Advantages and disadvantages of low voltage drop linear regulator in switching power supply

Power supply is an indispensable component of various electronic devices. The quality of its performance is directly related to the technical indicators of the electronic equipment, whether it can work safely and reliably, and its advantages and disadvantages.

Power supply is an indispensable component of various electronic devices. The quality of its performance is directly related to the technical indicators of electronic devices and whether they can work safely and reliably. Currently, the commonly used DC regulated power supplies are divided into two categories: linear power supplies and switching power supplies. Since the key components inside the switching power supply work in a high-frequency switching state, the energy consumed by the switching power supply itself is very low. The efficiency of the switching power supply can reach 80% to 90%, which is nearly doubled that of the ordinary linear regulated power supply. It has become the mainstream product of regulated power supplies.

2 Structure of switching regulated power supply

Figure (1) shows the schematic diagram and equivalent schematic diagram of the switching power supply, which consists of a full-wave rectifier, a switch tube Vi, an excitation signal, a freewheeling diode VD, an energy storage inductor and a filter capacitor C. In fact, the core part of the switching power supply is a DC transformer. Here we explain the DC converter and inverter as follows. Inverter, it is a device that converts DC into AC. Inverters are widely used in backup power supplies composed of level or battery. DC converter, it is a device that converts DC into AC and then converts AC into DC. This device is widely used in switching power supplies. A DC converter can be used to convert a DC supply voltage into multiple DC supply voltages with different polarities and values.

3 Advantages and disadvantages of switching regulated power supply

3.1 Advantages of Switching Power Supply

Low power consumption and high efficiency. In the switching voltage-stabilized power supply circuit in Figure (1), the transistor V, under the stimulation of the excitation signal, alternately works in the switching state of on-off and off-on, with a very fast switching speed, and the frequency is generally around 50kHz. In some technologically advanced countries, it can reach several hundred or nearly 1000kHz. This makes the power consumption of the switching transistor V very small, and the efficiency of the power supply can be greatly improved, and its efficiency can reach 80%.

Small size and light weight. From the principle block diagram of the switching voltage-stabilized power supply, it can be clearly seen that no bulky power frequency transformer is used here. Since the power dissipation on the adjustment tube V is greatly reduced, a large heat sink is omitted. Due to these two reasons, the switching voltage-stabilized power supply is small in size and light in weight.

Wide voltage regulation range. The output voltage of the switching voltage regulator is adjusted by the duty cycle of the excitation signal. The change of the input signal voltage can be compensated by frequency modulation or width modulation. In this way, when the power frequency grid voltage changes greatly, it can still ensure a relatively stable output voltage. Therefore, the voltage regulation range of the switching power supply is very wide and the voltage regulation effect is very good. In addition, there are two methods to change the duty cycle: pulse width modulation and frequency modulation. In this way, the switching voltage regulator not only has the advantage of a wide voltage regulation range, but also has many ways to achieve voltage regulation. Designers can flexibly select various types of switching voltage regulators according to the requirements of actual applications.

The efficiency of filtering is greatly improved, which greatly reduces the capacity and volume of the filter capacitor. The operating frequency of the switching power supply is currently basically 50kHz, which is 1000 times that of the linear power supply, which makes the filtering efficiency after rectification almost 1000 times higher. Even if half-wave rectification is used followed by capacitor filtering, the efficiency is also increased by 500 times. Under the same ripple output voltage, when using a switching power supply, the capacity of the filter capacitor is only 1/5001/1000 of the filter capacitor in the linear power supply.

The circuit forms are flexible and diverse. For example, there are self-excited and externally excited types, width-modulated and frequency-modulated types, single-ended and double-ended types, etc. Designers can give full play to the strengths of various types of circuits and design switching power supplies that can meet different application occasions.

3.2 Disadvantages of Switching Power Supply

The disadvantage of the switching power supply is the existence of serious switching interference. In the switching power supply, the power adjustment switch transistor V works in the switching state, and the AC voltage and current it generates pass through other components in the circuit to generate spike interference and resonance interference. If these interferences are not suppressed, eliminated and shielded by certain measures, they will seriously affect the normal operation of the whole machine. In addition, since the switching power supply oscillator is not isolated by the power frequency transformer, these interferences will be connected to the power frequency power grid, causing serious interference to other nearby electronic instruments, equipment and household appliances.

At present, due to the gap between domestic microelectronics technology, resistor and capacitor device production technology and magnetic material technology and some technologically advanced countries, the cost cannot be further reduced, which also affects the further improvement of reliability. Therefore, in my country's electronic instruments and mechatronics instruments, switching power supplies cannot be widely popularized and used. In particular, for high-voltage electrolytic capacitors, high-reverse-voltage high-power switching tubes, and magnetic core materials of switching transformers in switching power supplies without power frequency transformers, they are still in the research and development stage in my country. In some technologically advanced countries, although switching power supplies have made certain developments, there are still some problems in practical applications, which are not very satisfactory. This exposes another disadvantage of switching power supplies, that is, the circuit structure is complex, the failure rate is high, and maintenance is troublesome. In this regard, if designers and manufacturers do not pay enough attention to it, it will directly affect the promotion and application of switching power supplies. Today, the main reason why the promotion and application of switching power supplies is difficult is that its manufacturing technology is difficult, maintenance is troublesome, and the cost is high.

4 LDO Introduction

LDO is a micro-power low-dropout linear regulator, which usually has extremely low self-noise and high power supply rejection ratio PSRR (Power Supply Rejection Ratio).

The structure of the LDO low-dropout linear regulator is shown in Figure (2). It mainly includes a startup circuit, a constant current source bias unit, an enable circuit, an adjustment element, a reference source, an error amplifier, a feedback resistor network, and a protection circuit. The basic working principle is as follows: when the system is powered on, if the enable pin is at a high level, the circuit starts to start, the constant current source circuit provides bias for the entire circuit, the reference source voltage is quickly established, and the output continues to rise with the input. When the output is about to reach the specified value, the output feedback voltage obtained by the feedback network is also close to the reference voltage value. At this time, the error amplifier amplifies the small error signal between the output feedback voltage and the reference voltage, and then amplifies it to the output through the adjustment tube, thereby forming a negative feedback to ensure that the output voltage is stable at the specified value. Similarly, if the input voltage changes or the output current changes, this closed loop will keep the output voltage unchanged, that is: Vout=(R1+R2)/R2 ×Vref

The actual low dropout linear regulator also has other functions such as load short circuit protection, overvoltage shutdown, overtemperature shutdown, reverse connection protection, etc.