DC-DC Converter Circuit Design

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　What is a DC-DC converter? In fact, it is a voltage converter that effectively outputs a fixed voltage after converting the input voltage through a DC-DC converter. Generally, DC-DC converters are divided into three categories: boost DC-DC converters, buck DC-DC converters, and buck-boost DC-DC converters. According to different needs, it can be divided into three types of control methods. The PWM control type has high efficiency and good output voltage ripple and noise. The PFM control type has the advantage of low power consumption even when used for a long time, especially when the load is small. The PWM-PFM conversion type implements PFM control when the load is small, and automatically switches to PWM control when the load is heavy. At present, DC-DC converters are widely used in mobile phones, MP3, digital cameras, portable media players and other products.

　　What is DC?

　　It refers to a DC power source, such as a dry cell or a car battery. The 220V power source for home use is an AC power source (AC). If a DC voltage (3.0V) can be converted into another DC voltage (1.5V or 5.0V) through a converter, we call this converter a DC-DC converter, or a switching power supply or a switching regulator.

　　A: A DC-DC converter is generally composed of a control chip, an inductor, a diode, a transistor, and a capacitor. When discussing the performance of a DC-DC converter, it is not possible to judge its quality by focusing only on the control chip. The characteristics of the components in its peripheral circuits and the wiring method of the substrate can change the performance of the power supply circuit, so a comprehensive judgment should be made.

　　B: Modulation method

　　1. PFM (Pulse Frequency Modulation)

　　The switching pulse width is constant, and the output voltage is stabilized by changing the frequency of the pulse output.

　　2. PWM (Pulse Width Modulation)

　　The frequency of the switching pulse is constant, and the output voltage is stabilized by changing the pulse output width.

　　C: Generally speaking, the performance differences of DC-DC converters using two different modulation methods, PFM and PWM, are as follows.

　　How to select the frequency of PWM and the duty cycle of PFM.

　　DC-DC Converter Circuit Design Principles

　　DC-DC is a direct current-to-direct current conversion, which is generally divided into two types: boost (BOOST) and buck (BUCK). The output current of the buck DC-DC converter is relatively large, mostly hundreds of milliamperes to several amperes, so it is suitable for occasions with large output current. The basic working principle circuit of the buck DC-DC converter is shown in the figure below. VT1 is a switch tube. When VT1 is turned on, the input voltage Vi supplies power to the load RL through the inductor L1, and at the same time charges the capacitor C2. In this process, energy is stored in the capacitor C2 and the inductor L1. When VT1 is turned off, the energy stored in the inductor L1 continues to supply power to RL. When the output voltage is about to drop, the energy in the capacitor C2 is also discharged to RL to maintain the output voltage unchanged. The diode VD1 is a freewheeling diode to form a circuit loop. The output voltage Vo is divided by the voltage divider composed of R1 and R2, and the output voltage signal is fed back to the control circuit. The control circuit controls the on and off time of the switch tube to keep the output voltage unchanged.

　　The following conditions should be considered in DC-DC circuit design:

　　1. The range of external input power supply voltage and the size of output current.

　　2. DC-DC output voltage, current, and maximum power of the system.

　　The key points to consider when selecting a PWM IC are:

　　1. Maximum input voltage of PWM IC.

　　2. The frequency of PWM switching is related to the efficiency of the system. It also has a certain impact on the selection of the size of the energy storage inductor and capacitor.

　　3. The maximum rated current and rated power that the MOS tube can withstand. If the DC-DC IC has its own MOS, you only need to consider the rated current of the IC output.

　　4. The switching voltage Vgs of MOS and its maximum withstand voltage.

　　Selection of inductors, diodes, and capacitors

　　1. Inductance: The size is mainly determined by the switching frequency, and the size will affect the power supply ripple; the rated current and the internal resistance of the inductor are determined by the system power consumption.

　　2. Diode: Schottky diode is generally used. When selecting, the reverse voltage and forward current should be considered. Generally, the reverse voltage is twice the input power supply voltage, and the forward current is twice the output current.

　　3. Capacitor: The choice of capacitor is based on the switching frequency, system ripple requirements and output voltage requirements. The capacity and the equivalent resistance inside the capacitor determine the ripple size (of course, it is also related to the inductance).