Circuit principle
1. Buck circuit principle
The buck circuit, also known as the step-down circuit, is DC in-DC out. The circuit does not have a transformer, and the input and output share a common ground. The following diagram is a simplified schematic diagram of the buck circuit.
1. When the MOS transistor Q is turned on, the current flow direction is shown by the red arrow in the figure below.
Coming out of the positive pole of Vin on the left, passing through the MOS tube Q, after passing through the inductor Lf, a left-positive and right-negative induced electromotive force will be generated at both ends of the inductor Lf, which is used to store energy. Then it will power the load Vo. At the same time, Vin will charge the Cf capacitor. Finally the current is back at the negative pole of Vin. At this point, diode D has no current flow direction and is not on.
2. When the MOS transistor Q is turned off, the current flow direction is shown by the arrow in the figure below.
A negative left-right positive reverse electromotive force will be generated at both ends of the inductor Lf, at this time, the inductor Lf releases energy and forms a path through the load Vo through the diode D. At this time, diode D plays a freewheeling role. At the same time, the Cf capacitor will discharge the load Vo.
3.Buck circuit duty cycle
The figure below shows the hysteresis loop of a magnetic substance. Inductors and magnetic rods are magnetic substances. The hysteresis loop represents the closed magnetization curve of the hysteresis of a strongly magnetic substance when the magnetic field strength changes periodically. He showed the relationship between the magnetization intensity M or magnetic induction intensity B and the magnetic field strength H during the repeated magnetization of strongly magnetic substances.
Volt-second value: The product of volt-second, which is the product of the voltage V at both ends of the inductor and the time period T.
Volt-second value balancing: Volt-second balancing is generally used in switching circuits. When the switching circuit is stable, the current change of the inductance in one switching cycle is eventually zero. That is, when the switch is turned on, the current increase through the inductor is equal to the current decrease of the inductor when the switch is turned off, that is: △I*Ton=△I*Toff.
电感的电压公式:V=L*di/dt=L*△I/△T，所以V(导通)*△Ton=V(关断)*△Toff，即伏秒平衡。
In the buck circuit, as shown in the figure below, when the switch is turned on, the core excites and the inductor current increases. When the switch is turned off, the core is demagnetized and the inductor current drops.
如下图，根据伏秒值平衡原则，(Vin-Vo)*Ton=Vo*Toff; Vo=Vin*D;D=Vo/Vin。
实际占空比:D1=Ton/T
Ton: opening time; T: Period.
Theoretically, the buck circuit duty cycle can vary from 0-100%.
The actual duty cycle is theoretically larger. Because of the theoretical duty cycle, the voltage drop when the switch is turned on and the freewheeling diode is turned on is not considered.
2. Boost circuit principle
The essence of achieving a boost is the superposition of voltages (Ohm's law);
First, when the MOS transistor S is closed, the voltage source Vin charges the inductor, and after charging, a continuous current is applied to the inductor (inductance characteristics: the current on the inductor does not change abruptly).
When the switch is disconnected, there is still a current on the inductor, and if there is a current, it is thought that there will be a voltage at both ends of the inductor, and then the inductor voltage and the power supply voltage are superimposed to complete the output of the post-stage voltage is higher than that of the previous stage.
This voltage is not completed in a certain stage, it is in the whole cycle, many cycles are constantly superimposed and cycled, and finally from a lower voltage to a higher voltage.
Core: The inductor is stored and then released through the post-stage circuit, and repeated back and forth in different cycles to finally achieve a stable voltage output.