ICS Performance Opens New Perspectives for System Design
Here are four reasons integrated current sensors should be considered where accurate control, efficiency, and protection are essential.
This article is published by EEPower as part of an exclusive digital content partnership with Bodo’s Power Systems.
Current sensing is crucial in various electronic devices, including power supplies, battery management systems, e-motor drives, and renewable energy networks. Accurate and reliable current sensing is essential for these devices to be protected and operate efficiently.
Image used courtesy of Adobe Stock
However, there are challenges around current measurement as the power density of devices increases, and the goal is always to do more with less, including having minimal circuit board footprints. Within this environment of space constraints and high power density, integrated current sensors (ICSes) play a vital role.
Ideal for a range of automotive, industrial, or residential applications, ICSes are Hall-effect-based current sensors that incorporate the current conductor, sensing elements, signal treatment die, and some dedicated features such as fault detections and isolation in a single package.
Hall-effect sensing is one way to achieve contactless measurement of the current-induced magnetic field. The Hall cell is the sensing element that converts a change in the magnetic field into a change in its resistance. When a constant current passes through the Hall cell, the voltage output changes proportionate to the magnetic field.
Four key benefits illustrate why integrated current sensors could be a sound investment.
Traditional Hall-effect current sensors use a ferrite core around the current conductor and the sensing elements to concentrate the magnetic field. This core also protects undesired external magnetic fields and noise. Differential measurement makes it possible to remove the ferrite core using two sensing elements (the Hall cells), which both receive the magnetic field to be measured – one with a positive factor, the other negative. The difference between the two fields allows for canceling any additional unwanted magnetic fields.
Integrated current sensors use differential measurement to avoid using a ferrite core. Removing the core delivers several advantages in embedded applications. For example, the cost of the device is reduced, the power density on the sensing side is mechanically increased (up to 75A in 800V applications for LEM ICS products), and measurement is not affected by magnetic hysteresis (when an external magnetic field is applied to a ferromagnet and the atomic dipoles align themselves with it). Finally, frequency and bandwidth are not limited by the inherent saturation of the magnetic element of the core.
Some systems need specific isolation to protect the final user, which means the user interface has to be physically separated from the high-voltage (HV) network and cannot share the same voltage reference level. An ICS integrates the isolation function inside (galvanic isolation) and outside (creepage and clearance distances) of the device, meaning there is no physical connection between the primary conductor where the HV current flows and the secondary circuit with the application-specific integrated circuit (ASIC) chip and the secondary pins. These two sides communicate only through the magnetic field produced by the flowing current.
The ASIC in the ICS is produced using the CMOS semiconductor manufacturing process, allowing specific features to be integrated into the component without adding hardware. For example, all the analog and digital elements required to sense, amplify, and process the proportional voltage signal are manufactured on a single die with semiconductor materials, which also ensures low consumption and power dissipation.
Over-current detection (OCD) is also an important factor. With internal OCD, when the current crosses a threshold, it internally triggers a signal output sent to a dedicated fault pin. This allows the application’s microcontroller to receive the alert information with minimal delay. Otherwise, the action would have to be done internally and based on the current level sent by the sensor, which would take much longer.
Figure 1. OCD enables the microcontroller to react to overcurrent with minimal delay. Image used courtesy of Bodo’s Power Systems [PDF]
Compensation and Additional Integrated Functions
As for stress and temperature compensation, if the ASIC die is subject to mechanical stress from the package, this can create sensitivity drift (the same can occur with temperature variations of -40 °C to +125°C). Internal sensors in the die of the ASIC compensate for this drift to guarantee a linear and accurate sensitivity over a large range of conditions. In a discrete-based design, the temperature of the shunt varies widely with resistive losses, requiring an extra design step in the microcontroller to compensate for this accurately. By contrast, an ICS solution is plug-and-play.
Image used courtesy of Bodo’s Power Systems [PDF]
Figure 2. ICS embeds the primary conductor, two sensing elements on a die, etc. Image used courtesy of Bodo’s Power Systems [PDF]
Traditionally, the voltage output is always proportional to the measured current, but there are two possible reference voltages. In ratiometric mode, Vout is expressed as a percentage of the voltage supply Vcc and requires a stable voltage supply. In fixed (non-ratiometric) mode, Vout is compared to an external reference voltage Vref. The proportional signal is then Vout minus Vref, but when the current to be measured is 0A, Vout = Vref–in other words, the reference voltage sets the quiescent output voltage (zero current mode).
LEM has developed two families of ICS, the HMSR series and the GO series. LEM HMSR and GO-SMS ICSes feature internal and external OCDs for maximal system protection. They are also available with ratiometric and fixed voltage outputs on demand, depending on the system characteristics. While the LEM HMSR series provides extra immunity with its integrated core, the LEM GO series takes full advantage of differential measurements to offer all the performance of a Hall current sensor in a compact, surface-mounted small outline integrated circuit (SOIC)–SOIC8 or 16. For example, the LEM GO-SMS can guarantee a basic isolation of up to 2088V and a reinforced isolation of 1041V (DC or peak working voltage) according to IEC 62368-1.
Plug-and-Play by Design
In summary, integrated current sensors allow designers to realize the current sensing function with a plug-and-play approach and solve virtually all their challenges using a single component. Complete mechanical integration and very low power losses make an ICS’s footprint as small as possible with zero thermal challenges.
By design, contactless measurement with galvanic isolation and standard creepage and clearance distances make ICSes suitable for high-voltage applications and can support a reinforced isolation design strategy. Smaller packages with less isolation and un-populated features can bring the cost down to be cost-competitive where isolation is unnecessary (< 60Vdc). This flexibility in the product definition allows LEM ICSes to be suitable for various products, whether in cost-optimized applications or high-end isolated designs.
The performance of the ICSes is not compromised because all the signal treatment is done in the package with semiconductor elements. This enables the integration of ad hoc, specific system protection mechanisms such as fast overcurrent detection. Depending on the system architecture and design choices, the current-proportional voltage output can be referenced to the supply voltage Vcc or an external Vref.
Integrated current sensors are ideal in many applications where accurate control, efficiency, and protection are needed. LEM’s latest ICSes are suited to applications where space is at a premium and high power density is required.
LEM has an ambitious roadmap to develop even more ICS products that cater to specific customer needs. The next stage on this roadmap will be the launch of the HMSR DA, the first ICS with sigma-delta bitstream digital output.
This article originally appeared in Bodo’s Power Systems [PDF] magazine.