Home About us Products Technical Info Samples Sales Network News Contact Parametric Search
ImprintPrint page
Technical Information:
Basic Technical Data for Capacitors
Construction of Plastic Film Capacitors 
Dielectric 
Special Technical Subjects 


You are here: HomeTechnical InfoSelection of Capacitors for Pulse Applications

Selection of Capacitors for Pulse Applications

The maximum permissible AC voltage that can be applied to capacitors in sinusoidal waveform applications, can be determined from the graphs in the respective capacitor ranges.

However, where pulse conditions exists, the following procedure is to be observed to ensure that the correct capacitor rating is selected for a particular duty:

 

Rated voltage (Ur): The rated voltage of a capacitor against a zero potential reference point shall take into consideration that the dielectric strenght of the capacitor film diminishes with rising frequency. The calculation of the required rated voltage of a capacitor must therefore allow for the correction factor k; where k = dielectric strength of the film at the frequency f in % is shown in graph 1.

 

 
Graph 1: Dielectric strength of Polypropylene film as a factor of frequency (general guide).

 

The calculation of the required dielectric strength is shown in the following example(Umin, Umax have the same polarity).

 

 

Furthermore the rms voltage derived from the peak to peak voltage shall not be greater than the nominal AC voltage rating of the capacitor to avoid the ionization inception level:
Urms < UAC rated.

 

Maximum current: The voltage gradient or rise time of the pulse is taken as the reference point when calculating the maximum current rating of the end contacts. The maximum possible current load on the end contacts is calculated by means of the voltage rise of the pulse (pulse rise time F).
Imax = F x C x 1.6

The data of the rated pulse rise time Fr for pulses equal to the rated rated voltage figure in the technical data of the different types. With low voltage rise in operation (Upp) the permissible current load is calculated as follows:

 

 

for example Ur = 63 V, Upp = 12 V, Fr = 50 V/µsec.

hence Fmax = x 50 = 262.5 V/µsec.

When using maximum current ratings, self-heating must be taken into account at higher frequencies, and must not exceed 8 K.

 

Dissipation (heat losses): The heat dissipated by a capacitor when stressed by non-sinusoidal voltages or when under pulse conditions can be approximately determined from the following formula:

 

Pd = Urms2 x C x tan where
Pd = dissipation in Watts (see table 1 for the max.W per K).
Urms = root mean square value of the AC voltage share
= 2 x f, where f is the repetition frequency of the pulse waveform.
  C = capacitance in Farad
tan = dissipation factor corresponding to the frequency of the steepest part of the pulse.

 

pulse frequency =

The temperature rise is as follows:

Temperature rise in K = (see table 1)

 

Printed circuit module
PCM (in mm)
Specific dissipation in Watts per K
above the ambient temperature
2.5
5
7.5
10
15
22.5
27.5
37.5
0.0025
0.004
0.006
0.0075
0.012
0.015
0.025
0.03
 
Table 1: The data is for
ordinary assembly and
ventilation conditions avoiding
radiant heat within the chassis
of the equipment.
 
In applications where reliability is critical, it is recommended to measure the surface temperature of the capacitor and to take into account that the temperature within that capacitor will be approximately 5 K above the case temperature.

 

Determining the permissible AC voltage and AC current at given frequencies.
To determine the permissible AC voltage (sinusoidal) for applications in a higher frequency spectrum, graphs showing AC voltage derating with frequency are available for the respective WIMA series. The diagrams refer to a permissible self-heating of:


< 10 K.

 

For the WIMA MKP 10 / 0.01µF / 630VDC/400VAC, for example, this shows - when f = 50kHz - a permissible AC voltage of

 

Urms = 280 V (graph 2).

 

 
Graph 2: Permissible AC voltage in relation to frequency at 10°C internal temperature rise (general guide).

 

The AC voltage given in the diagrams can also be used to determine the maximum effective current

 

Xc = 318
   
Ic = 0.88 A
   
The calculated maximum value of the effective current
 
Ip = 1.24 A
   

must not exceed the maximum current rating specified in the maximum pulse rise time calculation (see Fmax above). In this case, the operating AC voltage is to be reduced accordingly.

 

Selection example for pulse application capacitors