Patent Publication Number: US-9887579-B2

Title: Power converter with digital current control circuit

Description:
FIELD OF THE INVENTION 
     The present invention relates to a control system and method for a power converter such as an uninterruptible power supply (UPS). 
     BACKGROUND OF THE INVENTION 
     An uninterruptible power supply (UPS) may be used to provide emergency power in situations where the main power supply fails or performs in an unusual manner. The UPS is designed to switchover in a near instantaneous or instantaneous fashion, due to the nature of the batteries used and the circuitry of the UPS. A UPS can be used to deal with a number of unusual events occurring at the mains power supply, such as: power failure; surge; sag; spikes in the power supply; noise; frequency instability; harmonic distortion; etc. 
     A UPS can be used to protect any type of equipment, however generally a UPS is most often found in computers, data centres, telecommunication systems and any other electrical equipment which could cause serious consequences such as damage to the person or to a business interest if there were no power. 
     A UPS can take many different forms and relate to various different technologies. The most common general categories of UPS are online, line interactive, and standby. Each of these is well-known in the art as are the other alternatives such as hybrid topologies and ferro-resistant technologies. 
     A UPS includes three main power converters. A DC to AC converter is used to supply the load with a clean AC voltage. That DC to AC converter is called an inverter and could be supplied by two different input sources. Firstly, the inverter could be supplied by an AC to DC converter used to convert an AC input source voltage into a DC voltage, which in turn is used to supply an output DC to AC converter. Secondly, a bi-directional DC to DC converter could be used in any circumstance where there is a failure in the input AC source to thus supply the DC to AC converter from the batteries. 
     The three power converters in a UPS are controlled by a control board in order to reach objectives defined for the UPS. One such objective may be summarised as: supplying an output customer load with a high quality voltage, whatever the state of the input AC voltage source, in a secure and safe manner. To achieve these objectives, certain physical signals need to be controlled using regulation algorithms or the like. On each converter, output voltage and driven current are regulated. Generally, all regulation algorithms used to control output voltage and driven currents in a UPS are embedded in the same control board. 
     One problem with using the same control board is that the control board has to be re-designed for any new UPS requirements (power topology, product architecture, etc.). This problem occurs each time there is a new project. The re-use of control boards from older products is difficult due to a high level of reliability expected for the control and UPS specific requirements. 
     In order to design a new product in which controls can be re-usable in future products, regulations or control functions have been classified into two categories. The first category contains regulations common to all UPS product requirements; the second category contains regulations specific to each new UPS product requirement. 
     In order to improve future re-use of control, the two categories of regulation or control function have to be embedded in two different circuit boards. The first is a common re-usable circuit board containing first category regulation (common to all UPSs) and the second is a specific circuit board containing second category regulation (specific to one particular UPS), which will be re-designed for each new product. 
     Generally, the specific circuit boards are of an on-chip design since the number of manufactured units will be low and may only be relevant to one product. 
     The common re-usable circuit board will contain all the common regulations and complex algorithms. As the manufactured number will be high, the cost optimisation will be a lower priority than is the case for the specific board. 
     The present invention provides a way of implementing current regulation in a cost effective manner in a specific control circuit board. 
     A number of proposals have been tried in the past in order to address the problem. One proposal relates to a fixed frequency linear analogue current controller, however this has been found to be difficult to tune, with a limited reliability and flexibility. Another proposal relates to a fixed frequency linear digital current controller. This requires expensive and fast A to D converters which have limited dynamic range. As a result, the solution is expensive and has reduced performance. A third proposal relates to a variable frequency sliding mode controller, with or without a variable hysteresis band. This is not suitable for situations where fixed switching frequencies are demanded. A fourth proposal relates to a fixed frequency peak current controller. However, this gives rise to dynamic error and requires a complex analogue implementation to deal with all the compensation requirements. 
     Generally, current controllers can be classified into two main types, these are linear and non-linear. 
     Linear current controllers compare a current reference with a current measure and compute an error. A linear function, usually comprising gain or integration, is applied to the error and the result of the linear function is provided to a control board to control the current. A binary command signal is generated by the control board to control the output current of an inverter and thereby controls a supply current. The command signal has a pulse width that is altered to control the inverter, but is set to 0 at a regular period and is therefore considered a “fixed frequency”. 
     Non-linear controllers determine the current error in the same way and switch a command control signal between 1 and 0 if the error exceeds certain boundary conditions. If the error is greater than an upper threshold then the command is set to reduce the current, and if the error is less than a lower threshold then the command is set to increase the current. In this case the command signal is considered to be of “variable frequency” as it is switched based only on the threshold values and without any timing considerations. 
     As previously mentioned, hybrid controllers also exist, which use non-linear control with a fixed frequency command. In this case there are two constraints on the command: staying within the error thresholds and the period of the command. An algorithm used in the control board considers the two constraints and decides whether to switch the command signal. 
     The above mentioned controllers are useful in many situations, but they do not address all the problems of the prior art. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to overcome at least some of the problems associated with the prior art. 
     It is a further object of the present invention to provide a current control system for a UPS using a digital controller, such as an FPGA. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and system as set out in the accompanying claims. 
     According to one aspect of the present invention there is provided a current control circuit for a power converter to control the switching thereof, wherein the current control circuit comprises a digital controller using a logical input signal to produce a logical control signal with a fixed fundamental frequency for the power converter. 
     Optionally, the circuit further comprises a current measurement device for measuring the current in the power converter (Imes), which current is compared with a current reference (Iref) to determine a current error value (Ierr), which current error value causes the generation of the logical control signal from the digital controller. 
     Optionally, the current error value is transformed into the logical input signal to cause the generation of the logical control signal by the digital controller. 
     Optionally, the circuit further comprises a digital to analogue converter or a pulse width modulated interface to convert a digital current reference to the current reference (Iref). 
     Optionally, the digital current reference is produced internally in the digital controller, or comes from an external source. 
     Optionally, the current values of the current in the power converter (Imes), the current reference (Iref) and the current error value (Ierr) are analogue values. 
     Optionally, the power converter is adapted to receive the logical control signal and respond thereto by closing or opening one or more switches therein. 
     Optionally, the digital controller is arranged to generate said logical control signal with a timing calculated from the timing of previous transitions of said logical input signal. 
     Optionally, the digital controller is a digital component such as a field programmable gate array or a microprocessor. 
     Optionally, the circuit is for use in power converter control, to control the switching thereof. 
     Optionally, the circuit is adapted to control power converter specific functions. 
     Optionally, the circuit is for use in conjunction with a further current control circuit which is adapted to control common functions for the power converter. 
     The invention further relates to a power converter having a current control circuit as defined in the claims. 
     Optionally, the power converter forms part of an uninterruptible power supply unit. 
     According to a second aspect of the present invention there is provided a method of controlling the switching of a power converter comprising:
         measuring the current in the power converter to determine a measured current (Imes);   comparing the measured current with a reference current (Iref) to determine a logical current error signal;   using the logical current error signal to cause a logical control signal with a fixed fundamental frequency to be generated from a digital controller to thereby control the switching of the power converter.       

     Optionally, the method further comprises transforming the logical current error signal into a logical value to cause the generation of the logical control signal by the digital controller. 
     Optionally, the method further comprises transforming the logical current error signal using a comparator. 
     Optionally, the method further comprises converting a digital reference current to analogue to generate the reference current. 
     Optionally, the method further comprises generating the logical control signal for the power converter to open and close one or more switches therein. 
     Optionally, the logical control signal is generated with a timing calculated from the timing of previous transitions of said logical input signal. 
     Optionally, the digital controller is a digital component such as a field programmable gate array or a microprocessor. 
     The invention provides a reduced analogue interface limiting the environmental sensitivity of the UPS. The digital (FPGA)-based control circuit provides flexibility and a digital current reference transmission. 
     There is no need to use an analogue to digital converter; thereby allowing cost-effective compact solutions that otherwise would not have been available. The fixed switching frequency with local control of the carrier phase in the FPGA offers further advantages. The design and architecture is robust to environmental variations and offers good dynamic and static performances. 
     By maintaining a majority of the functions in the analogue domain and using simple digital components such as zero crossing devices and FPGAs, the current control circuit in accordance with the present invention is relatively inexpensive to produce. 
     In circumstances where different circuits are used for controlling different control functions, the cost issue is a further advantage. The control circuit defined in the claims is useful for controlling power converter specific functions where cost must be optimised due to the relatively small numbers that are manufactured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a UPS topology, in accordance with an embodiment of the invention, 
         FIG. 2  is a circuit diagram of a power inverter topology, in accordance with an embodiment of the invention, and 
         FIG. 3  is a graph showing the current cycles of the  FIG. 2  topology, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The purpose of this invention is to deliver new ways of controlling driven current in the converters of a UPS or other power converter or power application. 
     The present invention proposes a non-linear fixed frequency current controller based on a digital controller, such as an FPGA for switching power electronic converter control. This does not suffer from the problems of the prior art proposals in a number of manners. Firstly, there is a low-cost control implementation and the number of components required to implement the solution is considerably reduced. Secondly, high static and dynamic control performances are achieved in a fixed frequency controller. The invention will now be described in greater detail below. 
     A typical architecture of a UPS is shown in  FIG. 1 .  FIG. 1  shows the batteries  100  and the UPS circuitry  102 . The UPS circuitry includes three converters: an AC to DC PFC (power factor correction) converter  104 , a DC to AC inverter  106  and a DC to DC converter  108  connected to the batteries  100 .  FIG. 1  also shows input 1 which passes through the PFC converter, the inverter and an interrupter  110 . A second input 2 passes through an interrupter  112 . The converter receives charge through a bus  114 . The energy to charge the batteries is taken from the DC bus which means that available power is limited, especially for low voltage inputs. Customers may have to wait several hours for the batteries to be fully re-charged. The typical architecture shown in  FIG. 1  also shows a second input (input 2) which can pass through interrupter  112  to output  116 . As a result, the classic architecture shown in  FIG. 1  has three different sources of energy, the first and second (input 1 and input 2) which may be a common input. The path which is used is determined by the switching of inverters  110  and  112 , as will be described below. The third source of power is the batteries in the UPS. 
     In a first situation, input 1 passes through the PFC converter, the inverter, and interrupter  110  to the output  116 . If either the PFC converter or the inverter ceases to operate, the interrupter  110  closes a switch which causes input 2 to be passed to the output  116 . If both input 1 and input 2 fail, the third source of energy, the batteries, is used and the power passes through the DC to DC converter, the inverter and the interrupter  110  to output  116 . The first and second power sources (input 1 and input 2) can be a common power supply. 
       FIG. 2  shows an example of an inverter and current control system  200  according to the present invention. The system  200  is connected to the inverter  106  which is connected in series with a current measurement device, such as a choke  204 . A capacitor  206  is also connected to the choke so that the current passing through the choke and the voltage across the capacitor  206  can be measured. 
     The circuit also includes a current control circuit  208  which deals with the specific UPS requirements. The current control circuit  208  includes a digital controller in the form of a digital component, such as a field programmable gate array (FPGA)  210 , which passes command signals  212  to the inverter. The FPGA is also connected across the capacitor on one side by means of a zero cross detector (ZCD) also referred to as a comparator to 0 voltage reference (first comparator)  214  on a first line  216 . The first line  216  is connected to the right-hand side of the capacitor and measures the voltage for the three main converters. A second line  218  on the left-hand side of the capacitor measures current and includes a second comparator device  220 . The second comparator device is connected to a filter  224  which is also connected to the FPGA. A digital current reference  226  is introduced into the FPGA from a voltage control loop (not shown) or could be generated inside the FPGA in some cases. The digital current reference  226  (Irefd) passes through a digital analogue converter  228  in the FPGA before passing through the filter to act as an analogue reference  230  (Iref) for the second comparator device  220 . 
     The digital current reference is usually produced in a UPS by a voltage control circuit (not shown). Where a multiple current circuit control architecture is being used, the digital current reference could come from a further current control circuit such as a re-usable circuit (not shown). The re-usable circuit may deal with the so called re-usable or common control functions which are common for all UPS systems and the circuit shown in  FIG. 2  may deal with the specific control functions for the UPS or power converter in question. If the circuit of the present invention is the sole control circuit, the digital current reference will come from a different source or could be generated locally in the FPGA. 
     The FPGA receives a current reference (Irefd) which is either digital or pulse width modulated from a source, such as the re-usable control circuit board mentioned above. The FPGA converts the Irefd into a local onboard analogue reference (Iref) using the digital to analogue converter. Instead of the digital to analogue converter, this function may be performed by a combination of a pulse width modulator and a filter. The current (Imes) is measured on the left-hand side of the capacitor and passed to the second comparator device. The reference current (Irefd) is obtained from the voltage control loop as previously described and is passed through the digital to analogue converter  228  and the filter  224  to produce Iref where it is compared with the current measurement (Imes) at the second comparator device  220 . This enables identification of any current error (Ierr). The error signal is a logical signal. The FPGA uses the logical signal and an internal clock to generate appropriate commands for the inverter and in so doing is able to follow the current reference (Iref) without any static error. 
     The objective of the regulation of the current is to generate appropriate commands for the inverter or other power converter to drive the real choke current (also know as Ichoke or Imes) to be equal to the analogue current reference (Iref). If the regulation is perfect, the remaining error is null or negligible. If this state is maintained for a specific time it is referred to as a static condition. If the error is large, the regulation has not worked correctly and the appropriate commands may be updated. 
     The commands generated by the FPGA for the inverter are logical commands (1 or 0), asking the inverter switches (IGBTs) to be closed or opened. The number of generated commands depends on the power converter topology and the number of switches. 
     The voltage measure is required to optimize the current regulation in some power converter topology controls, where the voltage sign is required. This is the case with topologies referred to as three level power topologies. 
     Referring now to  FIG. 3 , a graph of current against time for Imes and Iref is shown. The x-axis is the time axis and the y-axis is the current measurement axis. The FPGA has an internal clock signal (Tclk)  300  which is at a fixed fundamental frequency. The variation in Imes (also known as Ichoke) is shown. This is the current passing through the choke in the circuit of  FIG. 2 . The graph also shows the variation in a logical error signal  302  and the command signal  304 . 
     At a first step, the FPGA internal clock rising edge activates a switch command at t=0. An internal counter captures the value to determine the value of t1. The counter can then be reset and the analogue reference is updated. 
     At a second step, when the current increases and crosses Iref at t2, the error current (Ierr) becomes negative and a zero cross signal falling edge appears. The internal counter value is measured and thus defines the value of t2, which is then stored. The time t3 is then computed from the values of t1 and t2 with a crossing factor (CF) which is a function of t3 and t4 from a previous cycle. The crossing factor and the manner in which time t3 is calculated, using an algorithm, are described below. 
     At a third step, when the counter reaches a value corresponding to t2 plus t3 the command signal to the inverter is deactivated and the current starts to decrease again. 
     At a fourth step, when the current crosses the reference current (Iref), the counter value is captured to define a value for t4 which is also saved in an appropriate location. The crossing factor (CF) is calculated using the saved values of t2, t3, and t4. 
     At a fifth step, the next clock rising edge of the FPGA internal clock will cause the process to return to the start of the first step. 
     As previously mentioned, CF and t3 are calculated by a core of the control algorithm. In order to limit the static error and so obtain a choke current (Ichoke or Imes) centred on Iref on the falling slope, it is necessary to arrive at a situation where t4=t1. From that situation, the equations for CF and t3 can be defined. In order to achieve this, CF is defined as t3/t4. CF gives the ratio between crossing and falling slopes of the current and is calculated in step four. By using the value of CF at t2, it is possible to define t3 as follows: 
     
       
         
           
             
               t 
               ⁢ 
               
                   
               
               ⁢ 
               3 
             
             = 
             
               
                 CF 
                 
                   CF 
                   + 
                   2 
                 
               
               ⁢ 
               
                 ( 
                 
                   Tclk 
                   - 
                   
                     t 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                 
                 ) 
               
             
           
         
       
     
     Once the crossing factor has been calculated, it is used by the FPGA to determine the appropriate control signal for controlling the inverter. As a result, the FPGA controls the supply current of the UPS. The control of the converter in this respect relates to the current control functions provided by the present invention. Other current control functions may be carried out by different current controllers depending on whether the functions are common for all UPS systems or specific to one UPS. 
     The current control circuit  208  of the present invention is a part of the overall current control system  200 . The overall current control circuit may include a further part: the re-usable common current control circuit (not shown). 
     The current control system and/or the current control circuit of the present invention may be used in any power conversion application or any power conversion product. 
     It will be appreciated that this invention may be varied in many different ways and still remain within the intended scope of the invention as defined in the claims.