What is a boost converter?
A boost converter is called a parallel switching converter. Unlike a buck converter, the boost inductor is at the input (switch), while the buck inductor is at the output. The output voltage Vo of a boost converter is always greater than the input voltage Vi. The explanation is relatively simple: when the switch is on, the diode D is off, and the node voltage between the inductor L and the switch is 0. When the switch is off, the potential across the inductor L flips, so the node voltage between the inductor L and the switch is greater than the input voltage Vl. The inductor current freewheels through the diode D, making Vo greater than Vi. It can be proven that Vo = Vi * [T/(T-Ton)], where T is the switching pulse period and Ton is the on-time.
How the boost converter works
The main relationships and critical inductance of the Boost converter when operating in CCM and DCM
Based on whether the minimum current flowing through the inductor is zero (i.e., whether the inductor current is discontinuous during the S-turn-off period), Boost converters can be divided into two modes: Continuous Conducting Mode (CCM) and Discontinuous Conducting Mode (DCM). For a given switching frequency, load resistance, and input and output voltages, the Boost converter has a critical inductance Lc. When L > Lc, the converter is in CCM; when L > Lc, the converter is in DCM. The basic working principle is that under changes in input voltage, internal parameters, and external load, the control circuit uses the difference between the controlled signal and the reference signal to perform closed-loop feedback, adjusting the on (or off) time of the main circuit switch transistors to make the output voltage or current of the switching converter relatively stable.
To analyze steady-state characteristics and simplify the derivation of the formula, the following two assumptions are made:
(1) Switching transistors and freewheeling diodes are both ideal components. That is, they can be "turned on" or "turned off" instantaneously, and the voltage drop is zero when "turned on" and the leakage current is zero when "turned off".
(2) Inductors and capacitors are ideal components. Inductor I operates in the linear region without saturation, and its parasitic resistance is zero. The equivalent series resistance (ESR) of capacitor is zero.
Boost converter operating mode
The Boost DC-DC converter, also known as the StepUp Converter, has the circuit topology shown in Figure 2.1.
The basic circuit of a Boost DC-DC converter consists of a power switch VT, a freewheeling diode VD, an energy storage inductor L, and an output filter capacitor C. Because MOSFETs have a faster switching speed and relatively simpler control logic, the switching transistor VT is generally a MOSFET.
During the conduction period of the switching transistor VT, the current in the inductor rises; during the cutoff period of VT, the inductor current falls. If the current in the inductor drops to zero during the cutoff period of VT, and the energy stored in the inductor is also zero for the remainder of the cutoff period, then this switching power supply is said to operate in discontinuous current conversion mode (DCM); otherwise, it operates in continuous current conversion mode (CCM). The two operating modes of the Boost DC-DC switching converter are analyzed below to facilitate system design.
Boost converter operating range
Assuming the input voltage range of the Boost DC-DC switching converter is [V.min, V.max], and the load resistance range is [Rmin, RL.max], the operating range of the switching converter in the RL-V plane corresponds to a rectangle. Based on the expressions for the critical inductance Lc of the CCM and DCM of each switching converter, and the critical inductance Lk of the CISM and IISM, respectively, the curves describing these inductances are plotted. This divides the RL-V plane into two parts: CCM and DCM, and CISM and IISM, as shown in Figure 2.5. Where LcB...
As shown in Figure 2.5, for the Boost DC-DC converter, the critical inductance LK of CISM and IISM exhibits a monotonic relationship with the input voltage and load resistance, while the critical inductance Lc of CCM and DCM does not. In the RL-V plane, different inductance values cause the Boost converter to operate in different modes. (34.12-131)
