Tips on DC/DC Converter Circuit Design

1. Correctly understand DC/DC converters

A DC/DC converter is a voltage converter that effectively outputs a fixed voltage after converting the input voltage. DC/DC converters are divided into three categories: boost DC/DC converters, buck DC/DC converters, and buck-boost DC/DC converters. Three types of control can be used according to demand. 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. In terms of circuit type classification, it belongs to the chopper circuit.

2. DC/DC converter circuit design principle

DC-DC is a DC-DC 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.

Image description: Basic working principle diagram of DC/DC converter

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

4. The key points to consider when choosing 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, only the rated current of the IC output needs to be considered.

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

5. 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 usually 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).

6. How to get a DC-DC circuit with relatively small power ripple, relatively small interference to other circuits in the system, and relatively stable and reliable? The principle of the above circuit needs to be modified as follows:

1. Input part: Inductor and capacitor filtering are required at the power input. Purpose: The switching of the MOS tube and the instantaneous change of the inductor will cause the fluctuation of the input power supply, especially when the system power consumption fluctuates greatly, the impact is more obvious.

2. Output part:

(1) Assuming that the 100uF capacitor selected for C2 is correct, if we want to get a smaller ripple, we can change the 100uF capacitor into two 47uF capacitors (based on the same type of capacitor); if the 100uF capacitor is aluminum electrolytic, we can add a 10uF magnetic sheet capacitor or tantalum capacitor to the original one.

(2) Adding a capacitor and a capacitor at the output end to perform an LC filter on the original power supply will result in a power supply with smaller ripple.

In short, the DC-DC converter provides power to each circuit in the entire system. Only by mastering the skills of DC/DC converter circuit design and taking all factors into consideration can the overall performance of the system be improved and the performance of each circuit be reflected.