在新能源領(lǐng)域運用中，DCLink電容作為一個關(guān)鍵器件，不管是在風(fēng)力發(fā)電系統(tǒng)、光伏發(fā)電系統(tǒng)還是在新能源汽車系統(tǒng)中都要求高可靠性及長壽命，其選型顯得尤為重要。下面介紹薄膜電容與電解電容的特性對比及在DC-Link電容運用中兩者的分析對比：
In the application of new energy, dclink capacitor, as a key device, requires high reliability and long life in wind power generation system, photovoltaic power generation system and new energy vehicle system, and its selection is particularly important. The following is the comparison of the characteristics of thin film capacitor and electrolytic capacitor, and the analysis and comparison of the two in the application of DC link capacitor:
特性對比
Characteristic comparison
薄膜電容
Film capacitance
首先介紹薄膜金屬化的原理，薄膜金屬化技術(shù)的原理：在薄膜介質(zhì)表面蒸鍍上足夠薄的金屬層，在介質(zhì)存在缺陷的情況下，該鍍層能夠蒸發(fā)并因此隔離該缺陷點起到保護(hù)作用，這種現(xiàn)象被稱作自愈。
Firstly, the principle of film metallization is introduced. The principle of thin film metallization technology is as follows: a thin enough metal layer is evaporated on the surface of thin film medium. In the case of medium defect, the coating can evaporate and isolate the defect point to protect it. This phenomenon is called self-healing.
圖4為金屬化鍍膜的原理圖，蒸鍍前薄膜介質(zhì)先進(jìn)行前期處理(電暈或其他方式)以便金屬分子能夠附著在上面。
Figure 4 is the schematic diagram of metallization coating. Before evaporation, the film medium is pretreated (corona or other methods) so that metal molecules can adhere to it.
金屬通過在真空狀態(tài)下高溫溶化蒸發(fā)(鋁的蒸發(fā)溫度1400攝氏度~1600攝氏度，鋅的蒸發(fā)溫度400攝氏度~600攝氏度)，當(dāng)金屬蒸氣遇被冷卻的薄膜后凝結(jié)在薄膜表面(薄膜冷卻溫度-25攝氏度~-35攝氏度)，從而形成金屬鍍層。
Metal is dissolved and evaporated at high temperature in vacuum (the evaporation temperature of aluminum is 1400 ℃ ~ 1600 ℃, and that of zinc is 400 ℃ ~ 600 ℃). When the metal vapor meets the cooled film, it condenses on the surface of the film (film cooling temperature is - 25 ℃ ~ - 35 ℃), thus forming a metal coating.
金屬化技術(shù)的發(fā)展提高了單位厚度的薄膜介質(zhì)的介電強(qiáng)度，干式技術(shù)脈沖或放電運用電容設(shè)計可以達(dá)到500V/μm,直流濾波運用電容設(shè)計可以達(dá)到250V/μm.DC-Link電容屬于后者，根據(jù)IEC61071對于電力電子運用電容的要求可以承受較為苛刻的電壓沖擊，可以達(dá)到2倍的額定電壓。
The development of metallization technology improves the dielectric strength of thin film dielectric per unit thickness. The capacitance design of dry technology pulse or discharge can reach 500V / μ M, The design of DC filter capacitor can reach 250V / μ M. DC link capacitor belongs to the latter. According to the requirements of iec61071 for power electronic application capacitor, it can withstand severe voltage impact and can reach twice the rated voltage.
因此使用者只需考慮其設(shè)計所需的額定工作電壓就可以了。金屬化薄膜電容器具有較低的ESR,使其能承受較大的紋波電流;較低的ESL滿足逆變器的低電感設(shè)計要求，減少了開關(guān)頻率下的震蕩效應(yīng)。
Therefore, users only need to consider the rated working voltage required by their design. The metallized film capacitor has lower ESR, which makes it able to withstand larger ripple current; the lower ESL can meet the requirements of low inductance design of inverter and reduce the oscillation effect at switching frequency.
薄膜介質(zhì)的質(zhì)量、金屬化鍍層質(zhì)量、電容器設(shè)計及制造過程工藝決定了金屬化電容器自愈特性的好壞。Faratronic生產(chǎn)的DC-Link電容用的薄膜介質(zhì)主要為OPP薄膜。
The quality of thin film dielectric, metallized coating quality, capacitor design and manufacturing process determine the self-healing characteristics of metallized capacitor. The main thin film dielectric used for DC link capacitor produced by faratronic is OPP film.
電解電容
Electrolytic capacitor
電解電容使用的介質(zhì)為鋁經(jīng)過腐蝕形成的氧化鋁，介電常數(shù)為8~8.5,工作的介電強(qiáng)度約為0.07V/A(1μm=10000A)，按照計算對于900Vdc的電解電容需要的厚度為12000A.然而要達(dá)到這樣的厚度是不可能的，因為為了獲得好的儲能特性所用鋁箔要進(jìn)行腐蝕形成氧化鋁膜，表面會形成許多凹凸不平的曲面，鋁層厚度會降低電解電容的容量系數(shù)(比容)。另一方面，低電壓的電解液電阻率為150Ωcm,高電壓(500V)的電解液的電阻率則達(dá)到5kΩcm.
The dielectric of electrolytic capacitor is aluminum oxide formed by corrosion, and the dielectric constant is 8 ~ 8.5, The working dielectric strength is about 0.07v/a (1 μ M = 10000a). According to the calculation, the thickness required for 900vdc electrolytic capacitor is 12000A. However, it is impossible to achieve such thickness, because in order to obtain good energy storage characteristics, the aluminum foil is corroded to form alumina film, and the surface will form many uneven curved surfaces, and the thickness of aluminum layer will reduce the capacitance coefficient (specific capacitance) of electrolytic capacitor 。 On the other hand, the resistivity of electrolyte with low voltage is 150 Ω cm, and that of electrolyte with high voltage (500V) is 5K Ω cm
電解液較高的電阻率限制了電解電容所能承受的有效值電流，一般為20mA/μF.
The higher resistivity of electrolyte limits the effective value current that electrolytic capacitor can bear, which is generally 20 mA / μ F
基于上述原因電解電容的設(shè)計最高電壓典型值為450V(有個別廠家設(shè)計600V)。
Based on the above reasons, the typical design maximum voltage of electrolytic capacitor is 450V (some manufacturers have designed 600V).
因此，為了獲得更高的電壓必須用電容器串聯(lián)實現(xiàn)，然而因各個電解電容的絕緣電阻存在差異，為了平衡各串聯(lián)電容的電壓，各電容必須連接一個電阻。此外，電解電容為有極性器件，當(dāng)施加反向電壓超過1.5倍Un時，會發(fā)生電化學(xué)反應(yīng)。
Therefore, in order to obtain higher voltage, capacitors must be connected in series. However, due to the difference of insulation resistance of electrolytic capacitors, in order to balance the voltage of series capacitors, each capacitor must be connected with a resistor. In addition, the electrolytic capacitor is a polar device, when the applied reverse voltage is more than 1.5 times UN, the electrochemical reaction will occur.
當(dāng)施加的反向電壓時間足夠長，電容將發(fā)生爆炸，或冒頂電解液將外溢。為了避免該現(xiàn)象發(fā)生，使用的時候要在每個電容旁并上一個二極管。除此之外，電解電容的耐電壓沖擊特性，一般為1.15倍Un,好的可以達(dá)到1.2倍Un.這樣設(shè)計師在使用時就不但要考慮穩(wěn)態(tài)工作電壓大小，而且還要考慮其沖擊電壓大小。
When the reverse voltage is applied for a long enough time, the capacitor will explode or the top will fall, and the electrolyte will overflow. In order to avoid this phenomenon, a diode should be placed next to each capacitor. In addition, the withstand voltage impulse characteristic of electrolytic capacitor is generally 1.15 times UN, and the good one can reach 1.2 times UN. In this way, designers should not only consider the steady-state working voltage, but also the impulse voltage.
綜上所述，可以得出薄膜電容與電解電容如下特性對比表，見表1.
To sum up, the following characteristic comparison table of film capacitor and electrolytic capacitor can be obtained, as shown in Table 1
運用分析
Application analysis
DC-Link電容作為濾波器要求大電流和大容量設(shè)計。新能源汽車主電機(jī)驅(qū)動系統(tǒng)就是一個例子。在該運用中電容起到退耦作用，電路特點工作電流大。薄膜DC-Link電容具有較大優(yōu)勢，能承受較大的工作電流(Irms)。以50~60kW新能源汽車參數(shù)為例，參數(shù)如下：工作電壓330Vdc,紋波電壓10Vrms,紋波電流150Arms@10KHz.
DC link capacitor as a filter requires high current and large capacity design. The main motor drive system of new energy vehicles is an example. In this application, the capacitor plays the role of decoupling, and the circuit features high working current. Thin film DC link capacitors have great advantages and can withstand large operating current (IRMS). Taking the parameters of 50 ~ 60kW new energy vehicle as an example, the parameters are as follows: working voltage 330VDC, ripple voltage 10vrms, ripple current 150Arms@10KHz.
那么最小電容量計算為：
Then the minimum capacitance is calculated as:
這樣對于薄膜電容設(shè)計很容易實現(xiàn)。假設(shè)采用電解電容，如果考慮20mA/μF,那么為了滿足上述參數(shù)， 計算電解電容最小的容值為：
This is easy to implement for thin film capacitor design. Assuming that electrolytic capacitor is used, if 20mA / μ f is considered, then in order to meet the above parameters, the minimum capacitance value of electrolytic capacitor is calculated as follows:
這樣需要多個電解電容并聯(lián)獲得該容值。
This requires multiple electrolytic capacitors in parallel to obtain the capacitance value.
在過電壓運用場合，如輕軌、電動巴士、地鐵等，考慮這些動力通過受電弓連接到機(jī)車集電弓，在運輸行進(jìn)過程中受電弓與集電弓的接觸是間續(xù)的。當(dāng)兩者不接觸時通過DC-Link電容進(jìn)行支撐供電，當(dāng)兩者接觸恢復(fù)時過電壓就會產(chǎn)生。
In the case of over-voltage application, such as light rail, electric bus, subway, etc., considering that these power is connected to the locomotive pantograph through pantograph, the contact between pantograph and pantograph is intermittent during transportation. When the two are not in contact, the DC link capacitor is used to support the power supply, and when the contact is restored, the overvoltage will be generated.
最壞的情況是斷開時由DC-Link電容完全放電，此時放電電壓等于受電弓電壓，當(dāng)恢復(fù)接觸時，其產(chǎn)生的過電壓幾乎就是額定工作時的2倍Un.對于薄膜電容DC-Link電容可以處理不需額外考慮。
The worst case is that when the DC link capacitor is disconnected, the discharge voltage is equal to the pantograph voltage. When the contact is restored, the over-voltage generated by the DC link capacitor is almost twice the UN of the rated operation. The DC link capacitor can be treated without additional consideration.
如果采用電解電容，過電壓為1.2Un .以上海地鐵為例，Un=1500Vdc,對于電解電容要考慮電壓為：那么要用6個450V的電容進(jìn)行串聯(lián)連接。若采用薄膜電容設(shè)計在600Vdc到2000Vdc,甚至3000Vdc都容易實現(xiàn)。
If the electrolytic capacitor is used, the over-voltage is 1.2un. Take Shanghai Metro as an example, UN = 1500vdc. For the electrolytic capacitor, the voltage should be considered: then six 450V capacitors should be connected in series. If the thin film capacitor is designed, it can be easily realized from 600vdc to 2000vdc, even 3000vdc.
此外，在電容完全放電情況下能量在兩電極間形成短路放電，產(chǎn)生很大沖擊電流通過DC-Link電容，通常電解電容很難滿足要求。
In addition, when the capacitor is fully discharged, the energy forms a short-circuit discharge between the two electrodes, which generates a large impact current through the DC link capacitor, which is difficult to meet the requirements of the electrolytic capacitor.
另外，相對于電解電容DC-Link薄膜電容器通過設(shè)計可以達(dá)到很低的ESR(通常低于10mΩ，更低的<1mΩ)和自感LS(通常低于100nH,有的可以低于10或20nH)。
In addition, compared with electrolytic capacitors, DC link thin film capacitors can achieve very low ESR (usually less than 10 m Ω, lower than 1 m Ω) and self inductance LS (usually less than 100 NH, some can be lower than 10 or 20 NH).
這樣在運用時DC-Link薄膜電容器可直接安裝到IGBT模塊，可以把母線整合到DC-Link薄膜電容器中，因此采用薄膜電容器則不再需要專門的IGBT吸收電容，為設(shè)計者節(jié)約了一筆不小的費用。表2和表3為Faratronic C3A和C3B部分產(chǎn)品的技術(shù)參數(shù)。
In this way, the DC link film capacitor can be directly installed into the IGBT module, and the bus can be integrated into the DC link thin film capacitor. Therefore, the use of the film capacitor will no longer require a special IGBT absorption capacitor, which saves a lot of cost for the designer. Table 2 and table 3 show the technical parameters of some products of faratronic C3a and C3b.
作為直流支撐濾波用電容，DC-Link 電容早期考慮到成本及尺寸因素大部分選擇電解電容。然而電解電容受到耐壓、電流承受能力(相對薄膜電容ESR高很多)等因素的影響，為了獲得大容量和滿足高壓使用要求，則必須要用多個電解電容進(jìn)行串、并聯(lián)。
As a DC support filter capacitor, DC link capacitor mostly chooses electrolytic capacitor considering the cost and size factors in the early stage. However, the electrolytic capacitor is affected by the withstand voltage and current carrying capacity (much higher than the ESR of thin film capacitor). In order to obtain large capacity and meet the requirements of high voltage application, it is necessary to use multiple electrolytic capacitors in series or in parallel.
另外考慮到電解液材料的揮發(fā)，所以要定期進(jìn)行更換，新能源運用一般要求產(chǎn)品壽命要達(dá)15年，那么在這段時間內(nèi)必須更換兩到三次，因而在整機(jī)售后服務(wù)方面存在不小的費用和不方便性。
In addition, considering the volatilization of electrolyte materials, it is necessary to replace them regularly. Generally, the service life of new energy products should reach 15 years. Therefore, it is necessary to replace them two or three times during this period of time. Therefore, there is no small cost and inconvenience in the after-sales service of the whole machine.
隨著金屬化鍍膜技術(shù)及薄膜電容器技術(shù)的發(fā)展，采用安全膜蒸鍍技術(shù)已經(jīng)可以用超薄OPP膜(最薄2.7μm,甚至2.4μm)生產(chǎn)出電壓450V到1200V甚至更高電壓的大容量直流濾波電容。另一方面通過DC-Link電容與母排整合，使得逆變器模塊設(shè)計更加緊湊，大大降低了電路的雜散電感使電路更加優(yōu)化。
With the development of ultra-thin film (452.0 μ m) and the technology of thin film capacitor (1200 μ m), the most safe capacitor can be produced by using the technology of thin film evaporation with the voltage of 450 μ M. On the other hand, through the DC link capacitor and bus integration, the inverter module design is more compact, greatly reducing the stray inductance of the circuit, making the circuit more optimized.
以此同時，薄膜電容制作成本在不斷下降，相比電解電容更凸顯其經(jīng)濟(jì)性，在要求工作電壓高、承受高紋波電流(Irms)、有過電壓要求、有電壓反向現(xiàn)象、處理高沖擊電流(dV/dt)以及長壽命要求的電路設(shè)計中，選擇DC-Link薄膜電容替代電解電容將成為設(shè)計者今后設(shè)計選擇的一種趨勢。
At the same time, the production cost of thin-film capacitor is declining, and its economy is more prominent than electrolytic capacitor. In the circuit design that requires high working voltage, high ripple current (IRMS), over-voltage requirement, voltage reversal phenomenon, handling high impulse current (DV / DT) and long life, DC link thin film capacitor will be the designer's choice in the future A trend of choice.