With the development and innovation of power electronics technology, switch power supply technology is also continuously innovating. Currently, switch power supplies are widely used in almost all electronic devices due to their small size, light weight, and high efficiency, making them an indispensable power supply method in the rapid development of today's electronic information industry.
A switch power supply is a type of power supply that uses modern power electronics technology to control the on and off time ratio of the switch tube, maintaining a stable output voltage. A switch power supply generally consists of a pulse width modulation (PWM) control IC and a MOSFET.
The switch power supply is relative to linear power supplies. Its input directly rectifies AC power into DC power, and then, under the action of a high-frequency oscillation circuit, controls the current on and off with a switch tube, forming a high-frequency pulse current. With the help of an inductor (high-frequency transformer), it outputs a stable low-voltage DC.
Due to the inverse relationship between the size of the transformer core and the square of the operating frequency of the switch power supply, the higher the frequency, the smaller the core. This greatly reduces the size of the transformer, making the power supply lighter and more compact. Moreover, since it directly controls DC, the efficiency of this power supply is much higher than that of linear power supplies. This saves energy, which is why it is favored by people. However, it also has disadvantages, such as complex circuits, difficult maintenance, and serious pollution to the circuit. The power supply noise is high, making it unsuitable for certain low-noise circuits.
Characteristics of switch power supplies
A switch power supply generally consists of a pulse width modulation (PWM) control IC and a MOSFET. With the development and innovation of power electronics technology, switch power supplies are currently widely used in almost all electronic devices due to their small size, light weight, and high efficiency, highlighting their importance.
Classification of switch power supplies
According to the way the switching devices are connected in the circuit, switch power supplies can generally be divided into three main categories: series switch power supplies, parallel switch power supplies, and transformer switch power supplies.
Among them, transformer switch power supplies can be further divided into various types: push-pull, half-bridge, full-bridge, etc. Based on the excitation of the transformer and the phase of the output voltage, they can also be divided into various types: forward, flyback, single-ended, and double-ended.
Differences between switch power supplies and ordinary power supplies
Ordinary power supplies are generally linear power supplies, which refer to power supplies that operate in a linear state. In contrast, switch power supplies work differently; the switch tube (in switch power supplies, we generally refer to the adjustment tube as the switch tube) operates in two states: on — low resistance, off — high resistance.
Switch power supplies are a relatively new type of power supply. They have advantages such as high efficiency, light weight, adjustable voltage, and high output power. However, due to the circuit operating in a switching state, they tend to produce more noise.
For example: Buck switch power supply
Let's briefly discuss the working principle of a buck switch power supply: the circuit consists of a switch (in practical circuits, it is a transistor or a field-effect transistor), a freewheeling diode, an energy-storing inductor, a filter capacitor, etc.
When the switch is closed, the power supply provides power to the load through the switch and inductor, storing some electrical energy in the inductor and capacitor. Due to the self-inductance of the inductor, the current increases slowly after the switch is turned on, meaning the output cannot immediately reach the power supply voltage.
After a certain time, the switch opens. Due to the self-inductance of the inductor (it can be visualized that the current in the inductor has an inertial effect), it will maintain the current in the circuit, continuing to flow from left to right. This current flows through the load, returns to the ground, flows to the positive terminal of the freewheeling diode, passes through the diode, and returns to the left end of the inductor, thus forming a loop.
By controlling the time the switch is closed and opened (i.e., PWM — pulse width modulation), the output voltage can be controlled. If the output voltage is detected to control the on and off time to maintain a constant output voltage, this achieves the purpose of voltage regulation.
Both ordinary power supplies and switch power supplies have voltage adjustment tubes that use feedback principles for voltage regulation. The difference is that switch power supplies use switch tubes for adjustment, while ordinary power supplies generally use the linear amplification region of transistors for adjustment. Comparatively, switch power supplies have lower energy consumption, a wider applicable range for AC voltage, and better ripple coefficients for output DC, but they have the drawback of switching pulse interference.
The main working principle of an ordinary half-bridge switch power supply is that the upper and lower bridge switch tubes (at high frequencies, the switch tube is a VMOS) alternately conduct. First, the current flows through the upper bridge switch tube, utilizing the storage function of the inductor coil to accumulate electrical energy in the coil. Finally, the upper bridge switch tube is turned off, and the lower bridge switch tube is turned on, allowing the inductor coil and capacitor to continue supplying power externally. Then, the lower bridge switch tube is turned off, and the upper bridge switch tube is turned on to let the current flow in, repeating this process. Because the two switch tubes are alternately switched, it is called a switch power supply.
Linear power supplies are different. Without the intervention of switches, the upper water pipe continuously releases water. If there is excess, it will leak out. This is why we often see that some linear power supplies have a significant amount of heat generated by their adjustment tubes, converting all unused electrical energy into heat. From this perspective, the conversion efficiency of linear power supplies is very low, and when the heat is high, the lifespan of the components will inevitably decrease, affecting the final usage effect.
Main difference: Working method
The power adjustment tube of linear power supplies always operates in the amplification region, and the current flowing through is continuous. Due to the significant power loss on the adjustment tube, larger power adjustment tubes are required, along with large heat sinks, resulting in severe heating and low efficiency, generally between 40% and 60% (which is considered a very good linear power supply).
The working method of linear power supplies requires a voltage reduction device to convert high voltage to low voltage, which is generally a transformer, and there are others like KX power supplies, which then rectify to output DC voltage. This results in a large size, relatively heavy, low efficiency, and high heat generation; however, it also has advantages: small ripple, good adjustment rate, low external interference, suitable for use in analog circuits and various amplifiers.
The switch power supply operates its power devices in a switching state. When adjusting the voltage, energy is temporarily stored through the inductive coil, which reduces losses and increases efficiency, resulting in lower heat dissipation requirements. However, it also places higher demands on transformers and energy storage inductors, requiring the use of low-loss, high-permeability materials. Its transformer is quite small. The overall efficiency ranges from 80% to 98%. The switch power supply is efficient and compact, but compared to linear power supplies, it has certain discounts in ripple and voltage/current adjustment rates.
