1.Basic Working Principle
When the switch SW1 is closed, the current flows from the power supply VIN along the path: VIN → L1 → SW1 → GND. At this time, the inductor stores magnetic energy (the current gradually increases), while the capacitor C2 supplies power to the load (maintaining the VOUT voltage).
When SW1 is turned off, the current in the inductor cannot change suddenly. To maintain the current, an induced electromotive force (with the polarity of negative on the left and positive on the right) is generated across the inductor. The inductor voltage is connected in series with the power supply voltage and charges the capacitor C2 through the diode D1, while also supplying power to the load. At this time, the VOUT voltage is boosted to be higher than VIN.
When SW1 is closed again, the diode D1 is reverse - biased and cut off, preventing the capacitor C2 from discharging through SW1. The capacitor C2 can only discharge through the load at a rate determined by the RC time constant, thus maintaining the stability of the output voltage VOUT. By adjusting the duty cycle of the switching tube (the ratio of the on - time to the period), the inductor continuously stores and releases energy, and the capacitor is continuously charged and discharged. Finally, a stable step - up effect (VOUT > VIN) is obtained at the output.
2. Key Parameters in Circuit Design
2.1 Selection and Calculation of the Inductor
In the BOOST step - up circuit, the selection of the inductor is crucial for the circuit's working mode, output voltage stability, and efficiency. The inductor value and saturation current need to be carefully considered: The inductor value determines the rate of current change. If it is too high, the startup time will be prolonged; if it is too low, the current may drop to zero quickly when the switching element is turned off, causing the circuit to enter the discontinuous conduction mode (DCM). The saturation current of the inductor must be greater than the maximum current during normal circuit operation (including the steady - state peak current and transient spike current) to prevent saturation, which may lead to the deterioration of the inductor's performance and faults such as over - current.
The inductor value is usually calculated according to the following formula (∆IL is the change in inductor current). The actual value should be 30% - 50% larger than the theoretical value to provide sufficient design margin:
2.2 Selection and Calculation of the Output Filter
The output filter plays a key role in filtering out switching noise and maintaining the stability of the output voltage. A typical output filter consists of an output capacitor and a sensing resistor. The main points for selecting its parameters are as follows:
The capacitance of the output capacitor needs to be determined based on the required ripple voltage to ensure sufficient charge - discharge capacity to suppress voltage fluctuations.
The lower the equivalent series resistance, the better the filtering effect and the longer the capacitor's lifespan. It can effectively reduce the high - frequency components in the ripple voltage.
The sensing resistor value should be as small as possible to accurately measure the output current while reducing power loss, balancing measurement accuracy and energy efficiency.
The formula for calculating the output capacitor is (∆VOUT is the allowable ripple of the output voltage). The actual value should be 30% - 50% larger than the theoretical value to provide sufficient design margin:
3. Working Modes
According to whether the inductor current is continuous, the BOOST circuit can be divided into the following three working modes:
3.1 CCM (Continuous Conduction Mode)
Its working characteristics are that the inductor current is always greater than zero throughout the switching cycle. It is suitable for scenarios with a large load current and has high efficiency.
The working process is as follows: When the switch is turned on, the diode is reverse - biased and cut off. The input power supply charges the inductor, and the inductor current increases linearly. The load is powered by the output capacitor. When the switch is turned off, the inductor discharges to the load and the capacitor through the diode. The inductor current decreases linearly but remains positive, and at the same time, it charges the output capacitor. The voltage conversion relationship is Vout = Vin/(1 - D) (D is the duty cycle, with a value range of 0 < D < 1).
The conditions it needs to meet are:
3.2 BCM (Boundary Conduction Mode)
Since the inductor current has dropped to zero before the switching tube is turned on, zero - current switching (ZCS) can be achieved, effectively reducing switching losses. It has certain advantages in improving circuit efficiency and reliability.
The conditions it needs to meet are:
3.3 DCM (Discontinuous Conduction Mode)
Its working characteristics are that the inductor current drops to zero in each switching cycle. It is suitable for light - load or small - current situations, and its control characteristics are highly non - linear.
The working process is as follows: The switch - on stage is the same as in the CCM mode. The input power supply charges the inductor, and the inductor current rises linearly. The load is powered by the output capacitor. In the switch - off stage, the inductor discharges to the load and the capacitor through the diode. The inductor current decreases linearly to zero, and then the diode is cut off. The load is completely powered by the output capacitor. During the zero - current stage, the inductor current remains zero until the next cycle starts. This mode can avoid the efficiency drop that may occur in the CCM mode under light - load conditions, but the non - linear control characteristics require higher precision in circuit design.
The conditions it needs to meet are:
