According to the control principle, switching power supply can be divided into the following three working modes:

1) pulse width modulation (PWM). Its main feature is that the switching period is constant, and the duty ratio is adjusted by changing the pulse width to achieve the purpose of voltage stabilization. Its core is the pulse width modulator. Fixed switching period provides convenience for designing filter circuits. However, its disadvantage is that the output voltage cannot be adjusted in a wide range due to the limitation of the minimum on-time of the power switch; In addition, the output terminal is generally connected with a dummy load (also called preload) to prevent the output voltage when there is no load.

Stand up. At present, most integrated switching power supplies adopt PWM mode.

2) Pulse frequency modulation, abbreviated as pulse frequency modulation (PFM). Its characteristic is that the pulse width is fixed, and the duty ratio is adjusted by changing the switching frequency to achieve the purpose of voltage stabilization. Its core is the pulse frequency modulator. In the circuit design, the sawtooth wave generator in the pulse width modulator should be replaced by a fixed pulse width generator, and the frequency should be changed by a voltage-frequency converter (such as a voltage-controlled oscillator VCO). The principle of voltage stabilization is that when the output voltage Uo rises, the pulse width of the controller output signal remains unchanged and the period becomes longer, so that the duty ratio decreases and Uo decreases. The output voltage adjustment range of PFM switching power supply is very wide, and the output end can be free from dummy load.

3) Mixed modulation means that the pulse width and switching frequency are not fixed, but can be changed, which belongs to the mixed mode of PWM and PFM. It includes a pulse width modulator and a pulse frequency modulator. Because both and T can be adjusted independently, the duty cycle can be adjusted in the widest range, which is suitable for making switching power supplies with adjustable output voltage width for laboratories.

The above three working modes are collectively called "Time Proportional Control" (TRC) mode. It should be pointed out that the pulse width modulator can be used as an independent integrated circuit (such as UC3842 pulse width modulator), can also be integrated into DC/DC converter (such as LM2576 switching regulator integrated circuit), and can also be integrated into AC /DC converter (such as TOP250 single-chip switching power supply integrated circuit). Among them, switching regulator belongs to DC/DC power converter, and switching power supply is generally AC /DC power converter.

Development trend:

1) miniaturization, thinning, light weight and high frequency. The volume and weight of switching power supply are mainly determined by energy storage elements (magnetic elements and capacitors), so the miniaturization of switching power supply is essentially to reduce the volume of energy storage elements as much as possible. In a certain range, the increase of switching frequency can not only effectively reduce the size of capacitors, inductors and transformers, but also suppress interference and improve the dynamic performance of the system, so high frequency is the main development direction of switching power supply.

2) High reliability. The components of switching power supply are ten times that of continuous power supply, which reduces its reliability. From the point of view of life, the life of electrolytic capacitor, optical coupler and exhaust fan determines the life of power supply. Therefore, from the design point of view, we should use as few devices as possible to improve the integration, adopt modular technology to meet the needs of distributed power system and improve the reliability of the system.

3) Low noise. One of the disadvantages of switching power supply is high noise. Simply pursuing high frequency will increase noise. Adopting partial resonance conversion circuit technology can not only improve the frequency but also reduce the noise in principle, so reducing the noise influence as much as possible is another development direction of switching power supply.

4) Adopt computer-aided design and control. Using CAA and CDD technology to design the latest conversion topology and the best parameters, the switching power supply has the simplest structure and the best working state. The introduction of microcomputer detection and control in the circuit can form a multi-functional monitoring system, which can detect, record and alarm automatically in real time.

5) Low output voltage technology. With the continuous development of semiconductor manufacturing technology, the work of microprocessor and portable electronic equipment is getting lower and lower, which requires the future DC-DC converter to provide lower output voltage to meet the power supply requirements of microprocessor and portable electronic equipment. The development of switching power supply has been closely related to the development of semiconductor devices and magnetic components. The realization of high frequency requires corresponding high-speed semiconductor devices and high-frequency electromagnetic components with excellent performance. The development of new high-speed devices such as power M O S F E T and I G B T, the development of low-loss magnetic materials for high frequency, the improvement of the structure and design method of magnetic components, the increase of the dielectric constant of filter capacitor and the decrease of its equivalent series resistance have always played a great role in promoting the miniaturization of switching power supply.

In short, in the field of switching power supply technology, people develop low-loss loop technology while developing new components, which promotes each other and promotes the development of switching power supply to small, thin, high frequency, low noise and high reliability with an annual market growth rate of more than two digits.