The internal structure of a switching power supply IC (integrated circuit) typically includes the following core modules:
Core control module
A PWM (Pulse Width Modulation) controller is used to control the on-time of the power switching transistor by adjusting the pulse width, thereby achieving voltage regulation. For example, a DC/DC buck converter chip dynamically adjusts its duty cycle to maintain a stable output by detecting the output voltage feedback signal.
drive circuit
This includes a power amplifier or dedicated driver module that converts the control signal into a high-current signal sufficient to drive the power MOSFET. For example, the LM2675 chip amplifies the drive signal through internal operational amplifier circuitry to control the switching state of the external MOSFET.
Protection mechanism
It integrates overcurrent protection and undervoltage lockout (UVLO) modules, automatically shutting down the output or adjusting power when the input voltage is abnormal or the load changes abruptly. Some high-end chips also include EMI filtering modules to reduce electromagnetic interference.
Feedback loop
The output voltage is monitored using optocouplers or a resistor divider network, and the signal is fed back to the control module to achieve closed-loop regulation. For example, optocouplers convert output voltage changes into electrical signals, adjusting the PWM frequency to compensate for voltage fluctuations.
Different models of switching power supply ICs (such as DC/DC and AC/DC converters) differ in their internal structure, but their core design logic (PWM control + power output stage) is common.
Detailed Explanation of the Working Principle and Circuit Diagram of Switching Power Supplies
A switching power supply, as the name suggests, works similarly to a controllable door. Behind this "door" are components such as thyristors or switching transistors that perform the on/off operations. These components are controlled by pulse signals on their base or control electrode. When the positive half-cycle of the pulse signal arrives, the voltage on the control electrode rises, causing the switching transistor or thyristor to conduct. At this time, the 220V voltage, after rectification and filtering, outputs a 300V voltage, which is then transmitted to the secondary side through a switching transformer. Through the transformation ratio, this voltage is increased or decreased to supply power to various circuits.
When the negative half-cycle of the oscillation pulse arrives, the base voltage of the power supply regulator or the control electrode voltage of the thyristor will drop below the original set value, causing the power supply regulator to turn off. At this time, the 300V power supply will be shut off, and there will be no voltage on the secondary side of the switching transformer. The operating voltage required by each circuit is then maintained by the discharge of the rectifier and filter capacitors in the secondary circuit.
This process repeats itself until the positive half-cycle of the next pulse signal arrives, at which point the entire process restarts. This switching transformer is called a high-frequency transformer, and its operating frequency is much higher than the low frequency of 50Hz.
So, how is the pulse signal that drives the switching transistor or SCR generated? This requires an oscillating circuit. A transistor operates in amplification mode when the base-emitter voltage reaches 0.65-0.7V, enters saturation conduction mode above 0.7V, and oscillates between -0.1V and -0.3V. By adjusting its operating point and using negative feedback to generate a negative voltage, the oscillating transistor begins to oscillate. The frequency of the oscillating transistor is determined by the charging and discharging time of the capacitor at the base, which in turn affects the output voltage of the power supply regulator.
So, how is the operating voltage of the secondary output of a switching transformer regulated? Typically, we only wind one coil on the switching transformer, and the voltage obtained from its upper end is rectified and filtered to form a reference voltage. This reference voltage is then fed back to the base of the oscillator transistor via an optocoupler, thereby adjusting the oscillation frequency. When the transformer secondary voltage increases, the voltage output from the sampling coil also increases accordingly, leading to an increase in the positive feedback voltage obtained through the optocoupler. This increased voltage, applied to the base of the oscillator transistor, causes the oscillation frequency to decrease, thus stabilizing the secondary output voltage. The detailed working principle is too complex; we only need to understand the general process. It is worth noting that high-power voltage is transmitted through the switching transformer and isolated from the subsequent circuitry to ensure safety. Simultaneously, the returned sampling voltage is also transmitted via an optocoupler and isolated from the subsequent stages, thus achieving complete separation between the front-end mains voltage and the subsequent stages. This is what we call cold-plate technology, ensuring the safety of the switching power supply.
This schematic diagram of a switching power supply clearly shows the various components of the power supply and their interactions. Starting with the AC power input, the power is rectified and filtered, ultimately converting it into a stable DC power output to provide the necessary electrical energy to the load. This process involves not only basic circuit principles but also technologies related to power conversion, electromagnetic compatibility, and more. Through this schematic, we can gain a deeper understanding of the working mechanism and design considerations of switching power supplies.
This switching power supply schematic provides a brief overview of the power supply's workflow. Starting with the AC input, the power is switched by the transistor and transformed by the transformer, ultimately outputting a stable DC power supply. This process involves not only basic circuit knowledge but also complex technologies such as power conversion and electromagnetic compatibility. This schematic offers a more intuitive and comprehensive understanding of the working principle of a switching power supply.
When discussing the working principle of switching power supplies, we encountered a technical challenge: in current mode, to stabilize the slope and ensure performance at a duty cycle greater than 50%, slope compensation is required. This mechanism involves an additional compensation slope, the specific implementation of which is relatively complex and requires in-depth understanding. Furthermore, the error amplifier plays a crucial role in the switching power supply. It is responsible for sampling and processing the feedback voltage, thereby adjusting the PWM signal driving the power MOSFET to ensure a constant output current or voltage. This process is illustrated in the figure. Through precise control of the error amplifier, we can achieve stable output from the switching power supply.
The drive circuit has a relatively simple structure, with the core component being a large-area MOSFET, which has excellent current driving capability.
Other circuit modules play an indispensable role in chip design. Although they do not directly involve the core principles, they are crucial for the normal and reliable operation of the chip. Specifically, these circuit modules include the startup module and the overvoltage protection module (OVP). The startup module's task is to start the chip at the moment of power-on, preventing the transistor current from being zero and remaining constant at power-on, which would cause the chip to malfunction. Its working principle is similar to the "ignition" process, using a series of switching operations to ultimately start the chip and enter normal operating mode. The overvoltage protection module (OVP) activates when the input voltage exceeds a set value, shutting off the output through a switching transistor to protect the chip from damage. This module uses a comparator to set the protection point, ensuring a timely response in dangerous situations.
3. Over-temperature protection module (OTP)
To protect the chip from damage caused by abnormally high temperatures, we introduced an Over-Temperature Protection (OTP) module. Its working principle is based on the temperature characteristics of the transistor, using a comparator to set a protection point, thereby promptly shutting down the output when the temperature is too high, ensuring the chip's safety.
4. Overcurrent Protection Module (OCP)
In abnormal situations such as output short circuits, overcurrent protection measures are necessary to ensure circuit safety. The overcurrent protection module (OCP) monitors the output current in real time and uses feedback control of the output transistor based on the detection results to achieve shutdown or current limiting functions. Its working principle utilizes the characteristic that transistor current is proportional to its area for current sampling; typically, the area of the sampling transistor Q2 is only one-thousandth the area of the output transistor. Then, a voltage comparator compares the sampled signal with a reference voltage, thereby controlling the drive of the MOSFET and achieving overcurrent protection.
Other auxiliary module designs include constant current sources and current mirrors. Within an integrated circuit (IC), bias current plays a crucial role, determining the operating state of each transistor. The constant current source circuit, as the cornerstone of all circuitry, provides a stable foundation for generating the bandgap reference. The current mirror, on the other hand, is responsible for distributing current to various functional modules. Its operation is similar to a mirror image, precisely setting the required current magnitude based on the transistor's area ratio.
In-depth analysis of the internal structure of switching power supplies
When we turn on a switching power supply, the first thing that catches our eye is the dense array of electronic components.
These seemingly random small objects actually form a precise and complex circuit network.
In this network, every component plays a vital role, working together to fulfill the power supply's functional requirements.
So, specifically, what are the main components inside a switching power supply?
The input rectifier and filter circuit is the first line of defense in the entire switching power supply.
Its function is to perform preliminary processing on the input AC mains power, converting it into a relatively smooth DC voltage signal.
It typically consists of a bridge rectifier and a large-capacity electrolytic capacitor.
The bridge rectifier is responsible for converting alternating current into unidirectional pulsating current, while the electrolytic capacitor further filters out the ripple components, ensuring that the subsequent circuits can obtain relatively pure direct current.
Next is the power conversion circuit, which is the core part of the switching power supply.
Here, the energy transfer process from the primary side to the secondary side is realized through key components such as high-frequency transformers and power switching transistors.
Power switching transistors (such as MOSFETs or IGBTs) periodically turn on and off according to control signals, causing the current in the primary winding to form pulses, thereby generating an alternating magnetic field.
After the magnetic field is induced in the secondary winding, the required stable DC output voltage can be obtained through the rectification and filtering process.
To ensure the accuracy and stability of the output voltage, a feedback control circuit is also required.
This circuit detects the actual output value and compares it with a preset target value, then feeds the error signal back to the PWM controller.
The PWM controller adjusts the duty cycle based on the received information, thereby regulating the operating state of the power switching transistor in real time and ultimately achieving precise control of the output voltage.
In short, the feedback control circuit is like a dutiful housekeeper, always protecting the normal operation of the power system.
In addition, many auxiliary functional modules should not be overlooked.
For example, protection circuits can effectively prevent damage risks caused by overload, short circuit or overheating; startup circuits ensure that the power supply can start smoothly and enter normal working mode the moment it is connected to the power supply; and EMI filters are used to suppress electromagnetic interference and improve the overall system's immunity performance.
At this point, you might ask, "Since switching power supplies are so complex, how did they evolve step by step?"
In fact, as early as the early 1960s, the advancement of semiconductor technology, especially the invention and application of transistors, laid a solid foundation for the birth of switching power supplies.
Early switching power supplies were mainly used in military and aerospace fields, and they attracted much attention because of their advantages such as small size, light weight and high efficiency.
Subsequently, driven by the consumer electronics market, switching power supplies gradually became more consumer-grade and rapidly gained popularity.
With changes in market demand and continuous improvement in technology, the design concept of switching power supplies has also undergone several innovations.
From the initial hard-switching topology to the later soft-switching technology, each improvement has been aimed at increasing efficiency, reducing losses, and optimizing performance.
Meanwhile, the application of new materials has also brought new development opportunities to switching power supplies.
For example, the emergence of wide-bandgap semiconductor materials such as silicon carbide (SiC) and gallium nitride (GaN) has enabled power devices to operate stably at higher frequencies, further improving the overall efficiency of power supplies.
In the face of increasingly severe energy crisis and environmental pressure, how to further improve the energy efficiency of switching power supplies has become a focus of industry attention.
To this end, researchers are actively exploring various approaches, such as new topologies, advanced control algorithms, and intelligent management solutions, in an effort to achieve greener and more efficient energy conversion while maintaining existing advantages.
Having understood the functions of the main components inside a switching power supply, we will now focus on discussing one of the most crucial aspects—the working principle and importance of the power conversion circuit.
Power conversion circuit: the core of the system
As the heart of a switching power supply, the power conversion circuit bears the important mission of efficiently converting and transmitting input energy to the load.
Its working principle is based on fundamental physical laws such as the law of electromagnetic induction and Kirchhoff's laws. By cleverly applying these theoretical knowledge, it realizes the energy transfer process from the primary side to the secondary side.
Specifically, when the power switch is in the on state, the current in the primary winding begins to rise, forming an energy storage magnetic field; when the switch is turned off, the energy stored in the magnetic field is released and transferred to the load through the secondary winding.
This periodic charge-discharge cycle constitutes the most basic operating mechanism of a switching power supply.
It is worth noting that the high-frequency transformer played a crucial bridging role throughout the process.
It not only achieves electrical isolation between the primary and secondary windings, but also enables effective control of the output voltage amplitude through a reasonable design of the turns ratio.
At the same time, transformers themselves also face many challenges, such as leakage inductance and core saturation. If these problems are not handled properly, they will seriously affect the performance of the power supply system.
Therefore, in the actual design process, engineers need to comprehensively consider various factors, carefully select suitable magnetic core materials, and take corresponding compensation measures to ensure that the transformer can work stably and reliably for a long time.
