xCDT Family Sensor | Application NotexCDT Family Sensor – Application Note
May 2023
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TABLE OF CONTENTS
1.

HISTORY OF MODIFICATION .................................................................................................. 3

2.

GLOSSARY ............................................................................................................................. 4

3.

INTRODUCTION..................................................................................................................... 54.

SENSOR ARCHITECTURES ....................................................................................................... 6
4.1. CDT SENSORS ................................................................................................................................. 6
4.1. DCDT SENSORS ............................................................................................................................... 75.

SENSOR LOCATION IN CUSTOMER APPLICATION ................................................................... 8

6.

INTEGRATION IN CUSTOMER APPLICATION ........................................................................... 9
6.1. GENERAL SCHEMATIC ........................................................................................................................ 9
6.2. HIGH FREQUENCY SENSING ELEMENT INTERFACE CIRCUIT OF MEASUREMENT OUTPUT (DCDT ONLY) .............. 11
6.2.1.

Measurement using resistor ............................................................................................... 11

6.2.2.

Measurement using transimpedance amplifier circuit ...................................................... 126.3. TEST WINDING INTERFACE CIRCUIT FOR LOW FREQUENCY SENSING ELEMENT ............................................... 13

7.

6.3.1.

Disconnect injection circuit when not used ........................................................................ 13

6.3.2.

Test Circuit 1: AC and/or DC test capability ....................................................................... 13

6.3.3.

Test Circuit 2: DC or DC+AC test capability ......................................................................... 16

TESTING THE SENSOR .......................................................................................................... 19
7.1. VERIFICATION OF MEASUREMENT CHANNELS (AC AND/OR DC) ............................................................... 19
7.2. VERIFICATION OF LOW FREQUENCY SENSING ELEMENT TEST/INJECTION WINDING ..................................... 20
7.3. VERIFICATION OF HIGH FREQUENCY SENSING ELEMENT TEST/INJECTION WINDING (DCDT ONLY) ................. 21

8.

SENSOR PRIMARY CONDUCTOR ROUTING ........................................................................... 22

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1. History of Modification
Date
Type of Modification
05/08/2020 Creation
Update schematics related to latest version
07/12/2020 Add CDT Sensor paragraphs
Update graphs with SPI symbols
Correct test current values
26/03/2021 Add Paragraph 6.3 Test winding interface
circuit
14/06/2020 Correct test current values
Correct SPI bus recommended schematic.
Removed redundant CDT section.
24/08/2022
Add VCC supply filtering capacitor.
Added SW integration section
Updated architecture graphs.
Changed ‘testing the sensor generic test
setup’ to ‘integration in customer application’.
Changed pull-down/up R, line R, C values.
25/05/2023 TRIP output added in typical customer
application.
SW State Machine moved in SPI
specification.
Added Sensor Primary Conductor Routing

Who
DCO
DCO

DCO
DCO
DBa

DBa

Pages

Version
V0

all
all
all
12/18
19-24
12/18
9-13
9-13
9-13
22
6-7
9
9-10,
19-21

V1

V2
V3
V5

V6

22
22

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2. Glossary
AC: Alternating Current
CT: Current Transformer
DAC: Digital to Analog Converter
DC: Direct Current
EMC: Electro Magnetic Compatibility
GBW: Gain Band Width (used for operational amplifier)
MCU: Micro-Controller Unit
MOSFET: Metal Oxide Semiconductor Field Effect Transistor
OBC: On-Board Charger
PWM: Pulse Width Modulation
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3. Introduction
xCDT sensor family are leakage current sensors dedicated to measure AC and DC leakage
current in any single to 3 phase applications and up to 22kW of charging power.
This family is specially design for automotive On-Board Charger embedded in plug-in hybrid orelectric vehicle.
This application note aims to describe the environment in which the xCDT family sensors areused in customer or test environment.
It covers the following references of CDT sensors:
Item Number

HF Sensing
100 kHz (Y/N)

Safety Capable
Sensor (Y/N)

Number of Primary
Conductors

Primary
Conductor 1
Pin 13/17

CDT 0.3-S2

90.W3.A2.2xx.0*

N

N

2

MountedMounted

CDT-SF 0.3-S2

90.W5.A2.2xx.0*

N

Y

2

MountedMounted

CDT 0.3-S4

90.W3.A2.4xx.0*

N

N

4

MountedMountedMountedMounted

CDT-SF 0.3-S4

90.W5.A2.4xx.0*

N

Y

4

MountedMountedMountedMountedPrimary
Conductor 2
Pin 14/18

Primary
Conductor 3
Pin 15/19

Primary
Conductor 4
Pin 16/20

ReferencePrimary
Conductor 2
Pin 14/18

Primary
Conductor 3
Pin 15/19

Primary
Conductor 4
Pin 16/20

It covers the following references of DCDT sensors:
Item Number

HF Sensing
100 kHz (Y/N)

Safety Capable
Sensor (Y/N)

Number of Primary
Conductors

Primary
Conductor 1
Pin 13/17

DCDT 0.3-S2

90.W4.A2.2xx.0*

Y

N

2

Mounted

DCDT-SF 0.3-S2

90.W6.A2.2xx.0*

Y

Y

2

Mounted

DCDT 0.3-S4

90.W4.A2.4xx.0*

Y

N

4

MountedMountedMountedMounted

DCDT-SF 0.3-S4

90.W6.A2.4xx.0*

Y

Y

4

MountedMountedMountedMountedReference

MountedMountedThe datasheet of each reference of sensor shall be used in addition to this application note.

Note on DCDT variant:
DCDT sensor are generally used for two applications:••Non-isolated OBC topologies. In those designs, common mode current appears due to the loss of galvanic
separation. CT is used to inject a compensation current on primaries to fulfil normative max leakage
current requirements.
Norms that require protection up to 20 kHz, for instance DIN VDE 0664-400. CT allows to monitor leakage
at high frequency above 2 kHz.

CDT variant should be preferred if integrator is not concerned by one of these requirements.

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4. Sensor architectures
The sensor architectures are given as follow.
4.1. CDT Sensors
Non-Safety Version (CDT)

Safety Version (CDT-SF)

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4.1. DCDT Sensors
Non-Safety Version (DCDT)

Safety Version (DCDT-SF)

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5. Sensor location in customer application
The differential sensor placement inside client application, typically a car on-board charger,
must be chosen to minimize EMC interference. It shall be located after surge absorbers at
position 4 to 5.
Typical value of common choke in the customer circuit would be 1mH but any value higher
would be suitable.

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6. Integration in Customer Application
The following section describes typical integration in customer application.
6.1. General Schematic

Primary conductors (sensor jumpers) are part of customer power circuit. Primaries are usuallygalvanically insulated from sensor secondary signals.
5V power supply shall be able to provide the supply voltage and current to the sensor. CVCC
is a mandatory power supply filtering capacitor. CVCC shall be 47 uF.
The current source shall be able to source AC and DC current to be injected into the following
sensor inputs:-

Low Frequency Sensing Element Test/Injection Winding
High Frequency Sensing Element Test/Injection Winding (DCDT only).

Switches SW_TSTLF to SW_TSTHF are used to connect the current source to one of thesensor inputs at a single time.
R Current source shunt can be used to measure the current injected by the current source.
This resistance is not necessary in the case a non-intrusive sensing element is used tomeasure the current from the current source (hall probe…).

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3.3V SPI Communication Module is used to interface with the low frequency sensing element.
It should operate in MASTER mode and shall be compatible with 3.3V signals.
RSPI1 are resistances used as an interface between the SPI module and the sensor to reduce
circulating current due to 3V3 supply imbalance and limit current after SPI bit switch (max MCU
SPI output current is 15 mA). Their value can be chosen in the range of 220Ω to 1kΩ
considering the SPI clock frequency to be used. CSPI2 are 3x 100 pF capacitor set on CLK,
MOSI, /SS to GND internally to sensor for EMC stability. CSPI1 is a capacitor used to filter
MISO signal for EMC with recommended value 100 pF and recommended placement close tocustomer SPI device.
RSPI2 are resistances used as pull-up/pull-down resistors. Their recommended value is 47kΩ.
Recommended value for RTRIP1 is in range 1kΩ to 10kΩ. Recommended values for RTRIP2
is 47kΩ. Recommended values for CTRIP1 is 1 nF.
Different solutions can be connected to the measurement winding of the high frequency
sensing element channel. As this channel is a current transformer, the circuit used must
present a very low input impedance in order to ensure the proper operation of the CT especiallyin the low frequency range. A simple resistor is possible. The best solution would be to interface
the CT with a transimpedance amplifier circuit (this is detailed in a specific chapter below).

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6.2. High Frequency sensing Element interface circuit of measurement output(DCDT only)
The High Frequency Sensing Element is a Current transformer. Characteristics of the CT isgenerally a bandpass filter when looking at frequency response. Low frequency is limited by
the value of the magnetizing inductance. High frequency is limited by the parasitic elements
and magnetic material characteristics. Secondary impedance play a large role in the bandwidthof the CT and that is the reason why the following chapter describe interface circuit for this partof the sensor.
6.2.1.Measurement using resistor

The use of a resistor is possible and convenient for customer as voltage across the resistor
can directly be measured by an ADC.
The value of the resistor play a large role in the bandwidth of the sensor.
High value resistor will increase the voltage to be measured (easier for measurement) but
drastically reduce the frequency bandwidth as for proper operation the measurement windingsof a CT shall be in short circuit (or connected to a low impedance load).
Low value of resistor permits to properly bias the CT but direct voltage measurement is no
more reachable and voltage amplification circuit is required.
The following graph highlight attenuation (Secondary current vs primary current) for different
values of measurement resistors expressed in Ω:

Note: The bandwidth presented is based on a model used to show the impact of themeasurement resistance value. For actual bandwidth of the sensor, please refer to thedatasheet and the set-up used for validation.

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6.2.2.Measurement using transimpedance amplifier circuit

Transimpedance amplifier circuit uses standard operational amplifier circuit in which a current
source I1 is connected at the input and feedback resistor RGAIN is used as amplification.
The circuit is given as follow:

The output voltage is given as follow:
𝑉𝑂𝑈𝑇 = −𝐼1 ∙ 𝑅𝐺𝐴𝐼𝑁
The current source I1 corresponds to the sensor output (secondary of the current transformer).
Diodes D1 and D2 are used for clamping the operational amplifier inputs during di/dt of thejumpers. These diodes shall be low leakage diodes to not impact the measurement in
temperature.
Operational amplifier shall be selected with regards to the application requirement (bandwidth,
accuracy…). In such circuit, the operational amplifier offset shall be minimized as it can leadto saturation of the CT magnetic core.
The capacitor CFILTER is used to filter the high frequencies depending on the requirement.
Voltage output of the circuit is given as follow:
50 Hz

20kHz

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6.3. Test winding interface circuit for low frequency sensing element
As stated in both UL2231 and IEC62752 standards, it is mandatory to test the capability of thesensor to measure accurately leakage current and trip within a certain time prior any charging
cycle.
This chapter highlights some circuits that can be used by customers to inject DC and/or AC
current in the test winding and verify the proper behavior of the sensor for both measurement
and tripping specifications.
In order to properly select the injection circuit, the customer shall define the following
parameters for AC and/or DC characteristics to be tested by the Test/Injection winding:
Test n°AC

Test n°DC

Test 1: Primary Leakage Current value

TBD mARMS @ TBD Hz

Test 1: Primary Leakage Current value

TBD mADC

Test 2: Primary Leakage Current value

TBD mARMS @ TBD Hz

Test 2: Primary Leakage Current value

TBD mADC

…

…

…

…

This will help to define which test circuit must be used depending on the requirement.
6.3.1.Disconnect injection circuit when not used

When the test circuit is not supposed to be used to inject current, the customer shall disconnect
it from the sensor Test/Injection winding instead of setting the test voltage to 0V. It removes
any interference between the test circuit and the sensor that may degrade measurement
performances of the sensor (operational amplifier offset…)
6.3.2.Test Circuit 1: AC and/or DC test capability
6.3.2.1.

General Circuit

The following circuit is so called “Howland Current Pump”. It is the most generic circuit that
operate as a current source. This circuit permits to test the overall sensor capabilities as DC
range and AC range for positive and negative values of current with a limited number ofelectronic components:

The TST_Voltage is a voltage provided by the customer and used to set-up the current injected
through the test winding. When this voltage is positive, the current is sinked by the
Test/Injection winding (leakage current measured is positive) and when it is negative, thecurrent is sourced by the Test/Injection winding (leakage current measured is negative).

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6.3.2.2.

Supply Voltage

This circuit requires dual supply to operate correctly (positive and negative voltage). The supply
voltage shall be high enough to be able to source/sink current to the Test/Injection winding.
As a guidance, the following table summarize the testable primary referred leakage current
ranges versus the supply voltage of the circuit:
Supply Voltage

Testable primary referredleakage current range+/-5V+/-100mADC+/-6V+/-200mADC+/-7V+/-300mADC

The leakage current sensor shall be supplied within its nominal operating range to ensure
proper operation during the injection test.
6.3.2.3.

Leakage current as a function of Test Voltage

In the proposed circuit, 𝑅2 = 𝑅3 = 𝑅4 = 𝑅5 = 47𝑘Ω. The current through the Test/Injection
winding can be calculated as follow:
𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 =𝑉𝑇𝑆𝑇_𝑉𝑜𝑙𝑡𝑎𝑔𝑒= 0.01 ∙ 𝑉𝑇𝑆𝑇𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝑅1

With:
𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 :Current in the Test/Injection Winding expressed in Amperes.

𝑉𝑇𝑆𝑇_𝑉𝑜𝑙𝑡𝑎𝑔𝑒 : Test Voltage expressed in Volts.
𝑅1:

Resistor expressed in Ohms.

The resistor R1 shall be fixed to a value equal or higher than 100Ohms to ensure proper
operation of the sensor. This resistor limits the maximum current sinked/sourced by the test
circuit and the higher the resistor is, the lower the current range to be tested will be.
The leakage current measured by the sensor can be determined as follow:
𝐼𝐿𝐸𝐴𝐾𝐴𝐺𝐸 = 𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 ∙ 𝑁𝑠 = 𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 ∙ 16
With:
𝐼𝐿𝐸𝐴𝐾𝐴𝐺𝐸 :Leakage current measured by the sensor expressed at primary side in Amperes.

𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 :Current in Test/Injection winding expressed in Amperes.

𝑁𝑠 :Number of turns of the Test/Injection winding given in the datasheet.

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The following table summarize as an example several values of DC primary leakage current
corresponding to test/injection winding voltage with supply voltage of +/-7V.
Primary Leakage Current

Test/Injection winding
Leakage Current

Test Voltage-300mADC-18.75mADC-1.875VDC-100mADC-6.25mADC-0.625VDC-6mADC-0.375µADC-0.0375VDC

6mADC

0.375µADC

0.0375VDC

100mADC

6.25mADC

0.625VDC

300mADC

18.75mADC

1.875VDC

To inject AC current, the test voltage shall be AC and the amplitude will define the amplitudeof the current that will be injected in the Test/Injection winding.
6.3.2.4.

Operational Amplifier characteristics

The Operational Amplifier characteristic have strong impact on the test circuit.
As a guideline, the following requirements shall be applied to this component in order to ensure
proper operation of the circuit and the sensor:
Item

Value

Voltage supply

Dual and high enough to the current range to be tested

InputsRail to rail

OutputRail to rail

Output current sink/source≥ +/-25mA

Output Slew Rate

≥ 10V/µs

GBW≥ 20MHz

Input offset voltage< 1mV

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6.3.3.Test Circuit 2: DC or DC+AC test capability
6.3.3.1.

General Circuit

When customer does not have a dual supply voltage available, the following circuit is a unipolar
current source that permit to test only negative leakage current (sourced by the Test/Injection
windings through the injection circuit):

It permits to test DC and combined AC+DC values comprised in the measurement range of thesensor. It is not possible to test pure AC with such circuit due to the diode D1.
6.3.3.2.

Supply Voltage

The supply voltage VCC of the injection circuit must be 5V or higher in order to properly drive
the MOSFET M1.
The leakage current sensor shall be supplied within its nominal operating range to ensure
proper operation during the injection test.
6.3.3.3.

Leakage current as a function of Test Voltage and feeback measurement

The average current sourced by the Test/Injection winding can be approximated by thefollowing equation:
𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 ≈ −𝑘 ∙𝑉𝑇𝑆𝑇𝑉𝑜𝑙𝑡𝑎𝑔𝑒
𝑅6

1 𝑉𝑇𝑆𝑇𝑉𝑜𝑙𝑡𝑎𝑔𝑒
≈− ∙
2
200

With:
𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 :Current in the Test/Injection Winding expressed in Amperes.

𝑉𝑇𝑆𝑇_𝑉𝑜𝑙𝑡𝑎𝑔𝑒 : Setting voltage expressed in Volts.
𝑘:Coefficient linked to the unipolarity of the circuit

𝑅6:

Resistor expressed in Ohms.

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The resistor R6 shall be fixed to a value equal or higher than 200Ohms to ensure proper
operation of the sensor. This resistor limits the maximum current sinked by the test circuit andthe higher the resistor is, the lower the current range to be tested will be.
The coefficient 𝑘 represents the diode that is forward biased only 50% of the time. The negative
sign means that the current is going out of the LF_ITST_IN pin (sourced by the leakage current
sensor).
The leakage current measured by the sensor can be estimated as follow:
𝐼𝐿𝐸𝐴𝐾𝐴𝐺𝐸 ≈ 𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 ∙ 𝑁𝑠 ≈ 𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 ∙ 16
With:
𝐼𝐿𝐸𝐴𝐾𝐴𝐺𝐸 :Leakage current measured by the sensor expressed at primary side in Amperes.

𝐼𝐿𝐹_𝐼𝑇𝑆𝑇_𝐼𝑁 :Current in Test/Injection winding expressed in Amperes.

𝑁𝑠 :Number of turns of the Test/Injection winding given in the datasheet.

In the proposed circuit, the link between the leakage current measured and the Test Voltage isan approximation (that neglect some parameters). In order to verify accurately the operation of
the sensor, the customer shall measure the resistor R6 voltage that corresponds to the realcurrent sinked from the Test/Injection and shall compare this current with those one measured
by the measurement channels of the sensor.
The following circuit can be used for Test/Injection winding current measurement:

The 3rd order low pass filter removes the high frequency content of the current in order tomeasure only the average or low frequency content. It allows the customer to test the sensor
measurement and tripping capability for DC and DC+AC leakage currents with AC content of
50Hz or 60Hz.
The following table summarize as an example several values of DC primary leakage current
corresponding to test/injection winding voltage that can be used by the customer:
Average Primary Leakage
Current

Average Test/Injection
winding Leakage Current

Test Voltage-10mADC-0.62533mADC

0.2075VDC-50mADC

-3.1221mADC

1.1875VDC-100mADC-6.2248mADC

2.375VDC

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6.3.3.1.

Operational Amplifier characteristics

The Operational Amplifier characteristics have strong impact on the test circuit.
As a guideline, the following requirements shall be applied to this component in order to ensure
proper operation of the circuit and the sensor:

6.3.3.1.

Item

Value

Voltage supply

Dual and high enough to the current range to be tested

InputsRail to rail

OutputRail to rail

Output current sink/source≥ +/-25mA

Output Slew Rate

≥ 10V/µs

GBW≥ 20MHz

Input offset voltage

< 1mV

Other

Fast recovery of output from saturation
Phase reversal free

DIODE and MOSFET characteristics

The diode shall have the following characteristics:
Item

Value

Type

Schottky (fast switching with no recovery losses)

Reverse Voltage≥ 20V

Forward continous current≥ 200mA

Forward voltage≤ 300mV @ 10mA

The MOSFET will operate in both switching and linear modes. It shall have the following
characteristics:
Item

Value

Drain-Source Voltage≥ 20V

Drain Current≥ 200mA

Gate-Source Threshold Voltage

1V ≤ VGS ≤ 3V

Drain-Source ON state resistance≤ 1Ω

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7. Testing the sensor
7.1. Verification of measurement channels (AC and/or DC)
This test permits to validate the proper operation of all measurement channels. It consists ofinjecting current in the jumpers and measuring the outputs of the sensor. The test set-up shallbe as follow:

Any of the switches SW1, SW2, SW3 or SW4 shall be closed to inject current in the sensor
jumpers.
The 3.3V SPI Communication Module will get the currents (channel 1 and 2) from the low
frequency sensing element.
Current through R Measurement permit to validate the proper behavior of the High Frequency
Sensing Element.
Both test windings shall be open during this test.
The following table summarizes values that can be tested and expected on the product:

DC

AC(50Hz)

AC(20kHz)Current Source
Value

Ix_IN ValueLF_ITST_IN Value

HF_ITST_IN
Value

Low frequency
element
SPI

High Frequency
Element
HF_MES_IN

100mADC

100mADC00100mADC

0200mADC

200mADC00200mADC

0300mADC

300mADC00300mADC

0100mARMS

100mARMS

00100mARMS

1.25mARMS

200mARMS

200mARMS

00200mARMS

2.5mARMS

100mARMS

100mARMS

00

Not applicable

1.25mARMS

200mARMS

200mARMS

00

Not applicable

2.5mARMS

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7.2. Verification of Low Frequency Sensing Element Test/Injection Winding
This paragraph explains the way to verify the proper behavior of the Test/Injection winding of
the low frequency sensing element.
The following test set-up shall be used:

Primary jumpers are in open-circuit.
Current in injected through the Low Frequency Test/Injection winding using the current source.
3.3V SPI Communication module retrieve the measured current of the Low Frequency Sensing
Element.
The following table summarizes values that can be tested and expected on the product:

DC

AC(50Hz)

Current Source
Value

Ix_IN ValueLF_ITST_IN
Value

HF_ITST_IN
Value

Low frequency
element
SPI

High Frequency
Element
HF_MES_IN

6.25mADC

06.25mADC

0100mADC012.5mADC

012.5mADC

0200mADC018.75mADC

018.75mADC

0300mADC

06.25mARMS

06.25mARMS

0100mARMS

012.5mARMS

012.5mARMS

0200mARMS

0

Note: For the low frequency test winding, the ratio between current injected and measured
depends on the number of turns that is given in the datasheet of the product.

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7.3. Verification of High Frequency Sensing Element Test/Injection Winding (DCDT
only)
This paragraph explains the way to verify the proper behavior of the Test/Injection winding of
the high frequency sensing element.
The following test set-up shall be used:

Primary jumpers are in open-circuit.
Current is injected through the High Frequency Test/Injection winding using the current source.
The following table summarizes values that can be tested and expected on the product:

AC(50Hz)

AC(20kHz)Current Source
current

Ix_IN current

LF_ITST_IN
current

HF_ITST_IN
current

Low frequency
element
SPI

High Frequency
Element
HF_MES_IN

1.25mARMS

001.25mARMS

01.25mARMS

2.5mARMS

002.5mARMS

02.5mARMS

1.25mARMS

001.25mARMS

01.25mARMS

2.5mARMS

002.5mARMS

02.5mARMS

Note: For the high frequency test winding, the ratio between current injected and measured
depends on the number of turns that is given in the datasheet of the product.

xCDT Family Sensor – Application Note
May 2023
This document is the property of LEM Intellectual Property S.A. It shall not be reproduced without prior written approval.

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8. Sensor Primary Conductor Routing
An example of test board routing that is compatible with PCBA of minimum two layers andoffers creepage distances of 4mm is shown here below.

Another possibility is to have one phase per PCB layer in multilayer deisgn.

xCDT Family Sensor – Application Note
May 2023
This document is the property of LEM Intellectual Property S.A. It shall not be reproduced without prior written approval.

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