They are used to detect high frequency currents, in order to protect an electronic switch against overcurrents in switching converters. They are typically connected at the primary side of medium-high power high frequency transformers, but they are furthermore suitable for currents measurement, in applications which don't require high accuracy, but prompt response of the measurement equipment instead.
They are developed to detect alternate currents in the frequency range of some kHz. By connecting a burden resistance to the secondary side, you can read a proportional voltage signal, isolated from the mains. Depending on the application, other different signal processing are possible, either by using digital or analogue techniques. Anyway, the result is an information that defines for example the protection threshold for the electronic switch or can be used to perform current controls.
These current transformers can be built with any possible turns ratio, depending on what the application requires, but the typical ratios are the following ones: 1:50, 1:100, 1:200, 1:500, 1:1000. As the number of turns increases, the optimum working frequency range moves towards low frequencies, while the minimum working frequency is related to the capability of the core to transfer the current pulse, that is the transfer area of the transformer. The burden resistance sets the output voltage range, and after the designer has chosen the signal level, the minimum working frequency is fixed and for higher frequencies the detection circuit will not show any signal distortion. The maximum working frequency is influenced mainly by two factors: the core material behaviour versus frequency and the number of turns, in fact each pair of turns composes a parasitic capacitor and the total amount of this effect is a big capacity that limits the current transformer's bandwidth.
From a building point of view, high frequency current transformers can be wound either on toroidal cores or on E cores.
Toroidal models are available with passing-through hole, but some of them are feasible with the inserted primary turn too.
Linear models, based on E cores, are available exclusively with the primary turn inside, so they are especially recommended for control functions in equipments where the low cost is achieved by simply, strong and easy-mounting components. All standard current sense transformers are made according the safety standard IEC 742 for working voltage up to 600 Vrms, with patended bobbins that were developed by Sirio.
Typical working temperature is from -25 to +85°C.
The plastic material of the case is UL94-HB or UL94-V0 on request.
Main features of current sense transformers are the following ones.
- n secondary to primary turns ratio, that is the number of secondary turns
- RS secondary winding's resistance
- Ip rated primary current (rms value)
- LS rated secondary inductance
- fn optimum working frequency (or optimum working frequency range)
- Vt minimum secondary transfer area at 25°C
- Uis maximum working voltage primary/secondary
- Up isolation voltage primary/secondary
- D central hole diameter/primary turn diameter