Abstract:
A power converter includes a boost converter, a DC/DC converter and a coupler chain. The boost converter receives a rectified AC input and generates a high voltage output. The boost converter includes a zero-voltage-switching transistor that is operable to adjust the power factor of the rectified AC input to generate the high voltage output. The DC/DC converter is coupled to the high voltage output of the boost converter and is configured to generate a regulated output voltage. The DC/DC converter includes a complementary pair of zero-voltage-switching transistors that are operable to regulate the high voltage output of the boost converter to generate the regulated output voltage. The coupler chain is coupled between a current-carrying terminal of the zero-voltage-switching transistor of the boost converter and current-carrying terminals of the complementary pair of zero-voltage-switching transistors in the DC/DC converter The coupler chain is operable to reduce the voltage across the current-carrying terminals of at least one of the zero-voltage-switching-transistors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. application Ser. No. 10/040,841, filed on Jan. 7, 2002, and entitled “High Efficiency AC-DC Converter With Power Factor Corrector,” which claims priority from and is related to U.S. Provisional Application No. 60/262,186, filed on Jan. 17, 2001, and entitled “A High Efficiency AC-DC Converter With Power Factor Corrector.” 
     
    
     
       BACKGROUND  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to the field of power converters. Particularly, this invention relates to the field of AC to DC converters with Power Factor Correction (PFC).  
           [0004]    2. Description of the Related Art  
           [0005]    AC/DC converters need power factor correction in order to fulfill international standards of low input harmonic current content. A front-end boost PFC converter is one way to obtain good input harmonic current to meet these international standards. Generally, another DC/DC converter is cascaded from the front-end boost PFC converter to provide a steady output voltage.  
           [0006]    [0006]FIG. 1 shows a typical configuration of an AC-DC converter with power factor correction. Rectified AC is fed to input terminals of a boost converter  4  at nodes  0  and  1 . The boost converter  4  includes an inductor L 1    10 , MOSFET switch M 1    12 , diode D 1    14  and capacitor C 1    16 . A series of Pulse Width Modulated (PWM) voltage pulses are fed to the gate terminal G 1  of the MOSFET switch  12 . The pulse width of the voltage pulses are programmed to make the input current follow the shape of the input sinusoidal voltage and build up a voltage across capacitor  16 . A DC/DC converter  20  converts the voltage across capacitor  16  to a regulated DC voltage across output nodes  5  and  6 .  
           [0007]    A problem in boost converters is the reverse current of the diode  14  when the switch  12  turns on. When the switch  12  turns on, it draws reverse recovery current through the diode  14  and turns the switch  12  off abruptly to block the reverse voltage equal to the output voltage of the boost PFC converter  4 . The output voltage is always higher than the peak of the rectified AC and very often is close to 400V. This high output voltage causes a large amount of switching loss when the diode  14  is turned off. This switching loss increases with frequency. However, high switching frequency is often required to reduce the size and weight of the passive components. Thus PFC boost converter  4  generally are lossy circuits due to the high switching frequencies of the circuit. In fact, the switching loss is associated with every switch in the boost converter  4  and every switch in the DC/DC converter  20 .  
           [0008]    Previous work uses various techniques to reduce switching losses. In U.S. Pat. No. 5,313,382, Farrington discloses a boost converter with an auxiliary switch and a resonant network to achieve reduced voltage stress at a main power switch during turn on. The boost converter also enables a soft turn off of the boost rectifier. The auxiliary switch of the boost converter is turned on without reduced voltage condition, but it has a zero current condition. In U.S. Pat. No. 5,633,579, Kim discloses a boost converter with a stress energy reproducing snubber circuit in order to reduce the stress energy of the boost rectifier during turn off. The snubber circuit reduces the voltage stress on a main switch of the boost converter during turn on. In U.S. Pat. No. 5,748,457, Poon discloses a DC/DC converter which reduces voltage stress by means of zero voltage switching, but it has no boosting and power factor correction effect.  
           [0009]    In addition to soft switching, another problem with PFC converters is control of the switching. Some prior art techniques attempt to integrate the PFC converter and the DC/DC converter. Most of these prior art techniques include converters with fewer degrees of freedom which results in restrictions to operate the converters in certain modes, such as the discontinuous mode. These restrictions prevent maximized utilization of all the components.  
         SUMMARY OF THE INVENTION  
         [0010]    A power converter includes a boost converter, a DC/DC converter and a coupler chain. The boost converter receives a rectified AC input and generates a high voltage output. The boost converter includes a zero-voltage-switching transistor that is operable to adjust the power factor of the rectified AC input to generate the high voltage output. The DC/DC converter is coupled to the high voltage output of the boost converter and is configured to generate a regulated output voltage. The DC/DC converter includes a complementary pair of zero-voltage-switching transistors that are operable to regulate the high voltage output of the boost converter to generate the regulated output voltage. The coupler chain is coupled between a current-carrying terminal of the zero-voltage-switching transistor of the boost converter and current-carrying terminals of the complementary pair of zero-voltage-switching transistors in the DC/DC converter The coupler chain is operable to reduce the voltage across the current-carrying terminals of at least one of the zero-voltage-switching-transistors.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1(Prior Art) shows a typical configuration of an AC-DC converter with power factor correction;  
         [0012]    [0012]FIG. 2 shows a circuit comprising a first embodiment of the present invention;  
         [0013]    [0013]FIGS. 3A to  3 F show graphs of voltage and current during operation of the circuit of FIG. 2;  
         [0014]    [0014]FIG. 4 shows a circuit comprising a second embodiment of the present invention;  
         [0015]    [0015]FIG. 5 shows a circuit comprising a third embodiment of the present invention;  
         [0016]    [0016]FIG. 6 shows a circuit comprising a fourth embodiment of the present invention;  
         [0017]    [0017]FIG. 7 shows a circuit comprising a fifth embodiment of the present invention; and  
         [0018]    [0018]FIGS. 8A to  8 D show graphs of the driving waveforms for the switches of the circuit of FIG. 7. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    With respect to the drawing figures, a circuit comprising a first embodiment of the present invention is shown in FIG. 2. The invention includes various interconnected parts that define a plurality of devices that can function together. The circuit includes a boost converter, a DC/DC converter, a coupler chain, a discharging chain, and a soft switch inductor  28 . The boost converter includes an inductor  32 , a MOSFET switch  34 , a diode  36  and a capacitor  38 . The DC/DC converter includes MOSFET switches  40  and  42 , a capacitor  46 . A transformer  48  in the DC/DC converter, having windings W 11  and W 12 , couples the switches  40  and  42  to diodes  50  and  52  which are coupled to an inductor  54  and a capacitor  56 . The resistance  68  is the load of the converter. The coupler chain includes an inductor  58 , a diode  60  and a capacitor  62 . The discharge chain includes a diode  64  and an inductor  66 . Input terminals  30  and  31  are coupled to a rectified AC source. The input terminals  30  and  31  feed the boost converter. A series of Pulse Width Modulated (PWM) voltage pulses are injected into the boost converter on the gate G 11 , of the switch  34 . The PWM signal is programmed to make the shape of the averaged input current follow the shape of the input voltage and produce a high power factor. A boosted voltage across the capacitor  38  is then the input voltage of the DC/DC converter. The MOSFET switches  40  and  42  are programmed to turn on and turn off to give a regulated output voltage. These switches  40  and  42  are coupled to the switch  34  through the coupler chain, which couples the boost converter to the DC/DC converter. The discharge chain couples the coupler chain to the input of the DC/DC converter so that the discharge chain can discharge the capacitor  62  and the inductor  58  of the coupler chain.  
         [0020]    The soft switch inductor  28  is coupled to switches  40  and  42 . The circuit as a whole operates the switches  34 ,  40 , and  42  so as to soft switch each of the switches  34 ,  40 , and  42  in the circuit. When the switch  42  turns off from its on state, current in the inductor  28  continues to flow and exchanges charge in capacitance across the switches  42  and  40 . The voltage across the switch  40  then falls to zero.  
         [0021]    The switch  40  is programmed to turn on at zero voltage. When the voltage across the switch  40  falls, then the coupler chain of components is activated because the boost switch  34  is in the off state and the diode  36  is conducting. A resonant current flows through this chain and pulls current from the boost inductor  32 . Current through the diode  36  is reduced at a controlled rate and the turn off loss through the diode  36  is largely reduced. The resonant current will eventually become larger than the inductor current, which causes the diode  36  to turn off. The voltage across the switch  34  continues to fall under the influence of the resonant current in the coupler chain until it becomes substantially zero. Then, the switch  34  is programmed to turn on and the switching loss of the switch  34  is substantially reduced.  
         [0022]    After the switch  34  remains on for the period of time needed by the boost converter, the switch  34  may be turned off regardless of the state of switches  40  and  42 . These switches  40  and  42  are complementary, such that when one switch is on the other switch is off. A small time gap between switches avoids shoot through. The time gap is very small and it is regarded that  40  and  42  operate asymmetrically. Thus, as the switch  34  is turned off, there are two cases of operability for switches  40  and  42 . A first case where the switch  40  is on and the switch  42  is off, and a second case where the switch  40  is off and the switch  42  is on.  
         [0023]    In the first case, the capacitor  62  is settled to a voltage, when the switch  34  turns off, current is diverted to the capacitor  62  which acts like a snubber capacitor. Its voltage will eventually settle to the line voltage after the switch  34  has turned off.  
         [0024]    In the second case, the capacitor  62  has discharged to near zero and does not interfere with the switching off of the switch  34 . Current flows through the inductor  32  and the boost diode  36  like the current would in prior art boost converter.  
         [0025]    The states of switches  40  and  42  may also be changed at any time after the switch  34  has turned on at the beginning of a duty cycle. Again there are two cases when the switch  40  turns off and the switch  42  turns on. The first case is where the switch  34  is still on and the second case is where the switch  34  is off. In both cases, the diode  60  prevents the initiation of resonant current through the inductor  58 . When the switch  40  is turned off, energy in the inductor  28  attempts to raise the voltage across the switch  40  and reduce the voltage across the switch  42 . Resonance in the coupler chain discharges the charge on the capacitor  62 . When the voltage across the switch has fallen to substantially zero it is programmed to turn on. Thus the switch  42  can turn on at a voltage substantially close to zero. The switch  42  remains on for its designated duty cycle. When the switch  42  is turned off, its current has changed direction, the voltage of the capacitor  62  is zero and the whole switching process will repeat.  
         [0026]    With respect to drawing FIGS. 3A to  3 F, the graphs of voltage and current during operation of the circuit of FIG. 2 are shown. FIGS. 3A and 3B show typical asymmetric gate driving pulses for the MOSFET switches  40  and  42 . The duty cycle of the control pulses are programmed to maintain a regulated DC output voltage at the converter output terminal across the resistor  68  shown in FIG. 2. A small idling time period is inserted between the turn off and turn on of the switches  40  and  42 , as shown in the timing of the gate pulses to drive the switches in FIGS. 3A, 3B and  3 C. The switch  34  (FIG. 3A) in the boost converter is turned on shortly after the switch  40  (FIG. 3B) in the DC/DC converter has turned on but may turn off at any time in the cycle as explained above.  
         [0027]    In FIGS. 3D and 3E, when the switch  42  turns off, current in the inductor  28  pulls the voltage at the drain terminals of the switches  40  and  34  at nodes  17  and  13  respectively, although these voltages may not fall at the same time and rate. Also, FIG. 3D shows that when the switch  40  turns off, the current flowing in the inductor  28  will push the voltage at node  17  high and reduce the voltage across the switch  42 . The gate driving pulse will turn on the switch  42  when its drain source voltage has dropped to substantially zero. In FIG. 3F, current through the diode  36  decreases in this transient period until it reaches zero. The switch  34  is programmed to turn on after its voltage is substantially zero. The same switching applies to the switch  40  which is programmed to turn on after its voltage is substantially zero. Thus, both switches  34  and  40  have zero voltage turn on. The diode  36  can then switch off with less reverse current. FIG. 3F shows the current slope of the diode  36  has been limited, the reverse current can be controlled to be small which limits any significant losses. Thus, all switches  34 ,  40  and  42  may turn on at zero voltage state.  
         [0028]    With respect to FIG. 4, a second embodiment of the present invention is shown. This second embodiment differs from the first embodiment in the placement of a small inductor  80  for resonance to provide zero voltage switching of the switch  34  in the boost converter. In the first embodiment, the small inductor  58  is placed in the coupling chain of components connecting the boost converter and the DC/DC converter. Nevertheless, this is not the only location to place the inductor. In this second embodiment, the inductor  80  is placed in series with the switch  34  in the boost converter. The switches  34 ,  40  and  42  are controlled similar to the switches in FIG. 2, and the output of the circuit is similar.  
         [0029]    When the switch  34  switches off and the capacitor  62  has discharged to near zero, the energy stored in the inductor  80  is released through the diode  60  and flows through the coupling and discharging branches. The current through the inductor  80  settles to zero and the inductor current through the inductor  32  will flow through the boost diode  36  similar to other boost converters.  
         [0030]    A third embodiment of the present invention is shown in FIG. 5. This third embodiment differs from the first two embodiments in the placement of a small inductor  90  and a diode  92  for resonance to provide zero voltage switching of the switch in the boost converter. This third embodiment has the inductor  90  placed in between the input inductor  32  and the diode  36  of the boost converter. Another leg, which includes the diode  92 , is placed in the circuit between the inductor  90  and the diode  36 , and is then extended to the input terminal  30 . The switches  34  and  40  act substantially the same in this embodiment as they act in the first embodiment. When the voltage across the switch  40  falls, the voltage across the switch  34  falls simultaneously due to the coupling of the switches by the capacitor  62 .  
         [0031]    The aforementioned embodiments use two small inductors, one coupled to the boost diode  36  and another one placed near the DC/DC converter primary side switches  40  and  42 , to obtain less voltage stress during turn off of the diode  36  and turn on of the switches  40  and  42 . Nevertheless, it is possible to combine these two small inductors into one inductor to further reduce the converter component count.  
         [0032]    Turning now to FIG. 6, a circuit comprising a fourth embodiment of the present invention is shown. The embodiment is similar to the first embodiment, except the inductor  58  (FIG. 2) in the coupler chain and the soft switching inductor  28  (FIG. 2) are combined into a single inductor  94  coupled to the switches  40  and  42 . The inductor  94  provides soft turn off of diode  36  and also reduces voltage stress turn on of the switches  34 ,  40 , and  42 .  
         [0033]    When the switch  40  turns off, the current flowing in the series inductor  94  will continue to flow and discharge the stray capacitance across the drain source of the switch  40 . The voltage across the switch  42  will drop accordingly. The switch  40  then turns on when the voltage across the switch  40  drops to zero. The current flow direction of the inductor  90  then reverses and shunts the current flowing through the boost diode  36 . The inductor  94  also discharges the stray capacitance of the switch  34  via the path comprising the diode  60  and the energy limiting capacitor  62 . The voltage across the switch  34  then falls. When the voltage drops to essentially zero, then the switch  34  can be turned on with no switching loss. The rate of fall of current and the reverse recovery current during the turn off of the diode  36  is limited by the inductor  94 , therefore the turn off losses of the boost diode  36  can also be reduced.  
         [0034]    Turning now to FIG. 7, a circuit comprising a fifth embodiment of the present invention is shown. This embodiment functions similar to the first embodiment, but the component count of this circuit is less. The capacitor  62  in the coupler chain and the discharge chain of the first embodiment are replaced by an active switch  100 , which couples the boost converter to the DC/DC converter. The coupler chain, including the diode  60 , the switch  100  and the inductor  94 , can softly turn off the boost diode  36  and reduce the turn on voltage of the switch  34  by selecting proper gate driving timing of the four switches  34 ,  40 ,  42 , and  100 . The inductor  94  provides soft turn off of the diode  36  and reduces voltage stress turn on for the switches  34 ,  40 , and  42 .  
         [0035]    In the previous embodiments, an energy limiting capacitor  62  limits energy transfer from the PFC boost converter side to the DC/DC converter side. Another diode  64  and an inductor  66  release the stored energy of the capacitor  62  through the discharge chain. In the fifth embodiment, these components are replaced by the small active switch  100  which limits the energy transfer between the boost converter and the DC/DC converter during the switching transient by timing the gate signals of the switch  100  based upon the timing signals for the other gates  34 ,  40 , and  42 .  
         [0036]    With respect to FIGS. 8A to  8 D, graphs of the driving waveforms for the switches of the circuit of FIG. 7 are shown. FIGS. 8A to  8 C are similar to FIGS. 3A to  3 C, which are the gate drives for the switches  40 ,  42 , and  34 , respectively. As shown in FIG. 8D, the additional switch  100  is programmed to turn on before the switch  42  turns off. The switch  100  remains on throughout the transient during which the switches  34  and  40  switch on softly and the switch  42  turns off softly. Afterwards, this auxiliary switch  100  will then turn off and stop energy flow between the boost converter and the DC/DC converter side. This auxiliary switch  100  does not need to handle main power transfer, and it can be a very small MOSFET which operates for a short period of time.  
         [0037]    The embodiments described herein are examples of structures having elements corresponding to the elements of the invention recited in the claims. This written description may enable those skilled in the art to make and use embodiments having alternative elements that likewise correspond to the elements of the invention recited in the claims. The intended scope of the invention thus includes other structures that do not differ from the literal language of the claims, and further includes other structures with insubstantial differences from the literal language of the claims.