What EMC issues should be considered when designing drive modules?

As a driving power supply for high-power modules, main circuits such as switching circuits, amplification circuits, and inverter circuits may interfere with the electromagnetic environment. Therefore, when designing a drive module, electromagnetic compatibility must be considered to avoid interference from the drive unit to the outside world.

1. Basic principles of electromagnetic compatibility

Electromagnetic compatibility refers to the coexistence of electrical and electronic devices in a common electromagnetic environment to achieve their respective functions, to be able to work normally, and to achieve compatibility.

Electromagnetic interference can be expressed in time and frequency domains. Most interference signals are time-varying. In order to facilitate discussion and analysis, frequency domain analysis methods should be used. Typical signal representations are sinusoidal, non sinusoidal, periodic, non periodic, and pulses conducted through spatial radiation and wires. Non periodic signals and pulse signals are widely used in engineering, and interference signals are used using f (t), which means that the Fourier integral of non periodic signals is. You can also change the Fourier transform of the pulse signal to obtain a spectrum diagram.

Electromagnetic interference propagates through electric and magnetic fields. Therefore, its basic units can also be expressed in electric and magnetic field units. The decibel is widely used to measure electromagnetic interference in engineering, known as db.

2.Specific implementation of electromagnetic compatibility

When selecting the drive unit housing, comprehensive consideration should be given to materials with good shielding effects. In the design of the drive unit, the midpoint clamp type PWM inverter circuit is used as the main working circuit. When the power device is switched on, the circuit will generate harmonic currents, and high-frequency components will radiate into space. The working circuit of the PWM inverter is shown in Figure 1.

Therefore, electromagnetic compatibility should be fully considered in the selection of components, circuit board design, and interface design, so that the drive module can work normally without affecting other devices.

2.1 EMC considerations for components

In order to achieve electromagnetic compatibility, the method of reducing the parallel path impedance is usually used to reduce the noise voltage or increase the current path impedance to reduce the noise current. All components and interconnections can produce resonance phenomena, so when selecting components, it is necessary to fully ensure low impedance power distribution of the line.

The input impedance of a series resonant circuit is very low, but depending on the different q values of the circuit, the output voltage may be significantly higher than the input voltage. Due to the presence of large currents and high voltages, series resonance can easily generate high-level radiation or transmit to emission; Parallel resonance creates high circuit impedance and high current.

2.2 Board level EMC considerations for circuits

When designing a circuit schematic diagram, full consideration should be given to the placement of general components and power devices, with the following five main considerations:

(1) Relays need to match high-frequency capacitors;

(2) Each integrated circuit shall be equipped with decoupling capacitors;

(3) The useless end of the control chip should be connected to the ground or grounding terminal on the power supply or grounding integrated circuit through corresponding matching resistors, and should not be suspended;

(4) Do not use programmable devices on the watchdog circuit;

(5) Reduce the load capacitance to make the collector open circuit driver near the output terminal easy to pull up, and maximize the resistance value.

When designing a PCB, try not to use a single panel, and separate the layout of analog and digital circuits; In the middle, try not to use a single panel, and separate the layout of analog and digital circuits High speed circuits should also be laid out separately; The grounding wire should be clear and cannot be shared by all grounding wires; The PCB line should be connected in series with resistors to reduce the jump rate between the upper and lower edges of the control signal; The processor or heating device should be isolated from other chips through thermally conductive materials and grounded at multiple RF points around the processor.

In a PWM inverter circuit, in order to limit the harmonic current at the input end, a filter capacitor can be connected in parallel at the front end of the DC link, as shown in Figure 2; At the same time, DC reactors can be connected in series on the DC bus. In addition, a harmonic filter can be added to the input of the circuit, which is easy to install, low in cost, and easy to maintain, but susceptible to system parameters.

2.3 Grounding considerations

Grounding is an important measure for electromagnetic compatibility, which can reduce power module noise, reduce crosstalk, and prevent static electricity accumulation. The grounding principle of the drive unit is the separation of signal and power supply; High power single point grounding at the nearest location; At the same time, the AC safety should be connected to the unit housing.

2.4 Consider interface filtering

In order to facilitate communication, all drive units have an RS232 or RS485 interface, so the design of the interface circuit requires filtering processing. Differential circuit driving and receiving methods are commonly used to improve circuit interface immunity. Compared to noise pulses superimposed on a distorted signal, the voltage of noise pulses that are lower than the signal level and added to a non distorted signal cannot exceed the switching threshold at the receiver input. Therefore, adding a termination resistor at the receiver input can reduce reflection, improve signal quality, and improve circuit interface immunity. The differential receiving circuit is shown in Figure 3.

3. Conclusion

Electromagnetic compatibility (EMC) issues are frequently encountered and must be addressed by engineers in designing drive units. Several processing methods listed in this article have been verified in practical applications and have achieved good results.