Inquire
1. How does the self-resonant frequency (SRF) of a common-mode inductor affect its filtering effectiveness?
A: The self-resonant frequency (SRF) of a common-mode inductor is determined by its inductance and parasitic capacitance. When the noise frequency approaches the SRF, the impedance reaches its peak, resulting in optimal filtering performance. Above the SRF, parasitic capacitance dominates, the impedance decreases, and filtering performance deteriorates. During design, ensure that the SRF is above the target noise frequency band.
2. How does the Q value of a common-mode inductor specifically affect noise suppression?
A: A high Q value (quality factor) indicates strong energy storage capacity, but it produces a sharp impedance peak near the resonance point, potentially resulting in over- or under-suppression of noise in certain frequency bands. A low Q value provides a flatter frequency response, suitable for broadband noise suppression, but also reduces peak impedance.
3. How much does the parasitic capacitance of a common-mode inductor vary depending on the winding method?
A: Parallel winding: High inter-turn capacitance, with parasitic capacitance reaching tens of pF.
Slot winding: Reduces inter-turn coupling, reducing parasitic capacitance by 30%-50%.
Multi-layer winding: Inter-layer capacitance dominates, with parasitic capacitance 2-3 times higher than single-layer winding.
4. How is leakage inductance generated in common-mode inductors? What are the consequences of excessive leakage inductance?
A: Leakage inductance is caused by incomplete magnetic flux coupling (e.g., winding asymmetry, core gaps). Excessive leakage inductance transforms into differential-mode inductance, affecting signal integrity, causing high-frequency oscillation, increasing EMI risk, and reducing common-mode rejection ratio.
5. How does the core's permeability temperature coefficient affect common-mode inductor performance?
A: The permeability temperature coefficient (e.g., approximately -0.2%/°C for ferrite) results in: decreased inductance at high temperatures, drift in filtering performance, and core saturation at extreme temperatures. Therefore, a material with wide temperature stability (e.g., Mn-Zn ferrite) should be selected.
6. Why does the impedance curve of a common-mode inductor exhibit a "plateau" at high frequencies?
A: The plateau region (usually >10MHz) is caused by the following factors: parasitic capacitance and inductance forming an equivalent LC network, high-frequency loss of the core material (increased μ" component), skin effect and proximity effect of the winding.
7. What is the allowable range of symmetry deviation for dual windings? What are the consequences of exceeding this range?
A: The symmetry error is generally required to be <5% (e.g., inductance deviation and turns difference). Exceeding this tolerance can lead to: common-mode to differential-mode noise, increased current imbalance, and possible core bias saturation.
8. How is the DC superposition characteristic of a common-mode inductor tested?
A: Constant current source method: Apply the rated DC current and measure the inductance degradation curve (usually using an LCR meter). Critical saturation point test: Gradually increase the DC current until the inductance decreases by 10%-20%. The temperature rise must be controlled (ΔT ≤ 25°C).
9. What effect does the core air gap design have on the saturation characteristics of the common-mode inductor?
A: The air gap can: Improve anti-saturation capability (reduce effective permeance) (frequency), but reduces inductance (roughly inversely proportional to the air gap length). Typical air gap designs are 0.1-0.5mm (for power applications). A core air gap (a small gap left in the core's magnetic path) reduces the core's effective permeability and increases the core's saturation flux density.
Without an air gap, the core is susceptible to saturation under low DC or AC flux, causing a sharp drop in inductance and loss of filtering capability.
With an air gap, the core's saturation resistance is enhanced, allowing it to withstand higher DC currents or AC fluxes, ensuring stable inductance in high-current scenarios (such as power input circuits) and maintaining filtering effectiveness.
10. What performance does the loss tangent (tanδ) of a common-mode inductor reflect?
A: The loss tangent (tanδ = Rs/|Xs|) reflects: core loss (hysteresis + eddy currents), AC resistance (high-frequency skin effect). High-quality common-mode inductors should have tanδ < 0.1 (@1MHz). The larger the tanδ value, the greater the core energy loss: ① High frequencies can cause the inductor to heat up, reducing efficiency and even affecting reliability due to excessive temperature rise. ② High losses mean a greater ability to absorb noise energy, but excessive losses can weaken the inductor's impedance characteristics, reducing filtering effectiveness. Therefore, it is necessary to balance the tanδ value to find the optimal solution between suppressing noise and controlling temperature rise (high-frequency scenarios generally require a core with a low tanδ value).
