Step down refers to the switching power supply module power supply circuit in this way of work, some also apply to step-up and step-down two ways. But is commonly used in practice, which is divided into LDO and DC DC.
The principle of the external circuit of LDO power chip is simple, only the filter capacitance is added to the input and output. DC DC mode must be connected with capacitors and inductors. The power supply circuit of power chip is complex, but the conversion efficiency is high. The next part discusses how to choose the inductor of the step-down type.
For the buck 9V 2A 18w charger, the inductance must be confirmed with large input and output voltage, power switch frequency, large ripple current and duty cycle. It is assumed that the switching frequency of power supply is 400kHz, the input voltage range is 13V ± 12%, the output current 1a and the larger ripple current is 400mA.
Inductance is a device with magnetic element, and nature will have saturation state problems. Some use allowable saturation state, while some are not allowed, so it must be distinguished in main route. In most cases, the inductance is in line shape, and the inductance value is constant, and it does not change with the voltage and current.
However, there is a problem that can not be ignored in power transformer, that is, winding inductance will cause two main parameters or parasitic parameters. The first is the necessary winding resistance, the other is the distributed system stray capacitor related to the winding process and raw materials.
Stray capacitors are not harmful at high frequency, and will gradually show up with the frequency rising. When the frequency is higher than a certain value, the inductance may become the characteristics of capacitors.
The following aspects can be taken into account in the analysis of the working conditions of inductors in the route. First, when there is current I in inductance L, the energy stored by inductor is e=0.6 × L × I2 。
Secondly, in one power supply switching period, the correlation between the change of inductance current (peak peak ripple current) and voltage at both ends of inductor is v= (L × Di) /dt, so it can be seen that the size of ripple current is related to the inductance value.
The inductance has the whole process of charging and discharging voltage, and the current on the inductor is proportional to the integral of voltage. If the inductance voltage changes, the current change rate di/dt will also change.
The forward voltage increases the current line, while the reverse voltage decreases the current line. Therefore, it is very important to calculate the appropriate inductance value for the buck power module to select the suitable inductor and output capacitor to obtain the minimum output voltage ripple.
