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　　Analysis on the principle of high frequency switching power supply circuit

　　The key to power supply miniaturization is the miniaturization of the power supply, so the loss of the power supply circuit must be reduced as much as possible. When the switching power supply works in the switching state, the switching loss of the switching power supply inevitably exists, and the loss increases with the increase of the switching frequency. On the other hand, the losses of magnetic components and capacitive components such as transformers and reactors in switching power supplies increase with frequency. it

　　In the current market, most of the power transistors in switching power supplies are bipolar transistors, and the switching frequency can reach tens of kHz, and the switching frequency of MOSFET switching power supplies can reach hundreds of kHz. High-speed switching devices must be used to increase the switching frequency. For power supplies with switching frequencies higher than MHz, a resonant circuit can be used, which is called resonant switching mode. It can greatly increase switching speed. In principle, switching losses are zero and noise is very small. This is a method to increase the operating frequency of switching power supplies. Megahertz converter using resonant switching mode. Switching power supplies can solve this problem well through high-frequency switching mode. For high-frequency switching power supplies, the AC input voltage can be boosted before entering the transformer (generally 50-60KHz before boosting). As the input voltage increases, components such as transformers and capacitors do not need to be as large as linear power supplies. This high-frequency switching power supply is exactly what our personal PCs and devices like VCR recorders need. It should be noted that the switching power supply we often refer to is actually the abbreviation of high-frequency switching power supply and has nothing to do with the closing and opening of the power supply itself.

　　Introduction to switching power supply classification

　　Switching power supplies have a variety of circuit structures: (1) Depending on the driving mode, there are self-excitation and self-excitation. it

　　2) According to the working mode of the DC/DC converter: (1) single-ended forward and flyback, push-pull, half-bridge, full-bridge, etc.; 2) buck, boost and boost. it

　　(3) According to the composition of the circuit, there are resonance and non-resonance. it

　　(4) According to the control method, it is divided into: pulse width modulation (pWM), pulse frequency modulation (pFM), pWM and pFM hybrid. (5) Depending on the power supply isolation and feedback control signal coupling methods, there are problems such as isolation, non-isolation, transformer coupling, and photoelectric coupling. These combinations can form a variety of switch-mode power supplies. Therefore, designers need to effectively combine the characteristics of various modes to produce high-quality switching regulated power supplies to meet needs.

　　Analysis of switching power supply principle

　　Figures 3 and 4 below describe the pWM feedback mechanism of the switching power supply. Figure 3 depicts a cheap power supply without pFC (powerFactorCorrection, power factor correction) circuit, and Figure 4 depicts a mid-to-high-end power supply designed with active pFC.

　　By comparing Figure 3 and Figure 4, we can see the difference between the two: one has an active pFC circuit and the other does not. The former does not have a 110/220V converter, and there is no voltage doubler circuit. In the following, our focus will be on the explanation of active pFC power supplies.

　　In order to allow readers to better understand the working principle of the power supply, we provide a very basic diagram above. The diagram does not include other additional circuits, such as short-circuit protection, standby circuit, pG signal generator, etc. Of course, if you want to know a more detailed diagram, please see Figure 5. It doesn’t matter if you don’t understand it, because this picture is originally meant for professional power supply designers.

　　You may ask, why is there no voltage rectifier circuit in the design diagram of Figure 5? In fact, the pWM circuit has already taken on the job of voltage rectification. The input voltage will be corrected again before passing through the switching tube, and the voltage entering the transformer has become a square wave. Therefore, the waveform output by the transformer is also a square wave, not a sine wave. Since the waveform is already a square wave at this time, the voltage can be easily converted into DC voltage by the transformer. That is to say, when the voltage is re-corrected by the transformer, the output voltage has become a DC voltage. This is why switching power supplies are often called DC-DC converters.

　　The loop feeding the pWM control circuit is responsible for all required regulation functions. If the output voltage is wrong, the pWM control circuit will change the control signal of the duty cycle to adapt to the transformer, and finally correct the output voltage. This situation often occurs when the PC power consumption increases, and the output voltage tends to decrease, or when the PC power consumption decreases, the output voltage tends to increase.

　　Before looking at the next page, we need to know the following information:

　　★All circuits and modules before the transformer are called primary (primary side), and all circuits and modules after the transformer are called secondary (secondary side);

　　★The power supply using active pFC design does not have a 110V/220V converter and does not have a voltage doubler;

　　★For power supplies without pFC circuits, if 110V/220V is set to 110V, before the current enters the rectifier bridge, the power supply itself will use a voltage doubler to increase 110V to about 220V;

　　★The switching tube on the PC power supply is composed of a pair of power MOSFET tubes. Of course, there are other combinations, which we will explain in detail later;

　　★The waveform required by the transformer is a square wave, so the voltage waveform after passing through the transformer is a square wave, not a sine wave;

　　★The pWM control current is often an integrated circuit, usually isolated from the primary side through a small transformer, and sometimes it may be connected to the primary side through a coupling chip (a small IC chip with an LED and a phototransistor) isolation;

　　★The pWM control circuit controls the closing of the switch tube of the power supply according to the output load condition of the power supply. If the output voltage is too high or too low, the pWM control circuit will change the voltage waveform to adapt to the switching tube, thereby achieving the purpose of correcting the output voltage.

　　The technical area illustrates the simple principle of 48V DC high-frequency switching power supply and analyzes the principle of high-frequency switching power supply circuit. What is the principle of high-frequency switching power supply circuit? SCHURTER launches DG11 power input module and provides Ip67 dust-proof and waterproof protection. C&K launches new SSW detection switch to strengthen Functional safety of electronic systems

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