Abstract:
The present invention provides a power saving apparatus for a half bridge power supply. The main purpose of the present invention is to reduce power consumption during standby-mode. The present invention includes a switch-apparatus, which is designed such that a start-up resistor will only consume power during normal-mode operation. Another purpose of the present invention is to apply a balance-apparatus for automatically balancing a differential voltage between a high-side capacitor and a low-side capacitor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a power supply and more particularly relates to a half bridge switching power supply.  
         [0003]     2. Description of the Related Art  
         [0004]     Due to increasingly stringent environment regulations, computer and home appliance manufacturers are under increased pressure to improve power management and reduce energy consumption. US and European regulations regarding power consumption strictly limit the amount of power consumption permitted for supervising and remote-control purposes. Reducing power consumption under standby mode is a major concern.  
         [0005]      FIG. 1  shows the input stage of a traditional half bridge switching power supply. This prior-art half bridge switching power supply includes a transistor  30 , a transistor  40 , a power transistor  10 , a power transistor  20 , a diode  53 , a start-up resistor  55 , and a driver transformer  50 . The driver transformer  50  includes a current-winding N 1 , a drive-winding N B1 , a drive-winding N B2 , a control-winding N D1  and a control-winding N D2 . A standby power terminal V CC  provides the initial power for the driver transformer  50 . The driver transformer  50  is used to switch on the power transistors  10  and  20  via the diode  53  and the start-up resistor  55 . When the power transistor  10  is switched on, the current-winding N 1  of the driver transformer  50  will provide a proportional current I B1  to drive the power transistor  10 . When the power transistor  20  is switched on the current-winding N 1  of the driver transformer  50  will provide a proportional current I B2  to drive the power transistor  20 . The currents I B1  and I B2  are given by the following equations:  
         I   B1     =         T   NI       T   NB1       ×     I   P                     I   B2     =         T   NI       T   NB2       ×     I   P                    where I P  is the current flow through the current-winding N 1 , T N1  is the turn number of the current-winding N 1 ; and T NB1  and T NB2  are the turn numbers of the drive-windings N B1  and N B2  respectively.          
         [0007]     This circuit uses a technique known as proportional switching. By simultaneously turning on the transistors  30  and  40 , the control-windings N D1 , and N D2  will be short-circuited. This will terminate the proportional currents I B1  and I B2 , and turn off the power transistors  10  and  20 . One drawback of this technique is high power consumption during standby mode, while the switching circuit is off.  
         [0008]     In order to switch the transistors  30  and  40 , the resistance of the start-up resistor  55  must be low. However, if the resistance of the start-up resistor  55  is low, then power consumption will be high while the switching circuit is off.  
         [0009]     Furthermore, a bleeding resistor  85  and a bleeding resistor  95  cause significant power dissipation. A negative terminal of a high-side capacitor  80  is connected to a positive terminal of a low-side capacitor  90 . The negative terminal of the low-side capacitor  90  is connected to the ground reference. The voltages across the capacitors  80  and  90  are respectively given by,  
         V   C80     =         C   90         C   80     +     C   90         ×     V   IN           
         V   C90     =         C   80         C   80     +     C   90         ×     V   IN           
        where C 80  and C 90  are the capacitances of capacitors  80  and  90  respectively, and V IN  is the input voltage of the power supply.        
 
         [0011]     If the capacitance of the capacitor  80  and the capacitance of the capacitor  90  differ, then the voltages across each of them will differ as well. Each capacitor has a maximum voltage rating (e.g. 200V). The capacitor  80  or  90  may be easily damaged, if the capacitances vary significantly.  
         [0012]     Therefore, the bleeding resistors  85  and  95  are required to reduce the difference between the impedance of the capacitor  80  and the impedance of the capacitor  90 . The resistances of the bleeding resistors  85  and  95  should be kept relatively low if the capacitance difference between the capacitors  80  and  90  is high.  
         [0013]     If the voltages of the capacitors  80  and  90  are allowed to differ significantly enough, then the energy switched by the power transistors  10  and  20  will also differ. This will result in imbalanced energy switching across a power transformer  60 , which can easily damage power transistors and cause transformer saturation. In traditional power supplies, the bleeding resistors  85  and  95  are designed to cope with worst-case scenarios, and thus they consume significant amounts of power.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, one object of the present invention is to provide a power saving apparatus for a half bridge power supply to reduce power consumption under standby-mode. The present invention reduces power consumption by balancing the voltages of a high-side capacitor and a low-side capacitor. This reduces the amount of power consumed by the power supply&#39;s bleeding resistors.  
         [0015]     According to one aspect of the present invention, the power saving apparatus includes an N-current-sink connected in parallel with the high-side capacitor and a P-current-sink connected in parallel with the low-side capacitor.  
         [0016]     According to another aspect of the present invention, a differential voltage is used to regulate the operation of the N-current sink and P-current-sink in order to prevent voltage imbalance across the two capacitors from occurring. The differential voltage is generated by a resistor network in response to the voltage difference across the low-side capacitor and the high-side capacitor.  
         [0017]     According to another aspect of the present invention, to further reduce power consumption, the power supply includes a switch connected in series with a start-up resistor via a diode. The switch is turned off by a control signal while the power supply is under standby-mode. Thus, the start-up resistor only consumes power while the switching circuit operates under normal mode.  
         [0018]     The main advantage of the power supply according to the present invention is reduced power consumption under standby-mode. Furthermore, the power supply includes a method of automatically balancing the voltages across the high-side capacitor and the low-side capacitor.  
         [0019]     It is to be understood that both the foregoing general descriptions and the following detail descriptions are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0021]      FIG. 1  shows an input stage of a traditional half bridge switching power supply.  
         [0022]      FIG. 2  shows the input stage of a half bridge power supply with a power saving apparatus according to the present invention.  
         [0023]      FIG. 3  shows a switch-apparatus that controls the current flow across a start-up resistor of the half-bridge power supply according to a preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]      FIG. 2  shows the input stage of a half-bridge power supply with power saving apparatus according to the present invention. The power supply according to the present invention comprises a switch-apparatus  100  and a balance-apparatus  200 . The switch-apparatus  100  is connected in series with a start-up resistor  55  via a diode  53 . The switch-apparatus  100  is used to control the current flow through the start-up resistor  55 . The balance-apparatus  200  is connected in parallel with a high-side capacitor  80  and a low-side capacitor  90 . The balance-apparatus  200  is used to sink a current from either the high-side capacitor  80  or the low-side capacitor  90 . This is done to balance the differential voltage in between the high-side capacitor  80  and the low-side capacitor  90 .  
         [0025]     The switch-apparatus  100  comprises a switch  110  and an inverter  120 . A first input of the switch-apparatus  100 , which is also an input terminal of the switch  110 , is connected to a standby power terminal V CC . An output of the switch-apparatus  100 , which is also an output terminal of the switch  110 , is connected to an anode of the diode  53 . A voltage V A  is the voltage at the anode of the diode  53 . A cathode of the diode  53  is connected in series with the start-up resistor  55 . A control signal ON/OFF is supplied to a second input of the switch-apparatus  100 , which is also an input of the inverter  120 . An output of the inverter  120  controls the switch  110 . The control signal ON/OFF is low during normal operation. This closes the switch  110 , connecting the start-up resistor  55  with the standby power terminal V CC  via the diode  53 . During standby-mode, the control signal ON/OFF will turn off the switch  110  and disconnect the start-up resistor  55  from the standby power terminal V CC . In this manner, the start-up resistor  55  is prevented from consuming power during standby-mode.  
         [0026]      FIG. 3  shows the switch-apparatus  100  according to a preferred embodiment of the present invention. The switch-apparatus  100  comprises a switch-transistor  150 , an inverted-transistor  160 , a capacitor  190 , two diodes  171  and  172 , and four resistors  181 ,  182 ,  185  and  186 . The switch-transistor  150  acts in the same manner as the switch  110 . A collector of the switch-transistor  150  is connected to the standby power terminal V CC . An emitter of the switch-transistor  150  supplies the voltage V A . The control signal ON/OFF is supplied to a base of the inverted-transistor  160  via the resistor  181 . The capacitor  190  and the resistor  182  are connected in parallel from the base of the inverted-transistor  160  to the ground reference. The diode  171  is connected from an emitter of the inverted-transistor  160  to the ground reference. The inverted-transistor  160  is coupled with the diode  171 , the capacitor  190 , the resistors  181  and  182 , to act in the same manner as the inverter  120 . The resistors  185  and  186  provide the bias for the switch-transistor  150  and the inverted-transistor  160  respectively. The diode  172  provides reverse-bias protection for the switch-transistor  150 .  
         [0027]     Referring to  FIG. 2 , the balance-apparatus  200  comprises an N-current-sink, a P-current-sink and a resistor network. The resistor network comprises a high-side resistor  270 , a threshold resistor  280  and a low-side resistor  290 . The N-current-sink includes an n-p-n transistor  210 , a N-limit resistor  285  and a N-resistor  250 . The P-current-sink includes a p-n-p transistor  220 , a P-limit resistor  295 , and a P-resistor  260 . Via the N-limit resistor  285 , a collector of the n-p-n transistor  210  is connected to a positive terminal of the high-side capacitor  80 . The input voltage V IN  of the power supply is supplied to the positive terminal of the high-side capacitor  80 . The positive terminal of the low-side capacitor  90  is connected to a negative terminal of the high-side capacitor  80 . The voltage at the positive terminal of the low-side capacitor  90  is the voltage V B . A negative terminal of the low-side capacitor  90  is connected to the ground reference. An emitter of the n-p-n transistor  210  is connected to the negative terminal of the high-side capacitor  80  via the N-resistor  250 . The positive terminal of the low-side capacitor  90  is connected to an emitter of the p-n-p transistor  220  via the P-resistor  260 . A collector of the p-n-p transistor  220  is connected to the ground reference via the P-limit resistor  295 . The high-side resistor  270  and the low-side resistor  290  have the same resistance. The high-side resistor  270  is supplied from the input voltage V IN  of the power supply and is connected to the threshold resistor  280 . The low-side resistor  290  is connected from the threshold resistor  280  to the ground reference. A base of the n-p-n transistor  210  is connected to the junction of the low-side resistor  290  and the threshold resistor  280 . A base of the p-n-p transistor  220  is connected to the junction of the threshold resistor  280  and the high-side resistor  270 . This circuit generates a threshold voltage V TH  across the threshold resistor  280 . The magnitude of the threshold voltage V TH  is expressed as:  
         V   TH     =         R   280         R   270     +     R   280     +     R   290         ×     V   IN           
  V   TH   =V   E   −V   F  
        where R 270 , R 280  and R 290  are respectively the resistances of the resistors  270 ,  280  and  290 ; V E  is the voltage at the base of the p-n-p transistor  220 ; and V F  is the voltage at the base of the n-p-n transistor  210 .        
 
         [0029]     The purpose of threshold voltage V TH  is to save power. The n-p-n transistor  210  and the p-n-p transistor  220  will be turned off when the differential voltage of the capacitors  80  and  90  drops below the threshold voltage V TH . Once the differential voltage exceeds the threshold voltage V TH , either the n-p-n transistor  210  or the p-n-p transistor  220  will be activated to perform the adjustment. A current  1210  will be sunk from the high-side capacitor  80 . The magnitude of the current  1210  will be proportional to the voltage difference between V F  and V B ; it can be expressed as,  
         I   210     ≅       (       V   F     -     V   B     -     V   BE       )       R   250           
        where R 250  is the resistance of the N-resistor  250  and V BE  is the base-to-emitter voltage of the n-p-n transistor  210 .        
 
         [0031]     A current I 220  will be sunk from the low-side capacitor  90 . The magnitude of the current I 220  will be proportional to the voltage difference between V B  and V E ; it can be shown as,  
         I   220     ≅       (       V   B     -     V   E     -     V   BE       )       R   260           
        where R 260  is the resistance of the P-resistor  260 ; V BE  is the base-to-emitter voltage of the p-n-p transistor  220 .        
 
         [0033]     The N-limit resistor  285  and the P-limit resistor  295  are used for protecting the n-p-n transistor  210  and the p-n-p transistor  220  from over-current and/or other abnormal conditions.  
         [0034]     It will be apparent to those skilled in the art that various modifications and 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 cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.