The role of capacitor in LED power supply
The role of capacitance of LED power supply Capacitor capacity as one of passive components, its role is the following:
1, the application in the power circuit, the realization of the bypass, lotus root, filter and energy storage capacitor role, give details of the following categories: 1) filter for filtering is a very important part of the role of the capacitance.Almost all of the power supply is used in the circuit.Theoretically (i.e. assuming capacitance of pure capacitance) said, the greater the capacitance, impedance is smaller, by the higher the frequency also.But in fact most of the more than 1 uf capacitance for electrolytic capacitor, there is a lot of inductance component, so after high frequency impedance increases.Sometimes see a larger capacitance electrolytic capacitor in parallel with a small capacitance, then large capacitance at low frequency, small capacitance high frequency.Lao capacitance effect is low resistance, high frequency resistance at low frequency.The more easily through the low frequency, the greater the capacitance of the high frequency easier to pass.Specific use in filtering, large capacitor (1000 uf) filtering and low frequency, small capacitance (pf) 20 high frequency filter.Have net friend will filter capacitance compared to "pool".Due to the inability of the ends of the capacitor voltage mutation, therefore, the higher the signal frequency, the greater the attenuation, can say very image of the capacitance is like a pool, not caused by the addition of a few drops of water or the evaporation of water.It changes the voltage changes into current, the higher the frequency, the greater the peak current, so as to buffer the voltage.Filtering is charging and discharging process.
2) bypass the bypass capacitor energy storage device is provided for the local device energy, it can make the output of the voltage regulator homogenization, reduce the workload requirements.Like a small rechargeable battery, bypass capacitors can be charged, and to put the electric device.To minimize the impedance to bypass capacitors, try to close to the load device power supply pin and pin.It can effectively prevent the input value is too large and cause the earth potential of raises and noise.Play is ground connection at the voltage drop across the large current burr when.
3) go to lotus root lotus root, also called decoupled.From circuit for, always can be divided into driving source and driven load.If the load capacitance is large, driver circuit to the capacitor charging, discharge, to complete the signal jump, in the rise is steep, the current is large, this will absorb a lot of drive current of the power supply current, because in the circuit inductance, resistance (especially the inductance on the chip pins, can produce a rebound), the current relative to normal, in fact is a kind of noise, affect the normal work of the former level.This is the coupling.Decoupled capacitance is play the role of a battery, meet the changes of driving circuit current, avoid mutual coupling interference.Combine bypass capacitors and decoupled capacitance will be easier to understand.Bypass capacitors actually also to coupling, just bypass capacitors generally refers to the high frequency bypass, also is to give the switch of high frequency noise increase a low impedance leakage prevention way.High-frequency bypass capacitors tend to be small, according to the resonance frequency is generally 0.1 u and 0.01 u, etc., and coupling capacitance is compared commonly big, uf is 10 or more, according to the distribution parameters in circuit, and the change of the drive current to determine the size.Bypass is interference of the input signal as object, filtering and decoupling is the interference of output signal as the filter object, to prevent interference signal to return to power.This should be the essential difference between them.4) a storage capacitor charge collected through rectifier, and will be stored energy transmitted converter leads to the output of the power supply.Voltage rating for 40 ~ 450 VDC, capacitance between 220 ~ 150, 000 uf aluminum electrolytic capacitor (e.g., EPCOS B43504 or B43505) is relatively common.According to the requirements of the different power supply, the device is sometimes in the form of series, parallel, or their combination, for more than 10 kw power level of the power supply, usually adopt bulky cylindrical screw terminal condenser.
Applications in signal circuit, the main complete coupling, oscillation/synchronization and time constant:
Coupling, for example, a transistor amplifier emitter has a self-bias resistance, pressure drop it at the same time, make the signal feedback to the input form the input and output signal coupling, this resistance is produced a coupling element, if on both ends of the resistor in parallel to a capacitance, due to the appropriate capacity of capacitor on the impedance of the ac signal is small, thus reducing the resistance coupling effect, therefore calls the capacitance as decoupling capacitor.2) oscillation/synchronization including RC, the load capacitance of LC oscillator and crystal belong to this category.3) time constant This is the common integral circuit, composed of R, C series.When the input signal voltage on the input capacitance (C) the voltage rise gradually.As the voltage rises and the charging current is decreased.Electric current through A resistor (R), capacitance (C) the characteristics described by the following formula: I = V/R) e - (t/CR) we know that the capacitance effect later here to talk about matters needing attention in using a. what is A good capacitance.The greater the capacitance, the better.Many people tend to love in the capacitance of the replacement of using large capacitance.We know that although the capacitance, the greater the current compensation ability for IC.Not to mention the increase of the capacitance with larger volume, increase cost at the same time also affect the air flow and heat dissipation.The key is on parasitic capacitance inductance and capacitance discharge circuit will occur at some frequency resonance.At resonance point, the impedance of the capacitor is small.Therefore, the impedance of the discharge circuit of minimum energy effect is best.But when more than point of resonance frequency, the impedance of the discharge circuit starts to increase, capacitance provide current capacity then began to decline.The capacity of capacitance value, the greater the resonance frequency is lower, the frequency of the capacitor can effectively compensate current range.From the Angle of guarantee capacitor's ability to provide high frequency current, the greater the capacitance the better view is wrong, in the general circuit design has a reference value.The same capacity of capacitor, the better the more small capacitance in parallel Pressure value, heat resistance, capacitance value, the ESR (equivalent resistance) is capacitance of several important parameters, such as, for the lower ESR nature is, the better.The ESR and the capacity of capacitance, frequency, voltage, temperature, etc.When voltage is fixed, the bigger the capacity, lower the ESR.In the board design using multiple small capacitance and even more is the PCB space is limited, so some people think that the more parallel small resistance, the ESR is lower, the better the results.Is so in theory, but should consider the impedance of the capacitor pin solder joints, using multiple small capacitance in parallel, the effect is not necessarily outstanding.3. The ESR is lower, the better the results.The combination of the above we improve power supply circuit, for input capacitance, the input capacitance of the capacity to be a bit bigger.Relative capacity demands of the ESR, can reduce accordingly.Because the input capacitance mainly is the pressure, the second is to absorb the switch MOSFET pulse.For output capacitance, the requirement of the pressure and volume can be down a little bit.The ESR requirement is a little higher, because here is to ensure that enough electric current excess.But here is to pay attention to the ESR is not the lower the better, low ESR capacitor will cause switching circuit oscillation.While complex damping circuit will lead to increased costs.Board design, there generally have a reference, this as a component selection parameters, avoid damping circuit and lead to increased costs.
4. Good capacitor represents the high quality."Only capacitance theory" has reached its zenith, some vendors and media also deliberately put the things into a selling point.In the board design, circuit design level is the key.And some manufacturers can make than some vendors with two phase power supply more stable products, power is supplied by four phase blindly adopt high capacitance, may not be able to make a good product.To measure a product, be sure to all-round multi-angle thinking, the role of capacitance must not have intention to not intentional of exaggeration.B. the face of the capacitance blasting pulp interview of explosion pulp categories: two kinds of input capacitance blasting explosion pulp slurry and output capacitance.For input capacitance is C1, I mean C1 to filter by power receives the current.The quality of the input capacitance blasting pulp and power input current.Too much burr voltage, peak voltage is too high, the current instability and so on all too frequent charge and discharge, the capacitor of capacitance in this kind of work environment for a long time, the internal temperature rise quickly.More than mouth of the explosion limit will occur under explosion pulp.For output capacitance, C2, I said to filter was adjusted by the power module of current.The current through a filter, is smooth, the possibility of explosion pulp is relatively small.But if the environment temperature is too high, capacitance is also prone to detonation., also.Using waste things natural to explode, retribution.To realize the past due, now see the fruit;To realize the future fruit, see it by now.The cause of the electrolytic capacitor explosion pulp: capacitance blasting pulp have numerous causes, such as electric current is greater than the allowed wave current, use voltage over working voltage, the reverse voltage, charge and discharge frequently, etc.But the most direct reason is the high temperature.We know that the capacitance is one important parameter value of heat-resistant, means the capacitor inside the boiling point of the electrolyte.When the capacitance of the internal temperature reaches the boiling point of the electrolyte, electrolyte began to boil, capacitance internal pressure, when pressure exceeds the limit of the drain cracking-off the explosion pulp.So the temperature is a direct cause of the capacitance blasting pulp.Capacitance design service life is about 20000 hours, influenced by the environment temperature is very big also.The service life of the capacitance decreases with the increase of temperature, the experiment proves that the environment temperature rise every 10 ℃, the life of the capacitor will be cut in half.Main reason is that speed up chemical reaction and make the medium temperature degradation failure over time, such capacitor life end.In order to guarantee the stability of the capacitor capacitance in front of the board to after a long time high temperature environment test.Even at 100 ℃, the high quality of capacitance can also work for thousands of hours.At the same time, we talk about the life of the capacitor is refers to the capacitance in use process, capacitance will not exceed 10% of the standard range.Refers to the problem of capacitance, capacitance life rather than design life reached after explosion pulp.Only there is no guarantee that the design of capacitance capacity standards.So, a short period of time, the normal use of explosion pulp board capacitance is happen, this is capacitance quality problem.In addition, abnormal usage capacitance of explosion pulp is also likely to happen.Such as hot plug computer accessories can also lead to the card the dramatic change of local circuit current, voltage, triggering capacitance using fault.