**1. Power Line Noise**
Power line noise refers to electromagnetic disturbances that originate from various electrical devices within the power grid and propagate along the power lines. This type of noise is typically categorized into two main types: common-mode interference and differential-mode interference.
**1. Common-Mode Interference**
Common-mode interference occurs when the interference current on both wires has the same amplitude and direction, creating an unwanted potential between any conductor and ground. To combat this, a **common-mode choke** is often used. It works by generating opposing magnetic fields for normal signals, which cancel each other out, allowing the signal to pass with minimal resistance. However, for common-mode currents, the magnetic fields reinforce each other, increasing the impedance and effectively filtering out the noise.
Another method to suppress common-mode interference involves using **common-mode capacitors**, which short high-frequency interference to ground. In contrast, **differential-mode capacitors** are placed between the two lines to block high-frequency noise while allowing low-frequency signals to pass through.
To reduce common-mode interference, consider the following methods:
- Use twisted pairs and ensure proper grounding.
- In strong electric field environments, use shielded cables (e.g., zinc tubes).
- Keep signal lines away from high-voltage lines and avoid bundling them together.
- Avoid sharing the same power supply with sensitive electronic control units.
- Use linear power supplies or high-quality switching power supplies with ripple below 50mV.
- Implement differential signaling circuits.
**2. Differential-Mode Interference**
Differential-mode interference occurs when the interference current on the two wires has the same amplitude but opposite directions, resulting in an unwanted voltage difference between the two conductors. This type of interference is symmetrical and typically lower in amplitude compared to common-mode interference.
To suppress differential-mode interference, capacitors are commonly used. These capacitors have a low impedance at high frequencies, allowing them to short out the noise. The capacitance value usually ranges between 0.01µF and 0.47µF. Capacitors must be carefully selected to avoid excessive leakage current.
**3. Filters**
Filters play a crucial role in reducing both common-mode and differential-mode interference. A typical five-terminal filter includes two inputs, two outputs, and a ground connection. The inductor (L) helps suppress common-mode interference, while capacitors help filter out high-frequency noise.
The **ground leakage current** can be calculated using the formula:
$$ I_{cd} = 2 \pi f C U_c $$
Where:
- $ I_{cd} $ is the leakage current,
- $ f $ is the grid frequency,
- $ C $ is the capacitance,
- $ U_c $ is the voltage.
Leakage current is proportional to the capacitance, so it's important to balance safety and effectiveness. Typically, leakage current ranges from several hundred microamperes to a few milliamperes.
A poor grounding system increases the risk of electric shock if a person comes into contact with a faulty device. Proper grounding is essential for safety.
**Four Commonly Used EMI Devices**
Switching power supplies often use simple EMI filters, including common-mode chokes and capacitors. Some common configurations include:
- A basic filter with a common-mode choke and capacitors.
- A more advanced configuration with additional resistors and capacitors.
- A design that includes a discharge resistor (R) to safely discharge stored energy in the capacitors after power is turned off.
These components help reduce noise and improve overall system performance.
**2. Noise Caused by Input Current Distortion**
In power supplies without PFC (Power Factor Correction), the input current becomes distorted due to the non-linear behavior of rectifier diodes and the energy storage function of capacitors. This leads to spike-like current pulses with high peaks, causing harmonic distortion and lowering the power factor.
**3. Interference from Switching Transistors and Transformers**
The switching transistor is a key component in switching power supplies and also a major source of interference. As the switching frequency increases, so does the level of electromagnetic interference (EMI). Improper selection of components in the snubber circuit can further amplify this interference.
During operation, a high-frequency current loop forms between the input capacitor, the primary coil of the transformer, and the switch transistor. This loop can generate significant radiated noise.
**4. Noise from Output Rectifier Diodes**
When a diode is reverse-biased, it may experience a **reverse recovery current** as stored charge is released. This can create noise, especially in high-speed switching applications. Adding an RC snubber across the diode can help absorb this noise and improve stability.
**V. Noise from Distribution and Parasitic Parameters**
Parasitic elements such as distributed capacitance between the power supply and heat sink, or between the primary and secondary windings of a transformer, contribute significantly to EMI. At high frequencies, even small parasitic components can behave unpredictably—wires act as antennas, capacitors behave like inductors, and inductors act like capacitors. This makes careful layout and design critical for minimizing noise in switching power supplies.
Grid Scale Energy Storage,Grid Connected Pv Systems,Stand Alone Power Systems,Opzv Battery
EMoreShare International Trade (Suzhou) Co., Ltd , https://www.emoreshare.com