Patent Publication Number: US-2011057636-A1

Title: Method for Reducing Energy Loss in DC-DC Converter and Related Control Device and DC-DC Converter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to a method for reducing energy loss in a DC-DC converter and related control device and DC-DC converter, and more particularly, to a method capable of reducing energy loss in a DC-DC converter by adjusting a switching frequency thereof and related control device and DC-DC converter. 
     2. Description of the Prior Art 
     An electronic device generally includes various components requiring different operating voltages. Therefore, a DC-DC voltage converter is essential for the electronic device to adjust (step up or step down) and stabilize voltage levels. Based upon different power requirements, various types of DC-DC voltage converter, originating from a buck (step down) converter and a boost (step up) converter, are developed. Accordingly, the buck converter can decrease an input DC voltage to a default voltage level, and the boost converter can increase an input DC voltage. With advances in circuit technology, both of the buck and boost converters are varied and modified to conform to different system architectures and requirements. 
     For example, please refer to  FIG. 1 , which is a schematic diagram of a buck converter  10  of the prior art. The buck converter  10  includes an input end  100 , a lowpass module  110 , a control module  120 , a switch module  130 , an output end  140 , an output module  150  and a feedback module  160 . The input end  100  is utilized for receiving a first input voltage VIN 1 . The lowpass module  110  is composed of an input inductor  112  and an input capacitor  114 , and is utilized for filtering out high frequency components of the first input voltage VIN 1  to generate a second input voltage VIN 2 . The control module  120  is a pulse width modulation (PWM) controller for generating a PWM signal VPWM sent to the switch module  130  according to the second input voltage VIN 2  and a feedback signal VFB of the output end  140 . The switch module  130  includes an up-bridge switch transistor  132 , a down-bridge switch transistor  134 , an amplifier  136  and an inverter  138 , and is utilized for determining whether or not to enable the up-bridge switch transistor  132  and the down-bridge switch transistor  134  based upon the PWN signal (and an inverted PWM signal), so as to adjust a current of a node N 1 . The output module  150  coupled to the node N 1  is composed of an output capacitor  152  and an output inductor  154 , and is utilized for generating an output voltage VOUT by frequency response of the out inductor  152  and the output capacitor  154 . In short, the control module  120  adjusts the output voltage VOUT by varying duty cycles of the up-bridge switch transistor  132  and the down-bridge switch transistor  134 . 
     However, due to undesired effects caused by manufacturing process errors, physical properties of components, etc., parasitic components essentially exist in the switch module  130 , and lead to a performance decline in the buck converter  10 . For example, when the buck converter  10  operates in a light load state (with a low output current IOUT), a “switching loss” is the major cause of the performance decline in the buck converter  10 . In detail, when the switch module  130  performs switching operations, gate voltages of the up-bridge switch transistor  132  and the down-bridge switch transistor  134  cannot instantaneously hit a desired level due to parasitic gate capacitors thereof, but increase or decrease smoothly, implying a large resistor R DS  existing between drains and sources of the up-bridge switch transistor  132  and the down-bridge switch transistor  134  as well as an extra energy loss. Furthermore, the parasitic capacitors are charged and discharged during the switching operation, leading to another energy loss. Note that the switching loss is directly proportional the switching frequency. That is, the energy loss becomes more severe with increased switching times. 
     Thus, in order to reduce the switching loss, the buck converter  10  has to decrease the switching frequency of the PWM signal VPWM. However, without additional modifications, a load variation resistance of the output voltage VOUT drops with the switching frequency. For example, if demand for the output current IOUT explodes, a smoothly changing switching frequency leads to smoothly changing charging/discharging frequency of the output capacitor  152 , as well as an unstable output voltage VOUT. Moreover, in the buck converter  10 , the switching frequency of the PWM signal VPWM is determined by an oscillator (e.g. a crystal oscillator) of the control module  120 , and therefore is fixed. That is, the switching loss of the buck converter  10  cannot be reduced by adjusting the switching frequency. 
     Therefore, how to reduce the energy loss of the DC-DC converter to enhance performance by timely adjustment of the switching frequency has been a major area of research in industry. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the claimed invention to provide a method for reducing energy loss in a DC-DC converter and related control device and DC-DC converter. 
     The present invention discloses a method for reducing energy loss in a DC-DC converter. The method comprises detecting an output current of the DC-DC converter to generate a sensing signal, adjusting a frequency of an oscillation signal according to the sensing signal, comparing a reference signal and a feedback signal of the DC-DC converter to generate a comparison result, comparing the comparison result and the oscillation signal to generate a pulse width modulation (PWM) signal, and determining whether or not an input end of the DC-DC converter is electrically connected to an output end of the DC-DC converter according to the PWM signal. 
     The present invention further discloses a control device for a DC-DC converter comprising a sensor for detecting an output current of the DC-DC converter to generate a sensing signal, an oscillator, for adjusting a frequency of an oscillation signal according to the sensing signal, a first comparator for comparing a reference signal and a feedback signal of the DC-DC converter to generate a comparison result, and a second comparator for comparing the comparison result and the oscillation signal to generate a pulse width modulation (PWM) signal to the DC-DC converter, so as to determine whether or not an input end of the DC-DC converter is electrically connected to an output end of the DC-DC converter. 
     The present invention further discloses a DC-DC converter comprising an input end for receiving an input voltage, an output end for outputting an output voltage, a feedback module coupled to the output end for generating a feedback signal according to the output voltage, a switch module comprising a first end for receiving a pulse width modulation (PWM) signal, a second end, an up-bridge switch transistor coupled to the input end, the first end and the second end for determining whether or not the input end is electrically connected to the second end, and a down-bridge switch transistor coupled to the first end the second end and a ground end for determining whether or not the second end is electrically connected to the ground end according to an inverted signal of the PWM signal, an output module comprising, an output inductor comprising one end coupled to the second end of the switch module and another end coupled to the output end, and an output capacitor comprising one end coupled to the output end and another end coupled to the ground end, and a control device comprising a sensor for detecting the output current to generate a sensing signal, an oscillator for adjusting a frequency of an oscillation signal according to the sensing signal, a first comparator for comparing a reference signal and a feedback signal of the DC-DC converter to generate a comparison result, and a second comparator for comparing the comparison result and the oscillation signal to generate the PWM signal sent to the switch module. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a buck converter of the prior art. 
         FIG. 2  is a schematic diagram of a DC-DC converter according to an embodiment of the present invention. 
         FIG. 3  is a schematic diagram of a process according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 2 , which is a schematic diagram of a DC-DC converter  20  according to an embodiment of the present invention. The DC-DC converter  20  includes an input end  200 , an output end  210 , a feedback module  220 , a switch module  230 , an output module  240 , a control device  260  and a lowpass module  270 . The input end  200  is utilized for receiving a first input voltage VIN 1 . The lowpass module  270  comprises an input inductor  272  and an input capacitor  274 , and is utilized for filtering out high frequency components of the first input voltage VIN 1  to generate a second input voltage VIN 2 . The output end  210  is utilized for outputting an output voltage VOUT. The feedback module  220  is utilized for generating a feedback signal VFB according to the output voltage VOUT. The switch module  230  includes an up-bridge switch transistor  232 , a down-bridge switch transistor  234 , an amplifier  236  and an inverter  238 . The amplifier  236  is utilized for amplifying a pulse width modulation (PWM) signal VPWM. The inverter  238  is utilized for amplifying the PWM signal VPWM and generating an inverted signal VPWMB of the PWM signal VPWM. The up-bridge switch transistor  243  is utilized for determining whether or not the input end  200  is electrically connected to the output module  240  based upon the PWM signal VPWM. The down-bridge switch transistor  234  is utilized for determining whether or not the output module  240  is electrically connected to a ground end GND based upon the inverted signal VPWMB. The output module  240  includes an output inductor  242  and an output capacitor  244 , and is utilized for generating the output voltage VOUT. The control device  260  includes a sensor  262 , an oscillator  264 , a first comparator  266  and a second comparator  268 . The sensor  262  is utilized for detecting an output current IOUT of the DC-DC converter  20  to generate a sensing signal SEN. The oscillator  264  is utilized for adjusting a frequency of an oscillation signal VOSC based upon the sensing signal SEN. The first comparator  266  is utilized for comparing a reference signal VREF and the feedback signal VFB to generate a comparison result CMP. Finally, the second comparator  268  is utilized for comparing the comparison result CMP and the oscillation signal VOSC to generate the PWM signal VPWM sent to the switch module  230 , so as to determine whether or not the input end  200  is electrically connected to the output end  210 . 
     In short, according to the present invention, since a “switching loss” of the DC-DC converter  20  is directly proportional to a switching frequency of the switch module  230 , a frequency of the PWM signal VPWM is adjusted based upon load variation of the DC-DC converter  20  to adjust the switching frequency of the switch module  230 , so as to reduce the switching loss of the DC-DC converter  20 . In other words, the present invention “customizes” the switching frequency of the switch module  230  according to the output current IOUT to reduce energy loss during switching operations. 
     For example, since the switching loss is the major cause of energy loss when the DC-DC converter  20  operates in a light load state (with a low output current IOUT), the oscillator  244  preferably decreases the frequency of the oscillation signal VOSC to reduce the switching loss when the sensing signal SEN indicates that the output current IOUT lessens. The decreased switching frequency (decreased charging/discharging frequency of the output capacitor  244 ) does not lead to an unstable output voltage VOUT since level of the output current IOUT required is lower in the light load state. 
     In addition, when the sensing signal SEN indicates that the output current IOUT is zero, the oscillator  244  can preferably decrease the frequency of the oscillation signal VOSC to a minimum switching frequency when the sensing signal SEN indicates that the output current IOUT is zero. 
     Note that, in general, the sensing signal SEN is directly proportional to the output current IOUT, and the oscillation signal VOSC is a sawtooth signal. Methods for sensing the output current IOUT are well known to those skilled in the art and not further narrated herein. 
     Moreover, the feedback module  220  preferably includes a first resistor  222  and a second resistor  224 , and is utilized for generating a divided voltage of the output voltage VOUT to be the feedback signal VFB. Certainly, those skilled in the art can generate the feedback signal VFB through different methods based upon specific requirements, e.g. by directly feeding back the output voltage VOUT without any other processing. 
     Operations of the DC-DC converter  20  and the control device  260  can be summarized into a process  30 , as illustrated in  FIG. 3 . The process  30  is utilized for reducing energy loss of the DC-DC converter  20 , and includes the following steps: 
     Step  300 : Start. 
     Step  302 : The sensor  262  detects the output current IOUT of the DC-DC converter  20  to generate the sensing signal SEN. 
     Step  304 : The oscillator  264  adjusts the frequency of the oscillation signal VOSC according to the sensing signal SEN. 
     Step  306 : The first comparator  266  compares the reference signal VREF and the feedback signal VFB to generate the comparison result CMP. 
     Step  308 : The second comparator  268  compares the comparison result CMP and the oscillation signal VOSC to generate the PWM signal VPWM. 
     Step  310 : The switch module  230  determines whether or not the input end  200  is electrically connected to the output end  210  according to the PWM signal VPWM. 
     Step  312 : End. 
     Details of the process  30  can be referred in the above, and are not further narrated herein. 
     In the prior art, the switching frequency of the switch module  130  is fixed, and cannot be varied based upon different load states of the buck converter  10 . That is, without further modifications, the buck converter  10  cannot reduce energy loss caused by switching operations through adjusting the switching frequency. In comparison, according to the present invention, the switching frequency of the switch module  230  is varied based upon load variation, such that the energy loss caused by switching operations in the DC-DC converter  20  can be effectively reduced, especially in a light load state. 
     To sum up, the present invention reduces energy loss of the DC-DC converter by adjusting the switching frequency of the DC-DC converter during switching operations, so as to enhance performance of the DC-DC converter. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.