Some experiences and misunderstandings in the use of capacitors

When the polarity of the line cannot be determined in the circuit, it is recommended to use electrodeless electrolytic capacitors. The ripple current through the electrolytic capacitor cannot exceed its allowable range. If the specified value is exceeded, a capacitor with high ripple current resistance must be selected. The working voltage of the capacitor cannot exceed its rated voltage. When soldering the capacitor, the electric iron should be kept at a certain distance from the plastic shell of the capacitor to prevent the plastic sleeve from breaking due to overheating. And the welding time should not exceed 10 seconds, the welding temperature should not exceed 260 degrees Celsius.

Four myths:

● The larger the capacitance, the better

Many people often use large-capacity capacitors in the replacement of capacitors. We know that although the larger the capacitance, the stronger the current compensation capability provided for the IC. Not to mention that the increase in capacitance increases the volume, which increases the cost and affects air flow and heat dissipation. The key is that there is parasitic inductance in the capacitor, and the capacitor discharge circuit will resonate at a certain frequency.

At the resonance point, the impedance of the capacitor is small. Therefore, the impedance of the discharge circuit is the smallest, and the effect of supplementing energy is also the best. However, when the frequency exceeds the resonance point, the impedance of the discharge circuit begins to increase, and the capacity of the capacitor to provide current begins to decrease.

The larger the capacitance value, the lower the resonant frequency, and the smaller the frequency range in which the capacitor can effectively compensate the current. From the perspective of ensuring the ability of the capacitor to provide high-frequency current, the view that the larger the capacitor is, the better it is wrong. In general circuit design, there is a reference value.

● Capacitors of the same capacity, the more parallel the smaller the better

Withstand voltage value, temperature resistance value, capacitance value, ESR (equivalent resistance), etc. are several important parameters of the capacitor. Naturally, the lower the ESR, the better. ESR is related to the capacity, frequency, voltage and temperature of the capacitor.

When the voltage is fixed, the larger the capacity, the lower the ESR. In the design of the board, the use of multiple small capacitors in parallel is mostly due to the limitation of the PCB space. Some people think that the more parallel small resistors, the lower the ESR and the better the effect. This is theoretically true, but considering the impedance of the capacitor pin solder joints, using multiple small capacitors in parallel, the effect is not necessarily outstanding.

● The lower the ESR, the better the effect

Combined with our improved power supply circuit above, for the input capacitor, the input capacitor has a larger capacity. Relative capacity requirements, ESR requirements can be appropriately reduced. Because the input capacitor is mainly withstand voltage, followed by the absorption of MOSFET switching pulses. For the output capacitor, the requirements and capacity of the withstand voltage can be reduced appropriately.

The ESR requirement is a bit higher, because here we must ensure that sufficient current flow. However, it should be noted here that the lower the ESR, the better. Low ESR capacitance will cause the switching circuit to oscillate. The complicated vibration-elimination circuit will also increase the cost. In the design of the board, there is generally a reference value, which is used as a component selection parameter to avoid the cost increase caused by the vibration reduction circuit.

● Good capacitance represents high quality

"Capacitance-only theory" was very popular, and some manufacturers and media also deliberately made this a selling point. In board design, the level of circuit design is the key. Just as some manufacturers can use two-phase power supply to make products that are more stable than some manufacturers use four-phase power supply, blindly using high-priced capacitors may not necessarily make a good product. To measure a product, we must consider it from all angles and angles, and we must not exaggerate the role of capacitors intentionally or unintentionally.

var check_allow = "/d/Api/iscantalk.html"; var add_url = '/ d / article / write /'; function CheckLogin () {now_uid = ''; var ElecfansApi_checklogin = '/ webapi / passport / checklogin'; var logout_url = "{: U ('Login / logout')}"; var logout_url = 'http://bbs.elecfans.com/member.php?mod=logging&action=logout&refer=front'; $ .get (ElecfansApi_checklogin, function (data, textStatus) {if (data! = "") {EchoLoginInfo (data); CheckEmailInfo (data); data = $ .parseJSON (data); now_uid = data.uid; / * var login_content = 'write an article