Working Conditions, Characteristics and Modes of Switching Power Supply

Switching power supply products are widely used in industrial automation control, military equipment, scientific research equipment, LED lighting, industrial control equipment, communication equipment, power equipment, instruments and meters, medical equipment, semiconductor refrigeration and heating, air purifier, electronic refrigerator, liquid crystal display, LED lamps, communication equipment, audio-visual products, security monitoring, LED lamp bag and computer chassis, Digital products and instruments.

Operating conditions of switching power supply:

1. Switch: power electronic devices work in the switching state rather than the linear state

2. High frequency: power electronic devices work at high frequency rather than low frequency close to power frequency

3. DC: the output of switching power supply is DC rather than AC

Operating characteristics of switching power supply:

Low power consumption and high efficiency. In the switching power supply circuit, transistor V works alternately in the on-off and off-on switching states under the excitation of the excitation signal. The conversion speed is very fast, and the frequency is generally about 50KHz. In some technologically advanced countries, it can achieve hundreds or nearly 1000khz. This makes the power consumption of switching transistor V very small, and the efficiency of power supply can be greatly improved, and its efficiency can reach 80%.

Small size and light weight. It can be clearly seen from the principle block diagram of switching power supply that heavy power frequency transformer is not used here. Since the dissipated power on the adjusting tube V is greatly reduced, a large heat sink is omitted. Due to these two reasons, the switching power supply has small volume and light weight.

Wide voltage stabilizing range. The output voltage from the switching power supply is adjusted by the duty cycle of the excitation signal, and 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 more stable output voltage. Therefore, the voltage stabilizing range of switching power supply is very wide and the voltage stabilizing effect is very good. In addition, there are two methods to change the duty cycle: pulse width modulation and frequency modulation. Switching power supply not only has the advantages of wide voltage stabilizing range, but also has many methods to realize voltage stabilizing. Designers can flexibly select various types of switching power supply according to the requirements of practical application.

The filtering efficiency is greatly improved, and the capacity and volume of the filtering capacitor are greatly reduced. At present, the working frequency of switching power supply is basically 50 kHz, which is 1000 times that of linear regulated power supply, which almost improves the filtering efficiency after rectification by 1000 times; Even if half wave rectification is adopted and capacitor filtering is added, the efficiency is improved by 500 times. Under the same ripple output voltage, when switching power supply is used, the capacity of filter capacitor is only 1 / 500 1 / 1000 of that in linear regulated power supply. The circuit forms are flexible and diverse, including self-excited and other excited, width modulated and frequency modulated, single ended and double ended, etc. designers can give full play to the advantages of various types of circuits and design switching power supplies that can meet different applications.

Operating mode of switching power supply:

Switching power supply generally has three working modes: fixed frequency and pulse width mode, fixed frequency and pulse width variable mode, and variable frequency and pulse width mode. The former working mode is mostly used for DC / AC inverter power supply or DC / DC voltage conversion; The latter two working modes are mostly used for switching regulated power supply. In addition, the output voltage of switching power supply also has three working modes: direct output voltage mode, average output voltage mode and amplitude output voltage mode. Similarly, the former working mode is mostly used for DC / AC inverter power supply or DC / DC voltage conversion; The latter two working modes are mostly used for switching regulated power supply.

According to the connection mode of switching devices in the circuit, the widely used switching power supply can be generally divided into three categories: series switching power supply, parallel switching power supply and transformer switching power supply. Among them, transformer switching power supply (hereinafter referred to as transformer switching power supply) can be further divided into: push-pull, half bridge, full bridge and so on; According to the excitation of the transformer and the phase of the output voltage, it can be divided into forward type, flyback type, single excitation type and double excitation type; If we divide them in terms of purpose, they can be divided into more categories.

Next, we will briefly introduce the working principles of the three most basic module power supplies: series, parallel and transformer. Other types of switching power supplies will also be analyzed in detail step by step.

Series switching power supply

Working principle of series switching power supply

When the control switch K is turned on, the switching power supply outputs a pulse voltage up with pulse width ton and amplitude UI to the load R; When the control switch K is turned off, it is equivalent to that the switching power supply outputs a pulse voltage with pulse width of toff and amplitude of 0 to the load R. In this way, the control switch K keeps "on" and "off", and a pulse modulated output voltage uo can be obtained at both ends of the load.

The output voltage uo of the control switch K is a pulse modulated square wave, the pulse amplitude up is equal to the input voltage UI, and the pulse width is equal to the on time ton of the control switch K. therefore, the average value UA of the output voltage uo of the series switching power supply can be obtained as follows:

Where ton is the on time of control switch K and t is the working cycle of control switch K. The average value UA of the output voltage uo can be changed by changing the ratio of the on time ton of the control switch K to the off time toff. Generally, it is called duty, which is expressed by D, that is:

The amplitude up of the output voltage uo of the series switching power supply is equal to the input voltage UI, and the average value UA of the output voltage uo is always less than the input voltage UI. Therefore, the series switching power supply generally outputs the voltage with the average value UA as the variable. Therefore, Series modular power supply belongs to step-down switching power supply.

Series switching power supply is also called chopper. Because of its simple working principle and high efficiency, it is widely used in output power control. For example, electric motorcycle speed controller and light power controller are all applications of series switching power supply. If the series switching power supply is only used for power output control, the voltage output can directly provide power output to the load without connecting the rectifier filter circuit; However, if it is used for regulated output, it must be rectified and filtered.

The disadvantage of series switching power supply is that the input and output share the same ground. Therefore, EMI interference and backplane electrification are easy to occur. When the input voltage is the rectified output voltage of mains power, it is easy to cause electric shock and unsafe to people.