# Getra Transformers rotary current transformers, chokes,switchboard construction

Storage Chokes

Technical Description:

Storage chokes are typically used in switched-mode power supplies. Their function is to store the energy which is absorbed by the choke during the conduction phase. In the blocking phase they then supply the stored energy to the series-connected load or the smoothing capacitors. The constant rise and fall in the choke current results in an approximately triangular current waveform at the storage choke.

The storage energy W is derived from the inductance L and the choke current I according to the following formula:

W = ½ x L x I²

As a result of the increasing miniaturization of switched-mode power supplies and the requirement to set the clock frequency of the valves used in the switched-mode power supplies above the range of audibility, it is normal to work with switching frequencies in the range from 20 kHz to 100 kHz. These high frequencies which are applied to the chokes make it essential to use high-quality cores. The high frequencies result in high power losses which vary according to the core material and the core volume. These heat up the storage choke and put severe constraints on the conditions for use as far as the ambient temperature is concerned. GETRA therefore use cores made of high-quality materials such as MPP (molyper-malloy) and HF (High-Flux) which are suitable for high frequencies and are characterized by low core losses. As an alternative, however, low-cost cores of ferrous powder are also used.

The main parameters which are important for selecting the size of a storage choke are:
  • direct current preloading
  • current ripple
  • pulse frequency
  • ambient temperature
The direct current preloading results in a drop in the inductance at rated current compared with the no-load inductance. The permissible current ripple requires maximum saturation of the core material and is thus in direct relation to the core losses. The pulse frequency controls the inherent heating of the storage choke through the core losses and thus determines the maximum permissible ambient temperature.

Finally an example of how to select the right size of storage choke is shown below.

Given the following basic data for a switched-mode power supply:

UA = 12V; UV = 1,2V; IA = 5A; ∆I = 1A; f = 100kHz; vmin = 0,3


UA Output voltage
UV Voltage loss at the diode and the storage choke
IA Output current
∆IA Residual ripple of the choke output current
f Pulse frequency of the switched-mode power supply
Vmin Minimum pulse duty factor

The required inductance L can be determined with the following formula:


Storage Chokes:
GETRA Geräte- und Transformatorenbau GmbH
Osterwaal / Schulgasse 3-5
84072 Au / Hallertau
Tel.: +49 (0) 8752 8608 -0
Fax: +49 (0) 8752 8608 -60