With increasing global emphasis on energy issues, the energy consumption of electronic products is becoming increasingly prominent. Reducing standby power consumption and improving power supply efficiency have become urgent problems. While traditional linear regulated power supplies are simple in circuit structure and reliable in operation, they suffer from drawbacks such as low efficiency (only 40%-50%), large size, high copper and iron consumption, high operating temperature, and small adjustment range. To improve efficiency, switching power supplies have been developed. These power supplies achieve efficiencies of over 85%, have a wide voltage regulation range, and offer high voltage regulation accuracy without the need for a power transformer, making them a more ideal type of regulated power supply. For these reasons, switching power supplies are widely used in various electronic devices. A switching power supply uses a circuit to control the high-speed switching of transistors, converting direct current into high-frequency alternating current to supply a transformer for voltage transformation, thereby generating one or more required voltages.
Switching power supplies can be classified into three main operating modes based on their control principles:
1) Pulse Width Modulation (PWM) type. Its main characteristic is a fixed switching frequency, where the duty cycle is adjusted by changing the pulse width to achieve voltage regulation. Its core is the pulse width modulator. The fixed switching period facilitates the design of filter circuits. However, its disadvantages include being limited by the minimum on-time of the power switch, preventing wide-range adjustment of the output voltage; furthermore, a dummy load (preload) is generally required at the output to prevent the output voltage from rising under no-load conditions. Currently, most integrated switching power supplies use PWM.
2) Pulse Frequency Modulation (PFM). Its characteristic is a fixed pulse width, with the duty cycle adjusted by changing the switching frequency to achieve voltage regulation. Its core is the pulse modulator. In circuit design, a fixed pulse width generator replaces the sawtooth wave generator in the PFM, and a voltage-to-frequency converter (e.g., a voltage-controlled oscillator, VCO) is used to change the frequency. Its voltage regulation principle is: when the output voltage Uo increases, the pulse width of the controller output signal remains constant while the period increases, reducing the duty cycle and lowering Uo. PFM switching power supplies have a wide output voltage adjustment range, and a dummy load can be left unconnected at the output. In the modulation waveforms of PWM and PFM, tp represents the pulse width (i.e., the on-time tON of the power switch), and T represents the period. The difference between the two is relatively easy to see from this. However, they also have commonalities:
(1) Both adopt the voltage regulation principle of time ratio control (TRC). Whether tp or T is changed, the pulse duty cycle is ultimately adjusted. Although the methods used are different, the control objective is the same, and they can be said to have the same goal by different means.
(2) When the load changes from light to heavy, or the input voltage changes from high to low, the output voltage is kept stable by increasing the pulse width and increasing the frequency, respectively.
3) Hybrid modulation refers to a method where both the pulse width and switching frequency are not fixed and can be changed. It is a hybrid of PWM and PFM. It includes a pulse width modulator and a pulse frequency modulator. Since both pulse width and switching frequency (T) can be adjusted independently, the duty cycle adjustment range is the widest, making it suitable for manufacturing switching power supplies with a wide range of adjustable output voltage for laboratory use.
The above three operating modes are collectively referred to as "Time Ratio Control" (TRC). It should be noted that a pulse width modulator can be used as a standalone integrated circuit (e.g., the UC3842 pulse width modulator), integrated into a DC/DC converter (e.g., the LM2576 switching regulator IC), or integrated into an AC/DC converter (e.g., the TOP250 monolithic switching power supply IC). Switching regulators are a type of DC/DC power converter, while switching power supplies are generally AC/DC power converters.
A typical structure of a switching power supply is shown in Figure 2. Its working principle is as follows: Mains power enters the power supply and is first rectified and filtered into high-voltage DC. Then, it passes through a switching circuit and a high-frequency switching transformer to be converted into high-frequency low-voltage pulses. After further rectification and filtering, it finally outputs a low-voltage DC power supply. Simultaneously, a circuit in the output section feeds back to the control circuit, controlling the PWM duty cycle to achieve stable output voltage.
Switching Power Supplies - Analysis of Basic Principles and Development Trends of Switching Power Supplies
A switching power supply consists of the following four parts:
1) Main circuit: The main circuit from AC mains input to DC output. It mainly includes input filter, rectification and filtering, inverter, and output rectification and filtering.
(1) Input filter: Its function is to filter out the noise present in the power grid, and at the same time prevent the noise generated by the machine from being fed back to the public power grid.
(2) Rectification and filtering: The AC power from the grid is directly rectified into a smoother DC power for the next stage of conversion.
(3) Inversion: Converting rectified DC power into high-frequency AC power. This is the core part of high-frequency switching power supply. The higher the frequency, the smaller the ratio of volume, weight and output power.
(4) Output rectification and filtering: Provide a stable and reliable DC power supply according to the load requirements.
2) Control Circuit: On one hand, it samples the output, compares it with a set standard, and then controls the inverter to change its frequency or pulse width to achieve output stability. On the other hand, based on the data provided by the test circuit and identified by the protection circuit, it provides the control circuit with various protection measures for the entire machine. This includes the output sampling circuit, feedback circuit, and pulse width modulator.
3) Detection and protection circuits: Detection circuits include overcurrent detection, overvoltage detection, undervoltage detection, and overheat detection; protection circuits can be divided into various types such as overcurrent protection, overvoltage protection, undervoltage protection, clamping protection, overheat protection, automatic restart, soft start, and slow start.
4) Other circuits: such as sawtooth wave generators, bias circuits, optocouplers, etc.
Switching power supplies are much more efficient than linear power supplies. This saves energy, making them popular. However, they also have disadvantages: complex circuitry, difficult maintenance, and significant circuit pollution. They also generate more noise, making them unsuitable for certain low-noise circuits. The technological pursuits and development trends of switching power supplies can be summarized in the following five aspects:
1) Miniaturization, thinning, lightweighting, and high frequency.
2) High reliability.
3) Low noise.
4) Computer-aided design and control are adopted.
5) Low output voltage technology.
The development of switching power supplies has always been closely related to the development of semiconductor devices and magnetic components. Achieving higher frequencies requires corresponding high-speed semiconductor devices and high-performance high-frequency electromagnetic components. The development of new high-speed devices such as power MOSFETs and IGBTs, the development of low-loss magnetic materials for high-frequency applications, the improvement of the structure and design methods of magnetic components, and the enhancement of the dielectric constant of filter capacitors and the reduction of their equivalent series resistance have all played a significant role in promoting the miniaturization of switching power supplies.
Currently, switching power supplies are widely used in almost all electronic devices, including various terminal devices and communication equipment, with their small size, light weight, and high efficiency. They are an indispensable power supply method for the rapid development of today's electronic information industry. This has led to the requirement for switching power supplies to be small and lightweight, as well as to have higher efficiency, better performance, and higher reliability.
Continuous innovation in power electronics technology has given the switching power supply industry broad development prospects. To accelerate the development of my country's switching power supply industry, we must follow the path of technological innovation and forge a path of industry-academia-research collaboration with Chinese characteristics, contributing to the rapid development of my country's national economy.
