CURRENT SENSORS REFERENCE DESIGNS Application Note

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1 CURRENT SENSORS REFERENCE DESIGNS Application Note This document describes several reference designs for current sensing applications with either conventional or planar Hall sensors (Triaxis). The designs cover various current ranges (from 2 to 2000A) and conductor types (bus bar, PCB, cable). Also included is a quick selection guide for Hall effect current sensors and general magnetic design guidelines. REV006 Page 1 of 22 Jul-14

2 Table of Contents TABLE OF CONTENTS... 2 CURRENT SENSOR TYPES... 3 CONVENTIONAL HALL SENSORS... 3 PLANAR HALL SENSORS (IMC-HALL )... 3 CONVENTIONAL HALL SENSORS MAGNETIC DESIGN QUICK SELECTION GUIDE Main features Option code and sensitivity range... 6 PLANAR HALL SENSORS (IMC-HALL ) MAGNETIC DESIGN Magnetic field estimation (with ferromagnetic shield) Magnetic field estimation (without ferromagnetic shield) Typical system dimensions for bus bar applications Typical non-linearity at high field Canceling stray field without shield Avoiding cross-talk without shield Additional information QUICK SELECTION GUIDE Main features Option code and sensitivity range REFERENCE DESIGNS PCB application, 2-10A range, multi-layer/multi-turn solution PCB application, 2-10A range, ferromagnetic shield solution PCB application, 10-50A range Cable application, 10-40A range Bus bar application, A range Bus bar application, dual range 5A/200A Bus bar application, A range Bus bar application, A range FERROMAGNETIC MATERIAL SUPPLIERS FERROMAGNETIC SHIELDS Standard U-Shield Mechanical assembly FERROMAGNETIC CORES REV006 Page 2 of 22 Jul-14

3 Current sensor types Melexis provides two types of current sensors suitable for a broad range of applications. Conventional Hall sensors These current sensors are sensitive to the magnetic field perpendicular to the chip surface. They are meant to be used in combination with a ferromagnetic core. In a typical application, the core is wrapped around the current-carrying conductor and concentrates the magnetic flux on a small air gap (typically 2-5mm) where the sensor is inserted. Pros Strong magnetic gain from the core Very robust against cross-talk Suitable for medium to very high currents Cons Performance limited by the core (geometry and material): saturation, hysteresis, frequency response and thermal drift Bigger footprint (size, weight) than solutions based on Triaxis Hall sensors Planar Hall sensors (IMC-Hall ) With the patented integrated magnetic concentrator (IMC) technology, Hall sensors are made sensitive to magnetic fields parallel to the chip surface. The sensors can then directly measure the current flowing in a bus bar or a PCB trace below the package, without the need for a core. Pros Sensitive to magnetic field parallel to the chip surface, for easy integration and low footprint IMC is made of amorphous magnetic material featuring very high permeability and very low hysteresis Magnetic gain from IMC Cons Requires magnetic shield or specific design to avoid cross-talk and/or noise from external fields Less suitable for very high currents/magnetic fields due to IMC saturation limit REV006 Page 3 of 22 Jul-14

4 Conventional Hall Sensors REV006 Page 4 of 22 Jul-14

5 1.1 Magnetic design For this type of application, the sensor is typically enclosed in the air gap of a ferromagnetic core (ring or square), wrapped around the current-carrying conductor. d d I I The magnetic field seen by the sensor for a current I and a ferromagnetic core with air gap d can be approximated as: Equation 1: Magnetic field estimation formula (with core). REV006 Page 5 of 22 Jul-14

6 1.2 Quick Selection Guide Main features Sensitivity [mv/mt] *programmable 5-45* 5-150* Thermal sensitivity drift [ppm/ C] ±150 ±150 Thermal offset drift [mv] ±20 ±10 Non-linearity [%F.S.] ±0.5 ±0.5 Response time [µs] 8 3 Bandwidth [khz] Fast analog output Yes Yes PWM output Yes No Programmable Yes Yes Diagnostic functions Over/Under-voltage detection Broken-track detection Clamping Yes Yes Yes Yes No No Package SOIC-8 VA (SIP) Temp. range [ C] Table 1: Main features and specifications of conventional Hall effect current sensors Option code and sensitivity range Magnetic field Sensor Option code Sensitivity range [mv/mt] range [mt] CAA (25) (80) MLX91207 CAA (10) (200) MLX91209 CAA (50) (40) Table 2: Sensitivity and magnetic field ranges for conventional Hall effect current sensors. REV006 Page 6 of 22 Jul-14

7 Planar Hall Sensors (IMC-Hall ) REV006 Page 7 of 22 Jul-14

2.1 Magnetic design 2.1.1 Magnetic field estimation (with ferromagnetic shield) In a typical application, the current conductor and the sensor are enclosed in a U-shaped ferromagnetic shield to

8 2.1 Magnetic design Magnetic field estimation (with ferromagnetic shield) In a typical application, the current conductor and the sensor are enclosed in a U-shaped ferromagnetic shield to generate a homogeneous magnetic field around the sensor and protect it from external stray fields. W With such designs, the most important dimension is the inner width of the shield. The magnetic field seen by the sensor for a current I and an inner width W can be estimated as: Equation 2: Magnetic field estimation formula (with shield) Magnetic field estimation (without ferromagnetic shield) Depending on application environment and requirements, the ferromagnetic shield is not necessarily required. H W Without shield, the magnetic field seen by the sensor for a current I, a trace width W and a vertical position H can be approximated as: Equation 3: Magnetic field estimation formula (without shield). REV006 Page 8 of 22 Jul-14

2.1.3 Typical system dimensions for bus bar applications The table below displays suggested system dimensions for a typical design with a straight bus bar, a U-shaped ferromagnetic shield and various

Peak current [A] Bus bar Shield inner Magnetic Required sensor section [mm] width [mm] field [mt] sensitivity [mv/mt] 100 10x2 12 10.5 190 200 12x2 12 21 95 400 24x2 25 20 100 800 32x3 35 28.6 70 2.1.4 Typical non-linearity at high field Table 3: Typical system dimensions.

9 2.1.3 Typical system dimensions for bus bar applications The table below displays suggested system dimensions for a typical design with a straight bus bar, a U-shaped ferromagnetic shield and various current ranges. These dimensions can be easily scaled up or down for higher or lower currents according to the magnetic field estimation formula provided above. Peak current [A] Bus bar Shield inner Magnetic Required sensor section [mm] width [mm] field [mt] sensitivity [mv/mt] x x x x Typical non-linearity at high field Table 3: Typical system dimensions. The optimal magnetic field range for the high field current sensors is ±25mT. However, stronger magnetic field can also be applied at the expense of a few % of non-linearity. Magnetic field [mt] Typical sensor nonlinearity [%F.S.] Table 4: Typical non-linearity vs. applied magnetic field for high field current sensors Canceling stray field without shield In AC applications, external stray fields can be easily cancelled out by the microcontroller. Computing the difference between max and min sensor output values provides a signal independent of any parasitic DC field. REV006 Page 9 of 22 Jul-14

2.1.6 Avoiding cross-talk without shield Even without ferromagnetic shield, cross-talk between adjacent current tracks can be avoided by design.

10 2.1.6 Avoiding cross-talk without shield Even without ferromagnetic shield, cross-talk between adjacent current tracks can be avoided by design. Figure 1 shows an example of current trace layout with slots to locally change the current flow axis. The sensors are rotated by 90 with their sensitive axis (blue arrow) parallel to the current trace. In this configuration, there is virtually no cross-talk between phases. Figure 1: Current trace layout with slots and rotated sensors to avoid cross-talk between phases Additional information For more information and recommendations on the magnetic design, please refer to the following documents available on the Melexis website: Small current design guide High current design guide Shield design guide Current sensing in PDU applications Applications-6092.aspx REV006 Page 10 of 22 Jul-14

11 2.1 Quick Selection Guide Main features Sensitivity [mv/mt] High field version Low field version *programmable * * * * Thermal sensitivity drift [ppm/ C] ±200 ±150 ±150 Thermal offset drift [mv] ±50 ±20 ±10 Non-linearity [%F.S.] ±0.5 ±0.5 ±0.5 Response time [µs] Bandwidth [khz] Fast analog output Yes Yes Yes PWM output No Yes No Programmable No Yes Yes Diagnostic functions Over/Under-voltage detection Broken-track detection Clamping No No No Yes Yes Yes Package SOIC-8 SOIC-8 SOIC-8 Temp. range [ C] Yes No No Table 5: Main features and specifications of planar current sensors (IMC-Hall ) Option code and sensitivity range Sensor Option code Sensitivity range [mv/mt] Magnetic field (full scale) [mt] MLX91205 AAL AAH CAL (580) (3.4) CAL (380) (5.3) CAL (250) (8) MLX91206 CAH (270) (7.4) CAH (170) (11.8) CAH (110) (18.2) CAH (77.5) (25.8) MLX91208 AAL (250) (8) BAH (100) (20) Table 6: Sensitivity and magnetic field ranges for planar current sensors (IMC-Hall ). REV006 Page 11 of 22 Jul-14

2.2 Reference Designs Range [A] Conductor Solution

Single layer PCB with C-shaped ferromagnetic shield 20-50

10-40 Cable Simple PCB with clamp-on shield wrapped

shaped ferromagnetic shield 5 + 200 Bus bar Dual range

bar Simple PCB with 25mm U- shaped ferromagnetic shield

ferromagnetic shield Table 7: Overview of the reference

12 2.2 Reference Designs Range [A] Conductor Solution Illustration Multi-turn and multi-layer PCB 2-10 PCB Single layer PCB with C-shaped ferromagnetic shield PCB Single layer PCB with or without ferromagnetic shield Cable Simple PCB with clamp-on shield wrapped around the cable Bus bar Simple PCB with 12mm U- shaped ferromagnetic shield Bus bar Dual range sensor with U- shaped and C-shaped shields Bus bar Simple PCB with 25mm U- shaped ferromagnetic shield Bus bar Bus bar with slots and 12mm U-shaped ferromagnetic shield Table 7: Overview of the reference designs based on planar current sensors (IMC-Hall ). REV006 Page 12 of 22 Jul-14

2.2.1 PCB application, 2-10A range, multi-layer/multi-turn solution PCB with multiple

Can be used with or without ferromagnetic shield, depending on sensitivity and

Figure 2: PCB layout example for very high sensitivity with 6 windings on 3 layers.

Sensitivity (max) [mv/a] 210 350 420 700 Figure 3: Maximum achievable sensitivities

13 2.2.1 PCB application, 2-10A range, multi-layer/multi-turn solution PCB with multiple layers and trace windings (current loops) for very high sensitivity. Can be used with or without ferromagnetic shield, depending on sensitivity and accuracy requirements. Figure 2: PCB layout example for very high sensitivity with 6 windings on 3 layers. 3 windings 3 windings 6 windings 6 windings w/o shield w/ shield w/o shield w/ shield Sensitivity (max) [mv/a] Figure 3: Maximum achievable sensitivities with Melexis evaluation boards. 550mV/A 330mV/A Figure 4: Example of output functions for a six-winding evaluation board, with/without shield. REV006 Page 13 of 22 Jul-14

Closed ferromagnetic shield for high magnetic

one piece inserted through slots on the PCB

14 2.2.2 PCB application, 2-10A range, ferromagnetic shield solution PCB with one layer and a single current trace (no windings). Closed ferromagnetic shield for high magnetic gain. Assembly Concepts Figure 5: Shield in one piece inserted through slots on the PCB edge. Figure 6: Shield in two parts inserted in PCB slits and assembled together. 167mV/A Figure 7: Example of output function for a single-layer PCB with closed shield. REV006 Page 14 of 22 Jul-14

15 2.2.3 PCB application, 10-50A range PCB with one layer and a single current trace. To be used with or without ferromagnetic shield (U-shaped). Sensitivity: up to 60mV/A (without shield) and 170mV/A (with shield). Figure 8: Single-layer evaluation board without and with shield. Figure 9: Shield assembly through PCB slits. 65mV/A 40mV/A Figure 10: Example of output function for a single-layer PCB, with/without shield. REV006 Page 15 of 22 Jul-14

2.2.4 Cable application, 10-40A range The clamp-on shield gathers the magnetic field

Small air gap ensures high magnetic gain.

Figure 11: Cable clamp concept and evaluation board.

16 2.2.4 Cable application, 10-40A range The clamp-on shield gathers the magnetic field around the cable and concentrates it above the sensor package. Small air gap ensures high magnetic gain. Shield geometry can be adapted to match various cable diameters and current ranges. Figure 11: Cable clamp concept and evaluation board. Figure 12: Examples of cable clamp shield dimensions. 50mV/A Figure 13: Example of output function with a cable-clamp demonstrator calibrated for ±40A. REV006 Page 16 of 22 Jul-14

2.2.5 Bus bar application, 100-200A range The planar Hall sensor on its PCB can be mounted directly above the conductor.

17 2.2.5 Bus bar application, A range The planar Hall sensor on its PCB can be mounted directly above the conductor. A simple, low-cost and compact U-shaped shield is mounted around the sensor to protect it from stray fields and ensure good signal robustness against vibrations and displacements. With the dimensions demonstrated here, the linearity error is lower than ±0.5%F.S. up to ±250A. Figure 14: Demonstrator based on MLX91206, U12 shield and 12x2mm bus bar. Figure 15: Shield dimensions. 20mV/A Figure 16: Typical output and non-linearity of a sensor calibrated for ±100A. REV006 Page 17 of 22 Jul-14

2.2.6 Bus bar application, dual range 5A/200A Solution for applications with a wide dynamic range. One sensor with a C-shaped (closed) shield for high sensitivity at small currents (typ. ±5A).

18 2.2.6 Bus bar application, dual range 5A/200A Solution for applications with a wide dynamic range. One sensor with a C-shaped (closed) shield for high sensitivity at small currents (typ. ±5A). One sensor with a U-shaped shield for high saturation limit (typ. ±200A). Other combinations of ranges are possible depending on the application requirements. ±5A range ±200A range View Dimension [mm] thickness = 1mm S=400mV/A 20-25mA accuracy S=10mV/A 200mA accuracy Figure 17: Typical output of the 5A and 200A range sensors. REV006 Page 18 of 22 Jul-14

2.2.7 Bus bar application, 300-400A range A wider

19 2.2.7 Bus bar application, A range A wider ferromagnetic shield (20-25mm) is required to ensure optimal linearity for high currents. Figure 18: Simulation of the magnetic flux inside the shield. 6.7mV/A Figure 19: Typical output and non-linearity of a sensor calibrated for ±300A. REV006 Page 19 of 22 Jul-14

$effective solution is to use a bus bar with slots and measure only a fraction of the total current.$ This solution will only work for DC or constant frequency applications, due to the change of current distribution in the bus bar

This solution will only work for DC or constant frequency applications, due to the change of current distribution in the bus bar

20 2.2.8 Bus bar application, A range For higher currents, instead of scaling-up the whole design (bus bar and shield), an effective solution is to use a bus bar with slots and measure only a fraction of the total current. This solution will only work for DC or constant frequency applications, due to the change of current distribution in the bus bar over frequency. Figure 20: Ferromagnetic shield dimensions. 3.3mV/A Figure 21: Typical output and non-linearity of a sensor calibrated for ±600A. REV006 Page 20 of 22 Jul-14

Ferromagnetic material 3.1 Suppliers Melexis partnered with MagLab and PML India for ferromagnetic material supply. www.maglab.ch www.pmlindia.

This cooperation between maglab and PML offers an efficient and cost-effective solution for customers requiring magnetic shields.

Ferromagnetic shields 3.2.1 Standard U-Shield Figure 22: Ordering information for the standard U-shield from MagLab. 3.2.2 Mechanical assembly Ferromagnetic shields can be assembled by crimping, screwing or bonding (glue or tape).

21 Ferromagnetic material 3.1 Suppliers Melexis partnered with MagLab and PML India for ferromagnetic material supply. Recently, PML and maglab signed an exclusive collaboration in the field of contactless current sensing. This cooperation between maglab and PML offers an efficient and cost-effective solution for customers requiring magnetic shields. maglab takes care of the engineering side, while PML manufactures the products to our specifications. 3.2 Ferromagnetic shields Standard U-Shield Figure 22: Ordering information for the standard U-shield from MagLab Mechanical assembly Ferromagnetic shields can be assembled by crimping, screwing or bonding (glue or tape). The optimal solution depends on the application. In any case, care should be taken to avoid mechanical stress on the part of the shield involved in the magnetic measuring circuit. Figure 23: Several solutions for shield assembly. REV006 Page 21 of 22 Jul-14

22 3.3 Ferromagnetic cores Various material and geometries are available. As shown in Table 8, features vary substantially from core to core and the right choice ultimately depends on application requirements. Material type Saturation Speed Price Ferrite A Fast $ Nanocrystalline FeSi A Fast $$$ Laminated FeNi A Medium $$ Table 8: Some example of ferromagnetic core materials and their main properties. REV006 Page 22 of 22 Jul-14

A Novel Contact-less Current Sensor for HEV/EV and Renewable Energy Applications

Melexis Hall Presentation A Novel Contact-less Current Sensor for HEV/EV and Renewable Energy Applications ROR-MLX Contact-less Current Sensing Melexis markets a patented Hall technology under the brand