Choose the appropriate inductor for switching power supply | GETWELL

An inductor, also known as a choke, is characterized by the "great inertia" of the current flowing through it. In other words, due to the continuity of the flux, the current on the inductor must be continuous, otherwise it will produce a large voltage spike. The inductor is a magnetic component, so it naturally has the problem of magnetic saturation. Some applications allow inductance saturation, some applications allow inductors to enter saturation from a certain current value, and some applications do not allow inductors to be saturated, which requires a distinction in specific circuits.

In most cases, the inductor works in the "linear region", where the inductance is a constant and does not change with the terminal voltage and current. However, there is a problem that can not be ignored, that is, the winding of the inductor will lead to two distributed parameters (or parasitic parameters), one is the inevitable winding resistance, the other is the distributed stray capacitance related to the winding process and materials.

Stray capacitance has little effect at low frequency, but it gradually appears with the increase of frequency. When the frequency is above a certain value, the inductor may become a capacitive characteristic. If the stray capacitance is "concentrated" into a capacitor, the capacitance characteristics after a certain frequency can be seen from the equivalent circuit of the inductor.

The working condition of inductor in the circuit

Just like the capacitor has the charge and discharge current, the inductor also has the charge and discharge voltage process. The voltage on the capacitor is proportional to the integral of the current, and the current on the inductor is proportional to the integral of the voltage. As long as the inductor voltage changes, the current change rate di/dt will also change; the forward voltage makes the current rise linearly, and the reverse voltage makes the current linearly decrease.

It is very important to calculate the correct inductance to select the appropriate inductor and output capacitor to obtain the minimum output voltage ripple.

Inductance selection of step-down switching Power supply

When selecting inductors for buck switching power supply, it is necessary to determine the maximum input voltage, output voltage, power switching frequency, maximum ripple current and duty cycle.

Inductance selection of boost switching Power supply

For the inductance calculation of boost switching power supply, except that the relationship between duty cycle and inductance voltage has changed, the other process is the same as that of step-down switching power supply.

Please note that unlike the buck power supply, the load current of the boost power supply is not always provided by the inductor current. When the switch tube is on, the inductor current flows into the ground through the switch tube, and the load current is provided by the output capacitor, so the output capacitor must have a large enough energy storage capacity to provide the current needed by the load during this period. However, during the turn-off of the switch, the current flowing through the inductor not only provides the load, but also charges the output capacitor.

Generally speaking, when the inductance value becomes larger, the output ripple will become smaller, but the dynamic response of the power supply will also become worse, so the selection of inductance value can be adjusted according to the specific application requirements of the circuit to achieve the best effect.

The increase of switching frequency can make the inductance smaller, so that the physical size of the inductor becomes smaller and save circuit board space, so the current switching power supply has a trend to high frequency, in order to meet the requirements of smaller and smaller volume of electronic products.

The above is the introduction of choosing the appropriate inductor for the switching power supply. if you want to know more about the inductor, please feel free to contact us.