Abstract:
The present invention provides an apparatus and method for reducing the voltage stress in a single-stage single-switch (SSSS) converter by modulating the predetermined operating frequency of the converter lower responsive to increasing voltage stress. A control circuit ( 116 ) and associated cooperable frequency setting capacitance (CT) and resistance (RT) are coupled to the primary circuit ( 112 ) and the secondary circuit ( 114 ) of the SSSS converter via a switch ( 120 ). A frequency foldback device ( 180 ) is coupled to CT or RT and cooperable therewith to lower bus voltage stress by modulating the switch frequency. The operating frequency is modulated (i.e. reduced) from the predetermined operating frequency upon detection of a voltage threshold transition.

Description:
[0001]    This application claims the priority under 35 U.S.C. 119(e)(1) of copending U.S. provisional application No. 60/354,066, filed on Jan. 31, 2002 and incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field of the Invention  
           [0003]    The present invention relates to power electronics and, more particularly to a single stage single switch power factor correction circuit with reduced voltage stress.  
           [0004]    2. Description of Related Art  
           [0005]    A conventional method of solving the problem of harmonic current in a power supply is the power factor correction (PFC) circuit. For example, input AC power is rectified by a bridge-type rectifier device and input to a PFC circuit for power correction, then converted by a voltage converter for outputting to a load.  
           [0006]    In this single-stage approach, input current shaping and isolation are performed in a single step. Further, this single stage is implemented with only one controlled semiconductor switch. These single-stage single-switch circuits integrate a step-up inductor and an energy storage capacitor. When the single switch is switched on, the electric current flow through the inductor to a first transformer coil for providing energy to a secondary coil of a DC voltage transformer and storing energy in the inductor. When the switch is switched off, the energy stored in the inductor is delivered to the energy storage capacitor.  
           [0007]    The voltage on the bus or boost capacitor varies with the inputted AC power. Therefore, if the AC power is a wide-range AC input and the load has wide variations, then the voltage on the capacitor will vary enormously depending on the design thereof. In order to endure high voltage and have a sufficient capacitance for decreasing the variation in the output voltage, a capacitor with higher capacitance and higher voltage rating is required. However, this type of capacitor is very expensive and bulky. Because of this problem, currently used single-stage single-switch PFC circuits generally suffer from excessive voltage stress on their energy storage capacitor which limits their use and drives up cost.  
           [0008]    One approach to this problem includes the addition of a “feedback” winding to reduce the voltage stress as described in, “Single Stage, Single Switch Input Current Shaping Technique with Reduced Switching Loss” by Laszlo Huber and Milan Jovanovic, IEEE Volume 15, No.4, July 2000. With this feedback winding approach, two additional primary windings are employed to keep the voltage of the energy-storage capacitor below a desired level in the entire line and load ranges (e.g., 400 V at the universal line range of 90-264 Vrms). However, this feedback approach has shortcomings which leads to increased cost and increased complexity of the converter.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention achieves technical advantages as method and apparatus for reducing the voltage stress in a single-stage single-switch (SSSS) converter by modulating the predetermined operating frequency of the converter lower responsive to increasing voltage stress. A control circuit and associated cooperable frequency setting capacitance (CT) and resistance (RT) are coupled to the primary circuit and the secondary circuit of the SSSS converter via a switch. A frequency foldback device is coupled to CT or RT and cooperable therewith to lower bus voltage stress by modulating the frequency of the switch. The operating frequency is modulated (i.e. reduced) from the predetermined operating frequency upon detection of voltage threshold transition.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein:  
         [0011]    [0011]FIG. 1 illustrates a single-stage single-switch power factor correction circuit;  
         [0012]    [0012]FIG. 2 illustrates a circuit diagram of frequency foldback in accordance with an exemplary embodiment of the present invention;  
         [0013]    [0013]FIG. 3 illustrates an alternative circuit diagram of frequency foldback in accordance with an exemplary embodiment of the present invention; and  
         [0014]    [0014]FIG. 4 illustrates an alternate circuit diagram of frequency foldback in accordance with an exemplary embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses and innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features, but not to others.  
         [0016]    Throughout the drawings, it is noted that the same reference numerals or letters will be used to designate like or equivalent elements having the same function. Detailed descriptions of known functions and constructions unnecessarily obscuring the subject matter of the present invention have been omitted for clarity.  
         [0017]    Referring now to FIG. 1 there is illustrated a simplified diagram of a single-stage single-switch power factor correction (PFC) circuit in accordance with an embodiment of the present invention. AC power is received on the AC input terminals and DC power is output at output terminals Vo. The PFC circuit includes a primary circuit  112 , a secondary circuit  114  and a control circuit  116 . Rectifier bridge diodes  118  rectify the AC voltage received on the input terminals, from AC power mains for example. The PFC circuit further includes a power switch  120 , such as a power field effect transistor, and a magnetic component  122 , such as a power transformer or coupled-inductor. The transformer  122  includes a pair of primary windings and a secondary winding. The diode (D 1 ) of the primary circuit  112  determines which primary winding is energized when the power switch  120  is conductive. A storage capacitor (Cbus) provides energy to operate the PFC circuit when the mains voltage is low as well as providing intermediate energy storage required of PFC circuits.  
         [0018]    The secondary circuit includes a rectifier circuit (capacitor Co and diode D 2 ) for rectifying the current flowing from the transformer secondary. The control circuit  116  controls the On time of the power switch  120  to maintain a regulated DC voltage on the output terminals. The control circuit  116  can be a traditional pulse width modulation (PWM) control I.C. It varies the duty cycle (On time/Period=duty cycle) to the controlled switch  120 . It also contains an oscillator which sets the switching frequency, an error amplifier and other ancillary circuitry. It can also contain over-voltage and/or over-current protection circuits.  
         [0019]    The frequency is determined by the selection of resistor RT and capacitor CT. In this example, RT sets the charging current which is fed into CT. Once RT is set, the oscillator period is determined by the selection of CT through the relationship I=c*dv/dt. The dv is determined by the internal circuitry of the controller, the upper and lower oscillator threshold voltages, c is CT, I is the current established through RT and dt is the oscillator period. There may be a scaling of the currents and/or voltages and perhaps offsets as well, but this doesn&#39;t significantly change the oscillator operation.  
         [0020]    This type of circuit is generally know as a voltage source converter. A further description of the power stage topology for this type of voltage source converter is described in U.S. Pat. No. 5,301,095, Teramoto et al., issued Apr. 5, 1994, which is incorporated herein by reference. Although the present invention is describe in terms of a voltage source converter, it can be used for similar type circuits.  
         [0021]    In accordance with the present invention, a frequency foldback device  180  is coupled to CT or RT and cooperable therewith to lower bus voltage stress by modulating the operating frequency. The operating frequency is modulated (i.e. reduced) from a predetermined operating frequency when a monitored switch current or bus voltage meets a predetermined threshold.  
         [0022]    Referring now to FIG. 2 there is illustrated an embodiment of the frequency  15  foldback device  180  shown in FIG. 1. Resistors R 1  and R 2  are coupled in a voltage divider arrangement as an input to comparator U 1 . U 1  also has a voltage reference (V REF ) coupled to another input. Resistors R 1  and R 2  divide down the bus voltage (V BUS ) so that it can be compared to the relatively low V REF . U 1  changes states once the threshold established by V REF  has been reached. The output of U 1  is coupled to the gate of transistor Q 1 . Further, a resistor (Re) is coupled between the source of Q 1  and ground. The drain of Q 1  is coupled to a node between the I.C. Controller and capacitor CT. Resistor RT is also coupled to the I.C. Controller.  
         [0023]    Thus, when the voltage on Cbus rises above the threshold, the output of U 1  changes states from low to high, transistor Q 1  is turned On which effectively forces a constant voltage across Re. This effectively enables Q 1  to operate as a current sink. This current sink formed by Q 1  and Re pulls a fixed amount of current away from CT, i.e. the charge current to CT is reduced. This increases the time it takes for CT to reach its upper threshold thereby lowering the frequency which causes the voltage on Cbus to drop.  
         [0024]    Preferably, the frequency is lowered significantly, i.e. from the conventional 100 kHz to as low as approximately 20 kHz. A result of this magnitude of frequency drop is a reduction in bus voltage stress from approximately 455V to 419V. Since the maximum voltage rating of a typical Cbus capacitor used for this type of application is 450V, the present approach enables the converter to operate without overstressing or damaging the capacitor.  
         [0025]    It should be noted that the present technique could also work if U 1  was configure as an op amp. Instead of an abrupt change in state it would operate in a linear mode. This would have the effect of varying the frequency smoothly instead of abruptly.  
         [0026]    Referring now to FIG. 3 there is illustrated another embodiment of the frequency foldback device  180  shown in FIG. 1. Here, the source of Q 1  is coupled to ground and the drain is coupled to a resistor Rc which is further coupled in parallel with RT. The circuit of FIG. 3 functions similarly to the circuit of FIG. 2, however, instead of reducing the current available to CT by providing an alternate path through Q 1  (as in FIG. 2), Q 1  is turned On and driven into saturation when an appropriate trip point is reached (determined by the selection of R 1  and R 2  and the VREF input to U 1 ). This effectively places Rc in parallel with RT. This also reduces the charging current to CT which reduces the switching frequency which reduces the voltage on Cbus.  
         [0027]    It should be noted that this technique could also work if U 1  was configured as an op amp or comparator. Instead of an abrupt change in state it could operate in a linear mode. This would have the effect of varying the frequency smoothly instead of abruptly.  
         [0028]    [0028]FIG. 4 shows another method for implementing this technique. The circuit operation is essential the same as previously describe. Resistors R 1  and R 2  as well as U 1  have the same function. In FIG. 4, Q 1  and Q 2  are configured in a current sink arrangement. Rc sets up the reference current for Q 1  and Q 2 . These transistor then pull charging current away from Ct when the reference voltage of U 1  is exceeded thereby reducing the switching frequency.  
         [0029]    Further, it should also be noted that the description of FIGS. 2, 3, and  4  apply to discrete component implementations. The same techniques can be implemented within an integrated circuit in accordance with an embodiment of the present invention. In this case the circuit configuration would be a little different (by necessity) but the function would be the same.  
         [0030]    It should also be noted that the control I.C. can have an external separate Ct and Rt or one or both of these components can be internal to the control I.C. ( 116  in FIG. 1).  
         [0031]    There has been an attempt to reduce bus voltage stress by modulating operating frequency, however, it was implemented in converters which operated in the discontinuous conduction mode (DCM) and the frequency was increased (as opposed to decreasing the frequency) to reduce the stress. The present approach applies to converters which operate in the continuous conduction mode (CCM). Operating in CCM has the advantage of reducing the peak switch and inductor currents in the converter. For the same power level, DCM would have much higher peak currents and therefore have higher stress on the components. In some cases this also leads to higher losses i.e. reduced efficiency. This in turn can drive cost up due to higher cost components and/or more challenging thermal design.  
         [0032]    It is counter intuitive to lower the frequency. On first glance it would seem that raising the frequency would keep the converter in CCM and this would help reduce the voltage. Attempts have been made to reduce voltage stress by increasing the operating frequency because the thought was that at light loads the converter entered DCM operation and this was causing the voltage to rise. Raising the frequency would logically cause the converter to stay in CCM longer. So, reducing the frequency goes against the logic. Second, a converter operating in CCM is typically immune to frequency shifts. A converter operating in DCM has a dc transfer function which is load dependant. The dc transfer function for a converter operating in CCM is load independent. Therefore changing the frequency should have no effect on the converter. With the present approach however, it does.  
         [0033]    Other advantages of the present approach are that it is easily implemented and it lends itself to implementation in an integrated circuit which has the potential ofbeing low cost compared to additional magnetic components of other solutions.  
         [0034]    Although a preferred embodiment of the method and system of the present invention has been illustrated in the accompanied drawings and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.