Research on electromagnetic interference of variable frequency speed regulation system based on DSP-EEWORLD

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1 Electromagnetic Interference (EMI) Analysis

1.1 Concept and approach of electromagnetic interference

Electromagnetic interference is generated by interference sources. It is an electromagnetic phenomenon that comes from the outside and inside and damages useful signals. Interference passes through sensitive components, transmission lines, inductors, capacitors, space fields, etc. and acts in some form. Its interference effects and phenomena are ubiquitous and vary in form. It is called conducted interference. It can be divided into two categories according to whether it carries information or not: information conducted interference source and electromagnetic noise conducted interference source. Information conducted interference source refers to the interference of useless information on analog channels. Electromagnetic noise conducted interference source refers to the interference of electromagnetic noise without any information on the frequency conversion system.

Conducted electromagnetic interference transmission channels can be divided into capacitive conduction coupling (or electric field coupling), resistive conduction coupling (or common impedance coupling) and inductive conduction coupling (or mutual inductance coupling). Capacitive conduction coupling refers to the electromagnetic conduction coupling formed by the mutual linkage of the capacitance of the wires and components between the interference source and the signal transmission line (including printed circuit lines). Resistive conduction coupling refers to the conducted electromagnetic coupling formed by the mutual linkage of the current or voltage on the common impedance between the interference source and the signal transmission line (including printed circuit lines). Inductive conduction coupling is essentially magnetic field coupling.

1.2 Electromagnetic interference problem of digital frequency conversion speed regulation system

All three situations exist in the digital variable frequency speed regulation system, and the resistance conduction coupling and inductance conduction coupling are particularly obvious, mainly due to the design of the reference ground, the design of the printed circuit board and the isolation of high and low voltages. The analog interface circuit is easily affected by the power circuit. The electromagnetic compatibility characteristics of DSP are mainly reflected in the electrical characteristics of the pin signal. Most of the input and output signals of DSP are digital signals, and most of its external interface circuits are also digital circuits, including the power device IPM, which also works in the switching state. The entire system has obvious digital circuit characteristics. Only the current feedback loop is an analog signal. The analog signal is converted into a digital signal for processing through the DSP's on-chip A/D converter, and then the PWM output is controlled to achieve closed-loop control. The performance of electromagnetic interference in this design is specifically analyzed as follows.

(1) Impact of transient pulse interference on digital circuits

Digital signal processors are based on binary codes. High and low levels are used to represent binary data, and various circuits are used to describe signal characteristics, so as to achieve the purpose of controlling the object. Transient pulse interference will seriously affect data transmission and control status. Although the digital circuit itself has a strong anti-interference ability, it is susceptible to high-energy pulse interference in high-frequency circuits. The interference parts are manifested in the clock generator, bus data transmission, and PWM control signals. The IGBT inside the IPM will generate strong switching noise when working at high speed in the switching state, which is brought into the low-voltage digital circuit through the coupling of the ground wire, power line, distributed capacitance, and distributed inductance. Sometimes it seriously interferes with the operation of the TMS320F240 digital signal processor, which manifests as loss of control, program runaway, and crash.

(2) Impact of distributed inductance and capacitance on signals

In the system design based on DSP control, the control part usually uses TMS320F240EVM evaluation board, and the design of the control circuit printed board is simplified a lot. Only the circuit design and printed board of the IPM driver board need to carefully analyze the influence of the distributed capacitance and distributed inductance between signals. However, it should be noted that the influence of the distributed inductance of the signal bus between the evaluation board and the driver board may cause signal delay and lengthen the rise and fall time of the PWM control signal, thereby causing the upper and lower bridge arm IGBTs in the IPM module to be common, which will cause irreversible permanent damage to the IPM module.

(3) Impact of power supply on the system

The power supply is a channel for various interference signals to affect the normal operation of the system, mainly in the following aspects:

The internal resistance cannot be zero. All interference signals of the parts with a common power supply can be transmitted through the internal resistance of the power supply. The power grid line is a channel for external interference (such as lightning and electromagnetic emission) to enter. The disconnection and connection of the power load will have a great impact on the power grid, and the impact of the inductive load is even more serious. The power supply itself will generate many interference signals, especially the switching noise generated by the high-speed switching of IGBT.

There are two forms of interference signals in the system, namely common mode and series mode interference signals. In digital systems, this is often used to characterize the existence of interference, as shown in Figure 1. Series mode interference, also known as normal interference, refers to interference connected in series in the signal loop, which is generated by the mutual inductance of the transmission line. It is related to frequency and is often reduced by filtering and improving the sampling frequency. Common mode interference, also known as common-state interference, is interference that occurs when interference voltage is added to two signal lines at the same time. Therefore, maintaining a balanced line transmission structure can well suppress common mode interference. In addition, eliminating ground current can eliminate common mode interference. The method is to ground one point or float isolation (use a pulse transformer, choke or optocoupler to cut off the ground current).

Common mode and series mode interference

2 Hardware Anti-interference Technology

2.1 Electromagnetic compatibility design of power supply system

The power supply system includes low-voltage auxiliary power supply and main power supply. The low-voltage auxiliary power supply refers to the +5 V, ±12 V required by the DSP and its related interface circuits, and 4 groups of +15 V isolated power supplies for the IPM drive circuit. The main power supply refers to the AC/DC/AC power supply for motor drive with adjustable speed. It is isolated from the low-voltage auxiliary power supply and does not share the same ground. The anti-interference measures adopted by the power supply system are:

(1) The AC power input from the power grid should be equipped with an EMI suppression filter, that is, a low-pass filter composed of a common-mode choke L, a capacitor C, and a resistor R. It can not only prevent the series-mode and common-mode interference signals of the power grid from entering the power supply, but also effectively prevent the interference generated by the system itself from entering the power grid, which is beneficial to environmental protection.

(2) When the IGBT is switched on and off with high current, there are voltage and current surges in the circuit. Since du/dt and di/dt are very large, the high-frequency component of the surge interference signal is very high. A small-capacity high-frequency capacitor should be connected in parallel at the input end of the IPM power supply to eliminate parasitic oscillations.

(3) The power input and output power connection lines are connected by twisted wires, which can reduce the radiation of electromagnetic fields generated by the loop current.

(4) Low voltage and high voltage are isolated by mutual inductance, optocoupler signal and ground wire to block common mode interference. The suppression countermeasure is selected according to the duration Δt of the interference output by the power supply;

Δt>1 s is overvoltage, undervoltage, and power outage interference. Use uninterruptible power supply (UPS) and voltage stabilization to suppress it;

Δt>10 ms is surge, sag, and drop-out interference. This type of interference voltage has a large amplitude and changes quickly, which will either burn out the system or form oscillation. It is necessary to use fast-response surge absorbers and transient voltage suppressor diodes (TVS) to prevent it.

Δt is in microseconds, which is a spike voltage interference. Due to its short duration, it generally will not burn out the system, but it can destroy the operation of the DSP source program and confuse the logical function. The signal line should be kept away from the interference source and shielded;

Δt is in nanoseconds and belongs to radio frequency interference. It does not cause serious harm to DSP and digital signals. Generally, a high-frequency decoupling capacitor can be added to the power input terminal of the IC.

2.2 Anti-interference measures for interface circuits

The peripheral circuits of DSP work in different ways and at different speeds. The exchange of information is controlled by the DSP digital signal processor. Various tasks are completed by the interface. Interference of the interface signal will affect the control result of the system. The interference of the interface comes from the transmission line connected to it, including the circuit line design of the printed circuit board and the connection between the circuit boards. The most commonly used transmission lines for connecting circuit boards are flat cables, twisted pairs and coaxial cables. From the perspective of anti-interference, twisted pairs are a transmission line with good electromagnetic interference resistance. The round-trip current induction effect of the two lines in the intersecting loop is offset, so the effective distance is up to 10 m, which is used for the output and input parts of the power supply. The PWM control signal generated by the DSP acts on the IPM and is isolated by optocouplers. Because the common mode rejection ratio of the optocoupler is very high, it effectively prevents the common mode interference between the control circuit and the PWM converter. Its schematic diagram is shown in Figure 2.

Anti-interference measures for interface circuits

2.3 Anti-interference measures for bus

(1) Use a three-state gate bus to improve anti-interference capabilities. Since the DC load capacity of the DSP bus is limited, if it is not enough, it needs to be connected to the chip through a buffer;

(2) Measures to prevent data conflicts on the bus: The connection between the CPU and the RAM is realized by the bus transceiver through the internal bidirectional data bus. Conflicts may occur on the internal data bus at a certain moment. The solution is to shorten the time for the RAM to access data, that is, shorten the selection time.

(3) Measures to overcome instantaneous instability on the bus: When two control signals with opposite phases deviate in time, one changes from low level to high level, while the other does not have time to change from high level to low level, and both are in high impedance state. At this moment, if the load of the bus is a TTL circuit, it will make the bus voltage unstable due to its own leakage current; if the load is all CMOS or NMOS, there will be several hundred megohms of disconnection state, which is easy to couple interference. Use a pull-up resistor to connect to the power supply so that the bus is at a high potential at this moment, which enhances the anti-interference ability of the bus. The pull-up resistor is usually selected to be 1Ω.

2.4 Electromagnetic compatibility design of power output

The power output part is a power converter composed of IPM, drive circuit and pump-up circuit. Using PWM algorithm, 6 groups of PWM signals generated by DSP are transmitted through the isolation of photocoupler, and then controlled by IPM drive circuit to work the IGBT switch inside IPM. Adding absorption capacitor at the DC power supply end can suppress switching noise. In system design, the low-voltage control circuit should be kept as far away from the power circuit as possible to ensure that the low-voltage signal is not affected by electromagnetic radiation and coupling.

2.5 Printed circuit board anti-interference measures

Practice has proved that the design of printed circuit boards has an important impact on anti-interference and ensuring the stable operation of the system. After the printed circuit board is powered on, the current on the printed line will generate electromagnetic waves that radiate into space, and high-speed components, crystal oscillators and other devices in the circuit will also generate electromagnetic radiation. In high-speed digital systems, when the delay time is twice the rising or falling edge of the pulse, the digital signal transmission line in the printed circuit board should be considered for matching according to the requirements of the distributed parameter transmission line. For example, for TTL circuits with generally faster conversion speeds, terminal matching measures must be added when the printed line length is greater than 10 cm. The conversion speed of the COMS circuit is relatively slow, and the printed line length can be relaxed by 5 to 6 times. According to the electromagnetic radiation model formula:

E=263×10-6(f2AI)／r

Where: E is the radiation field strength at space r of the printed circuit board; f is the frequency of the working current on the printed circuit board; A is the loop area on the printed circuit board; I is the current on the printed circuit board.

It can be seen from the above formula that reducing f, A, and I can reduce the electric field emission on the printed circuit board. In order to better suppress interference, the following issues should be considered in the design of the printed circuit board:

(1) Wiring principles: digital signal lines and analog signal lines should be separated, strong and weak signals should be separated, DC power lines should be orthogonal, heating components should be kept away from integrated circuits, magnetic components should be shielded, each IC chip should have a decoupling capacitor from the power supply end to the ground end, and the leads should be short;

(2) The size of the printed circuit board should be moderate, with logical components close to each other and away from devices that are prone to interference. The ground line of the printed circuit board should be as wide as possible, not only because it can reduce losses, but also because it can reduce the inductance component of the line, thereby reducing common-mode interference. If it is a double-layer wiring or multi-layer wiring, the power supply and ground should be in the middle layer, and the wires on the top and bottom layers should be orthogonal to each other, and parallel lines should be avoided as much as possible.

(3) A 10-100 μF electrolytic capacitor is connected across the power input terminal of the printed circuit board. For devices that are easily affected by interference signals in the circuit and have transient current changes, a decoupling capacitor with good high-frequency characteristics is connected between the device and the ground. For example, when RAM and ROM chips are in operation, the current changes greatly, and a 0.01 μF ceramic capacitor should be added to the power supply terminal of each chip to bypass the high frequency.

3 Software Anti-interference Technology

Software anti-interference can not only improve efficiency and save hardware, but also solve problems that hardware cannot solve. Although a large number of interference sources cannot cause damage to hardware, they can make the system unstable, data unreliable, malfunctioning, and the program "runaway". In severe cases, it can cause DSP control failure and serious accidents. Since the fault is temporary, intermittent, and random, it is difficult to solve it with hardware, but software can solve it with the help of the following technologies:

(1) Use trap technology to prevent the disorder caused by interference from spreading;

(2) Use time redundancy technology to shield interference signals, that is, sample input and judge multiple times to improve input reliability; use multiple repeated outputs to judge to improve the reliability of output information; reinitialize and forcibly restore normal operation to avoid abnormal I/O input and output; query the status of the interrupt source to prevent false interrupts caused by interference; shield interrupts during unnecessary time to reduce false interrupts caused by interference;

(3) Fault-tolerant technology: using some specific codes to check the data, determine whether it is disturbed due to storage, and then correct the error logically;

(4) Instruction redundancy: multiple important instructions can be written repeatedly;

(5) Marking method: set characteristic marks and identification marks, which are often used in the protection of internal data areas;

(6) Digital filtering technology: mainly used to treat interference on analog signals.

Reference address：Research on electromagnetic interference of variable frequency speed regulation system based on DSP

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