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The power of modern technology of the capacitor in the correct selection
 

Abstract: analysis of the development of modern power technology of capacitor specific parameters of the request, focuses on the filter capacitor, absorbing capacitor, point out should pay attention to the use of voltage rating and conditions of life test.

<P src="關鍵詞:電容器等效串聯電阻阻抗頻率特性dv/dt"" 關鍵詞:電容器等效串聯電阻阻抗頻率特性dv/dt ">Key words: capacitor equivalent series resistance impedance frequency characteristics of dv/dt <P src=""" 電容器作為基本元件在電子線路中起著重要作用,在傳統的應用中,電容器主要用作旁路耦合、電源濾波、隔直以及小信號中的振蕩、延時等。以上電路對電容器參數的主要要求有:電容量;額定電壓;正切損耗;漏電流等,對其它參數沒有過多的要求 。">The capacitor as the basic component in electronic circuit plays an important role, in the traditional applications, mainly used as a bypass capacitor coupling, power filtering, blocking and small signal, delay oscillation. The above circuit of capacitor parameters of the main requirements are: capacitance; rated voltage; loss tangent; leakage current, the other parameters without too much requirement.

Along with the electronic circuit, especially the development of power electronic circuits for different applications of the capacitor presents different specific requirements.

<P src="1、濾波電容器"" 1、濾波電容器">In 1, a filter capacitor

Alternating current ( AC or RF ) via rectifier with capacitor filter so that the output voltage is smooth, requirements of capacitor capacity is large, the general use of aluminum electrolytic capacitor. Aluminum electrolytic capacitor application is the main problem of temperature and life, such as cheap environmental temperature is85 ℃, in the 1000h to ensure that the performance parameters, especially the capacity, more than 1000h, the performance indicators will be can not be guaranteed, even though in many cases can use. As the lower use temperature can prolong life, basically follows the rule of50. So many requirements in high temperature and high reliability of occasions, should choose long life ( such as 5000h or more, and105 ℃,5000h ) electrolytic capacitor. General small electrolytic capacitor, its relatively short life.

For DC/DC switching power supply input filter capacitor, for switching converter is in the form of pulse power supply of electrical energy to learn, so the filter capacitor through the larger high frequency current, when the electrolytic capacitor equivalent series resistance ( ESR ) is larger, will produce bigger loss, resulting in electrolytic capacitor fever. While low ESR electrolytic capacitor can significantly reduce ripple ( especially high frequency ripple current generated by the heating ).

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For switching power supply output rectifier of the electrolytic capacitor, the impedance frequency characteristics in300kHz or even500kHz still presents the trend of escalation, see figure 1. Electrolytic capacitor ESR is low, can effectively filter the switch power supply in the high frequency ripple and peak voltage. While the common electrolytic capacitor in the100kHz after the beginning of rise (see Figure 2), used for switch power supply output rectifying and filtering effect is relatively poor. The author found in experiments, ordinary type CDII4700μ F,16V electrolytic capacitors, used in switch power supply output filter ripple and spike is not better than

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Type CD03HF4700 μ F,16V high frequency electrolytic capacitor of low, while common electrolytic capacitor temperature is relatively high. When the load is mutation, with common electrolytic capacitor transient response than the high frequency electrolytic capacitor.

As a result of aluminum electrolytic capacitor in high frequency band can play a very good role, should be accompanied by good quality of high frequency ceramic or no sense of film capacitor, its main advantages are: good quality of high frequency, low ESR, such as MMK5capacity of 1μ F capacitor, the resonant frequency is more than 2MHz, the equivalent impedance of less than 0.02Ω, far below the electrolytic capacitor, and little capacity resonant frequency is higher ( more than 50MHz ), see Figure 3, that will be very good power output frequency response or dynamic response.

<P src="2、吸收與換相電容器"" 2、吸收與換相電容器">2, absorption and commutation capacitor

Semiconductor device with gate controlled rated power grew, the switching speed more and more quickly, the rated voltage is more and more high, the buffer circuit capacitor just enough pressure, capacity and excellent frequency characteristics is not enough.

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In the power electronic circuit, because the IGBT switch speed is less than 1μ s, requires absorbing circuit capacitor voltage change rate dv/dt> V/ μs is very normal, some require V/ μs and V/ μs. The capacitor and the voltage charging time of the basic relationship

I= C ·dv/dt (1)

As of1 μ F, V/ μs, by type (1) that the peak current of A. Even smaller capacity such as 10nF, V/μ s rate variation peak current of 100A. For ordinary capacitors, especially the common metallized capacitors of dv/dt<100V/μ s, special metallized capacitors dv/dt≤ 200V/μ s, special double metallized capacitor capacity of small ( less than 10nF) dv/dt≤ 1500V/μ s, large capacity ( less than 0.1μ F ) is 600V/μ s, in this kind of huge and repetition rate very high peak current impact is difficult to bear. So can often be seen in power electronic circuit due to absorption of capacitors used improperly capacitor breakdown or damage the power electronic circuit breaker, the phenomenon of.

The current absorbing circuit dedicated capacitor, whereby the metal foil electrode can bear larger current peak and RMS current impact, such as: smaller capacity ( below 10nF) can withstand100000V/ μs ~455000V/μ s voltage change rate,3700A peak current and up to 9A RMS current ( such as CDV30FH822J03); larger capacity ( more than10nF, less than 0.47μ F ) or larger size can withstand greater than 3400V/μ s and1000A peak current impact.

<P src="由此可見,盡管同是無感電容、金屬化和金屬箔電容,應用在吸收電路中將有不同的表現,外形相近但規格不同在這里是絕對不能互換的。"" 由此可見,盡管同是無感電容、金屬化和金屬箔電容,應用在吸收電路中將有不同的表現,外形相近但規格不同在這里是絕對不能互換的。">Thus, although with no sense of capacitors, metal and metal foil capacitors, used in absorption circuit will have different performance, different specifications of similar shape but here is absolutely not interchangeable.

Capacitor size will affect the capacitor dv/dt and the peak current tolerance, generally speaking, the greater the length dv/dt and the peak current is relatively small, with CDE WPP capacitance for example, as shown in table 1.

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Absorption circuit of capacitor 's work is characterized by high peak current duty ratio is small, RMS current is not very high, and the circuit is similar and the thyristor inverter commutation capacitor, the capacitor dv/dt requirements than absorption capacitor is small, but the peak current and effective current are larger, the ordinary capacitor in the current can not meet the requirements.

In some special application requirements of energy storage capacitor discharge and repeated shortness, the discharge circuit resistance is very low, the parasitic inductance is very small, in this instance will only use absorption capacitor in parallel to ensure long term reliability.

<P src="3、諧振電容器"" 3、諧振電容器">In 3, a resonant capacitor

Resonant converter, such as resonant switching regulated power supply and SCR medium frequency power supply with resonant circuit of the resonant capacitor, often through the large current work. As the parallel resonant type of Thyristor Intermediate frequency power supply, flowing through the resonant capacitor resonant current flowing through the thyristor current of 10 times or more, it must choose a special capacitor, can meet the requirements of. And as the electronic ballast resonant capacitor specifications choice is undeserved, can appear on the capacitor voltage never reaches the breakdown voltage but through larger resonant current and damaged phenomenon.

<P src="4、耐壓與壽命試驗"" 4、耐壓與壽命試驗">4, pressure and life test

Rated voltage of a capacitor and breakdown voltage is kept between a factor of safety, such as the voltage of400V electrolytic capacitor, breakdown voltage is not lower than 450V, while the film capacitor breakdown voltage rated voltage1.5times to 2 times, so we can not say that the capacitor reaches the rated voltage may breakdown.

Film capacitor life test is at its maximum operating temperature, applying 1.5 times rated voltage of500h or even1000h, ensure no damage. Electrolytic capacitor life test is at the maximum temperature and the rated voltage condition, the rated life period the main parameters to meet the requirements.

Electrolytic capacitor and capacitor life long work environment temperature has a direct relationship, the higher the temperature, the shorter the life of capacitor. Common electrolytic capacitor at an ambient temperature of 90has been damaged, such as: B41303, B43303and other types of electrolytic capacitor. But now there are many kinds of electrolytic capacitors working environment temperature is very high, as the B43502 in an ambient temperature of 90 ℃, through electrolytic capacitor of AC current and the rated impulse current ratio of 0.5, life is still 10000h, but if the temperature rises to 95 ℃, electrolytic capacitor that has been damaged. Therefore, in the selection of capacitor when, should according to the concrete environment temperature and other parameters and to select, if ignoring the environmental temperature on the capacitor life expectancy effects, then the power supply reliability, stability will be greatly reduced, or even damage the equipment and apparatus. Generally speaking, electrolytic capacitors working at an ambient temperature of 80, generally can reach 10000h life requirements.

On the other hand, the life of electrolytic capacitors and capacitor of long time working current and rated current pulse ( generally refers to85 ℃temperature environment test values, but there are some high temperature electrolytic capacitor is at 125 ℃on the ratio of test data ). Generally speaking, the larger the ratio, the shorter the life of electrolytic capacitors, when flowing through the electrolytic capacitor current is3.8 times of the rated current, electrolytic capacitors are generally already damaged. Therefore, an electrolytic capacitor having its safe operating area, for general applications, when the AC current and the rated impulse current ratio below 3 times, for the life of the requirements have been satisfied. Environmental temperature and current on electrolytic capacitor influences such as shown in figure 4.

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<P src="5、結語"" 5、結語">5, conclusion

To sum up, in the power of modern technology, different applications need different properties of the capacitor, cannot mix, abuse, misuse, with as far as possible to eliminate there should not be damaged, and ensure the performance of the product.

 
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