Technical Information
CAS / CASR / CKSR series Current Transducers
Insulated Highly Accurate Measurements
from 1.5 to 50 A
RMS
CAS / CASR / CKSR series Current Transducers
Future precision.
Future performances.
Now available.
CAS / CASR / CKSR series Current Transducers
Insulated Highly Accurate Measurements from 1.5 to 50 A
RMS
The Power Electronics market is in constant change and
always on the lookout for new technologies and better
performance driving our progress. To enable applications
with enhanced performance, current measurement must
always been made with the best possible performance
also allowing to the final application to differentiate itself
from all the others.
This is the human nature and allows the progress of
technology.
With the LTS / LTSR current transducers using the Closed
Loop Hall effect technology coupled with a dedicated
ASIC (Application Specific Integrated Circuit) specially
designed for these products, we thought we had reached
the optimal performance, but this was without taking the
eternal human nature into account.
The market required even better accuracy over the
temperature ranges maintaining a low price and LEM
decided to achieve this goal.
A few months later, LEM delivers the solution with the
CAS / CASR / CKSR current transducers series covering
nominal current measurements from 1,5 to 50 A
RMS
.
To respond to these new challenges, the Hall effect
technology was no longer the solution. Even if used
in a Closed Loop configuration and with the use of a
dedicated ASIC as done with the LTS family which allowed
a substantial performance improvement notably for the
accuracy and the size.
Fluxgate technology was selected enabling both possible
targets: the improvement in accuracy and the low price.
Without compromising the advantages of the LTS product
such as size, dynamic performances, high measuring
range, ect.
LEM has already been using multiple Fluxgate technologies
in the past and it was just a question to find the one making
the best comprise between price, size and performances.
In order for the products to be able to work in the typical
industrial applications, the insulation criteria needed to be
respected and a particular attention has then been brought
to the mechanical design of the product.
Although we were able to reduce the size even when
nobody thought it could still be done, the insulation
performances allow usage in standard industrial
applications without particular mounting with a rated
insulation voltage up to 1000 V
RMS
(Simple isolation
according to EN 50178 standard with following parameters:
OV 3, PD2).
The CAS / CASR / CKSR models have been specially
designed to respond to the technology advances in drives
and inverters in industrial environment requiring better
performances in areas such as:
• Common mode influence
• Thermal drift (offset and gain)
• Accuracy (in the whole temperature range)
• Response time
• Insulation
• Size
2
Closed Loop current transducers measure current over
wide frequency ranges, including DC. They provide
contact-free coupling to the current that needs to be
measured as well as safe galvanic isolation and high
reliability. Their output signal is an accurate, high-resolution
image of the primary current with a very short delay.
In higher frequency ranges these transducers function
exactly the same way as (passive) current transformers,
where a relatively small induced voltage in the secondary
winding is capable to drive the secondary current through
the secondary winding and, most important, through
the burden resistor. A low induced voltage equals low
magnetic flux in the magnetic core, which is the cause
for the good accuracy (low flux means a small difference
between primary and secondary current linkage
1
, too).
For DC and in low-frequency ranges, the induced voltage
is too low to be able to drive the secondary current, and
the error of simple current transformers will increase with
decreasing frequency. In this domain, the magnetic flux
density in the core is measured by a sensing element and
a voltage is applied to the secondary circuit that in the
end keeps the flux density near zero, effectively creating a
closed control loop.
The only basic difference between the CAS / CASR / CKSR
transducer series and standard Closed Loop transducers
of LEM is that the Hall element used for feedback is
replaced by a Fluxgate detector. The driving force behind
this choice is the need for a “better” feedback, which
basically means more voltage per current linkage, a
quantity that is called “Open Loop sensitivity”. Given
an equal electronic circuit, the zero output of a current
transducer (traditionally called “offset” in analogy to
operational amplifiers) will be less influenced by changes
in the electronics (e.g. offset variations of the amplifiers
used) if the Open Loop sensitivity is higher.
The complexity of a Fluxgate based current transducer
is comparable to the one of a transducer based on a Hall
effect IC (integrated circuit). Like there, some AC signal
processing and synchronous rectifying is applied. In
addition, the Fluxgate detector is needed. Fortunately,
this Fluxgate is a very simple small solenoid with a tiny
soft magnetic strip used as detector core. Because of
the complexity of the signal chain, an IC is used to stay
at a competitive cost level compared to Hall effect current
transducers. A circuit in this IC forms an oscillator together
with the Fluxgate, driving it into saturation each half cycle
at a frequency of several hundred kilohertz. The effect
that is used for the detection of a residual flux in the main
transducer core is the fact that in such a configuration a
change of the duty cycle of the driving voltage will occur
when a magnetic DC flux is present in the fluxgate core.
The signal processing stages in the IC comprise a duty
cycle demodulation, frequency response compensation,
an integrator and a bridge amplifier that provides the
secondary current. This output architecture can provide
a higher (doubled) voltage to the secondary circuit when
compared to a single output stage with the other side of the
circuit connected to a reference potential at typically 2.5 V.
In this configuration, the burden (or measurement) resistor
is floating, so in order to obtain an output signal referenced
to a fixed voltage, a difference amplifier is used which is
also part of the IC.
1
Current linkage is the technical term for current multiplied by turns count
Fig. 1. Closed Loop Fluxgate Technology used for the CAS / CASR / CKSR current transducers
Magnetic Core
R
S
Primary conductor
I
Compensation
I
COMP1
Driver
Diff
Amp
GND
+5V
V
OUT
V
REF
I
P
Fluxgate
Interface
Filter
Int. ref
I
COMP2
Compensation
Winding
Fluxgate
3
CAS / CASR / CKSR Transducers Technology
CAS / CASR / CKSR Transducers Technology:
Closed Loop Fluxgate technology
CAS / CASR / CKSR Series: Main Characteristics
Mechanical and dimensions
CAS / CASR / CKSR have been designed to provide
current measurements from 6 to 50 A
RMS
in a very
compact size compared to the existing current
transducers based on different technologies allowing to
reach similar electrical performances.
Moreover, the same compact design is used to cover
the complete current range from 6 to 50 A
RMS
with 4
standard models (6 A, 15 A, 25 A and 50 A models) for
each series CAS, CASR and CKSR.
The CAS / CASR / CKSR design is
30 % smaller
in
height than LTS transducer (Closed Loop Hall effect
chip technology using an ASIC): 16.5 mm height versus
24 mm. 7.5 mm won in height !
Where LTS and LTSR were limited
to 25 A
RMS
, with the respective
LTS 25-NP
and
LTSR 25-NP
models, as the highest nominal
current, the CAS / CASR / CKSR
models are offered with a model
expected to measure 50 A
RMS
as
nominal current. There has been
a requirement from the market for
years to have a
100% PCB mounted
50 A
RMS
current transducer with
single + 5 V power supply
in such a
class accuracy.
Transducers dimensions
Fig. 2. CAS / CASR / CKSR: 30 % smaller compared to the LTS / LTSR models
4
The CAS and CASR construction uses three U-shaped primary terminals integrated into the housing, providing the
designer with a great flexibility to perfectly adapt the measuring range of the current transducer to his application.
Fig. 4a shows the different connection possibilities.
Number
of primary
turns
Primary nominal
current rms
I
PN
[ A ]
Nominal*
output voltage
V
OUT
[ V ]
Primary resistance
R
P
[ mΩ ] ( typ. )
at +25° C
Recommended
connections
1
2
3
± 25
± 12
±8
2.5 ± 0.625
2.5 ± 0.600
2.5 ± 0.600
0.24
1.08
2.16
* Output voltage CASR 25-NP is used with internal reference.
Fig. 4a. Different nominal current ranges possible according to the primary current circuit configuration (as example: CAS or CASR 25-NP)
Variant 1
Variant 2
When all three U-shaped terminals are connected in
parallel (variant 1 Fig. 4b) the user can measure the
maximum nominal primary current.
Expected for:
I
PN
(I
PMAX
) = 25 A (85 A max)
Expected for:
I
PN
/ 3 (I
PMAX
/ 3) = 8.33 A (28.33 A max)
Fig. 4b. The 2 extreme possibilities for connecting the primary
current circuit (as example: CAS or CASR 25-NP)
The variant 2 (Fig.4b) corresponds to a series connection
of the primary terminals and leads to a reduction of the
nominal measuring range by a factor of 3, but offering a
3 times higher accuracy for low currents.
The
CAS / CASR models are 100% compatible
with the LTS and LTSR models
in regards to
the footprint mounting and also with all the
other models that are available on the market
with the same footprint as the LTS / LTSR.
Fig. 3. CAS and CASR transducers can be mounted at the
exact place of the LTS and LTSR transducers
5
CAS / CASR / CKSR Series: Main Characteristics
Multifunctional primary circuit
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