There are multiple causes of EMI in PCBs, such as RF currents, common-mode signals, ground loops, impedance mismatches, and magnetic flux. To effectively manage EMI, it's essential to gradually understand these sources and their effects. While the mathematical foundation of EMI can be found in electromagnetic theory, it’s often complex and challenging for most engineers. Therefore, a clear and practical explanation is more valuable for everyday application. This article will explore the "source of electricity" on a PCB, the role of Maxwell's equations, and the concept of minimizing magnetic flux.
**Source of Electricity**
Unlike magnetic sources, electric sources are modeled using time-varying electric dipoles. These consist of two closely spaced, oppositely charged points that change over time. The current flowing through the dipole creates a changing charge at each end. An oscillator can drive an unterminated antenna, which acts as an electric source. However, this circuit doesn't behave as expected in low-frequency models. Even though signal propagation has a limited speed based on the dielectric constant of materials, RF currents still arise due to this limitation.
The electromagnetic field from an electric source depends on four key factors:
1. **Current amplitude**: The strength of the electromagnetic field is proportional to the current flowing through the dipole.
2. **Polarity and measurement setup**: Similar to magnetic sources, the polarity of the dipole must match that of the measuring device.
3. **Size of the dipole**: The field strength increases with the length of the dipole, but only up to a certain fraction of the wavelength. Larger dipoles tend to resonate at lower frequencies.
4. **Distance**: Both electric and magnetic fields decrease with distance. In the far field, they behave like plane waves, while near the source, their dependence on distance becomes more significant.
The relationship between near-field and far-field components is illustrated in Figure 1. All electromagnetic waves consist of both electric and magnetic components, forming what is known as the “Poynting Vector.†A plane wave appears flat when observed from a small antenna at a few wavelengths away from the source.
The electric field is measured in volts per meter (V/m), and the magnetic field in amperes per meter (A/m). The ratio of E to H represents the free-space impedance, which remains constant regardless of distance or source type. For a plane wave in free space:
$$ Z_0 = \frac{E}{H} $$
This wave carries energy in watts per square meter (W/m²).
In many applications, noise coupling can be modeled using equivalent components. For instance, a time-varying electric field between two conductors resembles a capacitor, while a time-varying magnetic field corresponds to mutual inductance. Figure 2 illustrates these noise coupling mechanisms.
**Plane Wave Behavior**
For the lumped component model to work, the circuit size should be smaller than the signal wavelength. If not, we can still use lumped components for EMC analysis because:
1. Maxwell’s equations are hard to apply directly due to complex boundary conditions. Lumped models offer a reliable approximation.
2. Numerical models may not clearly show how noise is generated based on system parameters. Lumped components provide better insight into system behavior.
Understanding Maxwell’s equations is crucial for PCB design. It helps us identify how electromagnetic fields are created, allowing us to reduce unwanted RF currents. These currents affect signal distribution, bypass capacitors, and coupling. Minimizing loop areas in the signal return path reduces EMI. Bypass capacitors and power distribution networks must handle large currents, but they inherently create larger loop areas, increasing EMI risk.
Figure 3 shows the noise coupling mechanism, highlighting how different factors contribute to EMI in real-world PCB designs.
AC Power Supplies
AC power supply is a kind of power supply that converts the input mains or DC input into a pure sine wave output after AC-DC-AC or DC-AC conversion. The ideal AC power supply is characterized by stable frequency, stable voltage, zero internal resistance and pure sine wave output voltage wave (without distortion). Mainly used for power grid simulation tests conducted by various electrical appliance manufacturers on electrical appliances according to the voltage/frequency requirements of different countries, various AC motors, AC transformers, aircraft & mechanical equipment and other electronic devices that require pure, regulated and frequency-stabilized output.
Our AC power supply mainly includes three types
1. AC Power Supplies converting the input mains AC by AC-DC-AC, output voltage and frequency are stable and adjustable with pure sine wave output waveform, so called: Variable Frequency AC Power Supplies.
2. AC Power Supplies converting the DC output of the battery and other DC equipment by DC-AC, giving an AC sinusoidal output with stable output voltage and frequency. so called: Inversion AC Power Supplies.
3. AC Power Supplies converting the input mains AC by AC-DC-AC, giving a variable voltage AC output at constant 400Hz output frequency ultra-high output frequency stability. so called: Intermediate Frequency AC Power Supplies.
4. According to the difference in the number of output phases, the AC Power Supplies can be divided into single-phase output AC power supplies and three-phase output AC power supplies.
Different from Variac and AC voltage regulators, our AC power supplies support the setting of output voltage and frequency and has superior high precision, high stability, and high efficiency stable frequency AC output, not only for AC power conversion but also for AC high precision test purpose.
Through the friendly operation panel, you can read the output data such as output voltage, output current, output power, power factor, etc., providing accurate data records for your test, and can add RS485 interfaces as standard, following the MODBUS-RTU international communication protocol, which can realize remote control and operating status monitoring of the power supplies.
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