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Technical Paper

STP 98-1*

‘NON-INTRUSIVE’ HALL-EFFECT CURRENT-SENSING

TECHNIQUES PROVIDE SAFE, RELIABLE DETECTION

and PROTECTION for POWER ELECTRONICS

by Paul Emerald

Abstract

As systems extend and expand the exploitation of the latest

power semiconductors (IGBTs, MCTs, etc.) that manifest the

very relentless advance in power output limits, a prerequisite

(and parallel) demand for sensing these escalating current levels

is (increasingly) very apparent. Hall-effect ICs provide ‘non-

intrusive’ current sensing techniques and safe, isolated detection

of high current levels without dissipating the sizable amounts of

wasted power (and the resultant heating) associated with

resistive current-sensing methods. Further, Hall-effect current

sensing provides electrical isolation between the current-

carrying conductor; hence, a safe environment for circuitry,

operators, etc.

The proliferating current-sensing applications for Hall-effect

sensors continue; become even more diverse; plus expand and

grow as other designers endeavor to protect systems, create

more reliable ‘bulletproof’ equipment, and reconcile any safety

issues. The prime applications for cost-effective Hall-effect

sensors for current sensing include:

Current Imbalance

Current Monitoring

Operator/User Safety and Security

Overcurrent Detection/System Protection

System Diagnosis and Fault Detection

Test and Measurement

Background and Introduction

The discovery of the Hall-effect originated back in 1879;

however, any meaningful application of this Edwin H. Hall

finding awaited semiconductor integration that first occurred in

the late 1960s. Subsequently, further advances (particularly

those of the 1990s) have evolved further, more fully functional

integration plus an expanding series of application-specific Hall

sensor types. Yet the relentless progress of magnetic sensor

electronics continues to proliferate an increasing demand for

low-cost, reliable, and ‘non-contact’ Hall-effect circuitry for

sensing/detecting motion, direction, position, and measuring/

monitoring current.

Hall-effect sensor ICs (especially the ratiometric linear types)

are superb devices for ‘open-loop’ current-sensing designs.

However, there are limits to the operational range, accuracy and

precision, frequency response, etc. that may be realized.

Because many prospective users are ignorant of and/or oblivi-

ous to either the benefits or shortcomings of current-sensing

techniques using Hall-effect ICs, this paper endeavors to

provide a comprehensive discussion of the essential, basic

techniques of ‘non-intrusive’ current sensing with silicon Hall-

effect devices (HEDs) now available.

Most Hall-effect current-sensing requirements do not develop

adequate magnetic fields without the use of a slotted toroid to

concentrate (and focus) the induced flux field. Low-to-modest

currents (<≈15 amperes) require winding sufficient turns on the

slotted toroid (core) to induce usable flux strength and develop

a suitable signal voltage. A higher current level (>15 to 20

amperes) induces field intensities that allow passing the current-

carrying conductor straight through the center of the toroid (no

turns necessary at these higher currents).

Designs requiring a broad (or continuous) current range

mandate utilizing linear Hall-effect sensor ICs. However,

overcurrent protection and/or fault detection designs can be

accommodated by digital HEDs. Examples and particulars of

the essentials of current-sensing techniques, device parameters,

temperature stability, and other relevant concerns of Hall-effect

current sensing are covered in this treatise on HEDs for sensing

ac and dc currents.

Rival, Competing Technologies

Although there are many current-sensing methods, only three

are commonplace in low-cost, volume applications. The others

are expensive laboratory systems, emerging technologies

(magnetoresistive is an example), or seldom used. The com-

monly used techniques include: (1) resistive, (2) Hall-effect,

and (3) current transformers.

Resistive sensing is very widely used, low-cost, and easily

understood. However, the shortcomings are its insertion loss

(heating and wasted power) and lack of isolation. Also, the

series inductance of many power resistors constrains the

frequency range with low-cost components; hence, resistive

sensing is classed as either a dc or ac application per the

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Figure 1: Linear Hall Sensor Transfer Curve

Table 1: Commonplace, Inexpensive Current-Sensing Techniques

Widely Used Insertion Circuit External Frequency Offset Accuracy Rel.

Sensors Loss Isolation Power Range (Zero I) (Est.*) Cost

Resistive

dc Yes None None <100 kHz None >99% Lowest

ac Yes None None >500 kHz None >99% Low

Hall-Effect

Open-Loop None Yes Yes ≥20 kHz† Yes 90-95% Med

Closed-Loop None Yes Yes ≥150 kHz None >95% High

Current

Transformers Yes (ac) Yes None Constant‡ None >95%§ Highest

* (Estimated): accuracy and precision very dependent upon design implementation.

† 20 kHz to 25 kHz represents (typical) usable frequency limit.

‡ Current transformers (usually) designed for limited frequency range.

§ Accuracy very contingent upon component and cost factors.

0+B

OUTPUT VOLTAGE (VOLTS)

MAGNETIC FIELD (GAUSS)

Dwg. PRE-507

-B

QUIESCENT

OUTPUT

VOLTAGE

SATURATION

categories in Table 1. Low inductance, high-power resistors for

high frequency are more expensive, but allow operation beyond

500 kHz. Further, signal amplification is (usually) required

with resistive current-sensing techniques (either a comparator or

operational amplifier is needed).

Hall-effect sensor ICs (open- and closed-loop) represent the

next tier of commonplace solutions. Insertion loss (and related

heating, etc.) are not an obstacle. However, frequency range,

cost, dc offset, and external power represent the potential

disadvantages of Hall-effect IC technology when compared to

the resistive-sensing methods.

Current transformers close out the last low-cost technology, and

(as the term transformer should imply) are only useful with

alternating currents. Most low-cost current transformers are

designed for narrow frequency ranges, are more expensive than

resistive or Hall-effect, and cannot be used for dc currents.

However, current transformers avoid insertion loss, offer

electrical isolation, do not require external power, and exhibit

no offset voltage at the zero (null) current level.

Because this treatise focuses upon Hall-effect ICs, understand-

ing the elements of linear, ratiometric HEDs is imperative to

open-loop current sensing.

Linear Hall-Effect Sensor ICs

As the term implies, linear Hall sensors develop an output

signal that is proportional to the applied magnetic field. Nor-

mally, in any current-sensing application, this flux field is

focused by a ‘slotted’ toriod to develop an adequate field

intensity, and this magnetic field is induced by current flowing

in a conductor. A ‘classical’ transfer curve for a ratiometric

linear is illustrated in Figure 1. Note that, at each extreme of its

range, the output saturates.

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Inducing a Magnetic Field

As mentioned, Hall-effect current sensing usually necessitates

the use of a slotted toroid (made of ferrous materials). The

toroid both concentrates and focuses an induced magnetic field

toward the location of the Hall-effect element within the IC

package. Figure 2 typifies a classic example of ‘non-intrusive’

current sensing exploiting a slotted toroid. The conductor

current flows through the turns wound upon the toriod, and the

induced flux field is concentrated on the sensor in the gap (or

slot) in the toriod. Usually, this gap is made to closely match

the Hall IC package thickness ( approx. 0.060" or 1.52 mm),

and this provides optimal magnetic coupling. The current flow

(with this ‘tight’ magnetic coupling) induces a flux intensity per

the formulaB (gauss) ≈ N (turns) x 6.9 gauss/ampere

Note — 6.9 gauss/ampere is updated from the earlier 6 G/A.

Widening the (gap) slot reduces the flux coupling and can

increase the upper current limit, which is predicated upon the

Hall sensor sensitivity (more to follow). However, decoupling

the induced field to extend the maximum current limit may

affect linearity, usable range, etc. This ‘loose’ coupling is

under evaluation, but not yet complete; hence, no new formulas

for magnetic flux and conductor current (and larger gaps) have

been documented.

‘Calibrated’ Ratiometric Linear HEDs

The two newest linear Hall sensors, with dynamic dc offset

cancellation, provide a cornerstone for a discourse on linear

ratiometric HEDs and current sensing. The A3515 plot (Figure

3) and related data (Table 2) record the vital characteristics of

the most sensitive linear HED; its counterpart, the A3516,

properties are in Figure 4 and Table 3.

Presently, though seldom sold, ‘calibrated’ linear Hall-effect

ICs are superb circuits for setting up and measuring system

magnetic parameters, and represent an excellent entry to the

performance, characteristics, and limitations of ratiometric ICs.

Most recent linear Hall ICs provide a ratiometric output voltage.

The quiescent (i.e., null) voltage is (nominally) 50% of the

applied, stable supply. This quiescent output voltage signal

equates to no applied magnetic field and, for current sensing, is

equivalent to zero current flow. A south polarity field induces a

positive voltage transition (toward VCC), and a north polarity

results in a transition toward ground (0 V). Output saturation

voltages are (typically) 0.3 V (high/sourcing) and 0.2 V (low/

sinking) and are measured at ±1 mA.

Each linear Hall-effect IC integrates a sensitive Hall element

(also called a ‘plate’), a low-noise (bipolar) amplifier, and sink/

source output stage. Any systems problems associated with

low-level signals and noise are minimized by the monolithic

integration of magnetic sensor, amplifier, output, and allied

signal processing circuitry.

Existing very stable, linear HEDs exploit dynamic quadrature

offset cancellation circuitry and utilize electronic switching to

change the current path in the sensor element. Switching the

current paths, from 0° to 90°, at a high repetition rate offers a

new answer to the (intrinsic) dc offset that has long plagued

linear sensor operation and stability.

Figure 2: Current Sensing with Gapped Toroid

Sample-and-hold circuitry and a low-pass filter are exploited to

properly ‘recondition’ the internal dynamic signals of these

innovative linear HEDs.

Linear Hall-effect ICs can detect small changes in flux inten-

sity, and are (generally) more useful than digital Hall ICs for

current sensing. Linear HEDs are often capacitively coupled to

op amps, or dc connected to comparators, to attain system

design objectives. Also, microcontrollers (µCs) and micropro-

cessors (µPs) are being exploited to detect small signal changes

from linear Hall ICs, and are very suitable (with proper soft-

ware) of sensing/measuring either ac or dc currents.

≈

N x 6.9 G/A

(POSITIVE FIELD)

Dwg. AH-005-1A

BRANDED

SURFACE

CURRENT

FLOW

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Sensor Sensitivity

The elemental distinction between the A3515 and A3516 is

magnetic sensitivity. The nominal data for the two specific

sensors depicted in Figures 3 and 4 is listed in Tables 2 and 3.

Sensitivity is specified in millivolts per gauss (mV/G). Three

voltages are listed; however, most designs utilize fixed, low-

cost 5 V regulator ICs for stability. The nominal sensitivity

(and usable range) of the two linear HEDs is as follows

(VCC = 5 V):

A3515: Sensitivity: 5.0 mV/G

Range: ≥±400 G (≥±2.0 V)

A3516: Sensitivity: 2.5 mV/G

Range: ≥±800 G (≥±2.0 V)

Linearity and Symmetry

From these plots (Figures 3 and 4) it is apparent that neither

linearity nor symmetry (the deviation in the slope from the

quiescent (or null) voltage) is a vital design consequence as

neither surpasses 0.3 % for the A3515. The plots record

±400 G for the A3515, and ±800 G for the A3516, and output

voltage swings of ≥±2.0 V for both types.

-500 -300 -200-400 -100 0 +100 +200 +300 +400 +500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

MAGNETIC FLUX (GAUSS)

OUTPUT VOLTAGE (VOLTS)

Dwg. GH-071-3

Figure 3: Linear, Ratiometric Hall-Effect Sensor Characteristics (A3515 Output)

Table 2: Linear, Ratiometric Hall-Effect Sensor Characteristics Measurement Data (A3515)

Measured Over ±250 Gauss

VCC (Volts) VOQ (Volts) Sensitivity (mV/G) Non-Linearity (%) Symmetry (%)

❍4.500 2.217 4.350 ≤ 0.1 99.9

❏5.000 2.463 5.014 ≤ 0.2 99.9

∆

5.500 2.710 5.704 ≤ 0.1 99.7

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

-1000 -600 -400-800 -200 0 +200 +400 +600 +800 +1000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

MAGNETIC FLUX (GAUSS)

OUTPUT VOLTAGE (VOLTS)

Dwg. GH-071-4

Figure 4: Linear, Ratiometric Hall-Effect Sensor Characteristics (A3516 Output)

Table 3: Linear, Ratiometric Hall-Effect Sensor Measurement Data (A3516)

Measured Over ±500 Gauss

VCC (Volts) VOQ (Volts) Sensitivity (mV/G) Non-Linearity (%) Symmetry (%)

❍4.500 2.232 2.149 ≤ 0.1 99.9

❏5.000 2.475 2.481 ≤ 0.1 99.6

∆

5.500 2.723 2.820 ≤ 0.1 99.9

avoided; a sense resistor of 500 µΩ) and 200 A produces

20 watts. Obviously, this is a situation that a designer would

prefer to avoid. However, low-cost options are scarce (or non-

existent).

Linear, Ratiometric Hall-Effect ICs

The latest linear HEDs incorporating the dynamic quadrature dc

offset cancellation are illustrated in Figure 5. The Hall element

is a ‘single-plate’, and designated by its symbol (

). Sensor

current is switched from a 0° orientation (downward) to a 90°

path (across the Hall plate) at ≈170 kHz. This precludes most

of the earlier offset related factors (dc imbalances due to

resistive gradients, geometrical dissimilarities, piezoresistive

effects, etc.). A low-pass filter and a sample-and-hold circuit

are employed to recondition the signal fed to the linear,

ratiometric Hall sensor output.

Linear Current Range(s)

The practical current limit (maximum with ‘tight’ coupling) is

derived using the range and flux per turn in the prior formula

per the approximation:

A3515: ≥±400 G ÷ 6.9 G/A ≈ ±58 A

A3516: ≥±800 G ÷ 6.9 G/A ≈ ±116 A

Per a prior mention, current values beyond ≈115 amperes

mandate reducing the magnetic coupling, shunting higher

current levels (i.e., pass a portion of the total through the

toroid), or other methods that effectively ‘desensitize’ the

circuitry. There are many, growing and expanding applications

for ‘non-intrusive’ current sensing, especially at high currents

(>100 A). An ultra-low value resistor (<1 milliohm) dissipates

considerable power and heating at these currents, and the ‘non-

inductive’ resistors needed raise costs. I2R losses cannot be

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Maximum Output Voltage

Also itemized in Table 4; however, it should be noted that the

output must not be connected to a voltage either beyond the

supply or below the IC ground. Either might compromise the

Hall sensor reliability and/or affect system dependability.

Maximum Output Current

The newest linear HEDs specify a higher current than prior

devices. However, typical applications rarely involve more

than a trivial percentage of the 10 mA maximum listed in Table

4. The high-impedance inputs of today’s analog or conversion

circuitry (usually) necessitates microamperes not milliamperes

of Hall sensor output current.

Maximum Flux Density

Magnetic fields that exceed the linear range of these Hall-effect

ICs neither damage nor destroy the device. However, magnetic

fields beyond the usable range force the output into saturation

(and non-linear operation) without harm to the HED.

Package Power Dissipation

The maximum package power dissipation limit is based upon

operating with safe, reliable junction temperatures. The two

package types in use are specified below for their thermal

resistance (and maximum power with TA = +25°C).

‘U’ Package: RθJA = 183°C/W (PD = 683 mW)

‘UA’ Package: RθJA = 206°C/W (PD = 606 mW)

The maximum recommended junction temperature is +150°C,

and the dissipation should equal zero at this temperature.

However, the newest linears permit infrequent (i.e., transient)

excursions up to +200°C (ambient temperature, TA ≤ +170°C).

The internal power (PD) consists of two factors: (a) the HED

supply power (ICC x VCC) and (b) the IC output power (IOUT x

VOUT(SAT)). Normally, supply power (a) smothers output

dissipation (b), and for 5 V operation typical power dissipation

is ≤40 mW. With ≤40 mW, the device junction temperature

might rise ≈8°C above the ambient; (TJ ≤ TA + [PD x RθJA]).

Internal power is (usually) not a HED limitation, but designers

should comprehend the basic results of device power dissipation

and its relationship to elevating the sensor IC junction tempera-

ture. IC (and system) reliability is inversely correlated to the

temperature of all system components. Higher ambient and

junction temperatures reduce the life expectancy and depend-

ability of any system.

Dwg. FH-016

GROUND

OUTPUT

DYNAMIC

OFFSET CANCELLATION

LOW-PASS

FILTER

–

SUPPLY

Vcc

Vcc/2

Figure 5: Linear Hall-Effect Sensor with

Dynamic Quadrature Offset Cancellation

Powering Linear Hall-Effect ICs

Although the power requirements for linear HEDs are small,

external power is needed. The source must be stable and well

regulated; and with fixed voltage IC regulators (usually 5 V)

this design issue is easily (and inexpensively) resolved. The

linear sensors specify a maximum supply current of ≤10 mA

with 5 V (typical value ≈7 mA). Easy, on-board, ‘down’

regulation from a system supply is simple with low-cost IC

regulators.

A listing of absolute maximum limits for the new linear,

ratiometric sensors follows in Table 4.

Table 4: Absolute Maximum Limits (TA = +25°)

Supply Voltage, VCC ........................................... 8.0 V

Output Voltage, VOUT .......................................... 8.0 V

Output Sink Current, IOUT ................................. 10 mA

Magnetic Flux Density, B.............................. Unlimited

Package Power Dissipation, PD................... 600 mW*

* ‘UA’ package Rating of 183°C/W.

Operation beyond the above specified limits may affect device

operation, performance, or result in compromising (sacrificing)

circuit and/or system reliability and is (absolutely) not recom-

mended.

Maximum Supply Voltage

The recent linear HEDs, with offset cancellation, permit

operation at a higher supply than the prior generation (A3506,

etc.). These new linear ICs boost the maximum limit to that of

Table 4.

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Distinctive Linear HED Parameters

Various, numerous linear-HED characteristics are of concern in

current-sensing applications, and brief descriptions of these

follow. Subsequently, many of these characteristics and

parameters will be embodied in a focus on accuracy, tempera-

ture effects, linearity, symmetry, etc.

Voltage Output

As mentioned, the ratiometric, linear Hall sensors provide an

output voltage that is proportional to the applied magnetic field

induced by current as illustrated in Figure 2. The output is

specified to sink and source ±1 mA at guaranteed limits. Per

Figures 2, 3, and 4, the usable range is ≥±2.0 V with a 5 V

supply. Also previously mentioned, the quiescent output

voltage is 1/2 the supply when no magnetic field is present (or

current induced). A stable, well-regulated supply is very

necessary for proper operation, otherwise the output voltage

will fluctuate and follow any variations in supply.

Circuit Loading with Hall-Effect Sensors

The linear HEDs present no load to the conductor being sensed.

A ‘no-disconnect’, ‘non-intrusive’ technique is based upon

forming a ‘toriod’ around the conductor being sensed. Rather

than pass the wire through the toroid (Figures 6A & 6B), a soft

iron piece is formed around the conducting wire. This permits

sensing currents without the need for disconnecting any

conductors in the power system (‘no-disconnect’ formed toriod

in Figure 6C).

Figure 6A: Toroidal Current Sensor (<15 A)

Figure 6B: Toroidal Current Sensor (>15 A)

Figure 6C: ‘No-Disconnect Current Sensor

Tolerance to Current Overloads

As mentioned, a conductor current that exceeds the range of the

linear Hall IC forces the output into a non-linear, saturated

condition. Excessive current does not impair or damage the

sensor IC. However, extreme, sustained overcurrent could be a

fire or safety hazard if the conductor overheats and creates a

dangerous situation.

Response Time of Hall-Effect Current Sensors

A review of some of the current sensors utilizing Hall-effect-

based techniques and toriods reveals a rather broad range of

sensor response times. A majority of these (those including

amplifiers) fall within a range of ≈7 µs to ≈15 µs, though others

are below and above these limits. Testing is (normally)

specified with di/dt = 100 A/µs; and the specified linear current

ranges vary from rather low (<5 A) to the extreme (>20,000 A).

Obviously, the 20 kA variety are expensive and do not exploit

any low-cost toroid techniques.

115 Northeast Cutoff, Box 15036

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‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Hall-Effect Sensor Bandwidth

Today, the usable bandwidth of most linear Hall ICs is ≥20

kHz. Signal voltage changes little up to this frequency. How-

ever, noticeable phase shift becomes distinct at somewhat lower

frequencies. Some variation is apparent amongst different ICs

and vendors, but the rolloff is quite steep beyond ≈20 kHz.

Although the cutoff frequency for the -3 dB rolloff of all linear

HEDs is inconsistent, 20 kHz to 25 kHz is a valid approxima-

tion.

Representative oscilloscope plots show the effects of frequency

on the Hall sensor signal. From dc to 500 Hz (Figure 7) no

discernible phase shift materializes. The top signal is the HED

voltage, and the lower trace is the winding (coil) current.

The phase shift becomes quite noticeable with a 10 kHz input

rate (Figure 8), and very apparent at 20 kHz (Figure 9).

Note — testing performed with 20 turns on a gapped toriod; and

the voltage scales of the three plots are not identical. Other

intermediate-frequency plots exhibit similar phase shifts, but

were not included due to space limits.

Also, it should be mentioned that this bandwidth limitation is

correlated with the linear sensor. The magnetics (and induced

coupling) is definitely not a restricting factor to bandwidth

within this range of operating frequencies.

Obviously, with such bandwidth limitations, Hall sensors

cannot sense high-power PWM circuitry exploiting power

MOSFETs or IGBTs at normal, inaudible operating frequencies

(>20 kHz), but a linear HED is viable for dc and ‘mains’ power.

Dwg. WH-018-7

Figure 7: VOUT (Upper) vs IIN (Lower) at 500 Hz

Dwg. WH-018-8

Figure 8: VOUT (Upper) vs IIN (Lower) at 10 kHz

Dwg. WH-018-9

Figure 9: VOUT (Upper) vs IIN (Lower) at 20 kHz

Linear HED Response to Application of Power

Increasingly, systems designers confront stringent power

‘budgets’, and seek techniques to conserve current and power.

Battery-powered and battery ‘backup’ designs are particular

concerns, and any method capable of curtailing power is

scutinized.

A recurring technique is to (periodically) activate the sensor by

switching the power supply ON for brief intervals, and then

OFF for longer periods. Average power is related to duty cycle.

Thus, for low duty-cycle applications, the power consumed can

be decreased substantially. Fixed-voltage IC regulators (with

an ENABLE input) are one very viable circuit technique to

switch the HED supply and reduce average power.

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Clearly, the time required for a linear Hall IC to provide a

stable, usable signal is very important, and two different linear

HEDs were evaluated to ascertain their power-up response

characteristics. The devices exhibit dissimilar properties, and

the oscilloscope plots portray their dynamic operation upon

applying power to the linears. These plots include a 5%

window to compare the settling of the signals as the voltage

attains its final value.

The latest linear HEDs (with dynamic quadrature offset

cancellation) have a slower response than an earlier generation

that exploits the orthogonal Hall element. The previous series

(A3506, etc.) settles to above 95% of final voltage in less than

1 µs (per Figure 10), and takes approximately 15 µs (per Figure

11) to reach its final value. The very obvious tradeoff: speed vs

accuracy and resolution of the signal voltage at power-up.

The newest devices (A3515 and A3516) exhibit a slower

response (≈25 µs to ≥95%, and ≈40 µs for a final, stable voltage

level). These plots reveal basic tradeoffs in performance vs

response speed, and the latent potential for conserving power.

Dwg. WH-018-10

Figure 10: A3506 Power-Up (0.2 µs/Div.)

Dwg. WH-018-11

Figure 11: A3506 Power-Up (2.0 µs/Div.)

Dwg. WH-018-12

Figure 12: A3515 Power-Up (5.0 µs/Div.)

Linear Hall Sensor/Toroid Hysteresis

Tests executed at ±6 A, which induce a substantial output

voltage signal swing, reveal that any error involving hysteresis

is rather minor (≈1% for the combination of linear HED

(A3516) and gapped toriod. Inherently, linear Hall sensors

exhibit no hysteresis. However, different slotted toroids (and

varied magnetic materials) may possess differing hysteretic

properties.

The actual measured voltage differentials ranged from ≈16 mV

to ≈22 mV with ≥2.1 V changes. Hysteresis is a minor concern

when using ferrite cores, but other ferrous cores (such as

powdered iron) may exhibit different characteristics.

Thus, a complete, thorough evaluation of specific toroids and

the associated linear sensor IC would be a very prudent (and

recommended) suggestion.

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

up’ table, this dc parameter very tangibly affects accuracy of

any current-sensing system utilizing linear Hall ICs. By

referring back to both Figures 3 and 4, and Tables 2 and 3, the

significance of dc offset (VOQ, or quiescent output voltage) is

very plain.

The latest ratiometric Hall-effect sensors specify the dc quies-

cent output voltage limits as 1/2 supply ±0.2 V°. The quiescent

output voltage drift over the HED operating-temperature range

corresponds to ±10 gauss with the newest linear Hall ICs.

A significant facet of the static quiescent voltage is its tolerance

limits. Present specifications list ±0.2 V* from the nominal,

and this translates into a ±8% maximum error without any

temperature-induced effects (A3515/3516). Obviously, this

latent error factor poses a formidable constraint, and must be

given serious deliberation if accurate voltages are prerequisite

to system performance.

DC compensation for the quiescent output voltage is feasible by

regulating the supply to achieve the 2.5 V nominal, but this also

influences sensitivity and any interrelated offsets are likely to

prove intolerable in production. Per Figures 3 and 4, boosting

the supply offsets a low quiescent output voltage, and reducing

the supply compensates for a high quiescent voltage. However,

such offsets adversely influence sensitivity and counteract the

positive aspects of ‘nulling’ the quiescent voltage.

Because the sensitivity specifications for the newest linears

encompass a ±10% tolerance without any temperature effects,

‘nulling’ the quiescent output voltage (to 2.5 V) to escape a

±8% error in the quiescent output voltage seems rather absurd.

The dc drift of the earlier linears equated to ±20 gauss for the

‘premium’ type, and ranged to ±50 gauss for a ‘limited’

temperature unit. Also, the ranges of tolerances for quiescent

output voltage of prior ICs was broader (or very much broader)

than the newest ICs with offset cancellation.

This impedes the capacity to design an accurate, precise linear

sensing system that operates over a broad temperature range.

Designs necessitating tight current-sensing tolerances must

confront and reconcile any concerns linked to quiescent output

voltage (value and drift), and these are discussed in greater

detail under accuracy of linear HEDs.

Applying the drift relationships mentioned above, the maximum

quiescent output voltage drift error can be closely approxi-

mated. These calculations are based upon the (nominal) linear

sensitivities:

Core (Toroid) Saturation

Normally, the saturation of a core should not be an issue. A

current-sensor design that employs sufficient turns to drive the

output voltage of the HED to nearly full scale (at the maximum

design current) first induces saturation of the sensor IC. For

optimum accuracy, the number of turns used should induce

output voltage transitions that (just) fall short of saturating the

sensor (more on this).

Zero Crossover

With a linear Hall-effect sensor, zero crossover corresponds to a

zero magnetic field (no induced flux field as B = 0 with 0 A).

The HED output voltage with a zero magnetic field equates to

1/2 supply (i.e., the quiescent output voltage).

Wide-Band Output Noise of Linear HEDs

The wide-band noise of these linear Hall ICs is inconsequential,

and its value linked to the sensor chosen. The testing specifica-

tions for the recent, stable linear Hall IC series are:

B = 0, BW = 10 Hz to 10 kHz, IOUT ≤1 mA

Typical equivalent input noise voltage (Vn) values for the

two series of linears are:

A3506, A3507, A3508: 125 µV

A3515, A3516: 400 µV

Given that the lowest sensitivity of these HEDs is 2.5 mV/G,

plus that accurate measurement is not feasible at very low flux

strengths (more on this later), the consequences of wide-band

noise is (typically) a very minor consideration. Other factors

(particularly quiescent output voltage drift with temperature)

are much more significant.

The System Temperature

A crucial constituent to consider, the temperature range must be

well understood, properly specified (without inordinate mar-

gins), and controlling this very vital design element greatly aids

the ability to realize reasonable accuracy. Note — open-loop

designs cannot (easily) resolve small variations in current. A

core hysteresis of ≈1% precludes this without contemplating the

other (and more acute) effects of temperature upon a linear

HED output parameters and their relationship to performance.

Quiescent Output Voltage (DC Offset)

Essentially, the dc offset of a ratiometric, linear Hall IC relates

to its deviation from the nominal quiescent output voltage (i.e.,

1/2 supply). Lacking a system calibration or individual ‘look- * Refer to addendum.

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

A3515: ±10 G x 5.0 mV/ G ≈ ±50 mV

A3516: ±10 G x 2.5 mV/ G ≈ ±25 mV

A3506: ±20 G x 2.5 mV/ G ≈ ±50 mV

A3507: ±35 G x 2.5 mV/ G ≈ ±87 mV

A3508: ±50 G x 2.5 mV/G ≈ ±125 mV

Essentially, the list establishes the A3516 as the favored linear

when the quiescent voltage drift is an important criteria, and

maximum sensitivity is not the primary consideration. In

current-sensing applications this entails twice the number of

turns (vs. A3515) to attain the same voltage swing.

Over a full-scale voltage swing (≥±2.0 V) the maximum error

with the A3516 is ≤±1.3% but, consistently, quiescent voltage

drift is <±3 G (≈±7.5 mV with the A3516). This error factor is

dependent upon temperature; hence, sufficient turns should be

employed to drive the output near full-scale. This minimizes

the overall effect of temperature-related quiescent output

voltage drift. Therefore, operation near full-range is absolutely

advised as the ∆VOQ error percentage is lower.

Temperature Influence upon Sensor Sensitivity

The nominal sensitivities (and ranges) of both of the new linears

was mentioned previously. However, the circuit tolerances

were unspecified. The ICs have different nominal sensitivities;

however, the temperature-related maximum shifts are identical.

Reiterating sensitivity and range, plus adding the tolerances,

produces the following Hall-effect IC parameters and device

temperature shifts:

A3515:

Sensitivity ........................... 5.0 mV/G, ±10%

∆Sensitivity(∆T) at TA = Max

-2.5% (min), +2.5% (typ), +7.5% (max)

∆Sensitivity(∆T) at TA = Min

-9.0% (min), -1.3% (typ), +1.0% (max)

Magnetic Range ............... ≥±400 G (≥±2.0 V)

A3516:

Sensitivity ........................... 2.5 mV/G, ±10%

∆Sensitivity(∆T) at TA = Max

-2.5% (min), +2.5% (typ), +7.5% (max)

∆Sensitivity(∆T) at TA = Min

-9.0% (min), -1.3% (typ), +1.0% (max)

Magnetic Range ............... ≥±800 G (≥±2.0 V)

Temperature Ranges:

TA (min) ................................................. -40°C

TA (max) ............................. +85°C or +125°C

Essentially, the attainable accuracy of open-loop linear HEDs

involves dc offset and sensitivity.

Accuracy of Open-Loop Linear Hall Sensors

In any classic mystery, at this juncture the ‘plot’ thickens.

Because precise, exacting measurement demands are increasing,

a concise explanation of the interrelated elements associated

with attaining ‘accuracy’ and dependability is next. Accuracy,

repeatability, cost, etc. are very interrelated.

Though parametric maximums can be defined, the cumulative

impact on accuracy is quite nebulous. Also, it is improbable

that all worst-case errors occur coincidentally. Increasingly,

cost-sensitive designs are based upon typical specifications, and

this may precipitate a small (although tolerable) failure rate that

cannot (easily) be decreased.

Pinpointing the absolute accuracy of ‘open-loop’ current

sensing is beyond this treatise. However, reviewing the

essential factors supports analysis.

Hysteresis, hys ........................................... ≈±1%

Output Quiescent Voltage, VOQ ................. ±8%*

(A3515 or A3516 .............2.5 V ±0.2 V)

Output Quiescent Voltage Drift, ∆VOQ ..... ±10 G

(A3515.................................. ≤±50 mV)

(A3516.................................. ≤±25 mV)

Sensitivity, TA = Max ................................. ±10%

(A3515.................................. 5.0 mV/G)

(A3516.................................. 2.5 mV/G)

∆Sensitivity,

TA = Max ..................... -2.5% to +7.5%

TA = Min...................... -9.0% to +1.0%

Positive/Negative Linearity .................... ≈99.7%

Symmetry................................................. ≈99.7%

Wide-Band Noise, en............................... 400 µV

Clearly, some of these elements are very crucial to attaining

accurate current sensing, while others are rather inconsequen-

tial. Fundamentally, errors correlated to hysteresis, linearity,

symmetry, and wide-band noise become quite insignificant.

The factors linked to quiescent voltage and sensitivity are

(absolutely) essential to any implemention of an accurate and

precise current sensing design.

Errors linked to quiescent output voltage drift are range depen-

dent and device related. The ±10 G (typically <±5 G) shift

correlates to a potential error of 50% with a 10 gauss applied

magnetic field. However, the ±10 G drift represents less than

1.5% with a field strength >667 G. Thus, the quiescent voltage

error factor is ‘non-linear’ and is (substantially) diminished with

large output-voltage swings of the A3516 linear HED.

* Refer to addendum.

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

The quiescent output-voltage tolerance is listed as a percentage

(≤±8%)*. This is predicated upon a nominal ratiometric (1/2

supply = 2.5 V), and the specified limits of ≤±0.2 V*. Because

the majority of linear Hall sensors are much closer to nominal

(≤±0.1 V), the ±8% tolerance represents a very ‘worst-case’

quiescent output-voltage scenario.

The sensitivity parameters also pose considerable error poten-

tial. However, these listings equate to a worst-case analysis.

Further, the relationships between sensitivity and the effects of

temperature are not (as yet) completely specified. Whether a

consistent correlation between devices near either limit of

sensitivity and temperature-induced shifts exists is not speci-

fied. The temperature-related effects might be nil, or miniscule

(temperature cancels any cumulative deviations), or cumulative

(temperature further exacerbates the tolerances).

Based upon the published parameters and limits, open-loop

current-sensing designs cannot readily expect to attain results

below ≈±10% to ±15%. However, after reviewing recent plots

based upon test data (A3515/16), the prospect for boosting the

measurement accuracy (absolutely) improves.

Two plots (Figures 13 and 15) delineate VOQ vs temperature.

The +25°C data registers an A3515 minimum of 2.468 V; a

maximum of 2.512 V; the A3516 spans from a minimum of

2.464 V to a maximum of 2.501 V. This is much tighter than

specified. The -3 sigma limits for the ICs are: 2.457 V

(A3515), and 2.462 V (A3516). The +3 sigma data limits are

2.520 V (A3515) and 2.509 V (A3516), and these voltages

convert to well within the published ±8% tolerances* for the

quiescent output voltage of these linears.

Data for the A3515 provides the following:

VOQ -40°C +25°C +85°C +150°C

-3 σ2.448 2.457 2.463 2.472

min 2.461 2.468 2.473 2.481

mean 2.487 2.489 2.493 2.501

max 2.517 2.512 2.520 2.530

+3 σ2.525 2.520 2.523 2.531

in volts with VCC = 5 V and

VOQ -40°C +25°C +85°C +150°C

-3 σ-4.04 0.00 -1.15 -1.54

min -2.90 0.00 -0.60 -0.60

mean -0.59 0.00 0.74 2.38

max 2.60 0.00 2.40 5.50

+3 σ2.86 0.00 2.63 6.31

as a percentage drift from +25°C.

Data on the A3516 discloses similar properties:

VOQ -40°C +25°C +85°C +150°C

-3 σ2.454 2.462 2.462 2.466

min 2.458 2.464 2.467 2.472

mean 2.484 2.485 2.483 2.485

max 2.503 2.501 2.498 2.499

+3 σ2.514 2.509 2.504 2.504

in volts with VCC = 5 V and

VOQ -40°C +25°C +85°C +150°C

-3 σ-3.97 0.00 -3.36 -5.13

min -3.60 0.00 -1.60 -2.90

mean 0.12 0.00 -0.14 0.56

max 3.20 0.00 3.08 5.70

+3 σ4.22 0.00 3.60 6.25

as a percentage drift from +25°C.

The data and plots of ∆VOQ vs temperature also record better

performance than the specified limit of ±10% (earlier listed in

millivolts). Figures 14 and 16 show the VOQ drift is well within

range, and the drift is very small in any narrow temperature

band about +25°C. Clearly, temperature range affects the

output voltage shift tolerances.

Because these plots and data entail characteristics that fall

within certain HED specifications, some earnest deliberation on

the achievable accuracy is absolutely advised (particularly if the

temperature range is limited). Fundamentally, the effects of

temperature are the foremost consideration in any endeavor to

attain single-digit (<10%) precision without calibration and/or

compensation methods.

025 50 75 100

AMBIENT TEMPERATURE IN °C

-50

Dwg. GH-070-13

125

-25

2.55

2.50

2.45

2.40 150

2.60

QUIESCENT OUTPUT VOLTAGE IN VOLTS

+3 σ

-3 σ

MEAN

A3515

Figure 13: VOQ vs Temperature (A3515)

* Refer to addendum.

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

025 50 75 100

AMBIENT TEMPERATURE IN °C

-50

Dwg. GH-070-16

125

-25

5.0

0.0

-5.0

-10 150

+3 σ

-3 σ

CHANGE IN

QUIESCENT OUTPUT VOLTAGE IN GAUSS

MEAN

A3516

Figure 16: ∆VOQ vs Temperature (A3516)

Effects of Sensitivity upon Accuracy

The plots and data for sensitivity confirm that the new linear

HEDs are within published limits, and delineate another (albeit

secondary) constituent in the resolution of accuracy. The

device sensitivity and its interrelated variation over temperature

are conservative, albeit without extreme test margins. Figures

17 through 20 depict the sensitivity data, and the A3515 test

data provides the following:

Sensitivity -40°C +25°C +85°C +150°C

-3 σ4.408 4.683 4.795 4.842

min 4.454 4.793 4.930 4.927

mean 4.761 4.988 5.109 5.121

max 5.181 5.316 5.392 5.359

+3 σ5.113 5.293 5.423 5.400

in mV/G and

Sensitivity -40°C +25°C +85°C +150°C

-3 σ-7.6 0.0 -0.1 -0.7

min -7.1 0.0 -0.9 -1.0

mean -4.7 0.0 2.3 2.5

max -2.5 0.0 3.7 4.4

+3 σ-1.9 0.0 4.6 5.8

as a percentage drift from +25 C.

Data on the A3516 reveals similar properties:

Sensitivity -40°C +25°C +85°C +150°C

-3 σ2.174 2.313 2.393 2.410

min 2.263 2.401 2.465 2.476

mean 2.340 2.457 2.530 2.528

max 2.586 2.700 2.758 2.728

+3 σ2.506 2.600 2.667 2.646

in mV/G and

025 50 75 100

AMBIENT TEMPERATURE IN °C

-50

Dwg. GH-070-14

125

-25

5.0

0.0

-5.0

-10 150

+3 σ

-3 σ

CHANGE IN

QUIESCENT OUTPUT VOLTAGE IN GAUSS

MEAN

A3515

Figure 14: ∆VOQ vs Temperature (A3515)

025 50 75 100

AMBIENT TEMPERATURE IN °C

-50

Dwg. GH-070-15

125

-25

2.55

2.50

2.45

2.40 150

2.60

QUIESCENT OUTPUT VOLTAGE IN VOLTS

+3 σ

-3 σ

MEAN

A3516

Figure 15: VOQ vs Temperature (A3516)

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Sensitivity -40°C +25°C +85°C +150°C

-3 σ-7.1 0.0 1.1 -0.1

min -6.8 0.0 2.0 0.9

mean -5.0 0.0 2.7 2.6

max -4.0 0.0 3.7 4.3

+3 σ-2.9 0.0 4.2 5.3

as a percentage drift from +25°C.

Clearly, neither data nor plots reflect the overall distribution of

the ratiometric linear Hall sensors. This insight into accuracy is

intended to advise of a basic necessity to reconcile the attain-

able limits of precise current sensing with HEDs, but it does not

imply any definite constraint. Ultimately, the application of

innovative, thoughtful circuit-design techniques determines the

essential limits of open-loop Hall-effect current sensing.

Calibration and Compensation

Current-sensing designs endeavoring to realize an open-loop

accuracy below ±10% should consider alternatives. Implement-

ing ‘hardware’ calibration and/or compensation represents a

costly, complex option, and (for most designs) should be

ignored.

Whereas it is very feasible to establish trip points by using a

comparator (or multiple comparators) calibrating, or compen-

sating, for temperature and quiescent voltage to realize a full

range of linear operation is a formidable task. The comparators

can provide discrete current signals (overcurrent, normal

operation, etc.) with useful accuracy, but cannot (easily)

distinguish small current changes.

Increasingly, software is the solution to extending the accuracy

of HED current sensing. Typically, this involves

microcontrollers, µPs, or computers, and a software calibration/

compensation scheme.

Because the linearity, symmetry, and ratiometry of linear HEDs

is ≈100%, these error factors can (largely) be disregarded. The

temperature range is a definite factor if the system requires a

wide operating range. However, a benign environment with a

narrow temperature span alleviates design difficulties. The use

of software (and a µC/µP) to exploit a look-up table necessitates

measuring and storing sufficient data points to implement an

acceptable (and individual) calibration technique for each

current sensor. This (usually) involves the following calibra-

tion/compensation steps:

Measuring and storing VOQ (the null current),

Measuring and storing (specific) current points,

Computing sensitivity from VOQ and data, and

Measuring/storing temperature drift (if needed).

Determining the current level involves employing the ‘look-up’

data to calculate the current value via using the stored VOQ and

sensitivity data.

Measure VOUT and calculate current value

Measure system temperature and compensate for its drift

effects (if a system requirement).

In essence, the ‘look-up’ table corresponds to the ‘calibrated’

linear HEDs already mentioned. This software/look-up table

method can easily achieve <±10% accuracy, and its ultimate

limit (perhaps ≈±1%) is probably constrained by factors linked

to software development, the requisite calibration and compen-

sation (including equipment), and the associated costs and time

of increased accuracy.

025 50 75 100

AMBIENT TEMPERATURE IN °C

-50

Dwg. GH-070-17

125

-25

5.5

5.0

4.5

4.0 150

6.0

+3 σ

-3 σ

MEAN

A3515

SENSITIVITY IN MILLIVOLTS PER GAUSS

Figure 17: Sensitivity vs Temperature (A3515)

025 50 75 100

AMBIENT TEMPERATURE IN °C

-50

Dwg. GH-070-18

125

-25

5.0

0.0

-5.0

-10 150

+3 σ

-3 σ

MEAN

A3515

CHANGE IN

SENSITIVITY IN PER CENT

Figure 18: ∆Sensitivity vs Temperature (A3515)

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

025 50 75 100

AMBIENT TEMPERATURE IN °C

-50

Dwg. GH-070-19

125

-25

2.6

2.4

2.2

2.0 150

3.0

+3 σ

-3 σ

MEAN

A3516

SENSITIVITY IN MILLIVOLTS PER GAUSS

2.8

Figure 19: Sensitivity vs Temperature (A3516)

025 50 75 100

AMBIENT TEMPERATURE IN °C

-50

Dwg. GH-070-20

125

-25

5.0

0.0

-5.0

-10 150

+3 σ

-3 σ

MEAN

A3516

CHANGE IN

SENSITIVITY IN PER CENT

Figure 20: ∆Sensitivity vs Temperature (A3516)

Obviously, the data-storage demands non-volatile memory for

the parametric measurements, and an individual, initial calibra-

tion program. A look-up table compensates for the variations in

quiescent voltage, sensitivity, and temperature effects. The

latent errors associated with these constituents to system

accuracy can be minimized by a software calibration and

compensation technique. Although this may appear to be

complicated and costly, the other solutions are liable to be more

complex and more expensive than using a low-cost 8-bit µC.

Sorting of Hall-Effect Sensors

Although this approach could tighten device output parameters;

presently, only linears with published datasheet limits are

available for sale. Some ‘value-added’ sorting is provided by

others, but this procedure and service is neither common nor

inexpensive. Despite this, specific customers have elected to

solve formidable design issues by outside testing, sorting, and

selecting linear HEDs to specific, tightened device limits.

Clearly, any improvement in availability of presorted HED ICs

is a definite advantage to current-sensing designs, and the

availability of ‘sorted’ HEDs may change.

Size and Form of Sensor Assembly

Because various sizes of toroids with slots expressly cut to fit a

HED package are available (Eastern Components, Inc.), a

typical size cannot be identified. Figure 21 illustrates one basic

configuration that is provided in six different current ranges

(peak current ratings sensed are: 1 A, 3 A, 5 A, 8 A, 10 A, and

100 A). The length, height, and width vary somewhat, and the

largest version measures 0.950" long, 1.025" high, and 0.500"

wide; all versions are PCB through-hole form.

Figure 21: Hall IC Current-Sensing Assembly

Cost of a Current-Sensing ‘Sub-System’

Identifying the costs associated with a linear Hall IC-based

current sensor is virtually as difficult as the various issues

involved with system accuracy. The costs of the indispensable

components (linear HED and slotted toroid) can readily be

determined, and the prices of the complete assembly depicted in

Figure 21 start at ≈$8.00 (1000 quantity).

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Dwg. EP-006-40

Figure 22: ‘Full-Bridge’ with Current Sensors

Alternatively, the linear sensor in series with the winding

(center sensor) provides detection from shorted loads, and also

monitors the actual coil current. Current sensors in both

locations should preclude fire and safety hazards (and protect

any personnel); and high-speed ‘shut-down’ circuitry can

prevent damage to the power outputs (if the overcurrent results

from an external fault such as improper equipment servicing).

Clearly, overall circuit response speed (shutdown time) is

critical to protecting the system and providing safety.

Summary and Perspective

The applications for linear Hall-effect sensors in open-loop

current sensing continue to evolve and expand. Presently, the

devices available are far superior to any earlier linears, and

advancements in design, processing, packaging, testing, etc. are

incessant and relentless. As mentioned, present-day HEDs have

tolerances and temperature drifts that pose formidable chal-

lenges to those intending to design, develop, and implement

systems that demand dependable, single-digit accuracy over a

wide range of system operating temperatures.

Expect further progress in HED performance and temperature

stability, more functional integration, and other developments

that make linear HEDs more viable for higher resolution current

sensing.

Slotted ferrite cores (usually) cost <$1.00 (even in modest

quantities), and the linear Hall-effect sensor costs range from

<$2.50 to <$3.25 (1k pieces). This price span reflects the

various Hall-sensor types and the different temperature ranges.

Obviously, unit costs diminish in higher volumes, and the

combined sensor/toroid cost could easily fall (well) below $3.00

for volume production. A conversion from ferrite cores to

powdered iron toroids with a ‘cast’ gap can meaningfully

reduce overall cost. Rather than ferrites with an $0.80 to $0.85

cost, powdered-iron cores are estimated to be ≈$0.20 to $0.25 in

similar quantities.

However, other factors such as engineering time, software

programming, assembly labor, etc. vary (considerably) based

upon each individual design requirement. Clearly, every system

temperature, resolution, and accuracy are prerequisites that

affect the system cost. The outlays of developing and imple-

menting a high-resolution, very precise design with a wide

temperature range are greatly different than sensing only

excessive current. An overcurrent fault detection application

may allow a very broad tolerance (perhaps ±20%), and this

would not warrant any of the software ‘look-up’, stringent

device and temperature evaluation that a precise, full tempera-

ture design mandates.

Therefore, only the essential components (and the assembly of

Figure 21) can be identified. Costs associated with software

creation, system design engineering, etc. are (well) outside the

realm of utilizing linear Hall ICs for current sensing.

Protecting High-Power Electronics

An classical example of current-sensing detection and protec-

tion for high-power IGBTs is shown in Figure 22. This diagram

can relate to a single-phase of an adjustable speed drive (ASD)

for an ac induction motor or other power circuitry that requires

a full-bridge or triple half-bridge drive (for example, a 3-phase

PM brushless dc motor). Such a configuration can detect

excessive current in the supply rail (upper current sensor). This

can result from shorting the power rail to ground, or a shorted

output combined with a corrresponding IGBT that is activated.

Any combination of either a shorted lower or upper output with

an ON output in the opposite portion of the same ‘leg’ can

result in an (unsafe) overcurrent fault in the system.

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

Acknowledgement

The symbol for a linear Hall-effect current sensor was created

by Raymond Dewey of Allegro MicroSystems. Presently, no

standard or accepted schematic symbol exists for current

sensors utilizing Hall-effect technology.

References

Course: P. Emerald, “Open-Loop Current Sensing for Power

Conversion and Motion System Applications” in Principles of

Current Sensing, PCIM Power Electronics Institute, Chapter

six, PowerSystems World ’97; Baltimore, MD; plus various

contributors of the chapters comprising this one-day profes-

sional advancement course.

Workshop: P. Emerald and Joe Gilbert “Integrated Hall-Effect

Sensors for Motion Control and Positioning Applications”,

PowerSystems World ’95, Long Beach, CA.

Future linears may allow programming the sensor after HED

packaging. This would permit users to tune the gain (sensitiv-

ity), calibrate the output quiescent voltage (VOQ), and compen-

sate for the issues of temperature variations. Clearly, this

involves an innovative, more complex technique in the circuit

design and testing. However, the opportunities for applying

such Hall sensors expand exponentially.

Hall-effect sensors have undergone revolutionary changes since

their integration in the late 1960s. With further advancements

and improvements, the applications for new linear HEDs are

expected to expand and multiply to satisfy the many emerging

needs of future power electronics systems.

ADDENDUM

This tightened specification significantly enhances the ability to

realize more accurate measurements via utilizing these linear,

ratiometric Hall-effect sensors. This means that single-digit

accuracy is a reality for some designs (especially those with

limited temperature fluctuations).

Linear Current Range(s)

Per the original material (page 5) on Linear Current Range,

with ‘tight’ magnetic coupling (≈60 mil gap to match the sensor

package) the ranges are unchanged:

A3515: ≥±400 G ÷ 6.9 G/A ≈ ±58 A

A3516: ≥±800 G ÷ 6.9 G/A ≈ ±116 A

‘Desensitizing’ the magnetic coupling can readily be realized

via expanding (widening) the slot in the toroid. The first

endeavor to desensitize the magnetic coupling involved increas-

ing the slot to 120 mils (≈ twice the package body), and this

reduced the flux coupling and increased the upper current limit

as follows:

A3516: ≥±800 G ÷ 3.85 G/A ≈ ±210 A

Testing revealed that placement of the sensor had no effect

upon the magnetic coupling. Centering the ‘calibrated’ linear

Hall-effect sensor resulted in the same output signal as position-

Since this paper was written (December 1997), and presented, a

change to the specifications for the A3515 and A3516

ratiometric, linear Hall-effect sensors was made. In April 1998,

the new, and tightened, limits for quiescent output voltage were

changed from the original 2.5 V ±0.2 V to 2.5 V ±0.075 V. In

addition to this upgrade in the quiescent output voltage limits,

the effective linear current range can be extended by widening

the toroid gap (i.e., slot) to ‘desensitize’ the magnetic coupling.

Per the page 10 section headed Quiescent Output Voltage

(DC Offset), originally, the specifications listed the ratiometric

output as (nominally) 2.5 V. The limits were 2.3 V (min) and

2.7 V (max) with VCC = 5 V over the device operating tempera-

ture range. This improvement affects the achievable accuracy

of systems applying the ratiometric, linear Hall-effect sensors

(refer to the page 11 section that is headed Accuracy of Open-

Loop Linear Hall Sensors.

As mentioned, this paper shows the following output quiescent

voltage limits:

VOQ.................................. 2.48 V to 2.52 V (±8%)

The upgraded specification now shows this as:

VOQ.............................. 2.425 V to 2.575 V (±3%)

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

‘NON-INTRUSIVE’

HALL-EFFECT

CURRENT-SENSING

TECHNIQUES

ADDENDUM (cont’d)

ing the sensor against either face of the slot. Because many

users endeavor to attain higher current ranges, another evalua-

tion ensued (after new ferrite toroids were obtained from

Eastern Components, Inc.).

The next extension of the current range limit was undertaken

with toroids gapped at 250 mils (e.g., one-quarter inch and more

than 4x the package thickness dimension). This (very) ‘desensi-

tized’ magnetic coupling increased the maximum current limit

per the following calculation:

A3516: ≥±800 G ÷ 1.7 G/A ≈ ±470 A

Further evaluations are intended as toroids gapped with differ-

ing dimensions become available. This should offer a more

complete, albeit overlapping, set of current ranges with an

upper limit (as yet) unknown. Also, other toroid materials

(powdered iron in particular) are to be evaluated.

Summary

The tightened quiescent output voltage tolerance offers better

accuracy for the ratiometric, linear HEDs, and widening the

toroid slot increases the maximum current limitation of these

devices.

This paper was presented at the International Appliance

Technical Conference, Ohio State University, May 6, 1998.

Reprinted by permission.