The role of beads in the switching power supply adapter
When the switching frequency of the switching power supply adapter (100KHz and above), when the power switch is turned on, the high-frequency transformer primary side of the direction of the distributed capacitance and secondary output rectifier diode recovery process will be in Switch Collector peak current. The secondary output rectifier diodes also have a reverse peak current. The peak current may damage the power switch and rectifier diode, but also produce switching noise and electromagnetic radiation.
Although the RC components in series across the rectifier diode and RC snubber circuit, the switching noise can play a certain extent, but the effect is still not ideal Moreover, the resistance will result in power loss. The better solution is a bead in series with the power switch collector and the secondary output of the rectifier circuit.
Beads recent years, the development of fast growing anti interference components, low-cost, easy-to-use, to filter out high frequency noise effect is remarkable. It is an ultra-compact amorphous alloy magnetic material, its tubular shape, the lead through it and glue. Common beads Dimensions have 2.5 * 3mm, 2.5 * 8mm * 5mm and so on a variety of specifications. Also in the form of surface-mount, but rarely used in switching power supply circuit chip beads, chip beads are widely used: PDA, ISDN, ADSL, MP3, CD, DVD, mobile phones, computers, television sets, digital cameras, camera, tape recorders, walkie-talkies, EL backlight driver, remote control toys, fax machines, laser printers and electronic watches and clocks, and other communications and consumer electronics. Chip beads is composed of laminated monolithic structure composed of ferrite materials and conductor coil. Due to sintering at high temperatures, and thus good compactness and high reliability. Both ends of the electrodes are made of silver / nickel / solder three which can meet the requirements of reflow and wave soldering.
Used in switching power supply adapter beads, inductors typically a few ten to a few micro-Hang. The DC resistance of the bead is very small, is generally 0.005 to 0.01 ohms. Usually noise filter can only absorb noise, passive suppression; the bead role is different, can inhibit the generation of peak current, and therefore belong to the active suppression type, which is the fundamental difference between the two. Beads widely used in high-frequency switching power supply, but also used in other electronic appliances, electronic measuring instruments, and a variety of very strict noise requirements of the circuit. The beads are often used in the design of the EMC components, occupy a prominent position in the EMC countermeasures.