In various controllers of automotive electronics, there is often a design of BOOST boost circuit to ensure that some functions can still run after the voltage drops to a certain value.
The BOOST power supply is a switching power supply that uses the ratio of the time of switch on and off to maintain a stable output. It is an indispensable power architecture that is widely used in electronic devices in various industries due to its compactness, light weight, and high efficiency.
1. Topology
The following is the topology of the BOOST circuit, the main components are a MOS tube, an inductor, a diode, and several capacitors. Generally, the low-power BOOST type DCDC chip L1 and D1 are external, and the high-power DCDC chip MOS tube is external. The MOS tube is turned on and off using the PWM method.
2. MOS tube conduction (charging process)
After the MOS tube is turned on, the current flow direction is indicated by the red arrow in the figure below, the inductor starts to charge, converted into magnetic energy, at this time, the diode is reversed and cut off, and the output energy is all provided by the C2 capacitor, as shown by the green arrow below.
Switching on-time = duty cycle * switching period = D * T.
3. MOS tube closure (discharge process)
After the MOS tube is closed, the current flow direction is indicated by the green arrow in the figure below, and the inductor releases energy, at this time, the inductor is like a battery and Vin series to power the load, and at the same time charge the output capacitor C2. In this case, the load is powered by the output capacitor C2 and the diode D1. D1 generally chooses Schottky diodes with fast turn-on and small turn-on voltage drop.
Discharge time = (1-duty cycle) * switching cycle = (1-D) * T. At this time, Vin discharges to the load together with the inductor L1, and the positive and negative signs should not be confused (as shown in the figure above, the two are superimposed).
4. Boost formula
Depending on the volt-second characteristics of the inductor: the inductor is charged and discharged the same during the two times that the switch is closed and disconnected. That is:
That is:
After finishing, you can get:
According to this formula, we can clearly know that the boost factor is determined by the duty cycle of the PWM. When D = 0.5, the output is 2 times the input.