invention is also directed to a power supply, which consumes less power consumption than a conventional power supply. 
     The present invention provides a bootstrap circuit. The bootstrap circuit includes a transistor, a first capacitor, a first impedance and a regulator circuit. The collector and the emitter of the transistor serve as an input terminal and an output terminal of the bootstrap circuit respectively. A terminal of the first capacitor is coupled to the collector of the transistor, a terminal of the first impedance is coupled to another terminal of the first capacitor. The regulator circuit is coupled to another terminal of the first impedance and the base of the transistor for clamping the voltage at the above-mentioned base at a predetermined voltage level. 
     The present invention also provides a power supply. The power supply includes a bridge rectifier, a first capacitor, a transformer, a switch, a first diode, a second capacitor, a PWM control circuit and a bootstrap circuit. The bridge rectifier has two AC input terminals, a positive output terminal and a negative output terminal. A terminal of the first capacitor is coupled to the positive output terminal and another terminal thereof is coupled to the negative output terminal and a common voltage level. The transformer has a primary winding and a secondary winding, and a terminal of the primary winding is coupled to the positive output terminal. The switch has a first terminal, a second terminal and a control terminal. The first terminal of the switch is coupled to another terminal of the primary winding and the second terminal of the switch is coupled to a common voltage level. The anode of the first diode is coupled to a terminal of the secondary winding. A terminal of the second capacitor is coupled to the cathode of the first diode, and another terminal of the second capacitor is coupled to another terminal of the secondary winding and the common voltage level. The input terminal of the PWM control circuit is coupled to the cathode of the first diode and the output terminal thereof is coupled to the control terminal of the switch. The bootstrap circuit is coupled between the positive output terminal and the input terminal of the PWM control circuit. When a power voltage is produced on the positive output terminal, the bootstrap circuit provides the input terminal of the PWM control circuit with a starting voltage and the bootstrap circuit would be automatically turned off after a predetermined time. 
     In an embodiment of the present invention, the above-mentioned bootstrap circuit of the power supply adopts the above-described bootstrap circuit architecture. 
     The bootstrap circuit of the present invention is composed of a transistor, a capacitor, an impedance and a regulator circuit, wherein the capacitor and the transistor are used to produce a time constant. When the input terminal of the bootstrap circuit has a momentary variation of voltage, the capacitor makes the transient voltage coupled to the base of the transistor through the impedance, so that the transistor can be quickly turned on to establish a starting voltage at the emitter of the transistor. After the above-mentioned constant time, the bootstrap circuit enters a stable state. At this time, the capacitor takes open-circuit state to turn off the transistor, which makes the bootstrap circuit automatically turned off and the bootstrap circuit has no more power consumption. In addition, the power supply of the present invention can adopt the above-described bootstrap circuit architecture. Since the adopted bootstrap circuit consumes power only during the course of starting the power supply, therefore, the power supply of the present invention consumes less power in standby state and has an overall power consumption less than that of the conventional power supply. A computer system employing the power supply or the bootstrap circuit of the present invention can thereby easily meet the energy efficiency standards such as 80 Plus, Energy Star and the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objectives, features and advantages of the present invention will be further understood from the further technological features disclosed by the embodiments of the present invention wherein there are shown and described preferred embodiments of this invention, simply by way of illustration of modes best suited to carry out the invention. 
         FIG. 1  is a schematic circuit drawing of a power supply with a conventional bootstrap circuit. 
         FIG. 2  is a schematic circuit drawing of a power supply and the bootstrap circuit thereof according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
       FIG. 2  is a schematic circuit drawing of a power supply and the bootstrap circuit thereof according to an embodiment of the present invention. The power supply includes a bridge rectifier  202 , a capacitor  204 , a transformer T 1 , a switch  210 , a diode  212 , a capacitor  214 , a PWM control circuit  216  and a bootstrap circuit  218 . The bridge rectifier  202  has two AC input terminals  202 - 1  and  202 - 2 , a positive output terminal  202 - 3  and a negative output terminal  202 - 4 . A terminal of the capacitor  204  is coupled to the positive output terminal  202 - 3  and another terminal thereof is coupled to the negative output terminal  202 - 4  and a common voltage level GND. The transformer T 1  has a primary winding  206  and a secondary winding  208 , and a terminal of the primary winding  206  is coupled to the positive output terminal  202 - 3 . The switch  210  has a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to another terminal of the primary winding  206  and the second terminal of the switch  210  is coupled to the common voltage level GND. 
     The anode of the diode  212  is coupled to a terminal of the secondary winding  208 , a terminal of the capacitor  214  is coupled to the cathode of the diode  212  and another terminal of the capacitor  214  is coupled to another terminal of the secondary winding  208  and the common voltage level GND. The two terminals of the capacitor  214  are used to produce an output OUT of the power supply. The PWM control circuit  216  has an input terminal VDD, an output terminal OUT and a grounding terminal VGND, wherein the input terminal VDD is coupled to the cathode of the diode  212  and the output terminal OUT is coupled to the control terminal of the switch  210 . The bootstrap circuit  218  is coupled between the positive output terminal  202 - 3  and the input terminal VDD of the PWM control circuit  216 . When a power voltage VREC is produced at the positive output terminal  202 - 3 , the bootstrap circuit  218  provides a starting voltage VAUX to the input terminal VDD of the PWM control circuit  216  and the bootstrap circuit  218  would be automatically turned off after a predetermined time. 
     In the present embodiment, the bootstrap circuit  218  includes a capacitor  220 , two impedances  222  and  224 , a transistor  226 , two diodes  228  and  230  and a regulator circuit  232 . The collector of the transistor  226  is coupled to the positive output terminal  202 - 3  via the impedance  224 , and the emitter thereof is coupled to the input terminal VDD of the PWM control circuit  216  for outputting the starting voltage VAUX. A terminal of the capacitor  220  is coupled to the positive output terminal  202 - 3  and the impedance  224  and another terminal thereof is coupled to the regulator circuit  232  via the impedance  222 . The anode of the diode  230  is coupled to the common voltage level GND and the cathode thereof is coupled to the impedance  222 . The anode of the diode  228  is coupled to the regulator circuit  232  and the cathode thereof is coupled to the base of the transistor  226 . The regulator circuit  232  is coupled to the impedance  222  and the anode of the diode  228  for clamping the voltage of the above-mentioned anode at a predetermined voltage level. 
     The above-mentioned regulator circuit  232  includes two impedances  234  and  236  and a shunt regulator  238 . A terminal of the impedance  234  is coupled to the impedance  222  and the anode of the diode  228 , and the impedance  236  is coupled between another terminal of the impedance  234  and the common voltage level GND. The shunt regulator  238  has an anode terminal, a cathode terminal and a reference terminal, wherein the cathode terminal of the shunt regulator  238  is coupled to the anode of the diode  228 , the anode terminal of the shunt regulator  238  is coupled to the common voltage level GND and the reference terminal of the shunt regulator  238  is coupled to another terminal of the impedance  234 . The shunt regulator  238  determines the voltage between the anode terminal and the cathode terminal thereof according to the voltage of the reference terminal. The switch  210  herein can be implemented by using a metal oxide semiconductor transistor (MOS transistor), the impedances  222 ,  224 ,  234  and  236  can be implemented by using resistors and the transistor  226  can be implemented by using an NPN-type power transistor. 
     When the AC input terminals  202 - 1  and  202 - 2  of the bridge rectifier  202  receive an AC power, a job of converting AC into DC is executed, so that a pulse DC is output through the positive output terminal  202 - 3  and the negative output terminal  202 - 4 . The polarities of the DC power voltage are shown in the figure, where ‘+’ represents positive polarity and ‘−’ represents negative polarity. At the time, a momentary variation of the power voltage VREC at the positive output terminal  202 - 3  enables the capacitor  220  to couple the voltage to the node where the capacitor  220  and the impedance  222  are coupled to each other. After that, the high voltage is delivered to the base of the transistor  226  through the impedance  222  and the diode  228  in sequence, which further quickly turns on the transistor  226 . In this way, the transistor  226  is able to quickly produce the starting voltage VAUX at the emitter thereof and deliver the starting voltage VAUX to the input terminal VDD of the PWM control circuit  216 . The PWM control circuit  216  then starts to work and drives the switch  210  to enable the transformer T 1  to run. 
     After the transformer T 1  is operated, the power voltage required by the PWM control circuit  216  is received through the diode  212  and the capacitor at this time functions to keep the power voltage stable. As to the bootstrap circuit  218 , since the bootstrap circuit  218  has a certain operation time, i.e. a time constant preset by the capacitor  220  and the impedance  222 ; thus, after the capacitor  220  is charged for a while and gets a stable state, the capacitor  22   o  takes open-circuit state to turn off the transistor  226 , which automatically turns off the bootstrap circuit  218  to stop the consumption of power. In other words, as the power supply starts, the bootstrap circuit  218  is in charge of starting the transformer T 1 . Once the transformer Ti is started to produce the power voltage required by the PWM control circuit  216 , the bootstrap circuit  218  is automatically turned off. Therefore, the bootstrap circuit  218  consumes power only during starting the power supply, and therefore the power supply consumes less power in standby state and the power supply of the present invention thereby has a less overall power consumption than that of a conventional power supply. 
     Particularly, in addition to the function of the regulator circuit  232  in the above-mentioned bootstrap circuit  218  to clamp the voltage of the anode of the diode  228  at a predetermined voltage level so as to prevent the transistor  226  from a transient surge voltage resulting a possible damage, a user is allowed to adjust the ratio of the impedance  234  over the impedance  236  so as to determine the starting voltage VAUX. The diode  230  herein functions to protect other components from producing a negative voltage and the impedance  224  is served as a current-limiting resistor, which ensures the operation of the transistor  226  in a safe operation area (SOA). Note that in the bootstrap circuit  218 , the impedance  224  and the diodes  228  and  230  are optional parts only, and the user may decide whether or not to dispose the impedance  224  and the diodes  228  and  230  according to a real design need. 
     In summary, the bootstrap circuit of the present invention is composed of a transistor, a capacitor, an impedance and a regulator circuit, wherein the capacitor and the impedance are used to produce a time constant. When a momentary voltage variation occurs at the input terminal of the bootstrap circuit, the capacitor couples the transient voltage to the base of the transistor through the impedance so as to quickly turn on the transistor and to produce a starting voltage at the emitter of the transistor. When the bootstrap circuit enters stable state after the above-mentioned constant time, the capacitor takes open-circuit state to turn off the transistor, which enables the bootstrap circuit automatically to be turned off and not consume any power. In addition, the power supply of the present invention can adopt the above-described bootstrap circuit architecture. Since the bootstrap circuit consumes power only during the starting stage of the power supply, therefore, the power supply of the present invention consumes less power in standby state compared to the prior art. In this way, the power supply or bootstrap circuit of the present invention facilitates a computer system employing the power supply and the bootstrap circuit reaching the requirements of energy efficiency standards such as 80 Plus, Energy Star and the like. 
     The above are preferred embodiments of the present invention, however, they are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that various modifications and equivalent variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.