1. Boost circuit principle
The switching power supply boost circuit is a step-up DC-DC conversion circuit with an output voltage higher than the input voltage.
The topology of the boost circuit is shown in the figure below, which consists of a switch Q, an inductor L, an output filter capacitor C, a diode D, and a load R. Among them, the switch is usually driven by PWM waves to turn on and off, control the inductor energy storage, release energy, and then achieve voltage boost.
Boost circuit topology diagram
When the switch tube Q is on, the on-resistance RdsON is very small, which is equivalent to a short circuit, at this time, the left side of the diode is short-circuited to the ground, and the capacitance voltage on the right side cannot be abruptly changed, so the diode is in a cut-off state. The input power supply Vin charges the inductor L, and the inductor is stored for energy storage, because the current at both ends of the inductor cannot be abruptly changed (become larger), the inductor will produce a reverse electromotive force to prevent the current from becoming larger, the voltage polarity at both ends of the inductor L is left, positive and negative, and the voltage at both ends of the inductor is UL = Vin, and the equivalent circuit is shown in the figure below. Note that capacitor C supplies power to the load at this time, and as time increases, capacitor C continues to discharge, and the voltage at both ends of capacitor C (i.e., the output voltage) gradually decreases.
Boost circuit switch Q when the equivalent circuit diagram is turned on
When the switch is cut off at Q, it is equivalent to an open circuit, the diode is on, and the equivalent circuit is shown in the figure below. The inductor current can not be abruptly changed (smaller), the inductor will produce a reverse electromotive force to prevent the current from becoming smaller, the voltage polarity at both ends of the inductor is left negative and right positive, and the voltage at both ends of the inductor is UL = Vout-Vin, at this time the inductor is in a discharge state. It can be understood that the inductor is equivalent to a battery, which is connected in series with Vin (another battery) to supply power to the load and charge the capacitor C at the same time, so the output voltage is higher than the input voltage.
Equivalent circuit diagram of the Boost circuit at Q cut-off of the light opening tube
The current flowing through the inductor, the voltage across the inductor, and the output voltage are analyzed below. The switching period of the MOS tube is Ts, the on-time is Ton, the closing time is Toff, and the duty cycle D=Ton/Ts.
According to the law of volt-second equilibrium of inductance, that is, UL*Ton=UL*Toff.
对于Boost电路而言,UL*Ton = Vin*Ton,UL*Toff=(Vout-Vin)*(Ts-Ton)
Vin*Ton = (Vout-Vin)*(Ts-Ton), 通过公式推导,
占空比D = (Vout-Vin)/Vout。
After understanding the parameters, the output voltage of the inductor current, inductor voltage, and inductor voltage when the switch is turned on and off are analyzed, as shown in the figure.
As can be seen from the above figure, when the switch tube is turned on, that is, during the Ton period, the inductor current continues to increase, the voltage at both ends of the inductor is Vin, and the output capacitor is discharged to supply power to the load during the Ton period, and the voltage at both ends of the capacitor is continuously reduced, that is, the output voltage Vout is continuously reduced, and when the output voltage is less than the design value to a certain extent, the switch is closed. After the switch is closed, the current flowing through the inductor decreases during Toff, the voltage at both ends of the inductor is Vin-Vout, during the Toff period, Vin and the inductor supply power to the load together, the output voltage continues to increase, when it is higher than the design value to a certain extent, the switch tube is turned on again, and the switch tube is turned on and closed repeatedly, so as to control the inductor energy storage and release energy, and realize the voltage boost.
2. Synchronous type BOOST circuit
The above-mentioned circuit topology is an asynchronous Boost circuit, but the diode conduction voltage drop is large, and the power consumption is relatively high, so it does not have an advantage for applications with high efficiency requirements, so the synchronous Boost chip is selected, and the synchronous Boost circuit topology replaces the diode with a MOS tube, and the synchronous Boost circuit diagram is shown below.