鋁電解電容的特性
鋁電解電容的特性
1.1 Circuit model （等效模型）
The following circuit models the aluminium electrolytic capacitor’s normal operation as well as the over voltage and reverse voltage behavior. （此模型包含正常運(yùn)行，過(guò)壓，反壓時(shí)的特性）
Ca= Anode capacitance (陽(yáng)極電容)
Cc= Cathode capacitance(陰極電容)
Rp= Parallel resistance, due to dielectric (并聯(lián)電阻)
ESR= Series resistance, as a result of connections, paper, electrolyte, ect. 等效串聯(lián)電阻
L= Winding inductance and connections 等效串聯(lián)電感
D= Over and reverse voltage 等效穩(wěn)壓管
The capacitance Ca and Cc are the capacitance of the capacitor and is frequency and temperature depended. （Ca and Cc，它的容量是頻率及溫度的函數(shù)）
The resistance ESR is the equivalent series resistance which is frequency and temperature depended. It also increases with the rated voltage. （ESR是頻率及溫度的函數(shù)，隨著額定電壓的增加而增加）
The inductance L is the equivalent series inductance, and it’s independent for both frequency and temperature. It increases with terminal spacing. （L是頻率及溫度的函數(shù)）
The resistance Rp is the equivalent parallel resistance and accounts for leakage current in the capacitor. It decreases with increasing the capacitance, temperature and voltage and it increases with time. （Rp的大小決定了漏電流的大小，隨著容量溫度電壓的增加而降低，隨著使用時(shí)間的延長(zhǎng)而增加）
The zener diode D models the over voltage and reverse voltage behavior. Application of over voltage on the order of 50 V beyond the capacitor’s surge voltage rating causes high。（D模擬過(guò)壓及加反向電壓時(shí)特性）
Leakage current and a constant voltage-operating mode quite like the reverse conduction of a zenerdiode. Applications of reverse voltage much beyond 1.5 V causes high leakage current quite like the forward conduction of a diode. Neither of these operating modes can be maintained for long because hydrogen gas is produced, and the pressure built up will cause failure. （加到電容兩端的反向電壓不能大于1.5V）
1.2Capacitance （電容的容量）
The rated capacitance is the nominal capacitance and it is specified at 120 Hz and a temperature of 25°C. Capacitance is a measure of the energy storage capability of a capacitor at a given voltage. （額定容量： 標(biāo)稱電壓，120Hz, 25°C時(shí)測(cè)量）。
The capacitance decreases under load conditions and increases under no load conditions over time. When reverse voltage or excessive ripple current is applied, or when the capacitor is repeatedly charged and discharged, an aluminium oxide film is formed on the cathode foil. This film induces a sharp capacitance drop. Capacitance in aluminium electrolytic capacitors is also affected by frequency changes. For example, the capacitance falls as the frequency rises. Variation of magnitude depends on capacitor type. （電容上的紋波電流，頻繁地充放電導(dǎo)致陰極箔氧化，容量急劇下降）
1.3Equivalent series resistance (ESR)
The equivalent Series Resistance (ESR) is the sum of all the internal resistances of a capacitor measured in Ohms. It includes:
-Resistance due to aluminium oxide thickness
-Resistance due to electrolyte / spacer combination
-Resistance due to materials (Foil length; Tabbing; Lead wires; Contact resistance)
At low frequencies (10 – 100 Hz) the ESR is determined by the oxide thickness,
electrolyte / spacer combination and the materials. Above the 100 Hz electrolyte / spacer combination and the materials predominate.
The lower the ESR the higher the current carrying ability the capacitor will have. The amount of heat generated by ripple current depends upon the ESR of the capacitor.
ESR is both frequency and temperature dependent, increasing either will cause a reduction in ESR. The ESR is an important parameter in calculating life expectancy as the power dissipation (internally generated heat) is directly proportional to its value.
The limit is generally established at 120 Hz and 20o C.
The ESR of the electrolytic capacitor can cause another effect, especially above the 10 kHz where the ESR is the dominant contribution to the capacitors impedance.
When a current charges / discharges the capacitor, the voltage across the capacitor will increase / decrease:
and causes a voltage drop over the ESR （流過(guò)電容的充放電電流因?yàn)镋SR而產(chǎn)生紋波電壓）
V=I.ESR
如果電容由低占空比，高頻脈沖電流充電時(shí)， 比較典型的是fly-back電源的輸出濾波電容，ESR引起的紋波電壓是最重要的，導(dǎo)致必須選用合適ESR的電容來(lái)滿足要求。電容的容量不是主要考慮的問(wèn)題。
電容ESR的危害：（1）電容上的紋波電壓（2）紋波電流流過(guò)ESR發(fā)熱，使電容壽命縮短。
1.4Dissipation factor (tan?) 損耗角或損耗因子
影響損耗角或損耗因子的因素：電容的ESR，容量，工作頻率
The dissipation factor (also called Angle of loss or tangent delta) is a measure of the losses (or, the power factor) of a capacitor. When sinusoidal current is applied to an ideal capacitor, the phase of current should gain ?/2, however the phase of current actually delays from ?/2 on a realistic capacitor. The loss in angular velocity is called loss angle and represented by ”?”.
The tan? is decided by ESR (Equivalent Series Resistance), the capacitance (C) and the Frequency (F) usually at 120Hz and 20oC
1.5The impedance 阻抗
通常優(yōu)先選擇中頻段阻抗較低的電容。好電容具有較低的阻抗和ESR.
1.6Ripple current (紋波電流) --- 很重要
紋波電流 Ir ：電容上流過(guò)的交流電流的有效值
允許的最大紋波電流受限于：外殼的形狀，電容的表面積，ESR，最大允許的核心溫度
The ripple current Ir is the r.m.s. alternate current value that can be directly applied to the capacitor. The maximum permissible ripple current is dependent on the case style and surface area and ESR, and maximum allowed core temperature.
如果實(shí)際的紋波電流大于最大允許紋波電流，電容的壽命極大縮短
If the ripple current applied is higher than the specified maximum permissible ripple current, the life of the capacitor becomes shorter. In general, increasing ripple current is possible by changing the following parameters:
- Lower ambient temperatures （降低環(huán)境溫度）
- Lower ESR (note: the higher the Frequency, the lower the ESR) （降低ESR，頻率越高ESR越低）
- Improve heat transfer through heat sinking and / or forced air cooling （良好的散熱）
- Increase case size （加大電容的尺寸）
- Voltage derating （額定電壓降額使用,例如450V的電容用于300V的應(yīng)用下）
For more detailed information about calculating the ripple current see chapter 7 paragraph 7.4
1.7Voltage 電壓
-The rated voltage 額定電壓
峰值電壓（交流與直流的和）必須小于額定電壓
The rated voltage can be applied to a capacitor within temperature and reference margins. The sum of continuous and alternate voltage applied to the capacitor cannot exceed rated voltage. The maximum permissible ripple current limits the alternate voltage.
Derating the applied voltage will reduce the failure rate of the capacitor. Higher rated voltage capacitors may be substituted for lower rated voltage capacitors as long as can size, DF and ESR ratings are also compatible.
-The reverse voltage反向電壓
不能加反向電壓，能夠承受1.5V 1 min的反向電壓
反向電壓的危害：發(fā)熱，陰極氧化，產(chǎn)生大量氫氣，使電容爆裂
- The surge voltage 浪涌電壓
電容短時(shí)間能夠承受的最大過(guò)壓（5 min 的周期內(nèi)不超過(guò)30秒的脈沖過(guò)壓）
危害與反向電壓的危害一樣。
IEC的標(biāo)準(zhǔn)：
VP = 1.15 x Vrated for Vrated < 315VDC
VP = 1.10 x Vrated for Vrated > 315 VDC
最大允許的浪涌電壓與最大額定電壓的關(guān)系
Rated voltage(V)46.310162535506380100160180200250315350400450
Surge voltage (V)58132032446379100125200225250300365400450500
浪涌電壓應(yīng)用于電源上電的瞬間，電容上往往有過(guò)充電壓，特別是PFC電路。
- The transient voltage 瞬態(tài)電壓
Aluminium electrolytic capacitors can generally withstand extreme over voltage transients of limited energy. Applications of over voltage more than about 50 V beyond the capacitor’s surge voltage rating causes high leakage current and a constant voltage operating mode like the reverse conduction of a zener diode. The capacitor may fail short if the electrolyte cannot take the voltage stress, but even if it can, this operating mode cannot be maintained for long because hydrogen gas is produced by the capacitor, and the pressure built up will cause failure.
1.8Temperature 溫度（紋波電流與ESR的交互作用的后果）
熱阻可分成兩部分：內(nèi)部熱阻?（從核心到外殼）和 外部熱阻?（從外殼到周圍的環(huán)境）
內(nèi)部熱阻的大小取決于制造工藝。良好的散熱可以降低外部熱阻。
外殼的面積也很重要。
1.8.1Heat rise of case surface （外殼表面的溫升，從外殼到周圍的環(huán)境）
Tj= Heat rise at the core of the capacitor element從核心到外殼的溫升
a= Thermal resistance內(nèi)部熱阻（從核心到外殼）
內(nèi)部熱阻與外殼的尺寸密切相關(guān)，經(jīng)驗(yàn)結(jié)論：
外殼的直徑3 ~ 8 10 ~ 12.5 16 ~ 18 20~22 25 30 35
內(nèi)部熱阻 11.1 1.21.31.41.5 1.6
編輯：admin 最后修改時(shí)間：2023-04-07
