Current Sensing Circuit Concepts and Fundamentals

Abstract

There is a need to interface with the MPPT and sense current.

Types of Current Sensing

Indirect current sensing is based on Ampere’s and Faraday’s laws. By placing a coil around a current-carrying conductor, a voltage is induced across the coil that is proportional to the current. This allows for a non-invasive measurement where the sensing circuitry is not electrically connected to the monitored system. Since there is no direct connection between the sensing circuitry and the system, the system is inherently isolated. Indirect current sensing typically is used for load currents in the 100A-1000A range. This type of sensing, however, requires relatively expensive sensors and is not conducive to sensing currents on a PCB.
Direct current sensing is based on Ohm’s law. By placing a shunt resistor in series with the system load, a voltage is generated across the shunt resistor that is proportional to the system load current. The voltage across the shunt can be measured by differential amplifiers such as current shunt monitors (CSMs), operational amplifiers (op amps), difference amplifiers (DAs), or instrumentation amplifiers (IAs). This method is an invasive measurement of the current since the shunt resistor and sensing circuitry are electrically connected to the monitored system. Therefore, direct sensing typically is used when galvanic isolation is not required. The shunt resistor also dissipates power, which may not be desirable. Direct current sensing typically is implemented for load currents <100A.

Direct Current Sensing

High-side versus low-side current sensing

Low-side sensing is desirable because the common-mode voltage is near ground, which allows for the use of single-supply, rail-to-rail input/output op amps. The drawbacks to low-side sensing are disturbances to the system load’s ground potential and the inability to detect load shorts.

High-side sensing is desirable in that it directly monitors the current delivered by the supply, which allows for the detection of load shorts. The challenge is that the amplifier’s input common-mode voltage range must include the load’s supply voltage, or Vbus. This requirement frequently necessitates the use of DAs or dedicated CSMs, which allow for common-mode voltages outside their voltage supply range.

Operational amplifiers

The use of an op amp for current sensing is limited by input common-mode voltage. Due to the design of the input stage, input common-mode voltage of such a device is limited by the supply voltage (Voa). Additionally, the large open-loop gain of a traditional op amp requires the device to have feedback, which limits its use to single-ended input signals. This thus limits it to only low-side current sensing.

Difference amplifiers

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More Detailed Equations and schematics, http://ww1.microchip.com/downloads/en/AppNotes/01332B.pdf

Source: http://www.eetimes.com/document.asp?doc_id=1279404, http://www.eetimes.com/document.asp?doc_id=1279415