This invention relates to power factor corrected 3-phase ac-dc power converters, and in particular, 3-phase ac-dc converters operating from commercial power having fairly good regulation and having a fixed dc output voltage.
High efficiency and simplicity of control are desirable characteristics.
3-phase power factor correction (pfc) is much more efficient and much simpler than single-phase pfc because 3-phase power flow is continuous. Collectively, the power in a balanced 3-phase circuit is equivalent to dc. No energy needs to be shunted in and out of storage capacitors, an inefficient operation.
U.S. Pat. No. 7,633,782 teaches that a 3-phase buck converter has high efficiency when switched at “100% duty-ratio”. 100% duty-ratio cannot be true literally for all three phases. It is defined as having 100% duty-ratio for the dominant phase, that is, the phase having the highest voltage magnitude, negative or positive, and the sum of the duty-ratios of the non-dominant phases equaling 100%. The duty-ratio for the phase A switches of a 3-phase pfc 100% duty-ratio buck converter is shown in FIG. 3.
Expressed as equations, the duty-ratios for a 3-phase pfc 100% duty-ratio buck converter are:
      Da    =                  Sin        ⁢                                  ⁢        X                    max        ⁢                  {                                                                  Sin                ⁢                                                                  ⁢                X                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    120                                    )                                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    240                                    )                                                                            }                          Db    =                  Sin        ⁡                  (                      X            +            120                    )                            max        ⁢                  {                                                                  Sin                ⁢                                                                  ⁢                X                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    120                                    )                                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    240                                    )                                                                            }                          Dc    =                  Sin        ⁡                  (                      X            +            240                    )                            max        ⁢                  {                                                                  Sin                ⁢                                                                  ⁢                X                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    120                                    )                                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    240                                    )                                                                            }                    
In a practical converter, the sine functions are derived from the phase voltages.
Neither the voltage nor the total current is controlled when operating at 100% duty-ratio, but the proportioning of the phase currents is controlled, ensuring balanced currents.
A 100% duty-ratio buck converter is shown in FIG. 1, as an example, not a limitation.
U.S. Pat. No. 7,633,782 also teaches that a 3-phase pfc boost converter has high efficiency when switched at “0% duty-ratio”. 0% duty-ratio cannot be true literally for all three phases. It is defined as having 0% duty-ratio for the dominant phase, that is, the phase having the highest voltage magnitude, negative or positive, and the sum of the duty-ratios of the non-dominant phases equaling 100%. The duty-ratio for the phase A switch of a 3-phase 0% duty-ratio boost converter is shown in FIG. 5.
Expressed as equations, the duty-ratios for a 3-phase 0% duty-ratio boost converter are:
      Da    =                                    Sin          ⁢                                          ⁢          X                                    max        ⁢                  {                                                                  Sin                ⁢                                                                  ⁢                X                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    120                                    )                                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    240                                    )                                                                            }                          Db    =                                    Sin          ⁡                      (                          X              +              120                        )                                              max        ⁢                  {                                                                  Sin                ⁢                                                                  ⁢                X                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    120                                    )                                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    240                                    )                                                                            }                          Dc    =                                    Sin          ⁡                      (                          X              +              240                        )                                              max        ⁢                  {                                                                  Sin                ⁢                                                                  ⁢                X                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    120                                    )                                                                    ,                                                        Sin                ⁡                                  (                                      X                    +                    240                                    )                                                                            }                    
In a practical converter, the sine functions are derived from the phase voltages.
Neither the voltage nor the total current is controlled when operating at 0% duty-ratio, but the proportioning of the phase voltages at the inductors is controlled. Because even very small voltage errors will cause large current errors over time, current balance must be ensured by feedback.
A 3-phase pfc 0% duty-ratio boost power converter is shown in FIG. 2.
U.S. Pat. No. 7,633,782 also teaches using a current vector reference voltage Vcvr to ensure current balance. The current vector reference Vcvr is derived from a three-legged voltage divider of equal resistors from the three phase voltages, as shown in FIG. 2. The control algorithms are based upon the phase voltages, but because the power neutral is not connected even if it is present, the phase voltages are referenced to Vcvr. This largely compensates for phase voltage imbalance, if any, ensuring reasonably balanced currents even if the voltages are not balanced, as shown in FIG. 17.
For the 3-phase pfc 100% duty-ratio buck converter, the control algorithm controls the currents directly, ensuring current balance.
For the 3-phase pfc 0% duty-ratio boost converter the current control of the input currents senses any difference voltage between the current vector reference Vcvr and the transformer center-tap voltage Vct, with reference to FIG. 2. If the current vectors and the voltage vectors are not proportional, the voltage at the transformer center-tap will drift over time from that of the current vector reference. This difference voltage (error) can be used for feedback to modify the relative duty-ratios to correct the input currents with appropriate timing, gain and compensation for stability.
Used alone, both the 3-phase 100% duty-ratio buck converter and the 3-phase 0% duty-ratio boost converter have high output ripple voltages and must have post-regulators such as a variable dc-dc transformer to have a clean dc output voltage, as shown in FIGS. 1 and 2.