Control a three-phase full-wave rectifier with an FPGA
If your project has big-time power, you might want to consider using an FPGA to control your three-phase full-wave bridge rectifiers.
Three-phase fully controlled bridge rectifiers are well suited for several power electronics applications that require a large current such as brushless excitation systems for aircraft generators, high-voltage AC-to-DC converters, and DC-motor speed-control systems. Three-phase controlled rectifiers are preferred over single-phase rectifiers for these types of uses because the three-phase rectifiers can handle more current and output a smoother voltage than single-phase rectifiers. Therefore, small-sized filters can be used to produce pure DC, higher efficiency, and better use of the transformer.1, 2
The rectifier we constructed is a three-phase fully controlled bridge rectifier. Specifically, this rectifier converts (rectifies) into full-wave using a six-diode bridge. A user can control aspects of the rectifier's firing (hence the name "fully controlled") via a silicon circuit, in this case an FPGA.
Conventional three-phase full-wave rectifiers are based on either fixed output voltage or analog firing control. Silicon-controlled rectifiers (SCRs) have made it possible to vary the output voltage by varying the rectifier's firing angle and enabling digital firing control. Digital firing control3 has many advantages over an analog firing circuit as digital circuits are:
- Not much affected by noise
- Able to repeat their performance
- Easier to use on PCs and digital panels, thanks to a user interface
- Able to act as storing facilities
- Less likely to fail
Implementation of UART logic in an FPGA and an external MAX232 driver has enabled a GUI on the PC for testing and debugging. We developed our GUI using National Instruments LabVIEW software.4
Why use an FPGA
The advantages that FPGAs have over ASICs, microprocessors, and microcontrollers motivated us to opt for an FPGA in this application. FPGAs have come a long way from mere glue-logic applications of interconnecting discrete components to high-performance reconfigurable signal processors. DSP developers and design engineers know that FPGA-based processing outperforms conventional processors on a board-for-board comparison, resulting in improvements in processing speed, size, weight, power consumption, supply voltage, and costs.5
FPGAs are relatively fast. They can operate at frequencies in excess of 250MHz, whereas the clock frequency for general-purpose microprocessor/ microcontrollers commonly found in embedded applications is just 4 to 50MHz. FPGAs are also cheap: low-capacity FPGAs cost around $15 U.S. To reach the performance needed by this application, your FPGA would approach the $50 neighborhood of a processor with similar performance. Compared with basic microcontrollers that have built-in analog-to-digital converters, CAN, and other features, the FPGA seems resource-efficient for our application. Also, embedding soft processors in an FPGA offers a radical yet stable way to effectively eradicate the problem of processor obsolescence. The reconfigurability of the FPGA is a great advantage. Coding, compilation, simulation, and testing is more straightforward with an FPGA than with a microcontroller.
For our rectifier scheme, we used a Xilinx Spartan-series reconfigurable FPGA operating at 50MHz. We put a user-friendly digital control panel and display screen in the front so we could set the required output DC voltage. The FPGA-based electronics circuitry sits on a compact printed circuit board, and the scheme has a portable DC power-supply source that can handle increased current. Figure 1 shows an overview of the three-phase fully controlled bridge rectifier. In the circuit, an RS232 port is also available for connecting the rectifier to a PC; this interface enables you to debug and test the system and set voltage from the PC.
The block diagram in Figure 2 containing the hardware architecture of our three-phase fully controlled bridge rectifier helps illustrate how the rectifier functions.
A three-phase mains supply is provided to the unit. The user then sets the required DC voltage from the digital control panel using a display screen and the keypad shown in Figure 1. The unit interprets the voltage levels set based on the user's digital inputs from the keypad. The zero-crossing detector (ZCD) circuit produces pulses at zero crossings of the input R, Y, and B signals. The output voltage Vo varies by varying the firing angle α of the SCR (silicon-controlled rectifier). The line voltage VRY is described in Equation 1, where Vm is the peak value of the phase voltage.
In the case of resistive load, the average output voltage is given in Equations 2 and 3. For inductive load, Equation 2 alone holds good for the complete range of α.
The firing angle (α) of the SCR is in turn controlled by voltage VC, which is the output of a proportional integral (PI) controller. The inputs to the PI controller are voltages Vset representing the desired output voltage and the actual output voltage Vo of the bridge circuit. If Vo is less than the desired output voltage, the resultant error causes the output, VC, to increase, which in turn should advance the firing angle. As the firing angle is advanced, the output voltage of the bridge circuit increases. Vbr is the bridge output voltage. An inductor in the DC link reduces ripple in the output current of the bridge circuit, whereas the capacitor absorbs the ripple in output voltage. The inductor is designed such that it doesn't saturate even when it carries the maximum current; in other words, it has an air gap in the path of flux. The ripple current through the capacitor is also significant. Hence, for this application, an electrolytic capacitor is chosen that has the required ripple current rating. The differential equation that describes the DC-link inductor current is Equation 4:
Equation 5 expresses the capacitor voltage:
Equations 6 and 7 are related to closed-loop control. Let the output of the PI controller be VC(θ), where θ=ωt; VC(θ) is expressed by Equation 6:
In Equation 6, A is a constant to be evaluated, K is the proportional gain of the controller, and T is its time constant. Equation 6 is differentiated and represented as Equation 7, which is more convenient for use in simulation.
In Equation 7, Vset is the desired output voltage. The output of the controller is normally checked to ensure that it's within the set limits. From the output of the controller, the firing angle, α can be obtained. The maximum output voltage of the controller should correspond to a zero-degree firing angle and the minimum to a 120° firing angle. Hence we get Equation 8 for the firing angle:
This means that the range for VC(θ) is from 0 to VCmax. Figure 3 shows the waveforms for the three-phase fully controlled bridge rectifier.
Our software design process for this SCR involved using VHDL (VHSIC Hardware Description Language) for designing and programming the FPGA.6, 7 We designed the system using VHDL and Xilinx's ISE 5.1 Synthesis Software to create the floorplan and generate the bit format.8 We simulated the digital control logic using Xilinx's partner-tool ModelSim 5.8 Simulator. A JTAG port was provided in the FPGA for in-situ programming of the FPGA. Figure 4 shows the digital firing-control logic for our FPGA-based rectifier. The desired voltage output is set either by using the keypad and display control panel or by using the testing and control software through a GUI on a PC. The PC is connected to the system via the RS232 COM port.
We used a hardware switch to enable users to select the mode of control. In either mode (PC or keypad control), the output voltage is sent in digital format over the RS232 port to a PC so a user can monitor the rectifier's performance if needed. The digital keypad-control panel is connected to the FPGA controller board as a digital input with control signals On, Off, Start, Stop, and Clear. Hence users can set the required voltage using the digital control panel and view it on the display screen. The FPGA controller first performs keypad decoding and interprets the equivalent voltage using lookup tables. The set voltage is displayed on the display screen. The controller then computes the error input for the PI controller9 using the set voltage value and the actual output voltage that's fed back to the board through an analog-to-digital converter. Digital PI control10 logic is implemented in the FPGA to generate the control voltage from the resultant error. The control voltage is then fed to the firing-angle (α) converter logic, which computes the firing angle equivalent to the control voltage. Pulse width modulation (PWM)11 logic is implemented to generate the firing pulse duration for the SCR firing logic; finally, the SCR bridge circuitry fires.
Each function in the software code design is created as a subcomponent and called by the main function. The analog equivalent of this system has more circuits and uses some of the logic steps more than once. By using FPGA-based digital control, we can achieve flexibility and simulation facility employing the VHDL code and software. As a whole, we used the memory efficiently and reduced code complexities. FPGA code verification and analysis was done using Xilinx software.
The two major tasks involved in creating our three-phase fully controlled bridge rectifier were:
- Simulating and testing the proposed algorithms
- Establishing an interface with the PC for debugging, monitoring, and controlling the system
We used ModelSim software, Xilinx's partner tool, to simulate the FPGA's digital logic. We simulated and analyzed the control system and power electronics using MATLAB. The GUI software, shown in Figure 5, was developed using National Instruments' LabVIEW.12, 13
The application software helps the user debug, control, and monitor the output voltage. The user sets the required voltage in the Set Voltage control field and designates a baud rate and other parameters for the serial RS232 COM port. PC-control and keypad-control modes are shown as two LED indicators on the application screen. The circuit's hardware-switch input selection decides the control input, and the corresponding LED glows in the application screen. In the case of keypad control, the GUI software only receives the output voltage in digital format and is plotted in the graph and displayed as an indicator. The keypad-control LED glows in this case. For PC control, this GUI software transmits the required set voltage and also receives the output voltage with the PC-control LED glowing as shown in the application screen.
Because this design enables the flexibility to make routine changes and also improves the current control-logic scheme, we think an FPGA-based SCR is more advantageous than the conventional analog-controlled rectifier. Our proposed control routine is functionally checked by the simulated result for the digital control logic implemented in the FPGA. This scheme is quite simple and can be used for several other applications in the field of power electronics.
Manikandan Jayachandran has a BE in electronics and communication from Madras University and an ME in communication systems from Regional Engineering College in Trichy, India. He has been working as a scientist at Aeronautical Development Agency, Bangalore, India since 2002. His areas of interest are digital design and processor-based systems. You can reach him at firstname.lastname@example.org.
Maruthia Jayachandran has a BE in telecommunication from Regional Engineering College in Trichy, India, and an ME in radar and microwave from IIT Roorkee. He has worked at Research Centre Imarat, Hyderabad as head of the compact antenna test range. He also has served as a professor in the ECE department in Vellore Institute of Technology and as the head of ECE department in NHCE, Bangalore. He is currently head of communication at the Eritrean Institute of Technology, Asmara, Africa. He can be reached at email@example.com.
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