Current-Sen Transformer Application Design Guidelinestore
By Ariel General
Applications Engineer, Datatronic Distribution Inc.
The standard for preci current measurement in instrumentation and other high reliability equipment applications has been the current n transformer. 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 ud 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 lection of proper and cost-effective components for many applications.
anna paquinFigure 1. is a simple chart for the lection process
Figure 1: OTS current transformer lection flow chart
The Input Specifications
The lection of a current transformer must begin with the definition and verification of certain factors such as size, frequency, function, and the range of current being sampled – the accuracy and effecti
veness will esntially be dependent on the 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 cau circuit failures due to an uncontrolled ri 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 purpo. Typically, lecting a current-transformer that is rated approximately 30% above the expected maximum of the “sample current” is a prudent starting point.moonwalking
The Primary/Secondary Turns Ratio
Off the shelf current-transformers commonly have turns ratios ranging from 1:10 to 1:1000. The higher the turns ratio (r = N c /N pri ), the higher the resolution of the current measurement. However, care must be taken as too high of a turns ratio will necessitate an increa in distributed capacitance and leakage inductance which may decrea the transformer’s accuracy and capability to operate at higher frequencies (due to lf-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 cau instability in the control circuit and inaccuracies in measurements.
Inductance and Excitation Current
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The current transformer’s condary inductance will determine the fidelity of the output signal. The value of inductance is inverly proportional to the excitation current – which is then subtracted to the “nd current.”
The excitation current should be veral times less than the magnitude of the sample current (a maximum of 10% is ideal for most SMPS applications) – this will ensure the maximum error tolerance of the transformer. 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 t 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%.
In ca the excitation current is not specified in the manufacturer’s data sheet, it can be calculated with the equation:
dt
dI L e −=; L
e dt dI = Where e is the t output voltage (V), L is the inductance (H), and │dI/dt │ is the excitation current w/ respect to time (A/s).
The Output Voltage and “Burden Resistor”
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The output voltage (Vo) should be t as low as practically possible to minimize the inrtion loss. Assuming 0.5 V is the optimum condary output voltage in a circuit and the output current is 20 A, a 1:100 ratio transformer will yield a condary current of ≅ 200 mA. Per Figure 2, the burden resistor should be:
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===5.0Vo Ro 2.5 Ω 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:
hca2Using 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 loss), the primary excitation current must be less than 10% of the minimum input current – a maximum of 100 mA in this ca;
I exc (SEC) ≅ 1/f * e/L
≅ 1/100 kHz * 0.1/(2.5 mH)
≅ 0.4 mA
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This yields an approximate current of 40 mA on the primary, 60 mA less than the 100 mA maximum.
Burden Resistor Calculation:
sarah paulsonRo = Vo/Ic = 0.1V / (1A/100) = 10Ω
For the given requirement, the Datatronics CT317-100 is an acceptable lection. Conclusion
OTS components are inexpensive and instantly available, but as discusd in this article, there are functional limitations on their usage. There are applications where specific recommendations or even full customization may be required. It is therefore advisable to procure the components from
softkittyreputable manufacturers that have strong engineering, manufacturing, and customer rvice capabilities. References
Billings, Keith H., Switchmode Power Supply Handbook – 2nd ed., McGraw-Hill, New York, 1999.
McLyman, Colonel Wm. T., Transformer and Inductor Design Handbook, Marcel Dekker, New York, 1978.
