Why use inductance in switching power supply?

The inductance is usually an energy storage element, and is often used together with the capacitor in the input filter and output filter circuit to smooth the current. An inductance, also known as a choke coil, is characterized by the "high inertia" of the current flowing through it. In other words, because of the continuous nature of the flux, the current on the inductance must be continuous.

Inductance is magnetic element, have magnetic saturation problem naturally. Some applications allow the inductance to saturate, some applications allow the inductance to saturate from a certain current value, and some applications do not allow the inductance to saturate, which requires differentiation in specific lines. In most cases, the inductance works in the "linear region ", where the value of the inductance is a constant and does not vary with the terminal voltage and current. However, there is a problem that cannot be ignored in switching power supply, that is, the winding of the inductance will lead to two distributed parameters (or parasitic parameters), one is the inevitable winding resistance, the other is related to the winding process, materials distributed stray capacitance. Stray capacitance has little effect at low frequencies, but gradually becomes apparent as the frequency increases. When the frequency reaches above a certain value, the inductance may become capacitive. If the stray capacitance is "concentrated" into one capacitor, the capacitance characteristic after a certain frequency can be seen from the equivalent circuit of the inductance.

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

1. When current I flows through the inductance L, the stored energy of the inductance is E=0.5×L×I2 (1).

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

3, just like capacitance has charge, discharge current, inductor also has charge, discharge voltage process. The voltage on the capacitor is proportional to the integral of the current (Ann · s), and the current on the inductor is proportional to the integral of the voltage (v · s). As long as the inductance voltage changes, the rate of current change di/dt will also change; The forward voltage causes the current to rise linearly and the reverse voltage causes the current to fall linearly.

4, the size of ripple current will also affect the size of the inductor and output capacitance, ripple current is generally set at 10%~30% of the maximum output current, so for step-down power supply, the peak current through the inductance is 5%~15% larger than the output current of the power supply.

The calculation of the correct value of the inductance is very important for selecting the appropriate inductance and output capacitance to obtain the minimum output voltage ripple.