In a buck circuit, the inductor stores energy when the switch is on and continues to supply power to the load when the switch turns off. During this time, the inductor and capacitor work together as a second-order filter, helping to provide a more stable output voltage and current. The inductor helps reduce current ripple, which allows for a smaller capacitance value in the design.
As a magnetic component, inductors are naturally prone to magnetic saturation. Depending on the application, some systems allow for inductor saturation, while others require that the inductor operates within a linear region where its inductance remains relatively constant. However, in buck circuits, inductors can also introduce parasitic parameters—namely, winding resistance and distributed stray capacitance. While stray capacitance has minimal impact at low frequencies, it becomes more significant at higher frequencies. At a certain frequency, the inductor may begin to behave like a capacitor, which can affect circuit performance. This behavior can be observed in the inductor’s equivalent circuit when the stray capacitance is modeled as a single capacitor.
When selecting an inductor for a buck circuit, several key factors must be considered:
1. **Inductance and Tolerance**
The inductance value should match the required specification. Common units include Henry (H), millihenry (mH), microhenry (μH), and nanohenry (nH). Tolerances vary, with J (±5%), K (±10%), and M (±20%) being the most commonly used.
2. **Rated Operating Current**
The maximum working current is typically 1.25 to 1.5 times the rated current. It’s safer to derate by 50% for reliable operation. In practice, engineers often use 80% of the saturation current as the rated current.
3. **DC Resistance (DCR)**
Most inductors specify a maximum DC resistance, which should be as low as possible to minimize power loss.
4. **Test Frequency**
To accurately measure inductance (L), quality factor (Q), and DCR, the test frequency should closely match the inductor’s operating frequency.
5. **Inductance Stability**
Inductance can change with temperature, mechanical stress, and aging. The temperature coefficient (a1) is calculated as ΔL / (L * ΔT), and stability is crucial for consistent performance.
6. **Package Size**
Inductors come in various packages, such as chip or through-hole. Proper documentation in the BOM helps prevent errors during assembly.
When choosing an inductor, it's important to match the specifications to the design requirements. Some additional tips include:
- Select the inductor based on the maximum output current.
- Ensure the inductance value meets the theoretical calculation.
- Choose an inductor with low DC resistance to minimize losses.
- Opt for the appropriate core type and structure based on the application.
In DC-DC buck circuits, inductors typically operate in continuous conduction mode. Although the voltage across the inductor can change abruptly, the current remains smooth. However, the magnetic field generated by the inductor can cause electromagnetic interference (EMI), affecting nearby sensitive components. Shielding is therefore a critical consideration. Shielded inductors reduce external radiation but tend to be larger, have higher losses, and cost more. Unshielded inductors are compact, can handle higher currents, and are more cost-effective. If EMI is a major concern, shielded inductors are still the preferred choice.
Another trend is the increasing use of ring-shaped inductors, which offer better electromagnetic shielding and improved current handling due to their air gap design.
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