DC/DC converter circuit design skills

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It is well known that designing an ideal DC-DC converter involves numerous trade-offs. An increase in power density usually means an increase in overall power consumption, as well as an increase in junction temperature, case temperature, and PCB temperature. Similarly, optimizing a DC/DC power supply for medium to peak currents almost always means sacrificing light load efficiency and vice versa. This paper mainly makes a simple analysis of the DC-DC converter circuit design skills.

First, correctly understand the DC / DC converter
The DC/DC converter is a voltage converter that effectively outputs a fixed voltage after the input voltage is converted. There are three types of DC/DC converters: step-up DC/DC converters, step-down DC/DC converters, and buck-boost DC/DC converters. Three types of controls are available depending on the requirements. The PWM control type is highly efficient and has 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 at small loads. PFM control is implemented during PWM/PFM conversion type small load, and is automatically switched to PWM control at heavy load. At present, DC-DC converters are widely used in mobile phones, MP3 players, digital cameras, portable media players and other products. It is a chopper circuit in the classification of circuit types.

Second, DC / DC converter circuit design principle
DC-DC is a DC-DC converter, generally has two types of boost (BOOST) and step-down (BUCK). The output current of the buck DC/DC converter is large, ranging from hundreds of milliamps to several amps, so it is suitable for applications where the output current is large. The basic working principle circuit of the step-down DC/DC converter is shown in the figure below. VT1 is a switching transistor. When VT1 is turned on, the input voltage Vi is supplied to the load RL through the inductor L1, and at the same time, the capacitor C2 is also charged. In this process, energy is stored in capacitor C2 and inductor L1. When VT1 is turned off, the energy stored in the inductor L1 continues to supply power to the RL. When the output voltage is to be decreased, the energy in the capacitor C2 is also discharged to the RL, maintaining the output voltage unchanged. Diode VD1 is a freewheeling diode to form a circuit loop. The output voltage Vo is divided by a voltage divider composed of R1 and R2, and the output voltage signal is fed back to the control circuit, and the control circuit controls the on and off times of the switch tube to keep the output voltage constant.


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

Third, the DC-DC circuit design should consider the following conditions:
1. The range of the external input supply voltage and the magnitude of the output current.
2. DC-DC output voltage, current, system power maximum.

Fourth, the main points to consider when choosing a PWM IC are:
1. The maximum input voltage of the PWM IC.
2. The frequency of the PWM switch, the choice of this is related to the efficiency of the system. For the storage inductor, the choice of the size of the capacitor also has a certain impact.
3. The maximum rated current and its 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. MOS switching voltage Vgs size and maximum withstand voltage.

Five, the choice of inductors, diodes, capacitors
1. Inductance: The size selection is mainly determined by the switching frequency. 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. Diodes: Schottky diodes are usually used. When selecting, the reverse voltage and forward current should be filtered. Generally, the reverse voltage is twice the input power supply voltage, and the forward current is twice the output current.
3. Capacitance: The choice of capacitor is based on the frequency of the switch, the requirements of the system ripple and the output voltage requirements. The equivalent resistance inside the capacitor and capacitor determines the ripple size (of course also related to the inductor).

Sixth, how to get a DC-DC circuit with relatively small power supply ripple, relatively small interference to other circuits of the system, and relatively stable and reliable, the following principles need to be modified:
1. Input section: Insulation capacitor filtering is required at the power input terminal. Purpose: Due to the instantaneous change of the switch and inductance of the MOS tube, the input power supply will fluctuate, especially when the system consumes large fluctuations.
2. Output section:
(1) Assuming 100uF of C2 is correct, we want to get smaller ripple, we can change 100uF capacitor into two 47uF capacitors (based on the same type of capacitor); if 100uF capacitor is aluminum electrolysis, A 10uF magnetic or tantalum capacitor can be added to the original.
(2) Adding a capacitor and a capacitor to the output to perform an LC filter on the original power supply will result in a smaller ripple power supply.

In summary, the DC-DC converter supplies power to the various circuits in the overall system. Only master the skills of DC/DC converter circuit design, consider all the factors to be considered in order to improve the overall performance of the system and achieve the performance effect of each circuit.