Selection Example for Pulse Application Capacitors

Determination of the nominal voltage

Calculation is based on an operating temperature < +60°C unless other data is given by the user.

Ur > 350 V
Urms 85 V (referring to AC voltage share)
Selected nominal voltage: 400VDC / 250VAC lead spacing 27.5mm

Permissible voltage gradient

The voltage rise time is ~ 87.5 V/µsec.

Value from the table "pulse rise time WIMA FKP 1": 7000 V/µsec.

The calculated voltage gradient is lower than the permissible value shown in the catalogue for this capacitor.

Dissipation
Given:	Urms f C	= 85 V = 32 kHz = 0.1 µF

The frequency determined from the steepest part of the pulse is:
Pulse width = 15 µsec = 1 cycle.

Hence pulse frequency = ~ 66 kHz

The tan of WIMA FKP 1 at 66 kHz ~ 10 x 10^-4 (graph: Dissipation factor change versus frequency)
Pd = 85² x 2 x 32 x 10³ x 0.1 x 10^-6 x 10 x 10^-4 ~ 0.145 Watts.

The selected capacitor has a lead spacing of 27.5mm
(specific dissipation = 0.025 W/K according to table 1) and the temperature rise due to self-heating is:

Temperature rise = ~ + 6K

The temperature rise plus the max. ambient temperature = max. permissible operating temperature (taking into account the voltage derating in the Electrical Data). If the permissible temperature is exceeded, then select a capacitor with a higher voltage rating.