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Why do switch mode power supplies use inductors?

2024-06-26 10:28

Inductors are often energy storage components and are often used together with capacitors in input and output filtering circuits to smooth current. An inductor, also known as a choke, is characterized by a "significant inertia" in the current flowing through it. In other words, due to the continuous magnetic flux, the current on the inductor must be continuous, otherwise a large voltage spike will be generated.

Inductors are magnetic components, which naturally have the problem of magnetic saturation. Some applications allow inductance saturation, some allow inductance to enter saturation from a certain current value, and some applications do not allow inductance to saturate, which requires differentiation in specific circuits. In most cases, inductance operates in the linear region, where the inductance value is a constant and does not vary with terminal voltage and current. However, there is an undeniable issue with switch mode power supplies, which is that the winding of inductors will result in two distributed parameters (or parasitic parameters): one is the inevitable winding resistance, and the other is the distributed stray capacitance related to the winding process and materials. Stray capacitance has little effect at low frequencies, but gradually becomes apparent with increasing frequency. When the frequency reaches a certain value or higher, the inductance may become a capacitive characteristic. If the stray capacitance is "concentrated" into one capacitor, the capacitance characteristics presented after a certain frequency can be seen from the equivalent circuit of the inductor.

When analyzing the working condition of inductors in a circuit or drawing voltage and current waveforms, the following characteristics may be considered:

1. When there is current I in inductor L, the energy stored in the inductor is E=0.5 × L × I2 (1).

2. In a switching cycle, the relationship between the variation of inductor current (peak to peak ripple current) and the voltage at both ends of the inductor is: V=(L x di)/dt (2). From this, it can be seen that the magnitude of ripple current is related to the inductance value.

3. Just like capacitors have charging and discharging currents, inductors also have charging and discharging voltage processes. The voltage on the capacitor is proportional to the integral of the current (ampere second), and the current on the inductor is proportional to the integral of the voltage (volt second). As long as the inductance voltage changes, the current change rate di/dt will also change; The forward voltage causes a linear increase in current, while the reverse voltage causes a linear decrease in current.

4. The magnitude of ripple current also affects the size of the inductor and output capacitor. Ripple current is generally set at 10% to 30% of the maximum output current. Therefore, for step-down power supplies, the peak current flowing through the inductor is 5% to 15% higher than the output current of the power supply.

Calculating the correct inductance value is crucial for selecting appropriate inductors and output capacitors to achieve the minimum output voltage ripple.

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