Abstract: Ripple noise is an important indicator of power supply. A good power supply must control the output ripple noise within a reasonable range. But what are the effective countermeasures to reduce ripple noise? Let's take a look at the eight commonly used methods. 1, power PCB routing and layout The feedback circuit should avoid magnetic components, switching transistors and power diodes. Output filter capacitor placement and traces are critical to ripple noise. As shown in Figure 1, in the traditional design, the impedance of each capacitor is not the same, so the high-frequency current is unevenly distributed among the three capacitors. It can be seen that the length of each loop is equivalent, that is, the high frequency current is evenly distributed to each capacitor. figure 1 If the PCB is a multi-layer board, you can choose to cover the ground with the last layer of the main current loop layer. Overlay can effectively solve the noise problem. Pay attention to ensure the integrity of the overlay. 2. Add RCD between the field effect tube D level and the input positive Generally, the reverse recovery time of the FET is 2~3 times slower than that of the diode D1 to avoid the formation of a through current. This current will generate a strong magnetic field, which can increase the output noise interference, so the gate resistance can be artificially passed. R4 to slow down the switching speed of the switch. In order not to affect the turn-off speed, a diode D2 can be connected in parallel with the gate resistor as shown in Fig. 2. figure 2 3, FET DS parallel RC An RC circuit can be connected in parallel with the FET DS to effectively reduce the noise interference. As shown in Figure 2, the capacitor C2 is generally around 100P. If the capacitance value is too large, the switching loss of the FET is increased, and the resistor R2 is generally selected to be smaller. 10Ω resistor. 4, the output diode is connected in parallel with RC When the diode is turned on and off at high speed, the parasitic inductance and capacitance of the diode will generate high-frequency oscillation during reverse recovery. In order to suppress high-frequency oscillation, an RC snubber circuit can be added at both ends of the diode as shown in Fig. 2. The resistor R3 is generally In 1Ω~100Ω, the capacitor C3 is generally 100pF~1nF. If the power supply operating frequency is low and the efficiency meets the requirements, the diode D3 can select a diode with a slow reverse recovery time. 5, output plus secondary LC filter LC has obvious effect on noise and ripple suppression. According to the ripple frequency, the proper inductance and capacitance value are selected. However, due to the low price and small volume of the cylindrical inductor, the inductance of the LC is generally selected as the cylindrical inductor. However, the cylindrical inductor is The open magnetic structure will cause more serious magnetic interference to the surroundings. We can use two inductors placed side by side, and the current flows in the opposite direction, which helps to guide the magnetic flux from one magnetic column to the other, thus reducing electromagnetic Interference, as shown in Figure 3. image 3 6. Adding a Faraday shield between the primary and secondary of the transformer When the winding of the transformer passes the high-frequency current, the transformer will become an effective magnetic field antenna, and the transformer winding will withstand the jump voltage, that is, the transformer also becomes the electric field antenna. The Faraday shield between the primary and secondary can collect the noise at the isolation boundary. The current is transferred to the original side, but the copper foil should be a very thin copper foil strip. Try to avoid eddy current loss and reduce leakage inductance. Note that there should be no electrical connection at the end of the copper shield, otherwise a magnetic short circuit will be formed. 7, reduce transformer leakage The sandwich winding method can make the primary and secondary windings more tightly coupled, so that the leakage inductance can be reduced, thereby achieving the purpose of reducing noise. 8. Reasonable design of parallel connection of transformer output windings When the output current of the power supply is large, we usually use two windings in parallel. These two windings are usually placed inside and outside the primary winding, so the DC impedance will be slightly different, which may cause internal circulation, voltage. The waveform will also have severe ringing, and the electromagnetic interference will become more serious, and there may be unexpected large amount of heat. If the output windings must be used in parallel, it is better to use the recommended circuit as shown in Figure 4. Figure 4 Recommended Circuit The middle diode eliminates the negative effects caused by the unevenness of the two different windings. Figure 4 Summary : When designing the power supply, there are eight ways to reduce the ripple noise of the output. If the finished power supply is used, whether it is a module power supply, an ordinary switching power supply, a power adapter, etc., this part of the work is generally completed by the power supply design manufacturer, the customer only You need to pay attention to the output ripple noise written by the specification. For example, the ZLG module power supply will indicate typical ripple noise values, such as the E_FLS-1W series, with a typical value of 60mV VP-P.
Accessories for cameras are mainly for care, protection, special effects and functions.
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Lens hood: used on the end of a lens to block the sun or other light source to prevent glare and lens flare (see also matte box).
Lens cap: covers and protects the lens during storage.
Lens adapter: sometimes called a step-ring, adapts the lens to other size filters.
Lens filters: allow artificial colors or change light density.
Lens extension tubes allow close focus in macro photography.
Flash equipment: including light diffuser, mount and stand, reflector, soft box, trigger and cord.
Care and protection: including camera case and cover, maintenance tools, and screen protector.
Large format cameras use special equipment which includes magnifier loupe, view finder, angle finder, focusing rail /truck.
Battery and sometimes a charger.
Some professional SLR could be provided with interchangeable finders for eye-level or waist-level focusing, focusing screens, eye-cup, data backs, motor-drives for film transportation or external battery packs.
Tripod, microscope adapter, cable release, electric wire release.
Dew shield - Prevents moisture build up on the lens.