confident
Richard Dickinson and Shaun Milano 7/11/2002 Isolated Open Loop Current Sensing Using Hall Effect Technology in an Optimized Magnetic Circuit
Richard Dickinson & Shaun Milano
爬山虎翻译Allegro MicroSystems, Inc.
162 Pembroke Rd.
Concord NH, 03301 USAanhua
Tel: 001 (603) 228-5533
Fax: 001 (603) 224-2466
Abstract:
With the expected arrival of a 42V parallel bus power supply aboard automobiles and new energy efficiency standards being impod on most houhold appliances, there is a growing need for current nsing as a means of monitoring and controlling power consumption.
蒙文翻译公司There are three rival technologies that are typically ud for measuring current: n resistors, Hall effect nsors and current transformers. Each have attributes that differentiate them on a cost versus performance scale. Galvanic isolation, ea of implementation, robustness and low cost are a few attributes that urs demand when choosing a current nsor.
上海高级口译Galvanic isolation is needed to protect the nsing device from potentially damaging high power signals (over-current spikes) and to minimize the power dissipated (and heat generated) by n resistors. Becau Hall effect nsors measure the magnetic field strength in clo proximity to the current conductor, they can be parated by a few millimeters from the current signal, providing veral kilovolts of isolation. Other open loop Hall effect current nsor designs internalize the current carrying conductor, allowing the ur to optimize the current nsor package’s size and thermal characteristics.
While clod loop nsing can provide excellent accuracy, size and cost prohibit the u of the types of transducers in many applications. Open loop nsors usually have a more limited range of linearity and cannot compensate for offt and residual field errors. However, magnetic circuits and the linear Hall effect IC can be designed to reduce the types of errors without the external control circuitry that is typically required by clod loop systems.
This paper will explore magnetic circuit design, IC circuit designs, and integrated packaging techniques that attempt to minimize the errors associated with open circuit designs in order to achieve a nsor with accuracy equivalent to that of n resistors, but without the isolation tradeoff.
Introduction
Applications requiring current nsing are becoming ubiquitous, from battery management systems aboard automobiles to dc motor controllers within houhold appliances. While n resistors may have previously reprented the state of the art in current nsing, the need for safe, isolated detection of electrical current has spurred the development of non-intrusive current nsing methods and devices. Among a handful of alternative current-nsing methods, Hall-Effect nsors may be implemented in high volume with relatively low cost for many of the applications.石家庄少儿英语
Current Sensing Technology Overview
There are three technologies that are typically ud for measuring current: n resistors, current transformers and Hall effect nsors.
初始吸血鬼Sen resistors are simply a resistor placed in ries with the load. By ohms law, the voltage drop across the device is proportional to the current. For low currents, the provide very accurate measurement given the resistance value has a tight tolerance. Although n resistors with high performance thermal packages have been developed for larger currents, they still result in inrtion loss. In addition, they do not provide a measurement isolated from transient voltage potentials on the load. Sen resistors also require other circuitry such as instrumentation amplifiers to generate a distinguishable signal.
Current transformers are relatively simple to implement and are passive devices that do not require driving circuitry to operate. The primary current (AC) will generate a magnetic field that is coupled into a condary coil by Faraday’s Law. The magnitude of the condary current is proportional to the number of turns in the coil, which is typically as high as >1000. The condary current is then nd through a n resistor to convert the output into a voltage.
There are two techniques for nsing current using Hall effect devices. According to the Hall effect, a magnetic field passing through a miconductor resistor will generate a differential voltage proportional to the field (figure 1).
Figure 1: Reprentation of the Hall effect and its electrical parameters
Concentric magnetic field lines are generated around a current carrying conductor. Approximating the primary current conductor as infinitely long, the magnetic field
strength may be defined B = µo I/2pr, where µo is the permeability of free space, I is the current and r is the distance from the center of the current conductor. In order to induce a larger signal out of the Hall element; the current conductor may be wrapped around a
slotted ferrous toroid N number of times, such that B = µo NI/2pr. In an open loop topology, the Hall element output is simply amplified and the output is read as a voltage
that reprents the measured current through a scaling factor as depicted in figure 2.
Figure 2: Basic Topology of Open Loop Hall Effect Current Sensor
In a clod loop topology, the output of the Hall element drives a condary coil that will generate a magnetic field to cancel the primary current field. The condary current, scaled proportional to the primary current by the condary coil ratio, can then be measured as voltage across a n resistor.
Figure 3: Basic Topology of Clod Loop Hall Effect Current Sensor
By keeping the resultant field at zero, the errors associated with offt drift, nsitivity drift and saturation of the magnetic core will also be effectively canceled. Clod-loop Hall effect current nsors also provide the fastest respon times. However, with a condary coil that may be needed to drive up to veral milli-amps of current, power consumption is much higher in clod loop Hall effect devices than open loop topologies. The clod loop configuration also limits the magnitude of the current that
can be nd since the device may only drive a finite amount of compensation current. Figure 4 provides a simple comparison of different current nsing techniques.
Figure 4: Summary of current nsing techniques and attribute comparison i
Current-nsing Technique Accuracy Galvanic
Isolation
Power
dissipation1
Relative
Cost2
Typical current
ranges
Sen resistor >95% None High Low <20A, DC-100kHz Transformer ~95 Yes Moderate Med. Up to 1000A, AC
Open loop Hall effect nsor 90-95% Yes Low Med. Up to 1000A, DC-
20kHz
Clod loop Hall effect nsor >95% Yes Moderate-
High
High <500A, DC-150kHz
Design Considerations for Open Loop Hall Effect Current Sensors
jabber
The measurable current range, the output linearity and measurement accuracy are attributes that influence the design of a current nsor package. The linearity and accuracy are determined through the magnetic circuit that couples the primary current to the magnetic field nd by the Hall element. Offts and inaccuracy introduced in the magnetic circuit can often be minimized in the linear Hall effect IC design.
The measurable current range often dictates the size of an open loop current nsor. For current levels less than100A, a reduced package size can be achieved by internalizing the primary conductor. Combining the Hall effect nsor, flux concentrator and primary conductor into a single asmbly for PCB installation opens up applications that previously relied on n resistors. For current levels of100A and higher, any discontinuity in the bus-bar could prent reliability risks. Therefore, rather than soldering or welding on a current nsor, end urs desire a package that can be clamped/installed around the conductor. If the primary conductor is internalized into the package the design objective is to minimize the inrtion loss. Any power dissipated through the primary conductor will generate heat in clo proximity to the Hall effect IC and will be en as a temperature coefficient. Figure 5 provides an example of a propod 100A full-scale current nsor package integrating the primary conductor.
1 Including power loss through primary current carrying conductor & power consumed by device
2 Estimated, including cost of auxillary circuitry needed to implement device in operating environment
Figure 5: Open loop Hall effect current nsor integrating primary conductor Open loop Hall effect current nsor package under development at Allegro Microsystems, Inc.
In addition to the tradeoff between inrtion loss and size for a given current range, there is also a relationship between the flux concentrator and the measurable current range. The magnetic field nd by the Hall effect transducer is proportional to the product of the primary current and a gain factor related to a flux concentrator. In order to keep a magnetic circuit in its linear region for a given current, the dimensions of the flux concentrator and its effective magnetic path length, must be appropriately scaled.
A typical design rule is to u a toroid with a cross ctional area that is twice as large as the area en in the gap where the nsing element sits. Choosing a magnetic flux concentrator material with the appropriate effective permeability can be determined as a function of the primary current range,
µe = Bl e/0.4pNI
where N is the number of turns (typically 1), l e is the effective path length and I is the full scale current. The effective path length is
l g = l e(1/µe - 1/µI)(0.397)
jingba>what have i done
As the primary current range increas, it is often necessary to increa the air gap size to ensure a linear output. A material commonly ud as a flux concentrator is powdered iron, having magnetic permeability 2000 to 5000 times greater than air. Figure 6 provides a diagram of an open loop Hall effect nsor package, with a ferrous core concentrating flux onto a Hall effect IC.
