Components and methods for measuring current

There are various measurements that can generate signals that indicate "how big" or "too big", including:

• Resistive (direct)

—— Current sensing resistor

• Magnetic (indirect)

--Current Transformer

——Rogowski coil

——Hall Effect Device

• Transistor (direct)

—— R _{DS (ON)}

——Ratio ​

Each method has its advantages and is a valid or acceptable current measurement method, but each has its own advantages and disadvantages, which is critical to the reliability of the application. These measurement methods can be divided into two categories: direct or indirect. The direct method means that it is directly connected to the circuit being measured, and the measuring element will be affected by the line voltage. The measuring element of the indirect method is isolated from the line voltage. It is necessary to use the indirect method when the safety of the product requires it.

Resistive

Current sensing resistor

Measuring current with a resistor is a direct method with the advantages of simplicity and good linearity. The current-sensing resistor is placed in a circuit with the current being measured, and the current flowing through the resistor will convert a small part of the electrical energy into heat. This energy conversion process produces a voltage signal. In addition to the characteristics of ease of use and good linearity, the current-sensing resistor is also very cost-effective, with a stable temperature coefficient (TCR) of less than 100ppm/℃ or 0.01%/℃, and is not affected by potential avalanche multiplication or thermal runaway. In addition, low-resistance (less than 1mΩ) metal alloy current-sensing resistors have very good surge resistance and can achieve reliable protection in the event of short circuits and overcurrent conditions.

magnetic

Current Transformer

Current transformers (Figure 1) offer three distinct advantages: isolation from line voltage, lossless current measurement, and good noise immunity for large-signal voltages. This indirect method of measuring current requires a varying current, such as AC, transients, or switched DC, to generate a varying magnetic field that magnetically couples into the secondary winding. The secondary measured voltage is scaled based on the turns ratio between the primary and secondary windings. This measurement method is considered “lossless” because the circuit current has very little resistive losses as it passes through the copper windings. However, as shown in Figure 2, the transformer’s losses result in a small amount of energy being lost due to the presence of the load resistor, core losses, and the primary and secondary DC resistances.

Figure 1: Ideal current transformer circuit

Figure 2: Composition of current transformer losses

Rogowski coil

A Rogowski coil (Figure 3) is similar to a current transformer in that it induces a voltage in the secondary coil that is proportional to the current flowing through the isolation inductor. The Rogowski coil is a special design that uses an air core, which is completely different from current transformers that rely on a high permeability core such as laminated steel and magnetic coupling between the secondary winding. The air core design has a lower inductance, faster signal response, and a very linear signal voltage. Because of this design, Rogowski coils are often used on existing wiring such as handheld meters to temporarily measure current and can be considered a low-cost alternative to current transformers.

image 3

Hall Effect

When a conductor with current is placed in a magnetic field (Figure 4), a potential difference is generated perpendicular to the magnetic field and the direction of current flow. This potential is proportional to the current. When there is no magnetic field and current flowing, there is no potential difference. However, as shown in Figure 5, when there is a magnetic field and current flowing, the charge interacts with the magnetic field, causing the current distribution to change, thus generating the Hall voltage.

The advantage of Hall effect elements is that they can measure large currents with low power dissipation. However, this method also has many disadvantages that limit its use, such as the need to compensate for nonlinear temperature drift; limited bandwidth; the requirement to use a large bias voltage when measuring small current ranges, which can cause errors; susceptibility to external magnetic fields; sensitivity to ESD; and high cost.

Figure 4: Hall effect principle, no magnetic field

Figure 5: Hall effect principle with magnetic field

transistor

R _DS(ON) – Drain to source on-resistance

Since transistors are standard control devices for circuit design and do not require resistors or energy-consuming devices to provide control signals, transistors are considered to be an energy-free overcurrent detection method. Transistor data sheets give the drain-to-source on-resistance (R _DS(ON) ), and the typical resistance of power MOSFETs is generally in the milliohm range. This resistance consists of several parts, starting with the leads connected to the semiconductor die (Figure 6), which affects many channel characteristics. Based on this information, the current flowing through the MOSFET can be calculated using the formula I _Load = V _RDS(ON) / R _{DS(ON) .}

_{Each component of R DS(ON)} contributes to measurement error due to small changes in resistance in the interface region and the TCR effect . The TCR effect can be partially compensated by measuring the temperature and correcting the measured voltage with the expected change in resistance caused by temperature. Many times, the TCR of a MOSFET can be as high as 4000ppm/°C, which is equivalent to a 40% change in resistance for a 100°C temperature rise. Generally speaking, the signal accuracy of this measurement method is about 10% to 20%. From the application's accuracy requirements, this accuracy range is acceptable for providing overvoltage protection.

Figure 6: Simplified model of an N-channel enhancement-mode MOSFET

Ratiometric - Current Sense MOSFET

MOSFETs are made of thousands of transistor cells connected in parallel to reduce on-resistance. Current-sense MOSFETs use a small number of cells connected in parallel, connected to a common gate and drain, but with separate sources (Figure 7). This creates a second, isolated transistor, the “sense” transistor. When the transistor is on, the current flowing through the sense transistor is proportional to the main current flowing through the other cells.

The accuracy tolerance range depends on the specific transistor product, ranging from 5% to 15% to 20%. This method is usually not suitable for current control applications that generally require a measurement accuracy of 1%, but is suitable for overcurrent and short-circuit protection.

Figure 7

As can be seen from the summary table above, there are many ways to detect current in a circuit, and the appropriate method should be selected based on the specific needs of the application. Each method has its advantages and disadvantages, and these factors must be carefully considered in the design.