Design principles of boost circuit and buck circuit

The most widely used scenario of inductors is power supply. Both boost circuits and buck circuits require an inductor to store and release energy. Many novice friends are too familiar with the principle of inductor boost circuits. All boost and buck circuits use the important principle that "inductor current cannot change suddenly". That is, the current in the inductor has inertia, and this inertia is the energy stored in the inductor.

In the series backlight boost circuit of the example LCD screen, the boost IC mainly controls the switch on the inductor through the LX pin. In the boost circuit of the electrotherapy device, the switch on the inductor is controlled by the PWM port of the microcontroller. It is not easy to understand by just reading the text, so we use a diagram to indicate the direction of the current. (Here we emphasize the "unidirectional conduction" characteristic of the diode. The current in the diode can only flow in one direction, and the current cannot flow in the reverse direction.)

First, the switch is turned on, the inductor is short-circuited to the ground, and current is generated inside the inductor. (There is a switch inside the chip, and the transistor in the other picture also serves as a switch) Then, the switch is turned off, and the current from the inductor to the ground is cut off, but the current on the inductor cannot disappear immediately, and needs to find a way to discharge, so it runs to the load end. The load cannot consume so much current, so the current in the inductor becomes the voltage across the load, which increases the voltage.

In the next cycle, the switch is turned on, and the inductor generates current. Although the voltage on the right side of the diode is higher than that on the left side, it cannot flow in the opposite direction, so the high voltage is maintained. Then the switch is closed again, and the inductor releases energy to the load, and the voltage continues to rise. In this cycle, the inductor is constantly charged and discharged, providing pulse energy for the back end of the diode. By controlling the ratio of the time when the switch is turned on and off, it is possible to control how much energy is output from the inductor. This is to adapt to changes in the load by changing the duty cycle of the control signal so that the voltage is always maintained at the required value.

For ordinary boost circuits (left side of the above picture), there is a load, overvoltage protection (OVP), and voltage detection, and the voltage will rise to a stable value. For a simple boost circuit such as the electrotherapy circuit (right side of the above picture), the human body resistance is at the megohm level, which is basically equivalent to an open circuit. Every time the inductor is charged and discharged, the voltage of the rear section of the diode will be increased. If the rear section voltage is measured with an oscilloscope, it will be a step-like rise. By controlling the number of switches, the amplitude of the voltage increase can be controlled, and the maximum can exceed 200V. Because the power is very small, the human body will only feel a slight electric shock, which will not cause danger.

Boost circuit input and output relationship

A boost circuit is a commonly used circuit that can increase voltage from a lower level to a higher level to meet different application requirements. There are important relationships between the input and output of a boost circuit, and these relationships are very important when designing and using a boost circuit. In this article, we will discuss the input-output relationship of a boost circuit in detail, including its working principle, key components, and how to select appropriate circuit parameters.

1. Working principle of boost circuit

The working principle of the boost circuit is very simple. It increases the voltage through the process of charging and discharging. The most important components in the boost circuit are the inductor capacitor. When the capacitor is charged, the current in the inductor gradually increases, and the voltage in the capacitor also gradually increases. When the capacitor is fully charged, the current in the inductor begins to flow back to the capacitor, and this process causes the voltage in the capacitor to rise higher. In this way, the boost circuit can increase the voltage from a lower level to a higher level.

2. Key components of boost circuit

The key components of the boost circuit include inductors, capacitors, and switches. Among them, the role of the inductor is to store energy. When the capacitor is charged, the energy is stored in the inductor. When the capacitor is discharged, the energy in the inductor will be released, causing the voltage in the capacitor to increase. The role of the capacitor is to store charge and store electrical energy. The role of the switch is to control the switch of the circuit. When the switch is turned on, the capacitor starts to charge, and when the switch is turned off, the capacitor starts to discharge.

3. Boost circuit

When designing and using a boost circuit, you need to consider the circuit's parameters to ensure that the circuit can work properly. The following are some important parameters that require special attention.

1. Inductor parameters

The parameters of an inductor include inductance value and current saturation current, where inductance value refers to the ability of the inductor to store energy and current saturation current refers to the maximum current output of the inductor. When selecting an inductor, it is necessary to consider the required output voltage and current to ensure that the circuit can work properly.

2. Capacitor parameters

The parameters of capacitors include capacitance and operating voltage, where capacitance refers to the amount of charge that a capacitor can store and operating voltage refers to the maximum voltage that a capacitor can withstand. The selection of capacitors needs to take into account the required output voltage and current to ensure that the circuit can work properly.

3. Switch parameters

The parameters of a switch include switching frequency and maximum current. The switching frequency refers to the frequency at which the switch can perform switching operations, and the maximum current refers to the maximum current that the switch can withstand. When selecting a switch, you need to consider the required output voltage and current to ensure that the circuit can work properly.

Summarize:

A boost circuit is a commonly used circuit that can increase the voltage from a lower level to a higher level to meet different application requirements. There is an important relationship between the input and output of the boost circuit, which is very important when designing and using the boost circuit. The key components of the circuit include inductors, capacitors, and switches, etc., and appropriate parameters need to be selected to ensure that the circuit can work properly. Only when the circuit parameters are correctly selected can the boost circuit work stably and provide the required voltage and current output.