ASIC Based Closed-Loop Transducersfrom 6 A up to 25 A nominal

TechnicalInformation

Electronic Components

2

ASIC

IN

Closed-loop principle

The LTS series is based on the provenclosed-loop principle, used by LEMyears ago when building the firstcurrent transducer. Since that time, thisprinciple has been used with successfor electrically isolated measurementsof currents and voltages.Every current carrying conductor(Fig. 1) generates a magnetic field,which is concentrated in a magneticcircuit. This field can be measured inan air gap. For that purpose, a Hallelement is used. When it is suppliedwith a constant current it has theproperty of converting the magnetic fluxinto a voltage. When applying theclosed-loop principle, this voltage isused only for balancing the primary andthe secondary flux. An additionalsecondary compensating coil, forexample with 2000 turns, carries1/2000 of the primary current in orderto compensate exactly for the field ofthe primary conductor. The total fluxthen equals zero.

The compensation circuit accepts DCand AC currents up to the frequencylimit of the electronics. Above thatvalue, the transducer works as a nor-mal current transformer with a primaryand a secondary winding. This allowselectrically isolated measurements ofcurrents up to several hundred kHz.

The LTS series offers the first ASICbased closed-loop currenttransducers

The complete know-how and expe-rience of LEM has been used for thefirst time to create an ASIC. All activeelectronic components including theHall sensor are combined in the centralASIC (Fig. 2).

This integration makes it a lot easier tocompensate for the componenttolerances and the temperature shift.Moreover, it improves the EMC, andthe components design is optimised. Inconjunction with the new technology forthe construction of the magnetic circuit,

it has been possible to obtaindimensions for the housing (9.3 mm x22.2 mm x 24 mm, Fig. 3) which are3 times smaller than those of tradi-tional closed-loop current transducerswith similar current ranges.

Main characteristics

Table 1 gives an overview of the maincharacteristics of the current trans-ducers. The power supply voltage is0; +5 V which matches the mostcommonly used processors. In contrastto the existing closed-loop currenttransducers which generally exhibit afactor for current-range to nominal-current ratio of 1.5, a ratio of more than3 can now be obtained. This is a greatadvantage for most applica-tions. TheLTS 25-NP series can preciselymeasure currents up to 80 A for amaximum nominal current of 25 A. Thereference point without any primarycurrent is 2.5 V, which is exactly half ofthe supply rail voltage. The variationspan of the amplified output signal is

ASIC Based Closed-Loop Transducers from 6 A up to 25 A nominalBy Rüdiger Bürkel, Michel Friot, Hans Dieter Huber and François Mortier

The use of electronics is steadily increasing in all areas of dailylife. It starts in the home with domestic appliances, moderncommunication devices, intelligent HVAC equipment andcontinues in many areas ranging from computer andautomotive technologies to the fully automated control ofindustrial processes.In the field of power supplies, a fundamental change in circuittopologies has occurred during the 90’s. Here, digital controlcomponents are becoming more and more important. Thistrend can also be observed in power electronics. IGBT powermodules (IGBT = Insulated Gate Bipolar Transistor) arebecoming increasingly efficient and compact, thus allowingdesigners, in conjunction with other electronic components, tobuild smaller, more compact devices. The objective is alwaysan increase in power density (power per volume) and, at the

same time, a reduction in costs. This innovation trend is onlypossible, if the multitude of new processors is alsoaccompanied by the introduction of appropriate sensors insmaller and more cost-effective versions, equipped with anintegrated, electrically isolated interface for measuring theprocess variables.

Thanks to the creation of ASIC based closed-loop currenttransducers LTS series, LEM has opened new opportunitiesfor the use of galvanically isolated current measuringtechnologies in new market segments. The development ofthese current transducers combines the new constructiontechnology of the magnetic circuit and the integration of thecomplete electronics in a customer-tailored ASIC (ApplicationSpecific Integrated Circuit).

Figure 1 Construction of a closed-loop current transducer Figure 2 Block diagram of the LTS series

Measuring resistor

Operational amplifierClosed-looptransducer

3

- 15V

R

M

-5V 0 V

I

I

A/DI

I

+15 V +5 V +5 V

E DSP/�PRM

-15 V -5 V 0 V

+1 5V

- 15V

+ 5V

R

M

0 V

DSP

o der

P

A/D

I

I

A/D DSP/�PRMI

I

+15 V +5 V

-15 V 0 V

0.625 V/IPN, which results in an outputvoltage of 4.5 V at +80 A and 0.5 V at-80 A (e.g. LTS 25-NP). The currenttransducers also meet the usualstandards for Power ElectronicsSystems.

Excellent accuracy and temperaturestability

The series LTS current transducersachieve a total accuracy better than

�0.2 % at 25 °C. This value includes allkinds of transducer specific tolerancessuch as linearity errors, error of thenumber of turns and effects on thelong-term stability.

In contrast to the majority of closed-loop current transducers available inthe market today, which generallyhave a current output, here the mea-suring resistor has been integrated intothe transducer. LEM has chosenresistors with an accuracy of

�0.5 % and a temperature drift of50 ppm/K maximum.The built-in reference, which is alsonew, reaches a temperature stability of100 ppm/K max. The absoluteaccuracy of the reference is notimportant since, in most cases, it canbe compensated by a processor.

By adding up all tolerances in a tem-perature range from -10 °C to +85 °Cthe following accuracy is obtained forthe LTS series for �T = 60 K:

Main characteristics

Primary nominal current IPN

of LTS 6/15/25 A rms 6 - 15 - 25

Measuring range A 18 - 45 - 80

Accuracy of the transducerat +25 °C(Non-linearity + amplification +long-term stability) % x IN ± 0.2

Total error at +25 °C(0.2% + 0.5% of built-inmeasuring resistor) % x IN ± 0.7

Supply voltage V 0, + 5 (� 5%)

Reference voltage V +2.5 (� 1%)

Temperature drift of referencevoltage (typically) ppm/K 50

Response time @ 90% of IN ns < 200

Bandwidth, < 0,5 dB kHz 0...100

Test voltage, 50-60 Hz, 1 min kV 2.5

Standards pr EN 50 178/HD 625.1

Dimensions l x w x h mm 9.3 x 22.2 x 24

Mass g 10

Table 1 Technical data of the LTS series Figure 4 Block diagramme of existing digital systems for currentmeasurements

Accuracy at 25 °C �0.2 %Tolerance of the measuring resistor �0.5 %Temperature drift of the measuring resistor 50 ppm/K, �T = 60 K �0.3 %Temperature drift of the reference with regard to IN (50 ppm/K typ.) �1.2 %

Total error �2.2 %

Fig. 3 The compact LTS 25-NP

4

Frequency response of LTS 25 NP

-30-28-26-24-22-20-18-16-14-12-10-8-6-4-202468

10

100 1000 10000 100000 1000000Frequency [Hz]

Error [%] (Is-Ip)/Ip*100

-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1012

100 1000 10000 100000 1000000Frequency [Hz]

Phase [°]

CH1 50 mV/div normal 20 At /div 200 ns/div GeneratorCH2 500 mV/div normal 200 ns/div

CH1CH2

LTS 25-NP

DSP/�PA/DI

LTS+5 V

0 V

U

80 (A/�s)

Power supply with new voltagevalues

Digital control systems generallyoperated with a supply voltage of0; +5 V (in the future also 0; +3.3 V).This does not always apply to peri-pheral actif components, as is the casewith current transducers available inthe market today, since they normallyrequire �12V or��15 V to function. Sofar, the signals have been conditionedby using the A/D converter or analogueconverter circuits (Fig. 4).The LTS series (Fig. 5) can do withoutsuch conversion feeding directly into

Exact reproduction of the waveformat the transducer output

Fast power switching devices such asIGBT’s (IGBT = Insulated Gate BipolarTransistor) require a very fast detec-tion of overcurrents for their protection.At a current slope of 80 A/µs (Fig. 6),there is practically no delay to be seenwith regard to the primary current.Thanks to the optimum couplingbetween the primary circuit and thecompensating coil, the transformereffect can be optimised.

Wide frequency bandwidth

The excellent coupling characteristicsare also reflected in the bandwidth. The1 dB limit is situated at approx. 200 kHzand thus exceeds all values ofconventional state-of-the-art Hall effecttransducers. So far, the 3 dB limit ofclosed-loop current transducers hasbeen between 100 and 200 kHz.

Power supply with new voltage values

the signal processor. The user can nowsave costs both on the component itselfand on surrounding component costs:

• suppression of additional operatio-nal amplifiers, measuring resistorand external reference voltage nolonger required,

• smaller size of the printed circuitboard since some components areomitted

• the circuit can design may not nowrequire a �15 V supply voltage

Fig. 5 Block diagramme of a digital systemfor current measurements using the LTSseries

Figure 7 Frequency response of the LTS 25-NP type

Figure 8 Phase response of the LTS 25-NP

Figure 6 Behaviour of the LTS series at a current change.

5

Generator6 kV/�s

CH1CH2

LTS 25-NP

1000 V

15 % of IN

I1

I2

Multifunctional Primary Circuit

The construction uses three U-shapedprimary terminals and an additionalcircular hole in the housing, offering thedesigner greater flexibility to per-fectlyadapt the measuring range of thecurrent transducer to his applica-tion.Fig. 9 shows the different connectionpossibilities.

In Variant 1, all three U-shapedterminals are connected in parallel.This allows the user to measure themaximum primary current.

Variant 2 corresponds to a seriesconnection of the primary terminals andleads to a reduction of the mea-suringrange by a factor of 3, but offering anaccuracy which is 3 times better at lowcurrents.

The measurement of differentialcurrents is possible with Variant 3. Thecurrent measured is the difference ofthe currents I1-I2. The second current isintentionally fed through the holecreates any desired clearance distanceon the printed circuit board, dependingon the potential difference between thetwo phases.

Standards

During the design of the LTS series,the regulations of the EN 50178standard have been taken intoconsideration. All products have a safeinsulation up to a dimensioning voltageof 500 V in symmetrical networks. Thisvalue is valid for mains-circuits withpollution degree 2 and overvoltagecategory III.

All materials are UL-listed (UL =Underwriter’s Laboratories).The marking of the product with the CEmark testifies the conformity with theEuropean EMC Directive 89/336 CEEand the low voltage Directive 72/23EEC.

Behaviour at dv/dt noise

Any electrical component with agalvanic isolation between the primaryand the secondary circuit has a capa-citive coupling between the isolatedpotentials. In applications with highswitching frequencies and consequent-ly with steep switching slopes (i.e. fastvoltage changes on the primary side),this leads to undesirable EMIinfluences (EMI = Electro MagneticInterference).

On the secondary side, i.e. at the out-put of the component, an interferencesignal appears. A voltage change of10 kV/µs in combination with a 10 pFcoupling capacity generates a parasiticoutput current of 100 mA. For the LTSseries, this would be eight times thenominal current.

Figure 10 shows the behaviour at avoltage change of 6 kV/µs and anapplied voltage of 1000 V. Theinterference of 15 % of IN is mainly dueto the cabling layout of the test benchduring measurement. Note the veryshort duration of the disturbance of lessthan 200 ns which can be easilyfiltered. This is very important for theuse with digital regulating circuits usingpulse width modulation (PWM). In thiscase, a small filter is sufficient for theattenuation, in order not to reduce thedynamic characteristics.

Multifunctional Primary Circuit

Figure 9 The different possibilities forconnecting the primary current circuit(e.g. LTS 25-NP)

Figure 10 Immunity of the LTS series to dv/dt noise

Variant 1In/Imax = 25 A/80 A

Variant 2In/Imax = 8 A/27 A

Variant 3Differential currentmeasurementI = I

1-I

2

6

M

Practical examples

1. Electrically isolated currentmeasurements on a converter

The LTS series is open to allapplications in low-power electronicsystems. A typical application field isthe classic frequency inverter. Due toits excellent accuracy and immunity todv/dt noise, it is ideally suited for servo-drive applications.

Figure 11 gives an overview of thevarious possibilities of electricallyisolated current measurements.

Advantages

• excellent linearity for exactmeasurements of the motor currents

• fast response for obtaining shortswitch-off times in case of a faultcondition such as an earth leakageor a short circuit,

• good temperature stability allowsprecise repeatable measurements,

• immunity to high capacitive currentchanges which can result from longmotor cables.

2. Use in vehicles

For the use in vehicles (electricvehicles, fork lift trucks, cars etc.) withvoltages up to approx. 80 V, theelectrical isolation performance is notimportant. Here, other productadvantages are more decisive:

• construction meets requirements ofpick-and-place machines for PCB’s

• high availability, as only one activecomponent is integrated,

• the component is fully potted andthus immune to environmentalinfluences.

3. General current monitoring andregulation

The application possibilities are many:Where currents have to be preciselydetected, regulated and monitored, theLTS series offers possibilities which,perhaps, have not been seriouslyconsidered yet. This applies especiallyto systems in which, until now, only thealternating current has been measured.Non-linear loads are increasinglygenerating non-sinusoidal waveformswhich contain DC currents. Here, theLTS series offers a good alternative tothe classic transformers, because theycan measure both DC and AC currentswith the same device.

It can also be used in DC devices suchas power supplies, battery-poweredequipment or DC drives.In this case, the LTS series offers thefollowing advantages over a shuntresistor:

• much lower power losses,

• electrical isolation,

• better EMI immunity.

Figure 11: Possibilities of electrically isolated current measurements using the LTS series inan inverter

Figure 12 Use of the LTS series in an inverter for its regulation and protection.(Photo LEM, with the kind permission of REFU ELEKTRONIK GmbH)

Practical examples

Rectifier Inverter

LTS for short-circuitdetection

LTS for regulation and protection

LTS for Earth-leakagedetection

7

Advantages of the LTS series

● Using a unipolar power supply 0; +5 V, positive and negative currents can bemeasured.

● High temperature stability and low drift.

● Multirange concept allows a multitude of terminal wirings.

● Low power consumption.

● The closed loop principle provides an excellent linearity, a wide frequencyrange with a short response time, a wide measuring range and the capabilityof measuring short current pulses.

● Production-friendliness due to simple mounting.

● Cost-effective solution.

Applications

The LTS opens all applications in low-power electronic systems such asvariable speed drives, electrical drives for industrial use in heating,ventilation and air conditioning as well as in appliances and industrialdevices, servo drives, small uninterruptible power supplies (UPS), powersupplies and amplifiers, energy management systems and general applicationsof current monitoring.

Table 2 Advantages and applications of the LTS current transducer in an overview

Summary

Table 2 shows all advantages andapplications of the LTS series.This product is the result of a longdevelopment process based on manyapplication-specific solution details.These have been created inpartnership and co-operation withdevelopment and design engineers,where the exchange of ideas andinformation has played a major role.The objective was to improve thecustomers’ products in a verycompetitive environment.

Summary

This allows LEM to create innovativeand cost-effective product solutionswhich offer both the user and themanufacturer new possibilities ofautomated production with repeatableperformances and a high quality level.

Based on the technologies used for theLTS series, further ASIC based closed-loop current transducers offering highercurrent ranges or larger operatingtemperature ranges will follow.

8

991001

IPN = 6 - 15 - 25 AMulti-Range Current TransducerLTS 6-NP, LTS 15-NP, LTS 25-NPFor the electronic measurement of currents: DC, AC, pulsed, mixed,with a galvanic isolation between the primary circuit (high power)and the secondary circuit (electronic circuit).

Electrical dataIPN Primary nominal r.m.s. current 6/15/25 AtIP Primary current, measuring range 0 .. ±18/45/80 AtVC Supply voltage (± 5 %) 5 VVOUT Analog output voltage @ IP = 0 2.5 1) V

± IPN 2.5 ±0.625 VIC Current consumption @ IP = 0, VC = 5 V Typ 20 mAN S Number of secondary turns (± 0.1 %) 2000R L Load resistance � 2 k�R IM Internal measuring resistance (± 0.5 %) 208/83/50 �TCR IM Thermal drift of R IM < 50 ppm/KVd R.m.s. voltage for AC isolation test, 50/60 Hz, 1 min 2.5 kVVb R.m.s. rated voltage 525 2) V

Accuracy - Dynamic performance data

X Accuracy @ IPN , TA = 25°C ±0.2 %X G Global accuracy @ IPN , TA = 25°C ±0.7 3) %�L Linearity <0.1 %

Typ MaxTCVOUT Thermal drift of VOUT @ IP = 0 -10°C..+85°C LTS 6 200 300 ppm/K

LTS 15 100 180 ppm/KLTS 25 50 100 ppm/K

TC�G Thermal drift of the gain -10°C..+85°C 50 4) ppm/KVOM Residual voltage @ IP = 0

after an overload of 3 x IPN ±0.5 mV5 x IPN ±2.0 mV

10 x IPN ±2.0 mV

tra Reaction time @ 10 % of IP max < 50 nstr Response time @ 90 % of IP max < 200 nsdi/dt di/dt accurately followed > 100 A/µsf Frequency bandwidth @ 0 .. - 0.5 dB DC .. 100 kHz

- 0.5 .. 1 dB DC .. 200 kHz

General dataTA Ambient operating temperature - 10 .. + 85 °CTS Ambient storage temperature - 25 .. + 100 °Cm Mass 10 g

Standards EN 50178

Notes: 1) Absolute value @ TA = 25°C, 2.475 < VOUT < 2.525, 2) Pollution class 2,

category III, 3) 0.2 % + 0.5 % due to RIM accuracy, 4) Only due to TCRIM

Features

� Closed loop (compensated) currenttransducer using the Hall effect

� Unipolar voltage supply� Compact design for PCB mounting� Incorporated measuring resistance� Extended measuring range� Insulated plastic case recognized

according to UL 94-V0.

Advantages

� Excellent accuracy� Very good linearity� Very low temperature drift� Optimized response time� Wide frequency bandwidth� No insertion losses� High immunity to external

interference� Current overload capability.

Applications

� AC variable speed drives and servomotor drives

� Static converters for DC motor drives� Battery supplied applications� Uninterruptible Power Supplies (UPS)� Switched Mode Power Supplies (SMPS)� Power supplies for welding

applications.

Copyright protected.

9

6 5 4 OUT

IN 1 2 3

6 5 4 OUT

IN 1 2 3

6 5 4 OUT

IN 1 2 3

VOUT

54.5

3.1252.5

- IPmax - IPN 0 IPN IPmax IP [ At ]

[V]

1.875

○ ○ ○ ○ ○

○ ○

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○

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○

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○ ○ ○ ○ ○ ○ ○

○

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○

○

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○

○

○ ○ ○ ○ ○ ○

○ ○

○

○

○

○

0.5

LEM reserves the right to carry out modifications on its transducers, in order to improve them, without previous notice.

Mechanical characteristics� General tolerance ± 0.2 mm� Fastening & connection of primary 6 pins 0.7 x 0.8 mm

Recommended PCB hole ��1.3 mm� Fastening & connection of secondary 3 pins 0.5 x 0.35 mm

Recommended PCB hole ��0.8 mm� Additional primary connection hole ��3.2 mm

Remarks� VOUT is positive when IP flows from terminals 1, 2, 3 to

terminals 6, 5, 4

Dimensions LTS 6, LTS 15, LTS 25-NP (in mm)

Number Primary nominal Nominal Primary Primary Recommendedof primary r.m.s. current output voltage resistance insertion inductance connections

turns IPN [A] VOUT [V] [m�] [µH]

1 LTS 6-NP ±6 2.5 ± 0.625 0.18 0.013LTS 15-NP ±15LTS 25-NP ±25

2 LTS 6-NP ±3 2.5 ± 0.600 0.81 0.05LTS 15-NP ±7,5 2.5 ± 0.625LTS 25-NP ±12 2.5 ± 0.625

3 LTS 6-NP ±2 2.5 ± 0.600 1.62 0.12LTS 15-NP ±5 2.5 ± 0.625LTS 25-NP ±8 2.5 ± 0.625

Output Voltage - Primary Current

Operation principle

Closed loop Outputtransducer amplifier

Sta

ndar

d 00

Y

ear

wee

k

oder

N°

SP

..

Bottom view

Right view Back view Front view

10

Notes

As far as patents or other rights of third parties areconcerned, liability is only assumed for components per se,not for applications, processes and circuits implemented withcomponents or assemblies. For more details see theavailable data sheets.Terms of delivery and rights to change design or specifica-tions are reserved.

Technical Information "ASIC Based Closed-LoopTransducers from 6 A up to 25 A nominal"

Published by LEM Components��LEM Geneva, Switzerland 1999, e-mail: lsa@lem.com

All Rights reservedThe paper of this publication is produced with pulp bleachedwithout chlorine, neutral sized and non-aging.

11

5 Years Warrantyon LEM Transducers

LEM designs and manufactures high quality andhigh reliability products for its customers over the entire world.

Since 1972, we have delivered several million current and voltage transducers which are,for most of them, still in operation on traction vehicles, industrial motor drives,

UPS systems and many other applications requiring high quality standards.

Our 5 years warranty applies on all LEM transducers delivered fromthe 1st. of January 1996 and is valid in addition to the legal warranty.

The warranty granted on our Transducers is for a periodof 5 years (60 months) from the date of their delivery.

During this period we shall replace or repair at our cost all defective parts(provided the defect is due to defective material or workmanship).

Further claims as well as claims for the compensation of damages, which donot occur on the delivered material itself, are not covered by this warranty.

All defects must be notified to us immediately and faulty material must be returnedto the factory along with a description of the defect.

Warranty repairs and or replacements are carried out at our discretion.The customer bears the transport costs. An extension of the warranty period following

repairs undertaken under warranty cannot be granted.

The warranty will be invalidated if the buyer has modified or repaired, or has had repairedby a third party the material without LEM's written consent.

The warranty does not cover any damage caused byincorrect conditions of use and cases of force majeure.

No responsibility will apply except legal requirements regarding product liability.

The warranty explicitly excludes all claims exceeding the above conditions.

LEM, Geneva, January 1. 1996Business Area Components

Pierre Strübin, Business Area Manager

LEM Components8, Chemin des Aulx, CH-1228 Plan-les-OuatesTel. +41/22/7 06 11 11, Fax +41/22/7 94 94 78e-mail: Isa@lem.com; http://www.lem.com

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Proteus ElectricVia di Noghere 94/1I-34147 Muggia-AquiliniaTel. +39/40/23 21 88Fax +39/40/23 24 40e-mail: proteus@interware.it

Spain

SUMELECDalia, 387E-28109 Alcobendas (Madrid)Tel. 091/650 76 57Fax 091/650 76 63e-mail:abisum@santandersupernet.com

Sweden

Beving Elektronik AB.Jägerhorns Väg 8S-14105 HuddingeTel. 08/680 11 99Fax 08/680 11 88e-mail:information@bevingelektronik.se

Switzerland

SIMPEX Electronic AGWeiherweg 8CH-8604 VolketswilTel. 01/908 20 01Fax 01/908 20 20e-mail: contact@simpex.ch

Switzerland

LEM SA

8, Chemin des Aulx

CH-1228 Plan-les-Ouates

Tel. 022/706 11 11

Fax 022/794 94 78

e-mail: lsa@lem.com

Turkey

Özdisan Electronik PazarlamaGalata Kulesi Sokak N°34TR-80020 Kuledibi/IstanbulTel. 0212/245 40 34Fax 0212/244 59 43

United Kingdom and Eire

LEM U.K.Ltd

Geneva Court

1, Penketh Place, West Pimbo,

Skelmersdale

Lancashire WN8 9QX

Tel. 01695/72 07 77

Fax 01695/5 07 04

e-mail: luk@lem.com

Publication CH 99105 E/US (10.99 • 9.5 • CDH)