Switched-Mode Power Supply (SMPS) is a power conversion device widely used in various electronic devices. Its main function is to convert the alternating current (AC) of the mains into the stable direct current (DC) required by the device. Compared with traditional linear power supplies, SMPS is more efficient, smaller and lighter, so it has been widely used in various modern electronic devices. So, how does SMPS convert AC to DC? This article will explore in detail the working principle of the switching power supply and the conversion process behind it through several key questions.

What is the basic structure of SMPS? How is it different from traditional power supplies?

Basic components of SMPS

The core components of the switching power supply include rectification circuit, filtering circuit, switching transistor, high-frequency transformer and feedback control circuit. These components work together to achieve efficient conversion of electrical energy.

Rectification circuit: converts AC into unstable DC.

Filter circuit: removes the ripple after AC conversion and produces smooth DC.

Switching transistor: modulates DC into high-frequency signals through high-speed switching action.
High-frequency transformer: adjust the output voltage according to different needs.
Feedback control circuit: monitor the output voltage in real time to ensure the stability of the power supply output.

The difference between SMPS and linear power supply

The most significant difference between SMPS and linear power supply is the working method. Traditional linear power supply controls the output voltage by adjusting the impedance of the transistor, which is inefficient and generates a lot of heat. SMPS works by high-speed switching, which can control the on and off of the current in a very short time, thereby reducing energy waste and improving conversion efficiency.
In terms of energy conversion efficiency, the efficiency of linear power supply is usually only 50% to 60%, while the efficiency of SMPS can be as high as 80% to 90%. This means that under the same input power, SMPS can convert more electrical energy into effective output power rather than into useless heat.

How does SMPS perform preliminary rectification?

The first working step of SMPS is to convert AC into unstable DC. This step is usually completed by a rectifier circuit.

Working principle of rectifier circuit

The rectifier circuit is composed of diodes. AC is a periodic waveform with a voltage direction that constantly changes alternately. The purpose of rectification is to convert this alternating voltage into a single-direction voltage, usually called "direct current". The diode has the characteristic of unidirectional conduction, which only allows current to pass in one direction, so it can conduct the positive half-cycle of the alternating current, while the negative half-cycle is blocked.

In SMPS, a bridge rectifier circuit is usually used. The bridge rectifier circuit consists of four diodes, which can rectify the current of the positive and negative half-cycles at the same time, and convert the alternating current into pulsating direct current. Although the current is unidirectional, it still has large fluctuations, which is what we call "unstable direct current".

The role of the filter circuit

The pulsating direct current generated after rectification cannot be directly supplied to the load because its voltage changes greatly. At this time, the filter circuit plays an important role. Usually the filter circuit is composed of a capacitor, which can "smooth" the rectified current, reduce the ripple in the current, and convert it into a relatively stable direct current.

The working principle of the capacitor is to store charge. When the output voltage of the rectifier circuit reaches the peak, the capacitor begins to charge, and when the output voltage drops, the capacitor will discharge to compensate for the drop in the output voltage, thereby reducing fluctuations.

How to calculate the capacity of the filter capacitor?

The capacity of the filter capacitor directly affects the smoothness of the output voltage. We can calculate the required capacity of the filter capacitor by the following formula:

C=I/(f×Vripple）

Where:

C is the capacity of the filter capacitor (unit: Farad, F)

I is the output current (unit: Ampere, A)

f is the frequency of the AC power (unit: Hertz, Hz, usually 50Hz or 60Hz)

Vripple is the required ripple voltage (i.e. the allowable voltage fluctuation amplitude)

For example, assuming that the output current is 2A, the AC frequency is 50Hz, and the allowable ripple voltage is 1V, the capacity of the filter capacitor is:

C= 2A/(50Hz×1V)=0.04F=40,000μF

This calculation shows that the larger the capacity of the filter capacitor, the smaller the fluctuation of the output voltage.

How does SMPS regulate current through switching action?

After completing the initial rectification and filtering, the next step of SMPS is to convert DC power into a high-frequency pulse signal. This step is achieved by switching transistors.

The role of switching transistors

Switching transistors are one of the core components in SMPS. Commonly used switching components include semiconductor devices such as MOSFET and IGBT. These components can switch at high speed and complete the switching of "on" and "off" states in a very short time (usually at a frequency of several thousand hertz or even several megahertz).

When the switching transistor is in the on state, the current passes through the primary coil of the transformer; when the switching transistor is in the off state, the current is interrupted, and the electrical energy in the transformer is released to the secondary coil to continue to power the load. This high-speed switching action can modulate DC into a high-frequency pulse signal.

Why use high-frequency switching?

The reason why SMPS uses high-frequency switching is that high-frequency current can reduce the volume of energy storage components such as transformers and inductors. Traditional linear power supplies require larger transformers due to their low operating frequency (usually 50Hz or 60Hz). In SMPS, due to the switching frequency of several thousand hertz or even higher, the volume of transformers and inductors can be greatly reduced, making the entire power system more compact and lightweight.

Duty cycle adjustment

The output voltage of SMPS can be controlled by adjusting the duty cycle of the switch. The duty cycle refers to the proportion of time that the switch is in the on state. The higher the duty cycle, the greater the output voltage and current; conversely, the smaller the output voltage and current. The feedback control circuit adjusts the duty cycle in real time according to the load requirements to ensure that the output voltage is stable.

How does SMPS convert high-frequency pulses into stable DC power?

After the SMPS generates a high-frequency pulse signal, the final step is to convert these high-frequency signals back into stable DC power for the load to use.

The role of high-frequency transformer

In SMPS, the transformer is not only used for voltage rise and fall conversion, but also can achieve electrical isolation to ensure safety. When the high-frequency signal passes through the transformer, the electrical energy is transferred between the primary coil and the secondary coil. By designing coils with different numbers of turns, the transformer can adjust the output voltage to ensure that the output meets the load requirements.

For example, if the input voltage is 300V DC, and we need an output voltage of 5V, the ratio of the number of turns of the primary coil and the secondary coil of the transformer is:

Turns ratio = input voltage ÷ output voltage = 300V ÷ 5V = 60
This means that the number of turns of the primary coil needs to be 60 times that of the secondary coil.

Rectification and filtering again

After the transformer converts the high-frequency pulse signal into the required voltage, these signals are still pulse waveforms and cannot be used directly. Therefore, it is necessary to pass through the rectification and filtering circuit again to rectify the high-frequency pulse into a smooth DC.

Rectification: Through Schottky diodes or fast recovery diodes, the positive and negative currents in the high-frequency pulses are separated.

Filtering: Through the filter capacitor again, the remaining high-frequency components are removed and the output current is smoothed.

Stability of feedback control

In the entire conversion process, the feedback control circuit plays a vital role. The feedback control circuit monitors the output voltage in real time and compares it with the preset target voltage. If the output voltage deviates from the target value, the control circuit adjusts the frequency and duty cycle of the switch to ensure that the output voltage remains stable. For example, when the load increases, the circuit will increase the duty cycle of the switch to provide more current; when the load decreases, the duty cycle will decrease accordingly to avoid excessive current output.

Feedback control usually adopts a voltage feedback loop. By measuring the output voltage and feeding it back to the controller, the controller adjusts the switching frequency as needed. This dynamic adjustment capability enables SMPS to still provide stable DC output under different load conditions.

What are the advantages of SMPS in converting AC to DC?

In the process of converting alternating current (AC) to direct current (DC), SMPS shows many advantages that traditional linear power supplies do not have, especially in terms of efficiency, volume and heat dissipation.

High efficiency

Switching mode power supply regulates the flow of current through high-speed switching of switching transistors, avoiding the high energy consumption caused by resistance regulation in traditional linear power supplies. Since the switching device only works between the "on" and "off" states, most of the energy can be effectively converted into the required DC power instead of being wasted through heat generation. The efficiency of SMPS can usually reach more than 80% to 90%, which is much higher than the 50% to 60% of linear power supplies.

Calculate energy efficiency:

Efficiency = output power / input power × 100%

Assuming the output power of the SMPS is 300W and the input power is 330W, its efficiency is:

Efficiency = 300W/330W×100%=90.91%

Small size and light weight

Since the SMPS uses high-frequency switching operation, the size of the transformer and inductor can be greatly reduced. Compared with the heavy low-frequency transformer in traditional power supplies, the high-frequency transformer of the SMPS is significantly reduced in size and weight while maintaining the same power output. This makes the SMPS very suitable for applications with limited space such as portable devices, laptops, and household appliances.

Less heat dissipation

The high efficiency of the SMPS means that it generates less heat during operation. This not only reduces the need for heat dissipation devices such as heat sinks and fans, but also improves the reliability of the overall system. When working for a long time, the low heat characteristics of the SMPS can avoid failures caused by overheating and extend the service life of the equipment.

High output voltage stability

Since SMPS has a feedback control circuit, even if the input voltage fluctuates or the load changes, SMPS can still maintain a stable output voltage by adjusting the switching frequency and duty cycle. This feature makes it very useful in devices that require precise voltage, such as computer power supplies, communication equipment, etc.

What are the applications of SMPS in daily equipment?

Due to its high efficiency and compact design, SMPS has become an indispensable part of modern electronic devices. The following are several common SMPS application areas:

Computers and laptops

Modern computers and laptops are almost all powered by SMPS. Such devices require high power efficiency and stable voltage output. Switch mode power supply can quickly adjust the output voltage when power demand changes to ensure that the computer can operate normally. In addition, the miniaturization design of SMPS also makes laptop power supplies light and portable.

Home appliances

Many home appliances, such as televisions, refrigerators, microwave ovens, etc., also use SMPS to provide stable DC power. Since home appliances often need to run for a long time, the high efficiency and low heat characteristics of SMPS help reduce power consumption and improve the reliability of the equipment.

Mobile phone chargers

Modern mobile phone chargers are basically designed based on SMPS. The compact charger shape and efficient power conversion allow users to charge their mobile phones quickly and stably. At the same time, SMPS can also protect the life of mobile phone batteries by intelligent control to avoid overcharging or overheating of the battery.

Industrial equipment and communication equipment

In industrial equipment and communication equipment that require high power supply, SMPS is also widely used due to its high efficiency and stability. Such equipment has very high requirements for the reliability of the power supply. SMPS can provide stable and accurate voltage output through advanced feedback control to ensure the normal operation of the equipment.

Conclusion

The switch mode power supply (SMPS) successfully converts AC power into stable DC power through multiple steps such as rectification, filtering, high-frequency switching and feedback control. It not only has the advantages of high efficiency, small size and light weight, but also can provide highly stable power output, which makes SMPS widely used in modern electronic devices. From mobile phone chargers to industrial equipment, SMPS has become the core of modern power supply technology. By deeply understanding its working principle, we can better apply and optimize SMPS to provide reliable power support for various electrical equipment.