Do you understand the role of electronic components in power supplies? (Part 3)

　　The variable resistor is connected across the live wire and the ground wire at the rear end of the fuse. Its operating principle is that when the voltage difference between its two ends is lower than its rated voltage value, the body presents high impedance; when the voltage difference exceeds its rated value, the body resistance will drop rapidly, and LN will present an approximate short-circuit state. The current increased by the short circuit at the front end of the fuse will cause it to melt to protect the rear end circuit. Sometimes when the body is subjected to excessive power, it will also self-destruct to warn the user that there is a problem with the device.

　　Usually used at the AC input of a power supply, it can blow the fuse in time when the input AC voltage is over, thus avoiding damage to the internal components. Its color and appearance are very similar to Cy capacitors, but the difference can be distinguished by the words and models on the components.

　　■ Bridge rectifier

　　The bridge rectifier is composed of four interconnected diodes. Its function is to full-wave rectify the input AC for use in the back-end switching circuit.

　　Its appearance and size will vary depending on the rated voltage and current of the component. Some power supplies will fix it on a heat sink to help it dissipate heat for stable long-term operation. After rectification, it enters the switching circuit on the primary side of the power stage. The components here determine the maximum output capacity of each channel of the power supply and are a very important part of the power supply.
■ Switching semiconductor

　　As a contactless fast electronic switch in the switching circuit, it turns on and off according to the control signal to determine whether the current flows. It plays an important role in the active power factor correction circuit and the primary side circuit of the power stage.

　　The upper part of the photo shows the N MOSFET (N-type metal oxide semiconductor field effect semiconductor transistor) commonly used in power supplies, and the lower part shows the NPN BJT (NPN type transistor)

　　With the circuit composition of the switch components, different power level topologies such as dual-crystal forward, half-bridge, full-bridge, push-pull, etc. can be formed. In power supplies that emphasize high efficiency, there are also applications that use switching semiconductors to form synchronous rectification circuits and DC-DC buck circuits.

　　■ Transformer

　　The reason why it is called an isolated switching step-down power supply is that it uses a transformer to separate high and low voltages and uses magnetic energy for energy exchange. It can not only avoid the danger of leakage when the high and low voltage circuits fail, but also easily generate a variety of voltage outputs. Because of its higher operating frequency, the transformer is smaller than the general AC transformer.

　　Because the transformer is one of the power transmission paths, currently large output power supplies are designed with multiple transformers to avoid saturation of a single transformer and limit the power output. The smaller transformer on the top of the photo is a pulse transformer for auxiliary power circuits and signal transmission, and the larger one on the bottom is the main power transformer and the secondary side adjustment transformer of the ring.

　　With the transformer as the isolation boundary, the output voltage on the secondary side is already much lower than that on the primary side, but it still needs to go through circuits such as rectification, adjustment, filtering and smoothing before it can become the DC power of various voltages required by computer components.