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.

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.

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 (Fig.1).

	FIG. 1: BASIC CIRCUIT OF A FORWARD CONVERTER WITH A CST
The CST (T1) is connected into the source line of the transistor. Unlike a shunt resistor, the CST generates almost no losses.

The functional principle of a CST is shown in Fig. 2. The primary current to be measured (I_prim) 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 (N_s). The measured signal V_sense is linear to I_prim when the diode voltage dropping across D1 is neglected.

	FIG. 2: MEASUREMENT CIRCUIT WITH CST
The measurement signal Vsense is linear to Iprim when the diode voltage dropping across D1 is neglected.

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:

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 A_e:

EE5 core: 2,510^-6 m²

E4.2 core: 1,44^-6 m²

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

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:

After determining the maximum measurement voltage V_{sense_max} by selecting a CST with a sufficiently large number of turns, the maximum measurement current I_{sense_max} and R_T can be determined:

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 I_mag.

I_{sense_max} can be determined with the following equation:

where: 

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

Finally, R_T can be determined:

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.

Authors:

Bernhard Röllgen, Product Development Manager Transformers

Karl Stoll, Product Development Manager Transformers