Technical points and selection of current transformers

Current transformers have long been used as a standard device in instruments and equipment to measure precision currents. Even in harsh environments and high temperatures, this device is very accurate, easy to use and reliable.

In applications such as switching power supplies, motor current load detection, lighting and instrumentation, current transformers are generally used as control, circuit protection and monitoring devices. With the increasing number of current transformers available, how to choose a suitable current transformer requires consideration of many factors. This article introduces a simple selection method, which is very helpful for choosing the right cost-effective device in many applications. Although the off-the-shelf devices are cheap and available immediately, they have some functional limitations in use. Some applications may require special products or even complete customization.

Figure 1 The selection of a current transformer should take into account many factors, such as size, frequency, function and current range.

Input Current

First, the selection of the current transformer must clarify and verify multiple indicators, such as size, frequency, function and sampling current range. Its accuracy and efficiency actually depend on these parameters. In addition to the possible compromise in the accuracy of the current transformer, if the current transformer is used with a current exceeding the rated current specification specified by the manufacturer, its operating temperature will continue to rise and cannot be controlled, resulting in circuit failure.

In addition, if the rating of a current transformer is much higher than its "sampling current", the size of the device will inevitably be large and too expensive for its application. Generally speaking, it is a wise choice to select a current transformer with a rating about 30% higher than the maximum expected value of its "sampling current".

Turns ratio

Common current transformer turns ratios range from 1:10 to 1:1000. The higher the turns ratio (r=Nsec/Npri), the higher the resolution of the current measurement.

However, it is worth noting that a high turns ratio will increase the distributed capacitance and leakage inductance, thereby reducing the accuracy of the current transformer and its performance at high frequencies (due to self-resonance). However, if the turns ratio is too low (low inductance), the output signal may be distorted or "drooped" (single-stage input signals must be skewed), causing instability in the control circuit and inaccurate measurement results.

Inductance and Excitation Current

The secondary inductance of the current transformer determines the fidelity of the output signal. The value of the inductance is inversely proportional to the excitation current, which is commonly known as the "sensing current".

In order to ensure the maximum fault tolerance of the current transformer, the excitation current should be several times smaller than the amplitude of the sampling current. For most applications such as switching power supplies, it is ideal to take 10% of the sampling current as the maximum excitation current. For example, if a circuit must ensure a maximum loss of 10% for a sampling current of 1 to 20A at 100kHz, the maximum excitation current must be set to 100mA (i.e. 10% of the minimum sampling current value).

A sampling current of 1A will produce a 10% error, and a sampling current of 20A will produce a 0.5% error. If the manufacturer's data sheet does not indicate the excitation current, it can be calculated using the following formula:

e = CLdI/dt

|dI/dt| = e/L

Where e is the device output voltage (in V), L is the inductance (in H), and |dI/dt| is the ratio of the excitation current to time (in A/s).

Output voltage and load resistance

The output voltage (Vo) should be set as low as possible to reduce insertion loss. Assuming that the optimal secondary output voltage of a circuit is 0.5V and the output current is 20A, a current transformer with a turns ratio of 1:100 will produce a secondary current of about 200mA. As shown in Figure 2, the load resistance should be: Ro=Vo/Is = 0.5/0.2 = 2.5Ω.

Figure 2 A typical current sensing circuit with a load resistor