Abstract:
The present invention is a switching power supply having efficient light load regulation at high input voltage. The power supply comprises a pulse-width modulator integrate circuit along with two separate voltage buses. A transformer is connected to the first voltage bus via a first primary winding and is connected to the second voltage bus via a second primary winding both primary windings have a predetermined number of turns proportional to voltage supplied by the two voltage buses. The power supply further comprises a pair of switches connected to the two primary windings and driven by the integrated circuit.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates in general to power supplies and more specifically to a switching power supply.  
           [0002]    In the field IP telephony equipment, an IP phone can be powered by a fixed-frequency, switching power supply employing an industry-standard discontinuous mode flyback topology. With this topology, the power supply output voltage is regulated by controlling the pulse width of the switching waveform in response to changes in both a source voltage and a power supply load. The pulse width control method is commonly known as pulse-width modulation (PWM). Inherent with this method is a potential for very narrow pulse widths at a maximum input voltage and a minimum load when the input operating voltage range is wide. However, the problem with narrow pulse widths is that integrated power supply controllers have a certain propagation delay from their control inputs to their power switch control outputs and the power switch also suffers from its own delay. These delays can become significant at narrow pulse widths, particularly when the switching frequency is high and, as a result, the period of each cycle is relatively short.  
           [0003]    There are other problems which arise from propagation delays on narrow pulse widths. Firstly, output voltage may be difficult to regulate at high input voltage and light load. Also, a peak primary current limit threshold that is suitable for the rated load at the minimum input voltage results in an excessive output load current at the maximum input voltage.  
           [0004]    There are also problems that result from a wide input operating voltage range. One problem is that the blocking voltage rating of power supply output diodes may need to be high which preclude the use of Schottky diodes. Schottky diodes are beneficial for use at low output voltages due to their forward voltage drop, which is lower than standard fast-recovery diodes. This lower voltage drop results in a more efficient power supply, however, it also results in a lower reverse voltage rating. Standard Schottky diodes send to have a maximum reverse voltage rating of 40 V. Another problem is that the switching device must have both a high voltage rating, for operation at the high end of an input voltage range, and a high current rating, to conduct the larger currents associated with operation at a low end of the input voltage range. For efficient power supply operation this combination of high voltage and high current rating in a single device may necessitate the use of a physically larger and more expensive component than would otherwise be required if the input voltage range were narrower.  
           [0005]    Presently, with respect to IP phone applications; the power supply is required to operate from two independent voltage sources, VSL and VSH, which have distinctly different voltage ranges. VSL provides a voltage range from SVDC to 22 VDC while VSH provides a voltage range from 22 VDC to 56 VDC. These voltage ranges result in operation of the power supply over a source voltage range from about 8 VDC to 56 VDC. For a given power supply load, this 7:1 range in input voltage results in a 7:1 range of PWM pulse width. The 7:1 PWM pulse width ratio, in turn, results in the problems associated with power supply operation using narrow pulse widths and wide input operating voltage range described above.  
           [0006]    Prior art techniques have combined the two voltage sources together through coupling diodes to form a single voltage bus having an operating range spanning that of the two sources combined, which in this case would be 8 VDC to 56 VDC. A wide input voltage range power converter converts the bus voltage to the voltage required by the load. This has been implemented with standard power converter topologies such as the buck converter and the flyback converter. However, this technique still suffers from the problems described above.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention is a switching power supply that improves light load regulation at a high input voltage by doubling the pulse width, thereby overcoming, the problems of in the prior art. The present invention also improves the current limit performance at a high input voltage, provides lower reverse voltages for an output diode and optimizes the use of switching devices.  
           [0008]    This apparatus of the present invention comprises two distinct voltage buses and an isolating transformer having two primary windings, each with its own associated switching device. Each winding is fed from its own voltage bus with the number of turns on each winding chosen to be proportional to the magnitude of its particular voltage bus. In tis way, at any given time, power is supplied from whichever of the two buses is proportionately higher in voltage. The power supply operates from either voltage source alone, or with both sources present simultaneously, with transitions between sources being transparent to the output.  
           [0009]    By keeping the two voltage buses separate, the duty cycle range for a given load in this phone application is reduced to 2.75:1 for the low voltage bus and an even lower 2.55:1 for the high voltage bus. This results in the minimum pulse width being over twice as wide as that associated with the 7:1 duty cycle range. This doubling of the pulse width significantly reduces the impact of controller and switching device delays and thus significantly improves light load regulation at high input voltage.  
           [0010]    Also, the reduction of the impact of controller and switching device delays improves the current limit performance at high input voltage.  
           [0011]    Another advantage of the present invention is that utilization of two, narrow, input voltage ranges allow the transformer turns ratios to be adjusted such that the output diode is subjected to a lower reverse voltage. This allows the use of Schottky diodes fur output voltage rails of up to 5 V. The benefit of using a Schottky diode is lower power dissipation and reduced component stress.  
           [0012]    Finally, having two switching devices allows each device to be chosen so that its parameters are optimized for its particular operating conditions. For example, the low voltage bus device may conduct a high current without having to withstand a high voltage. The opposite is true for the high voltage bus device. This aligns well with switching device technology where the most easily fabricated, and therefore less expensive, devices optimize one parameter, either voltage or current, at the expense of the other. Thus to smaller devices, each optimized for their particular operating conditions, can replace one physically larger, more expensive, device.  
           [0013]    According to an aspect of the present invention, there is provided a power supply having an output voltage, comprising: a first voltage source supplying a first DC voltage that is switched on and off by a first switch; a second, voltage source supplying a second DC voltage that is switched on and off by a second switch; a transformer comprising a first primary winding connected to the first voltage source, a second primary winding connected to the second voltage source, and a secondary winding, where the secondary winding has an output for supplying the output voltage and where the first primary winding to the second primary winding has a turns ratio that is proportional to a voltage ratio of the first voltage source to the second voltage source; and a pulse-width modulator for switching the first switch and the second switch on and off at a duty cycle to control the output voltage.  
           [0014]    According to another aspect of the present invention, there is provided A power supply having an output voltage, comprising: at least three voltage sources, each voltage source supplying a DC voltage and a current that is switched on and off by a switch; a transformer comprising a primary winding connected to each of the voltage sources, and a secondary winding, where the secondary winding has an output for supplying the output voltage and where the primary windings have turns ratios that are proportional to voltage ratios of the voltage sources; and a pulse-width modulator for switching the switch of each of the voltage sources on and off at a duty cycle to control the output voltage.  
       
    
    
     GENERAL DESCRIPTION OF THE DETAILED DRAWINGS  
       [0015]    Embodiments of the present invention are described below with reference to the accompanying drawings, in which:  
         [0016]    [0016]FIG. 1 is a schematic diagram of a first embodiment of a switching power supply of the present invention; and  
         [0017]    [0017]FIG. 2 is a schematic diagram of a second embodiment of a switching power supply.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    A schematic diagram of the switching power supply is shown in FIG. 1. The flyback switching power supply  10  comprises a pulse width modulator (PWM) IC  12  and a transformer  16  having two primary windings, W Low  and W High , and a secondary winding (W Out )  18 . The primary winding W Low  and W High  are connected to two separate voltage buses  20  and  22 . Each of the primary windings W Low  and W High  is connected to associated switches Q Low  and Q High . The switches Q Low  and Q High  are driven by the PWM IC  12 , which also senses current via a resistor  24  to control the current. The secondary winding  18  is connected in parallel with a capacitor  28  via an output diode  26 . The capacitor  28  and the diode  26  function to rectify and filter an output voltage (V out ). As will be understood by one skilled in the art, the black dots within the windings W Low . W High  and  18  represent the positive terminal. PWM IC  12  further senses the V out  (not shown) to accordingly control such as will be understood by one skilled in the art.  
         [0019]    During operation, the signal output from IC  12  drives the two switching devices Q Low  and Q High  at a switching frequency such that both are either on or off at the same time. When the switching devices Q Low  and Q High  are on, the proportion of a primary current, I p , that each conducts is a function of the voltage supplied by voltage buses  20  and  22  and the number of winding turns, N. For example, if N Whigh :N Wlow =2.8:1, Q High  conducts significant current only when the voltage supplied by voltage bus  22  is more than 2.8 times higher than the voltage supplied by voltage bus  20 . Similarly, when, voltage bus  22  provides a voltage that is substantially lower than 2.8 times the voltage voltage bus  20 , Q Low  conducts the current. There is also a transition range slightly above and below the 2.8 times voltage source multiplier when both devices and their respective windings share the primary current, I p .  
         [0020]    When the switching devices Q Low  and Q High  are off, both must withstand a flyback voltage. The flyback voltage is equal to the output voltage multiplied by the primary to secondary turns ratio pluse the input bus voltage, or  
         
       V 
       Flyback 
       =V 
       Wout 
       ×N 
       primary 
       /N 
       Wout 
       +V 
       S 
     
         [0021]    where N primary  =N Whigh  and N Wflow  for Q High  and Q Low  respectively; and  
         [0022]    V Wout −V out /V Dout ; and  
         [0023]    VS=V SHigh +V SLow  (the voltages supplied by voltage buses  22  and  20  respectively).  
         [0024]    Therefore, with V out =5 V, V Dout =0.3 V, N WHigh :N Wout =2.15:1, and N WLow :N Wout =0.77:1, the switching devices Q Low  and Q High  are subjected to maximum voltages of 26.1 V and 67.4 V respectively, at a ratio of 2.58;1. With respect to current, assuming comparable efficiency, the ratio between the maximum currents conducted by Q Low  and Q High  is inversely proportional to their respective minimum input bus voltages supplied by voltage buses  20  and  22 . For example, with voltage bus  20  supplying a voltage of 8 V and voltage bus  22  supplying a voltage of 22 V, the maximum current conducted by Q Low  is therefore 22/8=2.75 times the current conducted by Q High . Thus, in comparing the two switching devices, Q Low  must carry 2.75 times the current, but Q High  must withstand 2.58 times the voltage. Therefore, each switching device Q Low  or Q High  can be chosen accordingly to optimize the power supply design.  
         [0025]    When Q Low  and Q High  are on, the output diode  26  is off and is required to withstand a reverse voltage equal to the input bus voltage multiplied by the primary to secondary turns ratio plus the output voltage. Therefore, if the voltage on voltage bus 22 is 56 V, output diode  26  is required to block a reverse voltage equal to 56/2.15+5=31 V, and if the voltage on voltage bus  20  is 22 V, the output diode  26  is required to block a reverse voltage of 22/0.77+5=33.6 V. As will be understood, since both voltages are below 40 V, a standard Schottky diode can be employed as the output diode  26 .  
         [0026]    In another embodiment, a two-resistor current sense network is implemented to tailor current limit to the voltage bug that is predominant at any given time. With reference to FIG. 2, the switch Q High  current is sensed by the PWM IC  12  via a pair of resistors  32  and  34 . The Q Low  switching device current is sensed by the PWM IC  12  via only the second resistor  34 . When switch Q Low  is conducting, a current I Low  flow through the second resistor  34  and generates a voltage (V sense ) that is sensed by the PWM IC  12 . The PWM IC  12  adjusts this voltage, as necessary, to control the pulse width, and also to fix a maximum value for the voltage that establishes a primary current limit threshold. When switch Q High  is conducting, a current I High  flows through a higher resistance value formed by the sum of the pair of resistors  32  and  34 . Since the resistance value is higher but the maximum voltage value (V semsemax ) remains unchanged, a lower peak current limit value for the high voltage source range threshold is achieved. This lower peak current limit counteracts the tendency for the output current limit value to increase with input voltage and reduces component stress under a high input voltage overload of the output.  
         [0027]    It will be appreciated that, although only two embodiments of the invention have been described and illustrated in detail, various changes and modification may be made. For example, additional input sources can be accommodated by adding one primary winding and one switching device for each source. This can be used wherever a low-power, multi-source power supply is required and would be suitable for both isolated as well as non-isolated applications. All such changes and modifications may be made without departing from the sphere and scope of the invention as defined by the claims appended herein.