Abstract:
The present invention relates to a bi-directional AC/DC converter having (i) a power stage, (ii) a sourcing control circuit and (iii) a recuperation control circuit. The converter operates with a Power Factor Correction in both directions, i.e. when transferring energy from the AC mains into the DC load as well as when it is transferring energy from an active DC load into the AC mains. Smooth transition between sourcing and recuperation is possible by allowing an active load to control the output voltage until the correct control circuit begins regulation.

Description:
CLAIM OF PRIORITY 
     This non-provisional patent application claims priority to U.S. provisional patent application Ser. No. 60/168,571, filed on Dec. 2, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates, in general, to power conversion and, more specifically, to a bi-directional AC/DC converter and a method of operation of the converter. 
     BACKGROUND OF THE INVENTION 
     Applicants are unfamiliar with any AC/DC power supply converter that provide the following characteristics: 
     1. Provide galvanic isolation between input and output sides of the converter unit; 
     2. Be able to work with an active load (a load that can sink or source energy, such as battery, for instance), by sourcing energy into the load or sinking energy generated by the load; 
     3. Recycle energy when working with an active load by returning the energy into the electrical main of the unit; and 
     4. Provide Power Factor Correction for the line current, regardless if the energy is taken from the mains or recycled into the mains. 
     Different variations of isolated converters that provide Power Factor Correction have been described in Hirachi et al&#39;s article entitled  Switched - Mode PFC Rectifier with High Frequency Transformer Link for High - Power Density Single Phase UPS , Proceedings of the PESC Conference, June 1997, p. 290-96; Cho et al.&#39;s article entitled Zero-Voltage-Transition Isolated PWM Boost Converter for Single Stage Power Factor Correction, Proceedings of the APEC conference, March 1997, p. 471-76; and Dalal&#39;s article 400 W  Single - Stage Current - Fed Isolated Boost Converter with PFC , Unitrode Power Supply Seminar 1999-00 Series, Manual SLUP002, p. 3.1-3.24. These articles disclose a process to integrate a Power Factor Correction circuit with a DC/DC converter stage to obtain circuits simpler than classic, two-stage approaches (a PFC regulator and DC/DC converter as a separate, basically independent units). 
     In particular, the Hirachi et al. reference presents an isolated AC/DC/AC converter for use in uninterruptible power systems. Comparison between conventional circuit configurations, which are based on non-isolated boost topologies and proposed solutions, which is basically an isolated boost converter, with a diode bridge and a separate MOSFET bridge on the primary side and a rectifier bridge on the secondary side. 
     Cho et al. disclose an isolated boost converter consisting of a diode rectifier bridge and a MOSFET bridge on the primary side and a diode bridge of the secondary side with additional circuitry located also on the secondary side, for minimizing switching loss in the AC/DC converter. 
     Dalal suggest a current fed isolated AC/DC converter topology based on the push-pull converter and typically consisting of a bridge rectifier on the input, center-tapped transformer and two MOSFET switches on the primary side and a diode rectifier bridge on the secondary side of the unit. 
     However, these articles do not disclose a system that can (1) be able to work with an active load (a load that can sink or source energy, such as battery, for instance), by sourcing energy into the load or sinking energy generated by the load; (2) recycle energy when working with an active load by returning it into the mains; or (3) provide Power Factor Correction for the line current, regardless if the energy is taken from the mains or recycled into the mains. 
     Furthermore, several solutions for non-isolated topologies (Wang et al. in the article entitled Some  Novel Four - Quadrant DC—DC Converters , Proceeding of the PESC Conference, June 1998, p. 1775-82;) and an isolated topologies (Reimann et al.&#39;s article entitled  A Novel Control Principle of Bi - Directional DC—Dc Power Conversion , Proceedings of the PESC Conference, June 1997, p. 978-84; and Huang et al.&#39;s article entitled  Novel Current Mode Bi - directional High - Frequency Link DC/AC Converter for UPS , Proceedings of the PESC Conference, June 1998, p. 1867-71) capable of transferring energy from the DC source to the DC or AC load and also in the opposite direction (bi-directional power flow) have been presented. The topologies disclosed in those article do not disclose systems capable of (1) recycling energy when working with an active load by returning it into the mains; or (2) providing Power Factor Correction for the line current, regardless if the energy is taken from the mains or recycled into the mains. Moreover, the Wang et al. reference fails to disclose a method for providing galvanic isolation between input and output sides of the unit. 
     In particular, Wang et al. disclose a family of four topologies capable of operating in all four quadrants. This is a family of non-isolated converters, operating from a DC source and capable of generating both positive and negative polarity of output voltages, in addition to positive and negative output current, as may be directed by the load. 
     Reimann et al. suggest an isolated DC/DC converter topology capable of controlling energy flow in both directions—from source to load and from load side to the source side of the unit. It is basically an isolated boost topology consisting of two bridges, one on the primary side and the other on the secondary side, each having four quasi-bidirectional switches. 
     Also, there are products on the market (such as BOP series from KEPCO, Inc.) which are capable of controlling active loads. These products, made by the applicant, lack the ability to (1) recycle energy when working with an active load by returning it into the mains; or (2) provide Power Factor Correction for the line current, regardless if the energy is taken from the mains or recycled into the mains. 
     Work described in Hui et al.&#39;s article entitled  A Bi - Directional AC - DC Power Converter with Power Factor Correction  (Proceedings of the PESC Conference, June 1998, p. 1323-29) presents a non-isolated topology providing a bidirectional link between AC line and DC source capable of recycling the energy, but it does not provide galvanic isolation between input and output. 
     As in inventor&#39;s knowledge, there has not been unit presented so far that can simultaneously satisfy the following requirements: (1) providing galvanic isolation between input and output sides of the converter unit; (2) be able to work with an active load (a load that can sink or source energy, such as battery, for instance), by sourcing energy into the load or sinking energy generated by the load; (3) recycle energy when working with an active load by returning the energy into the electrical main of the unit; and (4) provide Power Factor Correction for the line current, regardless if the energy is taken from the mains or recycled into the mains. 
     BRIEF SUMMARY OF THE INVENTION 
     An isolated, bidirectional AC/DC converter with Power Factor Correction function and capability to recuperate energy into the mains has been invented. It consists of a power stage and a control section. The power stage processes raw power from electrical mains to the power required by the load. The power stage also processes the power generated by an active load and delivers the energy to the mains during the recuperation phase. The power stage has an input filter inductor, at least four bidirectional switches that form a bridge configuration on the primary side of an isolation transformer, the isolation transformer, at least four quasi-bidirectional switches that form a bridge on the secondary side of the isolation transformer and an output filter capacitor. 
     The control section of the unit regulates the current on the primary side and voltage on the secondary side. The function of the control circuit is to satisfy load requirements and provide Power Factor Correction. The control section has two distinctive parts. The first part controls the unit when the power is being delivered to the load (sourcing). The second part controls the unit when an active load is present and the power gets recycled into the mains (recuperation). Each art of the control section is in control only when needed, which is ensured by utilizing a circuitry that automatically disables itself when conditions for each part to take over are met. 
     The present invention is capable of satisfying all four requirements of (1) providing galvanic isolation between input and output sides of the converter unit; (2) be able to work with an active load (a load that can sink or source energy, such as battery, for instance), by sourcing energy into the load or sinking energy generated by the load; (3) recycle energy when working with an active load by returning the energy into the electrical main of the unit; and (4) provide Power Factor Correction for the line current, regardless if the energy is taken from the mains or recycled into the mains. The output voltage in the present invention effectively has two levels, one during sourcing, and the other, slightly higher one, during recuperation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 presents the global block schematic of the invention; 
     FIG. 2 shows the schematics of the power stage, with all the primary and secondary switches, input inductor and output capacitor; 
     FIG. 3 illustrates a schematic of the sourcing control circuit; 
     FIG. 4 illustrates a schematic of the recuperation control circuit; 
     FIG. 5 shows driving signals for the main and secondary switches during recuperation for positive half-period of the input line voltage; and 
     FIG. 6 shows driving signals for the main and secondary switches during recuperation for negative half-period of the input line voltage. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention solves the above-discussed deficiencies of the prior art. The present invention provides a power converter system  10  capable of sourcing power to a load and also capable of recuperating power from an active load  12  (such as battery, for instance) into at least one source of electrical power (hereinafter the “mains”). The system  10  uses a bidirectional power circuit section  14  and at least two control sections  16 ,  18 . The first control section  16  controls the unit  10  during sourcing of the energy. And the second control section controls the unit  10  during recuperation. A global block-schematic depicting the major components of the system  10  are shown in FIG.  1 . In FIG. 1, the system  10  has mains  20 , the active load  12 , power transformer  22  acting as an isolation element, and a control section  24 . 
     Power section  14  is presented in greater detail in FIG.  2 . First, second, third and fourth bidirectional switches,  26 ,  28 ,  30 ,  32 , respectively, are located on the primary side of the isolation transformer  22 . In contrast, first, second, third and fourth quasi-bidirectional switches,  34 ,  36 ,  38 , and  40 , respectively, are located on the secondary side of the transformer  22 . A first inductor  42  is the input inductor, and an output filter capacitor  44  is shown. The power section  14  illustrated in FIG. 2 is just one embodiment known to those of ordinary skill in the art, since variations of this circuit have been presented in the prior art. 
     The sourcing control circuit  16 , as shown in FIG. 3, is a basic building block of the power factor regulator circuits  10 . As such, circuit  16  has also been presented in the prior art. However, its use in the present invention, in conjunction with the recuperation control circuit  18  (shown in FIG. 4) are, as in inventor&#39;s opinion, unique and at least one inventive step of the present invention. 
     SOURCING 
     The sourcing control circuit  16  (FIG. 3) operates in a controlled manner. An output voltage  46  (positive),  48  (return) is reduced to a lower value by first and second voltage dividers  50 ,  52 . The output voltage  46 ,  48  is then transferred across an isolation barrier  54  to the primary side of the unit. The isolation barrier  54  can be any conventional component such as an optocoupler or high frequency pulse transformer. From the isolation barrier  54 , the output voltage  46  is brought to a negative (inverting) input  56  of a first voltage amplifier  58 . A first voltage reference signal  60  connects to a positive (non-inverting) input  62  of the amplifier  58 . The first voltage reference  60  together with first and second voltage dividers  50 ,  52  determine the output voltage  46 ,  48  of the unit  10 . 
     The first voltage amplifier  58  generates a first output signal  64  that represents an amplified difference between the actual output voltage,  46 ,  48  (reduced by first and second voltage dividers  50 ,  52 ) and a given reference signal  60 —the voltage error signal (VES)  64 . A first input  66  of a multiplier  68  receives the first output signal  64 . While a second input  74  of the multiplier  68  receives an input voltage signal  70  that is transferred to a current by a third resistor  72 . This signal represents the template signal for the input current  78  of the unit  10 . 
     The multiplication component  68  generates a current reference signal  76 , which an input current  78  will follow, that is always in phase with the input voltage  70 , and that has the same shape. Thereby, the unit  10  provides a high power factor (Power Factor Corrected operation). The current reference signal  76  is further used as a reference signal at the positive input  80  of the current amplifier  82 , where it gets compared to the actual, measured input current  78  that is brought to a negative input  84  of the amplifier  82 . An output of the current amplifier  82  is the current error signal  86 , and the signal  86  is fed to one input  88  of a Pulse Width Modulated comparator  90  to generate a Pulse Width Modulated signal  92 . A separate oscillator  94  generates a sawtooth signal  96  which is received by a second input  98  of the comparator  90 . The resulting PWM signal  92  has a pulse width which is proportional to the value of the current error signal. 
     The closed loop circuit of the unit  10  operates in such a way that if the output voltage  46 ,  48  increases for some reason, for example increased input voltage or reduced output load, then the output voltage signal  64  of the voltage amplifier  58  will go in the opposite direction. Multiplication of the voltage signal  64  and the input voltage signal  70  will, therefore, decrease the first multiplier  68  output signal  76  (assuming that AC mains input voltage is constant) and, consequently, the width of the PWM pulses  92  will also be reduced. These pulses  92  directly control the first, second, third and fourth bidirectional switches,  26 ,  28 ,  30 ,  32 . With this control, the pulses  92  reduce the input current  78  and, consequently the output voltage  46 ,  48 , effectively canceling increase in the voltage. If the output voltage  46 ,  48  decreases, the circuit  10  operates in opposite manner, thus increasing input current  78  and output voltage  46 ,  48 , again canceling any disturbances. 
     TRANSITION FROM SOURCING TO RECUPERATION 
     If an active load  12  is connected to the output of the unit  10  and the load  12  starts delivering energy into the unit  10 , the output voltage  46 ,  48  will start increasing. The sourcing control circuit  16  will try to lower the output voltage  46 ,  48 , as described above. Voltage error signal  64  will start decreasing. At some point the VES  64  will reach the level set by a second voltage reference signal  106  created by the second voltage reference  104 . When VES  64  becomes slightly lower than the second voltage reference signal  106  at one input  108  of a shut-down comparator  110 , the comparator&#39;s output signal  112  will become zero and it will pull down the multiplier&#39;s  68  output  76  disabling it effectively. At that instant the PWM pulses  92  will cease and the unit&#39;s  10  output voltage  46 ,  48  will be controlled by the active load  12  only. 
     RECUPERATION 
     With control circuits  16 ,  18  out of function, the output voltage  46  will continue increasing. The output voltage  46  is reduced by fourth and fifth voltage dividers  114 ,  116  in the recuperation control circuit  18  (FIG.  4 ). The output voltage  46  is then directed to a second isolation barrier  118 , same type as described for the first isolation barrier  54 . On the primary side  120  of the isolation barrier  118 , the voltage  46  is fed to an inverting amplifier  122  with a gain of  1 . The inverting amplifier  122  generates an inverting signal  124 . The inverting signal  124  is directed to a first input  126  of a voltage amplifier  100 . In the voltage amplifier  100 , the inverting signal  124  is compared to a third reference signal  128 . The third reference signal  128  is generated by reference voltage  130 . The voltage amplifier  100  receives the third reference signal though a second input  132 . The resulting signal of the voltage amplifier  100  is an amplifier signal  102 . 
     Amplifier signal  102  is transmitted to a first input  134  of a second multiplier  136 . The second multiplier  136  receives at a second input  202  an input voltage signal  170  that is transferred to a current by a third resistor  172 . This signal represents the template signal for the input current  178  of the unit  10 . 
     The multiplication component  136  generates a current reference signal  176 , which an input current  178  will follow, that is always in phase with the input voltage  170 , and that has the same shape. Thereby, the unit  10  provides a high power factor (Power Factor Corrected operation). The current reference signal  176  is further used as a reference signal at the positive input  180  of the current amplifier  182 , where it gets compared to the actual, measured input current  178  that is brought to a negative input  184  of the amplifier  182 . An output of the current amplifier  182  is the current error signal  186 , and the signal  186  is fed to one input  188  of a Pulse Width Modulated comparator  190  to generate a Pulse Width Modulated signal  192 . A separate oscillator  194  generates a sawtooth signal  196  which is received by a second input  198  of the comparator  190 . The resulting PWM signal  192  has a pulse width which is proportional to the value of the current error signal. 
     When the output voltage  46 ,  48  increases, the voltage output  124  of the inverting amplifier  122  will decrease until it reaches a level set by the reference signal  128 . When it falls slightly below the reference signal  128  level, voltage error signal (VES) 102  at the output of the voltage amplifier  100  will increase. When the signal  102  goes above the level set by a fourth reference signal  140 , which is generated by reference voltage  142 , then shut-down comparator  144  will change the comparator output signal  146  from low to high and current reference signal  176  will not be tied to zero through diode  148  anymore. Thereby, the voltage amplifier&#39;s output  102 , multiplied by the input voltage signal  170  will result, similarly to the sourcing control circuit  16 , in increased width of the PWM pulses  192 . In difference to the sourcing control  16 , the recuperation control circuit  18  directly controls first, second, third and fourth quasi-bidirectional switches,  34 ,  36 ,  38 , and  40 , with primary switches being indirectly controlled through a conventional synchronization circuit  200  (shown in FIG.  1 ). Returning to FIG. 4, the increased pulse width will, therefore, transfer more energy being taken from the active load  12 , which will decrease the output voltage  46 ,  48 , and keep it regulated. The Synchronization circuit  200 , used to drive secondary switches  34 ,  36 ,  38 , and  40 , has two different patterns, one for positive half-period of the input voltage as shown in FIG. 5, and one for the negative half-period, as shown in FIG.  6 . 
     If the active load  12  stops delivering energy into the unit  12 , output voltage  46 ,  48  will start decreasing, inverting amplifier&#39;s output voltage  124  will start increasing, voltage error signal  102  will start decreasing, and eventually it will reach the level of set forth by reference signal  140 , which will disable the current reference  176  and PWM signals  192  will cease. The output voltage  46 ,  48  will stay uncontrolled until it reaches the level of reference signal  60  (FIG.  3 ), at which point the sourcing control  16  will become active again and it will start regulating the output voltage  46 ,  48 . 
     While preferred embodiments of the present invention have been disclosed, it will be appreciated that it is not limited thereto but may be otherwise embodied with the scope of the following claims.