Electromagnetic compatibility design of DSP digital control system

JasonYoo

Electromagnetic compatibility design of DSP digital control system [Copy link]

Abstract: Fully digital control has become an inevitable trend in the development of modern power electronic equipment control methods. In order to improve the accuracy and reliability of the control system, perfect electromagnetic compatibility design has become a crucial factor. The electromagnetic compatibility design of digital control systems is elaborated in detail. Digital control is conducive to improving the anti-interference ability of the control system and enhancing the stability and reliability of the control system.
Keywords: electromagnetic compatibility; filtering; anti-interference; digital control

EMC Design of Digital Control System Based on DSP
ZHONG He-qing, ZOU Yun-ping, XU Zhi-xin, CHAO Ze-yun
(Huazhong University of Science and Technology, Wuhan 430074, China)

Abstract: Using full digital control in modern power electronics devices has become the inevitable trend. In order to enhance the accuracy and reliability of the control system, the consummate design of EMC is very important. This paper expatiate the design of the digital control system's EMS in detail. It can not only improve the immunity but also can enhance the stability and reliability of the control system.
Key words: EMC; filter; immunity; digital control

0 Introduction
With the continuous development of control technology, digital control has become an inevitable trend in the development of power electronic equipment control systems. Only by adopting digital control systems can system indicators be better achieved and more complex functions be completed. At the same time, digital control can also improve system reliability and increase control flexibility, and can reduce costs and reduce size. At present, in terms of power supply and motor control, the core control chip of digital control mostly adopts TI's motor control dedicated chip TMS320C24X series chips. This series of control chips is a 16-bit fixed-point DSP core, which provides a digital solution for analog control designers without sacrificing the accuracy and performance of the original system. In fact, the system performance is enhanced by the use of advanced control algorithms such as adaptive control, Kalman filtering and state control. This series of DSP controllers integrates real-time processing capabilities and controller peripheral functions, providing an ideal solution for control systems. These digital and mixed signal peripherals include: timers, serial ports (SCI, SPI), analog-to-digital conversion (ADC), event managers, system protection (such as low voltage detection and watchdog timer), etc. Power electronic equipment combines strong and weak electricity closely together. However, due to the great difference in power, frequency and level between the strong and weak electricity parts, the weak electricity part has a low level, high frequency, high sensitivity and is very sensitive to electromagnetic interference signals. The electromagnetic interference of the strong electricity part may cause malfunction of the control equipment or even damage components. Therefore, suppressing electromagnetic interference has become an issue that must be considered in the design and application of control circuits, and it has also become one of the key factors for the reliable operation of control systems. This paper proposes an electromagnetic compatibility design method for the TMS320C24X control system. It includes circuit design to reduce electromagnetic interference of the TMS320C24X DSP, circuit board design, and the impact of the control scheme on electromagnetic compatibility.

1 Main sources of EMI and countermeasures for digital control circuits
The electromagnetic compatibility of electronic circuits depends largely on the designed components and the way they are connected to each other. The feedback signal line will form an antenna, generating electromagnetic energy radiation, the amplitude of which is determined by the current amplitude, frequency, and area of the current loop. There are three main sources of EMI in the control circuit: power lines, high-frequency signal lines, and crystal oscillator circuits.

(1) Power supply:
At any time, when a CMOS device changes its output state, the two complementary transistors will commutate, resulting in an increase in the power supply current. This peak current will pass directly through a more or less power line loop. Practice has shown that this peak current will cause very considerable electromagnetic interference.

Connecting a 100nF ceramic chip bypass capacitor to the power pin is a good decoupling method. However, circuit parasitic elements, such as the package pin impedance and the power line, will form an equivalent antenna. The bypass capacitor cannot completely and effectively reduce the current spike, and therefore, it cannot eliminate interference. In order to suppress the current spike, especially the current spike on the power line, so that the current spike does not affect other components, it can be improved by adding a core inductor (ferrite ring core) between the decoupling capacitor and the power line. The core inductor should be close to the integrated chip, which can reduce its interference.

(2) Signal line

Signal lines with high-frequency signals, low-bit address lines, clocks, serial ports, etc. are usually connected to CMOS input terminals, which are equivalent to a load of several 100kΩ resistors and 10pF capacitors in parallel. The charging and discharging of this load will generate spike currents, forming electromagnetic interference. A possible way to reduce this current is to connect a resistor of about 50Ω in series at the output. Transmission line theory shows that as long as the output resistance (internal resistance + external resistance) is less than or equal to the line impedance (its typical value is 70Ω~120Ω), this resistance has no negative impact on speed.
The second preventive measure is that the antenna (the return line of the signal and its response) should be as small as possible. The most effective and simplest method is that the key lines should be as short as possible. Such as clock lines, bit address lines, and other data lines. The TMS320C24X CPU clock is turned off by CLKOUT1 after reset. When this pin is not used in actual applications, it is recommended to turn off CLKOUT1. When there is no external memory, these pins are connected to pull-up or pull-down resistors to avoid internal interference caused by internal currents caused by floating input terminals.

(3) Crystal oscillator

In digital control systems, high-frequency oscillators usually appear in clock generators. When using a crystal oscillator in combination with the internal crystal oscillator of C24X, in order to reduce the EMI of this part, the area enclosed by the current path of this part should be reduced. Since the crystal oscillator has a high impedance (about 100kΩ) characteristic at the resonant frequency, the current at the resonant frequency point of the crystal oscillator is very small. However, the output voltage is the output voltage of the CMOS converter, which is a square wave signal containing a large number of harmonics. Therefore, the crystal oscillator is no longer high impedance, which will lead to a large harmonic current. The series resistor can reduce the corresponding current. The series resistor Rs should be within the range of 1kΩ.
At the oscillation frequency, the two bypass capacitors provide low impedance characteristics, so a considerable current flows through the bypass capacitor. At the same time, in order to reduce electromagnetic radiation, the area enclosed by this part of the circuit should be as small as possible.

2 Common anti-interference measures for digital control circuits [2][3]
(1) Inductor and capacitor filtering is used at the input end of the AC power supply to remove high-frequency and low-frequency interference pulses.
(2) The transformer adopts isolation measures. The primary input end of the transformer is connected in parallel with a capacitor, and the shielding layer between the primary and secondary coils is connected to the ground. This is a key means of hardware anti-interference.
(3) A low-pass filter is added to the secondary of the transformer to absorb the surge voltage generated by the transformer.
(4) Use an integrated DC regulated power supply because they usually have overcurrent, overvoltage, overheating and other protections. It is recommended to use a single-ended flyback power supply because the working principle of a single-ended flyback power supply is to first store the input energy in the transformer and then transfer the energy to the secondary side. Therefore, the differential mode interference at the input end of the power supply has no effect on the secondary side, so its ability to resist external electromagnetic interference is relatively strong.
(5) The I/O port is isolated by photoelectric, magnetic, and relay isolation, and the common ground wire is removed.
(6) Use twisted shielded pair cables for communication lines to eliminate parallel capacitor interference. Optical fiber should be used in strong interference environments. For example, optical fiber isolation is most effective in lightning protection environments.
(7) The power supply device housing is connected to the ground to ensure personal safety and prevent external electromagnetic interference.
(8) A reset voltage detection circuit is added to prevent the CPU from working due to insufficient reset, especially for circuits containing EEPROM devices. Insufficient reset will change the content of the EEPROM.
(9) Use fewer separate gate circuits and more highly integrated components to replace separate gate circuits. This will help reduce mutual interference and oscillation caused by more external connections.
(10) Printed circuit board anti-interference process
① Thicken the power cord; reasonably route and ground; separate the three buses to reduce mutual inductance oscillation.
② For major chips such as CPU, RAM, ROM, etc., electrolytic capacitors and ceramic capacitors are connected between VCC and GND to remove high and low frequency interference pulses.
③ Adopt an independent system structure to reduce plug-ins and connections, improve reliability, and reduce failure rate.
④ The integrated block has reliable contact with the socket. Use a double-spring socket or a military socket. It is best to solder the integrated block directly on the printed circuit board to prevent poor contact failures.
⑤ If conditions permit, use a printed circuit board with more than four layers, with the middle two layers being the power layer and the ground layer.

3. Electromagnetic interference shielding technology of control system
In power electronic equipment, due to the existence of severe di/dt, electromagnetic interference will be generated on the surrounding control circuit, causing abnormal operation of the control circuit. The larger the di/dt, the stronger the electromagnetic interference. In addition, transformers are often used in power electronic equipment for voltage conversion or isolation. Due to the leakage of the transformer, magnetic field interference will be formed in the surrounding area. Therefore, in order to ensure that the control circuit is not interfered, the control circuit needs to be shielded by electric field and magnetic field.

4 Software anti-interference design
1) Use more queries instead of interrupts to reduce interrupts to a minimum. The length of the interrupt signal connection should not exceed 0.1m to avoid false triggering and inductive triggering.
2) A/D conversion uses digital filtering to prevent sudden interference. For example, use the average method, comparative average method, etc.
3) Set a watchdog in the key places in the software so that even if the software goes wrong, it can start from the beginning.
4) Delay and de-jitter the input switch signal.
5) For the correct operation of the I/O port, the port execution command status must be checked to prevent external faults from not executing the control command.
6) Communication should add parity check or use query, voting, comparison and other measures to prevent communication errors. If necessary, reset the communication register settings to prevent communication errors from causing communication failures or other faults.

5. Selection of control schemes to resist electromagnetic interference
According to electromagnetic interference theory, the commonly used power waveforms, in terms of their electromagnetic interference, are Gaussian waveform, cosine wave, critical damping exponential wave, trapezoidal wave, sawtooth wave, triangle wave, and rectangular wave. Therefore, the use of soft switching circuits can reduce the electromagnetic interference of the main circuit to the control circuit. The use of space vector PWM or non-constant PWM generators can also reduce electromagnetic interference.
After the PCB is completed, the PWM unit of C24X can further reduce EMI by optimizing the switching mode. Take the three-phase DC full-bridge inverter circuit as an example.
There are three typical PWM modes: symmetrical, asymmetrical, and space vector. Different PWM modes have different effects on EMI radiation. All PWM modes are supported in the C24X PWM unit.
The electromagnetic interference generated by du/dt and di/dt of the symmetrical PWM control scheme is only about 66% of that of the asymmetrical PWM control scheme. The electromagnetic interference generated by the space vector PWM control scheme is reduced by 30% compared with the symmetrical PWM [1].
Research has shown [1] that when the PWM frequency is constant, the EMI and its harmonics generated by the circuit are very large. Slight changes to the PWM frequency can significantly reduce EMI, such as using a triangular carrier to randomly change the frequency within a certain range.

6 Conclusion
Whether the magnetic compatibility design is reasonable is directly related to whether the digital control system can work safely and reliably. Power supply designers should conduct comprehensive electromagnetic compatibility design based on the system's working environment, technical performance indicators of the power supply, process complexity, cost and other factors. This article discusses the commonly used electromagnetic compatibility design in detail, which is helpful for the software and hardware design of high-reliability digital control systems. [G]