As the world moves towards greener solutions for fulfilling its energy needs, power electronic devices have become an intrinsic necessity. For the proper switching of these devices, digital control is preferred. This is due to the overwhelming advantages offered by digital devices. Usage of these equipments results in greater flexibility and reduced complexity as compared to analog methods. Moreover, the implementation of very intricate strategies can be handled with ease and executed in real time.
For generating digital control signals the choice is between three families. The first of these are microcontrollers. A microcontroller is an integrated chip that contains programmable peripherals, a processor and a memory. The second group of devices are the digital signal processors. A DSP is a specialised semiconductor intellectual property core block. Its architecture is optimised for fulfilling the operational needs of mathematically manipulating an information signal. The third choice pertains to the Field Programmable Gate Arrays. An FPGA is a collection of combinational and/ or sequential logic blocks that are interconnected via reconfigurable links. Hence these devices have the facility of being reprogrammed again and again by the end user.
Linear Pulse Width Modulation
A popular strategy for controlling the voltage of converters is pulse width modulation. In this method, the amount of power sent to the load is varied by turning a switch on and off. Greater is the time duration for which the switch is ON, more is output power.
As a control strategy, PWM can be easily implemented by comparing a high frequency saw tooth or triangular waveform with a set DC level. By varying the level of the DC, the magnitude of the output can be changed.
Fig. 1: PWM using saw tooth wave…
Comparison OF Devices
For implementing the above method, any of the three families can be put to use. In terms of cost, a microcontroller will be the cheapest option. Moreover, as compared to the other two devices, it is the easiest to code. However, since it can only execute statements serially, implementation of more complicated methods like Sine Pulse Width Modulation (SPWM) will not be possible.
A DSP can be used for furnishing the desired output. But its cost will be greater or equal to that of an FPGA and much more than that of a microcontroller. Additionally the coding will be typical if not difficult. However, it has enhanced facilities and built in templates for generation of a plethora of schemes. But hard coded architecture limits its functionality.
An FPGA has the inherent quality that facilitates the end user to recode its architecture. This is a huge advantage as compared to DSPs. Furthermore, FPGAs have the resource of meting out massive parallel treatment. Consequently, even complex codes can be executed in real time at a very fast rate. The cost as compared to microcontrollers is more. With advances in technology and the availability of a wide range of products to choose from, FPGAs are making rapid penetration into the digital device market. The cost being comparable to that of DSPs, coupled with the increased degree of freedom, make FPGAs a versatile gadget for control engineers. So using FPGAs for implementing PWM will be a fruitful option.
Pwm Generation Algorithm
For generating saw tooth PWM in a programming language, three variables have to be used. The saw tooth is approximated by using one variable as a counter. The DC level data is obtained by assigning a value to the other variable. These two variables are compared and the result is stored in a third variable. Whenever the DC level is greater than the counter value, the third variable is assigned one; otherwise it is assigned zero. By varying the value stored in the second variable, the third variable output can be altered.
The frequency of PWM is obtained either from the clock of a dedicated device or by the use of an external oscillator.
The algorithm stated above can be modified and used for the generation of control signals of a three phase Voltage Source Inverter (VSI). The inverter is made to operate in the 180° and 120° mode.
Three Phase VSI
Fig. 2 Three Phase VSI…
Fig. 2 illustrates a three phase VSI. Among its various modes of operation are the 180° and 120° mode.
A. 180° Conduction
The switches of one leg are fired 180° apart. The upper/ lower switch of a leg and the respective subsequent switch is fired 120° apart. Switching frequency is 300 Hz for an output voltage of 50 Hz. Fig. 3 shows the line voltages of the three phases for a VSI in 180° mode.
Fig. 3: Line voltages of the three phases for a VSI in 180o mode
B. 120° Conduction
The switches of one leg are fired 180° apart. Each switch conducts for 120° . There is an off period of 60° for each switch. The upper/ lower switch of a leg and the respective subsequent switch is fired 120° apart. Switching frequency is 300 Hz for an output voltage of 50 Hz. Fig. 4 illustrates the line voltages of the three phases for a VSI in 120° mode
Fig. 4: Line voltages of the three phases for a VSI in 120° mode…
Platform And Kit Used
For coding, Verilog Hardware Definition Language is used. It is a language that facilitates the designing of digital logic chips. The hardware on which PWM is implemented is a Spartan 3E FPGA kit. This series provides a cost sensitive functionality. Fig. 5 shows a snapshot of the kit.
Fig. 5 Spartan 3E FPGA…
Result
Fig. 6 PWM output…
Fig. 6 illustrates the PWM output obtained on Spartan 3E FPGA for a duty cycle of 0.5 and a frequency of 5 KHz.
By varying the variable values, the duty cycle can be changed accordingly.
The frequency is set by assigning a counter against the inbuilt FPGA oscillator.
For the kit used, the output can be obtained till a frequency of 50 MHz.
Fig. 7 and fig. 8 are the output voltage of the three phase VSI operating in 180° and 120° mode respectively.
Fig. 7: Line voltage output for a three phase VSI in 180° mode of operation…
Fig. 8 Line voltage output for a three phase VSI in 120° mode of operation…
The laboratory prototype has been constructed using Insulated Gate Bipolar Transistors (IGBT) of FGA15n120 series. The control signals obtained from the FPGA have been isolated and amplified using TLP 250 driver IC. The actual hardware output obtained bears a strong co-relation with the simulation results.
Conclusion
A comparative analysis of the different means of generating digital control signals has been touched upon this article.
The advantages of FPGAs over DSPs and microcontrollers have been discussed.
Analysis of linear PWM and its implementation on a Spartan 3E FPGA kit using Verilog platform has been demonstrated.
This method can be put to use – for quick and easy construction of power electronic converters.
