Abstract:
A DC-DC converter to convert DC power input from an external source into a predetermined DC power required for an electric load. A switch part in which each of a pair of switches that cuts or supplies the input DC power is connected in parallel with at least one other switch. A current sensor senses a load current flowing through the electric load and a controller controls the switch part to enable a number of the switches according to an intensity of the load current sensed by the current sensor. The enabled switches are driven by a PWM signal to cut or supply the input DC power to the load. By changing the number of enabled switches, unnecessary switching is prevented for lower load currents.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of Korean Patent Application No. 2004-20845, filed Mar. 26, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a DC-DC converter and a control method thereof, and more particularly, to a DC-DC converter minimizes an unnecessary switching occurring when a light electric load is connected and which improves an efficiency of the DC-DC converter and a control method thereof. 
   2. Description of the Related Art 
   Generally, a DC-DC converter converts a DC power input from an external source to a predetermined DC power that an electric load requires. DC-DC converters may be classified into a boost type to raise a voltage of the input DC power and a buck type to drop the voltage of the input DC power. 
   Hereinbelow, a synchronous buck converter is taken as an example for a description. 
   As shown in  FIG. 1 , the synchronous buck converter comprises a pair of switches  300  and  302  which are alternately operable according to a pulse width modulation (PWM) signal output from a controller (not shown) and which supply/cut an input DC power V IN , an inductor  316  connected to a common node between the pair of switches  300  and  302  and a capacitor  318  connected between the inductor  316  and a ground potential. 
   According to the configuration described above, a description of an operation of the synchronous buck converter follows. 
   The synchronous buck converter is operated in two modes according to whether each of the switches  300  and  302  is turned on or off. 
   In a first mode, the switch  300  is turned on and the switch  302  is turned off. In the first mode, the DC power V IN  is supplied to an input end of the inductor  316 , so that an electric current flowing through the inductor  316  increases. Thus, energy is accumulated in the inductor  316  and the energy is supplied to an output end of the inductor  316 , so that an output voltage V OUT  across a capacitor  318  rises. 
   In a second mode, the switch  300  is turned off and the switch  302  is turned on, so that the inductor  316  and the capacitor  318  form a closed circuit. In the second mode, an electric current flowing through the inductor  316  flows continually through the closed circuit until the switch  300  is turned on in a next period of the PWM signal. Thus, an electric charge on the capacitor  318  decreases and the output voltage V OUT  drops. 
   The controller (not shown) senses the output voltage V OUT  output to the electric load  320 . If the output voltage V OUT  is low, the controller lengthens the turned-on time of the switch  300  and shortens the turned-on time of the switch  302  to raise the output voltage V OUT . If the output voltage V OUT  is high, the controller shortens the turned-on time of the switch  300  and lengthens the turned-on time of the switch  302  to reduce the output voltage V OUT . That is, the controller adjusts a duty ratio of the PWM signal output to each of the switches  300  and  302  according to the output voltage V OUT  to hold the voltage V OUT  supplied to the electric load  320  at a constant value. 
   A delay circuit (not shown) which is operative after the inverter  314  provides a dead time between the switch  300  and the switch  302  to prevent a so-called arm short phenomenon in which an electric current is conducted from the input V IN  directly to the ground potential as the switch  300  and the switch  302  are simultaneously turned on. 
   A central processing unit (CPU) of a mobile electronic device requires a relatively large current, thus in a CPU-voltage regulation module (CPU-VRM) a plurality of switches are connected in parallel to each of the switch  300  and the switch  302  shown in  FIG. 1 , to increase the current capacity. 
   As shown in  FIG. 2 , in a conventional CPU-VRM having the plurality of parallel switches, switches  341 ,  344  and  347  of a switch unit  340  are simultaneously turned on/off by a PWM signal output from a controller (not shown). Switches  351 ,  354  and  357  of a switch unit  350  are turned on/off alternately with the switches  341 ,  344  and  347 . A dead time due to a delay circuit (not shown) after the inverter  360  is operative between the switch units  340  and  350  in a similar manner as described above with respect to the conventional circuit shown in  FIG. 1 . 
   However, in the conventional circuits described above, some of the switches connected in parallel are unnecessarily switched although a light electric load is connected, thereby causing a switching loss and decreasing the efficiency. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an aspect of the present invention to provide a DC-DC converter to minimize an unnecessary switching which occurs when a light electric load is connected to improve an efficiency of the DC-DC converter. 
   It is another aspect of the present invention to provide a method of controlling a DC-DC converter to minimize unnecessary switching, thereby improving the efficiency of the DC-DC converter. 
   The foregoing and/or other aspects of the present invention are achieved by providing a DC-DC converter to convert a DC power input from an external source into a predetermined DC power required for an electric load and to supply the converted DC power to an electric load, the DC-DC converter comprising: a switch part in which each of a pair of switches connected in series is connected with at least one switch in parallel and that cuts or supplies the input DC power; a current sensor to sense a load current flowing through the electric load; and a controller to control the switch part to change a number of the switches to be enabled, according to an intensity of the load current sensed by the current sensor. 
   According to an aspect of the invention, the controller generates a PWM signal to drive the switches and selectively supplies the PWM signal to the switches to change the number of the switches to be enabled according to the intensity of the load current. 
   According to an aspect of the invention the controller comprises: a PWM controller to generate the PWM signal and enable/disable signals to enable/disable the switches according to the PWM signal and the intensity of the load current; a PWM signal line to transfer the PWM signal generated from the PWM controller to a predetermined number of switches; and at least one logic operator to logically operate the PWM signal and the enable/disable signals generated from the PWM controller and to output the operated result to the switches. 
   According to an aspect of the invention, the controller reduces the number of the switches to be enabled as the intensity of the load current is weaker and increases the number of the switches to be enabled as the intensity of the load current is stronger. 
   The foregoing and/or other aspects of the present invention are also achieved by providing a control method of a DC-DC converter to convert a DC power input from an external source into a predetermined DC power required for an electric load and supply the converted DC power to the electric load, the control method comprising: providing a switch part in which each of a pair of switches connected in series is connected with at least one switch in parallel and that cuts or supplies input DC power; sensing a load current flowing through the electric load; and controlling the switch part to change a number of switches to be enabled according to an intensity of the sensed load current. 
   According to an aspect of the invention, the controlling of the switch part comprises: generating a PWM signal to drive the switches; and selectively supplying the PWM signal to the switches according to the intensity of the load current. 
   According to an aspect of the invention, the controlling the switch part comprises reducing the number of the switches to be enabled as the intensity of the load current is weaker and increasing the number of the switches to be enabled as the intensity of the load current is stronger. 
   Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and/or other aspects and advantages of the present invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
       FIG. 1  is a circuit diagram of a conventional DC-DC converter; 
       FIG. 2  is a circuit diagram of a conventional DC-DC converter adapted to supply a large current; 
       FIG. 3  is a circuit diagram of a DC-DC converter according to an embodiment of the present invention; and 
       FIG. 4  is a control flow chart of the DC-DC converter according to the embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
   Referring to  FIG. 3 , a DC-DC converter according to an embodiment of the present invention comprises a switch part comprising a plurality of switch blocks  10 ,  20  and  30  (hereinbelow, referred to as “switch arms  10 ,  20  and  30 ”) connected in parallel, each switch arm comprising a pair of switches each operated to alternately cut or supply input DC power V IN ; an inductor  42  and a capacitor  44  to smooth electric power output by turning on/off of the switch part and supply the smoothen electric power to an electric load  40 ; a current sensor  46  to sense a load current I OUT  flowing through the electric load  40 ; and a controller  50  to control the switch part to change the number of switch arms  10 ,  20 , and  30  to be enabled, according to an intensity of the load current I OUT  sensed by the current sensor  46 . 
   The switch part comprises the plurality of the switch arms  10 ,  20 , and  30  connected in parallel, each switch arm comprising a pair of switches  11 ,  13 ;  21 ,  23 ; and  31 ,  33 , respectively, connected in series between the input DC power V IN  and the ground potential. The switches  11 ,  21  and  31  and switches  13 ,  23  and  33  of each of the switch arms  10 ,  20  and  30  are turned on/off according to a control of the controller  50 . Here, a Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) is useable as the switches  11 ,  13 ,  21 ,  23 ,  31  and  33 . 
   A low pass filter comprising the inductor  42  and the capacitor  44  is connected at a common node between the switches  11 ,  21  and  31  and the switches  13 ,  23  and  33 . 
   The current sensor  46  senses the load current I OUT  flowing through the electric load  40  and a variety of conventional techniques may be applied to the current sensor  46 . For example, a current mirror may be used as the electric current sensor  46  or the current may be sensed by sensing a voltage drop across a predetermined resistance caused by the load current I OUT . 
   The controller  50  generates a pulse width modulation (PWM) signal by adjusting a duty ratio according to an output voltage V OUT . The controller  50  comprises a PWM controller  48  to generate a B-ARM signal and/or a C-ARM signal to selectively enable/disable the switch arms  10 ,  20  and  30  according to the intensity of the load current I OUT  sensed by the current sensor  46 , PWM signal lines  54  and  60  to transmit the PWM signal generated by the PWM controller  48  to one of the switch arms  10 ,  20  and  30  and to a plurality of AND-gates  56 ,  58 ,  62  and  64  which AND-operate the PWM signal from the PWM controller  48  and the B-ARM and C-ARM signals to output a result of the operation to the switch arms  20  and  30 , respectively. 
   If the B-ARM signal input to the AND-gates  56  and  62  is a high signal, the PWM signal is output to the switch arm  20  and the switch arm  20  is enabled. If the C-ARM signal input to the AND-gates  58  and  64  is a high signal, the PWM signal is output to the switch arm  30  and the switch arm  30  is enabled. If the B-ARM signal and the C-ARM signal are both high signals, both the switch arms  20  and  30  are enabled. 
   The PWM signal is delayed by a delay circuit (not shown) after an inverter  52  and output to the switches  13 ,  23  and  33  in a similar manner as the conventional DC-DC converter. 
   If the intensity of the load current I OUT  is equal to or lower than a first predetermined level, the PWM controller  48  outputs B-ARM=0 and C-ARM=0. If the intensity of the load current I OUT  is higher than the first predetermined level and equal to and lower than a second predetermined level, the PWM controller outputs B-ARM=1 and C-ARM=0. If the intensity of the load current I OUT  is higher than the second predetermined level, the PWM controller outputs B-ARM=1 and C-ARM=1. Here, “0” is a low signal or a disable signal and “1” is a high signal or an enable signal. 
   If the PWM controller  48  outputs B-ARM=0 and C-ARM=0, the switches  11  and  13  of the switch arm  10  are enabled and the PWM signal is continually input to the switches  11  and  13  through the PWM signal lines  54  and  60 . However, the switches  21 ,  23 ,  31  and  33  of the switch arms  20  and  30  are disabled. 
   If the PWM controller  48  outputs B-ARM=1 and C-ARM=0, the switches  11  and  13  of the switch arm  10  are enabled and the switches  21  and  23  of the switch arm  20  are enabled by the B-ARM high signal applied to the AND-gates  56  and  62  to receive the PWM signal. 
   If the PWM controller  48  outputs B-ARM=1 and C-ARM=1, all of the switches  11 ,  13 ,  21 ,  23 ,  31  and  33  of all of the switch arms  10 ,  20  and  30  are enabled to receive the PWM-signal. The switches  31  and  33  are enabled by the C-ARM high signal applied to the AND-gates  58  and  64 . 
   The controller  50  controls the levels of the B-ARM signal and the C-ARM signal according to the intensity of the load current I OUT  sensed by the current sensor  46 , so that the number of the enabled switch arms  10 ,  20  and  30  becomes smaller as the intensity of the load current I OUT  is weaker and larger as the intensity of the load current becomes stronger. 
   The controller  50  senses the output voltage V OUT  and controls the duty ratio of the PWM signal to make the output voltage V OUT  constant. 
   According to the configuration described above, a description of a flow control of the DC-DC converter according to the embodiment of the present invention is described referring to  FIG. 4 . In the description below, I1 and I2 represent the first and second predetermined levels, respectively, described above and it is assumed that I1&lt;I2. 
   The current sensor  46  senses the load current I OUT  flowing through the electric load  40  at operation  100 . The PWM controller  48  determines whether the load current I OUT  sensed by the current sensor  46  is equal to or lower than I1 at operation  102 . 
   If the load current I OUT  is equal to or lower than I1, the PWM controller  48  outputs low signals as the B-ARM signal and the C-ARM signal at operation  104 , i.e., the PWM controller outputs B-ARM=0 and C-ARM=0. The PWM signal is supplied to the switches  11  and  13  of the switch arm  10  through the PWM signal lines  54  and  60  and each of the AND-gates  56 ,  58 ,  62  and  64  outputs low signals by an AND operation of the PWM signal with the B-ARM signal and the C-ARM signal, so that the switches  21 ,  23 ,  31  and  33  of the switch arms  20  and  30  are disabled. That is, only the switches  11  and  13  of the switch arm  10  are enabled at operation  106 . In  FIG. 4 , switches  11 ,  13 ,  21 ,  23 ,  31  and  33  are represented by symbols Q A1 , Q A2 , Q B1 , Q B2 , Q C1 , and Q C2 , respectively. 
   If the load current I OUT  is higher than I1 and equal to or lower than I2 at operation  108 , the PWM controller  48  outputs a high signal as the B-ARM signal (B-ARM=1) and a low signal as the C-ARM signal (C-ARM=0) at operation  110 . Thus, the PWM signal is output by the AND-gates  56  and  62  to which the B-ARM high signal is input. Thus, the switches  11 ,  13 ,  21  and  23  of the switch arms  10  and  20  are enabled at operation  112 . 
   If the load current I OUT  is higher than I2 at operation  114 , the PWM controller  48  outputs high signals as the B-ARM signal and the C-ARM signal at operation  116 . Thus, all of the switches  11 ,  13 ,  21 ,  23 ,  31  and  33  are enabled at operation  118 . 
   That is, the controller  50  makes the number of switch arms  10 ,  20  and  30  to be enabled smaller as the intensity of the load current I OUT  is weaker, thereby minimizing unnecessary switching if a light electric load is connected and makes the number of switch arms  10 ,  20  and  30  to be enabled larger as the intensity of the load current I OUT  is stronger, thereby distributing a stress on the switches across a greater number of the switches. 
   In the embodiment described above, the controller  50  comprises the PWM controller  48 , the PWM signal lines  54  and  60 , and the AND-gates  56 ,  58 ,  62  and  64 , but is not limited thereto. A variety of controllers may be employed as long as the controller controls the number of switches  11 ,  13 ,  21 ,  23 ,  31  and  33  to be enabled according to the intensity of the load current I OUT  sensed by the current sensor  46 . 
   Further, in the embodiment described above, an even number of the switches are enabled by enabling the switch arms  10 ,  20  and  30  according to the intensity of the load current I OUT . Alternatively, the number of the switches to be enabled according to the intensity of the load current I OUT  may be varied within the number of the switches. 
   In the embodiment described above, the switches  11 ,  13 ,  21 ,  23 ,  31  and  33  may be, for example, N-channel MOSFETs. Alternatively, the switches  11 ,  21  and  31  may be P-channel MOSFETs. Where P-channel MOSFETS are used, the inverter  52  is not required. 
   In the embodiment described above, control of the load current I OUT  is accomplished according to the number of the switches. 
   In the embodiment described above, a synchronous converter is employed. However, other converters, like a boost converter, may be used for the present invention. 
   In summary, the present invention controls the number of the switches to be enabled according to the intensity of the load current I OUT , thereby decreasing unnecessary switching occurred when the light electric load is connected and increasing the efficiency of the converter. 
   As described above, the present invention provides a DC-DC converter in which unnecessary switching is minimized and the efficiency of the DC-DC converter is increased and a method of controlling the DC-DC converter. 
   Although a few embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the accompanying claims and their equivalents.