Design of SPWM Frequency Conversion System Based on FPGA

Since pulse width modulation technology achieves the effect of voltage and frequency conversion of output voltage by adjusting the frequency and duty cycle of output pulses, it has been increasingly widely used in many fields such as motor speed regulation and inverters.

As an effective method of geophysical detection, electromagnetic method has been widely used in mineral exploration, geological disaster prediction and other fields. Electromagnetic instruments generally consist of two parts: transmitter and receiver. At present, the transmitter part of electromagnetic instruments generally directly adopts equal-width PWM technology, which has a large current harmonic distortion rate, low voltage utilization rate and low efficiency.

This paper uses FPGA technology and the principle of SPWM natural sampling to design an SPWM system for the transmitter of electromagnetic instruments. This system is applied to existing electromagnetic instruments and compared with the effect produced by the original PWM, and good results are obtained.

1 SPWM Technology Principle

The principle of SPWM signal is to compare a set of isosceles triangle waves with a sine wave, and the intersection point is used as the moment when the switch tube is "on" or "off". There are many ways to generate SPWM signal, such as harmonic elimination method, equal area method, sampling method, etc.

The method of using the intersection of the sine wave and the isosceles triangle to determine the switching mode of the switch tube is called the natural sampling method. It can be divided into unipolar triangle wave modulation method and bipolar triangle wave modulation method, and its principle diagram is shown in Figure 1. This article uses the bipolar modulation method.

2 Hardware Implementation of SPWM System

2.1 Overall system design

The system principle is shown in Figure 2. The system first generates a triangular wave signal and a sine wave signal, generates a pulse sequence by comparing the outputs of the two, and performs dead-time delay and digital filtering on the output pulses. The main modules are: frequency divider, triangular carrier generator, sine function table addressing, sine function table, dead-time delay module and digital filtering module.

2.2 Triangular Carrier Generator

In this design, a carrier triangular wave is generated by an addition and subtraction counter, and the amplitude of the triangular wave is 256. First, count from 0 to 256, then subtract from 256 to 0 to get a half-cycle of the triangular carrier, and then repeat the counting method of the first half cycle, and negate the obtained count value, so that a cycle of the triangular carrier can be obtained.

Figure 3 is a simulation diagram of the triangular carrier module. The frequency division of the triangular wave can be achieved by setting the value of triwave_fp. When the system clock is 10 MHz, Figure 3(a) sets triwave_fp to 0, and the period of the triangular wave is 102.4 μs; Figure 3(b) sets triwave_tp to 1, and its period becomes 204.8 μs. By changing the value of triwave_fp, carriers of different frequencies can be obtained.

2.3 Sine Wave Generator

This design uses Matlab software tools to divide the positive half-cycle sine wave into 512 equal parts and store the data in ROM. By calling the data in ROM, the positive half-cycle sine wave can be realized. Then, the value of the negative half-cycle can be obtained by negating the positive half-cycle. In order to make the pulse width adjustable, this design adds a sine amplitude multiplication adjustment module, and its module schematic diagram is shown in Figure 4.

Similarly, the module frequency division unit and amplitude modulation unit can be controlled to change the frequency and amplitude of the sine wave.

2.4 Comparison Module

After the triangular carrier and sine reference wave generation modules are designed, their output results are compared to generate SPWM pulse signals. This can be implemented using the Verilog hardware description language, and the comparison rule is set to output '1' when the carrier value is less than the sine wave function value, otherwise it outputs '0'.

2.5 Dead Time Delay Module

After the comparison module, two SPWM sequence signals (xl, xh) are obtained to control the switches of the upper and lower bridge arms of the circuit. In theory, these two signals are completely complementary. However, since the turn-on and turn-off time of the power device are not completely equal, the turn-off time of the device is actually longer than the turn-on time. Therefore, in order to avoid transient short circuits of the power devices on the upper and lower bridge arms, a delay must be provided to ensure that the corresponding switch tube has been turned off before the power switch tube is turned on.

The pulse delay is realized by the rising edge, and the delay time is mainly realized by a 10-bit up-down counter. Assuming the dead time is max, the counting rules of the delay counter are as follows:
(1) When the input is 0, if the count value is equal to 0, the count value remains unchanged; otherwise, it is counted down by 1;

(2) When the input is 1, if the count value is equal to max, the count value remains unchanged; otherwise, the count is increased by 1;

(3) When the input is 1 and the dead zone counter value is max, xl=0, xh=1, and the upper bridge arm is turned on;

(4) When the input is 0 and the dead zone counter value is 0, xl=1, xh=0, and the lower bridge arm is turned on;

(5) When the dead zone counter value is between 0 and max, xl=0, xh=0, and both the upper and lower bridge arms are cut off, forming a dead zone.

2.6 System Simulation

Finally, you can set the clock, frequency division, dead time, etc. as needed. The design is simulated, and the triangle wave frequency is set to 5 times the sine wave frequency. The simulation results are shown in Figure 5.

Observing the output signals xh, xl in FIG5 , it can be seen that the pulse width varies according to the sinusoidal law, so the design meets the requirements.

2.7 Filter Module

Since there are inevitably many interferences in the data collection process, the effective information is covered by them, so the data must be processed by digital filtering to improve the signal-to-noise ratio. In order to increase the speed of research and development, the filtering module is directly generated using the IP core of Altera.

By setting parameters and designing a digital filter with a passband frequency of 7.5 to 12.5 kHz, the Hanning window design structure is adopted, and the digital filter design and analysis tool of Matlab software is used to obtain the frequency attenuation diagram as shown in Figure 6. It can be seen that the passband effect is obvious and meets the system requirements.

3. Application of the system

The designed SPWM system is applied to an electromagnetic instrument designed by a company. Its main principle is to use special equipment to transmit an electromagnetic field to the dielectric body. This rapidly decaying magnetic field induces a new secondary field in the surrounding medium. Using this principle, the instrument is designed with one transmitting channel and three receiving channels. Figure 7(a) is the result obtained by the original instrument using a PWM wave with a transmitting frequency of 9.8 kHz. The first four channels are waveforms before filtering, and the last four channels are waveforms after filtering. The transmitting frequency of this system is controlled to 9.8 kHz for debugging, and the data of the transmitting channel and the receiving channel are uploaded to the host computer through serial communication for display. The waveform is shown in Figure 7(b). After filtering, the transmitting channel produces a relatively ideal sine wave. The three secondary fields produced can be compared with Figure 7(a) to show that the harmonic distortion is significantly reduced.

4 Conclusion

This paper designs an SPWM variable frequency system based on FPGA, and finally successfully applies the system to the transmitting module of the electrical instrument. It has been verified that the system is stable and reliable, and has a great improvement over the original PWM control. In addition, the system can modify the values ​​of the transmitting frequency, dead time, etc. online as needed, making the system more user-friendly. With a slight modification, the system can also be applied to motor drives or variable frequency power supplies.