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Posted on 01 November 2019

Measuring Currents without Losses

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Miniaturized current sense transformers

Two new series of miniaturized SMT current sense transformers are designed for no-loss current measurements in compact DC/DC converters.

By Bernhard Röllgen, Product Development Manager Transformers and Karl Stoll, Product Development Manager Transformers, EPCOS

 

Current sense transformers (CST) are used for controlling and protecting circuits as well as detecting loads in power supplies. EPCOS has developed two new CST series that are particularly suited for compact DC/DC converters thanks to their space-saving SMT design. The primary winding is already integrated in all types, thus saving development and placement times. Both series cover transfer ratios from 1:20 to 1:125.

Because they can measure currents while isolating the circuits electrically, current sense transformers have advantages over other forms of current measurement. The current to be measured is fed into the primary winding of a transformer while a voltage proportional to the primary current is sampled on the secondary side. The electrical losses are thus minimal compared with current measurement via a shunt resistor. CSTs are also very rugged and allow simple implementation of downstream electronics.

Image 1

These two new CST series are based on different ferrite cores. The first series is compatible with existing designs based on an EE5 core; it features a footprint of only 8.33 x 7.11 mm2 and an insertion height of 5.08 mm. The DC resistance of the primary winding is 0.8 mΩ in all types of this series. These transformers are designed for a maximum current of 20 A and a test voltage of 500 A. Depending on the transfer ratio, the series covers inductances between 80 and 3000 μH.

Image 2

The second series was developed as a miniature variant with the much smaller E4.2 core; its primary winding is integrated in the coil former in a space-saving way. The types in this series need only 33 percent of the area and 23 percent of the volume (4.7 x 3.5 mm2, insertion height 3.5 mm) of their EE5 counterparts. They enable inductances of between 33 and 1280 μH. Their DC resistance is 2.5 mΩ, their rated current is 7 A, and their test voltage 360 V AC.

Both CST variants have one turn on the primary side. Depending on the resolution needed to measure the current, customers can choose between 20, 30, 40, 50, 60, 70, 100 or 125 turns on the secondary side. If the aim is to measure high frequencies, for example, a variant with a correspondingly small number of secondary turns would be optimal. The natural resonant frequency, which has a limiting effect, is higher in variants with few secondary turns than in those with many. This is due to the parasitic capacitance of the secondary winding.

Function principle and correct dimensioning

CSTs are preferentially used in switching regulators. They replace the resistor in the source line of the switching transistor and considerably reduce the power dissipation in this part of the circuit (Figure 1).

Basic Circuit of a Forward Converter with a CST

The functional principle of a CST is shown in Figure 2. The primary current to be measured (Iprim) flows through the primary winding of the transformer. In both series, there is exactly one turn in the primary winding. As a result, the transfer ratio is determined directly by the number of secondary turns (Ns). The measured signal Vsense is linear to Iprim when the diode voltage dropping across D1 is neglected.

Measurement Circuit with CST

Correct dimensioning

Before selecting the type to use, the suitability of a particular CST type for a specific maximum primary current must initially be checked. Suitable types are those whose magnetic flux density does not exceed 200 mT:

Equation 1

where:
Bmax maximum flux density in the core
Ls inductance of the secondary winding
Ns number of secondary turns of the CST
Iprim_max maximum primary current
Ae effective magnetic cross-section of the core

Values for Ae:
EE5 core -    2.510-6m2
E4.2 core -    1.44-6m2

The transfer ratio needed to obtain the desired measurement signal Vsense can be calculated from Faraday’s law:

Equation 2

where:
Np number of primary turns of the CST (1 turn)
Ns number of secondary turns of the CST
Lp inductance of the primary winding
Ls inductance of the secondary winding
Iprim_max maximum primary current
fosc operating frequency of the switching regulator

From this, we obtain:

Equation 3

After determining the maximum measurement voltage Vsense_max by selecting a CST with a sufficiently large number of turns, the maximum measurement current Isense_max and RT can be determined:

Equation 4

where:
Imag_max maximum magnetizing current
Vsense_max maximum output voltage of the measurement signal
δmax maximum duty cycle
Ls inductance of the secondary winding
fosc operating frequency of the switching regulator

The variant with a maximum number of turns on the secondary side should be used wherever possible in order to minimize the undesired magnetizing current Imag.

Isense_max can be determined with the following equation:

Equation 5

where:
Isense_max maximum current through RT
Iprim_max maximum primary current
Ns number of secondary turns of the CST

Finally, RT can be determined:

Equation 6

where:
RT the magnitude of the load resistance

The two series are designed for IR values of 20 A (EE5) and 7 A (E4.2) respectively. This is sufficient for most compact DC/DC converters of medium power. The equations and rules described above can be used to identify a suitable CST type from these series. Both series are naturally RoHS compatible and are suitable for automatic placement. They are supplied in blister tape for this purpose.
 

 

 

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