Measuring a voltage in any system is a “passive” activity as it can be done easily at any point in the system without affecting the system performance. However, current measurement is “intrusive” as it demands insertion of some type of sensor which introduces a risk of affecting system performance.
Current measurement is of vital importance in many power and instrumentation systems. Traditionally, current sensing was primarily for circuit protection and control. However, with the advancement in technology, current sensing has emerged as a method to monitor and enhance performance.
Knowing the amount of current being delivered to the load can be useful for wide variety of applications. Current sensing is used in wide range of electronic systems, viz., Battery life indicators and chargers, 4-20 mA systems, over-current protection and supervising circuits, current and voltage regulators, DC/DC converters, ground fault detectors, programmable current sources, linear and switch-mode power supplies, communications devices , automotive power electronics, motor speed controls and overload protection, etc.
CURRENT SENSING PRINCIPLES
A current sensor is a device that detects and converts current to an easily measured output voltage, which is proportional to the current through the measured path.
When a current flows through a wire or in a circuit, voltage drop occurs. Also, a magnetic field is generated surrounding the current carrying conductor. Both of these phenomena are made use of in the design of current sensors. Thus, there are two types of current sensing: direct and indirect. Direct sensing is based on Ohm’s law, while indirect sensing is based on Faraday’s and Ampere’s law.
Direct Sensing involves measuring the voltage drop associated with the current passing through passive electrical components.
Indirect Sensing involves measurement of the magnetic field surrounding a conductor through which current passes.
Generated magnetic field is then used to induce proportional voltage or current which is then transformed to a form suitable for measurement and/or control system.
PASSIVE ELEMENT BASED CURRENT SENSING TECHNIQUES
1. Sense Resistors
Current sensing means developing a voltage signal which is representative of the current flowing at the particular place of interest inthe circuit. The traditional way of current sensing introduces a resistor in the path of the current to be sensed. The sense resistor can be placed in series with the inductor, switches, and the load.Thus, a current sensing resistor should be considered as a current-to-voltage converter.
The current sensing resistor should have following attributes
· Low value in order to minimize power losses
Value of the current sense resistors primarily depend upon the voltage threshold of the following circuitry which is going to operate based upon the sensed current information. In circuits where amplification is available, emphasis is to minimize the voltage drop across the resistor.
Typical resistance values utilized in various control ICs are 20m? to 25m? .
· Low inductance because of high di/dt.
Any inductance in the resistor, when exposed to high slew rate (di/dt), an inductive step voltage is superimposed upon the sense voltage and may be a cause of concern in many circuits. Hence sense resistors should have very low inductance.
· Tight tolerance
For maximizing the current supply within the limit of acceptable current, the tolerance of the sense resistor must be ±1% or tighter.
· Low temperature coefficient for accuracy
Normally specified in units of parts per million per degree centigrade (ppm/°C), temperature coefficient of resistance (TCR) is an important parameter for accuracy. Resistors with TCRs closer to zero, in the entire operating range should be used.
· High peak power rating to handle short duration high current pulses.
Power rating is a driving factor for the selection of appropriate technology for sense resistors. Though the device may be intended to sense DC current, it may often experience transients.
Power derating curve provides allowable power at different temperatures. But peak power capability is a function of energy; hence energy rating curve should be taken into account.
· High temperature rating for reliability
Pros and Cons of current sensing resistors include:
- Low cost
- High measurement accuracy
- Measurable current range from very low to medium
- Capability to measure DC or AC current
- Introduces additional resistance into the measured circuit path, which may increase source output resistance and result in undesirable loading effect.
- Power loss due to power dissipation. Therefore, current sensing resistors are rarely used beyond the low and medium current sensing applications.