Electromagnetic compatibility issues in drive module design

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

1 Basic principles of electromagnetic compatibility

Electromagnetic compatibility refers to the coexistence state in which electrical and electronic equipment can perform their respective functions in a common electromagnetic environment, and can work normally without interfering with each other, thus achieving a compatible state.

Electromagnetic interference can be represented in the time domain and frequency domain. Most interference signals are time-varying. For the convenience of discussion and analysis, it is appropriate to use the frequency domain analysis method. Typical signal representation methods include sinusoidal, non-sinusoidal, periodic, non-periodic and pulse, etc. They are all radiated through space and conducted through wires. Non-periodic signals and pulse signals are widely used in engineering. The interference signal is represented by f(t). The Fourier integral of the non-periodic signal is

Similarly, the pulse signal can be transformed by Fourier to obtain a spectrum diagram.

Electromagnetic interference is transmitted through electric and magnetic fields. Therefore, its basic units can also be expressed in units of electric and magnetic fields. Decibels, i.e. db, are commonly used in engineering to measure electromagnetic interference.

2 Specific implementation of electromagnetic compatibility

When selecting the drive unit housing, the cost-effectiveness should be considered for materials with better shielding effect. In the design of the drive unit, the midpoint clamped PWM inverter circuit is used as the main working circuit. When the power device is switched, the circuit will generate harmonic currents, and the high-frequency components will radiate into space. The PWM inverter working circuit is shown in Figure 1.

To this end, electromagnetic compatibility should be fully considered in various links such as component selection, circuit board design and interface design, so that the drive module can work in a normal state without affecting other equipment.

2.1 EMC considerations for components

Components used to achieve electromagnetic compatibility usually reduce the noise voltage by reducing the impedance of the parallel path or increase the impedance of the current path to reduce the noise current.

All components and interconnects will produce resonance, so when selecting components, it is necessary to fully ensure low impedance distribution of the line. The input impedance of the series resonant circuit is very low, but depending on the different q values ​​of the circuit, the output voltage may be much higher than the input voltage. Due to the emergence of high current and high voltage, series resonance is very likely to produce high-level radiation or transmission; parallel resonance will form a high line impedance and also generate high current.

2.2 Board-level EMC considerations for circuits

When designing the circuit schematic, the placement of general components and power devices should be fully considered, and the following five points should be paid attention to:

(1) The unused end of the control chip should be connected to the power supply or ground through the corresponding matching resistor. The ground or power supply end of the integrated circuit should be connected and should not be left hanging;

(2) The relay needs to be matched with a high-frequency capacitor;

(3) Each integrated circuit needs to be equipped with a decoupling capacitor;

(4) Reduce the load capacitance so that the open collector driver near the output end can be pulled up easily, and the resistance value should be as large as possible;

(5) Do not use programmable devices in watchdog circuits.

When designing a PCB, try not to use a single-sided board. At the same time, analog circuits and digital circuits should be laid out separately; medium and high-speed circuits should also be laid out separately; the grounding lines should be clear and not all groundings should be shared; resistors should be connected in series on the PCB traces to reduce the jump rate of the upper and lower edges of the control signal lines; the processor or heat-generating device should be isolated from other chips by thermally conductive materials, and multiple RF grounding points should be applied around the processor.

In the PWM inverter working 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, a DC reactor can be connected in series on the DC bus. In addition, a harmonic filter can be added at the input end of the circuit. This method is simple to install, low in cost, and easy to maintain, but it is easily affected by system parameters.

2.3 Grounding considerations

Grounding is an important measure for electromagnetic compatibility. Grounding can reduce power module noise, reduce crosstalk and prevent static electricity accumulation. The grounding principle of the drive unit is to separate the signal ground and the power ground; high power is grounded at a single point at the nearest location; and the AC safety ground should be connected to the unit housing.

2.4 Interface Filtering Considerations

To facilitate communication, the drive unit has an RS232 or RS485 interface. Therefore, the interface circuit in the design needs to be filtered. Differential line driving and receiving methods are usually used to improve the anti-interference of the line interface. Compared with the noise pulse superimposed on the distorted signal, the noise pulse voltage that is lower than the signal level and added to the non-distorted signal cannot exceed the switching threshold of the receiver input. Therefore, adding a termination resistor to the receiver input can reduce reflections and improve signal quality and improve the circuit interface anti-interference. The differential receiving circuit is shown in Figure 3.

3 Conclusion

Electromagnetic compatibility issues are often encountered and must be resolved by engineers when designing drive units. The several treatment methods listed in this article have been verified in actual use and have achieved good results.