In a buck circuit, the inductor stores energy when the switch is on and continues to supply current to the load when the switch is off. Simultaneously, it works with the capacitor to form a second-order filtering system, ensuring a stable output voltage and current. The inductor helps reduce current ripple, allowing for a smaller capacitance value in the design.
As a magnetic component, inductors are prone to magnetic saturation. Some applications can tolerate this, while others require the inductor to start saturating at a certain current level. In critical applications, saturation must be avoided entirely, which necessitates careful selection based on specific requirements. Most of the time, the inductor operates in a linear region, where its inductance remains constant regardless of the applied voltage or current.
However, in buck circuits, inductors have inherent parasitic parameters. These include winding resistance, which is unavoidable, and distributed stray capacitance, which depends on the winding process and materials. At low frequencies, stray capacitance has minimal impact, but as frequency increases, it becomes more significant. At a certain frequency, the inductor may exhibit capacitive behavior. If this stray capacitance were concentrated into a single capacitor, its capacitive characteristics would become evident in the inductor’s equivalent circuit.
When selecting an inductor for a buck circuit, several factors should be considered:
1. **Inductance Value and Tolerance**
The nominal inductance value must match the design requirements. Tolerances are usually specified as F (±1%), G (±2%), H (±3%), J (±5%), K (±10%), M (±20%), etc. The most commonly used are J, K, and M grades.
2. **Rated Operating Current**
It's recommended to use 1.25 to 1.5 times the rated current as the maximum working current. Engineers often derate by 50% for safety. Saturation current is also a key parameter, and 80% of that is typically used as the rated current.
3. **DC Resistance (DCR)**
Except for power inductors that don’t measure DCR, other inductors should specify a maximum DC resistance, ideally as low as possible to minimize losses.
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 stability is influenced by temperature, mechanical vibration, and aging. The temperature coefficient (a1) is calculated as ΔL / L * ΔT. Ensuring stability under varying conditions is essential.
6. **Package Size**
Inductors come in chip or plug packages. Clearly specifying the package in the BOM reduces the risk of using incorrect components. The package also reflects the physical size and shape of the inductor.
Most inductor specifications are similar, but the exact requirements depend on the circuit design. When choosing an inductor, it's important to follow the design parameters to ensure optimal performance.
Some tips for inductor selection include:
- Choose the inductor based on the maximum output current.
- Ensure the inductance value meets the theoretical calculation.
- Select an inductor with low DC resistance to reduce losses.
- Choose the appropriate structure and core material for the application.
In DC-DC buck circuits, inductors operate in continuous conduction mode. While 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 primary concern. As shown in Figure 4, shielded inductors reduce external radiation but are larger, have higher losses, and are more expensive. Unshielded inductors are compact, can handle higher currents, and are cost-effective. If EMI is a critical design factor, shielded inductors are still preferred. Ring-shaped inductors are gaining popularity due to their superior shielding and improved current handling capabilities.
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