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Posted on 01 September 2019

How to Select an Off-The-Shelf Current Transformer

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A simple guideline can greatly help

In electronic systems applications such as switch-mode power supplies, current transformers are generally used for control, circuit-protection, and monitoring features.

By Ariel General, Senior Design Applications Engineer, Datatronics, Inc.

 

Engineers have relied on current sense transformers as the ‘gold standard’ for precise measurements in instrumentation applications for decades. They are accurate, easy to implement, and reliable under harsh environmental and thermal conditions. In electronic systems applications such as switch-mode power supplies, current transformers are generally used for control, circuit-protection, and monitoring features. With the increasing availability of OTS (Off-The-Shelf) current transformers, a simple guideline can greatly help in the selection of proper and cost-effective components for many applications.

Below is a simple chart outlining the steps in the current transformer selection process:

OTS current transformer selection flow chart

Characterize the Input Current

The first step when selecting a current transformer is to define and verify key factors such as size, frequency, function and the range of current being sampled. These are important factors to verify since the accuracy and effectiveness will essentially be dependent on these parameters. Aside from the possibility of compromising the transformer’s accuracy, using a current transformer above the manufacturer’s rated current specification may saturate the transformer and may cause circuit failures due to an uncontrolled rise in operating temperature. On the other hand, a current transformer that is rated much higher than the “sample current” might be restrictively too large and expensive for its purpose. Typically, selecting a currenttransformer that is rated approximately 30% above the expected maximum of the “sample current” is a prudent starting point.

The Turns Ratio

The most commonly available OTS current transformers have turn ratios ranging from 1:10 to 1:1000. The higher the turns ratio (r = Nsec/Npri), the higher the resolution of the current measurement. However, care must be taken as too high of a turns ratio will necessitate an increase in distributed capacitance and leakage inductance which may decrease the transformer’s accuracy and capability to operate at higher frequencies (due to self-resonance). However, if the number of turns is too low (lower inductance), the output signal may distort or “droop” (in positively sloped unipolar input signal) which may also cause instability in the control circuit and inaccuracies in measurements.

Inductance and Excitation Current

To determine the fidelity of the output for a particular current transformer, you must evaluate its secondary inductance. The value of inductance is inversely proportional to the excitation current – which is then subtracted to the “sensed current.”

In order to ensure the maximum error tolerance of the transformer, the excitation current should be several times less than the magnitude of the sample current (a maximum of 10% is ideal for most switch-mode power supplies, SMPS, applications). For instance, if a circuit has to maintain a maximum of 10% loss for a sample current of 1 A to 20 A at 100 kHz, the excitation current must be set to a maximum of 100 mA (10% of the minimum sample current value). A 1 A sample current will yield an error of 10% while a 20 A sample will yield an error of 0.5%.

This equation can be used to calculate the excitation current, in the event that it is not specified in the manufacturer’s data sheet:

Equations 1

Where e is the set output voltage (V), L is the inductance (H), and |dI/dt| is the excitation current with respect to time (A/s).

The Output Voltage and “Burden Resistor”

Set the output voltage (Vo) as low as practically possible in order to minimize the insertion loss. Assuming 0.5 V is the optimum secondary output voltage in a circuit and the output current is 20 A, a 1:100 ratio transformer will yield a secondary current of ≅ 200 mA. Per Figure 2, the burden resistor should be:

Equation 2

Current sense circuit

Practical Example:

Definition of Requirements:
Input Current: 100 kHz, 1A - 5A
Output Voltage: 0.1 V →1 A
                       0.5 V → 5 A
                       @ 10% accuracy
Package: low-profile, surface-mount

Part Selection:
Using Datatronics´ standard OTS catalog on their website, the CT317-100 current transformer satisfies the input current and package requirements - only the accuracy level and burden resistance are left to be determined.

Error Level Approximation:
For 10% Error (neglecting coupling losses), the primary excitation current must be less than 10% of the minimum input current - a maximum of 100 mA in this case:
Iexc (SEC)  1/f * e/L
               1/100 kHz * 0.1/(2.5 mH)
               0.4 mA

This yields an approximate current of 40 mA on the primary, 60 mA less than the 100 mA maximum.

Burden Resistor Calculation:
Ro = Vo/Isec = 0.1V / (1A/100) = 10Ω

For the given requirement, the Datatronics CT317-100 is an acceptable selection.

Conclusion

Off-The-Shelf components are typically low cost and readily available. However, as wefve discussed, OTS components have definite functional limitations on their usage. There are applications where specific recommendations or even full customization may be required. It is therefore advisable to procure these components from reputable manufacturers that have strong engineering, manufacturing, and customer service capabilities.
 

References:

1) Billings, Keith H., "Switchmode Power Supply Handbook ," 2nd ed., McGraw-Hill, New York,1999.
2) McLyman, Colonel Wm. T., "Transformer and Inductor Design Handbook," Marcel Dekker, New York, 1978.

 

About the Author

Ariel General is Senior Design Applications Engineer for Datatronics, Inc.
28151 Highway 74, Romoland, CA 92585.
Phone: +1-951-928-7700

 

 

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