Abstract:
A switched-mode power supply includes a soft-burst circuit to minimize or prevent distracting audible noise. The power supply includes a control circuit for controlling switching of an output transistor to deliver a regulated output voltage to a load. The control circuit adjusts the operating frequency of the power supply based on a control signal. The soft-burst circuit discharges a storage device to minimize or prevent audible noise when the control signal reaches a particular level.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to electrical circuits, and more particularly but not exclusively to soft-burst circuits for switched-mode power supplies. 
         [0003]    2. Description of the Background Art 
         [0004]    A power supply is an electrical circuit that receives an input voltage to deliver a regulated output voltage to a load. In a switched-mode power supply, switching of a single transistor or a pair of synchronously switched transistors is controlled to maintain the output voltage to within a desired output voltage range. One problem with switched-mode power supplies is that undesirable audible noise may occur when varying the switching frequency of the output transistors. The audible noise is especially problematic in high-efficiency power supplies that use pulse-skipping techniques to turn off the output transistors at light load levels. 
       SUMMARY 
       [0005]    In one embodiment, a switched-mode power supply includes a soft-burst circuit to minimize or prevent distracting audible noise. The power supply includes a control circuit for controlling switching of an output transistor to deliver a regulated output voltage to a load. The control circuit adjusts the operating frequency of the power supply based on a control signal. The soft-burst circuit discharges a storage device to minimize or prevent audible noise when the control signal reaches a particular level. 
         [0006]    These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  schematically shows a conventional switched-mode power supply that may be adapted to take advantage of embodiments of the present invention. 
           [0008]      FIG. 2  schematically shows a soft-burst circuit in accordance with an embodiment of the present invention. 
           [0009]      FIG. 3  shows a timing diagram for an improved converter in accordance with an embodiment of the present invention during light load conditions. 
           [0010]      FIG. 4  shows a timing diagram for an improved converter in accordance with an embodiment of the present invention transitioning from light load conditions with pulse-skipping and bursting to normal mode of operation. 
       
    
    
       [0011]    The use of the same reference label in different drawings indicates the same or like components. 
       DETAILED DESCRIPTION 
       [0012]    In the present disclosure, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. 
         [0013]      FIG. 1  schematically shows a conventional switched-mode power supply that may be adapted to take advantage of embodiments of the present invention. In the example of  FIG. 1 , an LLC resonant half-bridge converter  100  is configured to step down an input voltage V IN  to a lower output voltage V O . For example, the converter  100  may step down an input voltage V IN  of 400 VDC to an output voltage V O  of 24 VDC, and generate an output current I O  ranging from 0 to 5 A. It is to be noted that the operation of an LLC resonant half-bridge converter, in general, is well known in the art. 
         [0014]    As is typical of modern switched-mode power supplies, the converter  100  includes an integrated circuit  130  to control switching of a pair of synchronously switched output transistors M 1  and M 2  to generate the regulated output voltage V O . The output transistors M 1  and M 2  are shown as metal-oxide semiconductor field effect transistors (MOSFET) and integrated with the integrated circuit  130  for illustration purposes. However, that is not necessarily the case. For example, the transistors M 1  and M 2  may be bipolar transistors and/or packaged separately from the integrated circuit  130 . The integrated circuit  130  may also control switching of a single output transistor rather than a pair of synchronously switched output transistors. The integrated circuit  130  may control the switching of the output transistors M 1  and M 2  by pulse frequency modulation (PFM). 
         [0015]    In one embodiment, the integrated circuit  130  comprises the FSFR-Series Fairchild Power Switch™ integrated circuit from Fairchild Semiconductor of South Portland, Me. The integrated circuit  130  includes a V DL  pin electrically connected to the transistor M 1  (also referred to as “high-side transistor”), a PG pin for the power ground, an SG pin for the control ground, an LV CC  pin for electrically connecting a supply voltage to the low-side gate drive circuit driving the transistor M 2  and to control blocks, an HV CC  pin for electrically connecting a supply voltage to the high-side gate drive circuit driving the transistor M 1 , a V CTR  pin electrically connected to the drain of the transistor M 2  (also referred to as “low-side transistor”), a CS pin for sensing current flowing through the transistor M 2 , an RT pin for setting the operating frequency of the converter  100 , and a CON pin for enabling/disabling protection features of the integrated circuit  130 . 
         [0016]    In the example of  FIG. 1 , an opto-coupler comprising a diode  151  and a transistor  152  allows for monitoring of the output voltage V O . The output of the opto-coupler transistor  152  is used as an input to the CON pin of the integrated circuit  130  to enable/disable the protection features of the integrated circuit  130  based on the output voltage V O . The operation of the integrated circuit  130  is enabled when the voltage on the CON pin is above a first protection value (e.g., 0.6V). The gate drive signals for transistors M 1  and M 2  are disabled when the voltage on the CON pin is below a second protection value (e.g., 0.4V) lower than the first protection value. The protection is triggered, i.e., the integrated circuit  130  is disabled and does not operate, when the voltage on the CON pin is above a third protection value (e.g., 5V) higher than the first protection value. 
         [0017]    The integrated circuit  130  includes a pulse-skipping feature wherein the switching of the transistors M 1  and M 2  are stopped during light load conditions to save energy. In the example of  FIG. 1 , the transistors M 1  and M 2  stop switching when the voltage on the CON pin drops below the second protection value (e.g., 0.4V), and resume switching when the voltage on the CON pin rises above the first protection value (e.g., 0.6V). The frequency that causes pulse skipping may also be configured by appropriate selection of the values of the resistors  155  and  157 . In one embodiment, the frequency at which pulse skipping is entered is equal to the maximum operating frequency. The integrated circuit  130  also includes a burst mode feature wherein the transistors M 1  and M 2  are switched for short periods of time (also referred to as “bursting”) to charge an output capacitor in between pulse skipping. 
         [0018]    In one embodiment, the gain of the converter  100  is inversely proportional to the operating frequency in the ZVS (zero-voltage-switching) region. The output voltage can thus be regulated by modulating the operating frequency of the converter  100 . In the example of  FIG. 1 , the output voltage V O  is monitored by way of the opto-coupler diode  151 . The output current of the opto-coupler transistor  152  is thus indicative of the output voltage V O . The monitored output voltage from the opto-coupler transistor  152  is applied to the CON pin to enable/disable protection features of the integrated circuit  130  and for pulse-skipping. The monitored output voltage from the opto-coupler transistor  152  is also applied to the RT pin as a frequency control signal for varying the operating frequency of the converter  100 , i.e., the frequency at which the transistors M 1  and M 2  are switched, to change its gain and thereby regulate the output voltage V O . 
         [0019]    The integrated circuit  130  may include a current controlled oscillator (not shown) that drives the transistors M 1  and M 2  at a frequency dictated by electrical current flowing out of the RT pin. The voltage on the RT pin may be kept at a constant voltage (e.g., 2 VDC). When the impedance on the RT pin decreases, current flowing out the RT pin increases to increase the operating frequency. When the impedance on the RT pin increases, current flowing out of the RT pin decreases and so does the operating frequency. The impedance presented by the opto-coupler transistor  152  to the RT pin changes depending on the output voltage and load current, allowing for control of operating frequency based on the output voltage. 
         [0020]    An RC series network comprising a resistor  156  and a capacitor  158  is electrically coupled to the RT pin to provide a soft-start function to limit in-rush current when the converter  100  is first powered up. When the converter  100  is first powered up, the current flowing out of the RT pin is determined by the resistors  155  and  156  because the capacitor  158  is still discharged. As the capacitor  158  is charged during startup, the current flowing out of the RT pin decreases, thereby causing the operating frequency to decrease. Because the gain curve of the converter  100  is inversely proportional to operating frequency, the gain of the converter  100  is controlled to smoothly increase from startup due to the charging of the capacitor  158 . Once the capacitor  158  is fully charged, the soft-start circuit formed by the capacitor  158  and resistor  156  no longer affects the operating frequency of the converter  100 . The operating frequency at that time is determined by the resistors  155  and  157  and the impedance presented by the opto-coupler transistor  152  to the RT pin. The minimum and maximum operating frequencies of the converter  100  may be set by appropriate selection of the values of the resistors  155  and  157 . 
         [0021]    While the converter  100  is more than suitable for its intended application, it may generate audible noise in light load conditions with pulse-skipping and bursting. Generally speaking, there is no audible noise during normal operation because the operating frequency of the converter is much higher than the range of frequencies that can be heard by human beings. However, when pulse-skipping or bursting, the operating frequency may drop down to audible frequency range (e.g., several kHz). Although the audible noise is potentially distracting to some consumers, it is heretofore relatively difficult and expensive to implement a circuit to prevent or minimize the audible noise. 
         [0022]      FIG. 2  schematically shows a soft-burst circuit  220  in accordance with an embodiment of the present invention. The soft-burst circuit  220  is configured to prevent or minimize audible noise in the converter  100 . In the example of  FIG. 2 , the soft-burst circuit  220  is implemented as part of a frequency setting circuit  200 . The circuit  200  may be used in lieu of the frequency setting circuit  120  of  FIG. 1 . That is, in the converter  100  of  FIG. 1 , the circuit  200  may be electrically connected to the integrated circuit  130  instead of the circuit  120 . The modified converter  100  that employs the circuit  200  (instead of the circuit  120 ) is also referred to herein as the “improved converter.” 
         [0023]    The circuit  200  includes the resistors  155 ,  156 , and  157 , the capacitor  158 , and the opto-coupler transistor  152 . These components provide the same functionality as in  FIG. 1 . 
         [0024]    In the example of  FIG. 2 , the soft-burst circuit  220  comprises a bipolar transistor T 1 , a bipolar transistor T 2 , and resistors R 1  and R 2 . The capacitor  158  is in series with the resistor  156 , which is electrically coupled to the RT pin of the integrated circuit  130 . The monitored output voltage, which serves as a frequency control signal by varying the impedance of the opto-coupler transistor  152 , is electrically coupled to the base of the transistor T 1  by way of the resistor R 1 . The base of the transistor T 1  is also electrically coupled to the RT pin by way of the resistors R 1  and  157 . The collector of the transistor T 1  is electrically coupled to the gate of the transistor T 2 . The gate of the transistor T 2  is also electrically coupled to a supply voltage V CC  by way of a resistor R 2 . The collector of the transistor T 2  is electrically coupled to the node of the capacitor  158  that is not electrically coupled to ground. When the transistor T 1  is ON, the transistor T 2  is turned OFF and thereby allows the frequency control signal to charge the capacitor  158  by way of the resistors  157  and  156 . When the transistor T 1  is OFF, the transistor T 2  is turned ON to discharge the capacitor  158 . 
         [0025]    The operation of the soft-burst circuit  220  is now further explained with reference to the timing diagrams of  FIGS. 3 and 4 . In  FIGS. 3 and 4 , the signals V CON , V b , and V SS  are voltage signals on nodes that are noted in  FIG. 2 . The signal I PRI  is the corresponding current through the primary winding of the transformer  160  shown in  FIG. 1 . The horizontal axis represents time. 
         [0026]      FIG. 3  shows a timing diagram for the improved converter with the frequency setting circuit  200  during light load conditions, wherein the load coupled to the converter is not drawing significant amount of output current I O . The example of  FIG. 3  is for an input voltage V IN  of 400 VDC, output voltage V O  of 24 VDC, and an output current I O  of zero. 
         [0027]    Referring to  FIGS. 2 and 3 , the control voltage V CON  is on the collector of the opto-coupler transistor  152 , and is a control voltage representative of the monitored output voltage. The control voltage V CON  serves as the frequency control signal because it corresponds to the impedance presented by the opto-coupler  152  to the RT pin, and thus controls the operating frequency of the improved converter. In the case of  FIG. 2 , the control voltage V CON  (and hence the frequency control signal) is used by the soft-burst circuit  220  to determine when to discharge the capacitor  158  to minimize or prevent audible noise. 
         [0028]    In light load conditions, the improved converter enters pulse-skipping mode as indicated in  FIG. 3  by the non-constant current flow through the primary winding of the transformer  160  (see waveform of current I PRI  in  FIG. 3 ). In pulse skipping mode, the output transistors M 1  and M 2  are not continuously switched. Instead, there are periods where switching of the output transistors M 1  and M 2  is stopped to save energy. Pulse skipping is so named because pulses that would otherwise drive the transistors M 1  and M 2  to switch are skipped. In-between pulse skipping, the improved converter may enter burst mode to deliver power to the load to maintain regulation while in light load condition. When the control voltage V CON  becomes sufficiently low, which may occur during light load conditions, the transistor T 1  turns OFF, which allows the transistor T 2  to turn ON and discharge the capacitor  158 . Discharging of the capacitor  158  lowers the gain of the improved converter, and allows the gain of the improved converter to increase back up in a controlled fashion as the capacitor  158  charges, thereby suppressing initial current peaks of the primary winding current I PRI  to minimize or prevent audible noise. 
         [0029]      FIG. 3  shows that that the amplitude of the sine wave of the primary winding current I PRI  increases smoothly. Because the amplitude of the primary winding current I PRI  is proportional to the amount of audible noise, the relatively small amplitude of the primary winding current I PRI , especially in the initial current peaks, indicates that the improved converter advantageously generates no or a relatively small amount of audible noise. 
         [0030]      FIG. 4  shows a timing diagram for the improved converter with the frequency setting circuit  200  transitioning from light load to high load condition. The example of  FIG. 4  is for an input voltage V IN  of 400 VDC, output voltage V O  of 24 VDC, and an output current lo transitioning from 0 to 5 A at a point in time indicated by the dashed marker  401 . The signals V CON , V b , V SS,  and I PRI  on the left side of the marker  401  are as previously described with reference to  FIG. 3 . 
         [0031]    When the load starts drawing more current, the improved converter goes from pulse-skipping and burst mode into normal mode of operation, wherein it resumes normal switching of the output transistors M 1  and M 2  (see  FIG. 1 ) to deliver power to the load. As shown in  FIG. 4 , the improved converter advantageously reduces the peak of the primary winding current I PRI  to minimize or prevent audible noise even during transitions from pulse-skipping and burst mode to normal mode of operation. 
         [0032]    In light of the present disclosure, one of ordinary skill in the art will appreciate that the above-described techniques for addressing audible noise problems in switched-mode power supplies are applicable to different power supply topologies and integrated circuit controllers. Embodiments of the present invention thus advantageously allow use of soft-burst circuits that have relatively small number of parts, allowing for ease of implementation even in cost-sensitive applications. 
         [0033]    While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.