INCAP LIMITED

The capacitance of a capacitor can be determined by measuring its AC Impedance (taking into account amplitude and phase) or by measuring the charge it will hold when a direct voltage is applied. The two methods produce slightly different results. As a general rule, it can be said that DC voltage based measurements (DC capacitance) yield higher values than the alternating current method (AC capacitance). The factors are approximately 1,1 to 1,5 and maximum deviations occur with capacitors of low voltage ratings.

Corresponding to the most common applications (e.g. smoothing and coupling), it is most usual to determine the AC capacitance of aluminium electrolytic capacitors.

The capacitance of an equivalent circuit, having capacitance, resistance and inductance in series, measured with alternating current of approximately sinusoidal waveform at a specified frequency is given below (refer to Fig. 1).

For this purpose the capacitive component of the equivalent series circuit (the series capacitance C_S)is determined by applying an Alternating voltage of < 0,5 V. As the AC capacitance depends on frequency and temperature, IEC 60384-1 and 60384-4 prescribe a measuring frequency of 100 Hz and a temperature of 25^oC (other reference values by special request).

There also applications (e.g. discharge circuit and timing elements) in which the DC capacitance is decisive. In spite of this fact capacitors for which the capacitance has been determined by the ac method are also used in such applications, where by allowances are made to compensate for the difference between the two measuring methods.

However, in exceptional cases it may be necessary to determine the DC capacitance. The IEC publications do not provide any corresponding specifications. Because if this a separate DIN standard has been defined. The standard, DIN 41 328, part 4, describes a measuring method involving one-time, non-recurrent charging and discharging of the capacitor.

DC Capacitance is given by the amount of charge which is stored in the capacitor.

The rated capacitance is the AC capacitance value for which the capacitor has been designed and which is indicated upon it. C_R is determined by specific measurement methods described in the relevant standards (IEC 60384-1 and 60384-4). Preferred capacitance values are taken from the E3 or E6 series.

INCAP specifies C_Rin μF as the AC capacitance measured at 100 or 120 Hz and 25^oC, in accordance with IEC 60384-4.

The capacitance tolerance is the range within which the actual capacitance may deviate from the specific rated capacitance. Where the capacitance tolerances are to be indicated on the components themselves, INCAP uses code letters in accordance with IEC 60062; this code letter is also part of the ordering code.

The maximum direct voltage, or peak value of pulse voltage which may be applied continuously to a capacitors at any temperature between the lower category temperature and the rated temperature.

The maximum voltage which may be applied continuously to a capacitor at its upper category temperature.

The temperature derated voltage is the maximum voltage that may be applied continuously to a capacitor, for any temperature between the rated temperature and the upper category temperature.

An alternating voltage may be applied, provided that the peak voltage resulting from the alternating voltage, when superimposed on the direct voltage, does not exceed the value of rated direct voltage and that the ripple current is not exceeded.

The maximum voltage applied in the reverse polarity direction to the capacitor terminations.

The maximum instantaneous voltage which may be applied to the terminations of the capacitor for a specified time at any temperature within the category temperature range.

The range of ambient temperatures for which the capacitor has been designed to operate continuously: this is defined by the temperature limits of the appropriate category.

The maximum ambient temperature at which the rated voltage may be continuously applied.

The minimum permissible ambient temperature which the capacitor shall withstand in the non-operating condition, without damage.

The ESR of an equivalent circuit having capacitance, inductance and resistance in series measured with alternating current of approximately sinusoidal waveform at a specified frequency.

The ESL of an equivalent circuit having capacitance, resistance and inductance in series measured with alternating current of approximately sinusoidal waveform at a specified frequency.

The power loss of the capacitor divided by the reactive power of the capacitor at a sinusoidal voltage of specified frequency:

The impedance (Z) of an electrolytic capacitor is given by capacitance, ESR and ESL in accordance with the following equation

The characteristics of the individual resistive and reactive components determine the total impedance of the capacitor.

- With increasing frequency, the capacitive reactance (Xc = 1/_w C_s) decreases until it reaches the order of magnitude of the electrolyte resistance.

- At even higher frequencies and unchanged temperatures, the resistance of the electrolyte predominates.

- When the capacitor's resonance frequency is reached, capacitive and inductive reactance mutually cancel each other.

- Above this frequency, the inductive resistance of the winding and its terminals (XL = wL) becomes effective and leads to an increase in impedance.

The resistance of the electrolyte increases strongly with decreasing temperature.

The current flows through a capacitor when a DC voltage is applied in correct polarity. It is dependent on voltage, temperature and time.

In accordance with international standards ("IEC 60384-4" and "EN130300") the leakage current ( I_L5) after 5 minutes application of rated voltage at 25^oC, is considered as an acceptance requirement.

The leakage current requirements for the majority of INCAP electrolytic capacitors, are lower than specified.

After prolonged storage and/or storage at excessive temperature (> 40^oC), the leakage current at the first measurement may be more than specified. Pre-conditioning shall be carried out in accordance with "EN130300 sub clause 4.1".

After continuous operation (1 hour or longer) the leakage current will normally decrease to less than 20% of the 5 minute value (I_L5).

Table 1 Typical multiplier of operational leakage current as a function of ambient temperature

Table 2 Typical multiplier of operational leakage current as a function of applied voltage

Any pulsating voltage (or ripple voltage superimposed on DC bias) across a capacitor results in an alternating current through the capacitor.

Because of ohmic and dielectric losses in the capacitor, this alternating current produces an increase of temperature in the capacitor cell.

The heat generation depends on frequency and waveform of the alternating current.

The maximum RMS value of the alternating current, which is permitted to pass through the capacitor during its entire specified useful life (at defined frequency anddefined ambient temperature), is called rated ripple current ( I_R ).

The rated ripple current is specified in the relevant detail specifications at 100 or 120 Hz (in special cases at 100 kHz ) and at upper category temperature.

Usually the rated ripple current will cause a temperature increases of the capacitor's surface of approximately 3^o or 5^oK(dependent on series) compared with ambient temperature. A furthur temperature increase of 3^o or 5^o K will be found in the core of the capacitor.

This temperature rise is the result of the balance between heat generated by electric losses:

The heat , generated by ripple current, is an important factor of influence for non-solid electrolytic capacitors for calculating the useful life certain circumstances.

In the detail specifications this factor is considered in the so-called 'life-time nomograms' ('Multiplier of useful life' graph) as a ratio between actual ripple current (I_A) and rated ripple current (I_R), drawn on the vertical axis.

Care should be taken to ensure that the actual ripple current remains inside the graph at any time of the entire useful life. If this cannot be realized, it is more appropriate to choose a capacitor with a higher rated voltage or higher capacitance, than originally required by the application.

The internal losses and the resultant ripple current capability of electrolytic capacitors are frequency dependent. Therefore, a relevant frequency conversion table ('Multiplier of ripple current as a function of frequency') is stated in the detail specifications.

The Useful life of Capacitor at maximum operating temperature is as follows.

☛ Failure rate curve

1.3 Factors Effecting life expectancy

The following misapplication will cause rapid deterioration to capacitor electrical parameters. In addition to rapid heating and gas generation within the capacitor pressure relief vent will operate and result in leakage of electrolyte.

1.3.1 Reverse Voltage

1.3.2 Charge/Discharge Applications






Figure 1 Individual Fuses smoothing capacitor



Figure 2 Protection by charging resistors

The protecting of capacitors connected in parallel is not suitable for capacitor banks designed for impulse discharges. Here, the capacitors should be protected during the charging process by means of appropriate resistors. The capacitors are then connected in parallel immediately before they are to be discharge. The principle is shown in figure 2.

Series Connection of AI. Electrolytic Capacitors

If two or more electrolytic capacitors are connected in series, balancing resistors are required; as the leakage current for capacitors shall not be identical, it shall vary from capacitor to capacitor. In such case when voltage is applied it shall not draw the appropriate voltage. There can have a chance of a capacitor drawing more than the rated voltage. Balancing resistors limit the difference in voltage across the capacitors under balancing DC conditions.

PI: Refer Fig. 3 for details of 2 capacitors in the series.

For practical purposes the following equation can be used to calculate the maximum possible resistor

Balancing Resistor Value

Values in ohms:

^{Here, U} _mis the maximum voltage that may be present on one of the capacitors and I _L5is the specified leakage current in amperes after 5 minutes (used as an approximation of the difference in leakage current between C1 and C2)

Figure 3 Balancing resistors two electrolytic capacitors in series


Figure 4 series connection (forced distribution Balancing)

Another method of series connection by using Electrically Isolated Voltage Source as shown in Fig. 4. This is known as Forced Voltage distribution Balancing.

The balancing measures described above may be omitted in cases where the total dc voltage to be applied is substantially lower than the sum of the rated voltages of the capacitors to be used.

Experience has shown that this possible for n = 2 to 3 single capacitors in series without any considerable risk if the total voltage does not exceed 0.8 . n . UR. However, this solution can only be implemented if the series circuit consists of matching capacitors (same type, same capacitance), so that the dielectric insulation resistance of the capacitors, which is the only factor determining the voltage distribution in this case, will not vary too greatly from one capacitor to the next.

^{Combined series / parallel connection}

The above mentioned rules for both series and parallel connection are accordingly valid for any combination of these two cases. If balancing resistors are to be used, it is advisable to allocate a separate resistor to each capacitor (Fig. 5).