Measuring a voltage in any system is a “passive” activity as it can be easily done at any point in the system without affecting system performance. However, current measurement is “intrusive” as it requires the insertion of some type of sensor that runs the risk of affecting system performance.
Current measurement is vitally important in many power and instrumentation systems. Traditionally, current sensing was mainly for circuit protection and control. However, with the advancement of technology, current sensing has emerged as a method to monitor and improve performance.
Fig. 1: A representative image of a current sensor
Knowing the amount of current supplied to the load can be useful for a wide variety of applications. Current sensing is used in a wide range of electronic systems, viz., battery life indicators and chargers, 4-20 mA systems, overcurrent supervision and protection circuits, current and voltage regulators, DC/DC converters. DC, earth fault detectors, programmable current sources, linear and switching power supplies , communication devices, automotive power electronics, motor speed controls and overload protection, etc.
CURRENT DETECTION PRINCIPLES
A current sensor is a device that detects and converts current into an easily measured output voltage, which is proportional to the current through the measured path.
When a current flows through a wire or circuit, a voltage drop occurs. Furthermore, a magnetic field is generated around the current-carrying conductor. Both phenomena are used in the design of current sensors. Thus, there are two types of current detection: direct and indirect. Direct detection is based on Ohm's law, while indirect detection is based on Faraday's and Ampere's laws.
Direct sensing involves measuring the voltage drop associated with current passing through passive electrical components.
Figure 2: A diagram explaining the principle of direct detection
Indirect sensing involves measuring the magnetic field around a conductor through which current passes.
Figure 3: A diagram illustrating the principle of indirect detection
The generated magnetic field is then used to induce proportional voltage or current which is then transformed into a form suitable for measurement and/or control system.
PASSIVE ELEMENT BASED CURRENT DETECTION TECHNIQUES
1. Sensing Resistors
Current sensing means developing a voltage signal that is representative of the current flowing at the specific location of interest in the circuit. The traditional way of sensing current introduces a resistor in the path of the current to be sensed. The sensing resistor can be placed in series with the inductor, switches and load. Thus, a current-sensing resistor should be considered as a current-to-voltage converter. 
Figure 4: An image of a current sensing resistor
The current sensing resistor must have the following attributes
· Low value to minimize energy losses
The value of the current sensing resistors depends mainly on the voltage limit of the following circuit, which will operate based on the detected current information. In circuits where amplification is available, the emphasis is on minimizing the voltage drop across the resistor.
Typical resistance values used in many control ICs are 20m ? at 25m ? .
· Low inductance due to high di/dt.
Any inductance in the resistor, when exposed to high slew rate (di/dt), an inductive step voltage is superimposed on the sense voltage and can be of concern in many circuits. Consequently, sense resistors must have very low inductance.
· Tight tolerance
To maximize current delivery within the acceptable current limit, the sense resistor tolerance must be ±1% or tighter.
· Low temperature coefficient for accuracy
Typically specified in units of parts per million per degree centigrade (ppm/°C), the temperature coefficient of resistance (TCR) is an important parameter for accuracy. Resistors with TCRs closer to zero must be used throughout the entire operating range.
· High peak power rating to handle short duration high current pulses.
Power rating is a determining factor in selecting the appropriate sensing resistor technology. Although the device may be designed to detect DC current, it can often experience transients.
The power derating curve provides allowable power at different temperatures. But maximum power capacity is a function of energy; therefore, the energy rating curve must be taken into consideration.
· High temperature rating for reliability
Pros and cons of current sensing resistors include:
Pros:
- Low cost
– High measurement accuracy
– Measurable current range from very low to medium
– Ability to measure DC or AC current
Cons:
– Introduces additional resistance into the measured circuit path, which can increase the source output resistance and result in undesirable loading effects.
– Energy loss due to energy dissipation. Therefore, current sensing resistors are rarely used beyond low and medium current sensing applications.
Current detection techniques
two. Current detection with copper resistor
Rather than using a separate discrete resistor for current sensing, it is often useful to use copper traces on a printed circuit board as a low-value resistor for current sensing purposes. This method will have lower power loss and will also save efforts to purchase and install a discrete resistor. But because the resistance of copper is very low, the detected voltage will also require significant amplification or an increase in resistor length at the expense of PCB area. Another significant factor is the TCR of copper (0.39% / °C), which equates to approx. 20% change for 50% temperature increase.
3. MOSFET-R DS
MOSFETs act as resistors when they are “on” and are biased in the ohmic (non-saturated) region. The current is determined by sensing the voltage at the source-drain of the MOSFET, if R DS of the MOSFET is known. The main disadvantages of this technique are the low accuracy and switching noise of non-zero gate currents during transients, the nonlinearity of R D S of the MOSFET, dependence of R D S on Cox, VT and temperature.
4. Sense-FET Technique
This method is a practical technique used for current sensing in many new power MOSFET applications. A current sensing FET is used in parallel with the power MOSFET. The effective width of the sensing MOSFET (sense FET) is significantly smaller (~10,000 times) than the power FET. The accuracy of the sense-FET technique is about ±20%. · Sensorless (observer) approach
This method uses the inductor voltage to measure the inductor current. Since the inductor voltage-current relationship is v=L*di/dt, the inductor current can be estimated by integrating the voltage over time. To avoid saturation in the integrator, it is reset periodically and therefore only the AC ripple current is estimated. The value of L must also be known for this technique.
Figure 5: A diagram explaining the Sense-FET technique
5. Average current
This current sensing technique uses an RC low pass filter at the junction of the converter switches. Therefore, the voltage at the filter output capacitor is the average voltage of the phase node. Consequently, the differential voltage at the input of the amplifier is the DC voltage across the inductor. V I-Average is a function of R VHS (Inductor Resistance) and I L_DC (Inductor DC Current).
Figure 6: A diagram explaining average current flow
6. Filter-Sense the inductor
This current sensing technique uses a simple low-pass RC network to filter voltage across the inductor and sense current through the equivalent series resistance (ESR) of the inductor.
Figure 7: A diagram explaining the direction of the inductor filter
Magn. Field-based detection technology.
CURRENT DETECTION TECHNIQUES BASED ON MAGNETIC FIELD
Although resistive current sensing techniques are useful in many applications, they have three inherent disadvantages:
• Voltage drop in the power supply line
• Insertion power loss
• Common mode errors
Most of these problems can be resolved by detecting low to moderate amounts of current on low-voltage power lines, but they can become significant as currents or voltages increase. When trying to measure currents at higher levels (>10A) or where the supply line is at a high voltage (e.g. 48V), the preferred solution is to use magnetic current sensors. One of the significant and obvious benefits of using magnetic coupling to detect current is electrical isolation.
In these sensors, a magnetically permeable core is used that concentrates the magnetic field of the conductor generated due to the current flow in the conductor. The magnetic field is detected using different methods:
· Hall effect sensors
The principle of the Hall Effect states that when a current-carrying conductor is placed in a magnetic field, a voltage will be generated perpendicular to the direction of the field and the flow of current.
When a constant current passes through a thin sheet of semiconductor material, there is no potential difference across the output contacts if the magnetic field is zero. However, when a perpendicular magnetic field is present, the current flow is distorted. The uneven distribution of electron density creates a potential difference between the output terminals. This voltage is called Hall voltage. If the input current is kept constant, the Hall voltage will be directly proportional to the magnetic field strength.
Figure 8: A diagram explaining the half-effect principle
Hall voltage is a low-level signal on the order of 20 to 30 microvolts in a one-gauss magnetic field. A signal of this magnitude requires a low-noise, high-impedance, moderate-gain amplifier.
Figure 9: A diagram explaining half voltage as a low-level signal in a magnetic field
Hall sensors are based on the following technologies. Can be used to measure DC, AC and impulse currents, with galvanic isolation between primary and secondary circuits
¨ Open Loop Hall effect technology
Current sensors based on this technology are electronic transformers. Primary current I p creates magnetic flux and the corridor
Fig. 10: A figure explaining current sensors based on open-loop Hall effect technology
probe placed in the air gap of the magnetic circuit provides a voltage proportional to the magnetic flux. This voltage itself is proportional to I p is amplified and used for further processing.
The linearity of the open-loop sensor is determined by the characteristics of the magnetic core and the Hall generator. The compensation deviation with respect to temperature is mainly determined by the temperature sensitivity of the Hall generator.
¨ Closed-loop Hall effect technology
Current sensors based on this technology are also electronic transformers. Primary current I p creates magnetic flux and the
Figure 11: A figure explaining current sensors based on closed-loop Hall effect technology
The Hall probe placed in the air gap of the magnetic circuit provides a voltage proportional to the magnetic flux. This voltage is fed into a push-pull drive stage that drives the oppositely wound coil in series on the magnetic core. Thus, it creates a magnetic field equal and opposite to the detected current field: keeping the central flux level close to zero. The secondary current cancels the primary magnetic flux that created it (counterreaction). The output of the closed loop sensor is proportional to the opening current and the number of turns of the coil.
The closed-loop approach enables significant improvements in sensor performance by eliminating the influence of nonlinearities in the magnetic core and reducing the effects of temperature sensitivity in the Hall element
¨ Electronic Technology
Figure 11: A figure explaining current sensors based on electronic technology
In contrast to open circuit and closed circuit technology, they do not use magnetic circuitry. Primary current I p creates magnetic flux and different Hall probes included in the sensor provide a voltage proportional to the magnetic flux.
Hall effect-based sensors do not suffer from insertion loss (and related heating, etc.). However, frequency range, cost, DC offset, and external power represent the potential disadvantages of Hall effect IC technology when compared to resistive sensing methods.
· Rogowski coil
This device consists of a single-layer coil, uniformly wound on a non-magnetic core that is flexible or formed into a circle that surrounds the conductor of the current to be measured. An AC current through the wire changes polarity. The change in polarity causes expansion and collapse of the magnetic field which in turn induces current in the windings. The current is then processed to make it suitable for measurement or control system.
Figure 12: A diagram demonstrating the Rogowski coil
Practical implementations of this technique typically also incorporate a low-frequency roll-off to eliminate thermal noise and drift. The main benefit of a Rogowski coil is that because the core is effectively air, there is no magnetic material to saturate and the coil's output remains linear for extremely high currents. This device is used to measure high-energy current pulses or transients with high-frequency harmonic content, since the upper bandwidth can extend into the megahertz range.
· Transformer Technique
The Transformer Technique is an extension of the Rogowski Coil technology in which the air core is replaced with a material that concentrates the magnetic flux within the coil. With the flux contained in the coil rather than passing through it, a direct relationship is obtained between the current in the coil and the current in the conductor that generates the field.
Current sensing transformers offer important benefits over simple resistive sensing. They offer electrical isolation, avoid insertion losses and do not require external power. The lower power dissipation of a current sensing transformer allows for a much higher signal level, significantly improving the signal-to-noise environment of the control system.
Current transformers (CT) are commonly used in high power systems to measure current. The main disadvantages are the large size and cost and also the inability to detect DC current.
Figure 13: A diagram explaining the use of current transformers in high power systems
· Fiber Optic Current Sensors
The development of magneto-optical sensors has demonstrated their use in current and magnetic field measurement applications. The principle of magneto-optical effects is based on the interaction between the magnetic field and the phenomenon of refraction and reflection of light in a transparent medium and on its surface. They offer inherent immunity against EMI and good isolation against high voltages
Current sensors employ the magneto-optical Faraday effect. The Faraday effect causes rotation of the polarization of the electromagnetic wave due to the intensity of the magnetic field in the transparent material. The magnetic field induced by the current leads to an angular rotation in the plane of polarization of the linearly polarized light propagating through the ferromagnetic material. Rotation is detected by polarizers and analyzers at the input and output. By monitoring the rotation of the incident polarization, the magnetic field and therefore the current can be estimated.
Figure 14: A diagram demonstrating fiber optic current sensors
The magnitude of the effect also depends on the constant of the magneto-optical material (Verdet constant) and the interaction length through which the wave travels in the magnetized material.
High side versus low side detection
DETECTION TECHNIQUES: HIGH SIDE vs LOW SIDE SENSING
There are two basic techniques for current sensing applications, namely low side current sensing and high side current sensing, based on the placement of the sensing resistor (either between the supply and the load or between the load and ground).
· Low side current detection
The low-side current sensing connects the sensing resistor between the load and ground. Typically, the detected voltage signal (V SEN = I SEN ×R SEN ) is so small that it needs to be amplified by subsequent op-amp circuits to obtain the measurable output voltage (V OUT ).
Fig. 15: A figure illustrating low-side current detection
a) Advantages:
¨ Low input common mode voltage
¨ Ground-referenced input and output
¨ Simplicity and low cost
b) Disadvantages:
¨ Disturbance in the soil path
¨ The load is lifted from the system ground since R SEN adds undesirable resistance to the ground path.
¨ High load current caused by accidental short is not detected
Low-side current detection should be chosen when short-circuit detection is not required and ground disturbances can be tolerated.
· High Side Current Detection
High-side current sensing connects the sensing resistor between the power supply and the load. The detected voltage signal is amplified by subsequent operational amplifier circuits to obtain the measurable V OFF .
Figure 16: A figure illustrating high-side current detection
a) Advantages:
¨ Eliminates soil disturbances
¨ The load connects the system ground directly
¨ Detects high load current caused by accidental shorts
b) Disadvantages:
¨ Must be able to handle very high and dynamic common mode input voltages
¨ Complexity and higher costs
High-side current sensing should be chosen when V CM the range of the differential amplifier is wide enough to support high common-mode input voltages
