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Hello and welcome to the TI Precision Labs series on magnetic sensors. My name is Louis Larsen and I'm the applications manager for current sensing products. In this video, we will discuss hall-effect current sensors, including how they work, the benefits they bring to current sensing applications, and key application challenges such as isolation, magnetic interference, and thermally-limited current capability.

Let's briefly review the different types of magnetic current sensing, or MCS. First, module-based sensing typically utilizes a magnetic toroid or other geometry to concentrate the magnetic field generated by the current carrying conductor. These systems are typically sold by a third-party manufacturer as there is a high degree of magnetic design required.

Next, ambient magnetic current sensing utilizes the ambient in air field generated by a PCB trace, bus bar, or their conductor to sense current. This is accomplished using a linear hall or other magnetic sensor at some fixed mechanical distance from the conductor. This type of solution can also utilize a magnetic concentrator or shield to improve signal levels or reduce the impact of stray fields.

Finally, the third type is hall-effect magnetic current sensing which we will focus on today. In this technology, the current to be measured actually passes through the device package and the magnetic field generated by the current flow through the lead frame is measured internally with an isolated sensor IC.

Let's talk a little bit about hall-effect MCS and how it fundamentally works. In this type of solution the current to be measured passes through the device package via a low impedance lead frame shown here on the left side of the package. The lead frame is the metal structure inside the package that carries signals between the die and the outside world.

The flow of current through a specific lead frame geometry, shown here is a loop or U in the left side of the lead frame, generates a concentrated magnetic field shown here. Following ampere's law, this magnetic field is proportional to current and thus can act as our signal representation of the current flowing through the package.

The lead frame is split into a high voltage isolated side and a low voltage side for the sensor. The sensor is assembled above the high voltage side of the lead frame which carries the current so there is no electrical connection between the lead frame and the sensor. This vertical spacing between the die and the current-carrying conductor provides an electrical barrier which provides isolation capability for the device.

The sensor itself is a precision linear hall sensor with its sensing element placed directly above the most concentrated portion of the magnetic field generated by the lead frame. This precision hall sensor converts the magnetic field signal back into a voltage where the output voltage is proportional to the current input by some sensitivity shown here by S, and which also can be offset by some offset voltage as required by the end application. This conversion from an electrical current to a magnetic signal and then back into a voltage output signal provides two signal domain transformations which is what creates some of the inherent challenges and features of the technology.

Let's dig a little deeper into the signal flow and domain transformation of hall-effect MCS. Remember that the input signal to be measured is a current that is passed through the package lead frame and in many cases through a loop in the lead frame. The lead frame itself has several signals that are either on the loop itself or generated from it.

It has the common mode high voltage of the input current signal. This could be a DC voltage or relatively low frequency AC voltage like that coming from an AC grid. The vertical gap between the lead frame and the sensor die is an electrical barrier blocking the common mode high voltage from the isolated side and is based upon the packaging construction.

There can also be significant voltage transient if the input voltage waveform has some switching in it like a PWM signal. A capacitive shield is integrated into the sensor die assembled above the lead frame loop. This capacitive shield stops the high DV/DT transients from passing into the signal chain and output signal. The result is a magnetic field signal void of electrical coupling between the lead frame and the sensor IC.

This results in excellent immunity to both common mode changes as well as voltage transients, enabling direct measurements of DC or AC signals. The completely integrated passive front end removes the need for any isolated supply, electronics, or external components on the high voltage side.

The resulting magnetic field is a relatively small signal requiring a high precision linear hall sensor IC in order to accurately be measured. A linearized hall sensor converts the magnetic field back into the electrical domain in the form of a small voltage signal on the order of microvolts.

Here, a precision signal chain very similar to the functionality of an INA-type current shunt amplifier is needed to recover this small signal and amplify it to a full scale analog output voltage which can easily be interfaced to the rest of the electronic system.

Because the entire signal flow is integrated into a single monolithic unit, the signal chain can be tuned and calibrated to match the sensor and package mechanics. This enables an out-of-the-box system-level performance based heavily on the performance and flexibility of the signal chain itself.

The integration of hall-effect MCS provides inherent system level benefits, especially in certain key applications. For example, as shown on the left, MCS devices are well suited to in-line pulse-width modulated PLUM current sensing, such as in brushless DC motor control circuits. Since the system quickly switches between high voltages, the high common mode rejection ratio and high common mode transient immunity, CMTI, of MCS devices ensure accurate operation.

Another application example, shown on the right, is isolated high voltage AC and DC power monitoring with overcurrent protection, OCP. Here, the fact that an MCS device does not need any hot-side components and can sense the AC/DC signals directly is very appealing to system designs.

Finally, many benefits of MCS devices are common to a wide variety of current-sensing applications. These include their inherent isolation and handling of high voltages, ease of use, small solution size, low power loss due to the low impedance of the lead frame, and high performance over temperature.

For hall-effect magnetic current sensors, the isolation barrier is typically a very narrow gap between the lead frame and the sensor IC. This is often determined by the quality of the assembly manufacturing process. High voltage isolation performance of an isolator is given at the component level by parameters including maximum repetitive peak voltage, VIORM, working voltage, VIOWM, maximum transient isolation voltage, VIOTM, isolation withstand voltage, VISO, and maximum surge isolation voltage, VIOSM.

These parameters represent the isolation barrier's ability to handle high-voltage stresses of different magnitude and transient profiles and have a direct mapping to realistic operating situations. The first two specs-- maximum repetitive, VIORM, and working voltage, VIOWM, are both intended to quantify the ability of an isolator to handle high voltage across its barrier on a continuous day-to-day basis throughout its lifetime.

The plot on the right shows an example of time dependent dielectric breakdown, or TDDB, the safe operating area, or SOA, shown as the region under the dashed blue line, defines the combination of voltage and lifetime which will result in failure rates of less than one part per million-- PPM. The device in this example achieves a working voltage of 1.5 kilovolt RMS and maximum repetitive voltage of 2.1 kilovolt peak for 40 years.

Maximum transient isolation voltage-- VOITM-- and the isolation withstand voltage-- VISO-- are both intended to quantify the ability of a isolator to handle high voltage across the isolation barrier for very short periods of time. This is tested during certification by stressing the isolator at VIOTM for 60 seconds, followed by a partial discharge test at 1.6 times VIORM for 10 seconds. This is called Method A testing.

Maximum surge isolation voltage, VIOSM, quantifies the ability of the isolator to withstand very high voltage impulses at a specific transient profile, including a charge to 90% of the surge voltage over 1.2 microseconds followed by a discharge to 50% of the surge voltage in 50 microseconds. Surge voltages can be caused by an installation due to direct or indirect lightning strikes, faults, and short circuit events. For more information on high-voltage isolation and the test used to qualify isolated devices, please read the whitepaper linked here.

Because magnetic current sensors measure current via the total magnetic flux at the sensor location they're susceptible to interference from external magnetic fields from multiple sources. If, for example, a current trace runs perpendicular to the device Y-axis it will generate a magnetic field at the sensor depending upon the trace geometry, current level, and distance from the current sensor.

As an approximation, an infinite length wire 10 millimeters away from the sensor and carrying 10 amps will generate a field of roughly 200 microtesla at the sensor location. To the sensor, the external field looks like it is generated by input current through the lead frame and can be referred to the input of the device as an additional error current. In order to do so, the external field should be divided by the magnetic coupling factor G-- how much field an amp of lead frame current generates, as well as the magnetic rejection ratio, BRR, if the sensor utilizes differential sensing to reject external fields.

This additional error current is then summed with the true input current and reflected in the output. Continuing our example with a 200 microtesla external field, if G is equal to 1.2 millitesla per amp and the sensor does not reject external fields for a VRR equal to one, the sensor will effectively see an error input current of 167 milliamps. This additional error can be calculated the same way as an offset error by dividing the actual input current. External fields can be dealt with in several ways in the application, whether by calibration, careful layout and orientation, or magnetic shielding of the device.

The input current capability of any hall-effect current sensor is fundamentally limited by thermal considerations of the device and the larger system. Shown here is a thermal model of a typical device with under 20 amps of continuous input current on a specific PCB. Resistive heating in the loop, due to both higher local resistance and current crowding due to the tight loop geometry, causes a large portion of the applied power to be dissipated right at the loop.

This makes high temperatures develop on the loop which results in elevated temperatures on the entire lead frame as well as the silicon die. In the simulation shown on the right, we see that the die temperature, about 70C, is roughly between the temperature of the loop, 93C, and the low voltage lead frame, 60C.

Maximum junction and package temperatures limit the allowable power dissipated in the current path. Because the lead frame has a resistive temperature coefficient, total power input is a function of both current and lead frame temperature. This power must be dissipated by the thermal resistance of the device to the ambient environment, known as R theta JA, and is effectively added to the ambient temperature.

This means that the continuous current capability of the device declines at higher ambient temperatures and is a strong function of the thermal environment such as the PCB layout, airflow, and heat sinks. It is also worth noting that the thermal time constraints of the device when on PCB is on the order of seconds. So shorter duration but higher amplitude pulses are tolerable so long as they are short enough in duration to prevent excessive heating of the lead frame.

Because the thermal capability of a hall-effect magnetic sensor is so dependent on its PCB layout and system environment, many characteristics of a sensor are characterized and specified to a particular board layout, usually that device's evaluation module or EVM. Please read the device data sheet to see how specs like maximum continuous current and pulse current capability are defined. Note that if you use a different PCB design in your application, you will observe different results.

The plot shown here are some typical examples of how a hall-effect magnetic current sensor's current capability is defined. The plot on the left shows continuous RMS current capability versus ambient temperature. For example, if the application has an ambient temperature of 85 degrees C, the maximum allowed RMS current is 25 amps. Again, this only applies when using the specified PCB layout.

The plot on the right gives us fused current versus current pulse duration. Fused current is the current at which the lead frame of the device begins to fuse open leading to damage and functionality loss. This information is useful both when designing for pulse current measurements greater than the device's continuous current capability or in trying to understand the conditions the device can survive if exposed to high current transients. For example, if ambient temperature is equal to 25 degrees C, given by the black curve, and a 100 amp pulse applied, the device can survive for 0.04 seconds, or 40 milliseconds, until it becomes damaged.

To improve the thermal capability of your PCB, follow these tips. Use thicker copper board layers, such as two ounce or three ounce. Place large radiation traces on both the high voltage and low voltage side. And farms of large amounts of thermal vias near the device pins to improve energy transfer out of the ICM package. Add airflow and heat syncs to the system to help dissipate and remove more heat from the environment. Finally, note that thermal behavior can be modeled and simulated using software like FEMM or ANSYS. While these tools can take time to learn, they can be invaluable in predicting and optimizing your thermal designs.

Achieving good performance with hall-effect magnetic current sensors while also ensuring good thermal capability and safety requires careful attention to be paid to the printed circuit board-- PCB-- layout. The example here with the TMCS1100EVM shows several of the most beneficial PCB layout techniques for these devices.

TI recommends these best practices for optimal thermal capability. Use large copper areas for the input current path, placing three ounce copper on both top and bottom layers of the PCB. Place an array of vias right at the IC pins to help sync heat out of the lead frame. Also, place an array of vias throughout the input current path to ensure a good thermal connection between top and bottom copper.

TI recommends these best practices for optimal magnetic current sensing performance. Place the input current lugs, or connectors, on a direct path straight to the IC input pins and design the input current copper areas to enter the IC at an angle of 80 degrees or less. Both of these techniques ensure balanced current distribution through the lead frame for best sensitivity error. Also, design the input current copper area symmetrically and side-by-side, which helps cancel stray fields from the input current flowing on the PCB. Finally, ensure that plenty of clearance is provided between the high voltage and low voltage sides of the PCB to meet any applicable isolation safety standards.

That concludes this video. Thank you for watching. Please try the quiz to check your understanding of the content. For more information and videos on magnetic current sensors, please visit ti.com/halleffect.

This video is part of a series