Measuring high currents with small current sensors

September 9th, 2019, Published in Articles: EE Publishers, Articles: Energize

Modern current measurement trends in uninterruptible power supplies (UPS) and renewable energy systems is towards smaller physical size, higher current ranges, faster response times and lower cost. The simplicity of open-loop sensors makes them an attractive solution for attaining these objectives.

This article discusses three new families of sensors which allow the nominal current to be as high as 800 A rms and the detection of an overcurrent up to seven times higher. The miniature HLSR xx/SP10 sensors are an extension of the existing HLSR family with an integrated primary conductor; they are can be mounted onto a printed circuit board (PCB) and are the best choice if small physical size is important. The two other families, HOYS and HOYL are busbar mounted devices which use new magnetic circuits optimised for weight and size. The open-loop HLSR and HO sensors previously introduced meet the objectives of size, accuracy, speed and cost but the nominal primary current of the smaller devices is limited to about 50 A by saturation of the ferrite magnetic circuit [1]

Fig. 1: HOYL (left) and HOYS (right) current sensors.

Often, closed-loop sensors are used to meet the accuracy and speed requirements of high current sensors but the secondary coil which cancels the magnetic field from the measured current adds to the size, current consumption, complexity and cost of the device. In high current devices, the secondary must be driven by a high supply voltage or complex electronics. A preferred approach is to use an open-loop architecture in which the imperfections inherent in an open-loop system are mastered by using a complex application-specific integrated circuit (ASIC) as the magnetically sensitive element. In older, lower current sensors any errors of sensitivity or offset, including drift with temperature, was measured during the manufacturing and test produces of the ASIC and stored within it. The corrections needed are applied continuously when it in use and most electrical parameters approach the level of the previous generation of closed-loop sensors.

Devices

HLSR xx/SP10 sensors have the same small physical dimensions and footprint as the existing family members, but the maximum nominal current has been extended from 50 to 120 A by a new FeSi magnetic circuit. In all cases the maximum current which can be measured is 2,5 times the nominal current. The HLSR has an integrated primary and is mounted on a PCB. The four other connections to it are for the secondary side supply, the output voltage Vout and a reference voltage Vref . Vout. Vref is proportional to the measured current.

Fig. 2 shows the new HOYS and HOYL sensors which are respectively “small” and “large” devices for mounting on busbars up to 21 x 12 mm and 39 x 12 mm respectively. Together, they cover the current range from 100 to 800 A with a maximum measurable current of 2000 A. These sensors include an overcurrent detection (OCD) pin. Their compact size is due to the optimisation of the magnetic circuit and package around the busbar and because there are no electronic components inside the sensors except the Hall effect ASIC and two decoupling capacitors.

Performance

The supply for the measuring (secondary) side of the sensors is 3,3 V or 5 V and the output is referenced to half that value, generated by the sensor (although other reference voltage values may be forced from an external source). Apart from the variations in size and mounting arrangement the differences in performance between the HLSR and HOY families are mainly due to their magnetic circuits.

Fig. 2: Example of linearity and magnetic offset of a HLSRxx/SP10 sensor.

In sensor applications, switching speeds are rising and therefore response times must be shorter so that unusually high current and short-circuit conditions are detected quickly. In the new sensors, the CMOS ASIC contains Hall cells as the magnetically sensitive elements and all the signal processing circuits. A high clock speed is used to give a fast response time, less than 3,5 µS, while filters minimise the noise at the sensor output by limiting the ASIC signal path bandwidth to that needed to pass the current waveform.

Two of the most important characteristics of high current sensors are linearity and, when the primary is part of the sensor, thermal dissipation. A comprehensive series of simulations and tests has been performed to validate these aspects of the new sensors.

The capability of the HLSR xx/SP10 magnetic circuit was validated by building a test sensor with an Ipn of 180 A, 50% above the highest production value, and measuring its linearity. The result for a current of Ipm (±450 A) is shown in Fig. 2. The curve shows the difference between the measured output and an ideal perfectly linear output. With a maximum of 0,5% of Ipn it can be seen that the linearity specification for the series sensors is attained with good margin. (Note that short current pulses were used for this test, 450 A is too high for a continuous current in the primary.)

Fig. 3: Example of linearity and magnetic offset of a HOYS sensor.

The capabilities of the HOYS and HOYL sensors are shown by their linearity in Figs 3 and 4 respectively. In these tests, the primary current covered the range ±Ipm, but the linearity error is expressed relative to Ipn, a more demanding specification.

Fig. 4: Example of linearity and magnetic offset of a HOYL sensor.

For the HLSR xx/SP10 sensor, whose primary is part of the device, it is important to know the thermal characteristics when a high current passes through it. Clearly the sensor heating depends on the PCB to which it is soldered as well as the sensor itself. Today’s PCB technology allows for maximum currents of around 100 A. In the example simulated here all of the four layers of the PCB are used; its design and cooling by natural convection maintain the solder joints at 100oC in an environment at 85oC. Fig. 5 shows the results. Only the sensor primary is shown. With a current of 120 A DC, the hottest part of the primary stabilises at 113oC, just lower than the 120oC maximum allowed.

Fig. 4: Thermal simulation of the primary of the sensor.

Overcurrent detection

A particularly useful feature of the sensor family is overcurrent detection (OCD). The input used for OCD detection is taken before the sensor output amplifier and filters – see the simplified block diagram of Fig. 5. This has two advantages: The signal is of lower amplitude so a current level higher than that which saturates the sensor output can be detected, and the OCD response time is faster than that at the output. By default, the OCD threshold is set at 2.93x IPN but 15 other multiples from 0,68 . Ipn to 7,06 . Ipn may be selected at the time of ordering the sensor. The exact multiples available are shown in the datasheets [2]. In the most extreme case, an OCD level of 5648 A may be chosen for the 800 A version of the HOYL sensor. Note that OCD levels are only accurate to about 10 or 20%, depending on the level chosen, but this is more than good enough for the fast warning function OCD performs.  

Fig. 5: Block diagram of the OCD system.

 

Fig. 6 shows an example of the OCD output: The primary current (yellow) is ramped up above the level which saturates the sensor output (red); 2,3 µS after the OCD threshold is crossed its output (blue) falls to zero. The OCD output is an open drain which allows several to be connected to a single warning line. The spread of OCD response times is due to the primary current not being synchronised with the sensor clock.

Fig. 6: Current and sensor waveforms when the OCD is triggered.

The new sensors have excellent isolation characteristics. In all of the HLSR and HOY sensors there is full galvanic separation between the primary and secondary circuits. For example, the 1,2/50 ms “impulse withstand” insulation test allows 8 kV with the HLSR sensors and 9,6 kV with the larger HOYL family.

Sensor reliability

The construction of open-loop current sensors is extremely simple, with only one active component – the Hall effect ASIC – and very few solder joints (none at all in the case of the HLSR family). The reliability of this sensor type offers a FIT rate of 3,4, corresponding to a MTTF of over 294 million-hours.

Conclusion

This article introduces new sensors which allow currents of up to 2000 A to be measured using a simple low-cost open-loop architecture. In many cases their performance will allow them to be used instead of more complex sensors. Their compact size, low supply voltage and the OCD feature will give designers new possibilities to implement efficient and economic systems.

References

[1] www.lem.com/en/file/3135/download

[2] www.lem.com/

Contact Hayley Porter, Diesel Electric Services, Tel 011 493-7079, hayley@dieselelectricservices.co.za

 

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